Nonlocal Magnetization Dynamics in Ferromagnetic Heterostructures

Abstract

Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents. On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. This review presents recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density-functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful for understanding the physics and for engineering the characteristics of small devices such as magnetic random-access memory elements. CONTENTS

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…We note that our data demonstrate an ultrafast transfer of angular momentum of longitudinal spin through a spin current on subpicosecond timescales. In contrast, previous works demonstrated spin-transfer torque, that is, a transfer of transverse angular momentum between noncollinear spins and a magnetic moment at microwave frequencies [30][31][32][33] , leading to important innovations such as spin-torque magnetic random access memory (RAM) for data storage, spin-torque oscillators for frequency-agile telecommunications, and spin-wave interconnects for spin-based logic. Similarly, we anticipate that superdiffusive spin transfer, which has a considerably larger magnetic moment, can find applications in moving domain walls, switching magnetic nano-elements on subpicosecond timescales or in spin-based electronics operating in the Terahertz frequency range.…”

Photodrive of magnetic bubbles via magnetoelastic waves

“…Bismuth-substituted iron garnet films (Gd 1 O 12 , 50 μm in thickness), grown by a liquid-phase epitaxy method on the (110) plane of (CaGd) 3 (MgGaZr) 5 O 12 (SGGG) substrates (350 μm in thickness), were used in the experiment. Although as-grown films have out-ofplane easy axis due to the growth-induced anisotropy, we prepared annealed films (6 h at 1,200°C in air), where the anisotropy is reduced and easily movable magnetic bubbles can be formed.…”

“…Therefore, the practical manipulation of the magnetic domain wall (DW) is now being realized by spin transfer torque generated from spin-polarized charge current in metals (3) and from flow of magnons in insulators (4), e.g., via the spin Seebeck effect (5,6). On the other hand, the optical control, aiming at ultrafast, nonthermal, and remote access to magnetic domains, remains elusive even after the discoveries of photomagnetic domain manipulation (7,8), laser-induced magnetization reversal (9), and directional generation of magnetostatic waves (10).…”

Antiferromagnetic spintronics

“…An alternative method, better suited to antiferromagnetic materials, is based on the absorption of a spin current created by spin pumping from a neighboring ferromagnet. This method has attracted considerable attention owing to its versatility (Tserkovnyak et al, 2005;Ando, 2014). The technique is applicable no matter the magnetic order (ferromagnetism, ferrimagnetism, or antiferromagnetism) and the electrical state (metal, insulator, or semiconductor) of the spin sink.…”