Magnetic imaging

Materials World magazine
,
28 Oct 2019

Diamond Light Source is producing magnetic imaging with polarised X-rays.

Anew generation of magnetic materials is being developed using polarised X-ray imaging at the synchrotron facility, Diamond Light Source, Oxfordshire, UK. The facility houses myriad laboratories for materials research, but the development of magnetic materials benefits in particular from the tuneable polarisation of the intense synchrotron X-ray beams. Today, the vast majority of magnetic materials in use are ferromagnets, which are well known for their strong interactions with magnetic fields. For instance, the orientation of ferromagnetic nanoparticles in a hard-disk bit are changed to write a 0 or 1 using magnetic field pulses, via a current through a coil. However, this is a dissipative process and produces a considerable amount of heat per operation. The development of new, more energy-efficient and robust magnetic materials is therefore a key driver for research, given the trillions of magnetic reversal operations involved daily in writing to hard-disks across the globe.

One route to more energy-efficient devices is to employ strain-mediated magnetoelectric effects to rotate the magnetisation in a ferromagnet film grown on a piezoelectric material that expands or contracts when subjected to an electric voltage. Another avenue is to exploit antiferromagnet materials, which are magnetically ordered like ferromagnets, but each atomic site has a specific orientation in relation to its neighbours such that the individual magnetic moments compensate each other perfectly. This leads to no net-magnetisation and hence no stray magnetic fields.

Antiferromagnets are difficult to manipulate, but also promise extremely secure storage since the data stored cannot be corrupted by a magnetic field. Recently, the combination of theoretical calculations, advanced device fabrication and X-ray imaging has led to the local magnetic ordering in an antiferromagnetic thin film being reproducibly controlled for the first time. This is a promising route to fast, efficient and secure storage devices.

A third possibility is to develop storage based on magnetic textures, known as skyrmions, which are robust swirls of localised magnetism that are easily displaced by low-current pulses and show potential for a new generation of racetrack memory devices.

Strained ferromagnets

The piezoelectric properties of the ceramic perovskite PMN-PT (0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3) are widely used in commercial actuators, where the strain generated varies continuously with the applied voltage. However, if the applied voltage is cycled appropriately, discontinuous changes of strain can arise. These discontinuous changes have been employed to drive magnetic switching in a thin overlying nickel ferromagnetic film, permitting magnetic information to be written electrically in a highly energy-efficient manner.

The composite Ni/PMN-PT structure is novel in that it mimics the rare magnetoelectric effects occasionally found naturally occurring in minerals. X-ray imaging of the nickel film is used to construct detailed vector magnetisation maps, showing two magnetic domain states that change into four after the application of a small voltage. X-ray images of the magnetisation initially suggested that all the magnetic domains in the film rotate by 90° as expected from the predictions of macroscopic laboratory measurements, but a closer look reveals the picture to be much more complex.

A pixel-by-pixel analysis of the X-ray vector magnetisation maps reveal that the magnetic domains rotate by about 62°, considerably less than 90°, which theoretical calculations ascribe to a combination of normal and shear strain components affecting the ferromagnetic nickel film. The results have significant consequences because all pervious measurements of ferromagnet-piezoelectric structures have systematically concluded a 90° rotation since the symmetry of the experimental setups were insensitive to the shear strain effects. Nanoscale imaging using polarised X-rays has thus uncovered a unique picture of the local magnetisation rotation under strain and unveiled effects hidden in macroscopic measurements. The discovery of shear components in strain controlled magnetism opens the opportunity to write data using media that can now be electrically and magnetically controlled.

Antiferromagnetic rotations

In the 1940s, the hidden magnetic structure of antiferromagnets, that was predicted by Louis Néel in the 1930s, was discovered using neutrons generated by the newly developed nuclear reactors. These materials remained a scientific curiosity for decades until the 1980s when atomic effects at ferromagnet-antiferromagnet interfaces in spin valves revolutionised the hard-disk industry.

