domain: chim.mate chim phys

text:
D Kumar

A Fouchet

A David

A Cheikh

T Suraj

O Copie

C U Jung

A Pautrat

M Ramachandra Rao

Wilfrid Prellier
email: wilfrid.prellier@ensicaen.fr

Enhancement of magnetic properties in compressively strained PrVO 3 thin films

Keywords: numbers: 81.15.Fg, 73.50.Lw, 68.37.Lp, 68.49.Jk

come

INTRODUCTION

The perovskite oxides with generic formula ABO 3 have attracted a great deal of scientic interest thanks to the strong correlation between orbital (electronic), spin (magnetic) and lattice degrees of freedom (structural). Consequently, owing to various functional properties such as ferroelectricity, ferromagnetism, antiferromagnetism, colossal magnetoresistance, superconductivity, the oxides are exploited in the diverse technological applications for electronics, data storage or sensing and so on [14]. While for bulk materials, these underlying properties can be expanded via lattice distortion by applying hydrostatic pressure [START_REF] Zhou | [END_REF]6], a substrate-induced biaxial strain in ABO 3 epitaxial thin lms proved to be an eective tool to modify the spin-orbit-lattice coupling [710]. Among RVO 3 (R: Rare earth element), the bulk PrVO 3 (PVO), at room temperature, adopts an orthorhombic P bnm crystal structure with the lattice parameters: a o = 5.487 Å, b o = 5.564 Å, and c o = 7.778 Å (o stands for orthorhombic) [11]. In the following, we will consider the pseudocubic unit cell of PVO that yields

a pc ≈ a o / √ 2 ≈ b o / √ 2 ≈ c o /2 ≈ 3.

Å (pc stands for pseudocubic). The bulk

PVO is an antiferromagnet associated with C-type Spin Ordering (C-SO), where V spins are staggered in ab-plane and aligned ferromagnetically along the c-axis, with T N 130 K [12].

However, for a PVO thin lm, the DFT calculations reveal a G-type Spin Ordering (G-SO) in the ground state (where V spins are aligned antiferromagnetically along all three crystal directions), associated with a C-type Orbital Ordering (C-OO) through KugelKhomskii mechanism [13,14]. In the course of our previous study, we revealed a new pathway to tune the magnetic properties of PVO thin lms grown on SrTiO 3 (STO) substrate by monitoring the concentration of oxygen vacancies in PVO lms [13]. A careful crystal structural investigation revealed that the tensile-strained PVO lm on STO substrate adopts a monoclinic P2 1 /m crystal lattice [10,13,15]. In addition, we have scrutinized the eect of substrateinduced strain on the structural and magnetic properties, by growing PVO lms on top of various lattice mismatched perovskite-oxide substrates [10]. The study revealed that a large compressive strain in PVO lms not only promotes the superexchange interaction i.e. VV interactions, but also changes the electronic structure of PVO. Recent investigations have unveiled that the layer-by-layer control of lm through minute deposition tunes the strain states in a lm, oering fascinating functional properties, namely, spin-glass behaviour in compressively strained BiFeO 3 thin lms [16], thickness dependent magnetic anisotropy in La 2/3 Ca 1/3 MnO 3 lms [17] and the dimensional crossover of magnetization from 3D to 2D in SrRuO 3 thin lms with decrease in lm thickness [18]. Even if the RVO 3 system is a good candidate to exhibit the spin-orbit-lattice coupling, only a few lm thickness-dependence studies have been conducted in thin lms [19,20].

In this paper, we study the thickness-dependent structural and magnetic properties of the PrVO 3 (PVO) thin lms grown on (La,Sr)(Al,Ta)O 3 (LSAT) and LaAlO 3 (LAO) substrates.

While thick PVO lms (100 nm) remain strained to the LSAT substrates, lms of ∼ 50 nm show partial relaxation on the LAO substrates. We have evidenced a dead-layer at the lm surface which possesses strong paramagnetic susceptibility, and is at the origin of observed magnetic properties. In addition, we have also explored the possibility to heal this layer by capping with few layers of LAO, in order to recover the magnetic properties of PVO.

II. EXPERIMENTAL

The PVO thin lms were epitaxially grown on top of (100)-oriented LAO and LSAT substrates, within thickness range of 10 to 100 nm, by using Pulsed Laser Deposition (PLD) technique. A KrF excimer laser (λ = 248 nm) with repetition rate of 2 Hz and laser uence of 2 J/cm 2 was focused on a PrVO 4 ceramic target, with substrate-to-target distance of 5

cm. The lms were deposited at a growth temperature T G = 650 • C and oxygen partial pressure P O 2 = 10 -6 mbar. To assure homogeneous oxygen vacancies throughout the growth, the samples were cooled down to room temperature in the same oxygen pressure. The crystallinity and the structure of the PVO lms were characterized using x-ray diraction technique (Bruker D8 Discover diractometer, Cu Kα1 radiation, λ = 1.5406 Å). The surface morphology of the lms was investigated using Atomic Force Microscopy (AFM) PicoSPM.

