Imaging an antiferromagnetic memory

We’re all familiar with ferromagnets - there’s probably one stuck to your fridge - and ferromagnetism underpins many digital storage devices, from floppy disks and cassette tapes, through to computer hard drives and the magnetic strips on our credit cards. Ferromagnets react to external magnetic fields, which allows us to use them for data storage, but also makes them vulnerable to being wiped by magnetic fields generated by other equipment. Antiferromagnets are internally magnetic, but the individual magnetic moments inside cancel each other out so that there is no external magnetism. They don’t produce magnetic fields and they don’t react to magnetic fields, and they have the potential to make smaller, faster, and more robust and more energy efficient data storage devices. However, writing data to antiferromagnetic memory is a big challenge. On Diamond Light Source’s I06 Nanoscience beamline, researchers used Polarised X-ray Photoemission Electron Microscopy (XPEEM) to demonstrate the first all-antiferromagnetic memory device. The highresolution XPEEM images allowed the team to show an antiferromagnet responding to electrical pulses in a way that would offer efficient control of the data writing process. This research could be extremely significant, as antiferromagnets have the potential to be up to 1000 times faster than the best current memory storage devices, and their unique properties make them particularly suited to space and aviation applications.

Figure 1

Figure 1: (a) Tetragonal crystal structure of CuMnAs. The two Mn sites (Mn A and Mn B) with opposite spin are inversion partners around the centrosymmetry position highlighted by the green ball; (b) Cross-sectional scanning transmission electron microscopy image; (c) Optical micrograph showing the geometry used for electrical writing (Jwrite) and measurement (Jread); (d) Transverse resistance signals after applying current pulses of amplitude Jwrite = 4x106 Acm-2 along the directions marked by red and black arrows in (c). Adapted from Science 351, 587 (2016).

Modern magnetic memory technologies rely on the flow of spin-polarised electron currents between ferromagnetic thin films for both the reading and writing of information. Recent studies have shown that the writing process can be made more efficient using spin-orbit torque: an electric current can pick up a net spin-polarisation on passing through materials with particular symmetries1. In either case, externally applied magnetic fields are not used for writing the information, and the sensitivity of the ferromagnet to external fields becomes a hindrance as neighbouring memory elements can interact with each other. Antiferromagnets are a class of magnetic material in which the magnetic moments alternate from atom-to-atom, giving zero net magnetisation. They are typically insensitive to magnetic fields of several tesla, which could substantially reduce this cross-talk effect in an antiferromagnetbased memory device. Also, the resonant frequencies of antiferromagnets are typically 1-2 orders of magnitude higher than for ferromagnets, potentially offering much faster writing speeds.

In the tetragonal crystal structure of CuMnAs, an antiferromagnetic compound, Mn atoms carrying oppositely oriented magnetic moments form inversion-symmetric partners (Fig. 1a). Due to this crystal symmetry, an electrical current produces a local spin polarisation, of opposite sign at each Mn site. The resulting interaction can induce a rotation of pairs of local magnetic moments, which maintain their antiferromagnetic orientation. This mechanism, referred to as Néel-order spin-orbit torque (NSOT)2, was first predicted for a Mn2Au alloy sharing the same coincidence of inversion-partner sublattices and magnetic moment sublattices as the CuMnAs films used in our study.

The CuMnAs films were grown by molecular beam epitaxy on GaAs or GaP substrates3. Fig. 1b shows a scanning transmission electron micrograph of a typical structure, showing high crystalline quality and chemical order. The films were patterned into devices using optical lithography.

First indications that electrical currents can be used to reconfigure the CuMnAs antiferromagnetic order were obtained from electrical signals in devices similar to the one shown in Fig. 1c. Electrical currents applied along the principal arms were used to set a particular state, while measurements along the diagonal arms were used to probe the resulting effect. The measurements, shown in Fig. 1d, demonstrate a clear, stable and reproducible change in the electrical signal after a current pulse, which is consistent with the predicted NSOT mechanism2.

