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 (formula presented) is determined based on the irreversibility in the temperature-dependent dc-susceptibility (formula presented) The Curie temperature (formula presented) is assigned to an inflection point in the (formula presented) 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 (formula presented) at which the magnetic phase transition disappears, are evaluated. For the FM-RSG transition, (formula presented) and (formula presented) are obtained. For the paramagnetic (PM)-FM transition, (formula presented) and a remarkably large value of the characteristic length (formula presented) (formula presented) are evaluated. This large value of (formula presented) 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 (formula presented) at which the FM phase disappears. Below (formula presented) 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 (formula presented) 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.