[Paper Review] Strain Dependent Spin Hall Magnetoresistance in the Multiferroic Antiferromagnet BiFeO$_3$

Daniel Sando, Si Chen+10 more|arXiv (Cornell University)|Aug 24, 2023

Multiferroics and related materialsMaterials Science3 citations

TL;DR

This study demonstrates that strain-induced structural phases in epitaxial BiFeO3 thin films—rhombhedral-like (R') and tetragonal-like (T')—exhibit distinct spin Hall magnetoresistance (SMR) responses. While both phases show positive SMR indicating canted ferromagnetic moments, T' BFO exhibits enhanced SMR at room temperature that decreases with cooling, whereas R' BFO shows the opposite trend, mirroring weak ferromagnetism. The difference arises from distinct field-induced magnetic moment contributions: in T' BFO, the applied field directly induces the moment, while in R' BFO, the moment is mediated by the Néel vector, leading to strain-tunable SMR for spintronic applications.

ABSTRACT

The spin Hall magnetoresistance (SMR) of epitaxial BiFeO$_3$ thin films is investigated. SMR consistent with ferromagnetic interfacial states for BiFeO$_3$ films fabricated on (001) SrTiO$_3$ (R' BFO) and LaAlO$_3$ (T' BFO) substrates is found, albeit with different temperature dependencies. For T' BFO, the SMR is enhanced at room temperature, and decays with reduced temperatures. By contrast, R' BFO shows a monotonic decrease in SMR response with increasing temperature, mirroring the trend of a weak ferromagnet. Density functional theory shows that this difference originates from the coupling of the applied magnetic field to oxygen octahedral rotation (R') and spin (T') degrees of freedom.

Motivation & Objective

• To investigate how epitaxial strain and structural phase (R' vs. T') in BiFeO3 thin films affect spin Hall magnetoresistance (SMR).
• To determine the origin of contrasting temperature dependencies of SMR in R'- and T'-phase BiFeO3.
• To explore the role of weak ferromagnetism and spin-lattice coupling in antiferromagnetic BiFeO3 for spintronic applications.
• To establish SMR as a sensitive probe of interfacial magnetic anisotropy and field-induced spin responses in multiferroic oxides.

Proposed method

• Epitaxial BiFeO3 thin films were grown on (001) SrTiO3 (R' BFO) and (001) LaAlO3 (T' BFO) substrates to induce distinct in-plane compressive strains.
• Spin Hall magnetoresistance (SMR) was measured as a function of temperature and magnetic field using Hall bar geometry with Pt capping layers.
• Field-dependent SMR measurements were performed to distinguish between domain reorientation, spin-flop transitions, and linear magnetic moment responses.
• Density functional theory (DFT) calculations were used to analyze the coupling between magnetic field, oxygen octahedral rotations (R'), and spin degrees of freedom (T').
• Mössbauer spectroscopy was employed to confirm magnetic states and validate the presence of weak ferromagnetism.
• Theoretical modeling linked the observed SMR behavior to the field-induced magnetic moment in T' BFO versus L-vector-mediated moment in R' BFO.

Experimental results

Research questions

• RQ1 How does epitaxial strain in BiFeO3 thin films influence the temperature dependence of spin Hall magnetoresistance (SMR)?
• RQ2 Why does T'-phase BiFeO3 exhibit enhanced SMR at room temperature while R'-phase shows maximum SMR at low temperatures?
• RQ3 What is the origin of the distinct SMR response in T' versus R' structural phases of BiFeO3?
• RQ4 How do the Néel vector and applied magnetic field couple to generate the observed SMR signals in antiferromagnetic BiFeO3?
• RQ5 Can SMR serve as a reliable probe for interfacial magnetic anisotropy and field-induced spin responses in multiferroic oxides?

Key findings

• T'-phase BiFeO3 exhibits enhanced SMR at 300 K, which monotonically decreases with cooling, indicating a field-induced moment response.
• R'-phase BiFeO3 shows maximum SMR at 5 K and decreases with increasing temperature, mirroring the behavior of weak ferromagnetism.
• Field-dependent SMR measurements for both phases show linear ∆ρxy vs. H, ruling out domain reorientation or spin-flop transitions as the origin of the signal.
• DFT calculations confirm that the SMR difference arises from coupling between the magnetic field and oxygen octahedral rotations (R') and spin degrees of freedom (T').
• The SMR response in T' BFO is dominated by the field-induced magnetic moment, while in R' BFO it is mediated by the Néel vector (L), explaining the contrasting temperature dependencies.
• The results demonstrate that strain engineering can be used to tune SMR magnitude and its temperature dependence in multiferroic antiferromagnets for spintronic applications.