Structure and magnetic properties of nanostructured Ni-ferrite

The structural and magnetic properties of NiFe2 O-4 ultrafine powders synth esized by coprecipitation, a nonconventional method of preparation, were in vestigated. The samples were obtained by annealing at relatively low temper atures (300-600 degreesC) and characterized by X-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. The average particle diam eter ranges from 4 to 15 nm, as determined by X-ray diffraction. All nanome tric powder samples presented strong superparamagnetic relaxation at room t emperature and reduced magnetic hyperfine fields at - 193 degreesC. Magneto metry measurements indicated different magnetic behavior related with cryst allinity of samples, coercivity as high as 168 Oe at 27 degreesC, value tha t is nearly two times higher than coercivity of bulk Ni-ferrite. (C) 2001 E lsevier Science B.V. All rights reserved.