From direct to inverse Giant Magnetocaloric Effect in Co-doped NiMnGa multifunctional alloys

We report the magnetic and magnetocaloric properties in Co-doped Ni–Mn–Ga Heusler alloys around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed

Leghe ferromagnetiche a memoria di forma Ni-Mn-Ga: una nuova sinergia tra struttura e proprietà

Il composto intermetallico Ni2MnGa appartiene alla famiglia delle leghe di Heusler, con formula generaleX2YZ. In generale le composizioni ternarie di tipo Ni-Mn-Z con Z appartenente agli elementi dei gruppi IIIA-VA,hanno recentemente catalizzato un crescente interesse nel campo scientifico internazionale. Questi materialimultifunzionali hanno infatti dimostrato di possedere un’ampia varietà di proprietà intrinseche che li rendeattrattivi per svariati campi di applicazioni [1]. Si annoverano infatti diversi effetti che possono essere controllaticon l’applicazione di un campo magnetico; deformazioni giganti (MFIS – Magnetic Field Induced Strain o MSM– Magnetic Shape Memory) [2], proprietà magnetocaloriche (MCE) [3, 4], comportamenti magnetoelastici emagnetoresistivi [5,6]. L’interazione tra struttura e magnetismo è all’origine di questa straordinaria fenomenologia.In questo contributo vengono presentati i principali fenomeni alla base dei comportamenti polifunzionali e unapanoramica dei più recenti avanzamenti in campo scientifico con l’obiettivo di portare le leghe Ni-Mn-Z ad unafase matura per uno sviluppo pre-industriale. In questa memoria verrà dato risalto a due fondamentali proprietà :l’effetto a memoria di forma magnetico e l’effetto magnetocalorico

preliminary investigation on a rotary magnetocaloric refrigerator prototype

Abstract Environmental legislations are currently imposing important restrictions to regulate the use of refrigerant fluids in order to reduce the greenhouse gases emissions and global warming potential. To overcome these issues, a valid alternative to replace conventional refrigeration systems can be represented by magnetic refrigeration. Since magnetic refrigeration is based on the magnetocaloric effect it represents an environmental friendly technology that avoids the use of Chlorinated refrigerants. In this paper a preliminary analysis of a novel magnetocaloric refrigerator is presented. The magnetocaloric refrigeration prototype uses Gadolinium as refrigerant and water as heat exchange medium, and relies on permanent magnets as magnetic field source. The device operates according to the active regenerative principle with a rotary movement. A detailed description of the main components included in the design of the prototype device is presented along with a schematic representation of the hydraulic circuit. Focusing on the regenerators beds, some simulations have been carried out to quantify the heat energy fluxes between water and gadolinium. The results of the simulations show a decrease on gadolinium temperature distribution cycle by cycle highlighting the actual effect of the regeneration

Pressure dependence of magneto-structural properties of Co-doped off-stoichiometric Ni2MnGa alloys

A strong effect of pressure on magnetization and paramagnetic moment of the Co-doped Mn-rich Ni 50-x Co x Mn 25+y Ga 25-y (x = 5,7,9 and y = 5,6,7,8) Heusler alloys is presented and compared with very weak pressure sensitivity of magnetization of the stoichiometric Ni 2 MnGa alloy. The effects of both, the pressure and the magnetic field, on temperature of the structural martensitic transition in the alloys are discussed with a use of the Clausius-Clapeyron relations. An analysis of pressure and field effects provides a possibility to evaluate structural and magnetic parts of latent heat of the martensitic transitions in the studied alloys. The Curie temperature of martensite phase of the Co-rich alloys is not affected by pressure

Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys

We report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy vs. temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material

Spin disordered resistivity of the Heusler Ni₂MnGa-based alloys

Electrical resistivity of the selected Heusler off-stoichiometric (NiCo)₂Mn(GaIn) alloys was studied in a wide range of temperature and magnetic field. A step-like change of resistivity (Δρ ≈24 μΩcm) was detected in the off-stoichiometric Ni_{1.85}Mn_{1.21}Ga_{0.94} alloy at temperature of martensitic structural transition. This Δρ is much more significant than one in the stoichiometric Ni₂MnGa alloy. In the case of the off-stoichiometric (NiCo)₂Mn(GaIn) alloys, an enormous change of resistivity, Δρ ≈ 200 μΩcm, accompanies the structural transition. Simultaneously, the maximum of the spin disordered resistivity ρ_{sd}(T) of austenite phase of the alloys is slightly dependent on composition of the alloy and vary from ≈30 μΩcm up to ≈45 μΩcm, in good agreement with theoretical calculations. Due to high sensitivity of the structural transition temperature of the alloys to magnetic field, the very pronounced magnetoresistance effects have been observed in the studied alloys