G. Porcari

Taming the First‐Order Transition in Giant Magnetocaloric Materials

Large magnetically driven temperature changes are observed in MnFe(P,Si,B) materials simultaneously with large entropy changes, limited (thermal or magnetic) hysteresis, and good mechanical stability. The partial substitution of B for P in MnFe(P,Si) compounds is found to be an ideal parameter to control the latent heat observed at the Curie point without deteriorating the magnetic properties, which results in promising magnetocaloric properties suitable for magnetic refrigeration.

Magnetocaloric effect, cyclability and coefficient of refrigerant performance in the MnFe(P, Si, B) system

MnFeP0.595Si0.33B0.075 has recently been presented as a top class magnetocaloric material combining a large magnetocaloric entropy change, a large temperature change, limited thermal hysteresis, and an enhanced mechanical stability. By providing practical rules to control the transition temperature in the MnFe(P,Si,B) system, we demonstrate that this new material was not a single composition and that a giant magnetocaloric effect (MCE) can be observed over a broad temperature range, a point of great interest for applications. As important prerequisite is the cyclability of the MCE. The thermal hysteresis and the recovery of the MCE during field oscillations have been addressed for MnFe(P,Si,B) materials. It is found that when the thermal hysteresis becomes about as large as the field induced shift of the transition, the MCE becomes partially irreversible and a strong decrease in the cyclic temperature change occurs. For an intermediate field change, typically 1 T, the limit for thermal hysteresis is about...

Co and In Doped Ni-Mn-Ga Magnetic Shape Memory Alloys: A Thorough Structural, Magnetic and Magnetocaloric Study

In Ni-Mn-Ga ferromagnetic shape memory alloys, Co-doping plays a major role in determining a peculiar phase diagram where, besides a change in the critical temperatures, a change of number, order and nature of phase transitions (e.g., from ferromagnetic to paramagnetic or from paramagnetic to ferromagnetic, on heating) can be obtained, together with a change in the giant magnetocaloric effect from direct to inverse. Here we present a thorough study of the intrinsic magnetic and structural properties, including their dependence on hydrostatic pressure, that are at the basis of the multifunctional behavior of Co and In-doped alloys. We study in depth their magnetocaloric properties, taking advantage of complementary calorimetric and magnetic techniques, and show that if a proper measurement protocol is adopted they all merge to the same values, even in case of first order transitions. A simplified model for the estimation of the adiabatic temperature change that relies only on indirect measurements is proposed, allowing for the quick and reliable evaluation of the magnetocaloric potentiality of new materials starting from readily available magnetic measurements.

Reverse Magnetostructural Transitions by Co and In Doping NiMnGa Alloys: Structural, Magnetic, and Magnetoelastic Properties

We review the composition dependence of the structural and magnetic properties of the Co-doped Ni–Mn–Ga Ferromagnetic Shape Memory Alloy around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects 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. The magnetic and structural properties display noticeable discontinuities across the martensitic transformation: remarkable values of the saturation magnetization jump at the transformation (DM), of the field dependence of the martensitic transformation temperature (dTM/dH), and of the crystalline volume change (DV/V) are reported, and are considerably enhanced by additional Indoping of the quaternary alloy. These properties give rise to a remarkable phenomenology which is of interest for multifunctional applications; magnetic superelasticity and high values of reversible strain are found.

Direct magnetocaloric characterization and simulation of thermomagnetic cycles

An experimental setup for the direct measurement of the magnetocaloric effect capable of simulating high frequency magnetothermal cycles on laboratory-scale samples is described. The study of the magnetocaloric properties of working materials under operative conditions is fundamental for the development of innovative devices. Frequency and time dependent characterization can provide essential information on intrinsic features such as magnetic field induced fatigue in materials undergoing first order magnetic phase transitions. A full characterization of the adiabatic temperature change performed for a sample of Gadolinium across its Curie transition shows the good agreement between our results and literature data and in-field differential scanning calorimetry.

Large Magnetization and Reversible Magnetocaloric Effect at the Second-Order Magnetic Transition in Heusler Materials

In contrast to rare-earth-based materials, cheaper and more environmentally friendly candidates for cooling applications are found within the family of Ni-Mn Heusler alloys. Initial interest in these materials is focused on the first-order magnetostructural transitions. However, large hysteresis makes a magnetocaloric cycle irreversible. Alternatively, here it is shown how the Heusler family can be used to optimize reversible second-order magnetic phase transitions for magnetocaloric applications.

Field Dependence of the Magnetocaloric Effect in MnFe(P,Si) Materials

The field dependence of the magnetocaloric effect (MCE) in Mn_1.22Fe_0.73P_0.47Si_0.53 is studied in terms of the entropy change (ΔS) and the temperature change (ΔT) for applied magnetic fields up to 5 and 14 T, respectively. The magnetic fields required to saturate the MCE in this system are ~1.7 and 4-5 T for ΔS and ΔT, respectively. The MCE field dependence is compared with the two approaches of the literature: 1) latent heat model and 2) the power law evolution predicted from the universal analysis of the MCE. It turns out that both of these methods are unsuitable to describe the MCE field evolution in MnFe(P,Si) materials.

Non-contact direct measurement of the magnetocaloric effect in thin samples

An experimental setup, based on a non-contact temperature sensor, is proposed to directly measure the magnetocaloric effect of samples few micrometers thick. The measurement of the adiabatic temperature change of foils and ribbons is fundamental to design innovative devices based on magnetocaloric thin materials or micro-structuring bulk samples. The reliability of the proposed setup is demonstrated by comparing the measurements performed on a bulk gadolinium sample with the results obtained by an experimental setup based on a Cernox bare chip thermoresistance and by in-field differential scanning calorimetry. We show that this technique can measure the adiabatic temperature variation on gadolinium sheets as thin as 27 μm. Heat transfer simulations are added to describe the capability of the presented technique.

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 versus 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. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

High field measurement of the magnetocaloric effect in MnFe(P, Si) materials

Recently, materials undergoing a first-order magnetic transition (FOMT) near room temperature have attracted much attentions due to the possibility to use their large magnetocaloric effect (MCE) for magnetic refrigeration. Among them, the MnFe(P, X) (X = As, Ge, Si, B) family turns out to be one of the most promising due to the large isothermal entropy change ΔS, adiabatic temperature change ΔTad, a tunable Curie temperature (TC) and the practical advantages. Till now, most of the MCE studies on MnFe(P, X) focused on the intermediate magnetic field range (B ≤ 2T) as it is the most relevant field for applications. However, extending the field range of the MCE derivation is important from both fundamental and practical points of view. On one hand, it allows one to address the field dependence of the MCE quantities, the possible influence of the critical point, etc; On the other hand, high field ΔS or ΔTad data are useful for the optimization of the MCE at intermediate field. Indeed, at first glance, one can consider for FOMT that the ΔS or ΔTad will saturate above a given field value (B*(ΔS) or B*(ΔT)). The point is that in Giant-MCE materials, it might be advantageous to bring these B* (often at high field) as close as possible to the field used in application. Understanding the field dependence of ΔS, ΔTd and quantifying the B* in MnFe(P, X) is required for further optimizations.