Magnetic field value measuring device and method for measuring magnetic field value

A magnetic field measuring device which detects a direct-current magnetic field originating from a specimen, the device comprising:

• a vibrational probe unit comprising a probe, the probe comprising one or more material whose intensity of magnetization is proportionate to an external magnetic field;
• a mechanical vibration source mechanically oscillating the probe;
• a vibration detector detecting a mechanical vibration frequency of the probe and a mechanical vibration amplitude of the probe;
• an alternating-current magnetic field generator applying an alternating-current magnetic field to the probe, the alternating magnetic field having a frequency different from the mechanical vibration frequency of the probe and having a nonzero gradient in a direction of the mechanical vibration of the probe;
• a direct-current external magnetic field generator applying to the probe a direct-current external magnetic field in the direction of the mechanical vibration of the probe;
• a frequency modulation detector detecting frequency modulation occurring in the mechanical vibration of the probe from the mechanical vibration frequency detected by the vibration detector;
• a direct-current external magnetic field adjuster adjusting intensity of the direct-current external magnetic field in the direction of the mechanical vibration of the probe applied by the direct-current external magnetic field generator to the probe;
• and
• a direct-current magnetic field determination unit determining a value of the direct-current magnetic field in the direction of the mechanical vibration of the probe originating from the specimen, based on an output value of the direct-current external magnetic field generator when intensity of the frequency modulation takes a minimum value or on an expected output value of the direct-current external magnetic field generator when the intensity of the frequency modulation is expected to take a minimum value.