Forcing Ferromagnetic Coupling Between Rare-Earth-Metal and $3d$ Ferromagnetic Films

Using density functional calculations, we have studied the magnetic properties of nanocomposites composed of rare-earth-metal elements in contact with $3d$ transition metals (Fe and Cr). We demonstrate the possibility to obtain huge magnetic moments in such nanocomposites, of order $10{\mu }_B/\mathrm{\text{rare-earth-metal atom}}$, with a potential to reach the maximum magnetic moment of Fe-Co alloys at the top of the so-called Slater-Pauling curve. A first experimental proof of concept is given by thin-film synthesis of $\mathrm{Fe}/\mathrm{Gd}$ and $\mathrm{Fe}/\mathrm{Cr}/\mathrm{Gd}$ nanocomposites, in combination with x-ray magnetic circular dichroism.

Figure 1

Left: Schematic diagram of the magnetic interactions between the rare-earth-metal ($R$) and the $3d$ transition metals Fe and Cr. Right: Structure of the considered system.

Figure 2

Calculated layer-projected magnetic moments of the $R/\mathrm{Cr}/\mathrm{Fe}$ ($R=\mathrm{Gd}$, Tb, and Dy) multilayers. The atomic orbital moments of the rare-earth-metal atoms were derived from Hund’s rules and added to the calculated spin moments.

Figure 3

Calculated total energy difference between an antiferromagnetic (AFM) and a ferromagnetic (FM) ordering within the rare-earth-element layer for $R/\mathrm{Cr}/\mathrm{Fe}$ ($R=\mathrm{Gd}$, Tb, and Dy) and $\mathrm{Gd}/\mathrm{Cr}/{\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$.

Figure 4

Upper panel: XMCD measurements at the Fe $L_{3,2}$ and Gd $M_{5,4}$ absorption edges at $T=11\mathrm{K}$ in magnetic remanence exemplarily shown for the directly coupled $\mathrm{Fe}/\mathrm{Gd}$ reference sample and the ferromagnetically coupled ${\mathrm{Gd}}_{13}/{\mathrm{Cr}}_5/{\mathrm{Fe}}_{15}$ sample. Lower panel: Temperature dependence of XMCD at the Gd sites in an external magnetic field of $\pm 1\mathrm{T}$. Experimental data are represented by symbols, solid lines representing a simulation assuming two regions with different Curie temperatures due to the coupling at the interface to Fe or $\mathrm{Fe}/\mathrm{Cr}$, respectively, as depicted in the inset. Contributions of the region with enhanced Curie temperature $T_C^{*}$ are plotted by dashed lines.