The magnetic measurements were performed using Superconducting Quantum Interference Device (SQUID) magnetometer, as a function of magnetic eld H (parallel conguration) and temperature T. The magnetization-magnetic eld (M-H) hyteresis curves were obtained at T = 20 K and magnetization-temperature (M-T) data were carried out at H in-plane = 50 Oe. The lattice constants of PVO, LSAT, and LAO in the pseudo-cubic structure are 3.901 Å, 3.868 Å, and 3.791 Å respectively, establishing PVO to be grown under nominal compressive strain with lattice mismatch of -0.8 % (LSAT) and -2.9 % (LAO).

III.

RESULTS AND DISCUSSION

Figure 1(a) and 1(b) display θ -2θ x-ray diraction measurements of a series of PVO lms grown on top of (100)-oriented LAO and LSAT substrates respectively, around (100) pc (where pc refers to the pseudo-cubic notation) of each substrate. The clear thickness fringes for PVO/LSAT lms conrm uniform thickness, and well-dened lm/substrate interface.

However, PVO lms grown on LAO substrates display suppressed oscillations, presumably due to presence of the twin domains in the LAO substrate [21]. The lm thickness (t) was calculated using these fringes, the details of which could be found elsewhere [10]. The AFM analysis was performed on each sample, and a surface roughness less than 0.5 nm was observed conrming the high-quality of the samples, without the presence of any islands even in the thicker lms (Fig. 1(d)). The out-of-plane and in-plane lattice parameters were calculated using XRD θ -2θ (Fig. 1(a), (b)) and reciprocal space map scans (Fig. 2), respectively. In order to further investigate the thickness-dependent structural evolutions, the asymmetrical reciprocal space maps (RSMs) were recorded for PVO lms, around (103) pc Bragg's peak (c) surface roughness (R q ) from 1 to 4 is: 0.18 nm, 0.43 nm, 0.22 nm and 0.42 nm, respectively.

of LAO and LSAT (see Figure 2(a) and 2(b) for PVO/LAO and PVO/LSAT, respectively).

We note that the thinner PVO lm (t ≤ 24 nm) is in-plane strained when grown onto LAO, whereas thicker lms start to relax (Figure 2 horizontal Q in axis shifts toward lower value. This is in perfect agreement with the highly mismatched PVO lms grown on the LAO substrate which tend to relax easily over small thickness, due to a decrease in the strain states as the thickness increases. The in-plane lattice parameters of PVO were calculated using these maps (within the instrumental error) (Fig. 2), and the results are plotted as a function of lm thickness in Figure 1(c). On the LAO substrate, the in-plane lattice parameter of lm increases linearly with the lm thickness. This behavior is typical of a partially relaxed lms, and is in accordance with the decrease of out-of-plane lattice parameter with increase in lm thickness, for a lm with an ideal Poisson's ratio [27]. On the LSAT substrates, the situation is dierent as the PVO lms are strained with the substrate, even at t ∼ 100 nm. This is clearly seen in Figure 2(b) where the horizontal position of lm peak coincides with the one of LSAT substrate.

Thus, the lm and substrate have same in-plane lattice parameter. Also, the lm peak width along q in -axis increases slightly with increase of lm thickness, and could be possibly due to the presence of dierent domains in the PVO lm, as shown in the earlier reports [10,13,15]. Moreover, a range of lm thickness analyzed by using electron microscopy [10] in combination with pole gures yield PVO [001] o -axis along in-plane of lm, and thus in- ducing [110] o -axis growth. A detailed microstructure of the lms will be published later. To summarize, the PVO lms are strained when grown on the LSAT substrates, even at t ∼ 100 nm, whereas lms of t ≥ 50 nm display partial relaxation when grown on LAO substrates. The magnetic moments are however canted, leading to a canted-antiferromagnetic state via antisymmetric spin-spin interaction (D ij .(S i × S j ), with D ij the DzyaloshinskiiMoriya term) [15]. Additionally, the magnetization of PVO lms could be described as combination of a soft and a hard magnetic phase, similar to what was observed in orthoferrite YFeO 3 [22,23], and is consistent with our previous observations [10]. In fact, we have shown earlier that the weightage of each magnetic phase can be tuned by the epitaxial strain imposed by dierent substrates [10]. In the present case, where strain is induced via dierent thicknesses, we observed a decreasing trend of soft magnetic phase with increase of lm's thickness. From Figure 3(a), (b) for PVO/LAO and 3(e), (f ) for PVO/LSAT, we extract that, the thinner lms indeed posses a larger soft magnetic phase (paramagnetic) with large saturation magnetization (M s ), and with the increase of lm thickness, the fraction of lm that is made up of paramagnetic phase reduces, and / or the relative contribution of PVO layer, associated with hard magnetic phase, increases. Moreover, we have shown previously that the soft magnetic component also depends upon the temperature, namely, it disappears at temperature T > 20 K for PVO/LSAT, and T > 80 K for PVO/LAO lms of t ∼ 50 nm [10].

