RHEOMAGNETIC PROPERTIES OF MIXED MAGNETIC PARTICLE SUSPENSIONS

Single-domain magnetic particles are the essential ingredient of magnetic tapes, particulate recording disks and magnetic stripes. The particles are single-domain gamma-Fe2O3, CrO2 or barium ferrite, and non-magnetic alpha-Fe2O3 mixture. Each of these particles has intrinsic coercivity, which should be matched with the magnetic field strength of the writing element of a particular device. In this study a magnetic inductance measurement with low field strength was employed to obtain the magnetic permeability of suspensions containing two of the particle types mixed together as a function of composition and volume fraction of particles. The bulk magnetic property B is a linear combination of the contributions from each particle type such that the ''excess'' inductance is L - L(s) = SIGMAphi(i)B(i) where phi(i) is the volume fraction and B(i) is the magnetic property of particle type i. For the non-magnetic alpha-Fe2O3, B(i) = 0. This allows the formulation of mixed particle suspensions to obtain a desired property for custom-designed magnetic particle coatings. However, mixing magnetic particle types will broaden or produce a bimodal switching field distribution. This may affect the squareness of the magnetic hysteresis loop. These properties should be taken into account for the design of a practical magnetic coating with mixed particle suspension. Another requirement of the magnetic particle suspensions is that they remain well dispersed, even though strong magnetic forces between the particles promote flocculation. An extension of the inductance measurement technique is employed to study the flocculation of a suspension containing magnetic gamma-Fe2O3 and non-magnetic alpha-Fe2O3. The presence of the alpha-Fe2O3 decreases the flocculation state of the suspension. Thus the suspension stability is enhanced by incorporating a small amount of non-magnetic particles in addition to surfactant.