Modified Frequency Modulation
It is, as the name says, a modification of the earlier Frequency Modulation scheme used in the same applications; instead of a clock transition and possibly also an additional data transition in each bit time, in MFM there is at most one transition per bit time. This allows twice as many bits to be encoded for a given maximum transition rate.
To be exect, in MFM there is a reversal (signal level, or magnetic flux) at the bit time for a '1', and no reversal for a '0'. A reversal between data bit times occurs with consecutive zeros.