Controlling magnetism by means of electric fields is a key issue for the future development of low-power spintronics1. Progress has been made in the electrical control of magnetic anisotropy2, domain structure3,4, spin polarization5,6 or critical temperatures7,8. However, the ability to turn on and o robust ferromagnetism at room temperature and above has remained elusive. Here we use ferroelectricity in BaTiO3 crystals to tune the sharp metamagnetic transition temperature of epitaxially grown FeRh films and electrically drive a transition between antiferromagnetic and ferromagnetic order with only a few volts, just above room temperature. The detailed analysis of the data in the light of first-principles calculations indicate that the phenomenon is mediated by both strain and field e ects from the BaTiO3. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics.

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Electric-field control of magnetic order above room temperature

A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice-Dirac fermions and flat bands-have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics.

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Dirac fermions and flat bands in the ideal kagome metal FeSn

The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice - Dirac fermions and topological flat bands - have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization- and termination-dependent angle-resolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

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Electrons, commonly moving along the applied electric field, acquire in certain magnets a dissipationless transverse velocity. This spontaneous Hall effect, found more than a century ago, has been understood in terms of the time-reversal symmetry breaking by the internal spin structure of a ferromagnetic, noncolinear antiferromagnetic, or skyrmionic form. Here, we identify previously overlooked robust Hall effect mechanism arising from collinear antiferromagnetism combined with nonmagnetic atoms at noncentrosymmetric positions. We predict a large magnitude of this crystal Hall effect in a room temperature collinear antiferromagnet RuO2 and catalog, based on symmetry rules, extensive families of material candidates. We show that the crystal Hall effect is accompanied by the possibility to control its sign by the crystal chirality. We illustrate that accounting for the full magnetization density distribution instead of the simplified spin structure sheds new light on symmetry breaking phenomena in magnets and opens an alternative avenue toward low-dissipation nanoelectronics.

Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets.

Spintronic devices based on antiferromagnetic (AFM) materials hold the promise of fast switching speeds and robustness against magnetic fields. Different device concepts have been predicted and experimentally demonstrated, such as low-temperature AFM tunnel junctions that operate as spin-valves, or room-temperature AFM memory, for which either thermal heating in combination with magnetic fields, or Neel spin-orbit torque is used for the information writing process. On the other hand, piezoelectric materials were employed to control magnetism by electric fields in multiferroic heterostructures, which suppresses Joule heating caused by switching currents and may enable low energy-consuming electronic devices. Here, we combine the two material classes to explore changes of the resistance of the high-Neel-temperature antiferromagnet MnPt induced by piezoelectric strain. We find two non-volatile resistance states at room temperature and zero electric field, which are stable in magnetic fields up to 60 T. Furthermore, the strain-induced resistance switching process is insensitive to magnetic fields. Integration in a tunnel junction can further amplify the electroresistance. The tunneling anisotropic magnetoresistance reaches ~11.2% at room temperature. Overall, we demonstrate a piezoelectric, strain-controlled AFM memory which is fully operational in strong magnetic fields and has potential for low-energy and high-density memory applications.

A Piezoelectric, Strain-Controlled Antiferromagnetic Memory Insensitive to Magnetic Fields

The anomalous Hall effect is allowed by symmetry in some non-collinear antiferromagnets and is associated with Bloch-band topological features. This topological anomalous Hall effect is of interest in the development of low-power electronic devices, but such devices are likely to demand electrical control over the effect. Here we report the observation of the anomalous Hall effect in high-quality thin films of the cubic non-collinear antiferromagnet Mn3Pt epitaxially grown on ferroelectric BaTiO3 substrates. We demonstrate that epitaxial strain can alter the anomalous Hall conductivity of the Mn3Pt films by more than an order of magnitude. Furthermore, we show that the anomalous Hall effect can be turned on and off by applying a small electric field to the BaTiO3 substrate when the heterostructure is at a temperature of around 360 K and the Mn3Pt is close to the phase transition between a low-temperature non-collinear antiferromagnetic state and a high-temperature collinear antiferromagnetic state. The switching effect is due to piezoelectric strain transferred from the BaTiO3 substrate to the Mn3Pt film by interfacial strain mediation.

Electrical switching of the topological anomalous Hall effect in a non-collinear antiferromagnet above room temperature

In this work, IrMn-based perpendicular magnetic tunnel junctions (MTJs) are investigated. By inserting a thin W layer at an antiferromagnet/ferromagnet (AFM/FM) interface, we enhance the annealing temperature to 355 °C and obtain a high tunnel magnetoresistance ratio of 127%. Subsequently, field-free spin–orbit torque (SOT) switching of perpendicular MTJ is realized thanks to the in-plane exchange bias generated at the AFM/FM interface. Moreover, by applying a gate voltage, a coercive field is effectively decreased due to the voltage-controlled magnetic anisotropy (VCMA) effect. Finally, through the interplay of the SOT and VCMA effects, the critical switching current density is dramatically reduced by 73% (to 2.4 MA/cm2) and the total writing power consumption is decreased by 84% when a gate voltage of 0.76 V is applied. These findings pave the way for the practical applications of the IrMn-based perpendicular MTJs in low-power magnetic random-access memory.

C. B. Jiang

Papers

Electrical switching of the topological anomalous Hall effect in a non-collinear antiferromagnet above room temperature

Voltage-gated spin-orbit torque switching in IrMn-based perpendicular magnetic tunnel junctions