IRON-RHODIUM MAGNETOCALORIC ALLOY RIBBONS FOR HIGH PERFORMANCE COOLING-HEATING APPLICATIONS AND PROCESS FOR MANUFACTURING THE SAME

Abstract

A polycrystalline magnetocaloric material based on thermally annealed Fe.sub.100-xRh.sub.x melt-spun ribbons with chemical composition x in the interval 48≤x≤52 at. % and the bcc CsCl-type crystal structure (B2) and method for manufacturing the same. The material has improved magnetocaloric properties associated to first-order magneto-elastic phase transition compared to bulk alloys of similar chemical composition manufactured by conventional melting techniques; exhibiting both low-magnetic field induced giant magnetocaloric effects and enhanced refrigeration capacity close to the room temperature range, due to the fast increase of a magnetic entropy change at low fields that is followed by a broad table-like magnetic entropy change as function in the temperature curve. The material is useful as a working substance for the applications involving heating or cooling upon the removal or application of an external magnetic field, such as magnetocaloric refrigeration, heat exchangers, controllable delivery and release of bio-active substances imbedded in a thermo-sensitive polymer and local heating which destroy malignant neoplasms.

Claims

1. A ribbon shaped magnetocaloric material comprising: binary alloys of Fe.sub.100-xRh.sub.x wherein x is in the interval 48≤x≤52 at. %.

2. The material according to claim 1, further comprising micronic grains having an average size ranging from 2 to 150 μm.

3. The material according to claim 1, wherein the ribbons of the magnetocaloric material have a thickness between 10 and 50 μm.

4. The material according to claim 1, wherein the magnetocaloric material shows a giant low magnetic field-induced magnetocaloric effect associated to a first-order phase transition in both heating and cooling directions.

5. The magnetocaloric material according to claim 1, wherein a first-order AFM.fwdarw.FM, or FM.fwdarw.AFM, magnetic structure change is caused by a change in a unit cell volume of around 1% induced by the temperature.

6. The material according to claim 4, wherein said transitions is tuned into a wide temperature interval around room temperature (260≤T.sub.t≤380 K), by varying x (within the established composition limit), by effect of a thermal treatment temperature and time and by the substitution of Rh by small amounts of Pd, Cu or Au.

7. The material according to claim 1, wherein said material shows a chemically ordered bcc CsCl-type crystal structure that at a given temperature undergoes a unit cell volume expansion (or contraction) on heating (cooling) of around 1% leading to a first-order AFM.fwdarw.FM (FM.fwdarw.AFM) transition.

8. The material according to claim 1, wherein a magnetic entropy change curve shows a working temperature range δT.sub.FWHM of 17 K and 18 K at μ.sub.oΔH=2 T for heating and cooling transitions, respectively.

9. A method for preparing the material according to claim 1 comprising the steps of: a) melt-spinning a bulk alloy to form melt-spun ribbons, and b) thermal annealing the ribbons, therefore showing a large low-magnetic field induced magnetocaloric effect (|ΔS.sub.M.sup.peak|/μ.sub.oΔH>22 Jkg.sup.−1K.sup.−1T.sup.−1 for μ.sub.oΔH=0.5 T), a large adiabatic temperature change of 14.6 K (12.1 K) and a refrigerant capacity RC-2 of 238 Jkg.sup.−1K.sup.−1 (231 Jkg.sup.−1K.sup.−1) associated to AFM.fwdarw.FM (FM.fwdarw.AFM) magneto-elastic transition for a magnetic field change μ.sub.oΔH=2 T.

10. The method according to claim 9, wherein the melt spinning step produces solidification ribbons and comprises an ejection of an induction-melted molten metallic alloy onto a surface of a rotating copper wheel under Ar or He at atmosphere or vacuum.

11. The method according to claim 9, wherein a linear speed at the surface of the rotating copper wheel of the melt spinner system varies between 10 and 50 m/s.

12. The method according claim 9, wherein thermal annealing step is performed in a furnace either under vacuum or highly pure Ar, or He atmosphere.

