Table 15.3 Saturation magnetostriction constants of
some polycrystalline ferrites
Composition A,,, (x 106)
Fe3O4
MnFe2O4
CoFe2O4
NiFe2O4
MgFe2O4
+40
— 5
-110
-26
-6
thin, long needle if its long axis is aligned parallel to the magnetizing field
than if it is perpendicular to it.
Orientational anisotropy is related to magnetostriction. When a material
is magnetized, it changes shape, which in turn introduces elastic strains in the
material. And since elastic properties are tensors, it follows that the penalty
for magnetizing crystals in different crystallographic directions is not equivalent.
Some crystallographic directions are easier to magnetize than others.
One measure of this effect is the magnetostriction constant Xm, defined as
the strain induced by a saturating field which is positive if the field causes
an increase in dimension in the field direction. Table 15.3 lists some values
for m for a number of polycrystalline ferrites. The values listed represent
the average of single-crystal values.
The details of these intriguing phenomena are beyond the scope of this
book, but the interested reader is referred to Additional Reading for further
details. What is important here is to appreciate how these phenomena can
and have been exploited to increase magnetic energy density by an order
of magnitude for every decade since the turn of the last century!
Table 15.3 Saturation magnetostriction constants of
some polycrystalline ferrites
Composition A,,, (x 106)
Fe3O4
MnFe2O4
CoFe2O4
NiFe2O4
MgFe2O4
+40
— 5
-110
-26
-6
thin, long needle if its long axis is aligned parallel to the magnetizing field
than if it is perpendicular to it.
Orientational anisotropy is related to magnetostriction. When a material
is magnetized, it changes shape, which in turn introduces elastic strains in the
material. And since elastic properties are tensors, it follows that the penalty
for magnetizing crystals in different crystallographic directions is not equivalent.
Some crystallographic directions are easier to magnetize than others.
One measure of this effect is the magnetostriction constant Xm, defined as
the strain induced by a saturating field which is positive if the field causes
an increase in dimension in the field direction. Table 15.3 lists some values
for m for a number of polycrystalline ferrites. The values listed represent
the average of single-crystal values.
The details of these intriguing phenomena are beyond the scope of this
book, but the interested reader is referred to Additional Reading for further
details. What is important here is to appreciate how these phenomena can
and have been exploited to increase magnetic energy density by an order
of magnitude for every decade since the turn of the last century!
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