NSRRC Activity Report 2022

Neutron Science 081 Manipulating the Spin Orientation of VdWMaterials The spin orientation of CrPS 4 , a candidate magnetic 2D van der Waals (vdW) material, can be manipulated through changes in temperature, magnetic field, and hydrostatic pressure. R esearch on atomically thin materials (ATMs) has emerged after the discovery of graphene, the first ATM produced in a laboratory; its discoverers were awarded the 2010 Nobel Prize in Physics. ATMs exhibit unique physical properties, and their atomically thin sheet geometry is favored for use in the semiconductor industry. Thus, experts regard ATMs as promising candidate materials for next-generation semiconductors, spintronic devices, rechargeable batteries, and quantum and optoelectronic devices. Transition metal chalcogenides (TMCs), such as MoS 2 , WSe 2 , MoTe 2, and SnS, are ATMs with well- researched electrical and optical properties. However, due to their nonmagnetic properties, these materials lack spintronic applications. Conversely, many of the ternary transition metal chalcogenides (TTMCs), such as CrSiTe 3 , FePS 3, and CrPS 4 , are magnetic, and they undergo a magnetic phase transition at low temperatures. Therefore, TTMCs are well-suited for spintronic applications. Due to the weak van der Waals (vdW) gaps between sulfur layers in CrPS 4 , the traditional method of mechanical cleavage can be employed to easily isolate thin flakes. Researchers have prepared and studied single-layer and multiple-layer CrPS 4 samples. Wenyun Yang (Peking University, China) and collaborators investigated the physical properties of the TTMC CrPS 4 . Yang and his team manipulated the spin orientation of CrPS 4 with externally controllable parameters, such as temperature, magnetic field, and hydrostatic pressure using neutron powder diffraction (NPD) at ECHIDNA , X-ray powder diffraction, magnetometry, and magnetic torque measurements. 1,2 Without a magnetic field, CrPS 4 exhibits an antiferromagnetic (AFM) structure in which the ferromagnetic layers are coupled antiferromagnetically along the c -axis. The magnetic anisotropy is temperature-dependent; it shifts from in-plane anisotropy (//b) in the vicinity of the T N (Néel temperature) to the ground state and then to out-of-plane anisotropy (in the ac -plane and mainly along the c -axis) upon cooling. The magnetic field can be used to tune the magnetic structure, as illustrated in Fig. 1 . After the magnetic field is applied, the AFM magnetic structure first undergoes a spin-flop transition to a canted AFM (cAFM) state with a net magnetization in the ab -plane. A further increase in the magnetic field causes the cAFM to undergo a spin-flip transition to the ferromagnetic state, where the spin orientation flips again to be along the c -axis. Existing experimental evidence also indicates that the transition of spin reorientation from the ac -plane to the b -axis is first-order in nature. T SR , the characteristic reorientation temperature, can be tuned using hydrostatic pressure, which provides an effective method of controlling the Néel vector in CrPS 4 . As presented in Fig. 2 (see next page), high hydrostatic pressure favors the in-plane anisotropy, and increasing pressure reduces T SR with an increasing temperature range of phase coexistence. The authors claim that the tunable magnetic anisotropy between the easy axis and easy plane provides an ideal platform for producing and investigating 2D Ising-type magnetism and the Berezinskii–Kosterlitz–Thouless regime of 2D magnetism. Specifically, CrPS 4 opens a pathway to the evolution of the universality classes of magnetism through the use of ATMs. Yang and his collaborators experimentally demonstrated the control of spin orientation and metamagnetic transition Fig. 1 : The two successive field-induced transitions of CrPS 4 . The antiferromagnetic magnetic structure first undergoes a spin-flop transition to a canted AFM state and then a spin-flip transition to the ferromagnetic state. [Reproduced from Ref. 1]