Editorial Feature

Switching Magnetism by Controlling Composition

An international team of Researchers has successfully shown that controlling the composition of ferrimagnetic materials offers novel ways of switching their magnetism, a finding which could massively improve the way computers handle information.

A computer hard drive stores information in a similar way a piece of iron surrounded by a coil takes on its own magnetic field when an external electric or magnetic field is applied to align groups of atoms in the metal – although much faster.

Ferrimagnetism is a special type of magnetism: a ferrimagnetic material has populations of atoms with opposing magnetic moments which are unequal. This happens when the populations contain different materials or ions, and means that spontaneous magnetism remains.

A team of Researchers – led by Physicists at Osaka University in Japan - has offered new insight into how the composition of ferrimagnetic materials can affect their interactions with light in a paper published in Applied Physics Express.

Ferrimagnetic materials are often thought of as a mixture of electrons spinning at different sites in the material. Some of the spins might cancel each other out, but some residual magnetism remains. Firing an ultra-fast laser pulse at the material could have one of two effects: either the spin direction flips completely which reverses the magnetism or the spin is disrupted, causing a wobble known as spin precession.

We know that laser pulses can reverse the magnetization in certain ferrimagnetic alloys, but light also affects other properties of the material. 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.

Hidenori Fujiwara, Co-author

In their study, the Researchers subjected perpendicularly magnetized ferrimagnetic gadolinium-iron-cobalt (Gd-Fe-Co) thin films with different compositions and multilayer arrangements to femtosecond laser pulses before systematically studying their magnetization responses. They were able to show that slightly varying the composition of an alloy dramatically changed its response to the laser pulse. If there was slightly more gadolinium in the films, the magnetic field was shown to flip, while slightly less gadolinium led to spin precession at room temperature.

The spin dynamics is known to differ depending on the angular momentum compensation temperature, or TA, of the films. At the angular momentum compensation point, the net angular momentum disappears: the compensation point is a crucial point for achieving high speed magnetization reversal in magnetic memory devices.

When the Gd content is 26% (TA>Texp), smooth spin reversal with strong damping is expected. When the Gd content is 22% (TA<Texp), the sample temperature does not intercept TA and long-lasting spin precession is expected.

“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 non-equilibrium state,” the Researchers explained in their paper.

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. Image Credit: Osaka University, Japan.

Their findings may enable broad-range tuning of magneto-optical responses of Gd-Fe-Co alloys, and facilitate advances in materials engineering.

The Researchers were also able to visualize the wave-like nature of the spin precession over a few nanoseconds following the laser pulse and showed the angle of precision – the angle of spin wobble – was the largest recorded 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. Understanding these behaviors is important from a fundamental physics standpoint, and essential for applying these material systems in advanced electronic devices.

Akira Sekiyama, Co-author


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