Domains and hysteresis কী?

A raw lump of iron is not magnetised: it shatters into domains, micron-scale regions each fully magnetised but pointing differently, so the exterior field (and its energy cost) nearly cancels. An applied $\vect H$ moves the domain walls — favourable domains grow — and then rotates whole domains into line. Walls snag on defects, so the process is irreversible: sweep $H$ up and down and $B(H)$ traces a loop, the hysteresis cycle. Switch the current off and a remanent field $B_{\text{r}}$ survives; cancelling it needs the reverse coercive field $H_{\text{c}}$. The loop’s enclosed area is energy dissipated per cycle and per unit volume. Soft materials (silicon steel, ferrites: thin loop, small $H_{\text{c}}$) make transformer and motor cores; hard ones (alnico, Nd$_2$Fe$_{14}$B: fat loop, huge $H_{\text{c}}$) make permanent magnets — and magnetic memory: the refrigerator magnet and the hard disk are hysteresis loops that refuse to forget.