PRESENTED TO:PRESENTED TO:Sir Mahmood AhmadSir Mahmood Ahmad

PRESENTED BY:PRESENTED BY:Ahmad Abdurehman Ahmad Abdurehman

Hafiz M. SajidHafiz M. SajidAdnan NawazAdnan Nawaz

M. JafarM. Jafar

What is Magnet?What is Magnet?

A magnet is a material or object that A magnet is a material or object that produces a magnetic field. This magnetic produces a magnetic field. This magnetic field is invisible but is responsible for the field is invisible but is responsible for the most notable property of a magnet: a force most notable property of a magnet: a force that pulls on other ferromagnetic materials that pulls on other ferromagnetic materials and attracts or repels other magnets.and attracts or repels other magnets.

Classes of Magnetic Classes of Magnetic Materials:Materials:

The origin of magnetism lies in the orbital The origin of magnetism lies in the orbital and spin motions of electrons and how the and spin motions of electrons and how the

electrons interact with one another. electrons interact with one another.

Types of magnetTypes of magnet

DiamagnetismDiamagnetismPara magnetismPara magnetismFerromagnetismFerromagnetismFerrimagnetism’sFerrimagnetism’sAnt ferromagnetismAnt ferromagnetism

DiamagnetismDiamagnetism

Diamagnetism is a fundamental property of all Diamagnetism is a fundamental property of all matter, although it is usually very weak. It is due matter, although it is usually very weak. It is due to the non-cooperative behavior of orbiting to the non-cooperative behavior of orbiting electrons when exposed to an applied magnetic electrons when exposed to an applied magnetic field. Diamagnetic substances are composed of field. Diamagnetic substances are composed of atoms which have no net magnetic moments atoms which have no net magnetic moments (ie., all the orbital shells are filled and there are (ie., all the orbital shells are filled and there are no unpaired electrons). However, when exposed no unpaired electrons). However, when exposed to a field, a negative magnetization is produced to a field, a negative magnetization is produced and thus the susceptibility is negative. If we plot and thus the susceptibility is negative. If we plot M vs. H, we see:M vs. H, we see:

Para magnetismPara magnetism This class of materials, some of the atoms or ions in the This class of materials, some of the atoms or ions in the

material has a net magnetic moment due to unpaired material has a net magnetic moment due to unpaired electrons in partially filled orbital. One of the most electrons in partially filled orbital. One of the most important atoms with unpaired electrons is iron. important atoms with unpaired electrons is iron. However, the individual magnetic moments do not However, the individual magnetic moments do not interact magnetically, and like diamagnetism, the interact magnetically, and like diamagnetism, the magnetization is zero when the field is removed. In the magnetization is zero when the field is removed. In the presence of a field, there is now a partial alignment of presence of a field, there is now a partial alignment of the atomic magnetic moments in the direction of the the atomic magnetic moments in the direction of the field, resulting in a net positive magnetization and field, resulting in a net positive magnetization and positive susceptibility. positive susceptibility.

In addition, the efficiency of the field in aligning the In addition, the efficiency of the field in aligning the moments is opposed by the randomizing effects of moments is opposed by the randomizing effects of temperature. This results in a temperature dependent temperature. This results in a temperature dependent susceptibility, known as the Curie Law. susceptibility, known as the Curie Law.

At normal temperatures and in moderate fields, At normal temperatures and in moderate fields, the paramagnetic susceptibility is small (but the paramagnetic susceptibility is small (but larger than the diamagnetic contribution). Unless larger than the diamagnetic contribution). Unless the temperature is very low (<<100 K) or the the temperature is very low (<<100 K) or the field is very high paramagnetic susceptibility is field is very high paramagnetic susceptibility is independent of the applied field. Under these independent of the applied field. Under these conditions, paramagnetic susceptibility is conditions, paramagnetic susceptibility is proportional to the total iron content. Many iron proportional to the total iron content. Many iron bearing minerals are paramagnetic at room bearing minerals are paramagnetic at room temperature. Some examples, in units of 10-8 temperature. Some examples, in units of 10-8 m3/kg, include:m3/kg, include:

