Anisotropic spin-density distribution and magnetic anisotropy of strained La$_{1-x}$Sr$_x$MnO$_3$ thin films: Angle-dependent x-ray magnetic circular dichroism

TL;DR

This study uses angle-dependent x-ray magnetic circular dichroism to reveal how strain influences the anisotropic spin-density distribution and magnetic anisotropy in La$_{1-x}$Sr$_x$MnO$_3$ thin films, clarifying surface orbital occupation effects.

Contribution

It provides direct experimental evidence of strain-dependent in-plane and out-of-plane spin-density anisotropy in LSMO thin films using XMCD, resolving previous ambiguities from XLD measurements.

Findings

Spin-density distribution anisotropy aligns with magnetic anisotropy.

Tensile strain induces in-plane spin-density anisotropy.

Compressive strain induces out-of-plane spin-density anisotropy.

Abstract

Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer.

Tables

Table 1. Table 1 : Best-fit parameters for the simulated curves in Figs.  4 a and 4 b. Errors have been estimated using the least squares method. Note that K u subscript 𝐾 𝑢 K_{u} represents the MCA energy excluding shape magnetic anisotropy.

Substrate	$M_{\text{sat}}$ (${\mu }_{\text{B}}$/Mn)	$K_u$ (kJ/m3)	$\left(7/2\right)⟨Q_{zz}⟩$
STO	$1.255\pm 0.007$	$-37.2\pm 0.8$	$+0.05\pm 0.01$
LAO	$1.206\pm 0.014$	$+40.4\pm 2.4$	$-0.12\pm 0.02$

Equations

$$E=$$

$$\hat{\mathbf{P}}\cdot {\mathbf{M}}_{\text{spin}}+(7/2)\hat{\mathbf{P}}\cdot {\mathbf{M}}_{\text{T}}$$

Full text

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Anisotropic spin-density distribution and magnetic anisotropy of

strained La1-xSrxMnO3 thin films: Angle-dependent x-ray magnetic circular dichroism

(Running title: Anisotropic spin density in La1-xSrxMnO3 thin films)

Goro Shibata

shibata@wyvern.phys.s.u-tokyo.ac.jp

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Miho Kitamura

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

  

Makoto Minohara

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

  

Kohei Yoshimatsu

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

  

Toshiharu Kadono

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Keisuke Ishigami

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Takayuki Harano

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Yukio Takahashi

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Shoya Sakamoto

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Yosuke Nonaka

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Keisuke Ikeda

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Zhendong Chi

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

  

Mitsuho Furuse

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan

  

Shuichiro Fuchino

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan

  

Makoto Okano

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan

  

Jun-ichi Fujihira

Fujihira Co., Ltd., Tsukuba, Ibaraki 305-0047, Japan

  

Akira Uchida

Fujihira Co., Ltd., Tsukuba, Ibaraki 305-0047, Japan

  

Kazunori Watanabe

Fujihira Co., Ltd., Tsukuba, Ibaraki 305-0047, Japan

  

Hideyuki Fujihira

Fujihira Co., Ltd., Tsukuba, Ibaraki 305-0047, Japan

  

Seiichi Fujihira

Fujihira Co., Ltd., Tsukuba, Ibaraki 305-0047, Japan

  

Arata Tanaka

Department of Quantum Matter, Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan

  

Hiroshi Kumigashira

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

  

Tsuneharu Koide

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

  

Atsushi Fujimori

Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Abstract and Keywords

**Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La1-xSrxMnO3 (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer. **

Keywords: Electronic properties and materials; Ferromagnetism; Surfaces, interfaces and thin films; Magnetic properties and materials

Introduction

Magnetic anisotropy is one of the most important properties of ferromagnets and its external control has been a major challenge both from the fundamental and applied science points of view Coey . From the application point of view, enhancement of the magnetic anisotropy is necessary to realize magnets with high coercive fields, which can be utilized as high-density energy-storage magnets. From the scientific point of view, elucidating the microscopic origin of magnetic anisotropy has been an important issue because it is generally governed by the complex interplay between spin-orbit interaction and microscopic electronic states such as spin and orbital magnetic moments, band structures, and anisotropy of charge/spin densities. Especially, the magnetic anisotropy of ferromagnetic thin films is of great interest and importance because it can be controlled, e.g., by changing epitaxial strain and film thickness.

