Abstract
Ferrimagnetic insulators have gained much attention as material platforms with efficient magnetization dynamics. To date, epitaxial iron garnet thin films are the most widely used materials in the emerging field of “insulator spintronics.” However, further advances in this field require overcoming the disadvantages of garnets—e.g., their complex structure, high growth temperature, incompatibility with other crystalline materials, and relatively weak perpendicular magnetic anisotropy. In this Perspective, we make the case that epitaxial thin films of spinel ferrites and hexagonal ferrites are viable materials for insulator spintronics with complementary advantages over the oft-used garnets. Specifically, spinel ferrites have a simpler structure, can crystallize at lower temperatures, and are more amenable to coherent integration with various materials; hexagonal ferrites possess enormous perpendicular anisotropy of bulk origin, in contrast to garnets where the strength of anisotropy is restricted by interfacial strain. The expanded repertoire of materials for insulator spintronics will enable new physical insights and potential applications, beyond what is currently possible with garnets.
Highlights
A flow of spin angular momentum injected into a magnetic thin film can exert torques on the magnetization.[1,2] Such spintransfer torque effects were initially studied in heterostructures, consisting of at least two layers of metallic ferromagnets.[3]
Rare-earth garnets have been developed that crystallize on Si and quartz without a seed layer, including sputter-deposited terbium iron garnet (TIG) and Bi-doped TIG (Bi:TIG).[116–118]
Those efforts have laid the foundation for fabricating magnetic insulator thin films with physical vapor deposition techniques that could be compatible with the CMOS processes
Summary
A flow of spin angular momentum injected into a magnetic thin film can exert torques on the magnetization.[1,2] Such spintransfer torque effects were initially studied in heterostructures (e.g., spin valves and magnetic tunnel junctions, MTJs), consisting of at least two layers of metallic ferromagnets.[3]. A recently reported spinel ferrite system[44] exhibits sufficiently low damping that enables spin pumping and SOT experiments giving insights into charge-spin interconversion in epitaxial Pt.[45,46]. III, we summarize recent experiments leveraging epitaxial thin films of hexagonal ferrites with PMA that is much stronger than that of typical garnet ferrites These out-of-plane magnetized hexaferrites have a strong anisotropy field of ∼17 kOe and a low intrinsic damping constant of
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