A magnetic core powder including granular powder A of Fe-based, magnetic, crystalline metal material and granular powder B of Fe-based, magnetic, amorphous metal material; the particle size d50A of granular powder A at a cumulative frequency of 50 volume % being 0.5 m or more and 7.0 m or less, and the particle size d50B of granular powder B at a cumulative frequency of 50 volume % being more than 15.0 m, in a cumulative distribution curve showing the relation between particle size and cumulative frequency from the smaller particle size side, determined by a laser diffraction method; the magnetic core powder meeting (d90Md10M)/d50M of 1.6 or more and 6.0 or less, d10M being a particle size at a cumulative frequency of 10 volume %, d50M being a particle size at a cumulative frequency of 50 volume %, and d90M being a particle size at a cumulative frequency of 90 volume %.

A spin transfer oscillator (STO) with a seed/FGL/spacer/SIL/capping configuration is disclosed with a composite seed layer made of Ta and a metal layer having a fcc(111) or hcp(001) texture to enhance perpendicular magnetic anisotropy (PMA) in an overlying (A1/A2).sub.YFeCo laminated field generation layer (FGL). The spin injection layer (SIL) may be laminated with a (A1/A2).sub.XFeCo configuration. The FeCo layer in the SIL is exchanged coupled with the (A1/A2).sub.X laminate (x is 5 to 50) to improve robustness. The (A1/A2).sub.Y laminate (y=5 to 30) in the FGL may be exchange coupled with a high Bs layer to enable easier oscillations. A1 may be one of Co, CoFe, or CoFeR where R is a metal, and A2 is one of Ni, NiCo, or NiFe. The STO is typically formed between a main pole and trailing shield in a write head.