K Younsi

V Russier

L Bessais
email: bessais@glvt-cnrs.fr

Structure and magnetic properties of nanocrystalline PrCo 3

Keywords: numbers: 75.50.Bb, 75.50.Tt, 76.80.+y

The structure and magnetic properties of nanocrystalline PrCo3 prepared by high-energy milling technique have been investigated by means of X-ray diffraction using the Rietveld method coupled to Curie temperature and magnetic measurements. The as-milled samples were subsequently annealed in temperature range from 750 to 1050 • C for 30 min to optimize the extrinsic properties. From x-ray studies of magnetic aligned samples, the magnetic anisotropy of this compounds is found uniaxial. The Curie temperature is 349 K and no saturation reached at room temperature for applied field of 90 kOe. The coercive field of 55 kOe and 12 kOe measured at 10 and 293 K respectively is obtained after annealing at 750 • C for 30 min suggests that nanocrystalline PrCo3 are interesting candidates in the field of permanent magnets. We have completed this experimental study by simulations in the micromagnetic framework in order to get a qualitative picture of the microstructure effect on the macroscopic magnetization curve. From this simple model calculation, we can suggest that the after annealing the system behaves as magnetically hard crystallites embedded in a weakly magnetized amorphous matrix.

I. INTRODUCTION

The intermetallic compounds, RCo 3 (R= rare earth element) have attracted great interest caused mainly by their wide applications as permanent magnets. These compounds present excellent magnetic properties such as large magnetocrystalline anisotropy and important saturation magnetization at room temperature. This interesting magnetic performances of the intermetallic compounds formed by alloying rare earth and 3d transition metals are due to the combination of the complementary characteristics of 3d-itinerant and 4f -localized magnetism. The coercivity is one of the most important extrinsic properties of a permanent magnet. This properties +depends on the microstructure of the material. Recently nanocrystalline compounds have been extensively investigated, their aim is to find the appropriate nanocrystalline state for a highest coercive field.

The most employed process to prepare these materials are rapid quenching by melt spinning [1] and mechanical alloying [2]. More recently high-energy milling technique, has also been used to produce high coercivity nanocrystalline powders. Rivera-Gómez et al are reported in [3] a high coercivity for PrCo 5 compounds milling and annealed at 800 • C for 1 min with H c = 17.3 kOe. A high coercivity of Sm 2 Co 17 nanocrystalline compounds has been obtained by using this method [4] where H c = 9.6 kOe after milling and annealed at 800 • C for 30 min.

In this paper, we report the elaboration of nanocrystalline PrCo 3 by high-energy milling, and their structural and magnetic properties in amorphous state and after re-crystallization. Hard magnetic properties of the as milled PrCo 3 have been improved by a controlled nanocrystallization, which optimize the size of grains in order to obtain a high coercivity.

We also propose a simple model for the system after annealing based on a simple lattice of crystallites embedded in an amorphous matrix. The simplest realization of such a model is considered here, in order to bring out at the qualitative level the microstructure effect on the magnetization curve. This model is treated in the framework of micromagnetism, i.e. through a continuous medium type of approach, and the calculations are performed by using the MAGPAR [5] code based on the finite elements method.

II. EXPERIMENT

A polycrystalline PrCo 3 was prepared by melting highpurity starting elements (> 99.9%) in an induction furnace under a protective environment(Ar atmosphere). Their homogeneity was insured with five times consecutive melting. These ingots were used as pre-alloys to manufacture samples by high-energy milling. All powder handling was performed in a glove box under a highpurity Ar atmosphere with an O 2 and H 2 O rate around 1 ppm with vial sealed glove box.

The mixtures were sealed in hardened steel vial together with 15 mm diameter steel balls. Milling was performed for 5 h under a high purity Ar atmosphere with a ball to powder ratio of 15 : 1, in a high-energy planetary ball mill Fritsch P7. These milling conditions correspond to kinetic shock energy, shock frequency and injected shock power values respectively equal to 0.81 J/hit, 62 Hz and 19.5 watt/g. After milling, the powder mixtures was wrapped in tantalum foil and sealed in silica tubes under a vacuum of 10 -6 Torr, then annealed for 30 min at temperature between 750 and 1050 • C followed by quenching in water.

The purity of the final phase was checked by X-ray diffraction (XRD) using Brucker diffractometer mounted with the Bragg-Brentano geometry and automatic divergence slit, using the CuKα radiation. The unit parameters were measured with Si as standard (a= 5.4308 Å) to insure a unit-cell parameter within the accuracy of ±1×10 -3 Å. The counting rate was 22 s per scanning step and step size of 0.04 • . To determine the microstructural alloy properties (crystallite size) and crystallographic properties (phase identification and lattice parameters), XRD patterns have been analyzed by Rietveld method.

The data treatment was carried out by a Rietveld refinement [6,7] as implemented in the FULLPROF computer code [8,9] with the assumption of a peak line profile of Thompson-Cox-Hastings allowing multiple phase refinement of each of the coexisting phases [10]. It takes into account the broadening of the diffraction lines and gives, via the Scherrer formula, the autocoherent diffraction domain size. The weight percentage of each of the coexisting phases is directly obtained with the approximation of a Brindley absorption factor equal to 1. The goodness-of-fit indicators R B and χ 2 are calculated as usual and described earlier [11,12].

The magnetic anisotropy of PrCo 3 was studied using X-ray diffraction. The X-ray diffraction pattern was recorded on powder sample (grain size < 50 µm) oriented under an external magnetic field. The magnetic field was oriented perpendicular to the surface of the sample and the grains are oriented and shape the needles parallel to the field applied, the powder so directed is analyzed by XRD at room temperature, thus the x-ray pattern gave information about the easy magnetization direction.

Curie temperature T c was measured on a differential sample magnetometer MANICS in field of 0.1 kOe and heating rate of 10 K/min with around 10 mg sample sealed under vacuum in silica tube in order to prevent oxidation under heating. The Curie temperature was determined from the M -T curves by extrapolating linear part of the M -T curves of the intersection with the extended base line. The M -H curves (Magnetic hysteresis measurement) were plotted at T= 10 and 293 K with a Physical Properties Measurement System (PPMS) Quantum Design equipment and a maximum applied field of 90 kOe on sample in epoxy resin.

III. RESULTS AND DISCUSSION

A. Structure analysis

The PrCo 3 compounds crystallize in rhombohedral PuNi 3 -type structure (space group: R 3m). The crystal lattice of PrCo 3 , can be regarded, approximately, as consisting of the consecutive arrangement of alternating blocks of PrCo 5 and PrCo 2 along the c axis with the ra-tion 1 : 2 (Fig. 1), having almost the same structure as in the corresponding compounds PrCo 5 and PrCo 2 . The unit cell contains two non-equivalent crystallographic site for Pr ions, 3a and 6c, and three sites for Co: 3b, 6c and 18h.

