V. Sánchez-Alarcos

Magnetocaloric effect linked to the martensitic transformation in sputter-deposited Ni–Mn–Ga thin films

In this letter, the analysis of the magnetocaloric effect (MCE) in a thin film of Ni–Mn–Ga alloy sputter deposited on alumina substrate is reported. This film has 0.4 μm thickness and exhibits merged martensitic and ferromagnetic transitions. The temperature dependence of the dc magnetization under different constant applied magnetic fields has been measured. The calculated MCE effect under applied magnetic fields up to 60 kOe shows the feasibility of these materials to be implemented in refrigeration system for functional microsystems. In addition, the shift of the martensitic transformation temperature, as a function of the applied magnetic field, has been determined.

Effect of atomic order on the martensitic and magnetic transformations in Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of long-range L2(1) atomic order on the martensitic and magnetic transformations of Ni-Mn-Ga shape memory alloys has been investigated. In order to correlate the structural and magnetic transformation temperatures with the atomic order, calorimetric, magnetic and neutron diffraction measurements have been performed on polycrystalline and single-crystalline alloys subjected to different thermal treatments. It is found that both transformation temperatures increase with increasing atomic order, showing exactly the same linear dependence on the degree of L2(1) atomic order. A quantitative correlation between atomic order and transformation temperatures has been established, from which the effect of atomic order on the relative stability between the structural phases has been quantified. On the other hand, the kinetics of the post-quench ordering process taking place in these alloys has been studied. It is shown that the activation energy of the ordering process agrees quite well with the activation energy of the Mn self-diffusion process.

Structural and magnetic properties of Cr-doped Ni?Mn?In metamagnetic shape memory alloys

The effect of the partial substitution of Mn by Cr on the structural and magnetic properties of Ni–Mn–In metamagnetic shape memory alloys is investigated. It is found that a Cr-rich second phase appears for quite low Cr concentrations, pointing out a very low solubility of Cr in Ni–Mn–In. Nevertheless, the martensitic transformation (MT) temperature of the doped alloys can be related to the variation in the electron concentration in the matrix phase, just as it occurs in the ternary Ni–Mn–In system. The effect of magnetic field on the structural transformation has been evaluated on both a ternary and a quaternary alloy. It is shown that the presence of the second phase reduces the magnetically induced shift of the MT and the associated magnetocaloric effect, thus limiting the potential applicability of Ni–Mn–In alloys. The obtained results prevent the addition of high amounts of Cr to Ni–Mn–In.

Vibrational and magnetic contributions to the entropy change associated with the martensitic transformation of Ni-Fe-Ga ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys undergo a martensitic transformation accompanied by a change in the magnetic and vibrational properties. However, these property changes are not independent. In this paper, the interplay between magnetic and vibrational properties in the martensitic transformation entropy change has been analyzed for Ni-Fe-Ga ferromagnetic shape memory alloys. The martensitic transformation entropy change has a magnetic and a vibrational contribution, ΔS(p−>m)=ΔS(vib)(p−>m) + ΔS(mag)(p−>m). Using a mean field approximation for the magnetic entropy, the full entropy ΔS(p−>m) has been decomposed and the magnetic contribution ΔS(mag)(p−>m) calculated. Upon removing the magnetic term, the vibrational entropy ΔS(vib)(p−>m) does not change substantially in the composition range where T(M) is below T(C). This latter contribution to the martensitic transformation entropy change has been analyzed using a Debye distribution for the density of states and a proportion of Einstein modes that account for the anomalous phonon mode of the austenite.

Long-range atomic order and entropy change at the martensitic transformation in a Ni-Mn-In-Co metamagnetic shape memory alloy

The influence of the atomic order on the martensitic transformation entropy change has been studied in a Ni-Mn-In-Co metamagnetic shape memory alloy through the evolution of the transformation temperatures under high-temperature quenching and post-quench annealing thermal treatments. It is confirmed that the entropy change evolves as a consequence of the variations on the degree of L21 atomic order brought by thermal treatments, though, contrary to what occurs in ternary Ni-Mn-In, post-quench aging appears to be the most effective way to modify the transformation entropy in Ni-Mn-In-Co. It is also shown that any entropy change value between around 40 and 5 J/kgK can be achieved in a controllable way for a single alloy under the appropriate aging treatment, thus bringing out the possibility of properly tune the magnetocaloric effect.

