[Paper Review] Metamagnetic Transitions in Few-Layer CrOCl Controlled by Magnetic Anisotropy Flipping

Minjie Zhang, Qifeng Hu+11 more|arXiv (Cornell University)|Aug 5, 2021

Electronic and Structural Properties of Oxides26 references4 citations

TL;DR

This study demonstrates field-induced metamagnetic transitions in few-layer CrOCl driven by magnetic anisotropy flipping, where an in-plane easy axis reorients to out-of-plane under perpendicular magnetic fields. The transition is signaled by a sharp increase in tunneling current and leads to a unique five-unit-cell-periodic ferrimagnetic superstructure, revealing a novel mechanism for controlling 2D magnetism via anisotropy engineering.

ABSTRACT

The pivotal role of magnetic anisotropy in stabilising two-dimensional (2D) magnetism has been widely accepted, however, direct correlation between magnetic anisotropy and long-range magnetic ordering in the 2D limit is yet to be explored. Here, using angle- and temperature-dependent tunnelling magnetoresistance, we report unprecedented metamagnetic phase transitions in atomically-thin CrOCl, triggered by magnetic easy-axis flipping instead of the conventional spin flop mechanism. Few-layer CrOCl tunnelling devices of various thicknesses consistently show an in-plane antiferromagnetic (AFM) ground state with the easy axis aligned along the Cr-O-Cr direction (b-axis). Strikingly, with the presence of a magnetic field perpendicular to the easy-axis (H||c), magnetization of CrOCl does not follow the prevalent spin rotation and saturation pattern, but rather exhibits an easy-axis flipping from the in-plane to out-of-plane directions. Such magnetic anisotropy controlled metamagnetic phase transitions are manifested by a drastic upturn in tun- nelling current, which shows anomalous shifts towards higher H when temperature increases. By 2D mapping of tunnelling currents as a function of both temperature and H, we determine a unique ferrimagnetic state with a superstructure periodicity of five unit cells after the field-induced metam- agnetic transitions. The feasibility to control 2D magnetism by manipulating magnetic anisotropy may open enormous opportunities in spin-based device applications.

Motivation & Objective

• To investigate the role of magnetic anisotropy in stabilizing long-range magnetic order in two-dimensional CrOCl.
• To explore whether magnetic anisotropy flipping can drive metamagnetic transitions distinct from conventional spin-flop mechanisms.
• To identify and characterize novel magnetic phases, including superstructures, in atomically thin CrOCl under magnetic fields.
• To establish a direct correlation between magnetic anisotropy and emergent magnetic ordering in 2D van der Waals magnets.
• To demonstrate the feasibility of tuning 2D magnetism through external control of magnetic anisotropy for spintronic applications.

Proposed method

• Angle- and temperature-dependent tunneling magnetoresistance (TMR) measurements were performed on few-layer CrOCl heterostructures using suspended graphene/CrOCl/graphene junctions.
• Non-contact atomic force microscopy (NC-AFM) and Raman spectroscopy were used to identify and confirm the thickness of exfoliated CrOCl flakes.
• Vibrating-sample magnetometry (VSM) and magnetic torque measurements were used to characterize bulk magnetic properties and confirm sample quality.
• Density functional theory (DFT) calculations, including spin-orbit coupling and van der Waals corrections (SCAN+rVV10), were used to compute electronic structure and magnetic anisotropy energy (MAE).
• 2D mapping of tunneling current as a function of magnetic field and temperature revealed phase transitions and superstructure formation.
• The use of a cryostat with a sample rotator enabled in-plane and out-of-plane magnetic field orientation control for angle-dependent measurements.

Experimental results

Research questions

• RQ1 Can magnetic anisotropy flipping induce metamagnetic transitions in few-layer CrOCl distinct from conventional spin-flop behavior?
• RQ2 What is the nature of the magnetic phase transition observed under perpendicular magnetic fields in CrOCl?
• RQ3 How does the magnetic anisotropy evolve with thickness and external field, and what role does it play in stabilizing long-range order?
• RQ4 What is the superstructure periodicity and magnetic ordering of the field-induced phase in CrOCl?
• RQ5 Can magnetic anisotropy engineering be used as a viable mechanism to control 2D magnetism in van der Waals materials?

Key findings

• Few-layer CrOCl exhibits an in-plane antiferromagnetic ground state with the easy axis aligned along the b-axis (Cr-O-Cr direction).
• Application of a magnetic field perpendicular to the easy axis (H∥c) triggers a magnetic anisotropy flipping transition, causing the easy axis to reorient from in-plane to out-of-plane.
• This transition is marked by a drastic upturn in tunneling current, indicating a field-induced metamagnetic transition distinct from spin-flop mechanisms.
• The anomalous shift of the current peak to higher fields with increasing temperature reveals non-monotonic behavior linked to anisotropy control.
• 2D mapping of tunneling current reveals a unique ferrimagnetic state with a five-unit-cell periodic superstructure following the field-induced transition.
• DFT calculations confirm the presence of competing in-plane and out-of-plane magnetic anisotropy, with spin-orbit coupling playing a key role in stabilizing the out-of-plane orientation under field.