As for oxide materials, the perovskite-type manganese oxide La1-xSrxMnO3 (LSMO) has been the most extensively studied ferromagnet due to its intriguing physical properties such as colossal magnetoresistance (CMR) and half-metallicity. The physical properties of LSMO can be controlled in various ways, e.g., by changing hole concentration $x$, temperature $T$, and external magnetic field $H$ (Ref. IFT, ). In the case of thin films, their properties are also strongly affected by epitaxial strain which originates from the lattice mismatch between the film and the substrate. For example, Konishi et al. Konishi have shown that ferromagnetic (FM) metallic LSMO ($x$ = 0.3-0.5) thin films enter the A-type antiferromagnetic (AFM) metallic phase under tensile strain from a SrTiO3 (STO) (001) substrate and the C-type AFM insulating phase under compressive strain from a LaAlO3 (LAO) (001) substrate. The magnetic anisotropy of the LSMO thin films also depends on the epitaxial strain: the magnetic easy axes are in-plane when grown on the STO substrate and out-of-plane when grown on the LAO substrate Tsui ; KwonJMMM97 . First-principles calculations have predicted that the $d_{x^{2}-y^{2}}$ orbital is preferentially occupied under the tensile strain and that the $d_{3z^{2}-r^{2}}$ orbital is preferentially occupied under the compressive strain Konishi . However, previous x-ray linear dichroism (XLD) experiments have shown that the $d_{3z^{2}-r^{2}}$ orbital is preferentially occupied for both STO and LAO substrates TebanoPRL08 ; ArutaPRB09 ; PesqueraNatCom12 . This apparent discrepancy with theory has been ascribed to the different orbital occupation between the surface and the bulk, that is, the spatial symmetry breaking at the surface leads to the preferential occupation of the $d_{3z^{2}-r^{2}}$ orbital TebanoPRL08 ; ArutaPRB09 ; PesqueraNatCom12 . Thus, the microscopic electronic and magnetic states of LSMO thin films and their relationship with the macroscopic magnetic properties have remained elusive so far.

In the present work, we have employed a method which directly probes the orbital occupation of spin-polarized electrons using angle-dependent x-ray magnetic circular dichroism (XMCD) in core-level x-ray absorption spectroscopy (XAS). In the XMCD spin sum rule SpinSum , in addition to the well-known term which represents the spin magnetic moment $\bm{M}_{\text{spin}}$, there is an additional term called ‘magnetic dipole term’ $\bm{M}_{\text{T}}$ which represents the spatial anisotropy of spin-density distribution, namely, the orbital shapes of the spin-polarized electrons. While XLD is sensitive to the orbital polarization of all the valence electrons, XMCD is sensitive to the orbital polarization of only spin-polarized electrons and, therefore, one can directly probe the orbital states of electrons which contribute to the ferromagnetism. In general, it is difficult to deduce $\bm{M}_{\text{spin}}$ and $\bm{M}_{\text{T}}$ separately from a single XMCD spectrum by using the sum rule. However, as we shall see below, one can separate the magnetic moment into the $\bm{M}_{\text{spin}}$ and $\bm{M}_{\text{T}}$ components from the angular dependence of the XMCD spectra, because they have different angular dependencies TXMCD_Stohr ; TXMCD_Durr ; vanderLaanJPCM98 ; vanderLaanPRB98 . Hence, the spatial anisotropy of the spin-polarized electrons in the ferromagnetic materials can be deduced in addition to the total spin magnetic moment. Especially, in the geometry where $\bm{M}_{\text{spin}}$ is perpendicular to the incident x rays [so-called transverse XMCD (TXMCD) geometry] TXMCD_Durr , one can extract the pure $\bm{M}_{\text{T}}$ component.

