T3sl4co1l Thank you for avalanche of top quality info.I didnt know about that negative permitivity,also I thought ferrite is highest frequency,what is this powder and garnet stuff? Is it like yttrium aluminum garnet aka YAG,the laser crystal?
Powder is just powdered iron, usually used in the VHF range. Small grains of iron (and often other metals), glued together to make solid cores. Grains small enough that skin effect isn't significant until much higher frequencies than other forms.
Well, to be fair, "ferrite" covers a lot more than just the most common (MnZn) power ferrite materials. Chemically speaking, YIG is a ferrite, too (i.e., a "salt" of "ferrous acid" H3FeO3 or such, as it were), but really, in practical terms, YIG is just a mixed oxide that happens to crystallize in the garnet family.
"Like YAG"? Yes, like -- but with Fe(III) replacing Al, which works out pretty nicely in most crystals.
Fe(III), Al, Cr(III), and a number of other ions, fit neatly into each others' spaces, so that suitable crystals can potentially contain any of them in any mixture -- called a solid solution. So, you get yttrium ferrite.
On a related note, colorful atoms like Cr can give pretty colors to otherwise-boring crystals. You can get Cr(III) substituting in Al2O3 (corundum), which gives ruby. Which can be excited with blue to UV light (which it absorbs, hence the red color appearance), and voila, ruby laser!
Or, same thing with Nd:YAG, or...
Conventional ferrites are in the spinel family, by the way -- ordinary spinel is MgAl2O4, but we can substitute Zn, Mn, Ni(II), Fe(II) and others for Mg, and Fe(III) for Al, and the resulting material is usually magnetic. As it happens, substituting a mixture of Mn and Zn (written (Mn,Zn)Fe2O4) gives a soft magnet (does not retain magnetization) with high permeability and reasonable saturation flux density, while substituting a mixture of Ni and Zn gives a higher frequency limit (usually 10-1000MHz) at the expense of lower permeability and saturation.
Incidentally, note that magnetite Fe3O4 is also Fe(II)Fe(III)2O4, i.e., ferrous ferrite, a naturally occurring ferrite, in the spinel family, that is a hard magnet (retains magnetization). Typical ferrite permanent magnets are made with strontium or barium, which don't fit very well in place of Mg, so crystallize in a different (hexagonal) form. And, for whatever reason, that form happens to give a hard magnet.
If chemistry makes your eyes glaze over, suffice it to say, it's a pinch of this and a dash of that, mixed together and heated to make a ceramic (or melted to grow a crystal, or..). Although exact formulations are rather more precise than that sounds, as it doesn't take much impurity to, say, wreck a mu_r >= 10k mix.
Tim