Switching snapping by controlling composition


An international team of researchers has successfully shown that controlling the composition of ferrimagnetic materials offers new ways to change their magnetism, a discovery that could dramatically improve the way computers process information.

A computer hard drive stores information in much the same way that a piece of iron surrounded by a coil picks up 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 opposite magnetic moments that are unequal. This occurs when the populations contain different materials or ions, and means that the spontaneous magnetism remains.

A team of researchers – led by physicists from Osaka University in Japan – have offered new insights 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 places in the material. Some of the spins may cancel out, but some residual magnetism remains. The firing of an ultra-fast laser pulse on the material could have one of two effects: either the direction of the spin reverses completely, which reverses the magnetism, or the spin is disturbed, causing an oscillation known as the name spin precession.

We know that laser pulses can reverse the magnetization in some ferrimagnetic alloys, but the light also affects other material properties. To learn more about the interactions of 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 ferrimagnetic ferrimagnetic thin films of gadolinium-iron-cobalt (Gd-Fe-Co) with different compositions and multilayer arrangements to femtosecond laser pulses before systematically studying their magnetization responses. They were able to show that a slight variation in the composition of an alloy considerably modified its response to the laser pulse. If there was a little more gadolinium in the films, the magnetic field reversed, while a little less gadolinium led to spin precession at room temperature.

The spin dynamics are known to differ depending on the angular momentum compensation temperature, or JA, movies. At the angular momentum compensation point, the net angular momentum vanishes: the compensation point is a crucial point for achieving high-speed magnetization reversal in magnetic memory devices.

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

“In the God26Fe66Co8 film, which has an angular momentum compensation temperature (T A) well above room temperature (T exp), a monotonic magnetization inversion occurred, while the Gd22Fe70Co8 movie (where T A is well below T exp) exhibited remarkable wave spin modulation with spatial inhomogeneity during laser-induced non-equilibrium state relaxation,“, explain the researchers in their article.

Time-dependent magnetic images of samples (a) Gd26% and (b) Gd22%, respectively. In the Gd26% sample, a sharp spin inversion is observed. However, in the Gd22% sample, the modulation of wave magnetization propagated isotropically along the radial direction. Image credit: Osaka University, Japan.

Their findings may enable large-scale tuning of the 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 after the laser pulse and showed that the precision angle – the angle of spin oscillation – was the largest recorded to date.

These are complex systems with many different interaction properties, but we have extracted clear relationships between the composition of a ferrimagnetic alloy and its magnetic interactions with light. Understanding these behaviors is important from a fundamental physical perspective and essential for the application of these material systems in advanced electronic devices.

Akira Sekiyama, co-author

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