RHEOMAGNETIC PROPERTIES OF MIXED MAGNETIC PARTICLE SUSPENSIONS

Single-domain magnetic particles are the essential ingredient of magne tic tapes, particulate recording disks and magnetic stripes. The parti cles are single-domain gamma-Fe2O3, CrO2 or barium ferrite, and non-ma gnetic alpha-Fe2O3 mixture. Each of these particles has intrinsic coer civity, which should be matched with the magnetic field strength of th e writing element of a particular device. In this study a magnetic ind uctance measurement with low field strength was employed to obtain the magnetic permeability of suspensions containing two of the particle t ypes mixed together as a function of composition and volume fraction o f particles. The bulk magnetic property B is a linear combination of t he contributions from each particle type such that the ''excess'' indu ctance is L - L(s) = SIGMAphi(i)B(i) where phi(i) is the volume fracti on and B(i) is the magnetic property of particle type i. For the non-m agnetic alpha-Fe2O3, B(i) = 0. This allows the formulation of mixed pa rticle suspensions to obtain a desired property for custom-designed ma gnetic particle coatings. However, mixing magnetic particle types will broaden or produce a bimodal switching field distribution. This may a ffect the squareness of the magnetic hysteresis loop. These properties should be taken into account for the design of a practical magnetic c oating with mixed particle suspension. Another requirement of the magn etic particle suspensions is that they remain well dispersed, even tho ugh strong magnetic forces between the particles promote flocculation. An extension of the inductance measurement technique is employed to s tudy the flocculation of a suspension containing magnetic gamma-Fe2O3 and non-magnetic alpha-Fe2O3. The presence of the alpha-Fe2O3 decrease s the flocculation state of the suspension. Thus the suspension stabil ity is enhanced by incorporating a small amount of non-magnetic partic les in addition to surfactant.