Although TXMCD has been theoretically studied since two decades ago TXMCD_Stohr ; TXMCD_Durr ; vanderLaanJPCM98 ; vanderLaanPRB98 ; TXMCD_Maruyama , there have been only few experimental reports Durr_Science97 ; TXMCD_Mamiya ; vanderLaanPRL10 ; TXMCD_Koide because the direction of the magnetic field is fixed parallel or nearly parallel to the incident x rays in conventional XMCD measurement systems. Recently, we have developed an apparatus for angle-dependent XMCD experiments using a vector-type magnet where the direction of the magnetic field can be rotated using two pairs of superconducting magnets Vector_Furuse . In this paper, we report on the angle-dependent XMCD and TXMCD experiments on ferromagnetic LSMO ($x=0.3$) thin films grown on STO and LAO substrates, and investigate the effect of epitaxial strain on the orbital states of spin-polarized electrons. We have revealed that the LSMO thin film under tensile (compressive) strain has $d_{x^{2}-y^{2}}$-like ($d_{3z^{2}-r^{2}}$-like) spin-density distribution, which is different from the charge-density distribution deduced from the XLD measurements. The origin of the difference between the spin- and charge-density distributions is attributed to the preferential occupation of both the spin-up and spin-down $d_{3z^{2}-r^{2}}$ orbitals at the surface, which suggests the formation of magnetic dead layers at the surface.

Results

Angular dependence of XMCD spectra and TXMCD.

Figure 1a shows a schematic drawing of the experimental setup for angle-dependent XMCD. One can change the direction of the external magnetic field using two sets of superconducting magnets orthogonally arranged. The experimental geometry is schematically drawn in Fig. 1b with the definition of the angles of incident x rays ($\theta_{\text{inc}}$), applied magnetic field ($\theta_{\bm{H}}$), and magnetization ($\theta_{\bm{M}}$). Note that in general $\theta_{\bm{M}}$ is not equal to $\theta_{\bm{H}}$ unless the applied magnetic field is large enough to fully align all the electron spins along the magnetic field direction. According to the XMCD sum rules SpinSum ; OrbSum , the ‘effective’ spin magnetic moment $\hat{\bm{P}}\cdot[\bm{M}_{\text{spin}}+(7/2)\bm{M}_{\text{spin}}]$ is proportional to $\Delta I_{3}+2\Delta I_{2}$, where $\hat{\bm{P}}$ is a unit vector along the x-ray incident direction, and $\Delta I_{3}$ and $\Delta I_{2}$ are the integrals of the XMCD spectra over the Mn $L_{3}$ ($2p_{3/2}\to 3d$) and Mn $L_{2}$ ($2p_{1/2}\to 3d$) absorption edges, respectively. Under the assumption that the orbital magnetic moment $\bm{M}_{\text{orb}}$ and the magnetic dipole moment $\bm{M}_{\text{T}}$ are small enough compared to $\bm{M}_{\text{spin}}$, the projected spin magnetic moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}$ is approximately proportional to the XMCD integrals $\Delta I_{3}$ or $\Delta I_{2}$. (For more information about angle-dependent XMCD, see Supplementary Fig. 1 and Supplementary Note 1.) In the present study, $\theta_{\text{inc}}$ was fixed to $45^{\circ}$ and $\theta_{\bm{M}}$ was varied through varying $\theta_{\bm{H}}$.

We have grown LSMO ($x=0.3$) thin films on the Nb-doped STO (tensile strain) and LAO (compressive strain) substrates by the laser molecular beam epitaxy method (See ‘Methods’ section for the detail of sample preparation, and Supplementary Figs. 2-4 and Supplementary Note 2 for sample characterization.) Figures 2a and 2b show the Mn $L_{2,3}$-edge ($2p\to 3d$) XAS spectra of the LSMO thin films grown on the STO and LAO substrates, respectively, taken at $\theta_{\bm{H}}=45^{\circ}$ (where the magnetic field is applied parallel to the incident x rays). Since the spectral line shape of XAS was almost independent of $\theta_{\bm{H}}$ (see Supplementary Fig. 5), only the XAS spectra for $\theta_{\bm{H}}=45^{\circ}$ are shown here. The spectral line shape of XAS is similar to those obtained in previous XMCD studies of bulk LSMO_Koide and thin-film ArutaPRB09 ; LSMO_Shibata samples, and absorption signals of extrinsic Mn2+ (Ref. Mn2plus, ) are hardly observed. Figures 2c and 2d show the Mn $L_{2,3}$-edge XMCD spectra of both the substrates for various $\theta_{\bm{H}}$’s. Systematic changes in the XMCD integrals at the Mn $L_{3}$ edge (approximately proportional to $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}$) can be seen which arise from the change in the magnetization direction $\theta_{\bm{M}}$ under varying $\theta_{\bm{H}}$. The XMCD integrals at the Mn $L_{3}$ edge reverse in sign around $\theta_{\bm{H}}=-15^{\circ}\;\mathchar 45\relax-20^{\circ}$ for the LSMO/STO film and around $\theta_{\bm{H}}=-50^{\circ}\;\mathchar 45\relax-55^{\circ}$ for the LSMO/LAO film. This means that the magnetization is directed nearly perpendicular to the incident x rays ($\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}\sim\hat{\bm{P}}\cdot\bm{M}\sim 0$) around these $\theta_{\bm{H}}$’s, namely, the TXMCD geometry is expected to exist around these angles.

