Experimental evidence for the non-existence of a quantum critical point in FeGa_3-xGe_x (x = 0.0–0.16)

FeGa_3-xGe_x (x = 0.0–0.16) was considered in literature as a candidate to display a ferromagntic quantum critical point at a Ge content of around x ≈ 0.15 [1]. This contribution (based on Ref. [2]) will present our investigation of crystallographically-oriented samples grown by the Czochralski method via measurements of AC and DC susceptibility, M(H) isothermal magnetization curves, electrical resitivity and specific heat down to temperatures of 0.4 K. We have observed that there is no direct transition from paramagnetism to ferromagnetism, but a transition via an intermediate spin-glass state. This intermediate spin-glass state exists in a narrow composition range (e.g. for x larger than cca 0.12) at low temperatures (spin freezing temperatures below 1.8 K) and is fragile with respect to applied magnetic fields. Analysis of the AC suscpetibility in the form of Cole-Cole plots indicates that the dynamics of FeGa_3-xGe_x remain thermally activated down to 0.4 K and thus even at 0.4 K the regime of predominantly quantum fluctuations is not yet reached.

[1] M. Majumder et al., Phys. Rev. B 93, 064410 (2016).

[2] S.Vrtnik et al., Phys. Rev. B 112, 104420 (2025).