Differential magnetic moment ($\Delta \mu =-\partial (E_2-E_1)/\partial B$)

With energy convention $E=-\mathbf{\mu }\cdot \mathbf{B}$, the difference of physical moments is the negative derivative of a transition energy. Some trapping conventions instead call the positive energy slope an effective magnetic moment. The sign convention must be stated; either gives the same absolute sensitivity of transition frequency to a field fluctuation.