Chemistry Reference
In-Depth Information
Table 4.1 Characteristics of main spinel ferrites with expected distribution of cations in tetra-
hedral (A) and octahedral (B) sites and theoretical values of magnetization
Ferrite
A-site
B-site
Magnetization (l
B
)
Fe
3+
Fe
2+
,Fe
3+
Fe
3
O
4
4.0
Fe
3+
Co
2+
,Fe
3+
CoFe
2
O
4
3.5
Fe
3+
Ni
2+
,Fe
3+
NiFe
2
O
4
2.2
Fe
3+
Zn
2+
,Fe
3+
ZnFe
2
O
4
0
Fe
3+
Mn
2+
,Fe
3+
MnFe
2
O
4
4.6
Fe
3+
0,5Mn
2+
, 0,5Fe
2+
,Fe
3+
Mn
0,5
Zn
0,5
Fe
2
O
4
7.0
Fe
3+
Cu
2+
,Fe
3+
CuFe
2
O
4
1.2
local symmetry. As listed in Table
4.1
, one can distinguish normal or direct
A
2
þ
B
3
þ
2
O
4
spinels where A
2
þ
ions occupy the tetrahedral sites and B
3
þ
ions
the octahedral ones and inverse B
3
þ
A
2
þ
B
3
þ
2
O
4
spinels where A
2
þ
ions
occupy the octahedral sites, half of B
3
þ
ions the tetrahedral ones and half of B
3
þ
ions the octahedral ones. But spinels with defective structure are usually evi-
denced: (A
k
B
1 - k
)[A
1 - k
B
1 ? k
]
2
O
4
, where inversion parameter k = 0 and 1
stands for the inverse and normal cases, respectively, and 1/3 for random. Spinel
ferrites are materials with fascinating magnetic, electronic and transport proper-
ties: they can be half metallic such as Fe
3
O
4
(magnetite as mixed valence system)
or insulating (most of spinel ferrites), ferrimagnetic (most of spinel ferrites) or
antiferromagnetic (ZnFe
2
O
4
) as ideal bulk state. Indeed, one has to pay attention to
the cationic distribution which strongly influences of the physical properties. Their
ferrimagnetic structure was first explained by Néel through two-sublattice model
resulting from superexchange interactions between cations (J
AA
,J
BB
and J
AB
)[
53
].
But cationic inversion and substitution with non-magnetic ions originate non
collinear up to spin-glass-like structures. For the last 50 years, numerous experi-
mental, theoretical and numeric studies have been devoted to model the magnetic
and electronic structures and to estimate the superexchange constants in micro-
crystalline ferrites. It is clear that in addition to the role of the chemical homo-
geneity, parameters as the surface anisotropy related to the surface state and
morphology, superparamagnetic relaxation phenomena and dipolar interactions
have to be considered to better understand intrinsic and extrinsic magnetic prop-
erties in the case of the nanoparticles of ferrites.
Consequently, the first question is to check whether the nanoparticles are
chemically homogeneous. Magnetite (Fe
3
O
4
) appears to be a first excellent
illustration: indeed, when cooling, this bulk mixed valent Fe oxide undergoes a
charge ordering, i.e. metallic-insulating (Verwey) transition at about 120 K, the
nature of which, together with the magnetic properties are related to the metal-to-
oxygen stoichiometry [
54
-
57
]. It is also important to emphasize that maghemite
(c-Fe
2
O
3
) has a similar structure but is insulating as it contains only ferric ions. As
is illustrated in Fig.
4.5
, the 300 K (and also above the Verwey transition)
Mössbauer spectrum of microcrystalline magnetite consists in two well resolved
magnetic sextets: from the values of hyperfine parameters, the outer sextet is
