Exploring the ferromagnetism of Fe(Ga1-xGex)3 near quantum criticality

Ferromagnetism is well described in two limits: at one end the limit of localised moments, and at the other the limit of itinerant moments. Understanding of the regime in between is limited and progress towards an understanding is impeded by scarcity of materials and experimental data.
FeGa$_3$ is a diamagnetic semiconductor, also proposed to be a Kondo insulator that becomes metallic with a small amount of germanium doping: a weak ferromagnetic moment develops at a critical doping of ~5%, and grows as the doping is increased to 10%. The magnetism in Fe(Ga$_{1-x}$Ge$_x$)$_3$ falls between the localised and itinerant limits, but although there has been theoretical discussion, there is no consensus on where it lies between these limits or how this changes with doping.
To approach these questions experimentally, we have used an AC field modulation technique applied to samples in the ferromagnetic phase of Fe(Ga$_{1-x}$Ge$_x$)$_3$ to characterise the temperature and field dependence of the magnetic susceptibility. A strong feature in the susceptibility at $T_{Curie}$ is quickly suppressed with a small magnetic field, similar to observations in the well-known weak itinerant ferromagnet ZrZn$_2$, but additional structure in the susceptibility indicates a more complicated magnetism in Fe(Ga$_{1-x}$Ge$_x$)$_3$. We have studied the evolution of this behaviour at several doping concentrations near and above the critical doping for ferromagnetism.