Lluís Mañosa

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys.

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials1,2,3. Here we report on an inverse situation in Ni–Mn–Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect4,5. However, the entropy change found for some ferromagnetic Ni–Mn–Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Magnetocaloric effect and its relation to shape-memory properties in ferromagnetic Heusler alloys.

Magnetic Heusler alloys which undergo a martensitic transition display interesting functional properties. In the present review, we survey the magnetocaloric effects of Ni-Mn-based Heusler alloys and discuss their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials. We show that all these effects are a consequence of a strong coupling between structure and magnetism which enables a magnetic field to rearrange martensitic variants as well as to provide the possibility to induce the martensitic transition. These two features are respectively controlled by the magnetic anisotropy of the martensitic phase and by the difference in magnetic moments between the structural phases. The relevance of each of these contributions to the magnetocaloric properties is analysed.

Giant solid-state barocaloric effect in the Ni–Mn–In magnetic shape-memory alloy

The search for materials showing large caloric effects close to room temperature has become a challenge in modern materials physics and it is expected that such a class of materials will provide a way to renew present cooling devices that are based on the vapour compression of hazardous gases. Up to now, the most promising materials are giant magnetocaloric materials. The discovery of materials showing a giant magnetocaloric effect at temperatures close to ambient has opened up the possibility of using them for refrigeration. As caloric effects refer to the isothermal entropy change achieved by application of an external field, several caloric effects can take place on tuning different external parameters such as pressure and electric field. Indeed the occurrence of large electrocaloric and elastocaloric effects has recently been reported. Here we show that the application of a moderate hydrostatic pressure to a magnetic shape-memory alloy gives rise to a caloric effect with a magnitude that is comparable to the giant magnetocaloric effect reported in this class of materials. We anticipate that similar barocaloric effects will occur in many giant-magnetocaloric materials undergoing magnetostructural transitions involving a volume change.

Giant Electrocaloric Strength in Single‐Crystal BaTiO3

Over the last fi fteen years, the discovery of giant magnetocaloric effects near room-temperature phase transitions in various magnetic materials [ 1 , 2 ] has led to suggestions of energy-effi cient and environmentally friendly household and industrial refrigeration. However, these large changes in isothermal entropy Δ S and adiabatic temperature Δ T require large changes in magnetic fi eld Δ H , which are challenging to generate economically. In contrast, it is straightforward to generate changes in electric fi eld Δ E in order to drive electrocaloric (EC) effects near ferroelectric phase transitions. Recently, giant EC effects near nominally second-order transitions have been reported in ferroelectric thin fi lms, [ 3 , 4 ] as thin fi lms can support large driving fi elds. However, two issues arise as follows. Firstly, measurements of heat Q and temperature change Δ T are typically indirect [ 3 , 4 ] as the direct measurement of fi lms is challenging. There is thus scope for error (e.g., because the possible role of thermal and electrical hysteresis is typically ignored). Secondly, the EC effects in fi lms are disproportionately small with respect to the large driving fi elds, and so EC strengths |Q |/| E | and | T |/| E | tend to be relatively small. Here we address both of these issues by presenting direct measurements of both Q and Δ T in single-crystal BaTiO 3 (BTO) near the ferroelectric phase transition at Curie temperature T C . We fi nd EC strengths |Q |/| E | and | T |/ | E | that are giant because the fi rst-order ferroelectric phase transition is very sharp. The observed EC effects are reversible at any temperature above the hysteretic transition regime. Giant EC strengths near sharp fi rst-order phase transitions with a large latent heat could therefore contribute to the future development of cooling devices with a high frequency of operation.

Elastocaloric effect associated with the martensitic transition in shape-memory alloys.

The elastocaloric effect in the vicinity of the martensitic transition of a Cu-Zn-Al single crystal has been studied by inducing the transition by strain or stress measurements. While transition trajectories show significant differences, the entropy change associated with the whole transformation (DeltaS_(t)) is coincident in both kinds of experiments since entropy production is small compared to DeltaS_(t). The values agree with estimations based on the Clausius-Clapeyron equation. The possibility of using these materials for mechanical refrigeration is also discussed.

Effect of Co and Fe on the inverse magnetocaloric properties of Ni-Mn-Sn

At certain compositions Ni-Mn-X Heusler alloys (X: group IIIA–VA elements) undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where X is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni50Mn37Sn13, where it reaches a value of 20 J kg−1 K−1 for a field of 5 T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 and 3 at % to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at % Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at % Fe leads to an increase in the value of the entropy change to about 30 J kg−1 K−1 at 5 T.

