Non-monotonic field-dependence of the ZFC magnetization peak in some systems of magnetic nanoparticles

TL;DR

This study investigates the non-monotonic dependence of the ZFC magnetization peak on magnetic field in nanoparticle systems, revealing the influence of particle size distribution and proposing a method to better understand barrier distributions.

Contribution

It introduces a combined FC and modified TRM measurement approach to directly access barrier distributions, clarifying the origin of non-monotonic ZFC peak behavior.

Findings

ZFC-peak position is highly sensitive to particle size distribution width.

Modified TRM measurements reveal a monotonic decrease in barrier values with increasing field.

Effective barrier distribution width slightly increases with field, explaining the initial ZFC-peak temperature rise.

Abstract

We have performed magnetic measurements on a diluted system of gamma-Fe2O3 nanoparticles (~7nm), and on a ferritin sample. In both cases, the ZFC-peak presents a non-monotonic field dependence, as has already been reported in some experiments,and discussed as a possible evidence of resonant tunneling. Within simple assumptions, we derive expressions for the magnetization obtained in the usual ZFC, FC, TRM procedures. We point out that the ZFC-peak position is extremely sensitive to the width of the particle size distribution, and give some numerical estimates of this effect. We propose to combine the FC magnetization with a modified TRM measurement, a procedure which allows a more direct access to the barrier distribution in a field. The typical barrier values which are obtained with this method show a monotonic decrease for increasing fields, as expected from the simple effect of anisotropy barrier lowering, in contrast with the ZFC results. From our measurements on gamma-Fe2O3 particles, we show that the width of the effective barrier distribution is slightly increasing with the field, an effect which is sufficient for causing the observed initial increase of the ZFC-peak temperatures.