Precise control of crystallography and magnetism in focused-ion-beam transformed iron-nickel thin films

Abstract

Focused ion beam irradiation of metastable Fe$_{78}$Ni$_{22}$ thin films grown on Cu(100) substrates results in the localized transformation of the originally paramagnetic, face-centered-cubic continuous film into ferromagnetic patterns with body-centered-cubic structure. The direction of the magnetic easy axis can be controlled by the focused ion beam scanning strategy, resulting in eight differently oriented crystallographic domains with different magnetic properties. We study the local crystallographic orientations of the transformed areas by electron backscatter diffraction and correlate these results with local magnetometry measurements. The observed magnetic anisotropy can be explained as a result of residual lattice strain after the fcc$\to$bcc transformation. These results extend the understanding of this material system and its transformation and allow for the patterning of high-quality magnetic nanostructures with precisely controlled magnetization landscapes.

Figures (5)

• Figure 1: A schematics of the transformation and characterization of the metastable FeNi film. (a) Aligning the sample in SEM. (b) FIB transformation. (c) EBSD crystallographic analysis. (d) Kerr microscopy magnetic analysis.
• Figure 1: Summary of magnetic and crystallographic parameters. For each domain, the table displays the fitted magnetic easy axis (Kerr EA), rotation angles around the $x$ and $y$ axes ($\omega_x, \omega_y$), the designated major rotation axis, and calculated values of $B_1\Delta\varepsilon$ and $B_2\varepsilon_{xy}$. Strains can be obtained by dividing by material-specific constants $B_1$, $B_2$.
• Figure 2: Crystallographic EBSD measurement. Magnetic 15×15 µm$^2$ structure fabricated by 30 keV single-scan FIB irradiation, which created eight bcc domains. These domains are named after the cardinal directions – for example NNE for north-northeast. The dashed-dot-dotted line represents the scanning strategy, which is inside-out and parallel to the fcc(100) directions. In the color-coded EBSD 2D map based on the IPF data, each color represents a specific crystallographic orientation. These orientations are depicted as oriented cubes in the vicinity of the corresponding domains. The rotation angles around the ($z$, $y$, $x$) axes are in brackets.
• Figure 3: Kerr microscopy measurements. (a) Typical hysteresis loops for three angles between external magnetic field and fcc$[1\bar{1}0]$ direction from the NNE domain. (b) Polar plots of the normalized remanence ${M_\mathrm{r}}/{M_\mathrm{r}^\mathrm{max}}$ for the eight domains (black rectangles). $0^\circ$ corresponds to the external field in the fcc$[1\bar{1}0]$ direction. Red lines represent fits according to the equation \ref{['anisofit']}. Fitted easy axis directions $\alpha_0$ are displayed by red numbers.
• Figure 4: Crystallography of bcc FeNi on fcc Cu. The side view (a) shows the Nishiyama-Wassermann (NW) and Kurdjumov-Sachs (KS) orientational relationships, under the assumption that the fcc Cu{111} planes continue as bcc{110}. For the NW case, this would result in a 7.5° tilt of the bcc unit cell. The top view (b) shows that this transformation would require a large 12% in-plane expansion along the fcc[110] direction. The direction of the main tilt corresponds to the WSW and WNW domains in Fig. \ref{['fig2']}.