PNR study of the phase transition in FeRh thin films

C Bull,A Caruana,C J Kinane,W Griggs,C W Barton,P W Nutter,T Thomson

doi:10.1063/1.5120622

Abstract

The first order antiferromagnetic to ferromagnetic metamagnetic phase transition of equiatomic FeRh offers new opportunities for novel antiferromagnetic memories and spintronic devices with the caveat that it can be utilized in thin film structures (&lt;50 nm). Here, we report a polarized neutron reflectivity (PNR) study for three representative film thicknesses (5, 20, and 50 nm) aimed at determining the physical and magnetic structure of FeRh at room temperature and partway through the transition. The PNR results are analyzed with reference to X-ray diffraction, X-ray reflectivity, and atomic force microscopy data which together provide a consistent description of the magnetic and physical state of the FeRh thin films. The data demonstrate that the nucleation of the ferromagnetic phase initiates at the MgO substrate, and results from structural and magnetic measurements demonstrate that the magnetic behavior and strain properties of FeRh correlate with the evolving topography of the three films investigated.

Highlights

The data demonstrate that the nucleation of the ferromagnetic phase initiates at the MgO substrate, and results from structural and magnetic measurements demonstrate that the magnetic behavior and strain properties of FeRh correlate with the evolving topography of the three films investigated
We report a polarized neutron reflectivity (PNR) study to determine the magnetic and physical structure of a range of representative film thicknesses likely to be incorporated into a multifunctional thin film stack
In order to fully understand the magnetic structure, it is necessary to consider both the physical structure and the film topography which we determine using a combination of X-ray diffraction (XRD), X-ray reflectivity (XRR), and atomic force microscopy (AFM)

Summary

Introduction

FeRh has attracted increasing research interest over the last few years due to the unique first-order metamagnetic phase transition from an antiferromagnetic (AF) to a ferromagnetic (FM) ordering it exhibits at a technologically useful temperature ∼370 K.1–3 This phase transition makes the material a suitable candidate for several novel applications such as antiferromagnetic spintronics, memory resistors, and magnetic refrigeration. When grown close to equiatomic (48 ≤ x ≤ 56 at. % Fe) composition, Fe1−xRhx in the AF α′′-phase assumes a B2 CsCl-like crystal structure in both the AF and FM phases. The phase transition is correlated with a 1% increase of the unit cell volume, and previous works have demonstrated that the onset and width of the phase transition are highly sensitive to the interatomic distance.8,9In this work, we report a polarized neutron reflectivity (PNR) study to determine the magnetic and physical structure of a range of representative film thicknesses likely to be incorporated into a multifunctional thin film stack. FeRh has attracted increasing research interest over the last few years due to the unique first-order metamagnetic phase transition from an antiferromagnetic (AF) to a ferromagnetic (FM) ordering it exhibits at a technologically useful temperature ∼370 K.1–3 This phase transition makes the material a suitable candidate for several novel applications such as antiferromagnetic spintronics, memory resistors, and magnetic refrigeration.. Whilst numerous PNR studies have focused upon fully AF and FM FeRh thin films or dopant-graded FeRh films, here we use PNR to measure FeRh thin films partway through the FeRh phase transition, where there is a sparsity of data in the literature This allows the manner in which the FM phase nucleates and develops through the thickness of the film to be investigated. One mechanism by which strain is induced comes from the epitaxial stress due to the substrate on which the FeRh thin film is fabricated. Any lattice mismatch between the two materials scitation.org/journal/apm imposes a strain on the first few atomic layers of FeRh, and this has been shown to increase its transition temperature with decreasing thickness when deposited onto a single crystal MgO substrate.

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Discussion

Conclusion