High frequency magnetization dynamics of ferromagnetic nano-structures

by Zohar, Sioan, Ph.D., COLUMBIA UNIVERSITY, 2010, 165 pages; 3420865

Abstract:

The development of smaller high frequency magnetic devices with new functionalities requires a more thorough understanding of magnetization dynamics. This thesis documents research into ultrafast magnetization dynamics in ferromagnetic nanoscale materials and summarizes the theoretical foundations and measurement techniques.

We present our investigation into the microwave properties of monodisperse, superparamagnetic Fe2O3 nanoparticle arrays using broadband ferromagnetic resonance. We identified a novel field-for resonance relationship in the films. Compared with ferromagnetic films of equal magnetization, resonance frequencies are decreased for in-plane magnetization and increased for out-of-plane magnetization, over the range 0–8 Ghz. The behavior identified is that of a superparamagnetic thin film, where thin-film dipolar fields act on a gradually saturating magnetization described by the Langevin function. Resonance linewidths can be described by the natural dispersion in properties of the system.

The second section addresses magnetization dynamics in metalic heterostructures, where the component ultrathin films have nanometer scale dimensions. We have searched for a signature of nonlocal magnetization dynamics, or magnetization dynamics driven by pure spin currents ("spin pumping"), in magnetically soft, polycrystalline Ni81Fe19/Cu/Co93Zr7 tri-layers using ferromagnetic resonance. An interface-related enhancement of damping is expected for each ferromagnetic layer when incorporated in a tri-layer; the enhancement should be absent where layer resonances overlap. While size effects in Gilbert damping have been identified, we note that expectations specific to spin pumping are not confirmed. We have also observed this effect in Ni81Fe19/Cu/Ni81Fe19/Mn 50Fe50 exchange biased spin valves with clearly defined giant magneto-resistance (GMR).

Finally, we have investigated the dynamic effects in these films using a novel time-resolved x-ray technique. The reciprocal effects of magnetization motion in opposite layers of a NiFe/Cu/CoZr "spin valve" have been isolated using ultrafast time-resolved x-ray magnetic circular dichroism (TR-XMCD), a layer-specific probe of dynamics. We first describe our instrumental advances in TR-XMCD, in which we have applied synchronous detection techniques to speed data acquisition, enabling measurements of weak coupling. In these measurements, we observe the CoZr responds to the NiFe precession with an in phase component, typically attributed to interlayer dipolar coupling, and a π/2 out of phase component which has been attributed to coupling via pure spin currents. We estimate an effective interface mixing conductance of [special characters omitted] of 8.68 ± 1.74 nm−2, very close to what has been observed in epitaxial Fe-based structures. This identification is made subject to the assumption of a phase offset in the Co layer precession, not explained at present.

We close with experiments demonstrating the feasibility of TR-XMCD in sub-micron and patterned structures. Synchronous detection techniques have been applied for the first time to measure domain wall motion in micron scale Ni81Fe19 squares.

AdviserWilliam Bailey
SchoolCOLUMBIA UNIVERSITY
Source TypeDissertation
SubjectsElectromagnetics; Condensed matter physics
Publication Number3420865

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