Thermomagnetic conversion efficiencies for ferromagnetic materials

The theory of thermomagnetic generation is reviewed and an efficiency analysis using experimentally measured magneto-thermal properties of 3d transitional and 4f rare earth ferromagnetic elements is presented in this study. While theoretical results suggest that 55% of Carnot efficiency is possible, experimental data indicate values smaller than 25% of Carnot efficiency unless large magnetic field (e.g., Ha ∼ 80 kOe) is applied. For smaller magnetic fields representative of NdFeB permanent magnets (e.g., Ha = 3 kOe), the largest efficiencies are obtained for operating ferromagnetic materials over a smaller temperature difference (ΔT = 5 K). Furthermore, single crystal materials are found to have superior efficiencies, as do elements that undergo an order-to-order phase transition. Both of these later results relate to increased magnetization changes over a given ΔT. These results are subsequently used to postulate that a single domain structure will produce larger efficiencies due to the higher magnetization present over a wide range of magnetic fields when compared to multi-domain materials. Calculations for a Gd single domain suggest efficiencies on the order of 30% are possible, representing a threefold increase from multi-domain Gd at relatively small magnetic fields.