Percolation concentration and saturation of the Pd moment in ferromagnetic Pd alloys

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Abstract
In PdFe, PdCo, PdNi, PdMn or PdGd alloy systems long-range ferromagnetic order always sets in whenever the concentration of magnetic units is about 0.11 at.%, irrespective of the type of solute or the total magnetic moment of the magnetic unit, the latter being a polarisation cloud seeded either by a single solute atom or by a statistically formed cluster of solute atoms. This observation, in conjunction with the facts (i) that in PdGd the d band polarisation is negative in zero or weak external fields with (ii) that for all the weakly ferromagnetic alloys the ferromagnetic Curie temperature, TC, is proportional to the solute concentration, c, is interpreted to mean that the spatial extent of the polarisation cloud cannot be as large as is currently assumed and that the onset of ferromagnetic order is through a percolation process involving ferromagnetic correlations between the otherwise isolated or independent magnetic units, rather than through any direct overlap of such units. Also, in appropriate cases, the concentration dependence of the bulk magnetisation, mu b, is approximately as predicted by the Landau theory of phase transitions for the weakly ferromagnetic alloys whereas for higher solute concentrations mu b is proportional to Tc, ironically a characteristic of very weak itinerant ferromagnetism. The average moment on a Pd atom, mu Pd, increases linearly with Tc up to Tc approximately=340K, above which temperature it assumes a constant value equal to the maximum possible Pd moment. This abrupt saturation of the Pd moment, indicative of some sort of phase transition, is discussed in relation to the theoretically predicted metamagnetic transition in Pd and more generally to the concept of a characteristic temperature for an exchange-enhanced but non-magnetic transition metal.