Researchers from High Magnetic Field Laboratory of the Chinese Academy of Sciences found field-dependent anisotropic magnetic coupling and two-dimensional Ising-like magnetic coupling in van der Waals bonded ferromagnets Fe3-xGeTe2(FGT) and Cr2Ge2Te6(CGT), respectively.

The work was done by ZHANG Lei and LIU Wei and was published in the Physical Review B of the American Physical Society.

In recent years, two-dimensional (2D) van der Waals (vdW) bonded systems have gained a great deal of attention for their highly tunable physical properties and immense potential applications in spintronics. The exploration of intrinsic ferromagnetism in 2D systems is extremely significant not only for uncovering the fundamental low-dimensional magnetism but also for developing the next-generation nanoscale spintronic devices.

In particular, 2D vdW bonded ferromagnets, such as Cr2Ge2Te6, CrI3, and Fe3GeTe2, have been investigated extensively owing to their excellent properties in magnetic, magnetoelectric, and magneto-optic areas.

The researchers performed systematic investigations on the critical behavior and magnetic coupling of 2D ferromagnetic materials FGT and CGT by many methods.

Fe3-xGeTe2 is an itinerant ferromagnet, by controlling the concentration of Fe vacancies, the Curie temperature TC ranged from 140K to 230K.

In addition, TC also depended on the number of layers, which could be raised up to room temperature by the ionic gate method.

Except for its high Curie temperature, layered FGT possessed a large magnetic anisotropy energy, which was required for practical application in storage.

The researchers studied the anisotropic behaviors by measuring the magnetization along easy axis and hard axis, calculating the anisotropic magnetic entropy change, and analyzing the critical behavior.

"The magnetic coupling along c axis belonged to three-dimensional(3D) Heisenberg model with a short-range magnetic coupling. However, that for when the magnetic field paralleled to ab plane followed mean-field model with a long-range magnetic coupling," LIU said.

"These results suggest the anisotropic magnetic coupling of Fe3-xGeTe2 is strongly dependent on orientations of the applied magnetic field. In fact, similar behaviors was also observed in CGT, which indicates that this field-dependent magnetic coupling may commonly exist in these systems", he said.

In addition, the researchers also investigated the critical behavior of CGT by magnetic entropy change. "2D-Ising-like magnetic coupling with a long-range ferromagnetic ordering existed in CGT. And the weak magnetic anisotropy was crucial for the stabilization of the long-range ferromagnetic ordering. These results are well consistent with the prediction renormalized spin-wave theory."

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, and the Users with Excellence Program of Hefei Science Center CAS, and et al.