Taming the First-‐Order Transition in Giant Magnetocaloric Materials F. Guillou, G. Porcari, H. Yibole, N. van Dijk and E. Brück, Adv. Mater. (2014) doi: 10.1002/adma.201304788 Solid-‐state magnetic cooling is a promising technology for developing a new generation of gas-‐free and highly efficient refrigerators. The magnetocaloric effect (MCE) is the entropy (Δs) or temperature (ΔT) change in isothermal or adiabatic conditions respectively, resulting from the perturbation of the system state due to the action of a magnetic field. This effect, at the basis of magnetic refrigeration, is maximum near magnetic phase transitions. For practical applications in domestic or industrial refrigeration, an obvious requirement is that this transition should occur near room temperature. The search for materials showing a large, reversible and long lasting MCE under a relatively low magnetic field span is fundamental. The idea of triggering the latent heat of first-‐order magneto-‐structural solid-‐solid transformations to enhance the material’s Δs has been the main concern of the scientific community during the last 15 years. Unfortunately first-‐order processes are characterized by two main drawbacks. First, the presence of a non-‐negligible hysteresis prevents MCE reversibility thus decreasing materials efficiency. Second, the occurrence of magnetic field induced fatigue, due to the continuous stress of the phase change during the working thermodynamic cycle, weakens the microstructure and in the short-‐term makes the material useless. MnFePSi systems are Fe2P-‐based materials showing a magneto-‐elastic first-‐order Curie transition (Tc) near room temperature. Mn and Fe atoms occupy different sites with respect to non-‐magnetic P/Si atoms. The loss of magnetism across the Tc has been recently ascribed to a discontinuous change of hybridization of the electronic structure within the ‘iron layer’ between two sites occupied respectively by Fe and P/Si. Ab initio calculations predicted that while manganese keeps its local magnetic moment above Tc, for iron a strong reduction is expected on going from the ferromagnetic to the paramagnetic state. A discontinuous variation of the c/a cell parameter is observed across the ∆! transition however keeping limited the volume change ! = 0.2% . This peculiar behaviour originates a nice MCE however not fully reversible (ΔT = 1.8 K for µ0ΔH = 1 T). Though this was a good starting point, the magnetocaloric properties and mechanical stability of MnFePSi system had to be improved by reducing the hysteresis, but without depressing the magnetization of the ferromagnetic state. High magnetization jumps at the Tc are well know from literature as the key features needed to drive high ΔT values. The previously described electronic mechanism at the origin of the transition inspired the taming of the latent heat by tuning the non magnetic site to keep unmodified the magnetic moment. The substitution of phosphorus with boron in MnFePSi increases the transition temperature while progressively depressing the latent heat and weakening the first-‐order character of the transition. Within the solubility limit of boron in the parent alloy (8% of boron) MnFe0.95P0.595B0.075Si0.33 is the composition showing the best magnetocaloric properties: Δs = 10 Jkg-‐1K-‐1 and ΔT = 2.6 K for a magnetic field change of 1 T. Figure 1: Adiabatic temperature change studied in operating conditions in MnFe0.95P0.595B0.075Si0.33 system for a magnetic field change of 1 T. Figure 1 proves that the reported temperature change is almost fully reversible (we checked this on more than 10000 magnetic field cycles) due to a huge reduction of the hysteresis from 75 K for the parent compound -‐ MnFe0.95P0.67Si0.33-‐ to only 1.8 K in the boron-‐substituted alloy. The reason of this enhanced reversibility is obtained from structural characterization. XRD spectra probed that in MnFe0.95P0.595B0.075Si0.33 no volume change occurs across Tc while a discontinuous evolution of the c/a still appears. Current literature mainly focuses the lattice as the main energy reservoir to obtain high MCE values at the transition. This is the reason why materials characterized by large volume change are pointed out as the most promising. We believe on the contrary that outstanding MCE materials must show no volume change at the transition therefore while they may be better engineered by triggering another energy reservoir: the electronic structure. This work introduces a novel approach to develop a new generation of MCE materials characterized by high and reversible magnetocaloric effect.  N. H. Dung, L. Zhang, Z. Q. Ou, L. Zhao, L. van Eijck, A. M. Mulders, M. Avdeev, E. Suard, N. H. van Dijk, E. Brück, Phys. Rev. B 86, 045134 (2012).  N. H. Dung, Z. Q. Ou, L. Caron, L. Zhang, D. T. C. Thanh, G. A. de Wijs, R. A. de Groot, K. H. J. Buschow, and E. Brück, Adv. Energy Mater. 1, 1215 (2011).  R. Chandra, S. Bjarman, T. Ericsson, L. Häggström, C. Wilkinson, R. Wäppling, Y. Andersson, S. Rundqvist, J. Solid State Chem. 1980, 34, 389.
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