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 TRSG is determined based on the irreversibility in the temperature-dependent dc-susceptibility χ(T). The Curie temperature Tc 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, λRSG=0.66±0.44 and DRSG0∼8 Å are obtained. For the paramagnetic (PM)-FM transition, λFM = 1.31±0.28 and a remarkably large value of the characteristic length DFM0, 43±13 Å, are evaluated. This large value of DFM0 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 Dc at which the FM phase disappears. Below Dc, 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 Dc, 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.
|ジャーナル||Physical Review B - Condensed Matter and Materials Physics|
|出版ステータス||Published - 2002 1 1|
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