13. The method according to claim 9, wherein an annealing temperature is between 900 and 1100° C.

14. The method according to claim 9, wherein an annealing time varies between few seconds and 72 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 shows the experimental room temperature X-ray diffraction pattern determined of thermally annealed Fe.sub.49.5Rh.sub.50.5 ribbons. As shown, all the Bragg reflections in the pattern were indexed on the basis of a single phase with the bcc CsCl-type (B2) crystal structure;

[0019] FIG. 2A shows the zero-field cooled (ZFC) and field-cooled (FC) magnetization as a function of temperature M(T) curves measured under a static magnetic field of 5 mT. The vertical dashed lines indicate the temperature range of the heating and cooling transitions;

[0020] FIG. 2B shows the heating/cooling differential scanning calorimetry (DSC) scans. The vertical dashed lines indicate the temperature range of the heating and cooling transitions; The ZFC and FC M(T) curves measured under static magnetic fields of 5 mT and 2 T (c) of thermally annealed Fe.sub.49.5Rh.sub.50.5 ribbons. Inset in FIG. 2B dM/dT vs T curves at 5 mT through the AFM.fwdarw.FM (blue open squares) and FM.fwdarw.AFM (brown open circles) transitions, respectively;

[0021] FIG. 3A shows a typical isofield magnetization curves measured under magnetic fields from 0.5 to 2.0 T (each 0.5 T) through the AFM.fwdarw.FM (heating);

[0022] FIG. 3B shows a typical isofield magnetization curves measured under magnetic fields from 0.5 to 2.0 T (each 0.5 T) through the FM.fwdarw.AFM (cooling);

[0023] FIG. 3C shows transitions and the corresponding ΔS.sub.M(T) curves for the same magnetic field change values;

[0024] FIG. 4A shows (a) the |ΔS.sub.M.sup.peak| versus μ.sub.oΔH dependence through AFM.fwdarw.FM (on heating) and FM.fwdarw.AFM (on cooling) transitions;

[0025] FIG. 4B shows the dependence of the refrigerant capacities RC-1, RC-2 and RC-3 on μ.sub.oΔH for the heating and cooling phase transitions;

[0026] FIG. 4C shows a dependence of the temperatures T.sub.hot and T.sub.cold that define the full width at half maximum of the ΔS.sub.M(T) curve with μ.sub.oΔH for both transitions;

[0027] FIG. 4D shows a thermal dependence of the hysteresis loss measured at μ.sub.oΔH=0.5 T for the FM.fwdarw.AFM transition. Inset in (d) shows the isothermal magnetization curves that were measured increasing the magnetic field up to 0.5 T and removing it down to 0 T. As shown, negligible hysteresis loss has been obtained; and

[0028] FIG. 5 shows: ΔS.sub.M(T) curves for magnetic field change values from 0.5 to 2.0 T through the AFM.fwdarw.FM (heating) and FM.fwdarw.AFM (cooling) transitions in thermally annealed Fe.sub.49.5Rh.sub.50.5 and Fe.sub.49Rh.sub.51 bulk alloys.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention is related to the abovementioned refrigeration and medical applications associated to the heating and cooling of a magnetocaloric material by the action of an external magnetic field.

[0030] The instant invention is related to a polycrystalline magnetocaloric material based on thermally annealed Fe.sub.100-xRh.sub.x melt-spun ribbons with a Rh content x in the interval 48×52 at. % and the chemically-ordered bcc CsCl-type crystal structure (also referred as B2) that shows a giant magnetocaloric effect upon the application of a low applied magnetic field change.

[0031] In the abovementioned general formula, the x increased (within the established composition limit), the thermal treatment temperature and time and the substitution of Rh by small amounts of Pd, Cu or Au, are effective to reduce the magneto-structural transition temperature T.sub.t that can be adjusted in a wide temperature interval around room temperature (260≤T.sub.t≤380 K). T.sub.t can be determined either from the peak of the endothermic (exothermic) DSC scan or the maximum of the |dM/dT(T)|.sup.max of the measured M(T) curve under a low magnetic field strength μ.sub.oH.

[0032] This material has improved magnetocaloric properties, i.e., giant low magnetic field-induced maximum magnetic entropy |ΔS.sub.M.sup.peak| and adiabatic temperature ΔT.sub.ad.sup.max changes, linked to its first-order magneto-elastic phase transition when compared with bulk alloys of similar chemical composition manufactured by conventional melting techniques. In addition, this shows an enhanced refrigerant capacity RC close to room temperature owing to its table-like magnetic entropy change as a function of temperature ΔS.sub.M(T) curve.

