Magnetic studies of LaFe9Si4 structure-type intermetallics

Abstract

The nature of the electronic ground state of itinerant d-band electrons in intermetallic compounds, whether it is para-, ferro- or antiferromagnetic, results from a competition between exchange interactions acting on spin degrees of freedom and the associated gain in band kinetic energy which for a given d-electron count basically dependents on the band width. In a family of isostructural compounds sharing the tetragonal LaFe9Si4 structure-type the bandwidth can be tuned e.g. by a replacement of Si by Ge or Ga whereas the substitution of Fe with Co or Ni controls the d-band filling and thus the strength of the exchange interactions. The most interesting member of this family is LaCo9Si4 which is on the borderline between para- and ferromagnetism and exhibits a metamagnetic transition at about 3.5 T. The influence of the magnetic field on the thermal expansion of LaCo9Si4 at small temperatures is studied by means of capacitance dilatometry. In the related newly characterized compound LaCo9Ge4 the isoelectronic replacement of Si by Ge leads to a narrowing of the bandwidth and in this work an antiferromagetic ground state is established. The non-isoelectronic substitution of Si with Ga leads to the novel compound LaCo9Ga4 which is characterized as a ferromagnetic metal which undergoes also a structural phase transition. In LaNi9Ga4 the filling of the d-band due to the replacement of Co by Ni leads to a decrease of the exchange interaction and thus to a Pauli paramagnetic ground state, in addition a high temperature structural transition is observed.