Journal of Magnetism and Magnetic Materials 54-57 (1986) 1697 1698 1697 A STUDY ON H~-ENHANCEMENT IN Co-MODIFIED 7-Fe20 3 A. BENSAOULA, C.W. CHU, P.H. HOR, A. IGNATIEV, J.S. LIU, R.L. MENG, A. MESARWI, J. RICHARDSON, C.S. TING, Y.Q. WANG and J. WOLFE Magnetic Information Research Laboratory, University of Houston, Houston, TX 77004, USA We have investigated the effects of valence states and Co-distributions on the coercivity of particulate and thin film Co-treated y-Fe203. A maximum H,. of 2 kOe has been obtained in thin fihns prepared by a new simple method. We found that Co-ions exist in the divalent states probably in the form of ferromagnetic CoFe204 and that simple CoO to y-Fe203 interfaces do not appear to enhance H r. On the other hand, we observed complex diffused interfaces in a thin film sample with enhanced H,,. The diffused interfaces may be responsible for the Hcenhancement in Co-modified "y-Fe20) recording media. 1. Introduction Various treatments [1] of the particulate g-Fe203 recording medium with Co-salts have been developed to enhance its H~.. Different suggestions have been made to account for the H~-enhancement by invoking im- proved magnetic anisotropy of the Co-treated y-Fe203 particles. Unfortunately, almost all of these suggestions are at best phenomenological and the published recipes sometimes do not yield the results expected. Therefore, we have intitiated a systematic study on the underlying mechanisms responsible for the Hcincrease in 7-Fe203 by Co-treatments. Results of our preliminary study are reported here. 2. Experimental Four sets of particulate samples (P1-4) were in- vestigated. Samples P1-3 [2,3] were obtained [2] by drying the slurry of Fe-oxides and Co(NO3)O 2 below 100°C; and heating the mixture first at 375°C in a H2-atmosphere for 1.5 h and then at 325°C in an air-N 2 atmosphere for 2 h prior to slow cooling to 100°C. The 1400 starting Fe-oxides used were the 99% FeOOH, 99.9% c~-F%O3, or 99.5% F%O 4 all from Alfa. Sample P4 were 1200 generated [3] by heating the 99.9% 7-FezO 3 in a Co(NO3) 2 solution at 100°C followed by drying below 1000 100°C. Two sets of thin film samples (F1,2) were pre- pared by first depositing c~-Fe203 films of 5000 8000 A. "~ 0 800 thick on glass substrates by reactive sputtering [4] from a 99.9% pure Fe-target. For the FI samples, the c~-F%O 3 :~ 6 O0 films were converted to 7-F%O 3, followed by sputtering an over-layer of CoO. On the other hand, the F2 sam- ples were obtained by first depositing a layer of CoO on 400 the ~-Fe203 films and then transforming the composite films to Co-modified ~,-Fe203 by reduction and then gO0 reoxidation processes. Magnetic properties of the samples were measured 0 with a PAR vibrating sample magnetometer, a Faraday balance and a ballistic flux meter. Results obtained by different magnetometers agreed with one another, when similar measurements were made. Crystal structures, crystallite sizes, chemical compositions, valence states and depth profiles were examined by the standard tech- niques of X-ray diffraction, XPS, EDAX, SEM, and AES. 3. Results and discussion All samples investigated displayed a single ¥-F%O 3 phase except for the samples with large Co-content (x = Co/Fe), which exhibited two extra weak lines with one assignable to Co304. In general, the quality of the X-ray data on film samples was poor. The aspect ratio of the P1 samples was = 7 whereas that of the P2 4 was = 1. The x-dependence of H c is shown in fig. 1 for P1 3 samples. Although H~ increases with x, in general agreement with previous results [1], all samples with x = 0 have a relatively low H c. It is evident that the Coefficiency in H~.-enhancement depends on the start- ing material and may be associated with the aspect ratio 52.2 5L4 _ ~7.1 60.5 66.9 PlgA ~.a A 70.9 Sl.561.71 55.7 I 52.1 5Z2 52_1 • p--I P-2 • P-3 • 48.9 A 50.e I I I I 0 4 8 [2 16 20 24 28 Co (at %) Fig. 1. The variation of H~ with x: The numbers denote the saturation moments in emu/g. 0304-8853/86/$03.50 © Elsevier Science Publishers B.V.