Proximity effects on dimensionality and magnetic ordering in Pd/Fe/Pd trialyers

The element-specific magnetization and ordering in trilayers consisting of 0.3–1.4 monolayer (ML) thick Fe layers embedded in Pd(001) has been determined using x-ray resonant magnetic scattering. The proximity to Fe induces a large moment in the Pd which extends $\sim 2\mathrm{nm}$ from the interfaces. The magnetization as a function of temperature is found to differ significantly for the Fe and Pd sublattices: The Pd signal resembles the results obtained by magneto-optical techniques with an $\mathit{apparent}$ three-dimensional (3D) to two-dimensional (2D) transition in spatial dimensionality for Fe thickness below $\sim 1\mathrm{ML}$. In stark contrast, the Fe data exhibits a $2\mathrm{D}$ behavior. No ferromagnetic signal is obtained from Fe below the $2\mathrm{D}$ percolation limit in Fe coverage $(\sim 0.7\mathrm{ML})$, while Pd shows a ferromagnetic response for all samples. The results are attributed to the temperature dependence of the susceptibility of Pd and a profound local anisotropy of submonolayered Fe.

Figure 1

Simultaneously collected structural reflectivity (top) and asymmetry ratio (bottom) from the 0.5 ML Fe sample recorded at the Pd $L_3$ edge [4-ID-D, APS] (points). Fits (line) are in excellent agreement with nominal growth parameters and show an induced Pd magnetic profile which decays on each side of the Fe interface to a distance of $\sim 2\mathrm{nm}$ (inset).

Figure 2

Element-specific hysteresis loops obtained from the 1.4 ML Fe sample at different values of $T/T_{\mathrm{C}}$. Fe (left) and Pd (right) experimental data (points) are fitted to a pair of arctan functions (line). Both elements show a ferromagnetic to paramagnetic transition at $T_{\mathrm{C}}$.

Figure 3

Normalized magnetization of the Fe (squares) and Pd (circles) sublattices for the 1.4 ML Fe sample as a function of $T/T_{\mathrm{C}}$. The best fit to an effective critical exponent is shown by solid lines with the low temperature behavior parameterized by a polynomial (broken line). Inset: Ratio of the two fitted curves shows a linear dependence with normalized temperature over the range $0.2\le T/T_{\mathrm{C}}\le 0.8$.

Figure 4

Power-law scaling behavior of the two magnetic sublattices as a function of reduced temperature plotted on base 10 logarithmic axes (left) with data offset for clarity. Effective scaling exponents as a function of nominal Fe thickness (right). The Pd exponents (circles) closely follow the exponents determined using MOKE (broken line). The Fe exponents (squares) could only be measured for samples with an Fe thickness $\ge 1$ ML.

Figure 5

Energy dependence of the asymmetry ratio (AR) determined by reversing the helicity in applied fields of $\pm 8\mathrm{mT}$ across the Fe $L_3$ and $L_2$ edges for the 0.5 (points) and 0.7 ML (line) samples at 30 K. Inset: Field dependence of AR as a function of temperature for the 0.7 ML sample.