Effect of orienting fields on agglomeration in dilute dispersions of magnetic fine particles

Static light scattering was used to investigate dilute dispersions of fine gamma -Fe/sub 2/O/sub 3/ magnetic particles. The particles were acicular and on average 0.39 mu m long and 0.056 mu m in diameter. They were dispersed in organic solvents at concentrations of approximately 10/sup -4/ vol%, corresponding to an average interparticle separation of 9 mu m. Computer programs using a coupled dipole algorithm to predict the scattering from the particles were written. Experimental results do not agree with computer calculations of the light scattering from single particles, but are consistent with scattering from thick ropelike agglomerates of particles. The size of these agglomerates was estimated after 10 s to be less than 100 particles, while the thickness of the ropes is at least 5 particles.<>     

Measurement of domain wall mobility in GdIG

The promise of memory and logic applications for magnetic bubble domains has heightened the general interest in the occurrence and behavior of cylindrical domains in various materials. A train of short magnetic field pulses transforms the usual demagnetized snakelike domain pattern into a minimum-energy circular domain pattern which is stable with no dc bias field. This work maps the limits on field pulse width and height for producing the array. Domain wall mobility studies have also been made making use of the simple domain structure. The mobility studies suggest that there are regions of the sample where the local mobility is as much as ten times the average value. This leads to a simple model for the formation of the bubble array from the labyrinth structure, involving the “pinching off” of the snakes. Similar experiments on orthoferrite having very uniform properties fail to produce bubble domains, tending to support the model. Detailed results are presented.     

Magnetic hysteresis model

A dynamical model is described which permits calculation of the excitation currentIas a function of time in a laminated grain-oriented (G-O) steel transformer core. The independent variable is the magnetic flux density or, equivalently, the coil voltage less theIRdrop associated with the resistanceRof the windings. Recent observations on flux reversal mechanisms in GO steel indicate that, in the range of magnetic field intensities typically present in transformer cores, the important reversal processes are the motion of long 180° domain walls continuous across grain boundaries and the motion of 90° walls within individual grains. These processes are represented in the model by two subcircuits connected in series. Each subcircuit consists of an inductive element in parallel with a linear resistor which accounts for the eddy current losses accompanying the flux change. The properties of each inductive element (flux vs. current) reflect the two wall motion mechanisms, respectively, in the limit of zero frequency. This model is capable of faithfully simulating minor loop behavior as well as the response to complex waveforms; e.g., the superposition of two or more frequencies. The circuit equations are solved, and some results of computer calculations using a program that implements this model are presented.     

Angular variation of coercivity in thin-film media by micromagneticmodel

A computer micromagnetic simulation model is employed to study the angular variation of coercivity (out of plane) in thin-film media. Each grain of the film is uniformly magnetized and switches coherently according to the S-W model. The M-H hysteresis loops for applied fields at different angles to the film plane are obtained for various magnetostatic interaction parameters, quantum exchange constants, and easy axis distributions. The angular variation of the coercivity curve exhibits a peak if there is magnetostatic interaction among the grains and qualitatively follows the experimental results well. The peak in the angular variation of coercivity is seen to result from the demagnetizing field. The rotational hysteresis process is also investigated by means of this model