Composition-dependent ratio of orbital-to-spin magnetic moment in structurally disorderedFexPt1−xnanoparticles

The ratio of orbital-to-spin magnetic moment ${\mu }_L^{\mathrm{eff}}/{\mu }_S^{\mathrm{eff}}$ averaged over the element-specific contributions of Fe and Pt has been measured for 3-nm ${\mathrm{Fe}}_x{\mathrm{Pt}}_{1-x}$ nanoparticles at room temperature using the multifrequency electron paramagnetic resonance method for different concentrations of Fe. From a detailed $g$-factor analysis we determine that the ratio decreases from ${\mu }_L^{\mathrm{eff}}/{\mu }_S^{\mathrm{eff}}=0.049\mathrm{}$ for $x=0.43\mathrm{}$ to ${\mu }_L^{\mathrm{eff}}/{\mu }_S^{\mathrm{eff}}=0.016\mathrm{}$ for $x=0.70\mathrm{}$ which is much smaller than the bulk iron value $({\mu }_L^{\mathrm{Fe}}/{\mu }_S^{\mathrm{Fe}}=0.045).\mathrm{}$ The observed concentration dependence is much stronger than the one calculated for ${\mathrm{Fe}}_x{\mathrm{Pt}}_{1-x}$ bulk samples and reveals likely changes of the confined electronic structure of the nanoparticle system. The ratio ${\mu }_L^{\mathrm{eff}}/{\mu }_S^{\mathrm{eff}}$ takes the lowest value at the concentration $\mathrm{}(x=0.70\mathrm{})\mathrm{}$ where the magnetic anisotropy energy vanishes in bulk alloys. For $x>\mathrm{}0.72\mathrm{}$ a phase transition from a fcc to the Fe bcc structure occurs resulting in the increased bulk ratio again.