Basic Magnetic Quantities and the Measurement of the Magnetic Properties of Materials Page: 6
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very small, not more than a few parts in a hundred
thousand. Therel are two kinds of paramagnetism
called strong and weak. Except for high fields of
the order of 104 gausses (1 tesla) or higher the
strong paranagnet ic susceptibility is constant
with field at a given te'lperature but varies in-
vefrsely with tcpeI)erature at a given field. Weak
paranllagretisni is due to the effect of conduction
electrons in conducting metals and is practically
il(depel(dlent of tlemnperat ure.
Ilerromagntic materials have a relative per-
ineability greater than unity and generally very
high. The perm ability is not constant but de-
pends upon thel degree of magnetization. Fer-
romlagnletic nuitecrials exhibit hysteresis, that is,
the i reduction corresponding to a given magnet-
izing iorce lep lds upon previous inagnetic history.
Furt lhenrore, the intrinsic i(lduction approaches a
limiting g or saturation value as the magnetizing
force is increased inrlefiiit lyv. Ferromagnetisrn
has some(ti(es been called a special case of para-
magnetislm l)ut in view of the differencess in the
processes involved, it should not be so classified.
Ferronagi etic i naterials are temperature-
sensitive and w hen the material is heat ed, the
temperature at which it is transformed from the
ferrolliagnetic conlditioln to the paramagietic
condition is called the Curie temperature.
In fcrrolnagnetic materials, tihe interatomlic
(listalices compared to the (tiamneter of the orbit
or shell ini which the unconmpensated spins are
located are such that neighboring atoms have
their magnetic moments alined parallel and in
the same direction. This is called positive
interaction. Not all of the moments in a body are
oriented in the same direction. Instead, they are
lined up in groups called domains which act as
inagnetic entities. Each( dorain is spontaneously
m agn (tized to saturation but because they are
oriented in various directions the resultant
magnetization in any direction may be anything
from zero to the saturation value at which all
domains would be oriented in the direction of the
applied field.
Antifcrrom agnetic miatrial,s are those whose
interatornic distances are less than the critical
value so that the magnetic moments of neighboring
atoms line up parallel to each other b)ut in opposite
directions, talt is, antiparallel. The suscepti-
bility of these materials is so low that they might
easily be mist aken for paramagnetic materials.
Tlhe experimentally distinguishable characteristic
is that t he susceptibility increases instead of
decreasing as the temperature is raised until the
thermal agitation destroys the interaction.
Above thlis temperature the material becomes
paranagnetic. Tis is analogous to the Curie
point in ferromagnetic materials but is called the
Neel point for tlis class of materials.
Ferrimagcetic materials are those in which
unequal magnetic moments are lined up anti-
parallel to each other leaving a net permanent
moment. Permeabilities are of thle same order ofmagnitude as those of ferromagnetic materials
but are lower than they would be if all the atomic
moments were parallel and in the same direction.7
Under ordinary conditions the magnetic charac-
teristics of ferrimagnetic materials are quite
similar to those of ferromagnetic materials.
3.4. Magnetic Hysteresis and Normal
Induction
One of the important characteristics of
ferromagnetic materials is the phenomenon of
magnetic hysteresis. This phenomenon is illus-
trated in figure 1. If a demagnetized specimen
is subjected to the influence of a magnetizing force,
H, which is increased from zero to higher and
higher values, the magnetic induction, B, also
increases but not linearly with H. This is shown
by a curve oabcd. This nonlinearity is another of
the characteristics of ferromagnetic material.
BMagnetizing Force
FIGURE 1. Normal induction curve and hysteresis loops.
If the increase in H is stopped at a point such as
b and then decreased, the induction does not
retrace the original curve in reverse order but lags
behind it as indicated by the curve bb, B,, Hl, etc.
This lag is called magnetic hysteresis. The point
where the magnetizing force is zero is called the
residual induction, B. The negative magnetizing
force at which the induction becomes zero is called
the coercive force, H7. The closed curve starting
froim b through Br, HC, etc. back again to b is called
a hysteresis loop. The loop does not always close
at the first reversal of the magnetizing force but
will close after enough reversals have been made.
There is a technically important class of material called ferrites. The
atomic interactions in these materils is mainly antiferromagnetic but there
is a net magnetic moment which gives them ferromagnetic characteristics.
The ferrites are ceramic bodies having extremely high electrical resistivity
which makes them particularly valuable for use at high frequencies.6
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Sanford, Raymond L. & Cooter, Irvin L. Basic Magnetic Quantities and the Measurement of the Magnetic Properties of Materials, report, May 21, 1962; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc13240/m1/12/: accessed May 7, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.