Figure 2
Figure 2: (a) Experimental geometry for the XPEEM measurements. The X-ray beam is at grazing incidence, with linear polarisation vector in the plane of the film. (b) Map of the antiferromagnetic domain structure after applying a current pulse of 6x106 Acm-2 along the path indicated by the black/red arrow. (c) Close up of the 4 μm square region shown by the yellow box in (b). (d) Close up of the 4 μm square region shown by the green box in (b). Adapted from Science 351, 587 (2016).

X-ray PhotoElectron Emission Microscopy (XPEEM) measurements, performed on beamline I06, provide more direct evidence of the electrical switching behaviour and an insight into its microscopic origin (Fig. 2). The images in Fig. 2(b-d) show maps of the antiferromagnetic domain structure for a CuMnAs device in which a current pulse has been applied along the path indicated by the arrow in Fig. 2b. A reconfiguration and coalescence of domains is observed after the current pulse. At the top of the image where the current is parallel to the X-ray polarisation, bright domains have enlarged at the expense of dark domains, as highlighted by the image in Fig. 2c. Vice versa, dark domains have enlarged at the expense of bright domains at the right-hand side of the image (Fig. 2d), where the current is perpendicular to the X-ray polarisation. By comparing measured and calculated X-ray magnetic linear dichroism (XMLD) spectra for CuMnAs, we find that the magnetic moments preferentially orient perpendicular to the current pulse direction, as predicted by Reference [2].

Further studies on a 4-terminal CuMnAs device showed that the domain structure could be repeatedly set and reset using orthogonal current pulses, with a close correlation between the electrical signal and the spatially averaged XMLD over the device centre (Fig. 3). Moreover, a local inhomogeneity of the electrical switching was observed at sub-micron lengthscales 4. Understanding and controlling this inhomogeneity will be crucial for maximising the electrical readout signal in future antiferromagnetic memory devices.

CAPTION


Figure 3: (a) Average XMLD signal over the centre of a 4-terminal CuMnAs device, after applying current pulses alternately in orthogonal directions. (b) Change in the electrical resistance over the same current pulse sequence. Adapted from Reference4.

References:

  1. Luqiao, L. et al. Spin-torque switching with the giant spin Hall effect in tantalum. Science 336(6081), 555-558, doi: 10.1126/science.1218197 (2012).
  2. Železný, J. et al. Relativistic Néel-order fields induced by electrical current in antiferromagnets. Phys. Rev. Lett. 113, 157201, doi: 10.1103/PhysRevLett.113.157201 (2014).
  3. Wadley, P. et al. Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs. Nature Communications 4, doi: 10.1038/ncomms3322 (2013).
  4. Grzybowski, M. J. et al. Imaging current-induced switching of antiferromagnetic domains in CuMnAs. Phys. Rev. Lett. 118, 057701, doi: 10.1103/PhysRevLett.118.057701 (2017).

Funding acknowledgement:

We acknowledge funding from EPSRC grant EP/K027808/1, the European Research Council grant 268066, and Diamond Light Source for the allocation of beamtime under proposal number SI-12504.

Corresponding author:

Dr Kevin Edmonds, University of Nottingham, kevin.edmonds (at) nottingham.ac.uk

Related publication: Wadley P, Howells B, Železný J, Andrews C, Hills V, Campion RP, Novák V, Olejník K, Maccherozzi F, Dhesi SS, Martin SY, Wagner T, Wunderlich J, Freimuth F, Mokrousov Y, Kuneš J, Chauhan JS, Grzybowski MJ, Rushforth AW, Edmonds KW, Gallagher BL, Jungwirth T. Electrical switching of an Antiferromagnet. Science 351(6273), 587-590, doi: 10.1126/science.aab1031 (2016).

Publication keywords: Spintronics; Antiferromagnetism; Magnetic domains