Abstract: Spin glass-ferromagnetic transition in disordered Ni-Mn

Abstract

The magnetization M of a disordered Ni-Mn alloy of ∼22 at. % Mn was measured as a detailed function of field H and temperature. For the alloy cooled to 4 K in zero H, M increases slowly as H is raised to a critical value (Hc∼200 Oe), where M increases rapidly and then slowly approaches ferromagnetic saturation as H is raised to 5 kOe. As H is subsequently lowered, M undergoes a similarly rapid drop at the same Hc and then slowly reaches zero at zero H. Since the isothermal remanence is zero, the low-M state below Hc cannot derive from simple ferromagnetic domain effects. That this is a spin-glass state is evidenced by field-cooling experiments which result in displaced hysteresis loops at 4 K. At higher temperatures after cooling in zero H, the field-induced transition to ferromagnetism is seen to occur at a progressively lower Hc. Hc reaches zero at ∼40 K, above which the spontaneous M varies like that of a ferromagnet with a Curie point of ∼350 K. However, the low-temperature values of the spontaneous M are well below the values expected from a ferromagnetic alignment of all the atomic moments. Similar experiments on a Ni-Mn alloy of ∼26 at. % Mn reveal it to be a typical spin glass with a susceptibility cusp at ∼80 K.1 A detailed determination of the magnetic phase diagram of the disordered Ni-Mn system is currently in progress. a) Work supported by NSF Grant. No. DMR78-12777. b) Presently at University of Colorado, Colorado Springs, CO 80907. c) Presently at Queens College-CUNY, Flushing, NY 11367. 1 ESR results for this alloy are reported at this conference by Hurdequint, Kouvel, and Monrod.