Rotational hysteresis in metal particle recording media: Remanent moment vectors

A collective dipole is written into a section of commercial magnetic recording tape containing metal particles by applying a saturating field in the plane of the tape at some particular angle theta(set) with respect to the tape axis. The section is then rotated for many cycles n in a lower magnetic field. The field is reduced to zero at the end of the rotation to the angle theta in the nth cycle. Then the components of the remanent dipole mu(x)(n,theta) and mu(y)(n,theta) are measured. The experiment is then started over beginning with saturation. By plotting the components on a mu(x),mu(y) diagram, one discovers a remarkable pattern of almost closed ellipses moving with cycle number n such that mu(cx)(n) and mu(cy)(n), the coordinates of the apparent centers of the ellipses, migrate toward the origin. The decay of the two components follows mu(x,y)(n,theta)=mu(x,y)(infinity,theta)-Delta mu(x,y)(theta)n(-p), with small values of p<0.2 in the critical field near in magnitude to the coercive field. The ellipses become centered about the origin after many cycles if the field is near or above the critical field, but not for lower fields. For theta(set)not equal m pi/2, the results depend on the sense of rotation (clockwise or counterclockwise) in the field. There are so many more particles in the tape than can be treated in a computer simulation that simulations only begin to suggest the effects observed. (C) 1997 American Institute of Physics.