The onset of ferromagnetism in disordered V-Ni, Rh-Ni, Pt-Ni and Pd-Ni alloys

It is shown that published magnetic susceptibility and magnetisation data for PdNi alloys are consistent with a homogeneous model in which the Ni atoms have a spin fluctuation temperature Ts approximately 55K and are coupled by ferromagnetic Ni-Ni interactions. Using a molecular-field model the equivalent classical RMS moment per Ni atom square root ( mu 2) is shown to be concentration independent for concentrations between 1.3 and 16.7 at.% Ni and for temperatures between the ordering temperature Tc and 700K. square root ( mu 2) exhibits a temperature-dependent exchange enhancement at high temperatures, and at low temperatures falls to zero due to intrinsic spin fluctuations. The exchange enhancement and the ferromagnetic coupling between Ni moments are shown to be related in accordance with the indirect-coupling model of Takahashi and Shimizu (1965). The critical concentration for the onset of ferromagnetism and the concentration dependence of the ordering temperature Tc are determined from the curve for square root ( mu 2) and are shown to be in excellent agreement with experiment. A simple model is presented which qualitatively reproduces the magnetisation curves for paramagnetic and ferromagnetic alloys.

Dilute PdNi-a homogeneous magnetic system of fluctuating moments

Publisher Summary This chapter discusses dilute transition metal alloys. The subject of dilute transition metal alloys may be divided into two classes of interest: (1) non-interacting or very dilute 3d-magnetic impurities dissolved in a non-magnetic host, which can be classified under the general heading of the Kondo effect—a localized antiferromagnetic interaction of the isolated impurity spins with the surrounding conduction electrons and (2) the impurity spin-spin interactions, which lead to giant moment ferromagnetism and spin glass freezing. These latter two cases represent different examples of magnetic ordering such as a long-range, but nonuniform, ferromagnetic alignment, and a random arrangement of frozen spin-orientations. Both of these effects depend strongly on the conduction electrons to propagate the magnetic interactions over large distances. The boundary between these two dissimilar states of magnetism is controlled by the magnetic behavior of these itinerant electrons. The chapter reviews the listing of alloy systems, experimental properties, and theory of random alloys.

Chapter 2 Dilute transition metal alloys; Spin glasses

The onset of ferromagnetism in PdFe and PdNi alloys

An earlier calculation (see ibid., vol.6, p.221 (1976)) based on the generalized Landau theory of phase transitions is extended to ferromagnetic disordered alloys for which the cubic term in the Landau equation is essential. To be tractable the cubic term is linearized and the linearization is tested for an ordered system where numerical solutions of the full cubic equation are available. The Green's function analogy is applied to the linearized equation to derive a general self-consistent system of equations for the susceptibility and magnetization of weakly ferromagnetic disordered alloys. This general system is solved in CPA for PdNi alloys in an applied field and the concentration and field dependences of the susceptibility and magnetization are calculated.

https://iopscience.iop.org/article/10.1088/0305-4608/6/7/015/pdf

A phenomenological theory of disordered ferromagnetic PdNi-type alloys

The generalized Landau model is applied to disordered strongly 'paramagnetic' alloys. It is shown that there is a close analogy between the susceptibility of such magnetic alloys and the Green's function of ordinary (nonmagnetic) disordered binary alloys. The Green's function analogy is used to prove that within the generalized Landau theory, disordered alloys similar to PdNi do not exhibit a critical concentration for ferromagnetism. The magnetization of such alloys is estimated on the basis of a simple statistical model and it is proved quite rigorously that a spontaneous magnetization exists even in the so called 'paramagnetic' region (below 2.3% Ni for PdNi). These general conclusions are illustrated by a CPA calculation of the susceptibility of PdNi alloys. In contrast to previous CPA calculations, it is found that the CPA susceptibility never diverges, thus indicating the absence of a well-defined critical concentration.

J Mathon

Papers

The absence of critical concentration for ferromagnetism in disordered PdNi-type alloys