Neutron spin echo evidence of mesoscopic spin correlations among Fe(Cu) ferromagnetic nanoparticles in a silver diamagnetic matrix

Abstract

We show that the neutron spin echo technique provides direct evidence of the interparticle spin correlations in the magnetic relaxation of a heterogeneous material of nanometric $4.6(1)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ferromagnetic Fe(Cu) particles, randomly embedded in a silver diamagnetic matrix. At the mesoscopic level sampled by neutron spin echo, a transition around $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is found, with an increased magnetic anisotropy. The small Fe(Cu) interparticle distance $(\ensuremath{\gtrsim}2\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ triggers giant spin dipolar interactions in a metallic matrix in which the Ruderman-Kittel-Kasuya-Yosida coupling is negligible.