The orange and green curves in Fig. 3a shows the expanded XMCD spectra for LSMO/STO at $\theta_{\bm{H}}=-20^{\circ}$ and for LSMO/LAO at $\theta_{\bm{H}}=-50^{\circ}$, respectively. (We note that we have chosen these angles by the comparison with theoretical TXMCD spectra, as described below.) Finite XMCD signals, which is expected to originate from the magnetic dipole term $\bm{M}_{\text{T}}$, are clearly observed. One may suspect that $\bm{M}_{\text{spin}}$ is not precisely aligned perpendicular to the x rays and yields this finite XMCD signals. This possibility, however, can be ruled out because the line shapes of the observed XMCD spectra are quite different from those of the conventional (longitudinal) XMCD (black curve in Fig. 3a). Furthermore, the spectral line shapes of LSMO/STO and LSMO/LAO are nearly identical but only the sign of the spectra is reversed. This suggests that the sign of $\bm{M}_{\text{T}}$, namely the anisotropy of the spin-density distribution, is reversed reflecting the opposite epitaxial strain.

In order to show that the obtained spectra arise from genuine TXMCD, we have calculated the TXMCD spectra under tensile or compressive strain using the Mn3+O6 cluster model with $D_{4h}$ symmetry (see ‘Method’ section for details). Here, only the Mn3+ ($d^{4}$) valence state has been considered since the anisotropy of the charge/spin density is negligible for the Mn4+ ($d^{3}$) valence state, where the $t_{2g\uparrow}$ levels are fully occupied and the $e_{g\uparrow}$ levels are empty. Using the parameter values listed in Supplementary Table 1, we have calculated the TXMCD spectra corresponding to both tensile and compressive strain, as shown in Fig. 3b. The calculated TXMCD spectra well reproduce the experimental ones, suggesting that the experimentally obtained spectra at $\theta_{\bm{H}}=-20^{\circ}$ for LSMO/STO and at $\theta_{\bm{H}}=-50^{\circ}$ for LSMO/LAO are the genuine TXMCD signals which reflect the anisotropic spin density on the Mn atom. Comparing the signs of the experimental TXMCD spectra with the calculated ones, it is clearly demonstrated that the spin-density distribution of the Mn 3$d$ electrons in the LSMO/STO (LSMO/LAO) thin film is more $d_{x^{2}-y^{2}}$-like ($d_{3z^{2}-r^{2}}$-like), consistent with the expectation for the tensile and compressive epitaxial strain from the substrates.

Quantitative estimate of magnetic anisotropy energy and anisotropic spin-density distribution.

We have seen in Figs. 2c and 2d that the sign change of $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}$ occurs around $\theta_{\bm{H}}\simeq-20^{\circ}$ for the LSMO/STO film and $\theta_{\bm{H}}\simeq-50^{\circ}$ for the LSMO/LAO film. If there were no magnetic anisotropy, $\theta_{\bm{M}}$ should be equal to $\theta_{\bm{H}}$ and the sign change should occur around $\theta_{\bm{H}}=-45^{\circ}$, where the incident x-ray beam is perpendicular to the magnetic field. The deviation of the sign change angle from $\theta_{\bm{H}}=-45^{\circ}$ in the present experiment indicates that $\theta_{\bm{M}}$ is not equal to $\theta_{\bm{H}}$ due to finite magnetic anisotropy. This offers the possibility to deduce the sign and magnitude of the magnetic anisotropy by fitting the measured angular dependence of the XMCD intensity to the theoretical one which incorporates the effect of magnetic anisotropy.

