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2017-9-13

Making a magnet from a piece of iron and a coil or wire, or another magnet, is a simple experiment. An external electric or magnetic field can align groups of atoms in the iron over time so that they take on their own permanent magnetic field. A similar accelerated process stores information on computer hard disks. A special case of magnetism, known as ferrimagnetism, could enable even faster switching of magnetism, leading to massive improvements in the way computers handle information.

Now, an international research group, led by Osaka University physicists, has provided new insight into how the composition of ferrimagnetic materials can affect their interactions with light. They recently reported their findings in Applied Physics Express.

“We know that laser pulses can reverse the magnetization in certain ferrimagnetic alloys, but light also affects other properties of the material,” coauthor Hidenori Fujiwara says. “To learn more about the interactions of the magnetism with light, we studied the spin dynamics of ferrimagnetic thin films containing different proportions of gadolinium.”

Ferrimagnetic materials can be thought of as a mixture of electrons spinning at different sites in the material. Some of the spins might cancel each other out, but a certain residual magnetization will remain. Firing an ultra-fast laser pulse at the material may completely flip the spin direction, reversing the magnetism, or may disrupt the spins, causing a kind of wobbling known as spin precession. The type of behavior shown strongly depends on the material’s temperature and composition.

The researchers used an advanced synchrotron measurement setup developed in their previous studies to show that slightly varying the composition of an alloy dramatically changed its response to the laser pulse. Slightly more gadolinium in the films led to flipping of the magnetic spin; slightly less led to spin precession at room temperature.

The researchers’ setup could also visualize the wave-like nature of the spin precession over a few nanoseconds following the laser pulse. They showed that the angle of precision, or the angle of the spin wobble, was the largest reported to date.

“These are complex systems with many different interacting properties, but we have extracted some clear relationships between the composition of a ferrimagnetic alloy and its magnetic interactions with light,” coauthor Akira Sekiyama says. “Understanding these behaviors is important from a fundamental physics standpoint, and essential for applying these material systems in advanced electronic devices.”

Abstract

Perpendicularly magnetized ferrimagnetic Gd–Fe–Co thin films with different compositions and multilayer arrangements were subjected to femtosecond laser pulses. The pulses triggered different magnetization dynamics in the various thin films. In the Gd26Fe66Co8 film, which has an angular-momentum-compensation temperature (T A) well above ambient temperature (T exp), monotonic magnetization reversal occurred, whereas the Gd22Fe70Co8 film (where T A is well below T exp) exhibited remarkable wavelike spin modulation with spatial inhomogeneity during relaxation of the laser-induced nonequilibrium state. These findings can enable broad-range tuning of the magneto-optical responses of Gd–Fe–Co alloys, facilitating advances in materials engineering.

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Figure 1. Schematic of magnetization reversals induced by laser pulses of Gd–Fe–Co thin films. The spin dynamics is known to differ depending on the angular momentum compensation temperature TA of the films. When the Gd content is 26% (TA>RT), smooth spin reversal with strong damping is expected. When the Gd content is 22% (TA<RT), the sample temperature does not intercept TA and long-lasting spin precession is expected.

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Figure 2. Time-dependent magnetic images of the (a)Gd26% and (b)Gd22% samples, respectively. In the Gd26% sample, clear spin reversal is observed. However, in the Gd22% sample, wave-like magnetization modulation propagated isotropically along the radial direction.

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Figure 3. Spin distribution of Gd–Fe–Co film containing 22% of Gd at 1500 picoseconds after the laser pulse duration. A line profile of spin distribution (main graph), a magnetic image (inset image), and calculated distribution of spin directions (lower drawings) are shown. One can see propagating spin waves whose precession angle ranging 15–20 deg.

To learn more about this research, please view the full research report entitled "Optical control of magnetization dynamics in Gd–Fe–Co films with differentcompositions" at this page of the Applied Physics Express website.


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