Ferrimagnetism
Form of ferromagnetism in crystalline material with two oppositely but
unequally magnetised sublattices. The quantum mechanical exchange
interaction favours a parallel or
antiparallel (via an anion) alignment of the magnetic moments as
domains and one
observes a resulting magnetisation. Ferrimagnetics
become paramagnetics above the Néel-temperature TN. Examples: magnetite
Fe3O4 (TN
= 585 °C) and maghaemite γ-Fe2O3
(TN
~ 620 °C). (see also ferromagnetism).
Ferrimagnetica have weak hysteretic properties.
Ferromagnetism
Magnetic state in a solid with unpaired electron spins (odd number of
electrons) and atomic distances such that the quantum mechanical
exchange interaction causes an alignment of the individual atomic magnetic moment as domains. Each
domain has a maximum magnetisation,
called spontaneous magnetisation. Ferromagnetics have a strong magnetic
susceptibility, a strong remanent magnetisation and show the
property
of hysteresis.
Ferromagnetics become paramagnetic above the Curie-temperature TC. The transition
elements iron (Fe), nickel (Ni) and cobalt (Co) are ferromagnetic.
Fire-tunnel
Part of a kiln,
connecting stokehole and combustion chamber.
Other names are flue
and sometimes, furnace or praefurnium. A part of the fire burnt at this
place, with the flames directed towards the combustion chamber.
Fisherian-statistics
Statistic applied to a population of vectors when they are normally or
Gaussian distributed in
azimuth. It is used in
archaeomagnetism to
describe the spatial distribution of magnetisation directions. The
probability density function is called Fisher-distribution (after Sir
Ronald Fisher), which is a bivariate distribution as these are two
variables: declination
and inclination. The
probability for an
observation falling in a small element of area δA at an angular
distance ψ from the true mean direction is given by PδA δA = κ / 4π sinh(κ) eκcos(ψ)
δA, with δA being sin(ψ) dψ dφ, where φ is
the azimuth (or longitude) of the observation about the mean direction
and ψ the co-inclination (or co-latitude). Application of the Fisher
distribution assumes that declinations are randomly distributed and
that the inclinations are exponentially distributed. The Fisher
statistics allows to define an error of true mean th ChRM direction of the population, called
the cone of confidence at given probability; in general 95% (see also concentration parameter,
confidence factor).
Fluxgate-magnetometer
In a fluxgate magnetometer the steady magnetic field due to the sample
is measured by a so called fluxgate sensor, which works on the
induction principle, similar to a transformer. The sensor consists of a
primary drive coil and a secondary pick-up coil, both winded around a
highly permeable core that does not exhibit hysteresis. Alternating
voltage is applied to the primary coil and induces a secondary
alternating voltage in the pick-up coil. The presence of a sample
causes an additional steady magnetic field. This field is superposed to
the field that is caused by the primary coil and the sum of both is
detected by the secondary coil. The voltage change per time at the
pick-up coil is analysed by splitting the signal with Fourier-analysis
into harmonics. The amplitude of the second harmonic is proportional to
the steady magnetic field produced by the sample and which is only
present if a steady field is present. Tri-axial fluxgate sensors can be
used to measure the three orthogonal components of the magnetic field
(depending on the position of the sensor), but also to measure the remanent magnetisation of a baked sample.