The effect of a weak ferromagnetic matrix on a system of nanomagnetic particles

TL;DR

This paper investigates how a weak ferromagnetic matrix, specifically graphitic carbon, influences the magnetic behavior of Ni nanoparticle systems, revealing unusual magnetic properties and interactions.

Contribution

It presents experimental evidence and interpretation of magnetic properties of Ni nanoparticles coated with graphitic carbon, highlighting the role of weak ferromagnetism in such systems.

Findings

High blocking temperature observed in Ni-graphitic carbon system

Fast relaxation times below the blocking temperature

Temperature-independent field-cooled magnetization

Abstract

The study of system of magnetic nano-particle has received increasing attention recently both because of the novel physical concepts involved and also because of their vast potential for application. The influence of background material (the substrate coating) on magnetic properties of such systems is a relatively open topic and often a full understanding is missing. In the present work we discuss our experiments and interpretation for two systems: Ni nanoparticles coated with graphitic carbon and Ni nanoparticles coated with Au. While the latter system exibits behavior typical of superparamagnetic particle systems the former shows several puzzling results such as extremely high blocking temperature ($T_{B}$), very fast relaxation time well below $T_{B}$, temperature independent field-cooled magnetization and very small coercivity and remanent magnetization. We interpret these findings as being a result of weak ferromagnetism, characteristic of the graphitic carbon. This induces strong magnetic interactions between the Ni particles in the presence of small magnetic fields. Such systems give rise to a dramatic difference in blocking temperature between measurements performed at zero field and those performed at very small magnetic fields.