‘Ferrites’ has become a generic term for all magnetoceramic materials widely used in magnetoelectronic applications.

1. ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY
SCHOOL OF APPLIED NATURAL SCIENCES
DEPARTMENT OF APPLIED CHEMISTRY
Material Chemistry PhD Program
Chem 7147 Course Seminar on: “Ferrite and Ferromagnetic Materials”
By: Ararso Nagari
Submitted to: Dr. Enyew
January, 2019
Adama, Ethiopia
1

2.  Outline
 Introduction
 Ferrites
 Crystal Structure, Properties and uses, Classification,
Applications, Synthesis
 Ferromagnetic Materials
 Properties, Types, Applications
 Summary
2
Ferrites and Ferromagnetic Materials

3.  One classification of magnetic materials—into
diamagnetic, paramagnetic, and ferromagnetic — is
based on how the material reacts to a magnetic field.
 In recent years, other types of magnetic ordering of
substances into antiferromagnets and Ferrimagnetics is
discovered
3
Introduction

4.  ‘Ferrites’ has become a generic term for all
magnetoceramic materials widely used in
magnetoelectronic applications.
 These are generally chemically inert materials, which lose
oxygen before melting at high temperatures.
 Ferrites are usually ferrimagnetic ceramic compounds
derived from iron oxides.
Ferrites
4

5.  Magnetite (Fe3O4) is a famous example.
 Like most of the other ceramics, ferrites are hard, brittle,
and poor conductors of electricity.
 Many ferrites adopt the spinel structure with the formula
AB2O4, where A and B represent various metal cations,
usually including iron (Fe).
….
5

6.  True ferrites contain iron(III) and are classified into two main
groups: ‘cubic ferrites’ (MFe2O4) with a spinel structure, which
are sometimes called ferrospinels, and ‘hexagonal ferrites’
(MFe12O19) with the magnetoplumbite structure (M = divalent
cation, e.g. Mn, Fe, Co, Ni, Cu, Mg).
 Spinel ferrites usually adopt a crystal motif consisting of cubic
close packed (fcc) oxides (O2−) with A cations occupying one
eighth of the tetrahedral holes and B cations occupying half of
the octahedral holes, i.e., A2+ B3+
2O2−
4.
Crystal Structure of Ferrites
6

7.  Ferrite crystals do not adopt the ordinary spinel structure, but
rather the inverse spinel structure: One eighth of the
tetrahedral holes are occupied by B cations, one fourth of the
octahedral sites are occupied by A cations, and the other one
fourth by B cation.
 Some ferrites adopt hexagonal crystal structure, like barium
and strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19
(SrO:6Fe2O3).
7
….

8.  It is also possible to have mixed structure spinel ferrites
with formula [M2+
1−δFe3+
δ][M2+
δFe3+
2−δ]O4 where δ is the
degree of inversion.
 The magnetic material known as "ZnFe" has the formula
ZnFe2O4, with Fe3+ occupying the octahedral sites and Zn2+
occupy the tetrahedral sites, it is an example of normal
structure spinel ferrite.
….
8

9.  This is a structure for small to medium sized cations whose
ideal composition takes the form AB2O4.
 The general chemical formula of a ferrospinel molecule is
M2+Fe2
3+O4
2-, where M2+ represents a divalent metal ion
such as Zn2+, Fe2+, Mg2+, Mn2+, Cd2+, etc.,
 The basic structure is derived from a cubic closest packing
of spheres with a doubled face-centered cubic unit cell.
Cubic Ferrites: The Spinel Structure
9

10.  Such a cell contains 32 octahedral holes and 64 tetrahedral
holes, which can be broken down into a number of
crystallographically different sublattices.
 Thus, in the spinel structure 50% of the octahedral holes
are designated as B positions and one eighth of the
tetrahedral holes as A sites, yielding the composition
A8B16O32 for the unit cell.
10
....