We will come back to this later in the MT section. The magnetic coercivity (H c ), extracted from the hysteresis loops (hard magnetic phase) and the M s are plotted as a function of lm thickness (See Figure 3(c) for LAO and Figure 3(g) for LSAT). A continuous decrease in M s and increase in H c are observed for both the substrates with increase in the lm's thickness, until a nominal value is reached, which remain nearly constant for thick lms.

Additionally, the shape of hysteresis loop changes from a square like MH to a paramagnetic like S-shaped as thickness is reduced. Here, we suggest that a large value of M s for thinner lms is reminiscent of the presence of a non-magnetic/paramagnetic layer at the surface of lm, namely a dead layer, similar to the previous observation in DyTiO 3 thin lms [25].

When the lm surface is exposed to air, the surface can get over-oxidized, and the magnetic V 3+ ions can indeed be replaced by V 4+ /V 5+ (non-magnetic) ions, decoupling Pr 3+ ions and thus unleashing their strong paramagnetic response. Therefore, the soft magnetic phase is due to presence of the dead layer at the lm surface. It is indeed true that a dead layer may also be present at the lm/substrate interface [24]. We hypothesize that the layer presents at the surface of lm dominates, due to excess oxidation, resulting from air exposure. To estimate the thickness of this layer, we use a model described by equation (1) [25], which assumes that this layer is paramagnetic, has null magnetization at remanence, and a huge magnetization at high elds. We then t the thickness-dependence of remanent magnetization (M R ) and saturation magnetization (M s ) with the proposed dead layer model, using the following equation:

M tot. = m tot. t = m P t P + m AF t AF (1)
m tot. = (m P -m AF )t P /t + m AF (2)
where, t = t P + t AF is the total thickness of lm, t P is the thickness of paramagnetic layer, t AF is the thickness of antiferromagnetic layer, m P is the moment per unit volume of the paramagnetic layer, m AF is the moment per unit volume of the antiferromagnetic layer. Setting m AF ∼ 0.7 µ B /f.u. (f.u.: formula unit) [26] (for bulk PVO) and m P = 0 (for M R ), a dead layer thickness t P of ∼ 6 nm for LAO (Figure 3(d)) and ∼ 4 nm for LSAT (Figure 3(h)) was obtained by tting the remanent magnetization. Furthermore, setting m P to the maximum magnetization of the thinnest sample and using t P as tting parameter, we evaluated t P ∼ 1012 nm from tting of M s (not shown). The inconsistency in the computation of t P from t of M R t and M s t can have two possible explanations. First, when the diamagnetic substrate contribution is subtracted, there is always some uncertainties concerning M s . Second, the magnetic moments associated with the paramagnetic layer, which typically consists of isolated Pr 3+ ions, may not necessarily be saturated at 5T (especially for thinner lms), and thus a large error in the calculation of t P from M s t t may be induced. Nevertheless, this error could be minimized at higher magnetic elds, as the moments would be close to saturation. Therefore, the proposed model can be improved at higher magnetic elds.

It is worth noting that, for low thickness lms (Figure 3a,e), the saturation magnetization remains low compared to 3.58 µ B /f.u. expected for isolated Pr 3+ ions, suggesting that there is still a small fraction of antiferromagnetic (AFM) interaction in the thinner lms.

