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Spin Hall magnetoresistance in antiferromagnet/heavy-metal heterostructures

Johanna Fischer, Olena Gomonay, Richard Schlitz, Kathrin Ganzhorn, Nynke Vlietstra, Matthias Althammer, Hans Huebl, Matthias Opel, Rudolf Gross, Sebastian T. B. Goennenwein, and Stephan Geprägs
Phys. Rev. B 97, 014417 – Published 17 January 2018
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  • INTRODUCTION
  • THEORY
  • EXPERIMENT
  • RESULTS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We investigate the spin Hall magnetoresistance in thin-film bilayer heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO. While rotating an external magnetic field in the easy plane of NiO, we record the longitudinal and the transverse resistivity of the Pt layer and observe an amplitude modulation consistent with the spin Hall magnetoresistance. In comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by 90∘ and its amplitude strongly increases with the magnitude of the magnetic field. We explain the observed magnetic field dependence of the spin Hall magnetoresistance in a comprehensive model taking into account magnetic-field-induced modifications of the domain structure in antiferromagnets. With this generic model, we are further able to estimate the strength of the magnetoelastic coupling in antiferromagnets. Our detailed study shows that the spin Hall magnetoresistance is a versatile tool to investigate the magnetic spin structure as well as magnetoelastic effects, even in antiferromagnetic multidomain materials.

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    • Received 13 September 2017
    • Revised 22 December 2017

    DOI:https://doi.org/10.1103/PhysRevB.97.014417

    ©2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    AntiferromagnetismElectrical conductivityMagnetic domainsMagnetoelastic effectSpin Hall effectSpin Hall magnetoresistanceSpin currentSpin-orbit couplingStrain
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Johanna Fischer1,2, Olena Gomonay3, Richard Schlitz4,5, Kathrin Ganzhorn1,2, Nynke Vlietstra1,2, Matthias Althammer1,2, Hans Huebl1,2,6, Matthias Opel1, Rudolf Gross1,2,6, Sebastian T. B. Goennenwein4,5, and Stephan Geprägs1,*

    • 1Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
    • 2Physik-Department, Technische Universität München, 85748 Garching, Germany
    • 3Institut für Physik, Johannes Gutenberg Universität Mainz, 55128 Mainz, Germany
    • 4Institut für Festkörper- und Materialphysik, Technische Universität Dresden, 01062 Dresden, Germany
    • 5Center for Transport and Devices of Emergent Materials, Technische Universität Dresden, 01062 Dresden, Germany
    • 6Nanosystems Initiative Munich, 80799 München, Germany

    • *stephan.gepraegs@wmi.badw.de

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    Vol. 97, Iss. 1 — 1 January 2018

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    • Figure 1
      Figure 1

      Spin Hall magnetoresistance (SMR) of a single-domain (a), (c), (e) collinear ferromagnetic insulator/heavy metal (FMI/HM) and (b), (d), (f) an antiferromagnetic insulator/heavy metal (AFI/HM) bilayer. The SMR is based on an interconversion of charge (Jc) and spin currents (Js) via the spin Hall effect. An increase of the resistivity ρlong of the HM is observed, if the spin polarization σ of Js is perpendicular to the direction of the order parameter of the magnetic layer [the magnetization direction m (FMI) or the Néel vector ℓ (AFI)]. This leads to a finite spin current Jsstt in the magnetic layer, which reduces the spin current backflow Jsback (a,d). For a collinear configuration between σ and m (ℓ), ρlong is approximately given by the normal resistivity of the HM layer (b) and (c). ρlong can be parametrized by the angle φ between m (ℓ) and the current density direction j. The expected angular-dependence of ρlong is sketched in (e) and (f) as a function of the angle α between the external magnetic field H and j for H larger than the anisotropy field (FMI: α=φ) or the spin-flop field (AFI: α=90∘+φ).

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    • Figure 2
      Figure 2

      Magnetic configurations in the magnetically easy (111) plane of NiO for an in-plane rotation of the magnetic field H with α representing the angle between H and the current density j for (a) H≃0, (b) 0<H<HMD/2, (c) HMD/2≤H<HMD, and (d) H≥HMD with the monodomainization field HMD. Top: Evolution of the antiferromagnetic multidomain state in NiO with the Néel vector ℓ(k) of each domain k (red double arrows) for an applied magnetic field along j (α=0∘). Middle: Same for H along t (α=90∘). Bottom: Expected angular-dependence of the total longitudinal resistivity ρlong of a NiO/Pt Hall bar within the SMR theory. The inset shows the orientation of the Pt Hall bar, the magnetic field H, and the Néel vector ℓ with respect to the NiO in-plane directions.

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    • Figure 3
      Figure 3

      Structural properties of the investigated NiO/Pt heterostructure fabricated on a (0001)-oriented Al2O3 substrate. (a) 2θ−ω-scan along the [0001]-direction of Al2O3. The inset shows the rocking curve around the NiO(111) reflection and the derived full width at half maximum value. (b), (c) Reciprocal space mappings around the NiO(402) and the Al2O3(303¯12) reflections. The reciprocal lattice units (rlu) are related to the Al2O3(303¯12) substrate reflection.

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    • Figure 4
      Figure 4

      Schematic drawing of the NiO/Pt Hall bar mesa structure with the coordinate system j, t, and n defined along the crystallographic directions [11¯0], [112¯], and [111] of the NiO thin film, respectively. In addition, the measurement scheme used for the magnetotransport measurements with the applied current I, the measured longitudinal voltage Vlong, and the transverse voltage Vtrans is illustrated. In the NiO(111) plane, the direction of the magnetic field H is defined by α (green) with respect to the current direction j. H is rotated counterclockwise.

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    • Figure 5
      Figure 5

      Angular dependent magnetoresistance of a NiO(111)/Pt thin-film heterostructure, measured at 300 K with in-plane external magnetic-field magnitudes of (a) 1 T, (b) 9 T, (c) 15 T, (d) 17 T. Normalized longitudinal resistivity ρlong (black symbols, left axis) and transverse resistivity ρtrans (red symbols, right axis) as a function of the magnetic-field orientation α. The lines are fit to the data using cos2α and sin2α functions [cf. Eqs. (16)].

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    • Figure 6
      Figure 6

      Field-dependent longitudinal resistivity ρlong(H) of the NiO(111)/Pt bilayer normalized to ρlong(H=0) measured at 300 K for α=90∘ (H∥t, black symbols) and α=0∘ (H∥j, red symbols).

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    • Figure 7
      Figure 7

      (a) SMR amplitude of the NiO(111)/Pt thin-film bilayer obtained from ADMR measurements at 300 K at different applied magnetic fields (cf. Fig. 5) using the longitudinal (black symbols) and transverse (red symbols) resistivities as well as data extracted from field-sweep measurements (cf. Fig. 6, blue symbols). The data are compared to the analytical model based on a magnetic-field-induced domain redistribution in NiO (green line). (b) Comparison of the normalized SMR amplitude of our NiO/Pt thin-film heterostructure to the data published by Hoogeboom et al. [46] measured on a NiO/Pt sample using a NiO single crystal. The magnetic field is normalized to the monodomainization field μ0HMDfilm=13.4T and μ0HMDcryst=4.1T, respectively.

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