Stefano Besseghini

Structural, magnetic and anisotropic properties of Ni2MnGa melt-spun ribbons

Abstract The structural and magnetic characterisation of the melt-spun Ni 2 MnGa alloy is presented. The rapid solidification technique produces nanocrystalline ribbons with composition very close to the master alloy, which are single phase, chemically homogeneous and not brittle. The samples are highly textured with the easy magnetisation axis of the martensitic phase in the ribbon plane.

Phase transitions and magnetic entropy change in Mn-rich Ni2MnGa alloys

Thermomagnetic and magnetocaloric (MC) properties of Ni2MnGa Heusler alloys are found to be very sensitive to small composition variations. It is shown that the temperature behavior of ac susceptibility mimics that of MC properties of the samples. The coincidence of magnetic and structural transformations is found for a Mn-rich composition. As in the case of Ni-rich alloys, the magnetocaloric effect is enhanced at the transitions co-occurrence. The increase of Mn content, with respect to the stoichiometric value, rapidly decreases anisotropy field and saturation magnetization.

Temperature dependence of magnetically induced strain in single crystal samples of Ni-Mn-Ga

We investigate the effect of temperature and applied magnetic field on the strain behavior of oriented single crystal samples of Ni2MnGa. It is shown that the martensite twins can be partially oriented by the application of a magnetic field during cooling from the austenite state, observing a strain of the order of 1%. Depending on the final cooling temperature, a different value of magnetically induced strain is observed in the martensite phase, an effect due to the irreversible alignment of a fraction of the martensite twins to the magnetic field direction. The results are discussed with reference to different experimental procedures and the shape memory characteristics of the alloy.

MFM Domain Imaging of Textured Ni-Mn-Ga/MgO(100) Thin Films

Magnetic, micromagnetic, and texture properties of the martensitic thin films prepared by magnetron sputtering of Ni49.5Mn28Ga22.5 target on MgO(100) wafer are studied as a function of film thickness. The films exhibit 220-fiber texture. Moreover, in contrast to thick films, a considerable in-plane texture component is giving rise to the pronounced in-plane magnetic anisotropy found in the thin film. The texture features together with a large magnetocrystalline anisotropy lead to the particular domain structure and different magnetic anisotropy components of films depending on the film thickness and substrate nature.

Incommensurate and Commensurate Structural Modulation in Martensitic Phases of FSMA

Magnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated.

Magnetic susceptibility of martensitic Ni–Mn–Ga film

The present work deals with the magnetic anisotropy of a textured Ni51.4Mn28.3Ga20.3 thin film sputter deposited on alumina ceramic substrate. This film is a ferromagnetic martensite with a strong uniaxial magnetocrystalline anisotropy. The magnetization versus magnetic field dependencies at different angles between the direction of magnetic field and the film plane are measured with a vibrating sample magnetometer. The initial magnetic susceptibility decreases with increasing inclination with respect to the film plane. A consistent procedure is introduced for the quantitative determination of (i) the angle of the easy-magnetic axis with respect to the film normal and (ii) the ratio between the magnetic anisotropy energy and the magnetostatic energy (quality factor). These values proved to be equal to 67°±4° and 2.9±0.3, respectively.

Magnetic and mechanical properties of Ni-Mn-Ga single crystals

A method is presented to obtain large field-induced strain in NiMnGa-oriented single crystals. The magneto-mechanical results here presented are measured on an oriented single crystal of composition Ni/sub 50/Mn/sub 29.5/Ga/sub 20.5/ with {100} and {110} faces. The method relies on the stress-strain characterization of the sample. Once a repeatable pseudoplastic behavior is obtained by a sequence of mechanical cycles and thermal treatments, a combination of stress and magnetic field is applied, achieving a 1.2% irreversible strain by applying a 450 kA/m magnetic field along the [100] direction and maintaining a [100] stress value to about 12.5 MPa.

Design and thermo-mechanical analysis of a new NiTi shape memory alloy fixing clip

In this work, a new NiTi shape memory alloy (SMA) bone fixator is proposed. Thanks to the shape memory effect, this device does not need any external tool for the fixation, as the anchorage is obtained only by the self-accommodation of the clip during the parent transformation. Calorimetry and thermo-mechanical tests were used to evaluate the phase transformation temperatures and to estimate the forces generated both during the fixing surgical procedure and after the surgical operation. An application on animal anatomical sample was also performed; an appropriate mechanical tightness as well as a good handiness has been found.

Field and temperature induced giant strain in single crystal Ni-Mn-Ga

We analyze the effect of temperature coupled to dc or ac magnetic fields on the strain properties of an oriented single crystal of Ni/sub 2/MnGa. The sample has an austenite-martensite phase transformation temperature T/sub M/=160 K and reverse transformation at T/sub A/=180 K. No external stress was applied before or during the tests. It is shown that the shape memory features may be deeply influenced by the presence of external magnetic fields: during heating or cooling through the transformation an ac field applied along the [100] direction is able to trigger an irreversible strain of 2.5% [100] at T/sub A/. This value can be compared with a 1.2% [100] strain with a dc field and about 0.17% in absence of field (both at T/sub A/). The peak cyclic strain at T/sub A/ is 0.45% when a cyclic field is applied at constant temperature. It is shown that, in this single crystal, twin rearrangements leading to large values of strain only occur in limited temperature ranges, in correspondence with the phase transformations at T/sub M/ and T/sub A/.

Assessment of the Peripheral Performance and Cortical Effects of SHADE, an Active Device Promoting Ankle Dorsiflexion

Acute post-stroke rehabilitation protocols include passive mobilisation as a means to prevent contractures, but this technique could also help preserve neuromuscular activation patterns through proprioceptive information. This paper presents SHADE, an active orthosis that provides repetitive passive motion to a flaccid ankle by using shape memory alloy wire actuators. SHADE was studied to assess the promoted passive range of motion (PROM), acceptability and cortical effects. PROM and tolerability was assessed on 3 acute post-stroke patients (58±5y/o) by means of optoelectronic cinematic analysis. EEG/MEG instrumentation was utilised to evaluate sensorimotor cortical involvement during passive mobilization induced by SHADE in 7 healthy subjects (30.3±6.9y/o). These data were compared with voluntary movement (VM). SHADE produced good mobilisation across the available PROM (typically, from 5° plantarflexion to 15° dosiflexion). Acceptability in patients was also good. Suitable functional sources were identified for the primary sensorimotor areas. Cortico-muscular coherence was high not only in primary motor but also in primary somatosensory cortex. The cerebral involvement in these regions during the use of SHADE was similar to VM and significantly different from rest. This suggests that passive stimulation with SHADE could have clinical implications in supporting recovery of active functionality in stroke patients.