(SrO:6Fe2O3). In terms of their magnetic properties, the different ferrites are often classified as "soft", "semi-hard" or "hard", which refers to their low or high magnetic coercivity, as follows. Soft ferrites Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese compounds. Soft ferrites are not permanent magnets. They have magnetism (much like mild steel), but when the magnetic field is removed, the magnetism decreases. Soft ferrites are commonly used as transformers (to change the voltage from primary to secondary windings). As a result, soft ferrites are also called transformer ferrites.They have a low coercivity. The low coercivity means the material's magnetization can easily reverse direction without dissipating much energy (hysteresis losses), while the material's high resistivity prevents eddy currents in the core, another source of energy loss. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies and loopstick antennas used in AM radios. The most common soft ferrites are: Manganese-zinc ferrite (MnZn, with the formula ). MnZn have higher permeability and saturation induction than NiZn. Nickel-zinc ferrite (NiZn, with the formula ). NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz. For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice. The exception is with common mode inductors, where the threshold of choice is at 70 MHz. Semi-hard ferrites Cobalt ferrite, CoFe2O4 (CoO·Fe2O3), is in between soft and hard magnetic material and is usually classified as a semi-hard material. It is mainly used for its magnetostrictive applications like sensors and actuators thanks to its high saturation magnetostriction (~200 ppm). CoFe2O4 has also the benefits to be rare-earth free, which makes it a good substitute for Terfenol-D. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. This can be done by magnetic annealing, magnetic field assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage to be ultra fast (20 min) thanks to the use of spark plasma sintering. The induced magnetic anisotropy in cobalt ferrite is also beneficial to enhance the magnetoelectric effect in composite. Hard ferrites In contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. Iron oxide and barium or strontium carbonate are