domain: chim.mate chim phys

text:
D Kumar

P Boullay

A Fouchet

A David

A Pautrat

W Prellier
email: wilfrid.prellier@ensicaen.fr

Strong magnetic anisotropy of epitaxial PrVO 3 thin lms on SrTiO 3 substrates with dierent orientations

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

We have probed the structural and magnetic properties of PrVO 3 (PVO) thin lms grown on the (001)-, (110)-and (111)-oriented SrTiO 3 (STO) substrates. By changing the substrate orientation,

(1) the out-of-plane orientation of lm can be tuned to [110], [100] / [010], and [011] / [311], (2) the number of crystal variants in the lm can be varied, for ex. we observe single domain lm on (110)oriented STO, whereas two domains in the lm grown on the (111)-oriented STO substrate. The lattice strain induced by using dierent oriented substrates has direct inuence on the magnetic properties of PVO lms. The magnetic moment of PVO lms radically enhances from 0.4 µ B /f.u. for STO (001) to 2.3 µ B /f.u. for STO (111). While, lms on (001)-oriented STO substrate display out-of-plane anisotropy, an in-plane anisotropy is observed for lms grown on the (110)-and (111)-oriented STO substrates. In addition, a strong uniaxial magnetic anisotropy is also extracted for a partially relaxed lm on the (110)-oriented STO substrate. Such ndings can help oxide community in the better understanding of magnetic anisotropy in vanadate thin lms, a subject that still lacks scientic investigations.

INTRODUCTION

The RVO 3 , where R is a trivalent rare earth element (R = La, Ce, Pr,.....Lu), have been intensively studied due to their wide range of physical properties [15]. In these compounds, the interaction between spin, orbit and lattice degrees of freedom remains the center of all intriguing physical phenomena, where one alters the ideal lattice of a compound in order to modify and even achieve new functionalities. These systems order with one of two dierent types of magnetic structures. For the compounds with larger rare earth radii (R = LaDy) the magnetic structure is of C-type (where, spins order antiferromagnetically along ab plane and ferromagnetically along the c-axis), whereas compounds with smaller rare earth radii (R = HoLu) have a G-type magnetic structure (spins order antiferromagnetically in all the direction) [6].

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) [7]. Bulk PVO is a canted antiferromagnet [8], but it has tendency to become a hard ferromagnet when grown under epitaxial strain in the form of a thin lm [9].

The impact of epitaxial strain imposed by the underlying substrate also plays a crucial role in governing the magnetism of the PVO lms. Namely, when PVO lms are grown on the LaAlO 3 (LAO) substrate, under a large compressive strain (2.9 %), the reduction in in-plane bond length between neighboring V atoms leads to promote the super-exchange interactions, and thus exhibit large Néel temperature (T N ). While, lms grown on the SrTiO 3 (STO) substrate under a moderate tensile strain (∼ 0.1 %) show reduced T N , in agreement with our theoretical calculations [2]. Recent advances in the technological devices allow one to realize materials in the realm of atomic resolution, and seperate the eect of epitaxial strain from microstructure and defects. Through X-ray Photoelectron Spectroscopy (XPS), the formation of an over-oxidized dead-layer (45 nm) on the surface of PVO lm was earlier revealed, consisting of isolated Pr atom. Furthermore, this dead-layer has a large paramagnetic contribution, whose impact gradually diminishes with the increase of lm thickness [10]. The over-oxidation of the lm surface is quite common in vanadium oxide thin lms, which tends to oxidize during growth process [11,12]. One therefore needs to be very careful during thin lm-fabrication and require additional precautions to avoid the surface oxidation, such as using of high vacuum during growth and reduction of lm surface by using a capping layer.

The preferential direction of magnetization in thin lms is mainly determined by the mag-netic anisotropy, which includes shape anisotropy, magnetoelastic anisotropy and magnetocrystalline anisotropy. There have been numerous studies in the past to explore magnetic anisotropy in other competitive complex systems such as manganites [1315], and ruthenates [16,17], and due to Tunneling Magnetoresistance properties of the former, they are being used as Magnetic Tunneling Junction devices. However, little is known of the magnetic anisotropy in vanadium oxide thin lms, especially PVO, and thus has yet to be understood.

