group » Research » Materials

The main challenger for applications of the magnetocaloric effect is the optimization of magnetic materials, which should be non-toxic, with good thermal conductivity, of low cost and, mainly, with magnetic entropy change as high as possible. In this direction, our group deals with several families of magnetic materials, below detailed.

» Pr(Ni_1-xCo_x)₅

This work focuses on the pseudobinary alloy Pr(Ni_5−xCo_x). From one side, the compound PrNi₅ does not show a magnetic order down to very low temperatures (0,40 mK). On the other side, PrCo₅ is a well know hard magnet, with a high magnetic ordering temperature (880K). The aim of this work is to search a composition that has its T_C near room temperature (RT) and therefore study its magnetocaloric properties. Following this approach, we prepared several samples between x=2 (T_C=115 K) and x=3 (T_C=538 K), namely: 2.3, 2.35, 2.40, 2.45, 2.50 and 2.55. For all samples we found a broad magnetic entropy change curve (100 K) leading to a RCP of 28 J/kg, value useful for applications. We found the Curie temperature around RT for the PrNi_2.7Co_2.3 sample. Based on these facts, Pr(Ni_5−xCo_x) is a competitive series, since, for instance, it has a RCP value larger than the promising Ni₂MnGa shape memory alloys. Summarizing, in this work we study the magnetocaloric, magnetic and structural properties of this series and therefore ratify that intermetallic compounds are also promising magnetocaloric materials.

Publications:

1. Not available yet

» MnP

In this work we deal with the magnetic and magnetocaloric properties of manganese monophosphate (MnP) single crystal; motivated by (i) the colossal MCE recently reported in MnAs compound [1] and (ii) the theoretical interest of the influence of magnetic anisotropy on the magnetocaloric properties of materials [2]. For low value of magnetic field, decreasing temperature, MnP becomes ferromagnetic (FM) at Tc =291 K, with its magnetic moment aligned along the c (easy) axis. Further cooling leads to a first order transition at Ts=47 K, from FM to a SCREW structure. This behavior is observed when the magnetic field is applied parallel to all three main crystal axis [a,b,c]. However, for higher values of magnetic field, different phase diagrams are observed with a rich variety of magnetic arrangements. We have characterized the magnetic properties of the MnP in a wide range of temperatures and various orientations by means of vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR). Thus, we study the magnetic entropy change through those rich magnetic phase diagrams (H vs T for the three main crystallographic axis) and therefore the influence of the magnetic anisotropy on the magnetocaloric properties of MnP single crystal. We found positive and negative magnetic entropy change, due to changes in the magnetic arrangement, and also an interesting flat entropy change as a function of temperature by rotating the crystal, instead of changing the magnetic field.

[1] A. Campos et al., Nature Materials 5 (2006) 802. ::: PDF

[2] P.J. von Ranke et al., Phys. Rev. B. (2007) in press.

 

Publications:

1. Not available yet.

» Manganites

    »» (Pr,Ca)MnO₃

We studied the magnetic entropy change DS_M of the Pr_1−xCa_xMnO₃ manganites, for a wide range of Ca concentrations (0.20<x<0.95). The results for the samples with 0.20<x<0.30 present the usual behavior expected for ferromagnetic systems, peaking at the Curie temperature T_C. In contrast, for the charge-ordered antiferromagnetic samples (0.30<x<0.90), an anomalous magnetic entropy change starts around the charge-ordering temperature T_CO, persisting for lower values of temperature. This effect is associated to a positive contribution to the magnetic entropy change due to the charge-ordering DS_CO, which is superimposed to the negative contribution from the spin-ordering DS_spin; that is described using a mean-field approximation. Supposing these contributions, we could also appraise DS_CO^max as a function of Ca content, which vanishes for the limits x~0.30 and 0.90 and presents a deep minimum around x~0.50, with two maxima at x~0.35 and 0.65. We conclude that for x~0.65 only the magnetic order governs the charge ordering, contrarily to x~0.65, where there are more than one mechanism ruling the charge ordering. Moreover, for the samples with phase coexistence (0.30<x<0.40), we found extremely large values for the magnetic entropy change at low temperatures. Finally, for x~0.90, we found usual magnetic entropy change curves, peaking at the Néel temperature T_N.

    »» (La,Sr)MnO₃

La_0.70Sr_0.30MnO₃ (LSMO) is an optimally doped ferromagnetic material manganite that presents considerable magnetocaloric effect. However, the operating temperature (T_C) of LSMO is near 375 K, which limits it's applicability in room-temperature refrigeration. In light of this, we have studied solid state and sol-gel produced samples of the LSMO system where La is substituted by Er and Eu, and show that the operating temperature can be significantly controlled by substitution to values near room temperature and also that the relative cooling power (RCP) of Er and Eu substituted samples is significantly increased. The regular dependence of T_C with the amount of rare-earth substitution and the increase in RCP allow the creation of composites or thermal cascades of chemically similar compounds to produce an active  magnetic element with a wide temperature interval of operation (from 260 to 370 K). This work presents relatively cost-effcient materials for application in room-temperature magnetic cooling.

Publications:

2. Physical Review B 71 (2005) 144413 ::: PDF

1. Materials Science Forum 514 (2006) 299 ::: PDF

3. Journal of Magnetism and Magnetic Materials 290 (2005) 697 ::: PDF

4. Journal of Magnetism and Magnetic Materials 290 (2005) 686 ::: PDF

5. Journal of Magnetism and Magnetic Materials 290 (2005) 694 ::: PDF

6. Acta Physica Polonica A 105 (2004) 163 ::: PDF

7. Applied Physics Letters 85 (2004) 4974 ::: PDF

8. Materials Science Forum 455 (2004) 148 ::: PDF

9. Journal of Magnetism and Magnetic Materials 272 (2004) 2385 ::: PDF

10. Journal of Magnetism and Magnetic Materials 272 (2004) 2393 ::: PDF

11. Journal of Magnetism and Magnetic Materials 242 (2002) 668 ::: PDF

» Ni₂MnGa

under construction...