Glassy Magnetic Behavior and Correlation Length in Nanogranular Fe-Oxide and Au/Fe-Oxide Samples

Abstract

In nanoscale magnetic systems, the possible coexistence of structural disorder and competing magnetic interactions may determine the appearance of a glassy magnetic behavior, implying the onset of a low-temperature disordered collective state of frozen magnetic moments. This phenomenology is the object of an intense research activity, stimulated by a fundamental scientific interest and by the need to clarify how disordered magnetism effects may affect the performance of magnetic devices (e.g., sensors and data storage media). We report the results of a magnetic study that aims to broaden the basic knowledge of glassy magnetic systems and concerns the comparison between two samples, prepared by a polyol method. The first can be described as a nanogranular spinel Fe-oxide phase composed of ultrafine nanocrystallites (size of the order of 1 nm); in the second, the Fe-oxide phase incorporated non-magnetic Au nanoparticles (10–20 nm in size). In both samples, the Fe-oxide phase exhibits a glassy magnetic behavior and the nanocrystallite moments undergo a very similar freezing process. However, in the frozen regime, the Au/Fe-oxide composite sample is magnetically softer. This effect is explained by considering that the Au nanoparticles constitute physical constraints that limit the length of magnetic correlation between the frozen Fe-oxide moments.

Experimental Comparison of Methods to Evaluate Heat Generated by Magnetic Nanofluids Exposed to Alternating Magnetic Fields

“…This indicates that higher anisotropy energy barriers are associated with the C26 MNPs, compared to C5 and C12, certainly due to their larger mean size and wide size distribution, good crystallinity, and stronger dipolar interactions also by virtue of the higher M S . In this last respect, the abrupt decrease of M ZFC below ~40 K is reminiscent of a long-range collective freezing of the MNPs magnetic moments under the action of dipolar interactions, probably involving the smaller MNPs of the assembly [82][83][84]. Above T ~40 K, the linear rising trend of M ZFC , typical of a ferromagnet, confirms the presence of non-relaxing MNPs, which coexist with the fraction of relaxing ones.…”

A Guideline for Effectively Synthesizing and Characterizing Magnetic Nanoparticles for Advancing Nanobiotechnology: A Review

“…This technique also has been proposed to probe the spin disorder in magnetic nanoparticles [126]. For example, Toro et al reported that the dips in the DeltaM plots, calculated by subtracting the remanence curves, are not necessarily due to the interaction fields as they could be due to spin disorder, an inhomogeneity of spin distributions in the magnetic nanoparticles [127].…”