The X-ray diffraction (XRD) of the PrCo 3 shows that the compound is single phase with the rhombohedral PuNi 3 -type structure (space group: R 3m). Fig. 2 presents the analysis result of XRD pattern of PrCo 3 ingot using the Rietveld analysis. The lattice parameters of PrCo 3 compound are a= 5.069 Å and c= 24.795 Å. The atomic positions, R B , χ 2 factors from Rietveld fit are given in Table 1. These values of structural parameters are in agreement with the results obtained previously by [START_REF] Burnasheva | Inorganic Materials, translated from[END_REF]. This ingot is an appropriate precursor for using as pre-alloys to manufacture samples by high-energy milling. Figure 3 shows the x-ray diffraction pattern of three samples of PrCo 3 , the as-milled powder and annealed at 750 and 1000 • C for 30 min.

The as-milled powder is typical of a quasiamorphous phase and does not show characteristic lines of diffraction of the crystallized state, however, we observe in the low angles a flattened and large line around 42 • . Such quasiamorphous matrix suggests the presence of various types of small crystallites. The diffraction peaks become more pronounced with increasing annealing temperature. The annealed samples at 750 and 1000 • C for 30 min reveal the presence of PrCo 3 phases crystallized. We have observed a small amount (around 0.5 wt%) of Pr 2 O 3 in all samples annealed after milling.

The mean diffraction crystallite size of the powder after annealing at 750 and 1000 • C for 30 min obtained by fitting the XRD diagram with Rietveld method, ranges between 31 and 45 nm respectively, this implies that synthesized materials are noncrystalline.

B. Intrinsic magnetic properties

The Curie temperature, also called the magnetic ordering temperature is a direct measure of the exchange interaction, which is the origin of ferromagnetism. This interaction depends markedly on the interatomic distance.

Generally, the Curie temperature in rare-earth transition-metal intermetallic compounds is governed by three kinds of exchange interactions, namely, the 3d -3d exchange interaction (J CoCo ) between the magnetic moments of the Co sublattice, 4f -4f exchange interaction (J PrPr ) between the magnetic moments within the Pr sublattice, and the intersublattice 3d -4f exchange interaction (J PrCo ). The interactions between the rareearth spins 4f -4f are assumed to be weak and negligible in comparison with the other two types of interactions [14]. There are two exchange interactions in PrCo 3 compounds, positive and negative. When the distance of the Co-Co pairs is smaller than 2.45 Å, the exchange interactions is negative, whereas at larger Co-Co distances the interaction is positive.

As an example, the thermomagnetic magnetization curve of PrCo 3 high-energy milling sample annealed at 750 • C for 30 min is presented in Fig. 4. This sample has a unique magnetic phase transition at the Curie temperature of 349 K. This result is in agreement with the data given by Lemaire [15].

The low Curie temperature in PrCo 3 compounds is due to the short Co-Co interatomic distances at the sites 6c -18h and at 18h -18h (Table 2), where the Co atoms couple antiferromagnetically. In this structure, the 6c -18h and at 18h -18h interactions are strongly negative. Isothermal magnetization curves M -H obtained at 293 K is represented in Fig. 5 for PrCo 3 . This figure shows that at 293 K the saturation is not reached, which gives evidence for a magnetocrystalline anisotropy among the highest known in those kinds of systems.

The pattern obtained on sample oriented under an external magnetic field, applied perpendicularly to the plane of the sample, shows only (00l) Bragg peaks (Fig. 6), which amount to suggest that the anisotropy strengthens the peaks of diffraction of type (00l) and at room temperature the easy magnetization direction is parallel to the c-axis. A similar result is obtained for YCo 3-x Fe x by [16] obtained by oriented powder under extern field.

IV. SIMULATION

In order to get a first picture of the link between the structure at the nanometric scale and the extrinsic magnetic properties at least at the qualitative level, we have performed micromagnetic simulations on a model for our system. Indeed, micromagnetic simulations provide a useful tool for this purpose and have been done for a wide range of nanocomposite magnetic systems [17][18][19][20][21][22]. It is worth mentioning that quite generally such simulations deal with models for heterogeneous systems including magnetic grains of different materials, typically hard and soft materials, the purpose being to characterize the exchange [17][18][19][20] or the magnetostatic coupling between phases [23]. The general finding is that the intergrains exchange coupling leads to a decrease of the coercivity and an increase of remanence when compared to the values of the hard material component. The magne-tostatic coupling may be more complex due to the long ranged and the anisotropy nature of dipolar interactions [24]. Here, we model a biphasic system, the phases of which are of the same chemical composition. Indeed, as we have seen in the experimental section, the as milled sample is amorphous, and then after annealing well crystallized grains whose size is controlled by the annealing temperature are formed. Our purpose, rather than to provide a systematic investigation, is to draw out some of the salient features concerning the sample structure which may explain the shape of the magnetization curve. We model the experimental sample as an assembly of well crystallized grains, represented by either spherical or cubic inclusions, embedded in the matrix made of the amorphous phase corresponding to the as milled material. The inclusion shape should be discriminated on physical grounds related to the crystallization growth at the temperature considered. This is beyond the scope of the present work, and we will be lead to discriminate the grain shape from the agreement with experimental results. We emphasize that the main difference between these two cases is the larger relative importance taken by the amorphous matrix, playing the role of grain boundaries, in the model with spherical inclusions.

Since the crystallites are well crystallized, they are characterized by the magnetic constants of the perfect massive crystal. We take the experimental value of the saturation magnetization per unit volume, J s = 0.723 T as deduced from both the saturation magnetization per unit formula [15], namely 3.8µ B and the volumic mass as obtained from the measured volume of the unit cell (see table I) which corresponds to Z = 9. The value of the exchange constant is set equal to that of the archetype of hard material (Nd 2 Fe 14 B), namely A ex = 7.7 × 10 -12 J/m and the magnetocrystalline anisotropy constant is fitted on the experimental value of the coercive field we have got at T= 10 K, with the result K 1 = 3.05 10 6 J/m 3 . The amorphous matrix is on the one hand non anisotropic (K 1 = 0), and on the other hand characterized by a very small value of the exchange constant, set somewhat arbitrarily to A ex = 3.8 × 10 -13 J/m. Indeed, since the coercivity remains very high we expect in our two phase model only a negligible exchange coupling between the matrix and the inclusions. Then, concerning the matrix saturation magnetization, we expect a much smaller value than for the crystallized phase of the inclusions [25]; therefore we consider either a non magnetic (J s = 0) or only weakly polarized (0.05 T ≤ J s ≤ 0.25 T ) matrix. As we shall see these two last points can be justified by the magnetization curve shape. The inclusions have all the same size, while their easy axis, ni are distributed either randomly or in a solid angle Θ ≤ Θ M centered on the direction of the external field, ĥext in order to model the effect of the texturation by the field. The influence of the preferential orientation has been investigated only in the case of spherical inclusions. The inclusions are located on the sites of simple cubic lattice. The inclusion packing fraction is therefore limited to values smaller than π/6 in the case of spherical inclusions; we set the nearest neighbor distance d = 2.106 R close to the minimum value d = 2 R, which leads to ϕ = 0.445. We have not such limitation when dealing with cubic inclusions. In this later case the inclusions are separated by a grain boundary of thickness, say δ, and the volume fraction is then ϕ = (1 + δ/2R) -3 where the inclusion hedge length is set to 2 R. In the framework of the micromagnetism, the value of δ cannot be reduced at will since δ must not reach the microscopisc scale; here we consider either δ/d = 0.10 or δ/d = 0.237 where d is the minimum intergrain distance which leads to ϕ = 0.73 or 0.445 respectively. The embedding matrix is a parallepipedic box including 4 layers of 64 lattice sites and the total number of particles taken into account is thus 256. The applied field is set normal to the layers. In order to simulate as far as possible the experimental case, we have chosen R = 17 nm for the spherical inclusions and 2 R = 32.5 nm for the cubic inclusions. Accordingly the size of the matrix is 288 nm ×288 nm ×144 nm and in between 297 nm ×297 nm ×158 nm and 340 nm ×340 nm ×170 nm in the case of spherical and cubic inclusions respectively. The micromagnetic calculations are performed by using the MAGPAR code [5], based on a finite elements scheme, from either the LLG equation or the total energy minimization. Most of the calculations have been performed with a mesh including 1632000 tetrahedra, and we have checked on a refined mesh of 3159000 tetrahedra the convergence versus the element size of the results in the case of spherical inclusions.