In this paper, we were motivated to study the substrate surface orientation-induced magnetic anisotropy in PVO thin lms, which were grown on (001)-, (110)-and (111)-oriented STO substrates. By changing the substrate-surface orientation, the out-of-plane orientation of PVO lms can be eectively controlled. For the lm grown on (111)-oriented STO substrate, we observe a striking enhancement in magnetization compared to its bulk counterpart, and, to our knowledge, is rst time ever recorded. Furthermore, a strong in-plane uniaxial anisotropy is seen for lm grown on the (110)-oriented STO substrate. Reciprocal Space Maps (RSM) of PVO lms on STO substrates were collected for the skew symmetrical reections of the substrates (Fig. 2). The inspection of these maps reveals that the lms have grown coherently on the STO substrates, with the same in-plane lattice The orthorhombic lattice of (110)-oriented STO (see Fig. 3(h)) may allow coherent growth of orthorhombic cell of PVO without any atomic reconstruction at the interface, therefore presenting sharp interface in combination with well-dened thickness fringes in the XRD scan. On the contrary, STO(111) substrate is known to undergo atomic reconstruction at the interface to suppress polar discontinuity and its divergent surface energy [18,19] monoclinic and trigonal distortions, respectively, due to unequal lattice mismatches along two in-plane orthogonal directions.

IV. MAGNETISM

Since the strain and lattice distortion depend on the orientations of the STO substrates, the magnetic properties of PVO lms are expected to dier for the three orientations of the substrate, due to change in the VO bond length and VOV bond angle. The primary magnetic results of 35 nm thick PVO thin lms are shown in Fig. 4. For PVO lm on the (001)-oriented STO, the saturated magnetization (M s ) is ∼ 0.3 µ B / f.u., and is close to our earlier observations [2]. Remarkably, the saturated magnetization for PVO lms on (110)-and (111)-oriented STO substrates has extensively enhanced, close to 1.0 µ B / f.u. and 2.3 µ B / f.u., respectively. These obtained values of M s are larger as compared to that of the bulk, which is ∼ 0.6 µ B / f.u. [8]. Due to the polar nature of the (110)-and

(111)-oriented STO substrates, they are more susceptible to the reconstruction. This could allow a quicker recovery of the spin moments, leading to an enhanced magnetization. To the best of our knowledge, this striking phenomenon where the saturated magnetization is increased by approximately 8-fold merely by changing the crystal surface orientation of the substrate, is perhaps observed rst time in the oxide thin lms. Indeed, a similar study focussed on SrRuO 3 thin lms grown on dierent oriented STO substrates reported nearly 2 times enhancement of saturated magnetization (between ( 001) and (111)-oriented STO), and the authors explained this peculiar behavior using a high spin state of Ru 4+ ions [20].

However, V 3+ ion has only two electrons in the 3d shell (t 2g orbital-active system), making them ineligible for high spin state context which typically require at least four electrons in the 3d shell. Nevertheless, a dead-layer present at the surface of lm and consisting of isolated paramagnetic Pr ions, could also explain the observed substantial magnetization.

In fact, we have recently shown that a dead layer of ∼ 5 nm in thickness, which is composed of mainly V 4+ ions, is responsible for the increase of magnetization for low thicknesses lms [10].

Remarkably, the steps in the hysteresis loops (especially for STO (111)) are also seen, and are almost similar to what were previously reported for PrVO 3 single crystals (below 3 K) [21]. In the stated reference [21], the steps (in hysteresis loop) strongly depended on the eld sweep rate, namely, they shifted to a higher magnetic eld when sweep rate was decreased. This unusual behavior was then proposed to be arisen on the account of ). We infer that, neither the steps in hysteresis loop depend upon eld sweep rate, nor the magnetic ordering temperature show variation as a function of frequency, a necessity for spin glass system. These observations suggest that the steps in the hysteresis loop may not essentially be related to the spin-glass structure, but perhaps to the magnetic domains in the lm. The magnetic anisotropy of the PVO lms was characterized by M-H curves after the subtraction of the diamagnetic contribution from the oriented STO substrates. The magnetic eld was applied along in-plane and out-of-plane of the sample, and the results are shown in Fig. 5. On one hand, for PVO lm on (001)-oriented STO, the largest saturated magnetization within the measured magnetic eld range is acquired when magnetic eld is applied along out-of-plane direction, and thus regarded as the magnetic easy axis. On the other hand, for lms on (110)-and (111)-oriented STO, a strong anisotropy is observed, where, the largest saturated magnetization is achieved for eld applied along in-plane of the sample. Therefore, the magnetic easy axis most probably remains normal to the lm plane, along PVO [110] o for (001)-oriented STO, while it changes along the lm plane for (110)-and (111)-oriented STO, along PVO[001] o and PVO [112], respectively. This could be due to the deformed VO 6 octahedra, which is extended (compressed) along out-of-plane direction for a compressive (tensile) strain. A summary of the magnetic parameters;

