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Register a new userThickness-dependent magnetic properties and strain-induced orbital magnetic moment in SrRuO3 thin films
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Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be ~ 0.1{mu}B/Ru atom, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. Mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.
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Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 Bohr-magnetron/Ru ion observed in bulk compound. Contrary to the expectation the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment. Strain disorder is shown to be an efficient mean to control the spin-orbit coupling.
Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both the tensile and compressive strain in the xy-plane could induce the abrupt change in the magnetic moment of Ru atom. In particular, under the in-plane ~4% compressive strain, a ferromagnetic to nonmagnetic transition is induced. Whereas for the tensile strain larger than 3%, the Ru magnetic moment drops gradually with the increase of the strain, exhibiting a weak ferromagnetic state. We find that such magnetic transitions could be qualitatively explained by the Stoner model. In addition, frozen phonon calculations at {Gamma} point reveal structural instabilities could occur under both compressive and tensile strains. Such instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains to be metallic in the range we studied. These might have influence on the physical properties of oxide supercells taking SRO as constituent.
We have performed x-ray linear and circular magnetic dichroism experiments at the Mn L2,3-edge of the La0.7Sr0.3MnO3 ultra thin films. Our measurements show that the antiferromagnetic (AF) insulating phase is stabilized by the interfacial rearrangement of the Mn 3d orbitals, despite the relevant magnetostriction anisotropic effect on the double-exchange ferromagnetic (FM) metallic phase. As a consequence, the Mn atomic magnetic moment orientation and how it reacts to strain differ in the FM and AF phases. In some cases a FM insulating (FMI) phase adds to the AF and FM. Its peculiar magnetic properties include in-plane magnetic anisotropy and partial release of the orbital moment quenching. Nevertheless the FMI phase appears little coupled to the other ones.
A quantitative mathematical model for the critical thickness of strained epitaxial metal films is presented, at which the magnetic moment experiences a reorientation from in-plane to perpendicular magnetic anisotropy. The model is based on the minimum of the magnetic anisotropy energy with respect to the orientation of the magnetic moment of the film. Magnetic anisotropy energies are taken as the sum of shape anisotropy, magnetocrystalline anisotropy and magnetoelastic anisotropy, the two latter ones being present as constant surface and variable volume contributions. Other than anisotropy materials constants, readily available from literature, only information about the strain in the films for the determination of the magnetoelastic anisotropy energy is required. Application of the epitaxial Bain path allows to express the strain in the film in terms of substrate lattice constant and film lattice parameter, and thus to obtain an approximate closed expression for the reorientation thickness in terms of lattice mismatch. The model can predict the critical spin reorientation transition thickness with surprising accuracy.
We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal $J_mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully strained up to a thickness of 108 nm. By using X-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out-of-plane magnetic correlation length and transform the magnetic anisotropy from two-dimensional (2D) to three-dimensional (3D) behavior by incrementing film thickness. These results provide a clearer picture for the systematic control of the magnetic degrees of freedom in epitaxial thin films of Sr$_2$IrO$_4$ and bring to light the potential for a rich playground to explore the physics of $5d$-transition metal compounds.
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