Magnetic energy is given by: $$ E(t) = \mathbf{M}(t)\cdot \mathbf{H}(t)$$

Hence, the magnetic power becomes: $$P(t) = \mathbf{M}(t) \cdot \frac{\mathbf{H}(t)}{dt} + \mathbf{H}(t) \cdot \frac{\mathbf{M}(t)}{dt}$$

However, (for ferromagnetic resonance) the absorbed power of the magnetic system is described by: $$P(t) = -\mathbf{M}(t) \cdot \frac{\mathbf{H}(t)}{dt} = \mathbf{H}(t) \cdot \frac{\mathbf{M}(t)}{dt}$$

Why is the absorbed power of a magnetic system not equal to the second equation?


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