coercive eld (H c ), remanent magnetization (M R ) and saturated magnetization (M s ), for eld parallel (in-plane) and perpendicular (out-of-plane) conguration, are listed in Table II.

We have further investigated the eect of strain relaxation in PVO lms on the uniaxial / biaxial magnetic anisotropy. For this, lms of thickness ∼ 75 nm were fabricated on the (001)-, (110)-and (111)-oriented STO substrates. The x-ray reciprocal space maps (not shown) recorded for skew symmetrical planes of the substrates show that the lms are partially relaxed. Fig. 6 shows the anisotropy results of 75 nm thick lms for the three orientations of the substrates. For lm on the (001)-oriented STO, the saturated magnetization is largest along the out-of-plane direction of substrate (or lm) (Fig. 6(a)), and could be the magnetic easy axis. For two orthogonal in-plane directions of the substrate, we observe similar shapes of the hysteresis loops, along with analogous magnetizations (both saturated and remanence). This indicates isotropic magnetism probably due to dominance of four-fold crystalline anisotropy in the PVO lm on (001)-oriented STO substrate.

For (110)-oriented STO substrate, we observe an obvious square-like hysteresis loop with the largest magnetization along in-plane [001] of STO, and perhaps is the magnetic easy axis.

While the hard axis lies along out-of-plane [110] of the STO, as evidenced by the linear hysteresis loop (inset of Fig. 5(b)). Therefore, PVO lm on the (110)-oriented STO substrate noticeably shows a perpendicular anisotropy. Interestingly, the magnetization along the two in-plane orthogonal directions dier extensively. This clearly shows the presence of uniaxial anisotropy for lm on (110)-oriented STO substrate, and can be account for the magnetoelastic anisotropy present in the lm due to unequal lattice mismatch along two in-plane directions. Finally, for the PVO lm on (111)-oriented STO, the magnetic easy axis is along compressive and tensile strain, respectively, can be at the origin of the observed magnetic anisotropy. Moreover, in the present case, the in-plane strains are unequal along the two in-plane orthogonal directions, producing further in-plane deformation of the octahedra, and thus causing in-plane uniaxial anisotropy. However, the direction of the magnetic easy axis along out-of-plane (probably) for STO (001), and along in-plane for STO (110) and STO (111), is still confusing. In general, the compressive strain enhances the out-of-plane magnetization, while tensile strain increases the in-plane magnetization [14,22,23]. Accordingly, we have estimated an approximate pseudocube expansion (compare with bulk) of the unit cell of PVO for the three orientations of the substrate, to be ∼ 1.2 % (STO (001)), ∼ 2.5 % (STO (110)) and ∼ 1.7 % (STO (111)). From our earlier report [2], we know that the tensile strain in PVO expands the unit cell, whereas, the compressive strain eectively reduces the unit cell volume of PVO. Therefore, in this sense, STO (110) and STO (111) substrates probably impose more tensile strain on the unit cell of PVO, forcing the magnetic easy axis to stay along in-plane direction. On the other hand, lm on STO (001) can favor a weak out-of-plane anisotropy (present case), or no anisotropy at all (in Ref. [1]) due to a negligible tensile strain .

V.

