Room-temperature magnetic refrigeration

To reject heat or to generate "cold", refrigerators typically rely on the vapor-compression cycle of fluids such as the chlorofluorocarbon Freon. Now emerging is a new class of solid-state materials known as magnetocalorics; under an applied magnetic field or with changing temperature, they experience a large and sharp magnetic entropy change with the first-order paramagnetic-ferromagnetic phase transition. That entropy change, known as the magnetocaloric effect, makes the material an effective heat sink. In fact, magnetocalorics have already been exploited for cryogenic cooling in research settings. Recently discovered magnetocaloric metal alloys containing either gadolinium or arsenic have shown large MCE values at ambient temperatures; however, the high cost of the former and the undesirable toxicity of the latter present challenges to their commercial deployment. Inspired by scientists at the Beijing University of Technology who replaced arsenic with the relatively benign germanium, an international collaboration has now measured the germanium-containing material's MCE under varying temperatures and magnetic fields. The team also conducted neutron-scattering studies at NIST to examine the material's crystal structure. Diffraction measurements revealed that the ferromagnetic structure, when compared with the paramagnetic phase, was noticeably contracted, confirming that the phase transition is driven by structural changes. The team also discovered that in the 250- to 270-K range, a 5-tesla magnetic field induced an MCE value in the germanium compound that is considerably higher than the arsenic-containing magnetocalorics. The researchers say the MCE can be pushed even higher with further improvements in sample purity. (D. Liu et al., Phys. Rev. B. 79, 014435, 2009.) — Jermey N.A. Matthews