US Patent 7172846 Carrier, developer, image forming apparatus and process cartridge

A carrier including a manganese ferrite core material having a layer on its surface such that 0.1≦K≦30, where K=(S/M)×100, S and M represent the standard deviation and average of M2/(M1+M2), respectively, M ranging from 0.05 to 0.45, and M1 and M2 represent the content of iron and manganese, respectively, in a carrier particle. The carrier has a magnetization ranging from 45 to 75 emu/g at 1,000 Oe, a mass-averaged particle diameter, D4, ranging from 25 to 65 μm and carrier particles having a particle diameter not greater than 12 μm are included in an amount not greater than 0.3% by weight, and a ratio, D4/D1, ranging from 1 to 1.3, where D1 is a number-averaged particle diameter of the carrier.