Introduction to Ferrite Nanoparticles

• Nanoparticles are those particles that are less than 100 nm in size .

• Nanotechnology is considered to be one of the most important future technologies involving several discipline of science including; solid state physics, solid state chemistry, materials engineering, medical science and biotechnology.

• The transition from bulk to nanoscale leads to a number of changes in structural, physical, magnetic and electrical properties.

• Due to high DC electrical resistivity and low dielectric losses, nanoparticles are potential candidates as high frequency electromagnetic wave absorbers.

• High transition temperature (ferrimagnetic to paramagnetic) makes them ideal for high temperature applications.

Ferrite nano materials are chosen due to their high (>106 -cm) DC electrical resistivity, good magnetic properties, chemically stable over wide temperature range and low eddy current losses. They have potential applications in:• Electromagnetic absorbers• Data storage• Microwave devices• Core Material• Drug delivery

• Ferrites are iron containing complex oxide with interesting magnetic and electrical properties and have been studied extensively because of their important in basic as well as in applied research.

• The DC electrical resistivity of ferrites can vary at room temperature from 10-2Ω-cm to 1011 Ω-cm.

• The knowledge of cations distribution and spin alignment is essential to understand the magnetic and electrical properties of spinel ferrites.

Ferrites can be classified into three different types• Spinel ferrites (MFe2O4)• Hexagonal ferrites (MFe12O19), • Garnets (Me3Fe5O12)

In spinel structure, cations rest on two types of interstitial sites to preserve the charge neutrality namely tetrahedral and octahedral sites.

• In a tetrahedral site the interstitial atom is in the center of a tetrahedral arrangement of four oxygen atoms at the lattice positions.

• Three of them, touching each other, are in plane; the fourth atom sits in the symmetrical position on top as shown in Fig. 1.

• There are 64 tetrahedral sites in a spinel structure.• Out of which 1/8 are occupied by the cations.

Fig. 1. Tetrahedral sites in unit cell.

• An octahedral position for an interstitial atom is the space in the interstices between 6 regular oxygen atoms at the face-centered positions forming an octahedral arrangement.

• Four regular atoms are positioned in a plane; the other two are in asymmetrical position just above or below as shown in Fig. 2.

• There are 32 octahedral sites in a spinel structure.• Out of which 1/2 are occupied by the cations.

Fig. 2. Octahedral sites in unit cell.

Octahedral

• The spinel ferrites have been further classified into three categories due to the distribution of cations on tetrahedral (A-sites) and octahedral (B-sites)

Types of spinel ferrites• Normal spinel ferrites

o [(De2+)A[D3+]BO4]• Inverse spinel ferrites

o [(D3+)A[De2+D3+]BO4 ]• Intermediate spinel ferrites

o [ (Deδ2+ D1- δ

3+)A[De1-δ2+ D1+ δ

3+]BO4]

Ferrimagnetic spinel

Classification of magnetic materials

• Diamagnetism• Paramagnetism• Ferromagnetism• Antferromagnetsim• Ferrimagnetism

Diamagnetism• The diamagnetic materials do not have any unpaired

electrons. So there are no net magnetic moments.• In response to an external applied fields, diamagnetic

material produces a weak magnetization that opposes the applied field giving rise to a negative susceptibility.

Examples; Gold, Copper, mercury, zinc

Paramagnetic

• Each atom possesses a permanent magnetic moment.

• When H=0, all the magnetic moment randomly oriented, so M=o

Paramagnetic• When magnetic field

is applied the magnetic moments tend to orient themselves in the direction of the field.

• This result net magnetization and positive susceptibility

• Example; Al, Cr, Zr

Ferromagnetism

• Materials that retain a magnetization in zero field

• Magnetic moment are parallel alignment in the absence of an external magnetic field

• Examples: iron, cobalt

Ferromagnetism

• Thermal energy can be used to overcome exchange interactions

• Curie temp is a measure of exchange interaction strength

Antiferromagnetism

• In some materials, exchange interactions favour antiparallel alignment of atomic magnetic moments

• Materials are magnetically ordered but have zero remnant magnetization and very low

• Many metal oxides are antiferromagnetic MnO

• CoO, MnS

Antiferromagnetism

• Thermal energy can be used to overcome exchange interactions

• Magnetic order is broken down at the Néel temperature (c.f. Curie temp)

Ferrimagnetism

• Antiferromagnetic exchange interactions

• Different sized moments on each sublattice

• Results in net magnetization

• Example: magnetite, maghemite

Magnetic domains

• Ferromagnetic materials tend to form magnetic domains

• Domain; The small region within which single direction of orientation of magnetic moment.

• Each domain is magnetized in a different direction

Magnetic domains

• Applying a field changes domain structure

• Domains with magnetization in direction of field grow

• Other domains shrink

Magnetic domains

• Applying very strong fields can saturate magnetization by creating single domain

Magnetic domains

• Removing the field does not necessarily return domain structure to original state

• Hence results in magnetic hysteresis

Magnetic susceptibility,

• Magnetic susceptibility is sometimes written as

• And sometimes as the slope of M vs H

How does M respond to H?