However, manipulating antiferromagnetism remained doggedly elusive until theoretical predictions in 2014 indicated a route to controlling antiferromagnetism using electrical current pulses in materials with specific local crystal symmetries. The right combination of crystal asymmetry and pulsed currents was predicted to generate staggered effective magnetic fields, at connected but oppositely magnetised atomic sites. These fields are capable of rotating the atomic moments by 90° in opposite directions. A thin film of semiconducting copper manganese arsenide (CuMnAs), grown by molecular beam epitaxy on a gallium phosphide (GaP) substrate, is a material that possesses the correct symmetry properties and can accommodate pulsed currents.

The change of the magnetisation axis in CuMnAs was monitored macroscopically via magnetoresistance effects. It indeed, displayed the expected resistance changes following current pulse injection across nanofabricated devices. Polarised X-ray imaging, however, is a unique probe that can visualise the rotation of the magnetisation vector in antiferromagnetic materials. Only detailed X-ray images could finally confirm the macroscopic magnetoresistance work.

The X-ray images also revealed a significant spatial inhomogeneity of the magnetisation rotation aiding further optimisation of the effect through thin film growth on different substrates. The first rotation effect detected was polarity independent meaning that no matter which direction the current pulses were applied along a particular crystallographic direction, the magnetisation always rotated in the same manner and could be attributed to a coherent reorientation of small magnetic domains.

However, with the aid of X-ray imaging on nanoscale areas of the CuMnAs, polarity dependent switching was observed for the first time with further investigations indicating that the effect was localised at the boundary between the antiferromagnetic domains. A polarity dependent change in the resistance of the thin CuMnAs film was then also measured for the first time following the hints gleaned from the polarised X-ray imaging.

Magnetic swirls

Vortex structures are common, from swirls in a cup of tea to spiral galaxies across the universe. For example, in a tornado, air circulates around a vertical axis forming a swirl, and once formed, the twisted air parcels can move, deform and interact with their environment without disintegrating. A skyrmion is the magnetic version of a tornado and can be pictured by replacing the air parcels that make up the tornado by magnetic compass needles representing the local magnetic moment direction, and then scaling the system down to the nanometre scale. Once formed, the ensemble of twisted magnetic moments can move, deform, and interact with the magnetic environment in a nanostructure without breaking up, which is an ideal characteristic for information carriers in memory and logic devices. What makes a tornado stable is not only its twist, but also comes from its three-dimensional properties, i.e. the wind current has extra twist along the column of turbulent flow. This leads to a tight bundling of the vortex sheets at different heights along the tornado column.

Similarly, such structures can also occur in magnetic skyrmions in which a circular magnetisation pattern, for instance, reverses from the centre to the perimeter guaranteeing an extreme stability arising from the topology of the magnetic structure. Up to now, skyrmions have been most commonly treated as two-dimensional objects, and their longitudinal tornado-like structure has remained completely unexplored. Copper oxyselenide (Cu2OSeO3) is a non-centrosymmetric multiferroic material that exhibits skyrmion structures over a limited magnetic field and temperature range corresponding to an ideal balance between the direct exchange, asymmetric exchange, dipole energy, magnetocrystalline energy and thermal energy.

The skyrmions in Cu2OSeO3 form in a plane orthogonal to an applied magnetic field and order into hexagonally packed arrays. The small size and complicated magnetism of these structures makes them extremely challenging to investigate. Resonant X-ray scattering, on the other hand, has revealed a continuous change from Néel-type winding at the surface to Bloch-type winding with increasing depth into the material. This demonstrates the power of X-ray profiling for microscopic studies of magnetic order and reveals the hidden energetics that make magnetic skyrmions so stable, which is crucial for skyrmion device engineering.

Future perspectives

Polarised X-ray imaging provides a powerful approach to understand the microscopic origin of macroscopic magnetic effects and in many cases shapes the direction of travel for improving material performance. The state-of-the-art in current polarised X-ray imaging is to build 2D vector maps of magnetic structures, but there are indications that synchrotron X-rays can also build high-resolution 3D magnetisation maps. The efficient generation of 3D magnetisation maps to follow, for instance, magnetisation reversal dynamics then promises to propel the development of magnetic materials to a new level. Diamond Light Source is currently developing plans for a facility upgrade, which will significantly reduce the X-ray beam size allowing unprecedented nanoscale insights into the materials of the future.