Figures 4a and 4b show the $\theta_{\bm{H}}$ dependence of the projected effective spin magnetic moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}\ (\equiv\hat{\bm{P}}\cdot[\bm{M}_{\text{spin}}+(7/2)\bm{M}_{\text{T}}]\simeq\hat{\bm{P}}\cdot\bm{M}_{\text{spin}})$ obtained by applying the sum rule SpinSum to the XMCD spectra in Figs. 2c (STO substrate) and 2d (LAO substrate), respectively. The obtained angular dependencies are different from the ones which assume $\theta_{\bm{H}}=\theta_{\bm{M}}$ (black dashed curves), indicating that the effect of magnetic anisotropy has to be taken into account. We have, therefore, simulated the obtained angular dependence of $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}\ (\simeq\hat{\bm{P}}\cdot\bm{M}_{\text{spin}})$ based on the Stoner-Wohlfarth model SWmodel . In this model, a single magnetic domain with uniaxial magnetic anisotropy of the lowest order (proportional to $\cos^{2}\theta_{\bm{M}}$) is assumed. Then, the magnetic energy (per volume) $E$ is given by an expression which contains $\theta_{\bm{M}}$. By minimizing $E$ with respect to $\theta_{\bm{M}}$ for each $\theta_{\bm{H}}$, one can deduce $\theta_{\bm{M}}$ as a function of $\theta_{\bm{H}}$, and can calculate the projected magnetic moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}\equiv\hat{\bm{P}}\cdot[\bm{M}_{\text{spin}}+(7/2)\bm{M}_{\text{T}}]$ using the deduced $\theta_{\bm{M}}$. It is also possible to deduce the uniaxial magnetocrystalline anisotropy (MCA) constant $K_{u}$, the saturation magnetization $M_{\text{sat}}$, and the electric quadrupole moment $\langle Q_{zz}\rangle\equiv\langle 1-3\hat{z}^{2}\rangle$ by taking these variables as parameters and fitting the simulated angular dependence to the experimental one (see the ‘Method’ section for more details). The results of the simulations are shown in Figs. 4a and 4b by blue solid curves, showing good agreement with the experiment. The best-fit parameter values are listed in Table 1. The $K_{u}$ values in Table 1 ($K_{u}>0$ corresponding to out-of-plane easy axis) clearly show that finite MCA is present in the LSMO/STO (LSMO/LAO) thin film which favors in-plane (out-of-plane) easy magnetization, consistent with the present (Supplementary Fig. 3) and previous Tsui ; KwonJMMM97 magnetic measurements. Table 1 also shows that the electric quadrupole moment $\langle Q_{zz}\rangle=\langle 1-3\hat{z}^{2}\rangle$ is positive (negative) for the STO (LAO) substrate. Since $(7/2)\langle Q_{zz}\rangle=+2$ for the $d_{x^{2}-y^{2}}$ orbital and $(7/2)\langle Q_{zz}\rangle=-2$ for the $d_{3z^{2}-r^{2}}$ orbital (as shown in the first column of Fig. 1 in Ref. TXMCD_Stohr, ), the positive (negative) $\langle Q_{zz}\rangle$ for the LSMO/STO (LSMO/LAO) film implies that the charge distribution of spin-polarized electrons, namely, the distribution of the spin density, is more $x^{2}-y^{2}$-like for the STO (tensile) substrate and more $3z^{2}-r^{2}$-like for the LAO (compressive) substrate. This supports the TXMCD result that the spin-density distribution in the strained LSMO thin films is anisotropic. The degrees of the preferential orbital polarization $\left|(7/2)\langle Q_{zz}\rangle/2\right|$ are estimated to be $\sim 2.5\%$ and $\sim 6\%$ for LSMO/STO and LSMO/LAO, respectively.