11.  The structure is difficult to view in three dimensions
 The A and B cations can have valences between 1 and 6,
the only requirement being that their sum adds up to 8.
 Although typically thought of as a structure for transition
metal ions, spinels form with a number of the smaller alkali
and alkaline-earth cations as well as post-transition metals
such as Zn+2, Cd+2, Ga+3, In+3, Ge+4, and Sn+4.
….
11

12.  Some examples include TiCo2O4, CdCr2O4, FeCr2O4, Fe3O4,
ZnAl2O4, FeGa2O4, NiV2O4.
 Assignment of A or B sites to cations in spinel type oxides based
upon their chemical formulae can often be misleading.
 Thus, for the mixed-valence oxide NiFe2O4, we might conclude
that the Ni+2 ions are on the tetrahedral A sites while the Fe+3
ions were on the B sites as implied by the formula.
….
12

13. 13
....
• Ferrites crystallize in the form of a cubic structure.
• Each corner of a ferrite unit cell consists of a ferrite molecule

14. 14
….
The small filled circles represent metal ions, the large open or
shaded circles represent oxygen ions: (a) tetrahedral or A
sites; (b) octahedral or B sites; and (c) one-fourth of the unit
cell of a cubic ferrite. A tetrahedron and an octahedron are
marked.

15. ….
 Normally there are two types of structures in ferrites-
ferrospinels:
 Regular spinel and
 Inverse spinel
15

16.  In this type, each divalent metal ion occupies 1 tetrahedral
site and each trivalent metal ion occupies 1 octahedral site.
 Totally in an unit cell, there will be 8 tetrahedral (8 A) sites
and 16 octahedral (16B) sites.
 Hence, the sites A and B combined to form a regular spinel
ferrite structures as shown in Fig below.
16
Regular spinel structure

17.  On the other hand, the mineral spinel, MgAl2O4 and Ni+2Cr2
+3O4,
have the site distributions predicted by the chemical formula
and are referred to as normal spinels.
 The Spinels, ‘zinc ferrites’ (Zn+2
xM+2
1−xFe+3
2O4), having the
second metal, M, in oxidation state two (M is frequently Mn or
Ni); when x = 1 it is that of a normal spinel in which all the Fe+3
ions occupy octahedral sites because Zn+2 displaces Fe+3 at the
tetrahedral sites.
17
....

18.  In a ferrite unit cell there are 8 molecules. Therefore in a
ferrite unit cell, there are 8 divalent metal ions, 16 ferric
ions and 32 Oxygen ions.
 If only the oxygen ions in ferrite crystal are considered, it is
found that they constitute a close packed face centered
cubic structure.
18
….

19.  The schematic representation of zinc ferrite molecule as shown
in Fig below
 Fig. Regular spinel structure
19
....

20.  In these arrangement it is found that for every four O2-
ions there are 2 octahedral sites (surrounded by 6 O2- ions)
and one tetrahedral site (surrounded by 4 O2- ions).
 The metal ions are distributed over these tetrahedral sites
(A sites) and octahedral sites (B sites).
 Thus in ferrites the number of octahedral sites is twice the
number of tetrahedral sites.
20
….

21.  In this type half of the B sites (8 sites) are occupied by
divalent metal ions and the remaining half of the B sites (8
sites) and all the A sites are occupied by the trivalent metal
ions
 For example, In fact, the actual distribution places all the
Ni+2 on octahedral sites and half of the iron in tetrahedral
sites giving a site preference formula of
Fe+3[Ni+2
0.5Fe+3
0.5]2O4. 21
Inverse spinel structure

22.  Magnetite, Fe2+Fe2
3+O4
2− is inverse spinel, with O2− ions forming
a fcc lattice and iron cations occupying interstitial sites. Half of
the Fe3+ cations occupy tetrahedral sites while the other half,
along with Fe2+ cations, occupy octahedral sites.
 The schematic representation of a ferrous ferrite molecule is
shown in Fig. 22
....

