M A T T E R A T E X T R E M E S

S C I E N T I F I C H I G H L I G H T S

2 8 H I G H L I G H T S 2 0 2 2 I

Ion-induced nanopattern formation in FeRh thin films

A combination of spectroscopy, microscopy and grazing-incidence nuclear resonance scattering experiments made it possible to reveal the 3D structure of tailored individual magnetic domains in FeRh thin films. Overall, the great customisability of the presented nanosphere-lithography technique in FeRh thin film provides opportunities for developing cutting-edge spintronic applications.

Grand societal challenges place expectations on materials science and nanotechnology to develop special alloys fulfilling particular requirements. Due to the great diversity of magnetic phases as well as magnetocaloric properties, FeRh is a candidate for manufacturing energy-efficient devices, hazardous-gas-free magnetic refrigerators and for medical applications [1-4]. The majority of these devices require well-tailored magnetic nanostructures [1,2].

FeRh films were deposited on MgO substrates by using molecular-beam-epitaxy technique. After annealing at 500°C for 40 minutes, the FeRh layer exhibited full

Fig. 18: The scheme of the experiment and scanning electron microscopy image of the actual polystyrene irradiation mask.

Fig. 19: Magnetic force microscopy images taken from the unirradiated,

1015 ion/cm2 and 1016 ion/cm2 Ne+ irradiated sample with different

applied masks. The projected schematics show the reconstructed 3D images of the formed magnetic

domains.

ferromagnetic (FM) ordering. Irradiation masks of a monolayer silica or polystyrene spheres with nominally 500 nm or 1000 nm diameter, respectively, were applied on the surface. Local modification of the magnetic order was performed by 110 keV Ne+ irradiation of fluences 1015 and 1016 ion/cm2 through both types of masks. The chosen ion energy enables to hinder the incoming ions by the mask, however, in uncovered regions, the Ne+ projectiles can penetrate deeply in the FeRh film. The scheme of the experiment is shown in Figure 18.

To reveal the change of the magnetic configuration in the FeRh layer, magnetic force microscopy images were recorded in unirradiated state and after both irradiations. The unirradiated sample was found to exhibit a characteristic FM domain structure. With ion irradiation, the formation of FM regions (B2-FeRh) in the paramagnetic (PM) (A1-FeRh) matrix was observed, reflecting the surface pattern of the applied mask (Figure 19).

To investigate the formed magnetic particles, grazing- incidence nuclear resonance scattering experiments were carried out at beamline ID18 [1], using 14.41 keV beam energy. The applied incident angles made it possible to gain information from different depth ranges of the film [1],

hence the magnetic depth profiles could be extracted. Based on the results, the 3D structure of the created individual magnetic domains was reconstructed (Figure 19). Common characteristics of the in-depth magnetic profiles show a lower FM concentration directly below the upper interface, followed by a sudden increase in the FM ratio, and, finally, a slow decrease of the FM phase component. In the case of perfect screening of the spherical mask, one expects cylindrically shaped magnetic domains. Still, the