• There is a variety of ways that M responds to H• Response depends on type of material• Response depends on temperature• Response can sometimes depend on the previous

history of magnetic field strengths and directions applied to the material

Non-linear responses• Generally, the

response of M to H is non-linear

• Only at small values of H or high temperatures is response sometimes linear

• M tends to saturate at high fields and low temperatures

Low field magnetic susceptibility

• For some materials, low field magnetic susceptibility is inversely proportional to temperature

• Curie’s Law

A high temperature AC magnetic susceptometer probe was fabricated, based upon the principle of mutual inductance. The instruments used for the construction of this apparatus include;• Pick-up coils (primary and secondary)• Heating element• DC power supply (up to 30 Amp)• A thick glass tube (8 mm)• An exhaust fan • A DC adapter (12 V)• Pt-100 temperature sensor (up to 800 0C)• Lock-in amplifier• Multi-meters• A wooden stand

Fig. 4: Block diagram representing the high temperature AC magnetic susceptibility measuring apparatus

• It was mandatory to calibrate the apparatus, which was newly developed and to make it more and more precise.

• The transition temperature for pureNi measurement is foundto be 633 ± 5 K. • That has an excellent

agreement with the reported value 631±2 K [Charles Kittle] for pure Ni.

Fig. 5. Variation of inverse AC magnetic susceptibility withtemperature for pure Ni.

Magnetic hysteresis• Ferromagnetic material

normally does not possesses net magnetization.

• For external magnetic field, magnetic domains grows in size thus the material get a net magnetization.

• At certain magnetic field, magnetization reaches a max value.

Magnetic hysteresis

• M depends on previous state of magnetization

• Remnant magnetization Mr remains when applied field is removed

• Need to apply a field (coercive field) in opposite direction to reduce M to zero.

Effect of temperature on remnant magnetization

• Heating a magnetized material generally decreases its magnetization.

• Remnant magnetization is reduced to zero above Curie temperature Tc

Effect of temperature on remnant magnetization

• Heating a sample above its Curie temperature is a way of demagnetizing it

• Thermal demagnetization

1. Solid State reaction Method2. Wet Chemical Methods• co-precipitation Method• Sol-Gel Method• Microemulsion Method• Hydrothermal Method

• Solid State reaction Method• Disadvantages

o High temperature (>1100 0C) and long sintering times for reaction to complete

o Slow reaction rateo Low homogeneityo Larger particle sizes (>100 nm)

Advantages• Low sintering temperatures are required• Short sintering time• High degree of homogeneous (mixing at atomic

level)• High reaction rate• Smaller particle size (<100 nm)

DisadvantagesThe method does not work well when:• When the reactants have different solubility in water• The reactants don’t precipitate at the same rate

In order to use co-precipitation as the synthesis route the followingfactors must be consider :

• Rate of Mixing of Reagents: • The rate of mixing of reagents plays a vital role in the size of the resultant

particles. • Co-precipitation consists of two processes: nucleation (formation of

centers of crystallization) and a subsequent growth of particles. • Less dispersed in size colloid is formed when the rate of nucleation is high

and the rate of particles growth is low. • To obtain ferrite particles of a smaller size, less dispersed in size and

more chemically homogeneous, the mixing of reagents must be performed as fast as possible.

• Influence of Concentration of Reagents:• Concentration of 0.1 to 0.2 mol/ is usually taken for the synthesis of

ferrite particles, .• This allows to obtain non-viscous primary suspension of particles that is

important for better mixing of the reacting volume. • At higher concentration (Me>0.4mol/ ) suspension becomes viscous,

and it is difficult to provide an intensive stirring.

Influence of Temperature:• An increase in temperature in the range 20-1000C significantly

accelerates formation of ferrite particles. • The activation energy for the formation of ferrites of different metals is

not equal.

• Activation energy calculated from kinetics of the formation reaction for three different ferrites in the temperature range of 20-1000C decreases in the following sequence EA(Ni-ferrite) > EA(Co-ferrite) > EA(Zn-ferrite).

• This sequence is also inagreement with the decreasing of dehydration temperature of the individual hydroxides of the corresponding metals.

Influence of pH of the Reaction:• For the formation of ferrite, the yield of ferrite grows when the pH of the reaction is

increased from 6.8 to 8.6 followed by a slight increase. • The most interesting fact is that a further increase of pH up to 12. -14 leads to a

significant growth of the yield. • At high pH values the time of formation of ferrites become very short.

Duration of Heating after Co-precipitation: Co-precipitation takes place in a concentrated system,

but no information is given on growing of particles size, which may occur in such a system during long heating process. In our case, we heated the precipitates for 30 min.

• The precipitates formed in a state which is easily filtered

• At first solid hydroxides of metals in the form of colloidal particlesare obtained by the co-precipitation of metal cations in alkaline medium.• For the case of Co-Ni ferrites this reaction is :

(1-x)CoCl2. 6H2O + xNiCl2 . 6H2O + 2Fe(NO2)3 .9H2O +8NaOH (1-x)Co(OH)2.xNi(OH)2.2Fe(OH)3 +2NaCl+6NaNO3

• Then this product is subjected to heating in the precipitation alkalinesolution to provide the transformation of metal hydroxides solution to the Co-Ni ferrites,

(1-x)Co(OH)2.xNi(OH)2.2Fe(OH)3 +2NaCl+6NaNO3

Co(1-x)NixFe2O4.nH2O + (4-n)H2O

where n is an integer.