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Thin film magnetization reversal dynamics

Thin film magnetization reversal dynamics

In studies of magnetization reversal dynamics we seek to understand the speed and character of the switching of a magnetic structure from one orientation of the magnetization to another. There are various routes to reversal, depending on structure size, ranging from fast, deterministic, quasi-coherent rotation in submicron-size magnetic elements to slower, stochastic domain wall nucleation and propagation in larger thin films. Present studies span a huge range of timescales, from magnetic relaxation measurements in static fields to experiments that probe spin dynamics with sub-nanosecond resolution. As well as being of fundamental interest, these studies are important for the understanding and control of switching in magnetic materials that is desired for data storage technology and other devices.

The goal of this project is to grow thin magnetic films in ultrahigh vacuum (UHV) by molecular beam epitaxy, and then to characterize their magnetic and structural properties. Currently, ferromagnetic metal films such as Fe and its alloys grown on GaAs are a popular choice due to their potential for use in magneto-electronic applications. The magneto-optic Kerr effect may be used to probe the magnetic properties of the films both in- and ex situ. It can measure magnetization reversal on a wide range of timescales, over several tens of seconds for a magnetic relaxation measurement, to only a few microseconds for a rapidly driven switching event. For the in situ measurement, the sample temperature may be stabilized at any value in the range 77 ? 300 K. Ex situ, a scanning Kerr microscope (SKM) is used to obtain both static domain images with a resolution of ~1 micrometre, and time resolved (~100 ns) measurements of magnetization reversal processes. For characterization of surfaces a scanning tunnelling microscope is being developed for use in UHV.


Fig. 1. Schematic representation of the Cambridge ac Kerr magnetometer. The longitudinal magneto-optic Kerr effect is used. A laminated-core electromagnet supplies sinusoidally varying fields with frequency extending to a few kHz. The thin film sample may be cooled to 77 K, in which case it is necessary to place the sample and electromagnet in a vacuum chamber to avoid condensation on the film surface. The applied field H(t) and Kerr intensity (magnetization M(t)) are recorded as a function of time and used to generate dynamic hysteresis loops.

We have used dynamic hysteresis measurements to understand magnetization reversal dynamics in the frequency range 10 Hz ? 1 MHz in thin Fe, NiFe and CoFe films on GaAs(001). Fig. 1 shows a schematic of our set-up for measuring dynamic hysteresis loops. At the fastest applied field frequencies (kHz), the switching process takes only a few microseconds. We found for high anisotropy materials such as Fe and CoFe that there are unexpectedly strong variations in coercive field with applied field frequency, in contrast to the behaviour of low anisotropy NiFe. Two dynamic regimes arise, one at low field frequency where domain wall motion is the dominant reversal mechanism, the other at high field frequency where domain nucleation dominates. The crossover between the two regimes is sensitively dependent on material composition (which defines the anisotropy strength), temperature and film structure (e.g. density of defects). Fig. 2 shows the switching field as a function of experimental timescale (which is inversely proportional to the applied field sweep rate and thus the frequency) for a variety of ferromagnetic films.

Fig. 2. Collected measurements of switching fields HS as a function of experimental timescale texp ~ Hc/(dH/dt) for NiFe, Fe and Co thin films. The measurements were made using either a pulsed field or a swept field to precipitate the magnetization reversal. The solid curve corresponds to the field Hprec necessary to switch a magnetic moment by precession on the timescale of texp. Gamma is the gyromagnetic ratio.

Using time-resolved Kerr microscopy we have captured single shot magnetization traces in thin magnetic films as a function of time, with magnetization switching times down to microseconds. This has allowed the study of domain wall velocity distributions as a function of applied field sweep rate and therefore a deeper understanding of the stochastic nature of thin film magnetization reversal. Fig. 3 shows a schematic of a domain wall passing through the laser spot in our Kerr microscope set-up.

magreversal3Fig. 3. Schematic of a domain wall passing through the laser spot in our Kerr microscope set-up.

Selected references:

T.A. Moore et al., J. Phys.: Condens. Matter 15, L407 (2003)
T.A. Moore et al., J. Phys.: Condens. Matter 16, L375 (2004)
T.A. Moore et al., J. Phys.: Condens. Matter 16, R1369 (2004)
T.A. Moore et al., Physica B 343, 337 (2004)
T.A. Moore et al., J. Appl. Phys. 96, 6546 (2004)

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