Efficient Room-Temperature Cooling with Magnets

Abstract

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential step for new and improved magnetic refrigerants

Kinetic-arrest-induced phase coexistence and metastability in(Mn,Fe)2(P,Si)

“…The Fe moment in the FM phase is almost twice as large as in the SDW phase, while the Mn moment is not significantly different in the FM and SDW phases. The derived magnetic moments for the FM and SDW phases from the neutron diffraction experiments are in good agreement with the values [43] obtained by density functional theory calculations for the FM and AFM phases, respectively.…”

“…This difference reflects the changes in the chemical environment around the Fe atoms during the magnetoelastic transition. Our recent studies [43] reveal that there is a significant redistribution of electronic density around the Fe atoms during the magnetoelastic transition. Also, a significant difference in the lattice parameters (see Fig.…”

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…Due to this, magnetic disorder leads to an increased DOS at E F , which improves the electronic screening of atomic displacements and thus softens the lattice . The itinerant magnetism of Fe is also observed at the heart of the excellent caloric properties of MnFe(P,Si)‐type materials . In the following, we will discuss the impact of the specific microscopic degrees of freedom on the magnetocaloric properties for three classes of first‐order materials, La–Fe–Si, Ni–Mn‐based Heusler alloys, and FeRh.…”

“…[53][54][55] Thei tinerant magnetism of Fe is also observed at the heart of the excellent caloric properties of MnFe(P,Si)-type materials. [56][57][58][59] In the following, we will discuss the impacto ft he specific microscopic degrees of freedom on the magnetocaloric properties for three classes of first-order materials,L a-Fe-Si, Ni-Mn-based Heusler alloys,and FeRh.…”

“…[74,76,77] To obtain more detailed information on the subtle interplay of itinerant magnetism and lattice entropyw ee xamined the VDOS at different temperatures above and below T t . Using temperature-dependent nuclear resonant inelastic Xray scattering (NRIXS) we measured the vibrational part of the entropy S lat .A tp resent,N RIXS measurements of the 57 Fe projected VDOS, g(e), have been performed at the Sector3 beam line at the Advanced Photon Sourcea tA rgonne National Laboratory.B yt uning the incident X-ray around the nuclear resonance of 57 Fe at 14.41 keV with a bandwidth as narrowa s1meV, [78] the 57 Fe-projected VDOS can be extracted from the measured NRIXS spectra. The spectra were acquired in the PM state and in the FM state of aL aFe 11.6 Si 1.4 powders ample,enriched with 10 % 57 Fe.…”