[0033] In said material the bcc CsCl-type (B2) crystal structure (Pearson Symbol: cP2; Space Group: Pm-3m) undergoes a temperature-induced unit cell volume change around 1% at 320-340 K that changes the magnetic structure on heating (cooling) from AFM to FM (FM to AFM). The first-order transition in both directions (i.e., heating or cooling), is accompanied by an abrupt change in magnetization, ΔM (120 Am.sup.2kg.sup.−1≤ΔM≤135 Am.sup.2kg.sup.−1). In addition, they are also sensitive to the application of an external magnetic field (i.e., both can be induced by the magnetic field). For the AFM to FM transition (this is, on heating), the material of the present invention displays a flattened magnetic entropy change curve at μ.sub.oΔH>1 T with |ΔS.sub.M.sup.peak| of 15.9 Jkg.sup.−1K.sup.−1 and a working temperature range δT.sub.FWHM=17 K, whereas the integral refrigerant capacity RC-2 and the estimated maximum adiabatic temperature change ΔT.sub.ad.sup.max are 238 Jkg.sup.−1 and 14.6 K, respectively, at μ.sub.oΔH=2 T. For the FM to AFM transition (this is, on cooling), said material at μ.sub.oΔH=2 T exhibits |ΔS.sub.M.sup.peak 1=14.4 Jkg.sup.−1K.sup.−1, δT.sub.FWHM=18 K, RC-2=231 Jkg.sup.−1 and ΔT.sub.ad.sup.max=13.1 K values. A distinctive feature of the alloys of the present invention is the large low-magnetic field which has induced a magnetocaloric effect, that has been evaluated through the relationship |ΔS.sub.M.sup.peak|/μ.sub.oΔH (Jkg.sup.−1K.sup.−1T.sup.−1).

[0034] The material according to the present invention is useful as working substance in magnetocaloric refrigerators or winter heaters, and for medical applications linked to heat release such as controllable delivery and release of bio-active substances imbedded in a thermo-sensitive polymer coating the same and local heating to destroy malignant neoplasms.

Alloy Constitution

[0035] The melt-spun samples of the present invention were produced from pure metallic elements (≥99.9%). In the melt-spun ribbons of the magnetocaloric alloys, that according to the present invention are represented by the general chemical formula Fe.sub.100-xRh.sub.x where x falls in the range 48≤x≤52 at. %, energy dispersive spectroscopy (EDS) analyses confirm that the starting chemical composition has been replicated in the ribbon specimens. In addition, melt-spun ribbon samples crystallize into a single-phase with the chemically ordered bcc CsCl-type crystalline structure; the indexed room temperature X-ray diffraction (XRD) pattern for alloy ribbons with a Fe.sub.49.5Rh.sub.50.5 composition, shown in FIG. 1, is an example of this; in this case, the cell parameter is a=2.987 Å.

Thermal and Magnetic Properties

[0036] FIGS. 2A and 2B show the low magnetic field ZFC and FC M(T) and heat flow DSC curves that were measured far in an alloy of the present instant invention, namely Fe.sub.49.5Rh.sub.50.5. These graphs reveal that the temperature T.sub.t for the AFM.fwdarw.FM and FM.fwdarw.AFM phase transitions [determined from both methods; for instance, the dM/dT vs T curves at 5 mT, shown at the inset of FIG. 2B] are approximately located at 352 K and 341 K, respectively. Notice that the obtained results from both methods are in good agreement. The ZFC and FC M(T) curves measured under a large magnetic field of 2 T. The insert in FIG. 2B, show a sharp magnetization change of 134 Am.sup.2kg at both phase transitions. For the sake of comparison, the M(T) curves at 5 mT are also plotted.

