Operando XMCD and EXAFS Spectroscopies for Orbital Control by Reversible Strain at Co₂FeSi/PMN-PT Interface

Artificial ferromagnetic/ferroelectric multiferroic heterostructures have been widely investigated from both fundamental and technological aspects. The electric field (<b><i>E</i></b>) control of magnetism through the interfacial strain modulates the magnetic anisotropy in the magnetic layer. Controlling the magnetic anisotropy by the interfacial strain related to the orbital magnetic moments has a potential for future devices using both spins and orbitals. The relationship between strain and orbital magnetic moments has not been clarified because there were few tools to probe the changes of orbital magnetic moments. We have developed the <b><i>E</i></b>-induced x-ray magnetic circular dichroism (EXMCD) technique in order to apply <b><i>E</i></b> to ferroelectric substrate Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT), which tunes the interfacial lattice constants of the Heusler alloy Co₂FeSi magnetic layer [1,2]. In this study, we discuss the microscopic origin of inverse magneto-striction effects or orbital-elastic effects concerning the orbital magnetic moments by using the EXMCD with the detection of element-specific local structural changes by extended x-ray absorption fine structure (EXAFS) analysis.<br/><br/>We prepared the samples of 10-nm-thick Co₂FeSi layer grown on single-crystal PMN-PT(011) substrates with thin Fe buffer layer insertion by molecular beam epitaxy at 300 °C. The <b><i>E</i></b>-induced modulation of the in-plane magnetic properties was characterized by magneto-optical Kerr effect (MOKE) and XMCD, where the XMCD measurement was performed at BL-7A in the Photon Factory (KEK). The partial fluorescence yield mode was adopted to probe the signals more than 10 nm below the sample surfaces. To apply an <b><i>E</i></b> to the PMN-PT substrate along the [011] direction, a Au(100 nm)/Ti(3 nm) electrode was deposited on the backside of the PMN-PT substrate, where the Co₂FeSi film was utilized as a top electrode [3]. EXAFS during applying <b><i>E</i></b> for Fe and Co K-edges were also performed at BL-12C in KEK for the same samples.<br/><br/>The MOKE measurements revealed that the magnetic easy axis is along the PMN-PT [100] in-plane direction at <b><i>E</i></b> = 0 when <b><i>E</i></b> is changed from +8 kV/cm to 0. By applying <b><i>E</i></b> of -8 kV/cm, the easy axis changes 90 deg. within the in-plane through the changes of lattice strain of 0.1 % order. The Fe and Co <i>L</i>₃-edge XMCD hysteresis curves also trace the same changes of magnetic anisotropy of 2.5×10⁴ J/m³, which corresponds to the changes of orbital magnetic moments (<i>m</i>_orb) of the order of 0.01 μ_B. The EXMCD spectra can detect the changes of <i>m</i>_orb by <b><i>E</i></b> for both Fe and Co <i>L</i>-edge XMCD, which suggests the relationship between the strain and the orbital moments. <i>Operando</i> EXAFS measurements also detect the change of nearest neighbor distance at Co site. These phenomena can be understood within the orbital moment anisotropy through the spin-orbit interaction which is controlled by applying <b><i>E</i></b>. We found that EXMCD clarifies the origin of the reversible changes of magnetic anisotropy and links the relationship between macroscopic inverse magneto-striction effects and microscopic orbital moment anisotropy.<br/><br/>This work was partly supported by JST CREST (JPMJCR18J) and JSPS KAKENHI (JP21K14196, JP19H05616).