Spatial Fluctuations of Loose Spin Coupling in $\mathrm{CuMn}/\mathrm{Co}$ Multilayers

A detailed investigation of magnetic impurity-mediated interlayer exchange coupling observed in ${\mathrm{Cu}}_{0.94}{\mathrm{Mn}}_{0.06}/\mathrm{Co}$ multilayers using polarized neutron reflectometry and magnetic x-ray techniques is reported. Excellent descriptions of temperature and magnetic field dependent biquadratic coupling are obtained using a variant of the loose spin model that takes into account the distribution of the impurity Mn ions in three dimensions. Positional disorder of the magnetic impurities is shown to enhance biquadratic coupling via a new contribution $J_2^{\mathrm{fluct}}$, leading to a temperature dependent canting of magnetic domains in the multilayer. These results provide measurable effects on RKKY coupling associated with the distribution of impurities within planes parallel to the interfaces.

Figure 1

(a) NSF (red circles, blue squares) and SF (green triangles, cyan stars) PNR data fitted to the model sketched in the inset. (b) Off-specular scattering at 30 K and 7 mT data (left) and simulations (right).

Figure 2

(a) Coupling angle and saturation field as determined by PNR (symbols) and theoretical fits (lines). (b) Experimental coupling terms (symbols) and model exchange coupling components (lines).

Figure 3

Mn $L_{2,3}$-edge absorption and x-ray magnetic circular dichroism as a function of temperature.

Figure 4

(a) Schematic coupling of a PM Mn spin between two FM layers. (b) Calculated lateral variation of $J_1^{\mathrm{LSC}}$ at 30 K. (c) RKKY exchange fields acting on loose Mn atoms derived by fits to the experimental data. Black dots indicate (111) planes in the fcc lattice. The extended plateau on either side of the plot indicates Mn impurities in the direct vicinity of the Co layers experiencing direct magnetic exchange.