Ferromagnetic Critical Behavior in U(Co$_{1-x}$Fe$_x$)Al ($0 \leq x \leq 0.02$) Studied by $^{59}$Co Nuclear Quadrupole Resonance Measurement

TL;DR

This study investigates the critical magnetic behavior in U(Co$_{1-x}$Fe$_x$)Al using $^{59}$Co NQR, revealing a first-order ferromagnetic transition above $x=1 ext{%}$ and critical fluctuations near the tricritical point.

Contribution

It provides experimental evidence of ferromagnetic transition order change and critical fluctuations in UCoAl with Fe substitution, supporting theoretical models.

Findings

First-order FM transition occurs at $x > 1 ext{%}$.

Magnetic fluctuations peak around 20 K.

Critical fluctuations are observed near the TCP.

Abstract

In order to investigate physical properties around a ferromagnetic (FM) quantum transition point and a tricritical point (TCP) in the itinerant-electron metamagnetic compound UCoAl, we have performed the $^{59}$Co nuclear quadrupole resonance (NQR) measurement for the Fe-substituted U(Co$_{1-x}$Fe$_x$)Al ($x$ = 0, 0.5, 1, and 2%) in zero external magnetic field. The Fe concentration dependence of $^{59}$Co-NQR spectra at low temperatures indicates that the first-order FM transition occurs at least above $x$ = 1%. The magnetic fluctuations along the $c$ axis detected by the nuclear spin-spin relaxation rate $1/T_2$ exhibit an anomaly at $T_\mathrm{max}$ $\sim$ 20 K and enhance with increasing $x$. These results are in good agreement with theoretical predictions and indicate the presence of prominent critical fluctuations at the TCP in this system.