Magnetic anisotropy is a figure of merit in many technical and industrial fields such as the information processing, power generation and distribution, communication devices and information storage devices. From the scientific perspective, unraveling the microscopic origin of magnetic anisotropy has been a significant issue since it is mostly governed by the complex interchange between spin-orbit interaction and the microscopic level electronic states such as the orbital and spin magnetic moments, and anisotropy of spin or charge densities. We studied magnetic anisotropy of Sr2IrO4 and TMPS3 (TM: Fe, Mn, and Ni).
The anisotropy was traced through the anisotropic exchange interaction among the spins in the crystal. The magnetic field dependent torque τ(H) of Sr2IrO4 display a very distinct transition from canted antiferromagnetic (CAF) to the weak ferromagnetic (WFM) state stemming from the Dzyaloshinsky–Moriya interaction(DMI). DMI is the antisymmetric super- exchange interaction which is a contribution to the total magnetic exchange interaction between two neighboring spins. In a magnetic system it favors spin canting and results in weak ferromagnetism in an otherwise antiferromagnetic (AFM) systems. An AFM to WFM transition occurs with the easy axis of FM component along b-axis evident from the angle-dependent torque τ(φ) result. We compared the coefficient of the magnetic susceptibility tensor and captured the anisotropy of the material. A tendency towards isotropic behavior was observed above WFM transition field in the in-plane τ(φ). A quite large and highly anisotropic magnetoresistance (AMR) was obtained by controlling the AFM isospin domains in Sr2IrO4 observed in the magnetic and transport measurements. Applying magnetic field along the in-plane aligns the net moments and removes domain boundaries which results in the exceptionally angle-sensitive AMR. The unusual AMR effect is an important basis for the applied and fundamental research on AFM based spintronics. Meanwhile, the adopted strain technique provides a more systematic way of strain engineering in single crystals by generating a homogenous strain. Tensile and compressive strain alters the CAF order and isospin domain boundaries, which generates a change in AMR. AMR associated with in-plane transport (Rab) is governed by iso-spin mismatch and domain wall scattering whereas AMR in c-axis transport (Rc) arises due to Giant magnetoresistance (GMR)-like effect stemming from layered structure of Sr2IrO4.
In-plane and out-of-plane magnetic anisotropy has been precisely determined for van der Waal (vdW) transition metal phosphorus trisulfides (TMPS3, where TM = Fe, Mn, and Ni) which exhibit AFM ordering: FePS3 shows Ising AFM, MnPS3 behaves like the Heisenberg model, and NiPS3 possesses XY AFM order. The angle-dependent torque measurements in FePS3 show an asymmetry and minute difference in magnetization strength for two opposite magnetic field directions along the same axis owing to different crystallographic and magnetic c-axes. The results indicate a large magnetic anisotropy between the out-of-plane and in-plane, and an imperfect AFM ordering along the out-of-plane c-direction. In MnPS3, magnetic field dependent torque and SQUID magnetometry show a spin-flop transition by applying external magnetic field along trigonal c-axis. Above the spin-flop transition, the out-of-plane spin orientation changes to the in-plane direction. We identified how in-plane and out-of-plane anisotropy changes above and below the spin-flop transition. The magnetization and torque results of NiPS3 shows a very small magnetic anisotropy along the in-plane direction with some projection along the c-axis. The fitting results of the in-plane angle-dependent torque τ(φ) show that sample possess easy-axis type anisotropy along the in-plane and spins are oriented more along b-axis than a-axis.
Thesis Advisor: Prof. Younjung Jo