A model of magnetic hyperthermia

A magnetic material exposed to a field that is cycled is observed to become warm. This arises because any misalignment between the field and the moment causes the generation of magnetostatic energy dissipated as heat. This effect is known as magnetic hyperthermia, and can be used as a medical therapy where fine particles are used as the magnetic medium. In a practical application where low fields (H < 250 Oe) are used, the mechanism of heating is not well understood and can be due to losses in a hysteresis cycle, susceptibility loss, or frictional heating due to particle rotation in a liquid environment. In this work a theoretical study has been undertaken of hysteresis loss using Monte-Carlo techniques. It has been found that there is a maximum in the power loss and therefore heat generated with frequency occurring in the range 1 to 10 kHz which depends only weakly on particle size. However, for small particles (Dm < 10 nm) the frequency of the peak depends strongly on packing fraction due to the effects of dipolar interactions. The hysteresis loss reduces significantly when a non-saturating field is used especially for high packing fractions where the field produced by dipolar interactions is stronger, which causes micromagnetic configurations to form that favour the demagnetised state.