Magnetic Properties and Growth‐Induced Anisotropy in Yttrium Thulium Iron Garnet Thin Films

Abstract

DOI: 10.1002/aelm.202100452 (PLD) and sputtering, thin films of rare earth (RE) iron garnet materials (REIG, formula unit RE3Fe5O12) have been developed with desirable properties for spintronic applications, including thulium (TmIG), terbium, europium, samarium and other REIG films with perpendicular magnetic anisotropy (PMA). Spinorbit torque switching,[1,2] chiral spin textures,[3–5] and a relativistic domain wall velocity approaching the magnon group velocity have been reported in Pt/TmIG and Pt/Bi:YIG heterostructures, making these materials promising candidates for memory or logic devices.[4–7] REIG materials are well-characterized, with studies having been performed from the 1950s to the present day. REIG materials have three magnetic sublattices, comprised of 3 RE3+ ions per formula unit (FU) on dodecahedral (c) sites, 2 Fe3+/FU ions on octahedral (a) sites, and 3 Fe3+/FU ions on tetrahedral (d) sites.[8] The strongest superexchange coupling is an antiferromagnetic interaction between the tetrahedral and octahedral iron. The dodecahedral moments are also coupled antiparallel to the tetrahedral iron, and hence the three sublattices form a collinear ferrimagnet, with the Fe3+ d ions opposing the combined moment of the RE3+ c and the Fe3+ a ions,[8] though noncollinear ordering can occur at low temperatures by canting of the RE moments.[9] Substitution of other cations has been explored extensively, with many cations exhibiting a site preference, for instance, Sc3+ on the a sites, Si on the d sites, and almost any RE element as well as yttrium (Y) on the c sites.[10] Tuning the magnetic properties of REIG thin films, including the magnetization, anisotropy, coercivity, magnetostriction, compensation temperature and damping, has been investigated through control of the RE ion, the RE:Fe ratio, oxygen content, and substrate strain.[11–13] In PLD or sputtered garnet films, PMA can be introduced due to magnetoelastic anisotropy,[11,13–15] and has been controlled through strain engineering by varying the substrate epitaxial mismatch strain[11,16] or the thermal mismatch strain[17,18] and by altering the magnetostriction coefficients by fully substituting the RE species on the c-site.[11,19,20] PMA of magnetoelastic origin has also been achieved in strained YIG[21] and in Bi-substituted YIG,[22] which exhibit lower damping than REIGs, and in garnets with multiple substitutions such as (Dy,Ce)3(Fe,Al)5O12. Rare-earth iron garnets (REIG) have recently become the materials platform of choice for spintronic studies on ferrimagnetic insulators. However, thus far the materials studied have mainly been REIG with a single rare earth species such as thulium, yttrium, or terbium iron garnets. In this study, magnetometry, ferromagnetic resonance, and magneto-optical Kerr effect imaging is used to explore the continuous variation of magnetic properties as a function of composition for YxTm3−x iron garnet (YxTm3−xIG) thin films grown by pulsed laser deposition on gadolinium gallium garnet substrates. It is reported that the tunability of the magnetic anisotropy energy, with full control achieved over the type of anisotropy (from perpendicular, to isotropic, to an in-plane easy axis) on the same substrate. In addition, a nonmonotonic compositiondependent anisotropy term is reported, which is ascribed to growth-induced anisotropy similar to what is reported in garnet thin films grown by liquidphase epitaxy. Ferromagnetic resonance shows linear variation of the damping and the g-factor across the composition range, consistent with prior theoretical work. Domain imaging reveals differences in reversal modes, remanant states, and domain sizes in YxTm3−x iron-garnet thin films as a function of anisotropy.