Taeg M. Kwon

Effects of shear rate and particle concentration on rheological properties of magnetic particle suspensions

Rheological characterizations were made for various types of magnetic particles (rod-like γ-Fe2O3, CrO2, and plate-like Ba-ferrite) and nonmagnetic (rod-like α-Fe2O3) suspensions, in terms of particle concentration and shear rate. Shear rate dependence on viscosity was accurately described by the Casson equation. The highest yield stress for Ba-ferrite among the four types of particles represents the sensitive flocculation characteristics of Ba-ferrite with respect to concentration. The effect of non-magnetic α-Fe2O3 and magnetic γ-Fe2O3 particle mixtures on suspension viscosity was also examined, and a negative deviation from the tie line of individual particle viscosities was observed.

Viscosity of magnetic particle suspensions

A rheological approach was used to study the effects of microstructure on the viscosity of magnetic particle suspensions as functions of concentration and shear rate. Empirical formulas derived from mean-field theory and the Mooney equation were used to relate viscosity with particle concentration for both rod- and plate-like particles. The Casson equation was employed to investigate the shear rate dependence of the viscosity.

Rheo-optical study of magnetic particle orientation under external fields

Abstract The orientation distribution of magnetic particle suspensions subjected to external magnetic and hydrodynamic fields is of considerable practical interest as it relates to the manufacture of particulate recording media. Polarization modulated linear dichroism, a non-invasive optical technique developed in recent years for studying suspension microrheology, is adopted in this paper to investigate the orientational ordering in ethylene glycol suspensions of rod-like γ-Fe2O3, and plate-like barium ferrite particles subjected to applied magnetic fields up to 2000 Oe. Particle concentrations range from a volume fraction φ of 10−5 to 10 −4, which is hydrodynamically but not magnetically dilute. Linear dichroism measurements /LDmax (where LDmax is the dichroism at uniaxial alignment), which reflect the degree of particle alignment about the field axis, are shown to be sensitive indicators of particle magnetic interactions (Δn″ vs. φ), of the effects of particle intrinsic magnetic properties (Δn″ vs. the coercivity Hc), of the effects of particle shape (γ-Fe2O3 vs. barium ferrite), and of qualitative microstructural changes during suspension aging. A scaling theory for uniaxial single domain systems is presented, which qualitatively reproduces the observed φ, Hc and shape dependences of the order parameter deduced from the data. The theory modifies the classical Debye—Langevin approach to paramagnetism by including magnetic anisotropy. Finally, we report preliminary observations which suggest an empirical equivalence between the magnitude of independently applied hydrodynamic and magnetic fields required to produce a suspension order for these particle systems.

A device for measuring the concentration and dispersion quality of magnetic particle suspensions

A technique, called rheomagnetic measurement, for studying the concentration and orientation of magnetic particles through inductance measurement is presented. The particles are oriented in a predominantly extensional flow field, and, because they are magnetic, their orientation can be detected with a weak magnetic sensing field. Because flocs of magnetic particles orient differently in a flow field than primary particles do, this method can be useful in obtaining information about the particle flocculation aspect of dispersion quality. A magnetic sensing field can also be used to detect the particle concentration in a quiescent flow. Experimental data on the effects of particle concentration and milling for rod-like gamma -Fe/sub 2/O/sub 3/ and plate-like Ba-ferrite suspensions are discussed. The results for Ba-ferrite magnetic markedly contrast with those for the rod-like magnetic particles but showed similarity with those for rod-like gamma -Fe/sub 2/O/sub 3/. >

Rheomagnetic properties of mixed magnetic particle suspensions

Abstract Single-domain magnetic particles are the essential ingredient of magnetic tapes, particulate recording disks and magnetic stripes. The particles are single-domain γ-Fe 2 O 3 , CrO 2 or barium ferrite, and non-magnetic α-Fe 2 O 3 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 = Σφ i B i where φ i is the volume fraction and B i , is the magnetic property of particle type i . For the non-magnetic α-Fe 2 O 3 , 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 γ-Fe 2 O 3 and non-magnetic α-Fe 2 O 3 . The presence of the α-Fe 2 O 3 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.

Rheomagnetic measurements on suspensions of magnetic recording particles

Measurement of magnetic particle flow orientation properties is of considerable practical and fundamental interest as it relates to the particulate recording tape and disk manufacturing industry. For production of high‐quality magnetic tapes and disks, the suspension must be well dispersed. Previous study focused on rodlike single‐domain magnetic particles for longitudinal magnetic recording. In contrast, platelike particles are considered here for use in perpendicular recording. Both theory and experiment have demonstrated that a magnetic sensing technique employed to measure the hydrodynamic orientation provides a measure of the flocculation state for rodlike magnetic particle suspensions. Here the previous theory is extended and new experimental results are shown to demonstrate that the hydrodynamic orientation measurement apparatus can also be used to measure the flocculation state of platelike particles. The key new experimental result is the contrast between the flow orientation properties of the ro...

A device for measuring concentration and dispersion quality of rod and plate-like magnetic particle suspensions

Rheomagnetic measurement, a technique for studying the concentration and the orientation of the magnetic particles through inductance measurement, is described. The method may complement conventional techniques. Compared to conventional techniques, the rheomagnetic method is uniquely suitable for process control during the early stages of slurry production in manufacturing, since it requires little test time, measures the dispersion quality of concentrated suspensions, operates automatically, and can be placed on-line. The concepts upon which the measurement technique is based are discussed. Experimental data on the effects of particle concentration and milling of rod-like gamma -Fe/sub 2/O/sub 3/ and plate-like barium-ferrite suspensions are discussed. This is the first observation reported on the rheomagnetic orientation measurement for plate-like magnetic particles and the results markedly contrast with those from the rod-like magnetic particles.<<ETX>>