Antiferromagnetic materials are a special class of materials having zero net magnetization in the ground state. In collinear antiferromagnets, the magnetic moments are parallel or antiparallel to the Néel vector, taking its name from the Nobel prize winner Louis Néel. Antiferromagnets are robust against magnetic field and do not generate any stray field, which limits the packing density of ferromagnetic elements. Only recently, it was shown that antiferromagnets hold potential for high speed all-electrical devices. In order to give an answer to the numerous open questions of the field, we study information storage, read-out and transport properties in novel exciting antiferromagnetic materials, both metallic and oxidic, performing combined magnetic and electrical characterizations.
The magnetic properties of antiferromagnets are studied both with state-of-art magnetometry and optical techniques, available in our lab (Superconducting Quantum Interference Device, Vibrating Sample Magnetometry, Magneto-Optical Kerr Effect, and in large scale facilities. Space resolved studies of antiferromagnets by X-Ray magnetic linear dichroism– photoemission electron microscopy (XMLD-PEEM), carried out in synchrotrons, allow to directly image the domain structure of antiferromagnets, estimating the critical temperatures and obtaining information on the orientation of the Néel vector.
The magnetotransport of antiferromagnets is fully characterized in several He cryostats in a magnetic field up to 12 T. Spin Hall magnetoresistance is a recently discovered type of magnetoresistance based on the interconversion between a spin current and a charge current. The charge current is converted to a spin current in a heavy metal layer, via spin Hall effect. The spin current is absorbed or reflected in the AFM based on the orientation of the Néel vector and it is afterwards reconverted to a charge current via the inverse spin Hall effect, as explained in fig. 1a [3,4]. This technique allows to study the spin current transmission properties at the interface between an insulating antiferromagnetic material and a heavy metal. We observed a negative spin Hall magnetoresistance (SMR) in a thin film of epitaxial antiferromagnetic NiO, without any ferromagnetic element, as shown in fig. 1. Our study paves the way to the electrical read-out of antiferromagnetic insulating thin films. For more information click here.
Storing information in spintronics devices occurs in most cases by Oersted fields, spin-transfer torques (STT) and spin-orbit torques (SOT). Alternatively, structural distortions modify magnetic properties. Such modifications provide means for voltage controlled switching of thin magnetic films on piezoelectric substrates, presenting new pathways for future spintronics with low energy consumption. Combined with new compounds, like the antiferromagnet Mn2Au with strong spin-orbit coupling and broken inversion symmetry on the spin sublattices, novel spintronics devices may emerge.
Figure 1: (a) Mechanism of the spin Hall magnetoresistance in an antiferromagnet. The spin current, generated via the spin Hall effect in a heavy metal layer, is reflected (absorbed) at the antiferromagnet/metal interface if the spin polarization σ is parallel (orthogonal) to L, yielding a lower (higher) resistance state via the inverse spin Hall effect. Adapted from Ryo Iguchi et al 2014 Appl. Phys. Express 7 013003. (b) Spin Hall magnetoresistance of NiO(001)/Pt when a magnetic field of fixed magnitude is swept in the plane of the sample. (c) Antiferromagnetic structure of NiO(001), and direction of the Néel vector in a stable configuration.
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- Baldrati, L. et al. Negative spin Hall magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films. arXiv 1709.00910 1–10 (2017).
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