SCANNING TUNNELING MICROSCOPE AND TUNNELING STABILIZED MAGNETIC FORCE MICROSCOPE CHARACTERIZATION OF MAGNETIC NANOCRYSTALLINE MATERIALS

Atomic scale surface as well as magnetic domain pattern studies have been carried out on nanocrystalline Fe86Zr7B6Cu and Fe73.5CuNb3Si13.5B9 melt cast ribbons by using scanning tunneling microscope and tunnel stabilized magnetic force microscope (TSMFM) techniques. In the micrometer range for the Fe-Cu-Nb-Si-B alloy we observe at the surface the typical smooth wavy and hilly structures expected for amorphous alloys. However, in nanocrystalline Fe-Zr-B-Cu we find uniformly distributed ellipsoidal bumps aligned along the direction of ribbon casting. At a nanometric range, on the other hand, both the alloys are found to contain homogeneously distributed nanocrystallites of 10+/-2 nm diameter as determined from an autocorrelation function analysis of our data. Such a determination of the nanocrystallite size with the implication that particle size is significantly smaller than the magnetic exchange length explains the unusually soft magnetic properties of nanocrystalline materials. Furthermore the smooth morphology of the nanocrystallites suggests that they are embedded homogeneously throughout the ribbon in an amorphous matrix. In our TSMFM studies for the Fe-Cu-Nb-Si-B ribbon we observe no magnetic contrast. In the case of the Fe-Zr-B-Cu ribbon, however, elliptic-shaped magnetic patterns are observed which probably arises from stray fields concentrating at the depressions between clusters of nanocrystals.