Angular dependence of reversible transverse susceptibility in magnetic nanoparticles

We present angular dependence measurements of reversible transverse susceptibility for two different single magnetic domain nanoparticle systems. A modified alternating gradient magnetometer was used for the measurements. This study supplements previous reversible transverse susceptibility measurements, usually performed at a limited number of angles by inductive techniques. The angular dependence measurements reveal several features in reversible transverse susceptibility of oriented magnetic nanoparticles that have previously gone undetected. The theoretically predicted peaks at the coercive magnetic field, [H.sub.C], appear only at certain narrow, primarily nonorthogonal angular orientations. In addition, we find that the magnitude of the reversible transverse susceptibility has a maximum at angles different from both the easy and hard nanoparticle axes, which might have important consequences for use of such nanoparticles in enhancement of magnetic resonance signals. Index Terms--Alternating gradient magnetometry, magnetic nanoparticle anisotropy, reversible transverse susceptibility.