Avoided quantum criticality in metallic ferromagnets under pressure: few recent examples

In this talk, a brief overview of our recent work on avoided quantum criticality under pressure will be presented. Current theoretical models [1] suggest that, when tuning a second-order ferromagnetic (FM) transition in metals towards zero temperature, for general reasons, the quantum criticality is avoided. In clean metallic systems, it is argued that either the FM transition becomes first order through a tricritical point or a long-wavelength AFM phase appears.
The variation of the latter scenario is possibly realized under pressure in LaCrGe$_3$ [2,3] that orders ferromagnetically at $T_C \sim 90$ K at ambient pressure and La$_5$Co$_2$Ge$_3$ [4] ($T_C \sim 4$ K at $P = 0$). In particular, in LaCrGe$_3$ the recent comprehensive study [3] suggested formation of FM clusters above $\sim 2$ GPa, in the proximity of the avoided FM quantum critical point. In La$_5$Co$_2$Ge$_3$ a new state, likely magnetic order with an antiferromagnetic component, appears above $\sim 1.7$ GPa. The experimental data for both compounds raise questions about the possible role of disorder in stoichiometric quantum materials.
The collapse of the Kondo state and FM quantum phase transition in YbFe$_2$Zn$_{20}$ [5] will be discussed as an experimental realization of the former scenario “in a reversed order”. At ambient pressure, YbFe$_2$Zn$_{20}$ is a non-magnetic heavy fermion with the Sommerfeld coefficient, $\gamma$ ~ 500 mJ/mol K$^2$, and Kondo temperature, $T_K \sim 30$ K. [6] Under pressure the hybridization strength decreases and apparent FM transition occurs above $P \sim 18$ GPa.
Outlook and potential future studies will be described.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358.

[1] M. Brando, et al., Rev. Mod. Phys., 88, 025006 (2016).
[2] V. Taufour et al., Phys. Rev. Lett., 117, 037207 (2016).
[3] E. Gati et al., Phys. Rev. B, 103, 075111 (2021).
[4] L. Xiang, et al, Phys. Rev. B, 103, 054419 (2021).
[5] U. S. Kaluarachchi, et al., Phys. Rev. B, 98, 174405 (2018).
[6] M. S. Torikachvili, et al., Proc. Natl. Acad. Sci. U.S.A., 104, 9960 (2007)

E-mail for corresponding author: [email protected]