Electronic structure, metamagnetism and thermopower of LaSiFe₁₂and interstitially doped LaSiFe₁₂

Abstract

We present a systematic investigation of the effect of H, B, C, and N interstitials on the electronic, lattice and magnetic properties of La(Fe,Si) 13 using density functional theory. The parent LaSiFe 12 alloy has a shallow, double-well free energy function that is the basis of itinerant metamagnetism. On increasing the dopant concentration, the resulting lattice expansion causes an initial increase in magnetisation for all interstitials that is only maintained at higher levels of doping in the case of hydrogen. Strong s-p band hybridisation occurs at high B,C and N concentrations. We thus find that the electronic effects of hydrogen doping are much less pronounced than those of other interstitials, and result in the double-well structure of the free energy function being least sensitive to the amount of hydrogen. This microscopic picture accounts for the vanishing first order nature of the transition by B,C, and N dopants as observed experimentally. We use our calculated electronic density of states for LaSiFe 12 and the hydrogenated alloy to infer changes in magneto-elastic coupling and in phonon entropy on heating through T C by calculating the fermionic entropy arXiv:1407.7975v2 [cond-mat.mtrl-sci] 13 Dec 2017Electronic structure, metamagnetism and thermopower of LaSiFe 12 and interstitially doped LaSiFe 12 2 due to the itinerant electrons. Lastly, we predict the electron thermopower in a spinmixing, high temperature limit and compare our findings to recent literature data.

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

“…They describe the enhancement of the free energy as a function of the magnetisation: at low temperatures the double well potential shows the deepening of the well associated with the FM state and a weakening of the barrier between the FM and PM state. In fact, the recent paper by Gercsi, Fujita and Sandeman looking at the influence of H and other interstitials on the density of states supports this explanation [11]. The band structure calculations show that the free energy well narrows as well as deepens, consistent with an increase of TC and reduction of spin fluctuations.…”

“…The first calculations were made by Kuz'min and Richter [9] on LaFe12Si, showing that the free energy had several shallow minima and maxima as a function of the magnetisation. Fujita and Yako [10] extended this work to study the influence of Si site occupancy as a function of Si concentration, and Gercsi et al have developed it further to look at interstitials [11]. Most recently Gruner et al [13], focused on understanding how the vibrational density of states is influenced by the magnetoelastic coupling across the transition.…”

“…In recent times there have been new efforts to examine the metamagnetic transition in La(Fe,Si)13 in terms of the itinerant electron metamagnetism (IEM). First principle calculations based on density functional theory (DFT) have been used to study the variation of Si content [9,10] and the effect of interstitial doping [11] and transition metal substitution [12] on the electronic structure and nature of the magnetic interaction. Most recent of these is a study of the thermodynamics of the transition with a particular focus on the pronounced magnetoelastic softening despite the large volume decrease at the transition [13].…”

“…Significant theoretical progress has been made to describe the electronic properties and the influence of substitution and interstitial occupancy [9][10][11][12][13] in La(FeSi)13, but detailed comparison with experiment is lacking. In order to make better comparison to theory we determine the behaviour of the low temperature specific heat and extract , α and D in a series of samples of LaFexMnySiz with and without hydrogen.…”

Element-Resolved Thermodynamics of MagnetocaloricLaFe13−xSix

“…The IEM picture gained further support from first-principles calculations [27][28][29] through the identification of metastable minima of the binding surface, which correspond to metastable magnetic configurations at distinct volumina. This is also a characteristic feature of Fe-Ni Invar, where the compensation of thermal expansion is linked to the redistribution of charge between non-bonding majority spin-states above and anti-bonding minority spin states below the Fermi-level [30].…”