Giant and Reversible Inverse Barocaloric Effects near Room Temperature in Ferromagnetic MnCoGeB_0.03.

Abstract

In the last three decades, numerous reports of giant magnetocaloric (MC) effects near room temperature first-order magnetostructural phase transitions have led to proposals for environmentally friendly cooling1-10. However, there are two matters arising. First, giant and reversible changes of isothermal entropy ΔS and adiabatic temperature ΔT necessitate large changes of magnetic field ΔH that are challenging to generate economically. Second, most giant MC materials display magnetostructural transitions in which large changes in volume can lead to cracking and also complete mechanical failure11-15. By contrast, it is straightforward to generate large changes of hydrostatic pressure Δp in order to drive giant barocaloric (BC) effects9,10,16 near non isochoric magnetostructural phase transitions17-23, and it is straightforward to exploit a fragmented BC working body by encapsulating it together with the pressure-transmitting medium. Here we use variable pressure calorimetry to investigate giant BC effects in the well-known24,25 MC material MnCoGeB0.03 near the ~290 K paramagnetic/hexagonal to ferromagnetic/orthorhombic phase transition (PM/H to FM/O). This transition is associated with a giant change of volume (~4%) that causes this brittle material to undergo a complete mechanical failure that would be problematic in MC cooling devices26. Moderate changes of pressure (|Δp| ~1.7 kbar) drive giant and reversible MC effects of |ΔS| ~ 30 J K 1 kg-1 and |ΔT| ~ 10 K. These BC effects are similar to the MC effects that would require impractically large changes of magnetic field (μ0ΔH ~ 10 T) in order to be reversible (μ0 is the permeability of free space). Our study therefore shows that hydrostatic pressure represents an inexpensive and practical method of driving caloric effects in brittle MC materials. More generally, our study incorporates MnCoGe based compounds into the growing family of multicaloric materials27.