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Dipartimento di Fisica e Scienze della Terra

Nanostructured magnetic materials

Involved People: Massimo Solzi, Raffaele Pellicelli, Massimo Ghidini (on leave to Cambridge, UK)

Micromagnetic modelling and experimental analysis of magnetization reversal processes in nano-composite exchange-spring systems

Exchange-spring (ES) magnets can be defined as systems in which a soft and a hard ferromagnetic phases are coupled at the nanometric scale through the exchange interaction. Hard/soft bi-and multi-layers have been realized with the aim of obtaining planar permanent magnets with large maximum energy product. Moreover, the ES coupling has recently found a viable application in the field of perpendicular magnetic recording. In that case, ES recording media constituted by isolated nanometric grains should help in reducing the writing field while maintaining a high thermal stability, mainly thanks to the hard phase magneto-crystalline anisotropy.

The main objective of this Research Line is the analysis of the magnetization reversal processes in ES systems, in particular the magnetic relaxation phenomena, in order to understand the influence of the exchange coupling at the hard/soft interface on the thermal stability over long time intervals. A specific expertise has been reached in the field of magnetic characterization and in that of micromagnetic and statistical modeling of magnetic hard films and hard/soft exchange-spring (ES) bilayers, structured on the nanometric scale. The development of analytical models and numerical simulations will be, on the one hand, a guide for the nanostructures design and, on the other hand, it will allow understanding the physical processes inherent to the observed phenomena that would be hardly accessible to direct observation. In real systems, the interface plays a crucial role at the nanometric scale in determining the exchange coupling properties and the reversal process features.

MIUR PRIN2003 - 2004-2005
MIUR PRIN2005 - 2006-2007
MIUR PRIN2008 - 2010-2011

New experimental techniques

Development of new experimental techniques, especially suited for the study of magnetic properties of nanostructured systems (thin films, multilayers, metastable phases, ...). The aim is to develop high sensitivity instruments capable to measure tiny specimens of complex and often metastable magnetic systems. The difficulties are connected with the necessity to perform the measurements either in very high or very weak magnetic fields, in a wide temperature interval and in very short times.

The Vibrating Wire Susceptometer (VWS) is based on the measurement of the vibration amplitude of a metallic thin wire, driven in resonance by an alternating force on the magnetic sample, which is fixed at the wire center.

The Singular Point Detection technique (SPD) was formerly developed for the measurement of anisotropy field of hard magnetic materials in polycrystalline samples. In principle there are possible extensions of this technique to different types of materials: non-collinear magnetic structures (for example canted two-sublattice systems) and nanostructured systems (especially exchange-spring magnets and magnetic multilayers). The usual method is to employ high pulsed magnetic fields. However it is also possible to utilize modulation technique in quasi-static fields.