Effect of magnetic field on Néel vector in two-dimensional van der Waals NiPS₃

Hung-Yan Chang¹, Li-Zai Tsai¹, Meng-Tien Chen¹, Ravish Kumar Jain^1*, Ngo Trong Hai¹, Raman Sankar¹, Jun-Zhi Liang², Shang-Fan Lee¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan
²Department of Physics, Fu Jen Catholic University, Taipei, Taiwan

* Presenter:Ravish Kumar Jain, email:ravish@gate.sinica.edu.tw

For a long time, antiferromagnetic (AFM) materials have been commonly considered insensitive to magnetic perturbations and just played a role as supporting material in Magnetic Tunnel Junction (MTJ) cells. However, recently AFM materials have attracted a lot of attention when a number of works indicates their potential advantageous properties such as ultrafast spin dynamics, zero stray field, magnetic moment compensation etc. Besides, a recent work on the AFM/Heavy metal bilayers, demonstrated that the effect of spin current generated by heavy metal can modify the orientation of the AFM Néel vector [1]. Therefore, characterizing the spin-orbit torque (SOT) associated with AFM/heavy metal is an emerging technology that enables new applications. In this work, we will investigate the SOT effect in the NiPS₃/Pt heterostructure. NiPS₃ was grown by employing the Chemical vapor transport (CVT) technique, exfoliated to nanoflake, and transferred to a silicon oxide substrate. The Pt Hall bar then was fabricated by using standard photolithography and sputtering deposition. NiPS₃ is a variation of transition metal dichalcogenides, a two-dimensional van der Waals material. The Ni ions form a honeycomb lattice with the a-axis along the zigzag direction. The spins of Ni ions lie collinearly in the a-axis forming ferromagnetic spin chains with antiferromagnetic inter-chain arrangement. Since antiferromagnetic moments in the NiPS₃ are considered insensitive to magnetic perturbations and would be invisible to common magnetotransport measurement techniques, we are investigating the samples under high applied field (from 5-9 T) and by employing the lock-in amplifier first and second harmonic technique. We found that under high applied in-plane field the Néel vector can be rotated by the field rather than only in the a-axis, as reported in the literature and the spin torque can be quantitatively determined. Further, photoluminescence (PL) emission from NiPS₃ has been reported to be spin-correlated and facilitated optical determination of the Neel vector [2] or read out local spin-chain directions [3]. In our PL experiments, polarization direction of emission from NiPS₃ is studied under zero field cooled (ZFC) and field cool (FC) conditions. Our findings will help in better understanding of the new behaviors of antiferromagnetic materials, making them more attractive for spintronic devices.

Keywords: Néel vector, Spin-orbit torque, spin chain, NiPS₃, Photoluminescence