Magnetic refrigeration techniques based on the magnetocaloric effect (MCE) have recently been demonstrated as a promising alternative to conventional vapour-cycle refrigeration. In a material displaying the MCE, the alignment of randomly oriented magnetic moments by an external magnetic field results in heating. This heat can then be removed from the MCE material to the ambient atmosphere by heat transfer. If the magnetic field is subsequently turned off, the magnetic moments randomize again, which leads to cooling of the material below the ambient temperature. Here we report the discovery of a large magnetic entropy change in MnFeP0.45As0.55, a material that has a Curie temperature of about 300 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 14.5 J K-1 kg-1 and 18 J K-1 kg-1 for field changes of 2 T and 5 T, respectively. The so-called giant-MCE material Gd5Ge2Si2 (ref. 2) displays similar entropy changes, but can only be used below room temperature. The refrigerant capacity of our material is also significantly greater than that of Gd (ref. 3). The large entropy change is attributed to a field-induced first-order phase transition enhancing the effect of the applied magnetic field.

We present low-temperature volume thermal expansion, β, and specific heat, C, measurements on high-quality single crystals of CeNi 2 Ge 2 and YbRh 2 (Si 0.95 Ge 0.05 ) 2 which are located very near to quantum critical points. For both systems, β shows a more singular temperature dependence than C, and thus the Grüneisen ratio Γ ∝ β/C diverges as T → 0. For CeNi 2 Ge 2 , our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spinfluctuations. By contrast, the observed singularity in YbRh 2 (Si 0.95 Ge 0.05 ) 2 cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

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