Aug 2, 2013
Competing interactions and stepwise magnetization observed in Fe nanoparticle films
The macroscopic behaviour of an assembly of randomly oriented magnetic nanoparticles (NPs) strongly depends on the type (exchange, dipolar) and strength of interparticle interactions, which determine, together with the intrinsic magnetic properties, the type of technological applications. For example, for applications in magnetic sensors, based on soft magnetic materials, large intergrain (interparticle) exchange coupling with respect to the intragrain (intraparticle) anisotropy is required, whereas for applications in magnetic recording media, consisting of hard materials, intergrain exchange coupling is detrimental because it increases the media noise, and high anisotropy is required to guarantee the thermal stability of the stored information. The intermediate situation, that is - the presence of a competition between interparticle exchange energy and intraparticle anisotropy energy (magnetocrystalline, shape, magnetoelastic, etc), can induce a random frustrated magnetic state. In such a case, a variety of magnetic phenomena can be observed which are worthy of a detailed investigation to understand the underlying physics and to tailor the properties for specific application areas.
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.
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.
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
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)
About the author
The study was carried out through a collaboration between various research teams from University of Naples "Federico II" and National Research Council (CNR) in Italy, and the Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems (IAMPPNM) in Greece. The team at the Physics Department of University of Naples "Federico II" and CNR-SPIN Institute has pioneered the use of femtosecond laser ablation for the generation of NPs of different materials, and the deposition of magnetic NP-assembled films displaying peculiar properties. Dr Vincenzo Iannotti is a researcher at the University of Naples "Federico II". He performed the magnetic characterization together with Dr Giovanni Ausanio, also at University of Naples "Federico II", and has developed a phenomenological model for the interpretation of the experimental results. His research interests focus on nanostructured magnetic materials, with emphasis on magnetic nanoparticles and magnetic nanoparticle assembled films, and elastomagnetic composites for sensors. Dr Ausanio also took part in the atomic force microscopy analysis. Dr Salvatore Amoruso, also at the University of Naples "Federico II", has a strong interest in laser ablation and performed samples deposition. Profs Riccardo Bruzzese and Luciano Lanotte are heads of the laser ablation and magnetism groups, respectively, based in Naples, and guided the project. Dr Alberto Barone is a researcher at the CNR-INSTEC Institute and performed additional analysis of atomic force microscopy and high resolution scanning electron microscopy. Dr Kalliopi N Trohidou is research director at the IAMPPNM. She has a strong interest in theoretical studies and computational modelling of the magnetic behaviour of nanostructured materials. Dr George Margaris is a PhD student at the IAMPPNM. Dr Margaris and Dr Trohidou performed Monte Carlo simulations. Dr Dino Fiorani is director of the CNR-ISM institute. His research interests focus on nanostructured magnetic materials for magnetic sensors and magnetorecording, and disordered magnetic materials such as spin-glasses. Dr Fiorani contributed to the interpretation of the experimental results.