FIRST-ORDER SPIN-PARAMAGNETIC TRANSITION AND TRICRITICAL POINT IN ULTRATHIN BE FILMS

We report measurements of the parallel critical magnetic field of thin superconducting beryllium films with T(c)similar to 0.6 K. The critic al field transitions were spin-paramagnetically limited and found to b e strongly hysteretic below a tricritical point at T-tri= 190 mK. The magnitude of the hysteresis and T-tri were observed to be completely s uppressed in field misalignments theta greater than or equal to 1 degr ees. Time relaxation measurements in the hysteretic regime showed simp le exponential decays in the highest quality films and nonexponential decays interspersed with avalanches in more inhomogeneous samples.