The magnetic domains in an oxide particle have an inherent resistance to change of their orientation

The magnetic domains in an oxide particle have an inherent resistance to change of their orientations, a sort of inertia, called hysteresis. Because of this, very-low-intensity signals in the record head coil do not produce a corresponding modulation in the tape. An extra signal, called the bias signal, or simply bias, is superimposed on the audio signal to overcome this magnetic inertia. The bias signal, a constant-amplitude sine wave with a frequency of about 100 kHz, well above the audible range, supplies an extra "push" or "shove" to the magnetic domains to overcome their magnetic inertia, which allows them to respond to the lower-amplitude audio-signal modulation. The bias signal is not heard on playback because of its extremely high frequency. Different types of tape respond differently to external magnetic fields and therefore require different bias-signal amplitudes. Most tape decks have switches that allow the user to choose the bias that is most suited to a particular type of tape. Ref: Berg R.E., Stork D.G. - The physics of sound (2005, Pearson PH).@Hearing_Aid