The results for the model with spherical inclusions are displayed in figure (8). First of all, we consider the case of the non magnetic matrix (J s = 0) with randomly oriented particles. As expected, the coercivity takes a large value, namely totally governed by that of the particles, since the exchange constant of the matrix is too low for the particles to be exchanged coupled to and via the matrix. We have checked that even with J s = 0 a significant value of the exchange constant for the amorphous matrix leads to a decrease of the coercivity, and this as already mentioned justifies our choice of a very small value of A ex for the matrix. However, the main qualitative characteristic which is missing in this first calculated magnetization curve when compared to the experimental one is the small kink in the vicinity of zero external field. Such a kink may be the signature of an uncoupled two phase magnetic behavior. Hence, we have increased the value of the matrix saturation magnetization in order to get such a behavior and we find that J s = 0.05 T leads to a qualitative agreement with the experimental curve. When the matrix J s value increases the kink amplitude becomes much larger than the experimental one. As a matter of fact, the kink relative amplitude behaves roughly as The agreement between the calculated hysteresis curve and the experimental one, for randomly distributed axes and J (mat) s = 0.05 T is qualitatively satisfying. We have to mention however, that we display in figure (8) the magnetization curve reduced by the total saturation magnetization, M s , of the system the value of which, M s = ϕJ

(1 -ϕ)J (mat) s (1 -ϕ)J (mat) s + ϕJ (incl) s
(incl) s + (1 -ϕ)J (mat) s
is significantly smaller than the experimental one. This is due both to the small value of the saturation magnetization of the amorphous matrix obtained to reproduce the amplitude of the kink at H ex = 0, and to the large fraction of the total volume occupied by the matrix. The model with cubic inclusions can remedy to this drawback since then the inclusion packing fraction is not limited as already mentioned. The results obtained with ϕ = 0.73 are displayed on figure (9) for the matrix saturation magnetization J mat s = 0.05 T, 0.125 T and 0.250 T. The first conclusion is that at the qualitative level, the hysteresis curve looks like that of the system with spherical inclusions which may be the consequence of the easy axis random orientation. Now, since the matrix volume fraction is strongly reduced, the amplitude of the magnetisation kink due to the matrix magnetization reversal compares to the experimental one with a larger value for J mat s than in the model with spherical inclusions, namely J mat s ∼ 0.125 T. However, increasing the matrix saturation magnetization leads to an important matrix/inclusion magnetostatic coupling resulting in a non negligible reduction of the coercivity (see figure 9). Thus the best agreement with the experimental results is obtained with the model with cubic inclusions, ϕ = 0.73 and J . The simulated magnetization curve of the model with cubic inclusions and ϕ = 0.445 is quite close to the that of the model with spherical inclusions and is thus not shown. We also get an important reduction of the coercivity with the increase of the matrix saturation magnetization. Then, and only for the model with spherical inclusions, we have examined the effect of the texturation via the easy axes distribution. In this calculation, the maximum value of the polar angle Θ of the axes ni is either π/15 or π/2.3 for a strong or a very weak preferential orientation respectively. Notice the strong sensitivity to the variation of Θ M ; indeed the random distribution corresponds to Θ M = π/2. We emphasize that this orientation effect may represent a consequence of a non spherical shape for the particles. Finally, we have also calculated the magnetization curve for particles of size R = 35 nm with nearly the same result at least in the case on a non magnetic matrix. Therefore, as expected, we conclude that the strong size effect observed on the coercivity of nanocomposite materials is mainly due to the dependence of the crystallite magnetic properties on the size.

V. CONCLUSION

In summary, the PrCo 3 intermetallic compound, crystallize in rhombohedral PuNi 3 -type structure (space group: R 3m). This compound is a highly anisotropic uniaxial ferromagnet with the easy magnetization direction is parallel to the c-axis. No saturation reached at room temperature for applied field of 90 kOe. High coercivities of about 12 kOe at 293 K and 55 kOe at 10 K can be reached in PrCo 3 alloy prepared by high-energy milling method followed by annealing at 750 • C for 30 min. High energy milled method following the controlled nanocrystallization brings high magnetic properties with smaller grain size more adequate to favor an available microstruc-ture to perform this properties, so these results show that PrCo 3 compounds are promising materials for permanent magnet applications. From the model calculations of the macroscopic extrinsic properties we can propose a simple picture of the microstructure. Indeed, the magnetization curve presents a signature of a system made of two uncoupled phases: the magnetically hard crystallites and the amorphous matrix. The presence of the small kink in the magnetization curve is analyzed as the reversal of the matrix magnetization; the localization of this kink at nearly zero applied field and the its small amplitude are indications that the matrix is, as expected, magnetically very soft and characterized by a small saturation magnetization. This latter point must be taken with care, since here we propose a simple lattice for the crystallite which may lead to an overestimated fraction of the total volume occupied by the matrix also with the model with cubic inclusions. It is important to notice that when dealing with the reduced magnetization, M/M s we find that the influence of the grain shape is rather weak.