CONCLUSIONS

In summary, we have studied the magnetic anisotropy of PrVO 3 thin lms on the oriented SrTiO 3 substrates. We have found that the magnetization of PrVO 3 thin lm on (111)oriented SrTiO 3 substrate can be extensively enhanced compare to the bulk. All the lms clearly show magnetic anisotropy. While lms on the (001)-oriented SrTiO 3 show a weak out-of-plane anisotropy, a strong in-plane anisotropy is observed for lms grown on the (110)-and (111)-oriented SrTiO 3 substrates. Furthermore, an evident uniaxial anisotropy is also argued for lms on the (110)-oriented SrTiO 3 substrates, and would probably require further experiments, e.g., XPS/XAS, X-ray Magnetic Circular Dichroism (XMCD) in order to conrm a dead layer on the surface of the lms (especially for lm on STO(111)) and to observe the magnetism from individual cations, respectively.

3 ( 2 = 2 .

 322 PVO) thin lms were synthesized by using Pulsed Laser Deposition (PLD) on the (001)-, (110)-and (111)-oriented SrTiO 3 (STO) substrates supplied by CrysTec. A KrF excimer laser (wavelength 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 growth temperature and pressure during deposition were maintained at 650 • C, and 10 -6 mbar, respectively. X-ray diraction and reciprocal lattice mapping were performed on a Bruker D8 Discover diractometer (Cu Kα1 radiation, λ = 1.5406 Å) to study the crystallinity and strain states of the epitaxial layers. The lm thickness was estimated by tting the thickness fringes around Bragg peak of substrate to the Laue equation (Intensity of oscillations, I = sin 2 (N QC/2) sin 2 (QC/2) ; where N is the number of unit cells, Q is the reciprocal lattice vector, and C denotes the out-of-plane lattice parameter). The microstructure of lms on (110)and (111)-oriented STO was revealed through Transmission Electron Microscopy (TEM) (Description of TEM instrument). The eld-and temperature-dependent magnetization measurements were performed in a Quantum Design Magnetic Property Measurement System (MPMS) XL, as well as the Vibrating Sample Magnetometer (MPMS 3).The X-ray diraction results of 35 nm thick PVO lms grown on (001)-, (110)-and (111)oriented STO substrates are detailed in Fig.1. The Laue oscillations in the θ-2θ High Resolution X-ray diraction (HRXRD) patterns of PVO lms on (001)-and (110)-oriented STO are indicative of a well-dened lm-substrate interfaces, whereas, hardly an oscillation is observed for the lm grown on (111)-oriented STO substrate. For STO (111) substrate, the polar catastrophe and intermix termination could invoke major reconstructions, and this could lead to produce rough interfaces[18]. The pseudocube interplanar spacings derived from the lm peak positions in Figs.1(a), 1(b), 1(c), are: d 001 = 3.94 Å, d 110 = 3.98/ √ 814 Å and d 111 = 3.954/ √ 3 = 2.283 Å, for PVO lms on (001)-, (110)-and (111)-oriented STO substrates, respectively.

Figure 1 :

 1 Figure 1: (Color online) HRXRD θ -2θ scans of PrVO 3 (PVO) thin lms on (a) (001)-, (b) (110)and (c) (111)-oriented SrTiO 3 (STO) substrates, in the vicinity of (001) c , (110) c and (111) c Bragg's peak of STO, respectively. The subscript c refers to cubic.

Figure 2 :

 2 Figure 2: (Color online) X-ray diraction reciprocal space maps around (d) STO (103), (e) STO (130) and (f) STO (121) of (001)-, (110)-and (111)-oriented STO substrates, respectively. The dashed lines are only guide to the eyes and represent strained nature of lms with the substrates.

Figure 3 :

 3 Figure 3: (Color online)(Top panel) HRTEM images of PVO lms grown on (a) STO (111) and (b) STO (110) substrates. (Middle panel) (c), (d): Selected area electron diraction (SAED) patterns for the lm on (111)-oriented STO substrate, corresponding to domain I and domain II, respectively. The Zone Axis Point (ZAP) for (c) is along [011] o and for (d) is along [311] o (the subscript o refers to orthorhombic). (Middle panel) (e): SAED patterns for the lm on (110)oriented STO substrate, with ZAP along [100] o . (Bottom panel) The representative schematics of the observed crystallographic domains in the lm on STO (111) (f,g) and STO (110) (h).

