TlCLnATE RECORDING MEDU

F. T. Parker and A. E. Berkowitz Center for Magnetic Recording Research, R-001, University of California, San Diego

La Jolla, California 92093, USA

Determination of the anisotropy field HK in random uniaxial magnetic recording particles is obtained from an analysis of the Mossbauer polarization field dependence. The calculations are based on the Stoner-Wohlfarth model.1 One considers those particles with uniaxial axes between Bi and ei + dei with respect to the applied field, Ha. The moments are then at the calculated angle ei with respect to Ha. For this subset of particles, the relative intensities of the f i t and sixth lines in a magnetically split Mossbauer spectrum are then Ai 1 = Ai 6 = 1.5 (1 + COS2 pi 1, and the second and fifth lines to Ai 2 = Ai 5 DC 2.0 sin2 pi, where pi is the angle between the moment and the yray propagation direction. The third and fourth lines are one- third the intensity of the f i i t line. The total Mossbauer spectrum is obtained by summing over all e, Aj.= & i A i j M , where mi 05 sin2 ai is the weighting function, and ai is the angle between the uniaxial axis and the yray propagation direction. The resultant line areas are then in the ratio 3:2p:1:1:2p:3, where the polarization is given by p = 1.5*Az/Ai. The grid spacing dei in the calculations was taken as 0.01O. Calculations of the polarization were done for Ha along the y ray direction (longitudinal, pL), and for Ha perpendicular to the yray propagation direction (transverse, 4.). The results are shown in Fig. 1.

Fig.1 H a/H K

Relative magnetization and Mossbauer polarization B. applied field for random single domain uniaxial particles. The field is either along the yray propagation direction ( p , right hand scale) or transverse to this direction @T. left hand scale).

The figure shows that the transverse polarization develops more slowly than the moment, offering better sensitivity at high H;JHK. Note that the applied field must be much greater than the anisotropy field to obtain zero pL. The saturation moment Ms has to be determined accurately from magnetization measurements to obtain HK, whereas, the saturation polarization is already known. Since the Mossbauer technique can often show different phases as separate Mossbauer subspectra, anisotropy fields for each phase may be determined. Conversely, minor amounts of secondary phases can be ignored in the Mossbauer fitting routine.

Several samples of magnetic recording particles were examined by Mossbauer spectroscopy in small transverse fields. Some magnetic measurements have been shown elsewhere.2 These include a small yFe2O3 ( 2 5 0 ~ dia. by 2000A), a precursor material (partially reduced y), and two samples with a Co-modified precursor surface. The first, third, and fourth of these were from the same processing batch; the second was prepared similarly. Also included in this study, is a larger yFe2a ( 0 . 2 ~ dia. by lpm). Some room temperature results are shown in Table 1:

Table 1

small y Fe203 325 1.8 1.16(3) 1.5(2) small y Fe203 325 3.0 1.22(2) 2.0(2) precursor 298 1.8 1.21(3) 1.2(2) 4% CO 737 1.8 1.06(2) 2.2(2)

16% CO 735 1.8 1.08(3) 2.1(2) large Y 340 3 .O 1.20(2) 2.1(2)

4% CO 737 3.0 1.16(3) 2.5(3)

The fits were obtained by using independent pseudo-Lorentzians on the six Mossbauer lines. Corresponding peak areas were averaged (first and sixth, etc). With maximum percent effects typically about 7%, saturation corrections were small, not included in Table 1. They were estimated from the known 3:l thin absorber ratio in comparison with the observed ratio in the large y sample, and linearizing to correct the ratio A2/A1. The maximum correction is then a decrease in HK about half the size of the statistical error for A2/A1- 1.2, and zero for A2/A1 = 0. Interparticle interactions (demagnetizing fields) were ignored; these would tend to decrease the observed HK.

No change in polarization was seen in the small y before and after the fields were applied. Note that the increase in absorbed CO from 4% to 16% did not increase the anisotropy, consistent with earlier results2. For the materials based on small y, the coercivity is seen to increase with anisotropy field, but not proportionally.

References:

1. E. C. Stoner and E. P. Wohlforth, "A Mechanism of Magnetic Hysteresis in Heterogeneous Alloys," hoc. Roy. Soc. (London) m, p.599 (1948).

2. A. E. Berkowitz, F. T. Parker, E. L. Hall, and G. Podolsky, "Toward a Model for Co- Surface-Treated Fe-Oxides," 4th Joint MMM-Internag. Conf., Vancouver, B.C. (1988).

CH2731 - 8/89/0000 - EP-6 $01 .OO @ 1989 IEEE EP-6