FIG. 1 :

 1 FIG. 1: Crystal structure of PrCo3. Pr atoms occupy the 3a and 6c sites, and Co atoms occupy the 3b, 6c and 18h sites.

FIG. 2 :

 2 FIG. 2: Rietveld analysis for PrCo3 ingot.

FIG. 3 :

 3 FIG. 3: XRD pattern of the PrCo3 as-milled and annealed samples at 750 and 1000 • C for 30 min.

FIG. 4 :

 4 FIG.4: Thermomagnetization curves of PrCo3 obtained after high-energy milling and annealed at 750 • C for 30 min.

FIG. 6 :FIG. 7 :

 67 FIG. 6: XRD pattern of PrCo3 oriented.

FIG. 8 :

 8 FIG.8: Calculated de-magnetization curve in terms of the external field for the systems of spherical inclusions with ϕ = 0.45. Randomly distributed axes and matrix saturation magnetization Js = 0, dashed line; Js = 0.05 T, solid line. The whole hysteresis curve in this later case is obtained by using the symmetric of the de-magnetization curve. Js = 0.05 T and ΘM = π/2.3, crosses; ΘM = π/15, dotted line.

  172). With this set of parameters, we get for the saturation magnetization of the composite system J (tot) s = 0.777 J (incl) s

FIG. 9 :

 9 FIG.9: Calculated de-magnetization curve in terms of the external field for the systems of cubic inclusions with ϕ = 0.73. Randomly distributed axes and matrix saturation magnetization Js = 0.05 T, dashed line; Js = 0.125 T, solid line and Js = 0.25 T, dotted line.

TABLE I :

 I a and c cell parameters, atomic positions, RB and χ 2 factors from Rietveld fit.

	a( Å)	5.0690
	c( Å)	24.7950
	z(6c)Pr	0.1414
	z(6c)Co 0.3337
	x(18h)Co 0.5002
	y(18h)Co 0.4998
	z(18h)Co 0.0829
	c/a	4.8915
	V( Å) 3	551.42
	RB	5.050
	χ 2	2.300

TABLE II :

 II Interatomic distance ( Å) for PrCo3.

	Site	Co (3b) Co (6c) Co (18h) Pr (6a) Pr (6c)
	Co (3b)	5.07		4.12		2.54		2.99	5.07
	Co (6c)	4.12		2.92		2.42		4.56	2.92
	Co (18h)	2.53		2.42		2.41		2.92	3.50
	Pr (6a)	2.99		4.56		2.99		3.18	3.50
	Pr (6c)	2.99		2.92		3.26		3.50	5.07
		60
	Magnetization (uem/g)	15 30 45
		0
		0	10	20	30	40	50	60	70	80	90
					Applied field (kOe)
	FIG. 5: Magnetization curves of PrCo3 obtained after high-
	energy milling and annealed at 750 • C for 30 min, measured
	at 293 K.

Acknowledgements

Most of the calculations have been performed at the CINES (Centre Informatique National de l'Enseignement Supérieur).


Blaise Ravelo

Marc Le Roy
email: marc.leroy@univ-brest.fr

André Pérennec

Frequency-Independent Active Phase Shifters for UWB applications

This paper describes an active phase shifter, which exhibits a frequency-independent phase shift over the ultrawideband (UWB) frequency range. The design of this pure phase shifter relies on the use of a negative group delay (NGD) circuit. The topology of the active NGD cell is based on a Field Effect Transistor in cascade with a series resonant network. A 90°±5° phase shifter with a single NGD stage was designed, at first, and implemented in hybrid microstrip technology; its measurements showed a 75% relative bandwidth and validated the proposed approach. Then, a compact pure phase shifter with a multi-stage NGD cell was designed. It was aimed at meeting specifications in the UWB frequency range, i.e. 3.1-10.6 GHz (110% relative band). Over this band, the corresponding simulation results showed a constant transmission phase of -45°±3°, an insertion gain from 2 to 5.5 dB and insertion losses better than 9 dB.

I. INTRODUCTION

Further to the study by Veselago [START_REF] Veselago | The electrodynamics of substances with simultaneously values of ε and μ[END_REF] and those by Smith and Pendry [START_REF] Pendry | Negative Refraction Make a Perfect Lens[END_REF]- [START_REF] Shelby | Experimental verification of a negative index of refraction[END_REF], many authors have investigated, theoretically and by experiments, the properties provided by artificial materials, known as metamaterials. For example, some resonant artificial materials and their equivalent 1-D planar circuits [START_REF] Eleftheriades | Transmission Line for Negative Index Media and Associated Implementations Without Excess Resonators[END_REF]- [START_REF] Lai | Composite Right/Left-Handed transmission Line Metamaterials[END_REF] are able to generate a negative group velocity over a limited frequency band where anomalous dispersion occurs. Generally, these frequency bands are rather narrow and associated with high attenuation values in transmission. Electronic active circuits generating negative group delay (NGD) were proposed in [START_REF] Solli | Superluminal effects and Negative delays in Electronics and their Applications[END_REF]- [START_REF] Kitano | Negative Group Delay and Superluminal Propagation: An electronic circuit Approach[END_REF], at first as demonstrators to study this intriguing property, and finally to compensate for losses and to widen the relative operating frequency band. In these electronic circuits, the notion of group delay is preferred to that of group velocity because the circuit length may be undefined in the case of lumped, or integrated, components. Nevertheless, as these circuits use classical electronic components and particularly operational amplifiers, their operating frequency bands are restricted below a few hundreds of MHz [START_REF] Solli | Superluminal effects and Negative delays in Electronics and their Applications[END_REF]- [START_REF] Kitano | Negative Group Delay and Superluminal Propagation: An electronic circuit Approach[END_REF]. The idea of a negative group delay synthesizer was validated by Lucyszyn et al. [START_REF] Lucyszyn | Negative group delay synthesiser[END_REF], but as the device was operating in reflection, the range of possible applications was restricted by the transmission losses as observed for resonant planar metamaterial circuits.

To overcome these limitations and give the opportunity to apply this "non-commonly-found-in-nature" property, the authors have proposed in previous studies a new NGD active topology [START_REF] Ravelo | Synthesis of broadband Negative Group Delay Active Circuits[END_REF]- [START_REF] Ravelo | Active microwave circuit with negative group delay[END_REF]. It consists in an active microwave circuit that provides a significant NGD value and gain. The topology is based on a Field Effect Transistor (FET) so as to permit operation in different frequency bands or over broad bandwidths according to the type of the chosen FET. As the first realization completely fulfilled these objectives [START_REF] Ravelo | Synthesis of broadband Negative Group Delay Active Circuits[END_REF], two major application domains were identified: i) the initial microwave topology was converted into a baseband one dedicated to high-speed signals in order to equalize or compensate for interconnect or passive-circuit parasitic effects [START_REF] Ravelo | Negative group delay active topologies respectively dedicated to microwave frequencies and baseband signals[END_REF], [START_REF] Ravelo | Application of negative group delay active circuits to reduce the 50% propagation Delay of RC-line model[END_REF]. ii) In the second domain of applications, i.e. microwave devices, the negative group delay which corresponds to a positive phase slope, was directly used as a basic element to design a pure phase shifter [START_REF] Ravelo | Application of negative group delay active circuits to the design of broadband and constant phase shifters[END_REF]- [START_REF] Ravelo | Broadband Balun Using Active Negative Group Delay Circuit[END_REF]. A pure phase shifter is a phase shifter with a constant (i.e. frequencyindependent) transmission phase; this notion was introduced in 1920 for analogue modulations [START_REF] Hartley | [END_REF]. More recently, several pure phase shifters were proposed at higher frequency bands [START_REF] Wang | A Multioctave-Band Photonically-Controlled, Low-Profile, Structurally-Embedded Phased Array with Integrated Frequency Independent Phase-Shifter[END_REF]- [START_REF] Ko | Integrated RF MEMS Phase Shifters with Constant Phase Shift[END_REF]. But their transmission losses were non negligible, or they operated in narrow frequency bands or their implementation was difficult. In [START_REF] Abbosh | Ultra-Wideband Phase Shifters[END_REF], an ultra-wideband (UWB) phase shifter easy to implement was described but it presented a constant phase shift between two output accesses and not a constant transmission phase as proposed here. Moreover, this passive distributed circuit proved to exhibit transmission losses.

