Measurement of magnetic field by exposure of a magnetic crystal to the field and an applied rotating field

In the measurement of a magnetic field by placing a body of magnetic crystal material in the field to be measured, applying a further magnetic field of cyclically varying direction to the body and detecting cyclical fluctuations in the magnetic field in the body, the magnitude of which are proportional to the strength of the magnetic field to be measured, the crystal material (eg. LuxY3-xFe5-yScyO12 where y is 0.5 - 0.7, x is 0 - 1.2) defines a single magnetic domain (ie all the magnetic moments of the body are aligned) which remains constant. Body 3 is mounted on substrate 4 adjacent coils 1 and 2 so as to be exposed to a rotating field of fixed frequency established by passing current through the coils and to the field to be measured. The detecting means detects the cyclical fluctuations at twice the fixed frequency. The problem of noise swamping such fluctuations is alleviated.