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Magnetic viscosity (MV) effect is a disturbance to the transient electromagnetic method (TEM). This effect will cause slow decay at the intermediate-late time of the TEM responses, leading to erroneous interpretation. Although there are extensive researches on the MV distortion to TEM data, they are focused on conventional loop source configuration, while little has been studied for grounded-wire source configuration that has become the essential role for relatively deep-buried targets. Hence, we present the study of MV effect on the grounded-wire source TEM data. We first derived the formulas for calculating the secondary field due to MV effect, based on the Chikazumi model. The dependence of the MV effect on model parameters, such as resistivity, susceptibility, offset and geometry of the superparamagnetic layer, is then examined to illustrate the intrinsic physical mechanism of MV effect. We find that the MV effect on the grounded-wire source TEM data can be suppressed by adjusting the parameters of the survey design. Thereafter, we propose the method to quantify the MV effect when estimates of resistivity and susceptibility are available. Finally, the method for choosing the optimal offset to suppress the MV effect is proposed, which can be used as a guide for the installation of fieldwork of grounded-wire TEM survey.

In this work, we report on the determination of the optimum level of SiO2 in (CoCrPt)1-x (SiO2)x perpendicular recording media. We have determined the optimum level via studies of magnetic viscosity which have been compared to recording properties. We find that the optimum level of SiO2 is of the order of 10%. At this level, studies of the activation volume of reversal show that the grains become essentially exchange decoupled and that a further increase in the level of SiO2 produces no further improvement in the decoupling. We also find that this level of SiO2 corresponds to optimum recording performance and that increasing the level of SiO2 further results in the production of a system with smaller grains where thermal loss is observed to increase dramatically

Perpendicular magnetic recording at high areal densities and high data rates has been studied. Using ring heads and perpendicular media with soft underlayer, we have successfully demonstrated the areal density of 60 Gb/in/sup 2/ at the corresponding ultrahigh-linear density of 750 kbpi, a bit-aspect ratio (BAR) of 9.4, and a data rate of 300 Mb/s. Comparing the BAR of 5.7 in our recent 63.2 Gb/in/sup 2/ demonstration on longitudinal media, a larger BAR has been demonstrated with present perpendicular recording system. We have also demonstrated the data rate capability of 1 Gb/s by using both ring heads and modified-single-pole heads on perpendicular media with soft underlayer. Finally, a comparison of high-data-rate recording between ring heads and single pole heads has shown that neither magnetic viscosity nor gyromagnetic switching in perpendicular media is the limiting mechanism for magnetic recording at data rates as high as 1 Gb/s.

Remanence measurements have been made on a series of CoPtCr thin films of varying composition in order to correlate the interaction field with media noise. /spl delta/M versus h curves have been analyzed using a phenomenological model of Che and Bertram, in which the interaction field is given by h/sub int/=/spl alpha/M/sub R/+/spl beta/(1-M/sub R//sup 2/), where M/sub R/=M/sub r/ (or M/sub d/). We have shown analytically that /spl alpha/ and /spl beta/ can be simply determined respectively from the area under the /spl delta/M versus h curve (or /spl delta/h versus M/sub R/) and the zero crossing of the curve. The parameter /spl beta/, which is related to the strength of the fluctuation field, was found to increase with increasing reverse dc erase noise (measured at zero remanence). Surprisingly, perhaps, no correlation was found between this noise and /spl alpha/. By contrast, /spl alpha/, which characterizes the strength of the magnetostatic and/or exchange interaction, was positive for all samples and decreased with increasing dc erase noise (measured at saturation remanence). As expected, /spl alpha/ was also found to increase with the magnetic viscosity, S, and to decrease as the coercivity, H/sub c/, increased. >

When domain wall pinning is due to random arrays of inhomogeneities, the dynamical process of unpinning could be described by the weak or the strong domain wall pinning models. Based on these two models, the values of activation volume V/sub w/ (for weak pinning) and V/sub s/ (for strong pinning) of bowed domain wall of thermally annealed iron-rich FeAu specimens were estimated. The results were then used to calculate their corresponding fluctuation held H/sub fw/ (using V/sub w/) and H/sub fs/ (using V/sub s/), for each of the specimens. Comparing the values of H/sub fw/ and H/sub fs/ with the Barbier's plot which relates magnetic viscosity to the coercive field H/sub 0/ of a wide variety of magnetic materials, it was found that H/sub fw/ of the low age specimens plotted against their H/sub 0/ exhibited similar behaviour as in the Barbier's plot. However, when values of H/sub fs/ were used in the plotting, similarity with the Barbier's result was only found in specimens after prolonged aging. It is also found that when the localized demagnetizing effect on the bowed portion of the domain wall due to the presence of the surface free-poles is to be considered in the derivation, the fluctuation field is smaller than that without such consideration by a factor of F/2F/sub 0/ in the case when the material has a weak pinning regime, where F and F/sub 0/ are the pinning force per unit area with and without the demagnetizing consideration respectively, and by a factor of 1/4 in the case of strong pinning. >

The magnetic viscosity coefficient (S/sub v/) and the rotational hysteresis loss (W/sub r/) are measured for M-type hexagonal strontium ferrite (SrM) fine particles whose average diameter (D) and coercive force (H/sub cJ/) differ. The relationship between S/sub v/, H/sub cJ/, the activation volume (v) and W/sub r/ is studied, together with milling effects. This experiment shows that S/sub v/ is constant and independent of the magnetic field. Ln S/sub v/ is in proportion to In H/sub cJ/, and H/sub cJ/ approximately v/sup -2/ could be obtained in SrM fine particles that had less milling effects. It is found that Sv increases and v decreases with SrM fine particles (D=0.3 mu m) whose Wr curve increases in width with milling, even through their H/sub cJ/ significantly decreases. This can be explained as an increase in high magnetic field components of anisotropy field (H/sub A/) caused by milling. >

Theoretical predictions are made of the time-dependent behavior and the irreversible susceptibility of a system of uniaxial, single-domain particles. These results are used to predict the fluctuation field H/sub f/. These predictions are consistent with experimental data on particulate recording media. >

Defining the fields B/sub n1/ and B/sub n2/, where the magnetization reversal begins and ends, the simple linear relationship B/sub n2/=aB/sub n1/+b mu /sub 0/M/sub s//cos theta was observed, where a and b are coefficients. Furthermore, from the temperature dependence of the change in magnetic energy Delta E in the system, an activation volume was obtained as a function of temperature. The values are in good agreement with the results obtained from magnetic viscosity measurements. This indicates that magnetization reversal is thermally initiated in a small activation volume. >