Magnetocaloric Properties

[0037] The magnetocaloric effect was mainly evaluated from the temperature dependence of the magnetic entropy change, the ΔS.sub.M(T) curves, that were obtained for several magnetic field change values, μ.sub.oΔH, from 0.5 to 2 T. For such a purpose, sets of isofield M(T) curves were measured, the examples of such M(T) curves are shown in FIGS. 3A and 3B. ΔS.sub.M(T) curves were obtained from a numerical integration of the Maxwell relation,

[00001]$\Delta S_M\left(T,{\mu }_o\Delta H\right)={\mu }_o\underset{0}{\overset{{\mu }_o{H}_{\max }}{\int }}{\left[\frac{\partial M\left(T,{\mu }_o{H}^{\prime }\right)}{\partial T}\right]}_{{\lambda }_o{H}^{\prime }}{\mathrm{dH}}^{\prime }.$

[0038] The magnetic field was applied along the major length of the magnetically studied ribbon specimens in order to minimize the internal demagnetizing magnetic field. Moreover, the refrigerant capacity RC was estimated from the following criteria: (a) RC-1=|ΔS.sub.M.sup.peak|×6 T.sub.FWHM, where δT.sub.FWHM is the full-width at half-maximum of the ΔS.sub.M(T) curve, i.e., δT.sub.FWHM=T.sub.hot−T.sub.cold; (b) from the area below the ΔS.sub.M(T) curve between the temperatures T.sub.cold and T.sub.hot. This is, RC-2=∫.sub.cold.sup.hot[ΔS.sub.M(T,μ.sub.o,ΔH)].sub.μ.sub.o.sub.ΔH dT, and; (c) by maximizing the product |ΔS.sub.M|×δT below ΔS.sub.M(T) curve, usually referred as RC-3 or Wood and Potter method [M. E. Wood, W. H. Potter, Cryogenics, Vol. 25 (1985) 667-683]. RC assesses the amount of heat that the magnetocaloric material can transfer from the cold to the hot sinks if an ideal refrigeration cycle is considered.

[0039] FIG. 3C shows the ΔS.sub.M(T) curves linked to the AFM.fwdarw.FM and FM.fwdarw.AFM transitions for magnetic field changes of 0.5, 1.0, 1.5 and 2.0 T.

[0040] FIG. 4A shows the magnetic field change dependence of 1ΔS.sub.M.sup.peak| for both transitions. A large |ΔS.sub.M.sup.peak| value is obtained for a low μ.sub.oΔH value (between 0.4 and 1.0 T). For μ.sub.oΔH=2.0 T, |ΔS.sub.M.sup.peak| reaches 15.9 and 14.4 Jkg.sup.−1K.sup.−1, while the values estimated of ΔT.sub.ad.sup.max, assuming that ΔT.sub.ad.sup.max=(T.sub.t/c.sub.o)|ΔS.sub.M.sup.peak| (where T.sub.t is the temperature at which the magneto-elastic transition occurs and c.sub.o is the specific heat just before or after the transition [A. Chirkova, K. P. Skokov, L. Schultz, N. V. Baranov, O. Gutfleisch, T. G. Woodcock, Acta Materialia, Vol. 106 (2016) 15-21]), are 14.6 K and 13.1 K, respectively. RC-1, RC-2 and RC-3 as a function of μ.sub.oΔH are given in FIG. 4B; for μ.sub.oΔH=2 T, the obtained RC-2 values for heating and cooling transitions were 238 Jkg.sup.−1K.sup.−1 and 231 Jkg.sup.−1K.sup.−1, respectively. FIG. 4C shows magnetic the field change dependence of the temperatures T.sub.hot and T.sub.cold.

[0041] The inset of FIG. 4D shows the isothermal magnetization curves measured at the temperature range of the FM.fwdarw.AFM transition increasing the magnetic field up to 0.5 T and removing it down to 0 T. FIG. 4D shows the thermal dependence of the hysteresis loss HL(T) for μ.sub.oΔH.sub.max=0.5 T originated from the effect of the field on the transition. The average hysteresis loss <HL> in the δT.sub.FWHM temperature range is negligible (i.e., 1.0 J/kg), however, ribbons show a quite large ΔS.sub.M.sup.peak=11.4 Jkg.sup.−1K.sup.−1.

[0042] A summary of the magnetocaloric properties associated to the AFM.fwdarw.FM and FM.fwdarw.AFM transitions on thermally annealed Fe.sub.49.5Rh.sub.50.5 melt-spun ribbon, for several magnetic field changes ranging from 0.5 to 2.0 T, are given in TABLE I. The parameters listed are |ΔS.sub.M.sup.peak|, |ΔS.sub.M.sup.peak|/μ.sub.oΔH, RC-1, RC-2, <HL>, δT.sub.FWHM, T.sub.hot, T.sub.cold, RC-3, δT.sup.RC-3, T.sub.hot.sup.RC-3 and T.sub.cold.sup.RC-3.

