Strong Longitudinal Magnetic Fluctuations Near Critical End Point in UCoAl: A 59Co-NMR Study

We report 59 Co-NMR measurements in UCoAl where metamagnetism occurs owing to the enhancement of ferromagnetism by a magnetic field. The metamagnetic transition from a paramagnetic (PM) state to a ferromagnetic state is a first-order transition at low temperatures, but it changes to a crossover at high temperatures on crossing the critical end point (CEP) at T CEP ∼12 K. The contrasting behavior between the relaxation rates 1/ T 1 and 1/ T 2 suggests that the longitudinal magnetic fluctuation of the U moments is strongly enhanced, especially near the CEP. A wide diffusion of the fluctuation from the CEP can be confirmed even in the PM state where the magnetic transition does not occur.

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