Research progress of room temperature magnetic skyrmion and its application

Abstract

<sec>It has been found that many magnetic materials possess the properties arising from skyrmions at room temperature. In addition to the common interaction energy, chiral interaction is also needed to form the skyrmion in magnetic material. There are four chiral magnetic interactions, namely: 1) Dzyaloshinskii-Moriya (DM) interaction; 2) long-ranged magnetic dipolar interaction; 3) four-spin exchange interaction; 4) frustrated exchanged interaction. Through the competition between exchange interaction and chiral interaction, magnetic skyrmion can be realized in magnetic material subject to a certain magnetic field and temperature. The skyrmion generated by the DM interaction features small size (5–100 nm), which is easy to adjust. The skyrmion can be driven by magnetic field or ultralow current density. The magnetic materials with skyrmion can exhibit the properties related to the skyrmion Hall effect, the topological Hall effect and the emergent electrodynamics, which are closely related to the skyrmion number. The existence of skyrmion in the magnetic material can be indirectly measured by topological Hall effect. The movement of skyrmion can be driven by spin polarized current in the direction either parallel or perpendicular to the current direction. The movement of the skyrmion driven by spin polarized currents will continue when the current is present, and will disappear when the current disappears. </sec><sec>In previous studies, magnetic skyrmions were realized in a variety of materials. However magnetic skyrmions were found only in very limited types of single crystal materials at room temperature or near room temperature. In recent years, scientists have discovered a variety of magnetic skyrmion materials at room temperature, including film materials (such as multilayer materials, artificial skyrmion materials) and crystal materialssuch as <i>β</i>-Mn-type Co<sub>10–<i>x</i>/2</sub>Zn<sub>10–<i>x</i>/2</sub>Mn<i><sub>x</sub></i>, Fe<sub>3</sub>Sn<sub>2</sub>. Among all kinds of room temperature magnetic skyrmion materials, the most valuable one is the multilayer film material. The Skyrmion multilayer film has the advantages of small size, adjustable material type, simple preparation, good temperature stability, good device integration,etc. At the same time, skyrmion multilayer film is very easy to optimize by adjusting and constructing a special structure that has the wanted types of materials each with a certain thickness. Artificial skyrmion material obtains artificial skyrmion by constructing a micro-nano structure, therefore the artificial skyrmion with high-temperature stability can be realized by choosing high Curie temperature materials. There are a variety of materials which can realize the skyrmion above room temperature, such as Co<sub>9</sub>Zn<sub>9</sub>Mn<sub>2</sub> (300–390 K) and Fe<sub>3</sub>Sn<sub>2</sub> (100–400 K). These room temperature materials further widen the temperature application range of skyrmion. The room temperature materials can be prepared or characterized by a variety of techniquesincluding sputtering for fabrication and X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) for characterization. </sec><sec>The discovery of the magnetic skyrmion materials at room temperature not only enriches the research content of materials science, but also makes the skyrmion widely applicable in novel electronic devices (such as racetrack memory, microwave detector, oscillators). Because the skyrmion has the advantages of small size, ultra-low driving current density, and topological stability, it is expected to produce racetrack memory based on the skyrmion with low energy consumption, non-volatile and high density. The MTJ microwave detector based on skyrmion can be achieved with no external magnetic field nor bias current but with low power input (< 1.0 μW); the sensitivity of the microwave detector can reach 2000 V·W<sup>–1</sup>. The frequency of the oscillator based on skyrmion can be tuned by magnetic field or current, and moreover, the oscillato is very easy to integrate with IC. In this paper, first, the basic characteristic of magnetic skyrmion is introduced; and then room temperature magnetic skyrmion is reviewed; finally the advances of the racetrack memory, microwave detectors and oscillators are introduced, highlighting the development trend of room temperature magnetic skyrmion. </sec>