[0043] TABLE II compares the |ΔS.sub.M.sup.peak|, RC-2, δT.sub.FWHM and |ΔS.sub.M.sup.peak|/μ.sub.oΔH values of the magnetic field changes at and below 2 T obtained on thermally annealed Fe.sub.49.5Rh.sub.50.5 melt-spun ribbons at the AFM*FM and FM.fwdarw.AFM transitions with the available data reported in literature in bulk Fe.sub.100-xRh.sub.x alloys with x in the range 48≤x≤52 at. %. The method which was followed on estimating the ΔS.sub.M(T) curve, either from magnetization and calorimetric measurements, has been indicated. If a magnetic field change μ.sub.oΔH.sup.max≤1 T is considered, the material of the present invention shows a large value of the relationship |ΔS.sub.M.sup.peak|/μ.sub.oΔH in comparison with the reported in literature for bulk alloys.

TABLE-US-00001 TABLE I Annealed Fe.sub.49.5Rh.sub.50.5 AFM.fwdarw.FM transition FM.fwdarw.AFM transition melt-spun ribbon alloys (i.e., on heating) (i.e., on cooling). μ.sub.oΔH (T) 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 |ΔS.sub.M.sup.peak| (J kg.sup.−1 K.sup.−1) 11.4 14.7 15.6 15.9 11.0 13.2 14.0 14.4 |ΔS.sub.M.sup.peak|/μ.sub.oΔH (Jkg.sup.−1K.sup.−1T.sup.−1) 22.8 14.7 10.4 7.95 22.0 13.2 9.3 7.2 RC-1 (J kg.sup.−1) 49 122 194 266 51 117 184 252 RC-2 (J kg.sup.−1) 40 102 169 238 41 102 165 231 <HL> (J kg.sup.−1) — — — — 1 — — — δT.sub.FWHM (K) 5 8 12 17 5 10 14 18 T.sub.hot (K) 347 346 346 346 339 339 339 339 T.sub.cold (K) 342 338 334 329 334 329 325 321 RC-3 (J kg.sup.−1) 26 71 119 176 27 70 118 172 δT.sup.RC-3 (K) 3 6.9 11 13 3 7 11 15 T.sub.hot.sup.RC-3 (K)* 346 346 345 344 338 337 337 337 T.sub.cold.sup.RC-3 (K)* 343 339 334 331 335 330 326 322 Isofield M(T) *related to RC-3.

TABLE-US-00002 TABLE II |ΔS.sub.M.sup.peak| RC-2 δT.sub.FWHM |ΔS.sub.M.sup.peak|/μ.sub.oΔH Alloy μ.sub.oΔH (T) (Jkg.sup.−1K.sup.−1) (Jkg.sup.−1) (K) (Jkg.sup.−1K.sup.−1T.sup.−1) Method Transition REF. Fe.sub.49.5Rh.sub.50.5 0.5 11.4 40 5 22.8 M(T) AFM.fwdarw.FM Present ribbons 1.0 14.7 102 8 14.7 isofield (heating) invention 1.5 15.6 169 12 10.4 2.0 15.9 238 17 7.95 0.5 11.0 41 5 22.0 M(T) FM.fwdarw.AFM 1.0 13.2 102 10 13.2 isofield (cooling) 1.5 14.0 165 14 9.3 2.0 14.4 231 18 7.2 Fe.sub.50Rh.sub.50 2.0 16.3 201 15 8.2 M(μ.sub.oH) AFM.fwdarw.FM [1] bulk isotherms Fe.sub.49.3Rh.sub.50.7 0.5 6.1 40 8 12.2 M(T) AFM.fwdarw.FM [2] bulk 1.0 11.5 88 10 11.5 isofield 1.5 13.3 145 13 8.8 2.0 13.6 210 18 6.8 Fe.sub.49.2Rh.sub.50.8 2.0 14.7 189 15 7.4 M(T) AFM.fwdarw.FM [3] bulk isofield Fe.sub.48.9Rh.sub.51.1 2.0 11.9 199 19 6.0 M(T) AFM.fwdarw.FM bulk isofield Fe.sub.48.7Rh.sub.51.3 2.0 10.7 170 19 5.4 M(T) AFM.fwdarw.FM bulk isofield REFERENCES OF THE TABLE II [1] Radhika Barua, Félix Jiménez-Villacorta, L. H. Lewis, J. Appl. Phys., Vol. 115 (2014) 17A903. [2] A. Chirkova, K. P. Skokov, L. Schultz, N. V. Baranov, O. Gutfleisch, T. G. Woodcock, Acta Materialia, Vol. 106 (2016) 15-21. [3] A. Chirkova, F. Bittner, K. Nenkov, N. V. Baranov, L. Schultz, K. Nielsch, T. G. Woodcock, Acta Materialia, Vol. 131 (2017) 31-38