The advantage of the present method in deducing the magnetic anisotropy from the angle-dependent XMCD is that one can eliminate the effect of extrinsic spectral changes due to the saturation effect TEYdepth , because the incident angle of the x rays is fixed. In addition, this method can be used in principle for dilute magnetic systems such as ultrathin films and lightly-doped magnetic semiconductors, for which the conventional magnetometry is hardly applicable, offering the possibility of estimating the magnetic anisotropy of these systems more accurately.

Discussion

The deduced anisotropic spin distribution in the LSMO thin films ($x^{2}-y^{2}$-like in the case of the STO substrate and $3z^{2}-r^{2}$-like in the case of the LAO substrate) is consistent with the preferential orbital occupation expected from the strain from the substrate. It is also consistent with the preferential orbital occupation which has been suggested by the transport and magnetic measurements and the density-functional calculation Konishi . On the other hand, the results of XLD measurements TebanoPRL08 show that the $d_{3z^{2}-r^{2}}$ orbital is more preferentially occupied than the $d_{x^{2}-y^{2}}$ orbital even in the case of tensile strain (STO substrate), which has been attributed to the symmetry breaking at the surface and interface PesqueraNatCom12 . The reason why the preferential orbital occupation seen by XMCD is consistent with that expected from the strain, in spite of its surface sensitivity comparable to XLD, may become apparent if one notices that XMCD is sensitive only to the spin-polarized electrons while XLD is sensitive to all the $d$ electrons. If the majority part of the surface Mn atoms occupies the $d_{3z^{2}-r^{2}}$ orbital due to the symmetry-breaking effect but are not spin-polarized, the $3z^{2}-r^{2}$-like charge-density distribution at the surface and interface should be observed in the XLD measurements, while the $x^{2}-y^{2}$-like spin-density distribution from underneath layers should be observed in the XMCD measurements. Indeed, there have been several reports which suggest the presence of magnetic dead layers at the surface or the interface of the FM LSMO thin films LSMO_Yoshimatsu ; LSMO_Huijben . The present angle-dependent XMCD and TXMCD studies, therefore, indicate close connection between the magnetic dead layer and the $3z^{2}-r^{2}$-like preferential orbital occupation at the surface of LSMO thin films. Further experiment is needed in order to test this hypothesis, e.g., by XLD measurements in the fluorescence-yield mode, in which we expect similar orbital polarization as the present study due to the longer penetration depth of the fluorescence-yield mode than the electron-yield mode.

Methods

Sample preparation.

LSMO ($x=0.3$) thin films were grown on Nb-doped STO (001) and undoped LAO (001) (in the pseudo-cubic notation) substrates by laser molecular beam epitaxy LaserMBE . Since the lattice constant of bulk LSMO is smaller (larger) than that of STO (LAO), the film is supposed to be under tensile (compressive) strain from the STO (LAO) substrate. The thickness of the thin films was around 100 unit cells ($\sim 40\ \text{nm}$) for both the samples. The growth rate was estimated from the intensity oscillation of the specular spot in reflection high-energy electron diffraction (RHEED) during the growth. The LSMO films were deposited at the temperature of $1050\ ^{\circ}\text{C}$ on the STO substrate and $650\ ^{\circ}\text{C}$ on the LAO substrate, under the oxygen pressure of $1\times 10^{-4}$ Torr. Since the LSMO/LAO film tends to be fully relaxed at higher growth temperatures and be fully strained to become an antiferromagnetic insulator at lower growth temperatures Konishi , we have adjusted the temperature so that the film is partially strained while the ferromagnetic metallicity of LSMO is maintained. After the growth of the films, both the samples were annealed at 400 ${}^{\circ}\text{C}$ for 45 minutes under 1 atm of O2 to fill oxygen vacancies. The lattice constants of the films were evaluated by four-circle synchrotron x-ray diffraction (XRD) measurements at BL-7C of Photon Factory, High Energy Accelerator Research Organization (KEK-PF), and laboratory-based XRD measurements using the Cu $K\alpha$ line. The magnetization measurements were performed using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. The temperature dependence of the resistivity was measured by the standard four-probe method. The results of the XRD, magnetization, and resistivity measurements are summarized in Supplementary Figs. 2-4 and Supplementary Note 2.