Figure 4 :

 4 Figure 4: (a) Hysteresis cycles at 10 K for 35 nm thick PVO lms on (001)-, (110)-and (111)-oriented STO substrates, obtained by applying magnetic eld in the plane of sample. (b) Hysteresis cycles for PVO lm on (111)-oriented substrate at dierent eld sweep rates. Inset shows the AC moment of the same sample as a function of temperature (in the vicinity of magnetic ordering temperature) taken at dierent frequencies.

Figure 5 :

 5 Figure 5: (a)-(c) Hysteresis cycles for lms on three oriented STO substrates with eld applied along the sample plane and perpendicular to the sample.

Figure 6 :

 6 Figure 6: The hysteresis loops of 75 nm thick PVO lms on (a) (001)-, (b) (110)-and (c) (111)oriented STO substrates, along two in-plane and out-of-plane directions of the substrates.

[010] STO [001] STO [100] STO [110] PVO [001] STO [110] STO [001] PVO [010] PVO [100] PVO [100] PVO [011] PVO [11𝟐] PVO [111] STO

  , making this substrate less compatible to grow orthorhombic perovskite lattice. For PVO lm grown on (001)-oriented STO substrate, we have already observed two domains, oriented 90 • to each other with the epitaxial relationship: F[110] o // S[001] c and F[001] o // S[100] c / S[010] c [2, 9]. In general, the sign and strength of lattice mismatch vary with the orientations of the substrate, and thus dierent distortions may be induced in the lm, yielding dierent crystal structures. The calculated in-plane lattice mismatches and observed growth orientations of the lms for dierent oriented STO substrates are listed in Table I. The (001)-oriented STO exerts tensile stress on PVO [001] o -axis (see TableI), imposing tetragonal distortion on the unit cell[2]. On the other hand, (110)-and (111)-oriented STO substrates would impose

			STO[001]		STO [110]		STO [111]
		In-plane	Out-of-plane	In-plane	Out-of-plane	In-plane	Out-of-plane
	Directions	[001] o = [100] pc	[110] o = [010] pc	[110] o = [001] pc	[001] o = [001] pc	[010] o /[100] o = [110] pc	[100] o /[010] o = [110] pc	[112] o = [110] pc	[100] o /[010] o = [110] pc	[101] o /[011] o = [111] pc
	Lattice mismatch	0.41 %	-0.05 %		0.41 %	-0.74 % / 0.65 %		0.18 %	0.65 % / -0.74 %
	Growth
	Table I: (Color online) A detailed summary of the in-plane and out-of-plane lattice directions of
	PVO grown on STO substrates with dierent orientations. The lattice mists between in-plane
	directions of PVO and the substrate are shown. The experimentally observed growth orientations
	of the PVO lattice on dierent oriented STO substrates are also shown. Here, a positive lattice

mismatch between PVO and substrate indicates tensile stress, and negative indicates compressive stress.

Table II :

 II Crystallographic orientation and angular dependence of coercive eld H c , remanent magnetization M R , and the saturation magnetization M s for 35 nm thick PVO lms. Here, θ (angle between direction of magnetic eld and sample surface) = 0 • means eld is applied in-plane, and θ = 90 • means eld is orthogonal to sample surface.

	Orientation		0 • / in-plane 90 • / out-of-plane
	001	H c (T)	3.1	3.0
		M R (µ B / f.u.)	0.35	0.66
		M s (µ B / f.u.)	0.42	0.75
		axis	Hard	Easy
	110	H c (T)	2.4	0.2
		M R (µ B / f.u.)	0.9	0.06
		M s (µ B / f.u.)	1.0	0.15
		axis	Easy	Hard
	111	H c (T)	0.03	0.03
		M R (µ B / f.u.)	0.47	0.03
		M s (µ B / f.u.)	2.3	0.4
		axis	Easy	Hard

Acknowledgments DK thanks Maxime Hallot for preparing cross section of the samples and F. Veillon for his valuable experimental support. The authors also thank to S. Froissart for the AFM support and L. Gouleuf for technical support. DK thanks O Copie and U Lüders for illuminating discussions. This work is supported by Region Normandie, by french ANR POLYNASH (ANR-17-CE08-0012) and Labex EMC3.