The microwave frequency method

We present new results showing that when sufficient microwave power is employed the grains carrying the natural remanent moment (NRM) that are able to absorb microwaves reach their blocking temperature in less than 10 ms, and the sample temperature at the end of this very short time has only increased by about 30 °C. We also show that because large gaps exist in the spin-wave frequency of small particles, some grains are unable to absorb because they are too small, and others because they are too large. A large number of results have been published, that were obtained using frequencies close to 14 GHz, using what has been called the microwave Thellier method. However, as Biggin has pointed out [Biggin, A.J., et al., 2007. A comparison of a quasi-perpendicular method of absolute palaeointensity determination with other thermal and microwave techniques. Earth Planet. Sci. Lett. 257, 564–581] the microwaves constitute a marginal improvement on thermal methods, with the attendant alteration problems.We wish to propose a radically new method here, the microwave frequency method, where the microwave frequency is varied, selectively magnetizing and demagnetizing grains of different sizes thereby keeping the sample temperature so low that alteration is avoided completely.