[Paper Review] Magnetoresistance oscillations in vertical junctions of 2D antiferromagnetic semiconductor CrPS$_4$

Pengyuan Shi, Xiaoyu Wang+14 more|arXiv (Cornell University)|Oct 23, 2024

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study reports magnetoresistance (MR) oscillations in vertical junctions of 2D antiferromagnetic semiconductor CrPS₄, a system previously thought incapable of such quantum oscillations due to its insulating nature. The oscillations arise from spin-canted states and in-gap electronic states, with experimental evidence pointing to spin-selective interlayer hopping of localized states as the underlying mechanism, demonstrating that van der Waals magnets can host unconventional quantum transport phenomena.

ABSTRACT

Magnetoresistance (MR) oscillations serve as a hallmark of intrinsic quantum behavior, traditionally observed only in conducting systems. Here we report the discovery of MR oscillations in an insulating system, the vertical junctions of CrPS$_4$ which is a two dimensional (2D) A-type antiferromagnetic semiconductor. Systematic investigations of MR peaks under varying conditions, including electrode materials, magnetic field direction, temperature, voltage bias and layer number, elucidate a correlation between MR oscillations and spin-canted states in CrPS$_4$. Experimental data and analysis point out the important role of the in-gap electronic states in generating MR oscillations, and we proposed that spin selected interlayer hopping of localized defect states may be responsible for it. Our findings not only illuminate the unusual electronic transport in CrPS$_4$ but also underscore the potential of van der Waals magnets for exploring interesting phenomena.

Motivation & Objective

• To investigate the origin of magnetoresistance (MR) oscillations in insulating 2D antiferromagnetic semiconductors, specifically CrPS₄, where such oscillations are not expected in conventional insulators.
• To determine whether MR oscillations arise from intrinsic properties of CrPS₄ or from extrinsic effects such as proximity-induced states in graphene electrodes.
• To explore the role of in-gap electronic states and spin-canting in enabling non-monotonic MR behavior in a van der Waals heterostructure.
• To establish the robustness of MR oscillations under varying external parameters, including magnetic field orientation, temperature, voltage bias, and layer thickness.
• To clarify the transport mechanism behind the oscillations, particularly the contribution of spin-selective interlayer hopping in localized states.

Proposed method

• Fabrication of vertical junctions using CrPS₄ flakes encapsulated between graphene or NbSe₂ electrodes, with hBN layers inserted to isolate the CrPS₄ from direct contact with electrodes.
• Systematic measurement of current-voltage (IV) characteristics and magnetoresistance (MR) under varying magnetic fields, temperatures, and voltage biases at cryogenic temperatures (2 K).
• Use of multiple device configurations, including Graphene/hBN/CrPS₄/hBN/Graphene and NbSe₂/CrPS₄/NbSe₂, to rule out electrode-induced effects.
• Control experiments with inserted hBN layers to suppress proximity effects from CrPS₄ into graphene, confirming that MR oscillations originate from CrPS₄ itself.
• Magnetic field dependence measurements with field applied parallel and perpendicular to the c-axis to probe anisotropy and spin-canting effects.
• Analysis of temperature-dependent MR to correlate oscillations with the Néel transition temperature (~38 K) of CrPS₄.

Experimental results

Research questions

• RQ1 Can magnetoresistance oscillations occur in an insulating 2D antiferromagnetic semiconductor like CrPS₄, despite the absence of free carriers?
• RQ2 What is the origin of the observed MR oscillations—local electronic states, spin-canting, or proximity effects in electrodes?
• RQ3 How do the oscillations depend on magnetic field orientation, temperature, and layer thickness?
• RQ4 Is the transport mechanism in CrPS₄ consistent with spin-selective interlayer hopping of localized states?
• RQ5 What role do in-gap electronic states play in enabling non-monotonic MR in this insulating system?

Key findings

• MR oscillations are robustly observed in vertical junctions of few-layer CrPS₄, even at 2 K and with magnetic fields applied both parallel and perpendicular to the c-axis, indicating strong anisotropy resilience.
• The oscillations vanish above the Néel temperature (~38 K), confirming their correlation with long-range antiferromagnetic order and spin-canting.
• No MR oscillations are observed in monolayer CrPS₄, which is ferromagnetic, indicating that the oscillations are specific to the A-type antiferromagnetic phase.
• Inserting a thin hBN layer between graphene and CrPS₄ preserves MR oscillations, ruling out proximity-induced states in graphene as the origin.
• MR oscillations are consistently observed in NbSe₂/CrPS₄/NbSe₂ devices, confirming the effect is intrinsic to CrPS₄ and not dependent on electrode material.
• The data support a mechanism involving spin-selected interlayer hopping of localized in-gap states, with the oscillations arising from quantum interference in the spin-canted magnetic state.