Spintronics in low dimensional magnets

Shang-Fan Lee^1*

¹Institute of Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Shang-Fan Lee, email:leesf@gate.sinica.edu.tw

The development of spintronics is briefly reviewed, with key concepts applied to the study of low-dimensional van der Waals magnets. The giant magnetoresistance (GMR) effect was discovered in metallic magnetic multilayers, and two distinct measurement geometries are employed: current-in-plane (CIP) and current-perpendicular-to-plane (CPP). In the CIP geometry, the electron mean free path dominates, while in the CPP configuration, spin diffusion length is the key factor. The semi-classical Boltzmann diffusion equation, combined with a two-current model, is used to describe the underlying physics. Tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) shares the same geometry as CPP GMR and yields similar experimental results, though with a different conduction mechanism. Low-resistance GMR devices are used as sensors, while high-resistance devices serve as memory cells in the information industry.
When the current density is high enough that the spin angular momentum carried by electrons significantly affects magnetization stability, spin-transfer torque (STT) must be considered. In this case, current-related contributions are added to the Landau-Lifshitz-Gilbert (LLG) equation, which describes the dynamics of magnetic moments. The role of minority electrons becomes crucial when STT is employed as a zero-field writing scheme. A similar concept, spin-orbit torque (SOT), arises from pure spin currents generated by effects such as the spin Hall, spin pumping, or spin Seebeck effects. Since STT in MTJs generates substantial Joule heating, more efficient SOT mechanisms capable of deterministic switching in perpendicularly magnetized layers are under development.
Antiferromagnetic (AFM) materials have traditionally been regarded as insensitive to magnetic perturbations, serving mainly as supporting materials in spin valve MTJ cells. However, recent studies have highlighted the potential advantages of AFM materials, including ultrafast spin dynamics and zero stray fields. Current research is investigating whether SOT generated by heavy metals can modify the orientation of the AFM Néel vector. We follow a theoretical approach based on the Néel vector dynamic equation of motion derived from the LLG equation, specifically focusing on the case of uniaxial anisotropy.
Experiments were conducted using NiPS3/Pt heterostructures. NiPS3, a variant of transition metal dichalcogenides, is a two-dimensional van der Waals material in which Ni ions form a honeycomb lattice, with the a-axis aligned along the zigzag direction. The spins of the Ni ions are collinear and lie close to the a-axis, forming ferromagnetic spin chains with antiferromagnetic inter-chain alignment. To investigate these samples, we employed second harmonic Hall voltage measurements using a lock-in amplifier. Photoluminescence (PL) emission from NiPS3 has been reported to be spin-correlated, enabling optical determination of the Néel vector. The mechanism behind the ultra-sharp polarized PL signals in NiPS3, which do not appear in related materials, remains to be clarified.

Keywords: spintronics, spin-orbit torque, magnetic vdW materials