a soft ferrite
When magnetic engineers say a material is 'soft', they do not mean it is physically squishy — a soft ferrite is a hard, brittle ceramic. They mean it is magnetically soft: easy to magnetise and just as easy to demagnetise, so its magnetisation flips back and forth in step with an applied field and does not stay stuck once the field is gone. Picture a weathervane that swings freely to follow the wind and never jams — that is a soft magnet. Soft ferrites are the invisible cores hidden inside transformers, inductors, and chokes.
In the language of the hysteresis loop — the graph of magnetisation versus applied field — a soft ferrite has a very narrow, tall loop: high permeability (a small field produces a large magnetisation) and low coercivity (almost no field is needed to reverse it). The two workhorses are manganese-zinc (MnZn) ferrite, which has very high permeability and is used up to about 1 megahertz, and nickel-zinc (NiZn) ferrite, whose higher electrical resistivity lets it work from megahertz up to hundreds of megahertz. Both are spinel-structured. Their prized quality is low loss: a narrow loop wastes little energy per magnetisation cycle, and the high resistivity of the oxide smothers eddy currents, so very little of the energy passing through the core is dissipated as heat.
That low loss is why soft ferrites are everywhere current has to be transformed or filtered at high frequency: the tiny transformer in every switch-mode power supply and phone charger, the inductors that smooth power-supply ripple, the beads clamped on cables to strangle electromagnetic interference, the coils in wireless chargers, and antenna rods. Getting the loss low is a microstructural craft — large, uniform grains, very low porosity, and carefully controlled grain-boundary chemistry all reduce it. The honest limits: every ferrite has a Curie temperature above which it loses its magnetism entirely, and loss climbs steeply as frequency and magnetic flux are pushed higher.
Inside a phone charger a coin-sized MnZn ferrite transformer switches its magnetisation tens of thousands of times a second. Because the ferrite is magnetically soft and electrically almost insulating, each flip costs very little energy, so the transformer stays cool while stepping the voltage down.
A soft ferrite follows the field willingly and lets go the instant it is removed — ideal for cores that switch millions of times a second.
'Soft' and 'hard' here describe magnetic behaviour, not mechanical hardness — both soft and hard ferrites are brittle ceramics. A soft ferrite makes a poor permanent magnet precisely because it lets go of its magnetisation so easily; that is a feature for a core, a failing for a fridge magnet.