In this paper, after a brief recall of the NGD active circuit theory, the principle of the proposed frequency-independent phase shifter is detailed in the case of a single NGD cell circuit. The design and implementation of this 90° phase shifter are reported, and the results from simulations and measurements are compared and discussed. Then, to widen the operating bandwidth, a multistage NGD pure phase shifter was designed. The simulations showed a -45° constant transmission phase over the 3.1-10.6 GHz band with a gain within 2 and 5.5 dB and return losses better than 9dB. The advantages and limitations of this technique are discussed and some improvements are considered.

II. ACTIVE NGD CELL THEORY

The NGD unit cell, required by the design principle of the proposed phase shifter, simply consists of an RLC series network in cascade with a FET [START_REF] Ravelo | Active microwave circuit with negative group delay[END_REF]. In [START_REF] Ravelo | Active microwave circuit with negative group delay[END_REF], by considering the low frequency model of the FET, i.e. a voltage-controlled current source with a transconductance, g m , in cascade with the drain-source resistor, R ds , simple analytical expressions for the S-parameters of this cell were obtained. Moreover, it was demonstrated theoretically and experimentally that the group delay is always negative at the resonance frequency,

TEC R L C TEC R L C g m V gs V gs R ds
LC / 1 0 = ω : )] ( [ 2 0 0 0 0 ds ds ds R R Z R Z R Z LR + + - = τ , ( 1
)
where Z 0 = 50 Ω is the reference impedance port. For a given FET, the cell may also generate gain under certain conditions of compromise between L and C:

)] ( [ 2 ) ( 0 0 0 0 21 ds ds ds m R Z R R Z R g RZ S + + = ω (2)
Indeed, by inverting these equations, the network synthesis equations are expressed as:

21 0 0 0 21 ) ( 2 S Z R R Z g Z R S R ds ds m ds + - = , ( 2
) [ ] 2 21 0 0 2 0 21 ) ( 2 ) ( S Z R Z R g Z R g S L ds ds m ds m + - - = τ , (3)
where S 21 = |S 21 (ω 0 )| and the group delay ) ( 0 ω τ τ = are the objectives. C is simply deduced from the resonance frequency:

C = 1/(Lω 0 2 ). ( 4
)
A series resistor is usually used to match the output access and a parallel one for the input access.

III. PRINCIPLE AND DESIGN OF FREQUENCY-INDEPENDENT PHASE SHIFTERS

The proposed topology of the frequency-independent phase shifter is depicted in Fig. 2. In fact, a positive group delay (PGD) circuit, for example a transmission line, is cascaded with a negative group delay circuit. To get a frequencyindependent transmission phase, the group delay values of the PGD and the NGD devices must be alike, but of opposite sign. Theoretically, the total group delay is then close to zero. It also corresponds to cascading devices with identical phase slope of opposite signs to achieve an overall constant phase value over a specified frequency band.

PGD NGD

Fig. 2. Bloc diagram of the proposed phase shifter.

A. Design and Validation of a One Stage 90° Pure Phase Shifter

As proof-of-concept circuit, a one stage phase shifter (PS) [START_REF] Ravelo | Application of negative group delay active circuits to the design of broadband and constant phase shifters[END_REF] was designed by first using the synthesis relations. A single NGD cell presents at its resonance a phase shift close to 180° (Fig. 3) due to the drain-source current direction. Then, to achieve the objective of 90°, the line length is set in a value such that its phase shift is around -90° at this particular frequency.

FET TL (PGD) group delay NGD cell group delay (PGD+NGD) group delay

Then, the NGD positive phase slope is optimized to get the opposite of the line one over the widest frequency band. Figs.

3 and 4 illustrate this principle: Fig. 3 shows the simulated PGD and NGD transmission phases and Fig. 4 depicts the group delays of the three circuits (PGD, NGD and PGD+NGD) s well as the overall simulated transmission phase. Finally, a final slight optimization under an EM software was performed by taking into account the transistor and the passive component S-parameters. This circuit was implemented in planar microstrip technology and biased by a classical passive network. Figs. 6 and 7 compared simulation (run under Momentum software) and experimental results. They show that the agreement is globally satisfying. The measured group delay and constant phase show frequency characteristics widened by comparison to simulation results. It is worth noting that the measured gain is slightly lower than to the simulated one. The measured gain is kept within -2 and 2 dB. The measured phase is 90°±5° from 1.08 GHz to 2.42 GHz, which corresponds to a 76.5% relative band and the return losses are better than 10 dB over this band.

Moreover, the total group delay is effectively close to zero and the active phase shifter has a compact size of 2 cm by 2.3 cm.

B. Design and Simulations of a -45° UWB Pure Phase Shifter using Multistage of NGD Cells

Further to this proof-of-concept validation, a -45° phase shifter was designed by using the same approach as the one detailed in [START_REF] Ravelo | Synthesis of broadband Negative Group Delay Active Circuits[END_REF] to widen the operating frequency band. As put forward in [START_REF] Ravelo | Application of negative group delay active circuits to the design of broadband and constant phase shifters[END_REF], it is advised to use an even number of FETs to get phase values between 0 and -180°. Then, three different resonant cells and two EC-2612 PHEMT FETs were used to get -45° with gain for UWB applications, i.e. for a frequency band extending from 3.1 to 10.6 GHz. Each cell was designed according to both the synthesis equations and the previously detailed process. Then, a final optimization was performed by taking into account the FET S-parameters. Fig. 8 shows the ADS schematic. The group delay and the transmission phase are both plotted in Fig. 9, whereas the S-parameter magnitudes are presented in Fig. 10. One should note that, in the UWB, the gain lies within 2 and 5.5 dB, and the return losses are better than 9 dB, the S 21phase is close to -45° with a variation from -41° to -47°. Once again, it is worth underlining that the device is of compact size. The whole group delay is close to zero, and even slightly negative from 3.1 to 7.4 GHz. Due to the FET non-reciprocity, the PS also presents a high isolation value, which may be an advantage depending on the final application. Using lumped components in simulations up to 10.6 GHz may seem inconsistent with a hybrid implementation, particularly for inductances. Nevertheless, chip thin film inductors, operating up to these frequencies, are now available and an MMIC integration is also possible.

