Some explanation here:http://users.catchnet.com.au/~rjandusimports/tut_2a.html
Ferrite materials can be divided into two groups: those with initial permeabilities below 1000 which are nickel-zinc compounds and those above 1000 which are made from manganese-zinc compounds. Nickel-zinc ferrites exhibit high volume resistivity, moderate temperature stability and can offer high Q factors for the 0.5 to 100 MHz frequency range. They are well suited for low power, high inductance applications, and their high permeability factors make them very useful for wideband transformer applications. The manganese-zinc group have relatively low volume resisitivity and moderate saturation flux density. They can give high Q factors for the 1 kHz to 1 MHz frequency range, and some are sutiable for switched-mode power conversion transformers operating between 20 and 100 kHz. Incidently, the high permeability iron powder core made from 26 or 52 material is particularly suitable for the filter inductor in switched mode power supplies.
Some details here:http://www.bytemark.com/products/ferrmat.htm