Example 1

[0044] Hereinafter, the specific method for making the magnetocaloric alloys Fe.sub.100-xRh.sub.x (48≤x≤52 at. %) with ribbon shape according to the present invention will be described through the specific example of Fe.sub.49.5Rh.sub.50.5. It should be noted, however, that the present invention is in no way limited to the filling specific example.

Method for Preparing the Magnetocaloric Material.

[0045] The magnetocaloric materials of the present invention (ribbons), with a nominal composition of Fe.sub.100-xRh.sub.x (48≤x≤52 at. %), were produced from suction-casting, arc- or induction-melted bulk pellets of the same composition by rapid solidification using the melt spinning technique under the Ar (or He) atmosphere. The linear speed of the rotating copper wheel varied from 10 to 50 m/s, resulting in ribbons with thickness from 50 to 10 μm, respectively. A thermal annealing, that was carried out at temperatures between 900 and 1100° C. for a time ranging from few seconds to 72 hours, was performed in a furnace under vacuum, or Ar, or He atmosphere, or vacuum. This thermal annealing ended with a fast quenching into oil, iced or room temperature water.

Characterization Methods.

[0046] X-ray diffraction (XRD) patterns of ribbons samples were collected with a Rigaku Smartlab high-resolution diffractometer using Cu—K.sub.alpha radiation (λ=1.5418 Å), in the 28 interval 20°≤2θ≤90°), with a step increment of 0.01°. The heating and cooling differential scanning calorimetry (DSC) scans were recorded using a TA Instruments model Q200 differential scanning calorimeter in absence of the applied magnetic field (temperature sweep rate of 10 Kmin.sup.−1).

[0047] Magnetization measurements were performed in a Quantum Design PPMS© Dynacool system using the vibrating sample magnetometer option. The magnetic field μ01-1 was applied along the major ribbon length to minimize the internal demagnetizing magnetic field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization as a function of temperature M(T) curves were measured between 300 and 380 K under static magnetic fields of 5 mT and 2 T at a temperature sweep rate of 1.0 Kmin.sup.−1.

[0048] The magnetic entropy change ΔS.sub.M(T, μ.sub.oΔH) curves were determined from a numerical integration of the Maxwell relation, i.e.,

[00002]$\Delta S_M\left(T,{\mu }_o\Delta H\right)={\mu }_o\underset{0}{\overset{{\mu }_o{H}_{\max }}{\int }}{\left[\frac{\partial M\left(T,{\mu }_o{H}^{\prime }\right)}{\partial T}\right]}_{{\lambda }_o{H}^{\prime }}{\mathrm{dH}}^{\prime }.$

[0049] For such a purpose, the sets of isofield M(T) curves, were measured with a temperature sweep rate of 1.0 Kmin.sup.−1 under applied magnetic fields from 0.05 T to 2.0 T through both the AFM.fwdarw.FM and FM.fwdarw.AFM transitions. A fixed thermal protocol, referred elsewhere as “back and forward” [A. Quintana-Nedelcos, J. L. Sánchez Llamazares, C. F. Sánchez-Valdés, P. Álvarez Alonso, P. Gorria, P. Shamba, N. A. Morley, J. Alloys Compd., Vol. 694 (2017) 1189-1195.], was followed prior to measure each isofield M(T) curve in the temperature range of the phase transition. Considering, for instance, the AFM.fwdarw.FM transition, the thermal protocol was as follows: at a zero magnetic field, the sample was first heated to 380 K to stabilize the ferromagnetic phase, cooled down to 270 K to completely reach the antiferromagnetic state, and then a given magnetic field was set to record the corresponding M(T) curve on heating. In order to minimize errors in the ΔS.sub.M(T) estimation, the magnetization versus temperature curves were measured for a large number of μ.sub.oH values. The values of RC-1, RC-2 and RC-3 were obtained from the criteria stated above (see the magnetocaloric properties section).