IV. CONCLUSIONS

Design examples of pure phase shifters were presented. To get a frequency-independent transmission phase, two devices were cascaded with identical absolute phase slopes but of opposite signs. In theory, this means that the cascaded elements have opposite constant group delay value; this results in a total group delay close to zero. This kind of phase shifter should be distinguished from most of the available phase shifters: here, the transmission phase is constant but not the phase shift between two branches as in classical phase shifters. To provide a positive phase slope, a negative group delay circuit is needed. The authors proposed an NGD topology able to provide gain and NGD, and used this circuit to implement a 90° frequency-independent phase shifter in hybrid technology. The experimental results validated the principle of design and the synthesis relations. Indeed, this single-NGD-stage showed a 90°±5° transmission phase over a 75% relative bandwidth centred around 1.75 GHz, with a gain between -2 and 2 dB. Then, multi-stage NGD cells were used to design a -45° UWB pure phase shifter in order to widen the operating bandwidth. The simulation results confirm a -45°±3° transmission phase from 3.1 to 10.6 GHz with a gain within 2 and 5.5 dB. EM simulations of this latter circuit are in progress and its implementation is planned. Ongoing researches are focused on the design of tunable pure PSs.

Fig. 1 .

 1 Fig. 1. Unit cell of active NGD circuit and its low frequency model.

Fig. 3 .

 3 Fig. 3. Transmission phases of the transmission line and of the NGD cell.

Fig. 4 .

 4 Fig. 4. NGD cell-, transmission line-and both cascaded-group delays; cascaded (PGD+NGD) transmission phase.

Fig. 5 .

 5 Fig. 5. Detailed schematic of the 90° phase shifter: PHEMT EC2612 FET (gm = 98.14 mS and Rds = 116.8 Ω, bias network in thin lines (Vgs = -0.1 V, Vds = 3 V, Ids = 30 mA). R1 = 51 Ω, R2 = 22 Ω, R = 33 Ω, L = 4.7 nH, C = 1 pF, Cb = 22 µF, Lb = 220 nH, Rb = 1 kΩ. ansmission lines, TL: w = 952 µm and d = 6.2 mm.

Fig. 6 .

 6 Fig. 6. Transmission phase and group delay values obtained by simulations and measurements.

Fig. 7 .

 7 Fig. 7. Simulated and measured S21-magnitude and return losses.

Fig. 8 .

 8 Fig. 8. ADS schematic of the UWB phase shifter with the component values and the substrate characteristics.

FET

Fig. 9 .

 9 Fig. 9. Transmission phase and group delay of the -45° UWB phase shifter.

Fig. 10 .

 10 Fig. 10. Simulated S-parameters.


Jean-Francois Rahier
email: jfrahier@gmail.com

Yazdan Yazdanpanah

Nathalie Viget

Simon P L Travis

Jean-Frédéric Colombel

Jean-Francois Rahier

Jean-François Rahier

Yazdan Yazdanpanah²

Nathalie Viget²

Jean-François Rahier

Systematic review: influenza A (H1N1) virus in patients with inflammatory bowel disease

Keywords:

come

infection with influenza A (H1N1)v characterized by a unique combination of gene segments that had not been identified among human influenza A virus [START_REF]Swine influenza A (H1N1) infection in two children--Southern California, March-April 2009[END_REF]. Additional cases were rapidly reported leading the WHO to declare a pandemic phase level 6, indicating widespread human infection. This has caused widespread anxiety, especially among patients who are potentially immunocompromised and symptoms of serious systemic infection may be wrongly attributed to influenza A (H1N1)v.

Most cases of influenza A (H1N1)v currently seem to have uncomplicated influenza-like illnesses; the most common symptoms are cough and fever. Calculating the case fatality ratio related to influenza A (H1N1)v is highly dependent on estimates of the total number of people with the disease, which are not easy to obtain. Nevertheless, based on surveillance data and mathematical modeling, the influenza A (H1N1)v case fatality ratio seems to be higher than that of seasonal influenza, although it remains of the same order of magnitude [START_REF] Vaillant | Epidemiology of fatal cases associated with pandemic H1N1 influenza 2009[END_REF][START_REF] Fraser | Pandemic potential of a strain of influenza A (H1N1): early findings[END_REF]. The severity and deaths associated with seasonal influenza infection result in a large part from secondary complications such as secondary bacterial pneumonia (Streptococcus pneumonia or Staphylococcus aureus), primary viral pneumonia, and exacerbation of underlying chronic conditions [START_REF] Hageman | Severe community-acquired pneumonia due to Staphylococcus aureus, 2003-04 influenza season[END_REF][START_REF] Mccullers | Insights into the interaction between influenza virus and pneumococcus[END_REF]. Initial observation suggests that children and young adults may be more susceptible to influenza A (H1N1)v than are older persons [START_REF] Dawood | Emergence of a novel swine-origin influenza A (H1N1) virus in humans[END_REF]. It is still unclear whether the low incidence in people over 60 years of age is due to a partial immunity from former infections with H1N1 influenza viruses. Although the median age among the persons who died with influenza A (H1N1)v is lower than those who die from seasonal influenza, when infection does occur, the percentage of deaths in the elderly seems to be higher than in other age groups [START_REF] Vaillant | Epidemiology of fatal cases associated with pandemic H1N1 influenza 2009[END_REF][START_REF] Fraser | Pandemic potential of a strain of influenza A (H1N1): early findings[END_REF]. Pregnant women and patients with underlying conditions such as respiratory disease, cardiac disease, diabetes, obesity, or immune suppression are commonly reported in hospitalized cases and are considered at higher risk of complications [START_REF] Vaillant | Epidemiology of fatal cases associated with pandemic H1N1 influenza 2009[END_REF][START_REF] Dawood | Emergence of a novel swine-origin influenza A (H1N1) virus in humans[END_REF][START_REF] Baker | Pandemic influenza A(H1N1)v in New Zealand: the experience from April to August 2009[END_REF]. Patients with inflammatory bowel disease (IBD) are in large part of younger age and may be immunocompromised due to their treatment. Immunomodulators commonly used in inflammatory bowel disease are corticosteroids, azathioprine, methotrexate, calcineurin inhibitors, In this particular setting, strategies for care providers to prevent disease occurrence and its complications in IBD patients rely on general precautions to limit inter-human transmission of the virus, and the use of a vaccination against influenza A (H1N1)v, the use of pneumococcal polysaccharide vaccine. Post-exposure antiviral chemoprophylaxis is best implemented in specific situations and treatment of the influenza A (H1N1)v infection best adapted for patient with inflammatory bowel disease (Table 1).