Remarkably, a similar trend in H c and M s was also observed for ferrite thin lms, which likely indicated a change in the magnetization spin axis, in addition to a reorientation of the domains above critical thickness [28,29]. In PVO thin lms, however, a strong paramagnetic response for thinner lms is indicative of dead layer at the surface of lms, which consists isolated Pr 3+ atoms. In addition, a decrease of M s with increase in lm thickness is also evidenced by a decrease in the proportion of the paramagnetic phase. Likewise, the increase of H c with increase of lm thickness could be related to the increase in the density of pinning sites due to increase in the number of domains, and/or domain boundaries. Indeed, due to a partial strain-relaxation, the lm could energetically favor multiple domains, as shown earlier [10]. For thick PVO lms, the magnetization and coercive eld approach a nominal value, meaning that the magnetic contribution from each layer is static, and independent of lm thickness. Nevertheless, the magnetization remains lower than the bulk value (∼ 0.6 0.7 µ B / f.u.) for thick lms, and may be related to the presence of V 4+ / V 5+ , which can aect the electron hopping, and thus suppress the magnetization considerably.

Further magnetic analyses were carried out by performing magnetic measurements as a function of temperature T. For clarity, only Field Cooled (FC) measurements are shown in Alike PVO lms on LSAT substrates, the PVO lms on LAO substrates show an abrupt increase in T N with increase in lm thickness and approach a nominal value for the thicker lms (Fig. 4(d)). Moreover, it is observed that the transition at T SO2 is present only for thicker lms, and appear imperceptible for thinner lms (t < 55 nm) (see inset of Fig.

T S O 2 M / M 1 0 K T N ( a ) P V O / L S A T ( b ) T N [ K ] t h i c k n e s s ( n m ) P V O / L A O ( c ) M / M 1 0 K T [ K ]

4(c))

. This suggests a possible vanadium spin reorientation solely in the thicker PVO lms.

Similarly, we observed a clear magnetic feature at T SO3 only for the thicker PVO/LAO lms, and perhaps related to a dierent strain states between thinner and thicker lms. Notably, the T N of PVO lms decreases with the decrease of lm thickness, which is in contrast to the fact that the compressive strain enhances the magnetic exchange interactions in PVO lms, leading to an increase of T N . This discrepancy could be account for the absence of T SO2 and T SO3 for the thinner PVO lms, producing dierent magnetic ground states, and may be a change in the spin conguration, which, further, are arranged in a way as to decrease the exchange interaction between V-V neighbouring sites, and thus lowering T N of thinner PVO lms. Interestingly, for thinner PVO/LAO lms, the soft magnetic component in the MH hysteresis loop was seen to persists up to T ∼ 80 K, meaning that the Pr -V interaction is signicantly enhanced for the LAO substrate, in agreement with our previous observations [10]. This enhancement is possibly related to the presence of higher strain states in PVO/LAO lms, which raises the degree of Pr-V interaction in the same way as V-V interaction (T N ). In conclusion, we have investigated the eect of lm thickness on the structural parameters (in-plane and out-of-plane) and magnetic properties of compressively-strained PrVO 3

(PVO) thin lms grown on (La,Sr)(Al,Ta)O 3 (LSAT) (100) (lattice mist ∼ 0.8 %) and LaAlO 3 (LAO) (100) (lattice mist ∼ 2.9 %) substrates, and evidenced a dead layer of t ∼ 46 nm for both sample series. The less strained PVO/LSAT lms ( 110 ∼ 1.4 % for t ∼ 50 nm) show highest T N ∼ 120 K, lower than bulk value i.e. 130 K. On the other hand, the T N of highly strained PVO/LAO lms ( 110 ∼ 2.5 % for t ∼ 50 nm) raises up to ∼ 170 K, way above its counterpart bulk value. In addition, we have observed an increase in H c and decrease in M s with increase in lm thickness for both substrates, and explained by the reduction in the proportion of paramagnetic phase. A model based on dead layer is used to quantify the salient origin of the large paramagnetic M s for thinner PVO lms. Finally, we have attempted to cap the PVO lm with ∼ 8 nm LAO lm, in order to partially diminish the excess oxygen at the interface with PVO, and recover the magnetic properties related to the pure PVO. These observations suggest that the lm thickness can be used to tune the strain/lattice deformation and thus functional properties in PVO thin lms, and should be considered for other epitaxial perovskite thin lms.

Figure 1 (

 1 Figure1(c) illustrates the evolution of PVO lattice parameters as a function of the lm thickness, for both LAO and LSAT substrates. First, we see that the out-of-plane lattice parameter of all PVO lms has increased for both substrates as compared to that of the pseudo-cubic bulk PVO (solid red line in Figure1(c)), which is in perfect agreement with the in-plane compressive strain imposed by the LAO and LSAT substrate. Moreover, the outof-plane lattice parameter of PVO/LAO decreases continuously (in overall) with increase of lm thickness, although a slight increase between t = 19.63 nm and t = 24.19 nm, and also between t = 55.93 nm and t = 74.50 nm is observed, and believed to be because of uncertainty in the calculations of parameters. Second, PVO lms grown on the LSAT substrates show a nearly constant out-of-plane lattice parameter with increase of lm thickness, and a slight change is within the experimental accuracy. (The error bars were calculated by tting the lm peak using the Voigt function, and then using the relation between the uncertainty in lattice parameter (∆d) and Bragg's angle (∆θ): ∆d = ∆θ tan θ . d, directly derived from the Bragg diraction expression). This nearly constant behavior of out-of-plane lattice parameter of PVO lm on LSAT substrate can be rather anticipated due to a smaller mismatch between PVO and LSAT substrate (∼ -0.8 %).