XMCD measurements.

The XAS and XMCD measurements were performed using a vector-magnet XMCD apparatus Vector_Furuse with circularly polarized soft x rays at the helical undulator beam line BL-16A2 of KEK-PF. The measurement temperature $T$ for the LSMO/LAO film was 30 K, while it was set to 270 K for the LSMO/STO film. A lower $T$ was chosen for the LSMO/LAO film because the saturation magnetization at room temperature was low Konishi , while a higher $T$ was chosen for the LSMO/STO film because the magnetic anisotropy at low temperature was too large to saturate the magnetization along the magnetic hard axis (out-of-plane direction). The strength of the applied magnetic field was 0.7 T for the LSMO/STO film and 0.5 T for the LSMO/LAO film. The spectra were taken in the total electron-yield mode, which is a relatively surface-sensitive measurement mode (with a probing depth $\lambda$ of $\sim 3\ \text{nm}$) TEYdepth . When the magnetic field is applied nearly parallel to the film surface, photo-ejected electrons are absorbed back to the sample due to the Lorentz force and the photocurrent drops to almost zero. In order to avoid this, we applied a negative bias voltage of $\sim 200\ \text{V}$ to the sample holder to help the photo-ejected electrons escape from the samples. The measurements were performed at a pressure of $\sim 1\times 10^{-9}$ Torr. The intensity of the incident x rays was monitored by a photocurrent from the post-focusing mirror.

Cluster-model calculation.

The cluster-model calculation was performed based on the method described in Ref. TanakaCluster, , using the ‘Xtls’ code (version 8.5) developed by Arata Tanaka. A distorted Mn*3+*O6 octahedral cluster with $D_{4h}$ symmetry (elongated or shrunk along the [001] direction) was used (Fig. 3c). The energy levels of the Mn 3$d$ orbitals under this symmetry are schematically drawn in Fig. 3d. The Mn 3$d$, Mn 2$p$ core, and O 2$p$ orbitals were taken as basis functions. Charge transfer from the ligand O $2p$ to the Mn $3d$ orbitals was taken into account, and we considered three electron configurations for both the initial and final states: $2p^{6}3d^{4}$, $2p^{6}3d^{5}\underline{L}$, and $2p^{6}3d^{6}\underline{L}^{2}$ for the initial state, and $2p^{5}3d^{5}$, $2p^{5}3d^{6}\underline{L}$, and $2p^{5}3d^{7}\underline{L}^{2}$ for the final state. We adjusted the following parameters to reproduce the experimental TXMCD spectra: $U_{dd}$ (Mn $3d$-$3d$ Coulomb energy), $U_{pd}$ (Mn $2p$-$3d$ Coulomb energy), $\Delta$ (charge-transfer energy from O $2p$ to Mn $3d$), $(pd\sigma)$ (Slater-Koster parameter between Mn $3d$ and O $2p$), and $10Dq$ (crystal-field splitting between the Mn $e_{g}$ and $t_{2g}$ levels). The magnitude of the $D_{4h}$ crystal-field splitting $8Cp$ (splitting between the $x^{2}-y^{2}$ and $3z^{2}-r^{2}$ levels) vanderLaanPRL10 was fixed to 0.08 eV and only its sign was varied, because varying the magnitude of $8Cp$ only changed the magnitude of XMCD and did not change the spectral line shape. We neglected the anisotropy of transfer integrals due to the $D_{4h}$ symmetry of the MnO6 cluster and transfer integrals between the O $2p$ orbitals, in order to reduce the number of adjustable parameters. The x-ray incident angle was chosen to be in the [101] direction. In order to fully align the spins perpendicular to the incident x rays, a molecular field (an effective magnetic field corresponding to the exchange interaction) of 0.01 eV along the [$\bar{1}01$] direction was introduced. We note that this molecular field is strong enough to saturate the magnetization of the Mn ions.

Simulation of angular dependence of $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}$ based on the Stoner-Wohlfarth model.

