Size dependent magnetic phase transition in reentrant ferromagnet NiMn multilayer films

The size dependence of the magnetic phase transition in reentrant ferromagnet NiMn is investigated. Multilayer films of NiMn/Cu with thicknesses between 30 and 13 000 Å are prepared using the ion-beam sputtering method in an ultrahigh vacuum. The ferromagnetic (FM)-reentrant-spin-glass (RSG) transition temperature T_RSG is determined based on the irreversibility in the temperature-dependent dc-susceptibility _χ(T). The Curie temperature T_c is assigned to an inflection point in the _χ(T) curve. At thicknesses greater than 200 Å, the transition temperatures are analyzed based on finite-size scaling, and the shift parameter λ and the characteristic length D⁰, at which the magnetic phase transition disappears, are evaluated. For the FM-RSG transition, λ_RSG=0.66±0.44 and D_RSG⁰∼8 Å are obtained. For the paramagnetic (PM)-FM transition, λFM = 1.31±0.28 and a remarkably large value of the characteristic length D_FM⁰, 43±13 Å, are evaluated. This large value of D_FM⁰ is discussed in connection with the inhomogeneous spin fluctuation appearing in the FM phase, which is found based on previous Mössbauer observation. The PM-FM and FM-RSG transition temperatures, as functions of thickness, intersect at a critical thickness D_c at which the FM phase disappears. Below D_c, in addition, the spontaneous magnetization disappears in the low-temperature phase. This observation indicates that there is a vertical boundary line through the thickness of D_c, which separates the reentrant ferromagnet, having a low-temperature RSG phase with ferromagnetic correlation, from the pure spin glass. This is compared with the magnetic phase diagrams of reentrant ferromagnetic systems.