C.F. Sánchez-Valdés

Giant magnetocaloric effect in melt-spun Ni-Mn-Ga ribbons with magneto-multistructural transformation

Magnetic refrigeration based on the magnetocaloric effect (MCE) may provide an energy-efficient and environment-friendly alternative to the conventional gas compression/expansion cooling technology. For potential applications, low-cost and high-performance magnetic refrigerants are in great need. Here, we demonstrate that giant MCE can be achieved in annealed Ni52Mn26Ga22 ribbons with magneto-multistructural transformation. It yields a maximum magnetic entropy change of −30.0 J kg−1 K−1 at the magnetic field change of 5 T, being almost three times as that of initial melt-spun ribbons and comparable to or even superior to that of polycrystalline bulk alloys.

Microstructure and magnetocaloric effect of melt-spun Ni52Mn26Ga22 ribbon

Microstructural features and magnetocaloric properties of Ni52Mn26Ga22 melt-spun ribbons were studied. Results show that there are four types of differently oriented variants of seven-layered modulated (7M) martensite at room temperature, being twin-related one another and clustered in colonies. Due to the coupled magnetic and structural transformations between parent austenite and 7M martensite, the melt-spun ribbons exhibit a significant magnetocaloric effect. At an applied magnetic field of 5 T, an absolute maximum value of the isothermal magnetic entropy change of 11.4 J kg−1 K−1 is achieved with negligible hysteresis losses.

Enhanced refrigerant capacity in two-phase nanocrystalline/amorphous NdPrFe17 melt-spun ribbons

The magnetocaloric properties of NdPrFe17 melt-spun ribbons composed of nanocrystallites surrounded by an intergranular amorphous phase have been studied. The nanocomposite shows two successive second-order magnetic phase transitions (303 and 332 K), thus giving rise to a remarkable broadening (≈ 84 K) of the full-width at the half-maximum of the magnetic entropy change curve, ΔSM(T), with a consequent enhancement of the refrigerant capacity RC. For a magnetic field change of 2 T, |ΔSMpeak| = 2.1 J kg−1 K−1 and RC = 175 J kg−1. Therefore, the reversible magnetocaloric response together with the one-step preparation process makes these nanostructured Fe-rich alloy ribbons particularly attractive for room temperature magnetic refrigeration.

Texture-induced enhancement of the magnetocaloric response in melt-spun DyNi2 ribbons

The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group Fd3¯m) exhibiting a saturation magnetization and Curie temperature of MS = 157 ± 2 A m2 kg−1 and TC = 21.5 ± 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change |ΔSMpeak| = 13.5 J kg−1 K−1, and a refrigerant capacity RC = 209 J kg−1. Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications.

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the...

Refrigerant capacity of austenite in as-quenched and annealed Ni51.1Mn31.2In17.7 melt spun ribbons

The thermal dependence of the magnetic entropy change (ΔSM(T)) and refrigerant capacity (RC) of austenite in as-quenched ribbons of chemical composition Ni51.1Mn31.2In17.7 produced by melt spinning at a high cooling rate of 48 ms−1 is reported. The effect of annealing at 1073 K on the structure and the magnetic properties was studied. The as-quenched sample is a single-phase austenite that presents a B2 ordered structure. The annealing on the melt spun samples produced a L21-type ordered structure. Austenite is characterized by a broad ΔSM(T) curve that, for a field change of 5.0 T, exhibits a full-width at half-maximum δTFMHW of 107 K, a peak value of the magnetic entropy change ΔSMpeak of −3.1 Jkg−1 K−1, and RC = 345 Jkg−1. Although annealed samples show larger ΔSMpeak values the narrower ΔSM(T) curves leads to a reduction in RC. Thus, the as-quenched sample shows a higher efficiency for a refrigerant cycle.

On the broadening of the magnetic entropy change due to Curie temperature distribution

We have studied the correlation between the broadening of the isothermal magnetic entropy change and the Curie temperature (TC) distribution in nanostructured Pr2Fe17 and Nd2Fe17 alloys produced by high-energy ball-milling after milling times of 10, 20, and 40 h. The changes in the microstructure affect the Fe local environments and as a consequence the magnetic interactions, giving rise to TC distributions centered around 285 K and 330 K for the Pr2Fe17 and Nd2Fe17 alloys, respectively. The width of the distributions enlarges (up to 60 K) as the milling-time increases, and consequently, the isothermal magnetic entropy change curves show an extended full width at half maximum.

Thermal and magnetic field-induced martensitic transformation in Ni50Mn25−x Ga25Cu x (0 ⩽ x ⩽ 7) melt-spun ribbons

We have studied the phase transformation behavior of melt-spun ribbons with x = 0, 1, 2, ..., 7 (at%). It is shown that Cu substitution simultaneously increases and decreases the martensitic transformation temperature T M and the magnetic transition temperature of austenite , respectively. In Ni50Mn18Ga25Cu7 ribbons, the magnetic and structural transformations are coupled. The field-induced martensitic transformation from a paramagnetic austenite with lower magnetization to a ferromagnetic martensite with higher magnetization has been studied. The critical field μ o H cr above which the magnetic field can induce the martensitic transition is well defined and decreases linearly with the decrease of the temperature at a reduction rate of 1 T K−1. Under a magnetic field of 5 T, the starting temperature of martensitic transformation (M s) increases by ~9 K. Such a magnetic field-induced transformation is irreversible and temperature dependent, giving rise to the maximum magnetic entropy change of 17.8 J kg−1 K−1 and hysteresis losses of 43.6 J kg−1 under the magnetic field change of 5 T, respectively.

Enhanced refrigerant capacity in Gd-Al-Co microwires with a biphase nanocrystalline/amorphous structure

A class of biphase nanocrystalline/amorphous Gd(50+5x)Al(30−5x)Co20 (x = 0, 1, 2) microwires fabricated directly by melt-extraction is reported. High resolution transmission electron microscopy and Fourier function transform based analysis indicate the presence of a volume fraction (∼20%) of ∼10 nm sized nanocrystallities uniformly embedded in an amorphous matrix. The microwires possess excellent magnetocaloric properties, with large values of the isothermal entropy change (−ΔSM ∼ 9.7 J kg−1 K−1), the adiabatic temperature change (ΔTad ∼ 5.2 K), and the refrigerant capacity (RC ∼ 654 J kg−1) for a field change of 5 T. The addition of Gd significantly alters TC while preserving large values of the ΔSM and RC. The nanocrystallites allow for enhanced RC as well as a broader operating temperature span of a magnetic bed for energy-efficient magnetic refrigeration.

High-magnetic field characterization of magnetocaloric effect in FeZrB(Cu) amorphous ribbons

The magnetic and magnetocaloric properties of a series of Fe-rich FeZrB(Cu) amorphous ribbons were investigated under magnetic field values up to μ0H of 8 T. A correlation between the saturation magnetization and the maximum magnetic entropy change |ΔSMpeak| is clearly evidenced. Although these metallic glasses show relatively low |ΔSMpeak| values (from 3.6 to 4.4 J kg−1 K−1 for μ0ΔH = 8 T), the ΔSM(T) curve broadens upon the increase in μ0ΔH, giving rise to a large refrigerant capacity RC (above 900 J kg−1 for μ0ΔH = 8 T). Using the universal curve method for rescaling the ΔSM(T,μ0ΔH) curves, we found a collapse of the curves around the Curie temperature. However, in the low-temperature range the curves do not match into a single one due to the existence of magnetic frustration.