We have adopted the Stoner-Wohlfarth model SWmodel in order to simulate the angular dependence of the projected effective spin magnetic moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}\ (\simeq\hat{\bm{P}}\cdot\bm{M}_{\text{spin}})$ in Figs. 4a and 4b. By assuming that the film has only a single magnetic domain and that the magnetic anisotropy has only the uniaxial component of the lowest order, the magnetic energy (per volume) $E$ can be expressed as

[TABLE]

where $H$ is the magnitude of the external magnetic field, $M_{\text{sat}}$ is the saturation magnetization, and $K_{\text{u}}$ is the uniaxial anisotropy constant for MCA ($K_{u}>0$ for out-of-plane easy axis). The three terms in Eq. (1) represent the Zeeman energy due to the applied magnetic field, the shape magnetic anisotropy which originates from the demagnetization field in the film, and the MCA which originates from a conbined effect of microscopic electron occupation and spin-orbit interaction. By minimizing $E$ with respect to $\theta_{\bm{M}}$, we deduced $\theta_{\bm{M}}$ as a function of $\theta_{\bm{H}}$, $H$, $K_{\text{u}}$, and $M_{\text{sat}}$. Then, the projection of the effective spin magnetic moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}\equiv\hat{\bm{P}}\cdot[\bm{M}_{\text{spin}}+(7/2)\bm{M}_{\text{T}}]$ was calculated using the deduced $\theta_{\bm{M}}$ by the following equation:

[TABLE]

where $\langle Q_{zz}\rangle\equiv\langle 1-3\hat{z}^{2}\rangle$ is the electric quadrupole moment [For the derivation of Eq. (2), see Supplementary Note 1]. This gives the $\theta_{\bm{H}}$ dependence of the projected moment $\hat{\bm{P}}\cdot\bm{M}_{\text{spin}}^{\text{eff}}$ for a set of parameters ($K_{u}$, $M_{\text{sat}}$, and $\langle Q_{zz}\rangle$). The obtained $\theta_{\bm{H}}$ dependence was fitted to the experimental one (Fig. 4) to deduce $K_{u}$, $M_{\text{sat}}$, and $\langle Q_{zz}\rangle$ using the least-square method.

Data availability

The data supporting the findings of this study are available from the corresponding authors on reasonable request.

Acknowledgements.

We would like to thank Kenta Amemiya, Masako Sakamaki, and Reiji Kumai for valuable technical support at KEK-PF. We would also like to thank Hiroki Wadati for providing us with information about the XLD studies of LSMO thin films. This work was supported by a Grant-in-Aid for Scientific Research from the JSPS (22224005, 15H02109, 15K17696, and 16H02115). The experiment was done under the approval of the Photon Factory Program Advisory Committee (proposal No. 2016S2-005, No. 2013S2-004, No. 2016G066, No. 2014G177, No. 2012G667, and 2015S2-005). Part of this work was performed using a SQUID magnetometer at the Cryogenic Research Center, the University of Tokyo. G.S. acknowledges support from Advanced Leading Graduate Course for Photon Science (ALPS) at the University of Tokyo and the JSPS Research Fellowships for Young Scientists (Project No. 26.11615). A.F. is an adjunct member of Center for Spintronics Research Network (CSRN), the University of Tokyo, under Spintronics Research Network of Japan (Spin-RNJ).

Competing financial interests:

The authors declare no competing financial interests.

Author contributions

G.S., K.Y., T.Kadono, K.Ishigami, T.H., Y.T, S.S., Y.N., K.Ikeda, and Z.C. performed XMCD measurements with the assistance of T.Koide and A.F. M.K., M.M., and K.Y. grew and characterized thin films with the assistance of H.K. M.F., M.O., S.Fuchino, A.U., and J.-i.F. developed the vector-type superconducting magnet. J.-i.F., A.U., K.W., H.F., K.Ishigami, T.H., Y.N., T.Kadono, Y.T., S.S., K.Ikeda, Z.C., and G.S. were involved in the design, construction, and improvement of the XMCD measurement chamber, with assistance of S.Fujihira, T.Koide, and A.F. G.S. analyzed the XMCD data and performed the cluster-model calculation. A.T. developed the code for the cluster-model calculation (Xtls version 8.5). G.S. and A.F. wrote the manuscript with suggestions by M.K., M.M., K.Y., T.Koide, and all the other coauthors. A.F. was responsible for overall project direction and planning.