Element-resolved thermodynamics of magnetocaloric LaFe(13-x)Si(x)

Caloric materials for cooling and heating

Magnetically driven thermal changes in magnetocaloric materials have, for several decades, been exploited to pump heat near room temperature. By contrast, their electrocaloric and mechanocaloric counterparts have only been intensively studied and exploited for little more than a decade. These different caloric strands have recently been unified to yield a single field of research that could help combat climate change by generating better heat pumps for both cooling and heating. Here we outline the timeliness of the present activity and discuss recent advances in caloric measurements, materials, and prototypes.

Extraordinary magnetic properties of double perovskite Eu2CoMnO6wide band gap semiconductor

Some novel magnetic behaviours in double perovskite Eu2CoMnO6(ECMO) have been reported. The x-ray photoemission spectroscopy study shows the presence of mixed valence states of transition metal ions. The UV-visible absorption spectroscopic study suggests that the ECMO has a direct wide band gap. A second-order magnetic phase transition as a sudden jump in the magnetization curve has been observed around 124.5 K. The large bifurcation between the zero field cooling and field cooling, suggests existence of strong spin frustration in the system. The inverse DC susceptibility confirms the presence of the Griffiths like phase. Sharp steps in magnetization have been observed in theM-Hcurve at 2 K, which vanishes on increasing temperature. The AC susceptibility study demonstrates the Hopkinson like effect as well as the presence of volume spin-glass-like behaviour. The temperature dependent Raman spectrum shows the presence of spin-phonon coupling.

Magnetocaloric Effect, Magnetoresistance of Sc0.28Ti0.72Fe2, and Phase Diagrams of Sc0.28Ti0.72Fe2−xTx Alloys with T = Mn or Co

(Sc,Ti)Fe2 Laves phases present a relatively unique case of first-order ferro-ferromagnetic transition originating from an instability of the Fe moment. In addition to large magnetoelastic effects making them potential negative thermal expansion materials, here, we show that Sc0.28Ti0.72Fe2 and related alloys also present sizable magnetocaloric and magnetoresistance effects. Both effects are found substantially larger at the ferro-ferromagnetic transition (Tt1) than near the Curie temperature TC, yet they remain limited in comparison to other classes of giant magnetocaloric materials. We suggest a strategy to improve these properties by bringing the transition at Tt1 close to TC, and test its possible realization by Co or Mn for Fe substitutions. The structural and magnetic phase diagrams of Sc0.28Ti0.72Fe2−xTx alloys with T = Mn or Co are explored. Substitutions for Fe by adjacent Mn or Co elements give rise to a breakdown of the long-range ferromagnetic order, as well as a swift disappearance of finite moment magnetism.

Influence of the Ge distribution on the first order magnetic transition of the MnFe(P,Ge) magnetocaloric material

MnFe(P,Ge) is a promising magnetocaloric material for potential refrigeration applications near room temperature. However, its relatively large hysteresis and large temperature/field range of two-phase [paramagnetic (PM) and ferromagnetic (FM)] coexistence displayed in the cyclic first order magnetic transition (FOMT) cause energy losses and reduce the energy conversion efficiency. In this work, we explore the underlying causes of phase coexistence, hysteresis and structural transformation based on determination of the Ge distribution in MnFeP1-xGex (0.10 < x < 0.50) materials. We find that all the samples crystallize in the Fe2P-type structure [P6[combining macron]2m (No. 189), Z = 3] and Ge displays a strong preference for the 2c site. First principles total energy calculations confirm this site preference of Ge, and Ge entering the 2c site changes the electronic structures and enhances the Fe and Mn 3d exchange splitting across the Fermi level as well as the FM exchange interactions, consequently leading to a linear increase in the transition temperature with increasing Ge content. Scanning electron microscopy and energy-dispersive spectroscopy reveal the inhomogeneous distribution of Ge in grains, which makes the grains with larger Ge content transform from the PM to the FM phase first when cooling and thus causes the phase coexistence. Maximum entropy method electron-densities show that weakening the coplanar Fe-P/Ge(2c) and Mn-P(1b) bonding strengths across the PM to FM phase transition can release some 3d-electrons to enhance the Fe-Mn FM exchange interaction and result in coupling between the magnetic and structural degrees of freedom. This provides first direct evidence for the dominant role of Fe-Mn exchange interaction in the ferromagnetic ordering and may provide a method to observe the exchange interaction. Diminishing the variances in covalent bonding strengths across the FOMT gives rise to an exponential decay in the heat hysteresis when increasing the Ge occupancy at the 2c site. To the best of our knowledge, this is the first time a relationship between the variances in covalent bonding strengths and hysteresis is proposed. This material thus provides an example of a FOMT and hysteresis driven by reversible weakening and strengthening of covalent bonds. Based on these, a strategy of designing better magnetocaloric materials is suggested.

