(a)  Hysteresis loss
(b) Relaxation loss
Figure 1: Schematic diagrams of conventional models for magnetic loss. (a) Hysteresis loss equivalent to the area of loop, and the potential energy in magnetic fields. (b) Relaxation loss given by the out-of-phase component of AC susceptibility. As illustrated in the sketches, in Néel relaxation, the magnetic moment shown by the yellow arrow reverses (the particle does not rotate), while in Brownian relaxation, the magnetic core (the red sphere) rotates with absorbed molecules (the green chains) as a whole (the magnetic moment does not reverse).