23.  The Spinels ‘zinc ferrites’, Zn+2
xM+2
1−xFe+3
2O4, when x = 0
the structure is that of an inverse spinel (i.e. half the Fe+3
ions occupy tetrahedral sites in the close-packed array of
oxide ions, the other half of the Fe+3 ions occupying
octahedral sites)
23
....

24.  It is an interesting example of intergrowth structures
 Are a series of magnetic oxides based upon the structure
of PbFe12O19, magnetoplumbite.
 The structure can be described in terms of five HCP layers
of 4 oxygens each, in which one of the layers has one-
quarter of its oxygen replaced by the large Pb+2 ion to yield
a PbO19 framework.
Hexagonal Ferrites: Magnetoplumbite
structure
24

25.  The iron then fill octahedral and tetrahedral holes in a
spinel-like fashion.
 The layer sequencing is BABAB.
….
25

26.  Now a day the term ferrite is used for magnetic oxides
whose main constituent is iron oxide.
 In terms of their magnetic properties, the different ferrites
are often classified as "soft", "semi-hard" or "hard", which
refers to their low or high magnetic coercivity, as follows.
26
Classification of Ferrites

27.  Ferrites that are used in transformer or electromagnetic
cores contain nickel, zinc, and/or manganese compounds.
 They have a low coercivity and are called soft ferrites.
 The low coercivity means the material's magnetization can
easily reverse direction without dissipating much energy
(hysteresis losses), while the material's high resistivity
prevents eddy currents in the core, another source of
energy loss. 27
Soft Ferrites

28.  Because of their comparatively low losses at high
frequencies, they are extensively used in the cores of RF
transformers and inductors in applications such as
switched mode power supplies.
28
....

29.  The most common soft ferrites are:
 a) Manganese-zinc ferrite (Mn, Zn), with the formula
(MnaZn(1-a)Fe2O4). MnZn have higher permeability and
saturation induction than NiZn.
 b) Nickel-zinc ferrite (NiZn), with the formula (NiaZn(1-
a)Fe2O4). NiZn ferrites exhibit higher resistivity than MnZn,
and are therefore more suitable for frequencies above 1
MHz. 29
….

30.  Cobalt ferrite, CoFe2O4 (CoO·Fe2O3), is in between soft and
hard magnetic material and is usually classified as a semi-hard
material.
 It is mainly used for its magnetostrictive applications like
sensors and actuators thanks to its high saturation
magnetostriction (~200 ppm).
 CoFe2O4 has also the benefits to be rare-earth free, which
makes it a good substitute for Terfenol-D.
30
Semi-hard Ferrites

31.  In contrast, permanent ferrite magnets are made of hard
ferrites, which have a high coercivity and high remanence after
magnetization.
 These are composed of iron and barium or strontium oxides.
 The high coercivity means the materials are very resistant to
becoming demagnetized, an essential characteristic for a
permanent magnet.
 They also conduct magnetic flux well and have a high magnetic
permeability. 31
Hard Ferrites

32.  This enables these so-called ceramic magnets to store stronger
magnetic fields than iron itself.
 They are cheap, and are widely used in household products
such as refrigerator magnets.
 The maximum magnetic field B is about 0.35 tesla and the
magnetic field strength H is about 30 to 160 kiloampere turns
per meter (400 to 2000 oersteds).
 The density of ferrite magnets is about 5g/cm3.
32
….

33.  The most common hard ferrites are:
 Strontium ferrite, SrFe12O19 (SrO·6Fe2O3), used in small
electric motors, micro-wave devices, recording media,
magneto-optic media, telecommunication and electronic
industry. Strontium hexaferrite (SrFe12O19) is well known
for its high coercivity due to its magnetocrystalline
anisotropy.
33
....

34.  It has been widely used in industrial applications as
permanent magnets and, because they can be powdered
and formed easily, they are finding their applications into
micro and nano-types systems such as biomarkers,
biodiagnostics and biosensors.
 Barium ferrite, BaFe12O19 (BaO·6Fe2O3), a common
material for permanent magnet applications.
34
....

