Origin of Hysteresis in La 0.67 Ca 0.33 MnO 3
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Origin of Hysteresis in La0.67Ca0.33MnO3 K. Morrison1, A. Berenov2, and LF Cohen1 1 Blackett Laboratory, Prince Consort Rd, London SW7 2BZ, UK 2 Department of Materials Science, Prince Consort Rd, London SW7 2BZ, UK ABSTRACT Hysteresis is unattractive for magnetocaloric applications because it introduces loss in the cooling cycle. It is however usually associated with a first order transition and large entropy change. In this paper we review the sources of hysteresis in magnetocaloric materials and in particular in manganite systems where the nature of the transition in terms of whether it is indeed a first order transition remains elusive. INTRODUCTION The field of room temperature magnetic refrigeration is beginning to show signs of maturing. A number of promising refrigeration prototypes with large temperature span have been reported within the last twelve months. The most popular systems appear to use one of a small number of compounds including: Gd or Gd based alloys, Pr2Fe17, various manganite compositions, La(FeyCoxSi1-x-y)13, Mn1-δAs1-xSbx or MnFeP0.8Ge0.2, for example see [1]. Each material system has attractive aspects but there are some outstanding issues before a robust commercially viable product is developed. One major question is whether magnetic systems showing a first order or second order magnetocaloric transition should be used in these applications. Gd, the ideal prototype material has a second order transition. It has the disadvantage that it is a rare earth and therefore costly, its entropy change is modest although perfectly useable, but the resulting temperature span for a refrigeration cycle is more limited than systems where doping can be used to shift the transition temperature. La(Fe1-xSix)13 is certainly a tunable system with a Tc typically close to 200K that can be brought to room temperature with Co substitution for Fe[2]-[4], or hydrogenation[5]. It is low cost as it is mainly made of Fe, but there are questions of stability of the hydrogenation process and sample degradation on repeated cycling because of large volume changes associated with the transition itself. It is nevertheless a very promising system. Many of the first order systems suffer from the sample degradation on repeated thermal cycling and in addition many systems also show significant thermal and magnetic hysteresis. La(Fe1-x-yCoySix)13H is rather unique because it clearly shows a first order, field driven metamagnetic transition and yet the intrinsic hysteresis is small. Nevertheless extrinsic sources of hysteresis in this system can be significant, due in part to the large latent heat, large heat capacity and poor thermal conductivity at the metamagnetic transition, as discussed elsewhere [6]. The situation with the manganites R1-xAxMnO3 (where R is a rare earth) is interesting and their magnetocaloric properties have been investigated [7] and discussed widely [8]. The fundamental understanding associated with these materials was heavily researched in the 1990s and of course being ceramic oxides and components for f
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