Strategies to prevent influenza A (H1N1)v infection in patients with IBD:

Personal protective measures for reducing the risk of acquiring or transmitting human influenza do not differ for IBD patient than for the large public. Measures for self protection are mainly (i) avoid close contact with sick people, (ii) wash hands frequently and (iii) avoid touching his own eyes, nose or mouth [START_REF]Personal protective measures for reducing the risk of acquiring or transmitting human influenza[END_REF]. Additional details data are available on the European Center for Disease Control website http://ecdc.europa.eu/en/healthtopics/Pages/Influenza_A(H1N1)_Outbreak.aspx Prevention of influenza is usually achieved by vaccination. New vaccines directed toward the influenza A (H1N1)v infection are currently being manufactured. Two recent studies have shown reassuring data regarding the efficacy and safety of these novel vaccine, whether in an MF59-adjuvant form or not, in young and middle-aged adults [START_REF] Clark | Trial of Influenza A (H1N1) 2009 Monovalent MF59-Adjuvanted Vaccine --Preliminary Report[END_REF][START_REF] Greenberg | Response after One Dose of a Monovalent Influenza A (H1N1) 2009 Vaccine --Preliminary Report[END_REF]. These data are difficult to extrapolate to adults who have underlying immune suppression, which is the group for whom the influenza A (H1N1)v vaccine is particularly recommended.

However, data on influenza A (H1N1)v vaccination may be extrapolated from the seasonal influenza vaccine. The efficacy of influenza vaccination depends on the ability of a person's immune system to respond adequately to vaccine antigen. Immune dysfunction that can potentially compromises vaccine response and effectiveness. Two studies have, however, shown that immunocompromised patients with IBD generally have an adequate response rate to influenza vaccination [START_REF] Mamula | Immune response to influenza vaccine in pediatric patients with inflammatory bowel disease[END_REF][START_REF] Lu | Immune response to influenza vaccine in children with inflammatory bowel disease[END_REF]. An adequate response is observed in patients with rheumatic diseases, except for those on biological therapies, which may diminish the response to influenza vaccination [START_REF] Brezinschek | Immunization of patients with rheumatoid arthritis with antitumor necrosis factor alpha therapy and methotrexate[END_REF]. Interestingly, vaccination on the day of anti-TNF administration might facilitate seroconversion [START_REF] Elkayam | The Effect of Infliximab and Timing of Vaccination on the Humoral Response to Influenza Vaccination in Patients with Rheumatoid Arthritis and Ankylosing Spondylitis[END_REF]. Although influenza vaccination (nonlive vaccine) appears to be safe in the immunocompromised, the lack of safety data on vaccines against the novel influenza A (H1N1) virus in patients on immunomodulators means that the safety and efficacy of these vaccines should be continuously monitored.

Patients with IBD on immunomodulators or biological therapy are best advised to receive the appropriate vaccine as soon as it is available, following national guidelines on vaccination against influenza A (H1N1)v [START_REF]Use of specific pandemic influenza vaccines during H1N1 2009 pandemic[END_REF]. Recommendations regarding influenza A (H1N1)v vaccination may differ by country, according to the amount of vaccine available, the type of vaccine used, the particular in patients with immunosuppression caused by medication [START_REF]Use of influenza A (H1N1) 2009 monovalent vaccine: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009[END_REF]. Those patients not on immunomodulators or biological therapy are not believed to be immunocompromised and should be considered in line with the general population.

Pandemic influenza A(H1N1)v virus is likely to become the predominant circulating influenza virus as it has been demonstrated in New Zealand and Australia [START_REF] Baker | Pandemic influenza A(H1N1)v in New Zealand: the experience from April to August 2009[END_REF][START_REF] Kelly | Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination[END_REF]. As a result, patients with IBD on immunomodulators or biological therapy are best advised to receive the appropriate vaccine against influenza A (H1N1)v as soon as it is available. Seasonal influenza vaccine does not protect against influenza A (H1N1)v infection [START_REF] Kelly | Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination[END_REF]. However, seasonal influenza vaccination should take place as usual in immunocompromised patients with IBD [START_REF] Rahier | on behalf of the European Crohn's and Colitis Organisation (ECCO). European evidence-based Consensus on the prevention, diagnosis and management of opportunistic infections in inflammatory bowel disease[END_REF] because of the potential of co-circulation of other influenza strains along with A(H1N1)v [START_REF] Kelly | Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination[END_REF]. Pneumococcal vaccination could reduce the complication rate from influenza A (H1N1)v infection in high risk groups. Vaccination recommendations for all patients with IBD include pneumococcal polysaccharide vaccine, with repeat vaccination at 5 years [START_REF] Rahier | The european consensus on the prevention of opportunistic infections in IBD: What does it change for the clinician ?[END_REF]. Pneumococcal vaccination should be implemented regardless of H1N1 vaccine availability.

Post-exposure antiviral chemoprophylaxis

The currently circulating influenza A (H1N1)v is sensitive to neuraminidase inhibitors: oseltamivir (Tamiflu®) and zanamivir (Relenza®). Patients with IBD on immunomodulators or biological therapy who are close contacts of persons with confirmed or probable cases of novel influenza A (H1N1) virus infection should receive post-exposure antiviral chemoprophylaxis with either oseltamivir (75 mg daily) or zanamivir (2 inhalations of 5 mg daily). Antiviral chemoprophylaxis should be initiated within 48 hours after the last known exposure to the novel (H1N1) influenza and continued for 10 days after the last known exposure to novel (H1N1) influenza [START_REF]Public health use of influenza antivirals during influenza pandemics[END_REF][START_REF] Moscona | Neuraminidase inhibitors for influenza[END_REF]. The use of neuraminidase inhibitors during pregnancy must depend on advice from a specialist in infectious diseases. Preexposure antiviral therapy should not be considered in patients with IBD. In practice it may prove

Strategies for treating suspected or confirmed influenza A (H1N1)v infection in patients with

IBD

In the event of infection, the first measures are to protect family members and other close contact by (i) maintaining good respiratory hygiene, (ii) washing hands frequently and (iii) staying at home [START_REF]Personal protective measures for reducing the risk of acquiring or transmitting human influenza[END_REF].

IBD patients who are receiving immunomodulators or biological agents and who meet current casedefinition for confirmed or probable cases novel influenza A (H1N1) virus infection should be treated with antiviral drugs: oseltamivir (75mg twice daily), or zanamivir (two inhalations of 5mg twice daily) [START_REF]Public health use of influenza antivirals during influenza pandemics[END_REF], since they are at high risk of complications from influenza. The drugs should be given within 48 hours and the earlier the better. However, some data from studies on seasonal influenza indicate benefit for hospitalized patients even if treatment is started more than 48 hours after onset.

