Internal field corrections in perpendicular columnar structured Alumite films

An investigation of the remanent and hysteretic properties of perpendicular media such as Alumite involves calculation of internal fields which requires the demagnetisation factor to be known. Demagnetisation effects have been calculated for Alumite, which is an ideal model material, by considering it as a hexagonal matrix of columns. This predicts that the demagnetising effects are less than that of a continuous film at saturation but increase as the magnetisation decreases

DC modulation noise in clustered particulate media

Mallinson’s theory of recording media noise is extended to ensembles where the particles have an easy axis distribution or are clustered. The effect of clusters is that there is a critical value of packing fraction at which noise in the demagnetised state switches to a minimum. Comparison is made with measurements of DC modulation noise in double layer MP tape

Social tipping points and Earth systems dynamics

Recently, Early Warning Signals (EWS) have been developed to predict tipping points in Earth Systems. This discussion highlights the potential to apply EWS to human social and economic systems, which may also undergo similar critical transitions. Social tipping points are particularly difficult to predict, however, and the current formulation of EWS, based on a physical system analogy, may be insufficient. As an alternative set of EWS for social systems, we join with other authors encouraging a focus on heterogeneity, connectivity through social networks and individual thresholds to change

Small-angle X-ray scattering study of ferrofluids

Small-angle X-ray scattering from colloidal suspensions of fine magnetic particles has been studied as a function of applied magnetic field using one- and two-dimensional detectors at the Synchrotron Radiation Source (SRS) at Daresbury. Both the scattering distribution and the radius of gyration become anisotropic in an applied field. The anisotropy of the radius of gyration increases with increasing field

Polymeric stabilization of colloidal magnetite magnetic fluids

Stable colloidal suspensions of ~4.3-nm mean diameter magnetic particles have been prepared employing polystyrene-block-poly(ethylene/propylene) copolymer (Mn ~100000) for steric stabilization. The potential of the copolymers as both dispersants and inhibitors of long-term oxygen degradation of the magnetic properties of the particles has been assessed. Magnetic changes due to oxidation, aggregation, and sedimentation consequent to prolonged exposure to atmosphere and gravitational and magnetic field gradients have been studied employing transmission electron microscopy, vibrating sample magnetometry, and scanning column magnetometry techniques. The flow properties in magnetic fields have also been investigated. Stable fluids of typically 5 J-1T-1kg-1 have been obtained. It appears that the polymer competes with oxygen at the particle surface to decrease colloidal sensitivity to oxidation