The Fe₂P type Mn–Fe–P–Si alloys exhibit a giant magneto-elastic first-order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe₂P-type (Mn_1.17-xFe_0.73-yV_0.02) (P_0.5Si_0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V-substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn_1.17-xFe_0.73-yV_0.02P_0.5Si_0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet.

The phase diagram of the magnetocaloric Mn_x Fe_2−x P_1−y Si_y quaternary compounds was established by characterising the structure, thermal and magnetic properties in a wide range of compositions (for a Mn fraction of 0.3 ≤ x < 2.0 and a Si fraction of 0.33 ≤ y ≤ 0.60). The highest ferromagnetic transition temperature (Mn_0.3 Fe_1.7 P_0.6 Si_0.4, T_C = 470 K) is found for low Mn and high Si contents, while the lowest is found for low Fe and Si contents (Mn_1.7 Fe_0.3 P_0.6 Si_0.4, T_C = 65 K) in the Mn_x Fe_2−x P_1−y Si_y phase diagram. The largest hysteresis (91 K) was observed for a metal ratio close to Fe:Mn = 1:1 (corresponding to x = 0.9, y = 0.33). Both Mn-rich with high Si and Fe-rich samples with low Si concentration were found to show low hysteresis (≤2 K). These compositions with a low hysteresis form promising candidate materials for thermomagnetic applications.

The first-order magneto-elastic transition in the Mn–Fe–P–Si alloys can be tailored by vanadium substitution. Alloys with a suitable V substitution provide an excellent magnetocaloric effect with minor hysteresis in low magnetic fields up to 1.2 T. Mössbauer measurements show that the hyperfine field is reduced by V substitution. Neutron diffraction reveals that Fe is substituted by V on the 3f site and the magnetic moment on the 3f site is enhanced by the V substitution. The modified magnetic exchange field around the 3f and 3g positions in the lattice can be utilized to design suitable magnetocaloric materials that operate in low magnetic fields.

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.

According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron-scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where the magnetoresistance is enhanced. We find similar phonon renormalization and dynamic CE-type polaron correlations as in La0.7Sr0.3MnO3. However, quantitative comparison of the results for the two samples shows that only polaron lifetime is well correlated with the strength of the colossal magnetoresistance.

The family of rare-earth (R) tritellurides RTe3 features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [J. J. Hamlin, Phys. Rev. Lett. 102, 177002 (2009)10.1103/PhysRevLett.102.177002]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wave vector qCDW,2 [100] of almost the same magnitude but orthogonal to the ordering wave vector qCDW,1 [001] observed at higher temperatures [N. Ru, Phys. Rev. B 77, 035114 (2008)10.1103/PhysRevB.77.035114]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe3 In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at TCDW,1=308K. Surprisingly, these soft modes close to qCDW,2=(0.68,0,0) show strong softening near TCDW,1 but do not exhibit any response to the lower-temperature transition at TCDW,2=68K. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures.

The lattice dynamics in MnFe0.95Si0.50P0.50 were investigated experimentally using Fe57 nuclear inelastic scattering and inelastic x-ray scattering across the first-order magnetic transition which occurs close to room temperature. The lattice dynamics characterization was supported by a macroscopic magnetic characterization, an x-ray diffraction study, and a hyperfine interactions characterization using Mössbauer spectroscopy. The Fe specific and the x-ray generalized density of phonon states were obtained both in the ferromagnetic and in the paramagnetic state. A prominent shift, 2meV at 20meV, in the x-ray generalized density of phonon states across the first-order magnetic transition, that involves vibrations with essentially Fe character, is revealed corroborated by a change in the local environment quantified in the isomer shift and the quadrupole splitting. Above 35meV the vibrational modes are practically insensitive to the magnetic transition. The entropy change induced by a 1T magnetic field across the magnetic transition, ∼10J/K/kg, is only a fraction of the Fe vibrational entropy change, 62(21)J/K/kg.