Neuraminidase inhibitors have been shown to reduce complications associated with influenza, including the need for antibiotics [START_REF] Moscona | Neuraminidase inhibitors for influenza[END_REF][START_REF] Treanor | Efficacy and safety of the oral neuraminidase inhibitor oseltamivir in treating acute influenza: a randomized controlled trial. US Oral Neuraminidase Study Group[END_REF]. Although evidence is lacking, it seems sensible temporarily to withdraw immunomodulator therapy at least during the symptomatic phase of infection and probably until 24 hours after symptoms and signs of fever have resolved. Since patients not on immunomodulators are not considered high risk, there is no need for antiviral treatment for uncomplicated influenza A (H1N1)v infection. On the other hand, a severe or progressive clinical presentation should promptly be treated with antiviral therapy, with advice from an infectious diseases' specialist, regardless of their IBD or immunocompromising condition [START_REF]WHO guidelines for pharmacological management of pandemic (H1N1) 2009 influenza and other influenza viruses[END_REF]. For patients on multiple immunomodulators who are considered to be severely immunosuppressed, higher doses of oseltamivir may be considered, because of the risk that resistant strains may emerge. Likewise, longer treatment duration may be considered, because of longer viral shedding in severely immunosuppressed patients [START_REF]Oseltamivir-resistant novel influenza A (H1N1) virus infection in two immunosuppressed patients -Seattle, Washington[END_REF] . There is no known interaction between neuraminidase inhibitors and drugs used in IBD, although methotrexate plasma levels may increase with concomitant use of oseltamivir, since both drugs use similar metabolic excretion pathways. In vitro studies, however, suggest a minimal effect and no clinical consequence for those on low doses of methotrexate, such as those used in IBD [START_REF]Use of oseltamivir in patients taking methotrexate June 2009[END_REF]. It is particularly important that clinicians do not overlook conventional infection or inflammatory conditions that may mimic the symptoms of pandemic flu, because such an oversight may have severe consequences.

Conclusion

As Gastroenterologists, we believe that it is our responsibility to motivate our patients to receive immunisation and make sure that those measures are part of our management plan: otherwise our patients will be left unprotected. We believe that the opportunity should be taken to vaccinate against pneumococcal infection and that seasonal influenza vaccination should be performed as normal for those on immunomodulators [START_REF] Rahier | on behalf of the European Crohn's and Colitis Organisation (ECCO). European evidence-based Consensus on the prevention, diagnosis and management of opportunistic infections in inflammatory bowel disease[END_REF]. In the event of suspected infection with influenza A (H1N1)v in an immunocompromised person, or clinically serious infection in anyone with inflammatory bowel disease, then prompt antiviral treatment is appropriate. The situation is rapidly evolving and recommendations may change, but this appears the most practical advice at present.

Bullet points

• Patients with IBD and treated with immunomodulators are at high-risk from complications of novel influenza (H1N1) virus infection.

• General precautions to limit inter-human transmission of the virus should apply to all IBD patients regardless of their immune status. ³ Defined as uncomplicated febrile illness, with systemic symptoms, but no respiratory distress or prostration ⁴ In patients on multiple immunomodulators who are considered to be severely immunosuppressed, higher doses of oseltamivir may be considered, because of the risk that resistant strains may emerge. Likewise, longer treatment duration may be considered because of longer viral shedding in severely immunosuppressed patients [START_REF]Oseltamivir-resistant novel influenza A (H1N1) virus infection in two immunosuppressed patients -Seattle, Washington[END_REF].

  2009, the Centers for Disease Control and Prevention (CDC) identified two cases of human

  necrosis factor agents or other biologics, all compromising to some extent the patient's immune response. For this reason, they might be considered as susceptible to and at high-risk from complications of novel influenza (H1N1) virus infection. In this article, we provide guidance for physicians on the risk, prevention and management of influenza A (H1N1)v infection in patients with inflammatory bowel disease. This reflects expert opinions and may complement national guidelines when available. Medline was searched using the following key words, individually and in combination: 'swine flu', 'immunosuppression', inflammatory bowel disease', 'recommendations', 'immunization',' vaccination'. The European Centre for Disease Prevention and Control (ECDC), the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) were consulted for recent papers and recommendations regarding immunocompromised patients and influenza A (H1N1) virus infection. No definitive national guidelines are available at the time of writing.

  epidemiology and practical considerations. The European Medicines Agency (EMEA) has very recently recommended the authorisation of two vaccines (Focetria®, Pandemrix®) for influenza pandemic (H1N1) 2009. Additional information can be found on the following website: htpp://www.emea.europa.eu/influenza.home.htm . The US Advisory Committee on Immunization Practices recently recommended the administration of influenza A (H1N1) 2009 monovalent vaccine

  appropriately timed post-exposure prophylaxis to the majority of our patients with IBD.

•

  Patients with IBD and treated with immunomodulators should receive vaccinations against seasonal influenza, H1N1 virus and pneumococcal polysaccharide vaccine. Vaccination in immunocompetent patients with IBD should follow general recommendations.

•

  Post-exposure chemoprophylaxis usefully mitigates infection when used at an early stage in immunosuppressed patients.• Severe or progressive clinical infection with influenza A (H1N1)v should be treated with antiviral therapy, with advice from an infectious diseases' specialist, regardless of their IBD or immune status.

Table 1

 1 Immunomodulators used in inflammatory bowel disease are corticosteroids, azathioprine, methotrexate, calcineurin inhibitors, anti-tumor necrosis factor agents, or other biologics. For corticosteroids, doses of prednisone ≥20 mg/d for more than 2 weeks are considered as an immunomodulator. ² According to Rahier JF, Ben-Horin S, Chowers Y, et al.; on behalf of the European Crohn's and Colitis Organisation (ECCO). European evidence-based Consensus on the prevention, diagnosis and management of opportunistic infections in inflammatory bowel disease. JCC 2009;3:47-91.

	Clinical circumstances	Standard IBD	IBD on immunomodulators¹
	Vaccination against H1N1	in line with general	Yes
		recommendations
	Vaccination against seasonal influenza virus	in line with general	Yes
		recommendations
	Vaccination with Pneumococcal	Yes²	Yes
	polysaccharide vaccine
	Asymptomatic patient with IBD, exposed to	Measures for self-protection	Measures for self-protection
	or in close contact with H1N1 influenza virus		Neuraminidase inhibitors (eg
			oseltamivir 75mg daily) for 10
			days
	Patient with IBD and symptoms of mild³ flu	Measures to protect family	Measures to protect family and
		and close contacts	close contacts
		No treatment	Neuraminidase inhibitor (eg
			oseltamivir 75mg twice daily)
			for 5 days⁴
	Patient with IBD and symptoms of severe flu	Measures to protect family	Measures to protect family and
		and close contacts	close contacts
		Neuraminidase inhibitors (eg	Neuraminidase inhibitors (eg
		oseltamivir 75mg twice daily)	oseltamivir 75mg twice daily)
		for 5 days⁴	for 5 days⁴
		Take specialist advice	Temporarily withdraw IM
			Take specialist advice

¹