Enhanced Orbital Magnetism in ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ Nanoparticles

X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt $L_{3,2}$ edges were measured on wet-chemically synthesized monodisperse ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ particles with a mean diameter of 6.3 nm before and after complete removal of the organic ligands and the oxide shell covering the particles by soft hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the frequency of oscillations in the extended x-ray absorption fine structure at the Pt $L_{3,2}$ edges provides evidence for crystallographic changes towards the $L{1}_0$ phase.

Figure 1

Isotropic XAS, XMCD, and integrated XMCD spectra at 15 K for ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ nanoparticles after $H$ plasma treatment (a),(b) and after annealing in $H$ plasma (c),(d) measured at the Fe $L_{3,2}$ edges (left) and at the Pt $L_{3,2}$ edges (right). The thin lines in panels (b) and (d) are Au reference spectra with a shifted and stretched energy scale. XMCD spectra at the Pt $L_{3,2}$ edges are scaled up by a factor of 4.

Figure 2

Hysteresis loops of the XMCD for ${\mathrm{Fe}}_{50}{\mathrm{Pt}}_{50}$ nanoparticles at 15 K measured at the Fe $L_3$ edge for perpendicular ($\theta =0°$, solid lines) and grazing ($\theta =75°$, dotted lines) incidence (a) after $H$ plasma treatment and (b) after 30 min annealing at $600°\mathrm{C}$. The insets show SEM images of the sample before and after annealing.