35.  Barium ferrites are robust ceramics that are generally
stable to moisture and corrosion-resistant. They are used
in e.g. loudspeaker magnets and as a medium for magnetic
recording, e.g. on magnetic stripe cards.
35
....

36. Properties and Uses of Ferrites (M2+Fe2
3+O4
2-)
 The susceptibility () is very large and positive. It is
represented by,  = C / (T), when T > TN
 When T<TN, they behave as ferrimagnetic materials.
 Mechanically, they have pure iron character.
 They have low tensile strength and are brittle and soft.
 In these, all valence electrons are tied up by ironic bonding
and they are bad conductors with high resistivity of 1011 
m.
36

37.  They are soft magnetic materials and so they have low
eddy current losses and hysteresis losses.
 Most spinel ferrites used in device applications are not
simple ternary oxides but rather contain proprietary
mixtures of several different magnetic ions tailor made for
specific applications.
37
….

38.  So-called (Mn, Zn) ferrites of general formula
Zn+2
xFe+3
1−x[Mn+2
1−xF ]O4 and related (Ni, Zn) ferrites are used
as cores for inductors or transformers and in TV deflector
yokes.
 (Mg, Mn) ferrites and yttrium iron garnets including those
partially substituted by Al or Gd find numerous applications in
microwave circuitry
 One of the biggest applications of magnetic oxides is for
information storage. 38
….

39.  Computer memories typically utilize (Mn, Mg) based
spinels.
 Hexagonal barium ferrites are used in floppy discs.
 Fe3O4 is used as a magnetic toner in photocopiers and as a
magnetic printing ink while the defect spinel γ-Fe2O3 and
γ-Fe2O3 /Fe3O4 mixtures compete with ferromagnetic
CrO2 for the audio and video magnetic tape market.
39
....

40.  In addition to composition, the shape and size of the
particles plays a crucial role in the quality and performance
of the tape.
 In this respect CrO2 is superior because of its normal,
needle-like crystal habit.
40
….

41.  Ferrimagnetics materials, ferrites, have wide application
potential in different fields such as telecommunication,
electronic industries, due to their interesting electrical and
magnetic properties.
 The simultaneous requirement of optical, electric and magnetic
properties in the advanced electronics, Microwave and
computer technologies have focused the attention of research
works on ferrites.
41
Applications

42. ….
 It was also observed that these properties are significantly
be modified by substitution of divalent or trivalent cations,
to suit the material for particular application.
 Therefore, investigations of structural, electrical and
magnetic properties of the ferrites become a field of
interest for many researchers.
42

43.  Synthesis techniques play an important role in controlling the
size and surface area of materials.
 Generally, Ferrites are manufactured by powder metallurgical
process by mixing, compacting and then sintering at high
temperatures followed by age hardening in magnetic fields.
 The synthesis of magnetic materials (ferrites) has been
reported using different chemical methods; that is, sol-gel,
precipitation, solid-state reaction, micro-emulsion.
43
Synthesis of Ferrites

44.  In this method, the formation of a gel provides a high
degree of homogeneity and reduces the need for atomic
diffusion during the solid state calcinations.
 A solution of the appropriate precursors is formed first,
followed by conversion into a homogeneous oxide (gel)
after hydrolysis and condensation.
 Drying and sub- sequent calcination of the gel yields an
oxide product. 44
Sol-Gel Method

45.  In the precipitation method the precipitation of substances
normally soluble under the employed conditions.
 An inclusion occurs when the impurity occupies a lattice site in
the crystal structure of the carrier, resulting in a
crystallographic defect, which can occur when the ionic radius
and charge of the impurity are similar to those of the carrier.
 An occlusion occurs when an adsorbed impurity is physically
trapped inside the crystal as it grows.
45
Precipitation Method

46.  Solid-state synthesis methods are the most widely used.
 This method involves mixing of raw materials and can take
place with both wet and dry processes.
 Assisted by grinding or ball milling
 heating at high temperatures in refractory containers for
several hours or days to produce the desired product
 Often, the reactants are pressed into pellets to promote
internal reaction and to minimize contact with the container
46
Solid-State Reaction Method

