Robust isothermal electric control of exchange bias at room temperature

Journal name:
Nature Materials
Volume:
9,
Pages:
579–585
Year published:
DOI:
doi:10.1038/nmat2785
Received
Accepted
Published online

Abstract

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr2O3 has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr2O3 single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

At a glance

Figures

  1. Structural characterization.
    Figure 1: Structural characterization.

    a, θ–2θ X-ray diffraction pattern of chromia bulk single crystal (upper panel) and thin film (lower panel) showing the chromia (0006) and (00012) peaks, respectively. The film is deposited on a sapphire (0001) substrate, giving rise to (0006), (00012), Kα and Kβ(*) peaks and a weak structure-factor-forbidden (0009) peak. The inset shows a room-temperature low-energy electron diffraction pattern of the hexagonal chromia (0001) surface measured at an electron energy of 140 eV. b, Real-space topography of the chromia (0001) surface of bulk single crystal (upper panel) and thin film (lower panel) measured by AFM. The respective main frames show cross-sectional analysis along indicated lines. A r.m.s. roughness of 0.88 nm is calculated in the region between scanning position 0.15 and 0.81 μm for the bulk single crystal. The r.m.s. roughness of 0.19 nm of the thin film is measured between 0.04 and 0.50 μm. c, The spin structure of a Cr2O3 single crystal with a stepped (0001) surface is shown for one of its two antiferromagnetic single-domain states. Up (red) and down (dark blue) spins of the Cr3+ ions (green spheres) point along the c axis.

  2. Spin-polarized UPS measurements and layer-resolved DOS.
    Figure 2: Spin-polarized UPS measurements and layer-resolved DOS.

    a, The intensity of photoelectrons (occupied states) versus binding energy from a Cr2O3 (0001) surface measured at T=100 K after cooling in μ0H=30 mT and E=0 from T>TN. Spin-up and spin-down intensities are shown by red circles and blues squares, respectively. Inset: The result of a first-principles calculation of the layer-resolved DOS. Colour code follows the experiment. The green line indicates a surplus surface state with spin-up polarization. b, Spin-up (red circles) and spin-down (blue squares) intensities after magnetoelectric annealing in E=3.85×10−4 kV mm−1 and μ0H=30 mT. The lines are best fits of multiple-peak Gaussian functions. The diamonds show Cr 3d spin-up (red and green) and spin-down (blue) contributions extracted from the fits. The Gaussian fit shown by the green diamonds reflects specific surface states. Colour code matches the theoretical DOS data. The triangles show the contrast, P, in the spin-dependent intensities versus binding energy. The green triangles highlight the contribution from the surface state. Maximum absolute errors in P are indicated by bars.

  3. Isothermal electric switching of the exchange-bias field.
    Figure 3: Isothermal electric switching of the exchange-bias field.

    a, Exchange-biased hysteresis loops of Cr2O3 (0001)/Pd 0.5 nm/(Co 0.6 nm Pd 1.0 nm)3 at T=303 K after initial magnetoelectric annealing in E=0.1 kV mm−1 and μ0H=77.8 mT. Hysteresis loops are measured by polar Kerr magnetometry in E = 0, respectively. The red squares show the virgin curve with a positive exchange-bias field of μ0HEB=+6 mT. Isothermal-field exposure in E=−2.6 kV mm−1 and μ0H=+154 mT gives rise to a loop with a negative exchange-bias field of μ0HEB≈−13 mT (green triangles). b, The red squares show the same virgin reference loop. The blue circles show the hysteresis loop after isothermal-field exposure in E=+2.6 kV mm−1 and μ0H=−154 mT, giving rise to the same negative exchange bias of μ0HEB=−13 mT. c, μ0HEB versus number of repeated isothermal switching through exposure to E=+2.6 kV mm−1 (blue circles) and E=−2 kV mm−1 (red squares) at constant μ0H=−154 mT, respectively.

  4. Hysteretic electric-field dependence of the exchange-bias field.
    Figure 4: Hysteretic electric-field dependence of the exchange-bias field.

    μ0HEB versus E, measured at T=303 K from individual Kerr loops. Data are taken after initial magnetoelectric annealing of Cr2O3 (0001)/Pd 0.5 nm/(Co 0.6 nm Pd 1.0 nm)3 in axial fields E=0.1 kV mm−1 and μ0H=77.8 mT. Kerr loops are measured in E=0 after isothermal E-field and simultaneous H-field exposure of the sample. For a given μ0HEB versus E curve the magnetic field is constant. The three μ0HEB versus E data sets correspond to μ0H=−115 mT (circles), μ0H=−154 mT (triangles) and μ0H=−229 mT (squares), respectively. The solid squares in the insets show the data points of electric switching fields and corresponding magnetic fields μ0H=−115,−154 and −229 mT. The lines are single-parameter fits of the functional form H=const/Ec.

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Author information

Affiliations

  1. Department of Physics & Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588-0111, USA

    • Xi He,
    • Yi Wang,
    • Ning Wu,
    • Kirill D. Belashchenko,
    • Peter A. Dowben &
    • Christian Binek
  2. Department of Physics, 257 Flarsheim Hall, University of Missouri, 5110 Rockhill Road, Kansas City, Kansas 64110, USA

    • Anthony N. Caruso
  3. Brookhaven National Laboratory, National Synchrotron Light Source, Upton, New York 11973, USA

    • Elio Vescovo

Contributions

X.H. and C.B. designed the study, in particular conceiving the electrically controlled exchange bias and electrically controlled magnetism. Y.W. and X.H. collected and analysed the magnetic data. N.W. led the photoemission experiments and data analysis. A.C. and E.V. supported the photoemission experiments. K.D.B. conceived the concept of roughness-insensitive surface magnetization and directed the electronic structure calculations. P.A.D. directed and conceived the photoemission experiments. C.B. directed the overall study. All authors contributed to the scientific process and the refinement of the manuscript. C.B. and X.H. wrote most of the paper.

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The authors declare no competing financial interests.

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