Figure 1 :

 1 Figure 1: θ -2θ x-ray diraction measurements of a series of PVO lms grown on (a) LAO and (b) LSAT substrates, around (100) pc of each substrate. The asterisk ( * ) and plus (+) represents PVO lm and substrate, respectively. (c) Out-of-plane (square symbols in top panel) and inplane (triangle symbols in bottom panel) lattice parameters of PVO lms grown on LAO (blue open symbols) and LSAT (black close symbols) substrates, as a function of the lm thickness, plotted along with the error bars. ⊥ and || symbols represent the out-of-plane and in-plane lattice parameters of PVO, respectively. The dashed lines serve as guide to the eyes. Red line indicates PVO bulk pseudo-cubic lattice parameter. (d) Representative AFM images of PVO thin lms grown on LAO (10x10 µm 2 ) and LSAT (2.5x2.5 µm 2 ) substrates. Dierent thicknesses are shown. The

Figure 2 :

 2 Figure 2: Asymmetrical Reciprocal Space Maps (RSMs) performed for a series of PVO thin lms grown on (a) LAO and (b) LSAT substrate, around pseudo-cubic (103) of the substrate. Thehorizontal axis is Q in and vertical axis is Q out for all RSMs. The substrate and lm peaks are located in the upper and lower region of RSMs, and marked by the asterisks and solid square symbols respectively, for both substrates. The double peaks for the LAO substrate are due to twin domains in the substrate. The vertical dashed lines are only guide to the eyes.

Figure 3

 3 Figure 3 details the magnetization-magnetic eld (M-H) measurements recorded at T = 20 K. In order to clearly observe the PVO magnetic contribution, the diamagnetic signals of the substrates were subtracted from the total measured signal. The magnetic hysteresis cycles for the selected PVO samples are shown in Figure 3(a), (b) for LAO substrate and Figure 3(e), (f ) for LSAT substrate. A clear opening of the hysteresis loop is observed with increase in the lm thickness, evidencing a low temperature ferromagnetic-like behavior of the PVO lms.

Fig. 4 (

 4 Fig.4(a) and 4(c) for LSAT and LAO substrate, respectively. On the LSAT substrate, PVO lms show three distinct anomalies at temperatures T N , T SO2 and T SO3 (Fig.4(a)) while sweeping the temperature from 300 K to 10 K (further conrmed by plotting dM/dT and χ -1 (T)). The rising signal at temperature T N shows the onset of G-type spin ordered state of the vanadium moments, where these moments align antiferromagnetically (|| to [001] o -

Figure 4 :

 4 Figure 4: Normalized magnetization measurements for a series of PrVO 3 lms grown on top of (a) LSAT, and (c) LAO substrate, obtained at H || = 50 Oe, displaying dierent transitions at T SO1 , T SO2 and T SO3 . The inset is dM/dT for respective substrates. (b) T N (T SO1 ) as a function of lm thickness for (b) PVO/LSAT, (d) PVO/LAO lms. The dashed lines are only guide to the eyes.

Finally, we haveFigure 5 :

 5 Figure 5: Magnetic hysteresis cycles at 20 K of a ∼ 50 nm PVO/LSAT lm uncapped (a), and capped with ∼ 8 nm thick LAO (b).

Acknowledgments

The authors thank F. Veillon for his valuable experimental support. The authors also thanks to S. Froissart for the AFM support and L. Gouleuf for technical support. This work is supported by Region Normandie, by french ANR POLYNASH (ANR-17-CE08-0012) and Labex EMC3. D. K. received his fellowship from Region Normandie.

P V O / L S A T ( e )

where M s and H c almost freeze and does not change with lm thickness. Thickness-dependence of the remanent magnetization (M R ) for (d) PVO/LAO, and (h) PVO/LSAT lms. The red line is a t to M R vs. t plot, using equation (2). The right down-inset in (a),(e) represents schematic of magnetization measurements, where the directions refer to the orthorhombic symmetry of PVO.

The inset of (h) is a scheme of dead-layer on the surface of lm. The dashed lines are only guide for the eyes.