Antiferromagnetic-ferromagnetic phase domain development in nanopatterned FeRh islands

The antiferromagnetic-to-ferromagnetic phase transition in B2-ordered FeRh is imaged in laterally confined nanopatterned islands using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. The resulting magnetic images directly detail the progression in the shape and size of the FM phase domains during heating and cooling through the transition. In 5-$\mu \mathrm{m}$-square islands this domain development during heating is shown to proceed in three distinct modes—nucleation, growth, and merging—each with subsequently greater energy costs. In 0.5-$\mu \mathrm{m}$ islands, which are smaller than the typical final domain size, the growth mode is stunted and the transition temperature is found to be reduced by 20 K. The modification to the transition temperature is found by high-resolution scanning transmission electron microscopy to be due to a 100-nm chemically disordered edge grain present as a result of ion implantation damage during the patterning. FeRh has unique possibilities for magnetic memory applications; the inevitable changes to its magnetic properties due to subtractive nanofabrication will need to be addressed in future work in order to progress from sheet films to suitable patterned devices.

Figure 1

FeRh nanopatterned islands. (a) Diagram of the nanofabrication process; see the text for a full explanation. (b) SEM image of the array of 0.5-$\mu \mathrm{m}$ squares. Inset: Closeup image of one of the islands taken at a ${45}^{\circ }$ angle to the film plane. The damage to the NiAl layer from the ion milling is shown. (c) In-lens detection SEM image of a 5-$\mu \mathrm{m}$ island highlighting the grain edges. (d) Cross-section TEM on the edge of a 5-$\mu \mathrm{m}$ island using annular dark-field contrast.

Figure 2

X-ray diffraction of the NiAl/FeRh sheet film in the Bragg-Brentano geometry showing the B2 crystal structure.

Figure 3

XPEEM imaging of FM domains. (a) Magnetic dichroism images of an island of side $5\mu \mathrm{m}$ shown as a function of the temperature during the cooling process (upper row of images) and the heating process (lower row). The beam direction and magnetic sensitivity are shown in the top left image. (b) Integrated absolute XMCD signal as a function of the temperature and in comparison to a VSM measurement. Both signals are normalized to the saturation value. The threshold signal is also shown; see text for details. (c), (d) Vector magnetization maps of the bottom right portion of the 5-$\mu \mathrm{m}$ island during heating. The scale bar in (c) is $1\mu \mathrm{m}$. The color represents the vector direction indicated in the color wheel and the length of an arrow represents the local XMCD magnitude. A vortex state is highlighted in (c).

Figure 4

The 363 K XMCD image from the heating arm (green) and 352 K XMCD image from the cooling arm (light red) made binary using a threshold value and overlaid for comparison (overlap, dark red). The full 5-$\mu \mathrm{m}$ island is shown. A high percentage of the domain sites is shown to overlap.

Figure 5

Characteristic sizes of FM domains. (a) Area-weighted histogram of the domain size at various temperatures during the heating process. See text for details. (b) Peak centers of Gaussian peak fits to the histograms in (a). Inset: Peak full width half-maxima (FWHM) are plotted.

Figure 6

Phase transition measured by XMCD. (a) Absolute dichroism integrated over the entire island for 0.5-, 1-, and 5-$\mu \mathrm{m}$ island sizes shown on the same scale. For the smaller two sizes an average signal over four FeRh islands has been plotted. (b) Integrated dichroism signal vs temperature for the 5-$\mu \mathrm{m}$ island. The integral is over the edge and central area of the island as indicated.

Figure 7

Size dependence of the phase transition temperature. (a) Map of the phase transition temperature for a 5-$\mu \mathrm{m}$ island. The absolute signal is used so only phase domains are shown. (b), (c) Dichroism images from a 1-$\mu \mathrm{m}$ island using the same color scheme as in Fig. 3 at the respective indicated temperatures. (d) Temperature map of the transition using the same color scale as for (a). (e), (f) Dichroism images for the 500-nm island size including four separated islands with edge effects less evident. (g) Temperature map of the highlighted island in (f), with a slightly altered color scale. Domain nucleation sites are isolated bright regions; domain propagation directions from the nucleation sites are indicated by arrows.

Figure 8

STEM analysis of the patterned island edge. (a) High-resolution STEM image of the right-hand edge of the 5-$\mu \mathrm{m}$ island shown in Fig. 1. Inset: Vertical line scan over the indicated region showing lattice fringes; these fringes are not observed in the edge grain region above and to the right of the dashed gray line. (b) Bright-field image with diffraction contrast over a larger region and dashed gray line showing continuation of grain. (c)–(e) Nanodiffraction images of points indicated in (b).