J.L. Sánchez Llamazares

Martensitic phase transformation in rapidly solidified Mn50Ni40In10 alloy ribbons

Heusler alloy Mn50Ni40In10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering. A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains. The characteristic temperatures of the martensitic transformation were MS=213K, Mf=173K, AS=222K, and Af=243K. Austenite phase shows a cubic L21 structure (a=0.6013(3)nm at 298K and a Curie point of 311K), transforming into a modulated fourteen-layer modulation monoclinic martensite.

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni50.3Mn35.5Sn14.4 ribbons around both the structural and the magnetic transitions for a field of 20kOe. The maximum entropy changes at the structural and magnetic transitions were of 4.1 and −1.1Jkg−1K−1. Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition.

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.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

Magnetocaloric effect in preferentially textured Mn50Ni40In10 melt spun ribbons

Inverse and direct magnetocaloric properties were evaluated in preferentially textured Mn50Ni40In10 ribbons applying the magnetic field H∥ along the ribbon length and perpendicular H⊥ to the ribbon plane (ΔH=30 kOe). Maximum magnetic entropy change, hysteretic losses, and refrigerant capacity were not significantly affected by crystallographic texture. Refrigeration capacity around structural transition is strongly reduced by the large hysteretic losses associated to the metamagnetic field-induced reverse martensitic transformation and narrower working temperature range making the interval around the magnetic transition more efficient for a refrigerant cycle (RCstruct=71 J kg−1 versus RCstructeff≈60 J kg−1, and RCmagn=89–86 J kg−1, for H∥ and H⊥, respectively).

Magnetic and morphological study of BaZn2Fe16O27 hexagonal ferrite prepared by chemical coprecipitation method

Polycrystalline samples of the BaZn2Fe16O27 (BaZn2‐W) ferrite prepared by the chemical coprecipitation method have been examined by measuring the magnetic properties (saturation magnetization, coercive and anisotropy field, Curie temperature), thermomagnetic analysis, Mossbauer spectroscopy, and scanning electron microscopy. The correlations between the magnetic properties, the grain morphology, and the relative amounts of the various phases present in samples subjected to different heat treatments are discussed. The samples prepared by optimal heat treatment show a saturation magnetization value σs =77.7 emu/g and an anisotropy magnetic field Ha =12.5 kOe. The magnetic coercive field value is low (iHc ∼150 Oe); this may be related to the large‐grain dimension (≥12 μm) of the material obtained. The Curie temperatures measured for the BaZn2‐W phase vary from 370 to 410 °C depending on the heat treatment of the samples.

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.

Martensitic transformation in Ni50.4Mn34.9In14.7 melt spun ribbons

Single phase microcrystalline ribbon flakes with the average elemental composition Ni50.4Mn34.9In14.7 were produced by rapid quenching using melt spinning technique. Fracture cross section micrographs of ribbons show the formation of a columnar-like microstructure, with the longer axis of grains aligned perpendicular to ribbon plane. X-ray diffraction and thermomagnetic analysis show that samples are single phase with L21-type austenite as high-temperature parent phase (Curie point of 284 K). At low temperatures austenite transforms into a ten-layered structurally modulated monoclinic martensite. The characteristic phase transition temperatures and thermal hysteresis of the reversible martensite–austenite transformation were MS = 262 K, Mf = 245 K, AS = 262 K, Af = 270 K and ΔT = 10 K. The crystalline directions [2 2 0] of austenite and [1 2 5] of martensite were found preferentially oriented normal to the ribbon plane. The measurement of magnetization isotherms up to 80 kOe confirmed the occurrence of the field-induced reverse martensitic transformation.