B-H Curve.

When tested experimentally, a ferromagnetic (i.e.
strongly magnetic) material such as iron will
produce a curve similar to that shown above.
Firstly, notice that here is an upper/lower limit to
the magnetic flux density which may be achieved,
which occurs at positive or negative saturation ,
respectively. This is related to the crystalline
structure of the iron, where each crystal has its
own – initially random – magnetic orientation.
Increasing the magnetic field strength in either
direction causes more and more magnetic
'domains' to align with the external magnetic field,
but once almost all of the domains have aligned
themselves, then little further increase in magnetic
flux density is possible. The ferromagnetic material
is said to be saturated.
A second key observation is that the curve
demonstrates magnetic hysteresis or 'lag' as the
sample is alternatively magnetised in the positive
and negative directions. When initially magnetised,
the curve follows points a – b on the graph, but on
reducing H to zero, some residual magnetism
remains (point c - also known as the remanent
flux density ). In order to fully demagnetise the
specimen, it is necessary to apply a negative
magnetic field strength (point d - called the
coercive force). Making H increasingly negative
leads to negative saturation (point e ). If H is
reduced back to zero, point f is reached (negative
residual magnetism). As H becomes positive, the
flux density reduces to zero (point g) and then
becomes positive, finally returning back to point b
(positive saturation), after which the cycle b– g
repeats.