Magnetic properties of the martensitic phase in Ni-Mn-In-Co metamagnetic shape memory alloys

The magnetic ground state of the martensitic phase in metamagnetic shape memory alloys seems to be dependent on the analyzed system. In Ni2Mn1+xZ1−x (Z = In, Sn, and Sb) alloys, ferromagnetic and antiferromagnetic interactions coexist in the martensitic state. Different mechanisms, i.e., reentrant spin glasses, superparamagnetism, or superspin glasses have been proposed to explain the martensitic magnetic behavior. In this letter, the magnetic properties of the martensitic phase in Ni-Mn-In-Co alloys have been determined. The martensitic phase shows the presence of superparamagnetic domains inside a paramagnetic matrix. On cooling, superspin glass features occur when interacting clusters are frozen below a critical temperature.

Influence on the martensitic transformation of the β phase decomposition process in a Cu–Al–Ni shape memory alloy

The decomposition of the metastable β3 phase and its influence on the martensitic transformation (MT) has been analysed in a Cu–14.37Al–4.2Ni (wt%) shape memory alloy processed by powder metallurgy by differential scanning calorimetry, resonant ultrasound spectroscopy and in situ neutron powder diffraction. The pro-eutectoid γ1 precipitation, the L21 to B2 disordering process and the eutectoid decomposition have been observed during heating, followed by its subsequent dissolution at higher temperatures. The γ1 precipitation limits high-temperature applications. The evolution of the kind of induced martensite, hysteresis and transformation temperatures has been analysed during the precipitation of the γ1 phase. The elastic stresses stored during quenching and the ordering degree play an important role in the MT evolution below 300 °C. At higher temperatures, the γ1 precipitation process induces an increase of the MT transformation temperatures and hysteresis.

Vacancy dynamic in Ni-Mn-Ga ferromagnetic shape memory alloys

Vacancies control any atomic ordering process and consequently most of the order-dependent properties of the martensitic transformation in ferromagnetic shape memory alloys. Positron annihilation spectroscopy demonstrates to be a powerful technique to study vacancies in NiMnGa alloys quenched from different temperatures and subjected to post-quench isothermal annealing treatments. Considering an effective vacancy type the temperature dependence of the vacancy concentration has been evaluated. Samples quenched from 1173 K show a vacancy concentration of 1100 ± 200 ppm. The vacancy migration and formation energies have been estimated to be 0.55 ± 0.05 eV and 0.90 ± 0.07 eV, respectively.

Influence of Long-Range Atomic Order on the Structural and Magnetic Properties of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

This chapter presents a review of the most recent and systematic works performed on the study of the effect of atomic order on the structural and magnetic properties of Ni-Mn-Ga ferromagnetic shape memory alloys. It is shown that a correlation between long-range atomic order and the martensitic and Curie temperatures can be established from the analysis of its evolution under high and low temperature thermal treatments. In particular, it is demonstrated that, irrespectively of the thermal treatment, both transformation temperatures increase with the increasing L21 atomic order degree, showing practically the same linear dependence on the nextnearest- neighbors atomic order parameter, in such a way that the effect of the atomic order on the relative stability between austenite and martensite can be quantitatively determined. In this respect, it is shown that the effect of atomic order on the martensitic (and also the premartensitic) transformation is directly related to the variation of the magnetic exchange coupling.

Direct evidence of the magnetoelastic interaction in Ni2MnGa magnetic shape memory system

The martensitic transformation in stoichiometric Ni2MnGa alloys is preceded by a weakly first order transformation from a high temperature cubic phase to a near-cubic modulated intermediate phase related to the presence of a soft phonon mode. This transformation has been proposed to appear as a consequence of the magnetoelastic coupling. Inelastic neutron scattering experiment performed under external magnetic field shows a temperature shift of the characteristic energy dip at ζ ≈ 0.33. Furthermore, an enhancement of the long-wavelength limit (C′) of this branch with the applied magnetic field has been observed. Both results evidence a strong magnetoelastic interaction at the intermediate transition.