NONMONOTONIC FIELD-DEPENDENCE OF THE ZERO-FIELD COOLED MAGNETIZATION PEAK IN SOME SYSTEMS OF MAGNETIC NANOPARTICLES

We have performed magnetic measurements on a diluted system of gamma-F e2O3 nanoparticles (d similar to 7 nm), and on a ferritin sample. In b oth cases, the zero-field cooled (ZFC) peak presents a nonmonotonic fi eld dependence, as has already been reported in some experiments, and discussed as possible evidence of resonant tunneling. Within simple as sumptions, we derive expressions for the magnetization obtained in the usual ZFC, field cooled (FC) and thermoremanent magnetization (TRM) p rocedures. We point out that the ZFC-peak position is extremely sensit ive to the width of the particle-size distribution, and give some nume rical estimates of this effect. We propose to combine the FC magnetiza tion with a modified TRM measurement, a procedure which allows a more direct access to the barrier distribution in a field. The typical barr ier values that are obtained in this procedure show a monotonic decrea se for increasing fields, as expected from the simple effect of anisot ropy barrier lowering, in contrast with the ZFC results. From our meas urements on gamma-Fe2O3 particles,we show that the width of the effect ive barrier distribution is slightly increasing with the field, an eff ect that is sufficient for causing the observed initial increase of th e ZFC-peak temperatures.