V. Recarte

Entropy change linked to the magnetic field induced martensitic transformation in a Ni–Mn–In–Co shape memory alloy

Experimental results on the temperature dependence of the entropy change induced by magnetic field in a Ni–Mn–In–Co ferromagnetic shape memory alloy have been analyzed. Different behaviors of the entropy change ΔS versus temperature have been observed, depending on the value of polarizing magnetic field. In addition, the magnetocaloric effect shows over certain temperature range, a limit value corresponding to the transformation entropy ΔStr. To explain the experimental results, a model, which takes into account the value of the martensitic transformation temperature shift and the transformation temperature range, has been proposed. The model allows to estimate the entropy change as a function of temperature and applied magnetic field from a few experimental data and therefore a first estimation of the refrigerant capacity of the system can be done.

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.

Ordering temperatures in Cu-Al-Ni shape memory alloys

The ordering temperatures Tc1 (disordered β to B2 order) and Tc2 (B2 to L21 order) have been obtained in Cu–Al–Ni shape memory alloys with different concentrations by electrical resistivity. The dependence of the ordering temperatures on the concentration has been established. Also, a modification of the theoretical calculations has been proposed to predict the ordering temperatures in Cu–Al–Ni ternary alloys. A good agreement between the theoretical ordering temperatures and the experimental results has been found.

Magnetocaloric effect in Ni–Fe–Ga shape memory alloys

The magnetic entropy change in three different polycrystalline Ni53+xFe20−xGa27 (x=0.5,1,2) alloys was analyzed as a function of temperature under different applied magnetic fields. The temperature dependence of the ac magnetic susceptibility (χ) and the magnetization of the alloys have been used to characterize the different structural and magnetic transformations. In spite of the different magnetic states, the alloys show comparable magnetic entropy values. For x⩽1 the martensitic transformation takes place in the ferromagnetic state for measuring temperatures below room temperature, whereas the alloy with x=2 displays the martensitic transformation above room temperature between two paramagnetic phases. Maximum values of the magnetic entropy change are correlated with the martensitic transformation, irrespective of the particular magnetic state (ferromagnetic or paramagnetic) during the transformation.

Magnetic field induced martensitic transformation linked to the arrested austenite in a Ni-Mn-In-Co shape memory alloy

The so-called metamagnetic shape memory alloys transform from a ferromagnetic austenite into a weak magnetic martensitic phase, thus the application of a magnetic field, stabilizing the high magnetization phase, can induce the reverse martensitic transformation. Moreover, the martensitic transformation itself becomes arrested as its temperature range is lowered by the application of high enough magnetic fields. In this work the effect of the magnetic field on a Ni-Mn-In-Co metamagnetic shape memory has been studied by SQUID magnetometry. The arrest of the transformation produced by the field results in metastable states, whose evolution when the field is removed or reduced, follows logarithmic time dependence. The observed behavior is interpreted in terms of the magnetic contribution to the total entropy change associated with the magnetostructural transformation.

Precipitation of the stable phases in Cu-Al-Ni shape memory alloys

The Cu-Al-Ni based shape memory alloys are developed as an alternative to the classically Cu-&-Al and Ti- Ni used alloys. The main interest of these alloys horn a technological point of view is their possible use at temperatures near 200°C (l), in advantage over the Cu-&Al and Ti-Ni alloys whose maximum use temperatures are limited to 100°C (1) by several reasons. The main problem that must be solved to guarantee the Ilability of the Cu-Al-Ni alloys at high temperatures is to determine the limit of stability of the overcooled S and martensite metastable phases. In this work we study the precipitation kinetics of the y , pro-eutectoid phase and the y ,+a eutectoid decomposition between 400 “C and 482 o C. It should be noted that the y , phase has been generally named y 2 (2,3) , although we will denominate that phase y , according with the last reviews (4,5). The study of these precipitation kinetics will allows us to obtain the values of the microstructural parameters that control these processes and to estimate the stability of the 8 phase at the using temperatures of these alloys.

Entropy change linked to the magnetic field induced Morin transition in Hematite nanoparticles

The most stable form of iron oxide is Hematite (α-Fe2O3), which has interesting electronic, catalytic, and magnetic properties showing size dependent characteristics. At room temperature, Hematite is weakly ferromagnetic with a rhombohedral corundum structure. Upon cooling, the structure undergoes a first order spin reorientation, in which the net magnetic moment is lost. This transition is called the Morin transition. In this work, the first order Morin transition has been analyzed as a function of the temperature and applied magnetic field in Hematite nanoparticles. The magnetization was measured in the temperature range of the transformation at different applied magnetic fields to evaluate the entropy change linked to the Morin transition. The magnetic field promotes a shift of the transformation temperature. The change of entropy has been estimated on the basis of Clausius-Clapeyron type equation.

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.