Role of diffused Co atoms in improving effective exchange coupling in $\mathrm{Sm}\text{−}\mathrm{Co}∕\mathrm{Fe}$ spring magnets

In $\mathrm{Sm}\text{−}\mathrm{Co}∕\mathrm{Fe}$ exchange-spring magnet films, the magnetization reversal processes of constituent elements and layers were studied with an emphasis on the role of diffused Co atoms. Enhanced coupling effectiveness was observed in a film with a graded interface where significant Co diffusion into the Fe layer was observed by means of electron microscopy. Comprehensive insight into the magnetization reversal processes was obtained by combining micromagnetic simulation with element- and depth-resolved x-ray resonant magnetic scattering. The approach unambiguously identifies distinctive composition profiles across the graded interface and provides the magnetization behavior of the diffused Co.

Figure 1

In-plane easy and hard axis VMS measurements at room temperature for the $T100$ and $T400$ samples are shown in (a) and (b). (c) Easy axis demagnetization curves.

Figure 2

(Color online) Cross-sectional TEM images. The white arrows indicate the location of the $\mathrm{Fe}∕\mathrm{Sm}\text{−}\mathrm{Co}$ interface. A bright field (a) and a dark field (b) image of the $T100$ sample. A bright field (c) and a dark field (d) image of the $T400$ sample. The contrast in the dark field images is due to the alignment of the grains. (e) TEM diffraction pattern for the $T400$ sample. The absence of separate Fe diffraction spots indicates that the Fe spots most likely overlap with the identified Cr spots, suggesting a good alignment between the Fe and the Cr buffer layers.

Figure 3

(Color online) Cross-sectional EDS line scan profiles from the $T100$ and $T400$ samples.

Figure 4

Element-specific magnetization curves for the $T100$ and $T400$ samples measured at $200\mathrm{K}$ using XRMS. (a) Fe, (b) Co, and (c) Sm magnetization. (d) SQUID magnetization curves at $200\mathrm{K}$.

Figure 5

(Color online) (a)–(c) Calculated electric field intensity profiles for the $T100$ sample at the $\mathrm{Fe}{L}_3$, $\mathrm{Co}{L}_3$, and $\mathrm{Sm}{M}_4$ resonant energies. (d) Concentration profiles used in the calculations. The concentration change across the $\mathrm{Sm}\text{−}\mathrm{Co}∕\mathrm{Fe}$ interface is modeled by an error function initially and then refined to better match the micromagnetic result to the SQUID and XRMS results.

Figure 6

(Color online) (a)–(c) Calculated electric field intensity profiles for the $T400$ sample at the $\mathrm{Fe}{L}_3$, $\mathrm{Co}{L}_3$, and $\mathrm{Sm}{M}_4$ resonant energies. (d) Concentration profiles used in the calculations. The concentration change across the $\mathrm{Sm}\text{−}\mathrm{Co}∕\mathrm{Fe}$ interface is modeled by an error function initially and then refined to better match the micromagnetic result to the SQUID and XRMS results.

Figure 7

(Color online) (a),(d) Comparison between micromagnetic calculations and SQUID demagnetization curves for the $T100$ and $T400$ samples. (b),(e) Simulated demagnetization curves for each 1-nm-thick sublayer. The layers $L1–L20$ represent $\mathrm{Sm}\text{−}\mathrm{Co}$ sublayers, and the layers $L21–L40$ represent Fe sublayers. (c),(f) Weighted simulation and XRMS experimental curves.