A series of nanogranular Co/Nb samples has been prepared using an unfiltered beam of Co nanoparticles preformed by inert gas condensation. The preparation technique is shown to be a simple and effective method for fabricating, in a single deposition step, a sample series across which both particle size and concentration vary. We estimate the presence of weak interparticle (dipole-dipole) interactions ranging from 7 to 19$\%$ in strength (normalized to the median anisotropy energy barrier) across the present series. With the aim of elucidating the effect of such interactions on the blocking behavior of such nanogranular material, we have studied the field and temperature dependence of the magnetization in the films. For each sample, the temperature of the maximum in the zero-field-cooled magnetization curve ($T_{\text{MAX}}$) is found to lie between the values of blocking ($T_{\text{B}}$) and freezing ($T_{\text{F}}$) temperature estimated from the experimentally determined particle size and concentration; i.e., $T_{\text{B}}<T_{\text{MAX}}<T_{\text{F}}$. Furthermore, the deviation of $T_{\text{MAX}}$ with respect to $T_{\text{B}}$ correlates with the estimated strength of the interparticle interaction. These results support the Dormann-Bessais-Fiorani model, which predicts an enhancement of the effective particle anisotropy barrier in the weak-interaction regime. Our study also provides information on (i) the oxidation of nanoparticles in granular systems and (ii) the size-dependent divergence of nanoparticles ejected from a cluster source.

• Received 21 September 2011

DOI:https://doi.org/10.1103/PhysRevB.85.054429