The giant magnetocaloric effect is widely achieved in hexagonal MnMX-based (M = Co or Ni, X = Si or Ge) ferromagnets at their first-order magnetostructural transition. However, the thermal hysteresis and low sensitivity of the magnetostructural transition to the magnetic field inevitably lead to a sizeable irreversibility of the low-field magnetocaloric effect. Here, we show an alternative way to realize a reversible low-field magnetocaloric effect in MnMX-based alloys by taking advantage of the second-order phase transition. With introducing Cu into Co in stoichiometric MnCoGe alloy, the martensitic transition is stabilized at high temperature, while the Curie temperature of the orthorhombic phase is reduced to room temperature. As a result, a second-order magnetic transition with a negligible thermal hysteresis and a large magnetization change can be observed, enabling a reversible magnetocaloric effect. By both calorimetric and direct measurements, a reversible adiabatic temperature change of about 1 K is obtained under a field change of 0-1 T at 304 K, which is larger than that obtained in a first-order magnetostructural transition. To gain a better insight into the origin of these experimental results, first-principles calculations are carried out to characterize the chemical bonds and the magnetic exchange interaction. Our work provides an understanding of the MnCoGe alloy and indicates a feasible route to improve the reversibility of the low-field magnetocaloric effect in the MnMX system.

Approaching the border of the first order transition and second order transition is significant to optimize the giant magnetocaloric materials performance. The influence of vanadium substitution in the Mn_1.2-xV_xFe_0.75P_0.5Si_0.5 alloys is investigated for annealing temperatures of 1323, 1373 and 1423 K. By tuning both the annealing temperature and the V substitution simultaneously, the magnetocaloric effect can be enhanced without enlarging the thermal hysteresis near the border of the first to second order transition. Neutron diffraction measurements reveal the changes of site occupation and interatomic distances caused by varying the annealing temperature and V substitution. The properties of the alloy with x = 0.02 annealed at 1323 K is comparable to those found for the MnFe_0.95P_0.595Si_0.33B_0.075 alloy, illustrating that Mn_1.2-xV_xFe_0.75P_0.5Si_0.5 alloys are excellent materials for magnetic heat-pumping near room temperature.

We report on a study on a representative set of Fe₂P-based MnFePSi samples by means of ⁵⁵Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the 3f sites than at the 3g sites on approaching TC, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the 3f spin density instead of enhanced spin fluctuations. The temperature-driven 3f spin extinction is demonstrated to evolve into a truly nonmagnetic state of the 3f Mn ions well above TC, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides "normal" 3f Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at 3f sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average 3f moment previously reported at large Mn concentrations.

(MnFe)₂(P, Si)-type compounds are, to date, one of the best candidates for magnetic refrigeration and energy conversion applications due to the combination of giant magnetocaloric effect (MCE), tunable working temperature range and low material cost. The giant MCE in the (Mn, Fe)₂(P, Si)-type compounds originates from strong magnetoelastic coupling, where the lattice degrees of freedom and spin degrees of freedom are efficiently coupled. The tunability of the phase transition, in terms of the critical temperature and the character of the phase transition, is essentially attributed to the changes in the magnetoelastic coupling in the (Mn, Fe)₂(P, Si)-type compounds. In this review, not only the fundamentals of the magnetoelastic coupling but also the related practical aspects such as magnetocaloric performance, hysteresis issue and mechanical stability are discussed for the (Mn, Fe)₂(P, Si)-type compounds. Additionally, some future fundamental studies on the MCE as well as possible ways of solving the hysteresis and fracture issues are proposed.

Given the potential applications of (Mn,Fe₂(P,Si))-based materials for room-temperature magnetic refrigeration, several research groups have carried out fundamental studies aimed at understanding the role of the magneto-elastic coupling in the first-order magnetic transition and further optimizing this system. Inspired by the beneficial effect of the addition of boron on the magnetocaloric effect of (Mn,Fe₂(P,Si))-based materials, we have investigated the effect of carbon (C) addition on the structural properties and the magnetic phase transition of Mn _1.25Fe _0.70P _0.50Si _0.50C _z and Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds by x-ray diffraction, neutron diffraction and magnetic measurements in order to find an additional control parameter to further optimize the performance of these materials. All samples crystallize in the hexagonal Fe ₂P -type structure (space group P-62m), suggesting that C doping does not affect the phase formation. It is found that the Curie temperature increases, while the thermal hysteresis and the isothermal magnetic entropy change decrease by adding carbon. Room-temperature neutron diffraction experiments on Mn _1.25Fe _0.70P _0.55Si _0.45C _z compounds reveal that the added C substitutes P/Si on the 2c site and/or occupies the 6k interstitial site of the hexagonal Fe ₂P -type structure.

