Magnetic behaviour of cobalt sponges in cemented carbides

Abstract

The measurement and analysis of magnetisation curves, including anhysteretic and initial curves of heterogeneous and anisotropic materials (sintered tungsten carbides of constant grain size containing 3–25 wt% Co and Co-rich alloys) are presented. Néel's law of approach to saturation magnetisation for ferromagnetics containing non-magnetic inclusions was compared with the results from an empirical Lamont-Frölich-Kennelly equation and the standard law of approach to saturation magnetisation. Internal demagnetising factors N if, where f refers to the magnetic fraction obtained from inverse anhysteretic susceptibilities ( H ≈ 0) X af -1 were compared with those from high-field inverse susceptibilities A f (LFK equati on) and calculated high-field demagnetising factors a/M sf (Néel theory); all obtained as a function of volume fraction V of WC. A f and a/M sf both increase to a maximum at V ≈ 0.9 and then decrease. A f is twice the magnitude of a/M sf at the maximum, having a smaller difference at lower V. A f is also twice the magnitude of N if being an increasing function of applied field and is interpreted to be a high-field demagnetising factor. Assuming the Néel equation to account for voids, the LFK equation is interpreted to measure the effects of voids, of demagnetisation by hard directions in hcp Co (at low V) and of demagnetising fields in Bloch walls in very thin Co films at high V. The Néel equation was also fitted directly to the results, showing that the theory derived for isotropic materials is inadequate in dealing with large magnetocrystalline anisotropies. A relationship between H c, j r (remanence ratios) and N if for presintered (pressed powder) and sintered specimens with a range of Co contents and WC grain sizes was established.