Ferromagnetic critical behavior in U(Co1-xFex)Al (0 ≤ x ≤ 0.02) studied by 59Co nuclear quadrupole resonance measurements

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 Co-59 nuclear quadrupole resonance (NQR) measurement for the Fe-substituted U(Co1-x Fe-x) Al (x = 0, 0.5, 1, and 2%) in zero external magnetic field. The Fe concentration dependence of Co-59-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-max similar to 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.