LETTERS Ambient pressure colossal magnetocaloric effect tuned by composition in Mn 1−x Fe x As ARIANA DE CAMPOS 1 , DANIEL L ROCCO 1 , ALEXANDRE MAGNUS G. CARVALHO 1 , LUANA CARON 1 , ADELINO A. COELHO 1 , SERGIO GAMA 1 *, LUZELI M. DA SILVA 1 , FL ´ AVIO C. G. GANDRA 1 , ADENILSON O. DOS SANTOS 1 , LISANDRO P. CARDOSO 1 , PEDRO J. VON RANKE 2 AND NILSON A. DE OLIVEIRA 2 1 Instituto de F´ ısica Gleb Wataghin, Universidade Estadual de Campinas - UNICAMP, Caixa Postal 6165, 13083-970 Campinas, S. Paulo, Brazil 2 Instituto de F´ ısica, Universidade do Estado do Rio de Janeiro—UERJ, Rua S ˜ ao Francisco Xavier, 524, 20550-013, RJ, Brazil *e-mail: gama@ifi.unicamp.br Published online: 3 September 2006; doi:10.1038/nmat1732 T he magnetocaloric effect (MCE) is the basis for magnetic refrigeration, and can replace conventional gas compression technology due to its superior efficiency and environment friendliness 1–3 . MCE materials must exhibit a large temperature variation in response to an adiabatic magnetic-field variation and a large isothermal entropic effect is also expected. In this respect, MnAs shows the colossal MCE, but the effect appears under high pressures 4 . In this work, we report on the properties of Mn 1-x Fe x As that exhibit the colossal effect at ambient pressure. The MCE peak varies from 285 K to 310 K depending on the Fe concentration. Although a large thermal hysteresis is observed, the colossal effect at ambient pressure brings layered magnetic regenerators with huge refrigerating power closer to practical applications around room temperature. The magnetocaloric effect (MCE) is important because of its potential applications in the domestic and industrial refrigeration markets. The effect is evaluated by two parameters, the adiabatic temperature variation T ad , and the isothermal entropic variation S iso for a material subjected to a magnetic-field variation 1 .A large S iso is important, because it is proportional to the material refrigerating power 1 . The possibility of tuning the transition temperature is also a key point to develop efficient active magnetic regenerator refrigerators 1–3 . For the materials exhibiting the conventional MCE, where the magnetic transitions are of the second-order type, the contribution to S iso is only of magnetic origin 1–3,5 . On the other hand, when a first-order transition occurs, the MCE is giant 6–10 and S iso also includes a considerable contribution from the lattice through the latent heat 5,11,12 . Up to now, no material exhibiting either the conventional or the giant MCE (GMCE) shows an entropic effect surpassing the magnetic limit posed by the relation R ln(2J + 1) (refs 1–3), where R is the gas constant and J is the total angular momentum of the magnetic ion. Recently, we disclosed the colossal MCE (CMCE) in MnAs under pressure 4 , which exhibits a maximum S iso 2.6 times the magnetic limit for MnAs. To account for such a great value for S iso , we proposed a model where a large contribution to the MCE is extracted from the lattice by the field variation through 12 x = 0.003 x = 0.006 x = 0.010 x = 0.015 H = 0.02 T 10 8 6 4 2 0 240 260 280 Temperature (K) Magnetization (A m 2 kg –1 ) 300 320 Figure 1 The effect on magnetization from Fe substitution for Mn in MnAs. Magnetization curves as a function of temperature for an applied magnetic field of 0.02 T (the lines are guides for the eye). the strong magnetoelastic interaction present in MnAs 4,13,14 . This lattice effect, however, is not to be mistaken as the latent heat lattice contribution 11,12 observed in the case of the GMCE. In the case of the CMCE materials the latent heat contribution is only a fraction of the observed entropic effect 4 . The colossal effect is of prime importance for applications because of the huge potential refrigerating power of the material. In addition, tunable CMCE will allow the use of layered regenerators 15 with overall refrigerating powers far greater than possible with materials exhibiting conventional or GMCE. Other materials such as Gd 5 Ge 2 Si 2 (ref. 16), MnFeP 0.8 Ge 0.2 (L. Caron et al., to be published) and the manganite La 0.8 Sr 0.2 MnO 3 (ref. 17) exhibiting similar properties (for example, strong magnetoelastic interaction) were studied under pressure, but none showed the CMCE. Measurements for La(Fe 1−x Si x ) 13 H y (ref. 18) show that pressure decreases the Curie temperature, T C , and 802 nature materials VOL 5 OCTOBER 2006 www.nature.com/naturematerials Nature Publishing Group ©2006