Magnetic Properties of Platinum Metal Alloys

A number of the papers presented at the International Conference on Magnetism held at the University of Nottingham in September dealt with alloys of the platinum metals. The proceedings will be published shortly by the Institute of Physics and the Physical Society. Papers describing neutron diffraction studies demonstrated the powerfulness of some of the recently developed techniques. Thus G. G. Low of AERE, Harwell, described how the detailed distribution of magnetisation in dilute solutions of iron in palladium can be studied. In an alloy containing only 0.25 atomic per cent iron the magnetic structure at liquid helium temperatures shows a magnetisation of palladium atoms extending many interatomic distances from an iron atom. At about 4 per cent iron the “spheres of influence” overlap and the alloy consists of iron moments in an almost uniformly magnetised palladium matrix. Other workers have studied ordered alloys, examining the magnitude and symmetry of the spin distribution about the different types of atom. Shirane, Nathans, Pickart and Alperin find moments of 3.1 Bohr magnetons (pB) for Fe and 1.0 pB for Rh in FeRh, the spin distribution being spherically symmetric around iron atoms but with a preference for the [001] direction around rhodium atoms. In Pd,Fe both distributions were approximately spherical and corresponded to 0.34 pB for Pd and 2.86 pB for Fe. A group from the Centre for Nuclear Studies in Rome described similar work on CoPt, (ppt=0.23 _t .04 pB, pco’ 1.56h.08 pB) where indications of a nonspherical spin distribution around cobalt atoms were found, and on FePd which showed approximately spherical distributions. Detailed information about atomic magnetism can also be provided by magnetic resonance studies. V. Jaccarino of the Bell Laboratory surveyed nuclear magnetic resonance and presented results for the temperature dependence of the shift (Knight shift) in this resonance from that in non-metallic environments for pure palladium; the Knight shift shows a temperature dependence very like that of the susceptibility, and is evidence against the existence of low temperature antiferromagnetism. In platinum the Knight shift is negative, but the change in its magnitude on alloying shows that it is associated with the susceptibility of the d-electrons. Froidevaux, Gautier and Weisman gave results for PtAu alloys, and a group from the Tata Institute in Bombay for intermetallic compounds of platinum with tin, lead and mercury. More conventional magnetic properties of more direct practical importance were described for platinum-cobalt alloys. McCurrie and Gaunt (University of Sheffield) studied magnetisation as a function of order in the 50 atomic per cent alloy where a well-known cubic to tetragonal transformation takes place, and discussed the locking of domain walls by tetragonal lamellae. Clark and Phillips (University of Nottingham) examined the behaviour of 50 atomic per cent platinum alloys in which chromium was substituted for up to four-fifths of the cobalt. In a 40 per cent Co, 10 per cent Cr alloy the largest BH, (Z x 106 gauss oersted) after annealing at 650°C was associated with an intermediate stage in the ordering; domain patterns were also studied. Associated electrical resistivity and magnetic susceptibility behaviour were described