PREMARTENSITIC TRANSITION DRIVEN BY MAGNETOELASTIC INTERACTION IN BCC FERROMAGNETIC NI2MNGA

We show that the magnetoelastic coupling between the magnetization and the amplitude of a short wavelength phonon enables the existence of a first order premartensitic transition from a bcc to a micromodulated phase in Ni2MnGa. Such a magnetoelastic coupling has been experimentally evidenced by ac susceptibility and ultrasonic measurements under an applied magnetic field. A latent heat around 9J ymol has been measured using a highly sensitive calorimeter. This value is in very good agreement with the value predicted by a proposed model. [S0031-9007(97)04508-0] Martensitic transitions (MT) are first order displacive structural phase transitions accompanied by a significant strain of the unit cell. In general, homogeneous and intracell (short-wavelength phonons) strains are necessary in order to describe the path followed by the atoms at the transformation. An interesting feature displayed by the systems undergoing this kind of first order transitions is the existence of precursor effects [1,2]. They reflect that, in a sense, the system prepares for the phase transition before it actually takes place. For instance, in bcc materials the TA2f110g phonon branch has low energy, the corresponding elastic constant (C 0 ) is very low and softening of all these phonons and C 0 occurs on cooling [2]. The Ni2MnGa Heusler alloy is investigated in the present Letter. At high temperature it is ferromagnetic (the Curie temperature is Tc › 381 K), displays a bcc structure with an L21 atomic order (space group Fm3m), and transforms martensitically at TM › 175 K. This alloy is unique in the sense that (i) it is the only known bcc ferromagnetic material undergoing a MT and (ii) the MT is preceded by a structural phase transition (intermediate transition) to a micromodulated phase (the cubic symmetry is preserved) resulting from the freezing of a q › 0.33TA2 phonon which becomes the intracell strain characterizing the new phase. Such a phase transition has been evidenced by neutron scattering [3], x-ray [4], electron microscopy [5], and ultrasonic measurements [6,7]. We have recently suggested that this transition to the premonitory (intermediate) phase is a consequence of the magnetoelastic interplay between the phonon and the magnetization [7]. At the MT the system transforms to a modulated structure with tetragonal symmetry (homogeneous strain) [8]. The modulation of the martensitic structure is different from that of the premartensitic phase. It has been argued [7] that the intermediate transition has to be first order because there is no complete softening of the frequency of the soft phonon; nevertheless, attempts in measuring a latent heat have not been successful [9] and a small thermal hysteresis has been detected [10] only in samples subjected to external stresses (which can result in a modification of the characteristics of the transition). In this Letter we present a phenomenological model for the intermediate transition based on a Landau expansion, which includes a magnetoelastic coupling. The primary order parameter is the amplitude h of a TA2 f110g phonon, and secondary order parameters are « ,a s110 df 110g homogeneous shear suitable to describe a cubic to tetragonal change of symmetry, and M, the magnetization (considered here to be a scalar). In terms of these three order parameters we assume the free energy function to have the following general form: F sh, «, Md › Fstr sh, «d 1 FmagsM d 1 Fmesh, «, Md ,

Advanced materials for solid-state refrigeration

Recent progress on caloric effects are reviewed. The application of external stimuli such as magnetic field, hydrostatic pressure, uniaxial stress and electric field give rise respectively to magnetocaloric, barocaloric, elastocaloric and electrocaloric effects. The values of the relevant quantities such as isothermal entropy and adiabatic temperature-changes are compiled for selected materials. Large values for these quantities are found when the material is in the vicinity of a phase transition. Quite often there is coupling between different degrees of freedom, and the material can exhibit cross-response to different external fields. In this case, the material can exhibit either conventional or inverse caloric effects when a field is applied. The values reported for the many caloric effects at moderate fields are large enough to envisage future application of these materials in efficient and environmental friendly refrigeration.

Cooling and heating by adiabatic magnetization in the Ni50Mn34In16 magnetic shape memory alloy

We report on measurements of the adiabatic temperature change in the inverse magnetocaloric Ni50Mn34In16 alloy. It is shown that this alloy heats up with the application of a magnetic field around the Curie point due to the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric effect associated with the martensitic transition results in the unusual decrease of temperature by adiabatic magnetization. We also provide magnetization and specific heat data which enable to compare the measured temperature changes to the values indirectly computed from thermodynamic relationships. Good agreement is obtained for the conventional effect at the second-order paramagnetic-ferromagnetic phase transition. However, at the first-order structural transition the measured values at high fields are lower than the computed ones. Irreversible thermodynamics arguments are given to show that such a discrepancy is due to the irreversibility of the first-order martensitic transition.

Vibrational properties of shape-memory alloys

Publisher Summary This chapter discusses vibrational properties of shape-memory alloys. The crystal structure of a given material depends on its energy and also its entropy, which plays an increasingly important role as temperature is raised. In the case of nonmagnetic metallic systems, the entropy has two major contributions: the electronic contribution arising from electronic states near the Fermi level and the vibrational contribution, related to the vibrations of the atoms around their equilibrium positions in the crystal lattice. The present chapter is concerned with a class of bcc-based metallic alloys, commonly called shape-memory alloys. These materials have the unique property of being able to recover from large deformations by slightly increasing their temperature. The physical mechanism behind this effect is a diffusionless, first-order structural transition, usually referred to as the martensitic transition. Martensites form when a high-temperature crystalline phase undergoes a first-order phase transition to a lower symmetry crystalline phase. This structural phase transition usually proceeds by means of the nucleation of the new phase within the initial one, and may be induced either by changing the temperature or by applying a mechanical stress.