A study on the fundamental properties of Gd 5Àx La x Si 1.8 Ge 2.2 pseudo-binary compounds with x ¼ 1, 2, 3, 4, and 5 is performed by means of X-ray powder diffraction (XRD) and magnetic measurements. An evolution from a monoclinic (M) Gd 5 Si 2 Ge 2 -type to a tetragonal (T) Zr 5 Si 4 -type structure, with the addition of La, is observed, finding that the T phase is stable for x ! 2. A monotonic expansion of the unit cell volume with a rate of 28 Å3/La by increasing the content of the non-magnetic La ions in the system influences significantly the overall magnetic behavior. For instance, T C decreases in a rate of dT C /dx ¼ À47.8 K La À1 ; and a considerable hysteresis reduction is observed as a result of the change on the nature of the phase transition from a first to a second-order one. Hence, these results reveal the impact of a non-magnetic La substitution on both magnetic and crystallographic properties of this series.

The magnetocaloric effect is often largest within the neighbourhood of a first-order phase transition. This effect can be utilised in magnetocaloric refrigeration, which completely eliminates the need for the greenhouse gases utilised in conventional refrigeration. However, such transitions present unique dynamical effects and are accompanied by hysteresis, which can be detrimental for such refrigeration applications. In this work, a Landau theory-based relaxational model is used to study the magnetic hysteresis and the dynamics of LaFe13– xSix’s first-order magnetic transition. Fitting the experimental magnetization data as a function of applied magnetic field under different field sweep rates with this model provided the Landau parameters (A, B and C) and the kinetic coefficient of the studied material. We demonstrate the tendency of the magnetic hysteresis to increase with magnetic field sweep rate, underlining the importance of studying and minimizing the magnetic hysteresis in magnetic refrigerants at practical field sweep rates. While evaluating the temperature dependence of the time required for a complete transition to occur, a non-monotonic behaviour and a sharp peak were found for temperatures near the transition temperature. Such peaks occur at the same temperature as the peak of the magnetic entropy change for low fields, whereas for higher fields the two peaks decouple. This information is critical for technological applications (such as refrigerators/heat pumps) as it provides guidelines for the optimization of the magnetic field amplitude in order to reduce the transition time-scale and consequently maximize the machine operational frequency and the amount of heat which is pumped in/out per second.

In this report, a successful thermodynamical model was employed to understand the structural transition in Er5Si4, able to explain the decoupling of the magnetic and structural transition. This was achieved by DFT calculations, which were used to determine the energy differences at 0 K, using a LSDA+U approximation. It was found that the M structure is the stable phase at low temperatures, as verified experimentally with a value of ΔF0= -0.262 eV. Finally, a variation of Seebeck coefficient (~6 µV) was determined at the structural transition, which allows to conclude that the electronic entropy variation is negligible in the transition.

In this report the magnetic, atomic structures and spin-lattice coupling have been thoroughly studied through high magnetic field magnetometry, Synchrotron X-ray diffraction under applied magnetic field and magnetostriction measurements in the Tb 5 Ge 4 compounds. A field induced phase transition from an antiferromagnetic towards a ferromagnetic ordering was confirmed but with absence of structural transformation. This absence has been confirmed experimentally through synchrotron x-ray diffraction under applied field (up to 30T). Moreover, this absence was explained via a thermodynamic free energy model: first principles calculations determined a large energy gap (ΔE= 0.65 eV) between the two possible structures, O(I) and O(II). From magnetic and structural properties, a H-T phase diagram has been proposed for Tb 5 Ge 4. Finally it was observed a large magnetostriction (up to 600 ppm) induced by ∆H =7 T.