Tailoring of magnetic anisotropy and interfacial spin dynamics
Keywords: Magnetism
Static and Dynamic Exchange Coupling
X-ray Magnetic Circular Dichroism
Magnetic Anisotropies
Topological Insulators
X-ray Detected Ferromagnetic Resonance
Spin Pumping
Synchrotron

Spin transfer in magnetic multilayers offers the possibility of a new generation of ultra-fast, low-power spintronic devices. New ways to control the resonance frequency and damping in ultrathin films are actively sought, fuelling study of the precessional dynamics and interaction mechanisms in such samples. One effect that has come under particular scrutiny in recent years is the spin-transfer torque, wherein a flow of spins entering a ferromagnet exerts a torque on the magnetisation, inducing precession. A flow of spin angular momentum is usually generated through a spin-polarised electrical current, but a promising alternative is the pure spin current emitted by a ferromagnet undergoing ferromagnetic resonance (FMR). This allows spins to be transferred without a net charge flow. The physics of the generation, transmission and absorption of pure spin currents is a developing field, and holds great promise for both industrial applications and as a means to study fundamental physical phenomena in exotic materials.
This thesis presents an investigation into the magnetodynamics of ferromagnetic thin films and heterostructures grown by molecular beam epitaxy and studied using vector-network analyser ferromagnetic resonance (VNA-FMR), x-ray magnetic circular dichroism, vibrating sample magnetometry and x-ray detected ferromagnetic resonance (XFMR). Particular attention is paid to the anisotropy of damping processes that occur in thin films, and the different coupling mechanisms that can exist across non-magnetic spacer layers in spin valves and magnetic tunnel junctions.
It is first shown that the static and dynamic magnetic properties of thin Fe films can be effectively tailored by dilute doping with Dy impurities, which introduces a sizeable anisotropy of Gilbert damping. The mechanism underlying this effect is discussed, as is the concurrent modification of the spin and orbital contributions to the magnetic moment.
The focus then turns to magnetodynamics of ferromagnetic films coupled across a nonmagnetic spacer layer, examining how different materials permit different interactions. First, an insulating MgO layer is used to separate the FM layers; it is found that this attenuates a spin current in under 1~nm, but permits a static interaction for at least 2 nm. XFMR measurements are used to ascertain the different contributions of the two interactions, and shed light on their interplay. Next, the same techniques are applied to spin valves with a spacer layer of the topological insulator (TI) Bi2Se3. TIs are the subject of much attention in the physics community, as they hold the potential for dissipationless transport, extremely high spin-orbit torques, and a host of novel physical effects. Here, their ability to absorb and transmit a pure spin current is studied, testing their suitability for incorporation into existing device schemata. VNA-FMR measurements confirm that the TI functions as an efficient angular momentum sink. XFMR measurements, however, demonstrate the presence of a weak interaction between the two ferromagnets, able to persist up to at least 8~nm, and possibly mediated by the topological surface state.
Finally, the angle-dependence of spin pumping through a Cr barrier is examined, finding that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter in the spin sink layer. VNA-FMR measurements show that anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. XFMR results confirm induced precession in the spin sink layer, with isotropic static exchange and an anisotropic dynamic exchange.
Taken together, these studies provide an insight not only into the magnetisation dynamics of thin films (and ways to modify them) but a demonstration of the power of ferromagnetic resonance techniques, and their applicability across materials and concepts. The results offer valuable information on the transmission and absorption of spin currents by different materials, and several mechanisms by which enhanced spin torques and angular control of damping may be realized for next-generation spintronic devices.