By observing the properties of Fe nanoparticle (NP) assembled films, researchers have shown that at low temperature the anisotropy energy density can prevail over the exchange energy density, thus frustrating the exchange interactions. This leads to an intriguing stepwise behaviour of the zero field cooled magnetization curve, indicating a nonmonotonic dependence of the magnetic correlation length on the temperature.

In the recent study, the scientists from academic and research institutes in Italy and Greece used femtosecond laser ablation, in vacuum, to generate iron nanoparticles (NPs), and a second UV laser pulse to control the median size and the size dispersion of Fe NPs.

Stepwise dependence

To their surprise, by measuring the zero field cooled (ZFC), and field cooled (FC) magnetization curves with a vibrating sample magnetometer, the researchers found, above a threshold value of the applied magnetic field, a stepwise dependence. This is suggestive of an iterated cross-over between a state where the anisotropy energy density prevails, characterized by plateaus, and another state with a predominance of the exchange energy density, characterized by jumps.

The presence of this competition between the anisotropy of clusters and the intercluster exchange interactions was ascribed to a system morphology characterized by the presence of some voids between clusters of a few tightly coupled NPs, as evidenced by high-resolution scanning electronic microscopy analysis, which leads to intercluster exchange interactions that are weaker than usual.

The resulting ZFC stepwise dependence on temperature was rationalized by taking into account the quasi-two-dimensionality of the system, due to film thickness and shape anisotropy of NPs, and the slow change of the anisotropy and exchange energy density with increasing temperature. Monte Carlo simulations, which model the competing effects between the anisotropy of clusters and intercluster interactions, reproduce the observed stepwise behaviour well.

Tunable interactions

The group’s results provide a new way to control the interparticle potential and to conceive nanostructured materials with peculiar and tunable properties for potential applications in sensor devices, nanoscope tips and magnetic read heads, such nanostructures being very sensitive to the local magnetic field gradient.

Full details can be found in the journal Nanotechnology 24 165706

Further reading

Magnetic recording creates complex nanoparticle patterns (May 2012)
Tiny magnets revealed under the microscope (Aug 2010)
Review: preparing magnetic nanoparticles for biomedical applications (Nov 2009)