Low-temperature specific heat in hydrogenated and Mn-dopedLa(Fe,Si)13

Abstract

It is now well established that the paramagnetic to ferromagnetic transition in the magnetocaloric La(FeSi)13 is a cooperative effect involving spin, charge and lattice degrees of freedom. However, the influence of this correlated behaviour on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of LaFexMnySiz samples with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature and the spin wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced d band electronic interactions, at play. The Sommerfeld coefficient is found to be 90-210 mJmol -1 K -2 unusually high compared to that expected from band structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping, but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly, and provides a basis for future theoretical development.

Electronic structure, metamagnetism and thermopower of LaSiFe12and interstitially doped LaSiFe12

“…We have further calculated thermopower, in good agreement with experiments on non-hydrogenated material. Thermopower values in the hydrogenated material seem to require a model that includes other scattering effects, although these are unlikely to spin waves, which are thought to be heavily suppressed [65].…”

“…It should be noted that this level of suppression of λ P M = 0.5 is not the lowest possible (which is λ P M = 0); further suppression of λ P M would result in more negative changes in phonon entropy while higher λ P M values would (contrary to Table 1. Calculated (DFT) and experimental [65] Sommerfeld γ factors and electronic entropy changes on cooling through the Curie transition of LaSiFe 12 (T C = 195 K) and LaSiFe 12 H 3 (T C extrapolated to 488 K). All values are given in JK −2 kg −1 .…”

“…We nonetheless note three aspects of our comparisons of calculated thermopower with experiment: (i) the relative success of thermopower modelling in the nonhydrogenated compound; (ii) the observation of theoretical features at low temperatures that can arise solely from band structure effects; and (iii) that scattering effects beyond our model are likely to play a larger role in the accurate modelling of the thermopower in the hydrogenated La-Fe-Si samples. It has already been suggested that experimental features in the low temperature thermopower of the hydrogenated material are due to the presence of such effects [67,65]. Further work will be required to separate the effect of band structure from additional scattering mechanisms.…”

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…The density of states can be obtained from electronic structure calculations . It can also be measured by photoemission spectroscopy (PES) or absorption spectroscopy for the occupied or unoccupied states, respectively, and by low‐temperature calorimetry . The lattice entropy S lat is given by the velocity and amplitude of the atomic vibrations (phonons).…”

Moment‐Volume Coupling in La(Fe1−xSix)13

“…For the knowledge‐based optimization of these materials, a basic understanding of their intrinsic properties on the electronic scale is of fundamental importance. This includes first‐principles calculations of the electronic structure of ternary and quaternary compositions, Mößbauer spectroscopy for the magnetic properties, structural properties from neutron diffraction and even spin wave stiffness or Debye temperature from measurements of the low temperature specific heat . Thus apart from materials engineering such as optimization of microstructure and thermodynamic properties, large efforts have been dedicated to fundamental research.…”