MAGNETIC VISCOSITY INVESTIGATIONS OF NANOGRAIN IRON-POWDER

Magnetic properties of three kinds of nanogram Fe powder were studied. The first two, designated as NM(Ar) and NM(N), were obtained by ball milling the iron powder under an argon or nitrogen atmosphere. The thi rd, designated NN, was obtained by evaporation and condensation. They have grain sizes of 20, 7, and 60 nm, respectively. The particle sizes of the first two were in the micrometer range, whereas those of the N N were in the nanometer range. The NN powder exhibited a coercivity of 79 mT; its magnetic moment at saturation was lower than that of bulk iron. The coercivities of the NM(N) and NM(Ar) were smaller and their magnetic moment values at high fields were closer to that of bulk iron (210 A . m(2)/kg). The NM(N) and NN samples exhibited marked time dep endences of the magnetic moments following a relatively abrupt change of the applied field. The moment changes by more than 0.5% of the satu ration moment within 50 min and continues to change for many hours. Mu ch smaller effects were detected in the NM(Ar) sample. The viscosities were investigated as a function of temperature and jump field. The re sults showed differences in the behavior of the three samples. The pow ders were consolidated by cold isostatic pressing followed by sinterin g at around 600 degrees C for several hours. The magnetic behavior of the sintered samples approach that of bulk iron. (C) 1996 American Ins titute of Physics.