Neutron diffraction, Mössbauer spectroscopy, magnetometry, and in-field x-ray diffraction are employed to investigate the magnetoelastic phase transition in hexagonal (Mn,Fe)2(P,Si) compounds. (Mn,Fe)2(P,Si) compounds undergo for certain compositions a second-order paramagnetic (PM) to a spin-density-wave (SDW) phase transition before further transforming into a ferromagnetic (FM) phase via a first-order phase transition. The SDW-FM transition can be kinetically arrested, causing the coexistence of FM and untransformed SDW phases at low temperatures. Our in-field x-ray diffraction and magnetic relaxation measurements clearly reveal the metastability of the untransformed SDW phase. This unusual magnetic configuration originates from the strong magnetoelastic coupling and the mixed magnetism in hexagonal (Mn,Fe)2(P,Si) compounds.

The high theoretical energy density of Li-O₂ batteries as required for electrification of transport has pushed Li-O₂ research to the forefront. The poor cyclability of this system due to incomplete Li₂O₂ oxidation is one of the major hurdles to be crossed if it is ever to deliver a high reversible energy density. Here we present the use of nano seed crystallites to control the size and morphology of the Li₂O₂ crystals. The evolution of the Li₂O₂ lattice parameters during operando X-ray diffraction demonstrates that the hexagonal NiO nanoparticles added to the activated carbon electrode act as seed crystals for equiaxed growth of Li₂O₂, which is confirmed by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX) elemental maps also showing preferential formation of Li₂O₂ on the NiO surface. Even small amounts of NiO (∼5 wt %) particles act as preferential sites for Li₂O₂ nucleation, effectively reducing the average size of the primary Li₂O₂ crystallites and promoting crystalline growth. This is supported by first principle calculations, which predict a low interfacial energy for the formation of NiO-Li₂O₂ interfaces. The eventual cell failure appears to be the consequence of electrolyte side reactions, indicating the necessity of more stable electrolytes. The demonstrated control of the Li₂O₂ crystallite growth by the rational selection of appropriate nano seed crystals appears to be a promising strategy to improve the reversibility of Li-air electrodes.

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.

The spatial and temporal correlations of magnetic moments in the paramagnetic regime of (Mn,Fe)2(P,Si) have been investigated by means of polarized neutron diffraction and muon-spin relaxation techniques. Short-range magnetic correlations are present at temperatures far above the ferromagnetic transition temperature (TC). This leads to deviations of paramagnetic susceptibility from Curie-Weiss behavior. These short-range magnetic correlations extend in space, slow down with decreasing temperature, and finally develop into long-range magnetic order at TC.

(Mn, Fe)₂(P, Si)-type compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of highly tunable giant magnetocaloric effect (GMCE) and low material cost.[1, 2] The GMCE of these compounds originates from the first-order magnetoelastic transition around the magnetic phase-transition temperature T_C. However, the phase-transition temperature shows a peculiar thermal-history dependence in these compounds. As-prepared (Mn, Fe)₂(P, Si) displays a significantly lower T_C upon first cooling than on second and subsequent cooling processes. Since this behavior is only observed in as-prepared samples it is called the 'virgin effect'. The difference in T_C between the first and second cooling processes of the as-prepared sample, hereafter referred to as ΔT_C0, is taken as a measure of how strong the virgin effect is. The virgin effect is not exclusive to (Mn, Fe)₂(P, Si) compounds being observed in other GMCE materials[3, 4], however its origin was for a long time unknown. In this study, we report our high-resolution neutron diffraction experiments that finally shed light on the origin of the virgin effect. Additionally, recovery of the virgin effect induced by thermal activation was observed experimentally.