Magnetic Viscosity under Discontinuously and Continuously Variable Field Conditions

For a ferromagnetic specimen exhibiting magnetic viscosity, the intensity of magnetization increases continuously with time under steady external field conditions An account is given here of investigations to determine the influence on magnetic viscosity of changes in the magnetic field applied to the specimen, two cases of discontinuous and continuous changes in field being considered. The latter case is of practical importance when direct measurements of magnetic viscosity are made using specimens with demagnetization coefficient other than zero since the effective field acting in the specimen is then continuously variable in time. It is found that both cases can be adequately described by extending the previously proposed formal theory of magnetic viscosity in which it was supposed that the domain processes responsible for magnetization can be activated by thermal agitation The required extension of the theory consists in assuming that the activation energy of the domains is a function of the field. The experimental results show that the changes in domain activation energy are linearly proportional to the changes in field strength, if the latter are small Denoting the constant of proportionality by q , and considering a possible domain model, it is shown that q is related to the elementary volume of the material in which thermal agitation leads to activation. Measurements on alnico show that the linear dimensions of this elementary volume are of the same magnitude as the width of a domain boundary wall Results of the variation of q and other parameters characteristic of magnetic viscosity as functions of the temperature of the specimen are also presented and it is shown that the observed behaviour is consistent with the present views on the metallurgical structure of alnico.