Biomedical Engineering Reference
In-Depth Information
so that each neighbor is “anti-aligned,” the substance is
antiferromagnetic. Antiferromagnets have a zero net mag-
netic moment, meaning no i eld is produced by them.
Antiferromagnets are less common compared to the other
types of behaviors and are mostly observed at low tempera-
tures. In varying temperatures, antiferromagnets can be seen
to exhibit diamagnetic and ferrimagnetic properties. If the
A and B sublattice moments are exactly equal but opposite,
the net moment is zero. h is type of magnetic ordering is
called antiferromagnetism. h e clue to antiferromagnetism
is the behavior of susceptibility above a critical temperature,
called the Néel temperature (T
N
). Above T
N
, the susceptibil-
ity obeys the Curie-Weiss law for paramagnets but with a
negative intercept indicating negative exchange interactions.
13.5.3 Dif erent Types of Magnetic Measurement Process
A magnetic measurement is the measurement of the characteristics of a
magnetic i eld or the magnetic properties of substances. Among the char-
acteristics of a magnetic i eld that are measured, are the vector of magnetic
induction B, the i eld intensity H, the l ux of the induction vector (magnetic
l ux) and the magnetic i eld gradient. h e magnetic state of a substance is
dei ned by the magnetization J, which is the magnitude of the resultant
magnetic moment per unit volume or mass of the substance; the magnetic
susceptibility
χ
; the magnetic permeability μ and the magnetic structure.
Among the most important properties of the most common magnetic
materials, ferromagnets, are the induction curve
B(H
) and the magneti-
zation curve
J(H
)—that is, the dependence of
B
on the i eld intensity
H;
the coercive force; energy losses during magnetization reversal; maximum
magnetic energy per unit volume (or mass) and the demagnetization fac-
tor (coei cient of demagnetization) of a ferromagnetic sample.
h e methods used to measure magnetic properties are the ballis-
tic, magnetometric, electrodynamic, induction, ponderomotive, bridge,
potentiometer, wattmeter, calorimeter, neutron-dif raction and resonance
methods. All these methods have been discribed in brief below.
•
Ballistic Method:
h is method is based on the measurement
using a ballistic galvanometer, of the quantity of electricity
induced in a measuring coil upon a rapid change in the mag-
netic l ux coupled to the coil. In addition to ballistic galva-
nometers, magnetoelectric and photoelectric webermeters