Example 2

[0050] After thermal annealing, bulk alloys with the chemical compositions of Fe.sub.49.5Rh.sub.50.5 and Fe.sub.49Rh.sub.51 showed a giant magnetocaloric effect in a relatively low magnetic field change associated to the first-order magneto-elastic transition in both directions (this is, through the AFM.fwdarw.FM transition, and vice versa). Bulk samples of both alloys can be produced by suction casting, arc melting or induction melting under an inert atmosphere (i.e., Ar or He). FIG. 5 shows the thermal dependence of the magnetic entropy change in bulk Fe.sub.49.5Rh.sub.50.5 and Fe.sub.49Rh.sub.51 alloys in both directions, whereas a summary of their magnetocaloric properties appear in TABLE III and IV, respectively. The estimated ΔT.sub.ad.sup.max values at μ.sub.oΔH=2 T are 13.9 K (13.3 K), and 13.3 K (12.7 K) across the in AFM.fwdarw.FM (FM.fwdarw.AFM) magneto-elastic phase transition in bulk Fe.sub.49.5Rh.sub.50.5 and Fe.sub.49Rh.sub.51 alloys, respectively.

TABLE-US-00003 TABLE III Annealed Fe.sub.49.5Rh.sub.50.5 AFM.fwdarw.FM transition FM.fwdarw.AFM transition bulk alloys (i.e., on heating) (i.e., on cooling). μ.sub.oΔH (T) 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 |ΔS.sub.M.sup.peak| (J kg.sup.−1 K.sup.−1) 9.5 12.8 14.0 14.7 9.5 12.8 13.9 14.5 |ΔS.sub.M.sup.peak|/μ.sub.oΔH (Jkg.sup.−1K.sup.−1T.sup.−1) 19.0 12.8 9.3 7.35 19.00 12.80 9.26 7.25 RC-1 (J kg.sup.−1) 45 112 187 253 42 107 175 246 RC-2 (J kg.sup.−1) 38 99 164 228 35 92 156 221 <HL> (J kg.sup.−1) — — — 66 — — — 73 δT.sub.FWHM (K) 4 8 13 17 5 9 12 17 T.sub.hot (K) 335 335 335 335 343 343 342 343 T.sub.cold (K) 331 327 322 318 338 334 330 326 RC-3 (J kg.sup.−1) 25 73 128 182 22 63 111 165 δT.sup.RC-3 (K) 3 6 11 15 3 7 10 14 T.sub.hot.sup.RC-3 (K)* 335 334 334 334 342 342 341 341 T.sub.cold.sup.RC-3 (K)* 332 328 323 319 339 335 331 327 Isofield M(T) *related to RC-3.

TABLE-US-00004 TABLE IV Annealed Fe.sub.49Rh.sub.51 AFM.fwdarw.FM transition FM.fwdarw.AFM transition bulk alloys (i.e., on heating) (i.e., on cooling). μ.sub.oΔH (T) 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 |ΔS.sub.M.sup.peak| (J kg.sup.−1 K.sup.−1) 10.8 12.9 14.4 14.8 10.4 13.3 14.3 14.6 |ΔS.sub.M.sup.peak|/μ.sub.oΔH (Jkg.sup.−1K.sup.−1T.sup.−1) 21.6 12.90 9.60 7.40 20.80 13.30 9.53 7.30 RC-1 (J kg.sup.−1) 45 112 187 253 45 113 186 254 RC-2 (J kg.sup.−1) 38 99 164 228 39 98 165 230 <HL> (J kg.sup.−1) — — — 87 — — — 85 δT.sub.FWHM (K) 4 8 13 17 4 9 13 18 T.sub.hot (K) 335 335 335 335 325 325 325 325 T.sub.cold (K) 331 327 322 318 321 316 312 307 RC-3 (J kg.sup.−1) 25 73 128 182 26 70 127 184 δT.sup.RC.sup.-3 (K) 3 6 11 15 4 7 11 16 T.sub.hot.sup.RC.sup.-3 (K)* 335 334 334 334 325 324 324 324 T.sub.cold.sup.RC-3 (K)* 332 328 323 319 321 317 313 308 Isofield M(T) *related to RC-3.