Materials with Giant Mechanocaloric Effects: Cooling by Strength

The search for materials with large caloric effects has become a major challenge in material science due to their potential in developing near room-temperature solid-state cooling devices, which are both efficient and clean, and that can successfully replace present refrigeration technologies. There are three main families of caloric materials: magnetocaloric, electrocaloric, and mechanocaloric. While magnetocaloric and electrocaloric materials have been studied intensively in the last few decades, mechanocaloric materials are only very recently receiving a great deal of attention. The mechanocaloric effect refers to the reversible thermal response of a solid when subjected to an external mechanical field, and encompasses both the elastocaloric effect, corresponding to a uniaxial force, and the barocaloric effect, which corresponds to the response to hydrostatic pressure. Here, the state of the art in giant mechanocaloric effects is reviewed and a critical analysis of the thermodynamic quantities that characterize the major families of barocaloric and elastocaloric materials is provided. Finally perspectives for further development in this area are given.

Two different critical regimes enclosed in the Bean–Rodbell model and their implications for the field dependence and universal scaling of the magnetocaloric effect

In the last few years power laws and universal scaling have been extensively used to study the field dependence of the magnitudes involved in the magnetocaloric effect of materials.

Ab initio mechanical and thermal properties of FeMnP1−xGaxcompounds as refrigerant for room-temperature magnetic refrigeration

We report here the FeMnP_1−xGa_xcompounds could be a possible candidate refrigerant for room-temperature magnetic refrigeration.

Impact of magnetic fluctuations on lattice excitations in fcc nickel

The spin-space averaging formalism is applied to compute atomic forces and phonon spectra for magnetically excited states of fcc nickel. Transverse and longitudinal magnetic fluctuations are taken into account by a combination of magnetic special quasi random structures and constrained spin-density-functional theory. It turns out that for fcc Ni interatomic force constants and phonon spectra are almost unaffected by both kinds of spin fluctuations. Given the computational expense to simulate coupled magnetic and atomic fluctuations, this insight facilitates computational modeling of magnetic alloys such as Ni-based superalloys.

Effect of ferromagnetic ordering on phonons in KCo2Se2

Results of the density functional theory studies of the phonon dynamics in the ternary layered cobalt diselenide are reported. The partial phonon densities of states due to vibrations of K, Co, and Se atoms are analysed in detail. They indicate that phonons associated with the dynamics of Co and Se ions within the [Co2Se2] structural blocks span the entire spectral range extending to 260 cm(-1), whereas phonons from the K-sublattice remain limited to the frequency range of 80-150 cm(-1). The phonons conform with structural features of the quasi-2D layered structure of KCo2Se2. Ferromagnetic order in the Co-sublattice is shown to determine to a great extent the phonon densities of states, the Raman and infrared spectra of KCo2Se2. The in-planar magnetic interactions are responsible for pronounced softening of the high-frequency phonon modes and lead to disappearance of the low-frequency Raman-active mode of the E g symmetry. The observed behavior of the Raman-active and infrared-active modes suggests rather strong spin-phonon coupling in KCo2Se2. Results of the present investigations allow to clarify the origin of substantial differences between dynamical properties of the ferromagnetic Co-based and the paramagnetic Ni-based ternary layered dichalcogenides, both adopting the ThCr2Si2-type structure.