José L. Sánchez Llamazares

Relative cooling power enhancement in magneto-caloric nanostructured Pr2Fe17

The magneto-caloric effect (MCE) of arc-melted bulk and 10 h ball-milled nanostructured Pr2Fe17 powders has been investigated. The maximum value for the magnetic entropy change, |�S M|, in the milled alloy is 4.5 J kg −1 K −1 for µ0H = 5 T, at around room temperature. The full width at half maximum, δTFWHM ,o f|�S M|(T ) for the nanostructured powders is about 60% greater than that of the starting bulk alloy, thus giving rise to large relative cooling power values of 573 J kg −1 (4.5 J cm −3 ) for µ0H = 5 T estimated from the product of |�S M| max × δTFWHM. These results have been compared with those of well-known magnetic materials that exhibit a large or giant MCE effect. The potential for using these low-cost iron based nanostructured Pr2Fe17 powders in magnetic refrigeration at room temperature is also discussed. (Some figures in this article are in colour only in the electronic version)

Enhanced refrigerant capacity and magnetic entropy flattening using a two-amorphous FeZrB(Cu) composite

The temperature dependence of the isothermal magnetic entropy change, ΔSM, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 − x)B, based on Fe87Zr6B6Cu1 (A) and Fe90Zr8B2 (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of ΔSM(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x ≈ 0.5. Moreover, a flattening over 80 K in the ΔSM(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration 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.

Annealing Effect on Martensitic Transformation and Magneto-Structural Properties of Ni-Mn-In Melt Spun Ribbons

We report the effect of a short-time vacuum annealing (1073 K during 10 minutes) on structural phase transition temperatures and magneto-structural properties of as-quenched ribbons of the Heusler alloy Ni50.6Mn34.5In14.9. This alloy crystallizes in a single phase cubic B2-type austenite with a Curie point of TCA=284 K that with the lowering in temperature transforms into a martensite with TCM185 K. The direct and reverse martensitic phase transition temperatures were MS=257 K, Mf = 221 K, AS = 239 K, and Af = 266 K. After annealing austenite shows the highly ordered L21-type structure while the average chemical composition as well as the structural and magnetic transition temperatures were shifted to Ni50.2Mn34.3In15.5 and MS = 253 K, Mf = 238 K, AS = 257 K, Af = 265 K, ΔT = 13 K, TCA = 299 K and TCM207 K. In the annealed samples the magnetization changes associated to the magnetic and structural transitions are more abrupt and magnetization isotherms in both the austenitic and martensitic existence region show higher initial magnetic susceptibility and faster approach to saturation. Field-cooled hysteresis loops at 10 K were shifted along the negative H-axis for both samples, but a significant anomaly was evident on the left side of the hysteresis loop for as-quenched ribbons.

The magnetocaloric effect in Er2Fe17 near the magnetic phase transition

Recent investigations in R2Fe17 intermetallic compounds have evidenced that these materials present a moderate magnetocaloric effect (MCE) near room temperature. A series of accurate magnetization measurements was carried out to show that the value of the demagnetizing factor has a significant influence on the absolute MCE value of Er2Fe17. In addition, the critical exponents determined from heat capacity and magnetization measurements allow us to describe the field dependence of the observed MCE around the Curie temperature.

Magnetostructural transitions and magnetocaloric effects in Ni50Mn35In14.25B0.75 ribbons

The structural, thermal, and magnetic behaviors, as well as the martensitic phase transformation and related magnetocaloric response of Ni50Mn35In14.25B0.75 annealed ribbons have been investigated using room-temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. Ni50Mn35In14.25B0.75 annealed ribbons show a sharper change in magnetization at the martensitic transition, resulting in larger magnetic entropy changes in comparison to bulk Ni50Mn35In14.25B0.75. A drastic shift in the martensitic transformation temperature (TM) of 70 K to higher temperature was observed for the annealed ribbons relative to that of the bulk (TM = 240 K). The results obtained for magnetic, thermal, structural, and magnetocaloric properties of annealed ribbons have been compared to those of the corresponding bulk alloys.

Magnetic structure and magneto-volume anomalies in Er2Fe17 compound

Neutron powder diffraction shows that the intermetallic Er2Fe17 compound with hexagonal crystal structure has a ferrimagnetic ground state (TC = 303 K). At T = 5 K the magnetic moments of Fe sublattice (μ ~ 2 μB) are therefore antiparalell to those of the Er one (μ ~ 9 μB), all of them lying on the basal plane. This compound exhibits strong magneto-volume effects up to temperatures in the vicinity of TC. Neutron thermo-diffraction experiments also show an anomalous temperature dependence of the cell volume, including a negative thermal expansion coefficient below 300 K. In addition, a positive spontaneous volume magnetostriction is observed up to T ~ 400 K, with a maximum (ωS ~ 0.02) located at T = 5 K.

Nanocrystalline Pr2Fe17 studied by neutron powder diffraction

We have used neutron powder diffraction to make a comprehensive study of the crystal structure in Pr2Fe17 nanocrystalline compounds. These nanostructured materials have been obtained by high-energy ball milling from an arc-melted bulk polycrystalline alloy. The Th2Zn17-type rhombohedral crystal structure of the starting bulk alloy is maintained after milling for 10 and 20 hours. The studied alloys are ferromagnetic below TC = 286?1 K (bulk) and TC ? 300?20 K (milled). These materials exhibit strong magneto-volume anomalies below TC, such as a negative coefficient of thermal expansion (?V ? -30 ? 10?6 K?1).

Thermal Annealing Influence on Magnetic and Structural Properties of Cu56Ga28Mn16 Microwires

We report on the crystalline structure, morphology and thermomagnetic properties of glass-coated magnetic microwires with Cu 56 Ga 28 Mn 16 composition, as well as the thermal annealing influence on its magneto-structural properties. As-cast CuMnGa microwires exhibit a majority cubic B2 phase, and upon annealing at temperatures up to 573 K a new hexagonal phase appears coexisting with the cubic B2 major phase. Thermal annealing treatments also shift the Curie temperature about 150 K with respect to the one for the as-cast microwire. Furthermore, the signature of a structural phase transition is observed for the microwire annealed at 523 K

Magnetocaloric Properties of Rapidly Solidified Ni 51.1 Mn 31.2 In 17.7 Heusler Alloy Ribbons

Magnetic entropy change and refrigerant capacity have been determined for a field change of 20 kOe around the second-order magnetic transition of austenite in as-quenched Ni51.1Mn31.2In17.7 alloy ribbons produced by melt spinning technique. Samples crystallize in a single-phase austenite with the highly ordered L21-type crystal structure and a Curie temperature of 275 K. The material shows a maximum magnetic entropy change of ∆SM = 1.7 JkgK, an useful working temperature range of 78 K (δTFWHM) and a refrigerant capacity of RC=132 Jkg -1 (RC= │∆SM│ x δTFWHM). The considerable RC value obtained together with the fabrication via a single-step process make austenitic Ni-Mn-In ribbons of potential interest as magnetic refrigerants for room temperature magnetic refrigeration.