47. Ferromagnetic Material
 Some materials become spontaneously magnetized at low
temperatures because of the interaction between the magnetic
atoms called ferromagnetic.
 A type of material that is highly attracted to magnets and can
become permanently magnetized
 The relative permeability is much greater than unity and are
dependent on the field strength.
 These have high susceptibility.
47

48.  E.g., Fe, Co, Ni, Cr, Mn
48
....

49.  Ferromagnetic materials, such as iron, contain unpaired
electrons (domains), each with a small magnetic field of its
own, that align readily with each other in response to an
external magnetic field.
 This alignment tends to persists even after the magnetic
field is removed.
49
....

50.  CrO2 is a metallic ferromagnet which can be used as a magnetic
recording medium
 Thermal decomposition of CrO3 is the most common way to
obtain this interesting material.
 The decomposition proceeds sequentially, while at ambient
atmosphere it is almost impossible to freeze the rapid process
at CrO2 before Cr2O3 forms.
50
CrO2

51.  Ferromagnetic materials, including iron and nickel, lose their
normal strong residual magnetism at a characteristic high
temperature, called the Curie temperature.
 Electrical resistance usually decreases with decreasing
temperature
 These and many other phenomena observed in solids depend
on energy quantization and can best be described in terms of
effective “particles” such as phonons, polarons, and magnons.
51
Ferromagnetic Materials Properties

52.  in nuclear magnetic resonance imaging, an important diagnostic
tool used by doctors
 in today's most powerful particle accelerators to keep the
accelerated particles focused and moving in a curved path
 Scientists are developing magnetic levitation trains that use
strong magnets to enable trains to float above the tracks,
reducing friction
52
....

53. Types of Ferromagnet
Actinide ferromagnets
 A number of actinide compounds are ferromagnets at room
temperature or exhibit ferromagnetism upon cooling.
 PuP is a paramagnet with cubic symmetry at room temperature,
but which undergoes a structural transition into a tetragonal
state with ferromagnetic order when cooled below its Tc=125 K.
 In its ferromagnetic state, PuP's easy axis is in the <100>
direction.
53

54.  In NpFe2 the easy axis is <111>.
 Above Tc ≈ 500 K NpFe2 is also paramagnetic and cubic.
 Cooling below the Curie temperature produces a rhombohedral
distortion wherein the rhombohedral angle changes from 60°
(cubic phase) to 60.53°.
 An alternate description of this distortion is to consider the
length c along the unique trigonal axis (after the distortion has
begun) and a as the distance in the plane perpendicular to c.
54
....

55.  In the cubic phase this reduces to c/a = 1.00.
 Below the Curie temperature which is the largest strain in any
actinide compound.
 NpNi2 undergoes a similar lattice distortion below Tc = 32 K,
with a strain of (43 ± 5) × 10−4.
 NpCo2 is a ferrimagnet below 15 K.
55
....

56. …..
Lithium gas
 A lithium gas cooled to less than one kelvin can exhibit
ferromagnetism.
 Fermionic lithium-6 was cooled to less than 150 nK (150
billionths of one kelvin) using infrared laser cooling.
 This demonstration is the first time that ferromagnetism
has been demonstrated in a gas.
56

57. Tetragonal ruthenium
 Body-centered tetragonal ruthenium exhibits
ferromagnetism at room temperature.
57
....

58.  is the basis of the electric motor and the transformer.
 important in the computer revolution. Computer memories can
be fabricated using bubble domains, thus serving as the units of
the binary number system used in computers.
 Magnetic materials are also important constituents of tapes
and disks on which data are stored.
58
Applications

59.  Magnetic Materials are those materials in which a state of
magnetization can be induced.
 Such materials when magnetized create a magnetic field in the
surrounding space.
 They have many application in electronics specially for data
storage and in medical for drug delivery
59
Summary