Elena Olivetti

Stripe domains and spin reorientation transition in Fe78B13Si9 thin films produced by rf sputtering

Magnetic thin films have been obtained by rf sputtering from an amorphous Fe78B13Si9 target. The samples have been produced with thickness t ranging in the interval 25–1000 nm. Microstructural investigations indicate that the films have different microstructures varying from fully amorphous to partially nanocrystalline with increasing t. Magnetic hysteresis loops have been measured by means of high-sensitive magnetometry. A tailorable spin reorientation transition (SRT) from in-plane single-domain-like to out-of-plane multidomain state with increasing film thickness was observed. Magnetic force microscopy images have been obtained for all samples indicating that for t≤80 nm the magnetization lies in the film plane. For larger thickness, a stripe domain pattern has been observed, indicating the presence of a magnetic anisotropy axis perpendicular to the film plane. In this work, SRT and stripe domain structure have been studied as a function of thickness and sample microstructure.

Local field loop measurements by magnetic force microscopy

Magnetic force microscopy (MFM) is a valuable technique to investigate the reversal mechanisms of the magnetization in micrometric and sub-micrometric-patterned thin films that cannot be studied by means of magneto-optical methods because of their limited resolution. However, acquiring tens or hundreds of images consecutively at different applied magnetic fields is often impossible or impractical. Therefore, a field-dependent MFM-derived technique is discussed and applied on square and circular dots of different materials (Ni80Fe20, Co67Fe4Si14.5B14.5, Fe78Si9B13) having sizes ranging from 800?nm to 20??m. Experimental local hysteresis loops are obtained by properly analysing the phase signal of the MFM along a selected profile of the studied patterned structure, as a function of the applied magnetic field. Characteristic features of the magnetization process, such as vortex nucleation and expulsion, transition from C-state to saturated state or domain wall motion in Landau-like domain configuration are identified, and their evolution with the applied field is followed. The necessity to combine experimental and theoretical analyses is addressed by micromagnetic simulations on a model system (a Ni80Fe20 square dot with a lateral size of 800?nm), comparable to one of the studied samples. The agreement between experimental and simulated MFM maps, at different applied fields, and hysteresis loops provides the necessary validation for the technique. Additionally, the simulations have been proven to be necessary to understand the magnetization reversal processes occurring in the studied sub-micrometric structures and to associate them with characteristic features of the hysteresis loops measured with the proposed technique.

Magnetization Properties of FeTb Thin Films

The magnetic properties of FeTb rf-sputtered thin films have been investigated. While the as-prepared films are mainly amorphous, a successive annealing process caused the development of successive crystal phases. We observed the emergence of a variety of different domain structures, as the system crystal fraction evolves. The hysteretic behavior is shown to change as the internal stresses are released by annealing.

Ferromagnetic resonance and superparamagnetic behavior of iron oxide nanoparticles injected in porous anodic alumina

Iron oxide nanoparticles with diameter around 10nm were produced and injected in 60μm thick anodic alumina membranes with pore diameters of 20 and 100nm. The structure, magnetic properties, and the ferromagnetic resonance frequency of the nanoparticles before and after injection into the columnar arrays were measured as a function of the out-of-plane applied field. The effect of dipolar interactions and clustering mechanisms of the injected nanoparticles on the static and radio frequency magnetic response is discussed.

Rapid solidification of alloys

This paper reviews some applications of rapid solidification processing of alloys. With reference to original work of the authors, the production and properties of amorphous alloys, mainly iron based, of nanocrystalline aluminium alloys and of granular materials are described. A section is devoted to the latest advancement of thermodynamic modelling of metastable systems using CALPHAD softwares.

Fabrication of new Magnetic Micro-Machines for minimally invasive surgery

Magnetic micro-machines (MMM) have been proposed as effective tools for minimally invasive surgery. An innovative and low cost MMM manufacturing process, based on casting of a mixture of SmCo powders and acrylic resin into silicone moulds, is presented. A MMM prototype has been manufactured with this new process and its swimming ability tested by using different siliconic oils of known kinematic viscosity. The propulsion efficiency, given by the velocity and frequency ratio, has been found comparable with other MMM reported in the literature.

Fabrication of New Magnetic Micro-Machines for Minimally Invasive Surgery

Magnetic micro-machines (MMM) have been proposed as effective tools for minimally invasive surgery. An innovative and low cost MMM manufacturing process, based on casting of a mixture of SmCo powders and acrylic resin into silicone moulds, is presented. A MMM prototype has been manufactured with this new process and its swimming ability tested by using different siliconic oils of known kinematic viscosity. The propulsion efficiency, given by the velocity and frequency ratio, has been found comparable with other MMM reported in the literature.

Analysis of Magnetic Domain Patterns and Vector Hysteresis Loops in Dot/Antidot Structures

Dot and antidot structures have been prepared by means of optical lithography on Co-Fe-Si-B and permalloy thin films with thickness of 100 nm and 50 nm respectively. Magnetic force microscopy has been employed to study their surface magnetization configuration. Vector vibrating sample magnetometry has been exploited to measure hysteresis loops with the application of an in-plane magnetic field directed at different angles with respect to the dot/antidot arrays. Depending on microstructure geometry and chosen material, different and complex domain structures have been observed and discussed with the support of vector magnetization measurements.

Magnetocaloric Properties of MnBi Compound as a Function of Grain Size

MnBi is a magnetic material with many peculiar properties, interesting for different applications such as rare-earth-free permanent magnets and magnetic refrigeration. In this work the entropy change and the transformation kinetics of the magneto-structural transition are investigated, for two sets of samples with different grain size (“as-annealed” and “compacted pwd”). Both sets show a logarithm dependence on the temperature scan rate of the transition temperature upon cooling. This suggests that thermal activation effects are present. The estimated activation volume v0 of the magneto-structural transition corresponds to around 120 atoms.

Nanometer scale correlation of magnetic and structural features in Ni2MnGa

In this work we present experimental results and a discussion of the structural and magnetic features leading to a minimum magnetic correlation length of the order of 20 nm thick films of Ni2MnGa deposited on MgO substrates. The study is conducted using a vibrating sample magnetometer (VSM) for the magnetic characterization and an atomic force-magnetic force microscope (AFM-MFM) setup, which allows for simultaneous imaging of the structural and magnetic characteristics of the sample surface. The magnetization curves show a coercivity of the order of 4-10 kA/m and an anisotropy field of the order of 160-300 kA/m.