Page 66 - Book Hosokawa Nanoparticle Technology Handbook

1.12 MAGNETIC PROPERTIES FUNDAMENTALS
amount and kind of magnetic ion are different against The interaction between positive ion spins is super-
antiferromagnetic distribution. Ferrite is a typical exchange type, not directly act on itself. If super-
ferrimagnetic substance, and has formation of exchange interaction is positive, its oxide compound
2
M O·Fe O (M 2 is magnetic ion) generally. shows ferromagnetism. In case it is negative, it would
2
3
Ferromagnetism arises when all the spins of mag- appear antiferromagnetism or ferrimagnetism.
netic substance line in parallel and show their sponta-
neous magnetization. Ferromagnetism is observed in (1) Monoxide
3d-transition metals such as Fe, Co and Ni, rare earth Typical examples of monoxide compounds are MnO,
metals such as Gd, Tb, Dy and their alloy. Also it is FeO, CoO and NiO. CoO belongs to tetragonal system,
observed in compounds such as CrO and EuO. and MnO and FeO belong to trigonal system. Both
2
systems are warped slightly, and then magnetic
anisotropy would appear.
1.12.2 Magnetism of metal materials
(2) Spinel oxide
There are free conduction electrons in metal sub-
stances, and then their magnetic property is much dif- Generally spinel oxide has ferrimagnetism. Oxide com-
ferent from that of insulator. pounds with a spinel structure especially ferrites have
Transition metals and rare earth metals, which have been studied for many years. Magnetite, which has been
d-unpaired electron, show complex magnetic charac- known as a ferromagnetic substance, has the spinel
teristics. Magnetism of metals is classified as follows: structure. Industrial ferrite materials are often spinel
structure compounds. Ferrite formula is defined as
(1) Transition metals (I)
Fe, Co, Ni MFe O (M Mn, Fe, Co, Ni, Cu, Zn etc.). There are
4
2
Ferromagnetism two kinds of ion distribution. One is normal spinel
(M[Fe O ]) and the other is inverse spinel (Fe[MFe]O ).
(2) Transition metals (II) Cr, Mn, Fe 2 4 4
Ferrite formula is generally described as M Fe
Antiferromagnetism 1-x x
[M Fe ]O . In the case of x 0 and x 1, it is normal-
2-x
4
x
(3) Transition metals (III) Others spinel and inverse-spinel, respectively. It is known that x
Paramagnetism value of Cu-ferrite and Mg-ferrite can be varied by sev-
(4) Rare earth metals Ferromagnetism or eral heat treatments [13–15].
Ion distribution of spinel ferrite at high tempera-
Antiferromagnetism
ture can be partly controlled, and then various satu-
(5) Other metals Paramagnetism or
ration magnetizations can be derived by suitable heat
Antiferromagnetism treatment.
Many kinds of alloys are suggested by using magnetic (3) Perovskite oxide
characteristics of transition metals. Useful industrial Oxides with MTO structure have perovskite structure
materials have been produced for many years. 3
when M is a cation with a large radius and T is a metal
such as Cr, Mn or Fe. When T is trivalent metal such
1.12.3 Magnetism of oxide material as Cr or Fe, M are often trivalent metal such as La or
rare metal. If T is tetravalent Mn, M is alkali earth
Conduction electron plays an important role in the metal such as Ca and Sr.
magnetism of metals and alloys. However, effect of Typical materials are antiferromagnetic substance
conduction electron can be almost neglected in the such as LaFeO , LaCoO and LaNiO . LaMnO is usu-
3
3
3
4
case of oxides. Many kinds of metal oxide sub- ally antiferromagnetic substance. When La is replaced
3+
4
stance can be regarded as ionic crystal. Metal atoms by Ca, Mn changes to Mn . It was reported that fer-
in ionic crystal are almost in ionic conditions. They romagnetism appeared in these composite perovskite
are enclosed by positive and negative ions, and oxide [16].
influenced by the crystal electric field.
There must be exchange interaction between (4) Garnet structure oxide
magnetic ions in order that ionic crystal has ferro- Magnetic substance (3M O ·5Fe O ) which has the
3
2
3
2
magnetism. Metal ions and oxygen ions are located same structure as garnet (Mg Al (SiO ) ) has been
2
4 3
3
alternately in oxide compounds. Iron ions are likely studied [17,18]. First garnet structure oxides whose M
buried in the dense structure of oxygen ions in iron is Y was studied. Next new magnetic materials whose
oxide compounds. In oxide compounds, it is impos- M was replaced by rare earth metal have been studied.
sible to apply the model of metal type exchange When M is replaced by rare earth metals such as Gd,
interaction. Negative ions affect on exchange inter- Tb and Dy, these garnet oxides are able to derive large
action in oxide compound. P.W. Anderson suggested magnetic moment by super-exchange interaction at
super-exchange interaction between positive ion low temperature. It is known that Yttrium–Iron–Garnet
spins carried by negative ions, and described the (YIG) remarkably reduces frequency loss on applica-
magnetism of oxide compounds [12]. tions for high frequency device.
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