Thermal Fluctuation Field Research Articles

Time effects in exchange anisotropy-affected metal-evaporated tapes

The time-dependent magnetic behavior in metal evaporated tapes for high-density magnetic recording is studied at low temperatures, where the coupling between the ferromagnetic Co and the antiferromagnetic CoO, in the obliquely evaporated CoNiO film, induces a unidirectional exchange anisotropy, and it is compared with the effects obtained at room temperature, where no coupling occurs. At low temperature a contraction of the hysteresis loop as a function of the number of repeated cycles N is observed, with a remarkable decrease of the measured coercive field. No contraction occurs at room temperature. The time dependence of the magnetization is larger at low temperatures, with increase of magnetic viscosity and thermal fluctuation field, than at room temperature. The observed anomalous increase of thermal instability of magnetization and coercivity with the decrease of the temperature is imputed to the presence of exchange coupling between the ferromagnetic and antiferromagnetic phases at low temperature and to the effect that such coupling can exert on the reversal of the magnetization in the ferromagnetic grains.

Influence of the doping on the time-dependent magnetic behaviour of fine particles

Time dependent magnetic phenomena are analysed by measuring the changes of magnetization with time in the presence of a constant field on fine particles containing various percentage of dopants. The samples analysed are: iron oxides (Co-doped), CrO/sub 2/ (Fe/Sb- and Ir-doped) and Ba ferrite (Co/Ti-doped) particles. The doping causes a large increase of the coefficient S which characterizes the magnetic viscosity, and an increase of the thermal fluctuation field H/sub f/. The "activation volume" V/sub ac/, which is considered as the region of greatest magnetic instability in the particle and the "switching volume" V/sub c/ decrease with doping. These effects are principally attributed to the increase of the anisotropy directions distribution inside the particles and to the evolution of the magnetization reversal mode in the modified particles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of crystallite size and shape on the viscous magnetization of λ-Fe 2O 3 particles

The usefulness of the normalized thermal fluctuation field in magnetic granulometry is confirmed with data for λ-Fe 2O 3 and CrO 2. The variation of the normalized fluctuation field with crystallite diameter in the λ-Fe 2O 3 follows a similar trend for both isometric and longitudinal crystallite growth even though in the second case the diameter is not a good guide to the volume of the crystallites. This suggests that the smallest dimension of the crystallite rather than its volume is critical for the activation volume; after correction for the differences in coercivity a long, thin particle is more subject to thermal activation than a short, thick one of the same volume.

Thermal fluctuation field in NdFeB permanent magnets

Measurements of temperature dependence of the coercive field Hc are reported for Nd-Fe-B sintered magnets. The relationship between log Hc and log Hf was deduced from Gaunt's localized weak-domain-wall pinning model, which can be written in the following form, log Hc=log Hf+log(25 ((31rb2/25kT)-1)), where r is the domain-wall energy and b is the range of interaction between wall and pin. The values of log Hc were calculated using the above equation, and are in good agreement with experiment.

Thermal fluctuation field in RE-TM-B permanent magnet materials

The temperature dependence of the coercive field (H/sub c/) and the thermal fluctuation field (H/sub f/) of Nd-Fe-B sintered magnets was measured from 100 K to 500 K. It was found that a linear relationship exists between l/sub g/H/sub f/ and l/sub g/H/sub c/ in this temperature range. The relationship between l/sub g/H/sub c/ and l/sub g/H/sub f/ was deduced from Gaunt's localized weak pinning model. Values of l/sub g/H/sub c/ calculated from the derived relationship are in very good agreement with experimental values.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal fluctuation field in an inhomogeneous nonisothermal medium

Thermal fluctuation field in an inhomogeneous nonisothermal medium

Increased Barkhausen noise of hard-drawn 50% nickel-iron at low temperatures

To resolve the question whether thermal fluctuation fields have an influence on the Barkhausen noise, its energy spectrum in the frequency range from 0.1 Hz to 80 kHz was measured at low temperatures. In particular, we investigated the noise of a hard-drawn wire of 50NiFe, the spectrum of which depended on the field rate d H/d t at low frequencies, because there was a weak coupling between subsequent Barkhausen pulses. We found that the noise at medium frequencies was much higher after the sample had been cooled to temperatures below 77 K. Several interpretations of the effect will be discussed, but the most probable appears to be that thermal fluctuation fields do indeed disturb the strong coupling which exists between neighbouring divisions of a Bloch wall at a sufficiently low temperature.

Velocity and Temperature Fluctuation Measurements in a Turbulent Boundary Layer Downstream of a Stepwise Discontinuity in Wall Temperature

Abstract Results are presented of an experimental investigation of the coexisting thermal and velocity fluctuation fields in a turbulent boundary layer downstream of a small stepwise discontinuity in wall temperature. The statistical behaviors of several relevant fluctuating quantities have been obtained at a typical cross section of the boundary layer in the region where the thermal boundary layer has not yet reached the free stream. The instantaneous surface of demarcation between heated and unheated fluid was found to be sharp and distinct at all points, resulting in intermittent temperature fluctuation signals well within the fully turbulent region of the momentum boundary layer. From the measurements at one section, the balances of turbulent kinetic energy and of turbulent temperature fluctuations across the boundary layer have been evaluated. In addition, the distribution of the local turbulent Prandtl number (ratio of the turbulent diffusivities of momentum and heat) through the boundary layer has been obtained; it is not constant and varies between limits of 0.8 and 1.2.