Time Decay Of Magnetization Research Articles

Time decay of magnetization in particulate media

Intrinsic coercivities, Hc, measured in several particulate recording media are observed to fall as the sweep speed of the field through Hc decreases. Signal amplitudes in magnetic tape are also observed to decay with time; this decay is a function of writing density, increasing with wavelength to 7.3% in 1 hr. at 4 kfc/cm in one case. These phenomena are explained by time decays of magnetization, M, which are attributed to a thermal-fluctuation after effect proposed by Néel [1], involving the irreversible switching of interacting particles. In an external field, several types of magnetic recording media exhibit a decay of magnetization, with time constants from 20 to 1500 seconds. Particulate magnetic materials, such as gamma-iron oxide, Co-modified gamma-iron oxide, isotropic iron, acicular chromium dioxide, and barium ferrite show somewhat different decay characteristics. Longitudinally oriented chromium dioxide media exhibit a faster and greater decay in the direction of orientation than in the transverse direction, both measured at Hc. In general, however, the fall of magnetization is maximum at Hc, with little decay at H=O and at fields greater than twice Hc.

Decay of a post-depositional remanent magnetization in wet sediments due to the effect of drying

Summary. The decay of the post-depositional remanent magnetization (post-DRM) during desiccation in magnetic field free space is measured as a function of the loss of water. The decay is ascribed to the drying effect and the time decay of viscous remanent magnetization (VRM). The VRM forms only 10 per cent of the total of loss of remanent magnetization. The decay due to the drying effects depends both on the loss of water and on either the evaporation rate or the period of storage. The percentage of loss of magnetization is independent of its intensity. A critical drying stage appears (about 60 per cent in water content on a dry basis) which is characterized as a vanishing point of mobile particles or particle units. The mobile particles or units play an important role both in acquisition and demagnetization through physical rotational motion within wet sediments before the critical drying stage. More than 80 per cent of the total loss of the post-DRM is destroyed before the desiccation proceeds to the critical drying stage. The decay of post-DRh4 is concluded to be mainly due to the physically random rotation of the magnetic particles trapped in shallow energy wells which are overcome by the torques caused by the application of the alternating magnetic field less than 200 Oe.

On the Nature of Pulsars. I. Theory

view Abstract Citations (685) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Nature of Pulsars. I. Theory Ostriker, J. P. ; Gunn, J. E. Abstract We present in this paper the initial installment of a quantitative exploration of one particular pulsar model. We first make plausible and then assume that the seat of the pulsar phenomenon is a rotating neutron star having a dipolar magnetic field which is not parallel to the rotation axis. We then show that such stars may be expected to emit large amounts (1050_1052 ergs) of magnetic-dipole and gravitational- quadrupole radiation, that these energy losses are inevitably associated with losses of angular momentum and increases in the rotation periods, and that the emitted low-frequency magnetic-dipole radiation is extremely efficient at accelerating charged particles to relativistic energies. An explicit expression for the period as a function of time allows us to calculate the age of the Crab Nebula (with 20 percent accuracy) and to predict the so far unobserved second derivative of the period (d2P/dt2). We also de- termine the luminosity of the nebula and the highest-energy electrons presently being injected into it- both numbers found to be in good agreement with independent observations. In extreme cases the ac- celeration mechanism can produce protons with energies up to m~c2(e2/Gm~)'/3 or 1021 eV, which is somewhat in excess of the most energetic cosmic rays yet observed. The theory predicts a relation be- tween period, F, and rate of change of period, dP/dt 3 X 10'5/P(sec), which is well observed, in the mean, for pulsars. Finally, after determining the magnetic decay time to be about 4 million years, we predict that few pulsars should be found with periods in excess of 1.5 seconds, also in good accord with observations. We do not discuss the origin of the pulses themselves Publication: The Astrophysical Journal Pub Date: September 1969 DOI: 10.1086/150160 Bibcode: 1969ApJ...157.1395O full text sources ADS | data products SIMBAD (2) Related Materials (1) Part 2: 1970ApJ...160..979G