ExamplesExamples

Montmorillonite (clay) 13Montmorillonite (clay) 13 Nontronite (Fe-rich clay) 65Nontronite (Fe-rich clay) 65 Biotite (silicate) 79Biotite (silicate) 79 Siderite(carbonate) 100 Siderite(carbonate) 100 Pyrite (sulfide) 30Pyrite (sulfide) 30 The Para magnetism of the matrix minerals in The Para magnetism of the matrix minerals in

natural samples can be significant if the natural samples can be significant if the concentration of magnetite is very small. In this concentration of magnetite is very small. In this case, a paramagnetic correction may be case, a paramagnetic correction may be needed.needed.

FerromagnetismFerromagnetism::

When you think of magnetic materials, you When you think of magnetic materials, you probably think of iron, nickel or magnetite. Unlike probably think of iron, nickel or magnetite. Unlike paramagnetic materials, the atomic moments in paramagnetic materials, the atomic moments in these materials exhibit very strong interactions. these materials exhibit very strong interactions. These interactions are produced by electronic These interactions are produced by electronic exchange forces and result in a parallel or exchange forces and result in a parallel or antiparallel alignment of atomic moments. antiparallel alignment of atomic moments. Exchange forces are very large, equivalent to a Exchange forces are very large, equivalent to a field on the order of 1000 Tesla, or field on the order of 1000 Tesla, or approximately a 100 million times the strength of approximately a 100 million times the strength of the earth's field. the earth's field.

The exchange force is a quantum mechanical The exchange force is a quantum mechanical phenomenon due to the relative orientation of phenomenon due to the relative orientation of the spins of two electron. the spins of two electron.

Ferromagnetic materials exhibit parallel Ferromagnetic materials exhibit parallel alignment of moments resulting in large net alignment of moments resulting in large net magnetization even in the absence of a magnetization even in the absence of a magnetic field.magnetic field.

The elements Fe, Ni, and Co and many of their The elements Fe, Ni, and Co and many of their alloys are typical ferromagnetic materials. alloys are typical ferromagnetic materials.

Two distinct characteristics of Two distinct characteristics of ferromagnetic materials are theirferromagnetic materials are their

(1) Spontaneous magnetization and the (1) Spontaneous magnetization and the existence of Magnetic ordering existence of Magnetic ordering temperaturetemperature

Spontaneous MagnetizationSpontaneous Magnetization The spontaneous magnetization is the net The spontaneous magnetization is the net

magnetization that exists inside a uniformly magnetization that exists inside a uniformly magnetized microscopic volume in the absence magnetized microscopic volume in the absence of a field. The magnitude of this magnetization, of a field. The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic at 0 K, is dependent on the spin magnetic moments of electrons.moments of electrons.

A related term is the saturation magnetization A related term is the saturation magnetization which we can measure in the laboratory. The which we can measure in the laboratory. The saturation magnetization is the maximum saturation magnetization is the maximum induced magnetic moment that can be obtained induced magnetic moment that can be obtained in a magnetic field (Hsat); beyond this field no in a magnetic field (Hsat); beyond this field no further increase in magnetization occurs. further increase in magnetization occurs.

Curie Temperature:Curie Temperature:

Even though electronic exchange forces in Ferro Even though electronic exchange forces in Ferro magnets are very large, thermal energy magnets are very large, thermal energy eventually overcomes the exchange and eventually overcomes the exchange and produces a randomizing effect. This occurs at a produces a randomizing effect. This occurs at a particular temperature called the Curie particular temperature called the Curie temperature (TC). Below the Curie temperature, temperature (TC). Below the Curie temperature, the Ferro magnet is ordered and above it, the Ferro magnet is ordered and above it, disordered. The saturation magnetization goes disordered. The saturation magnetization goes to zero at the Curie temperature. A typical plot of to zero at the Curie temperature. A typical plot of magnetization vs. temperature for magnetite is magnetization vs. temperature for magnetite is shown below.shown below.

The Curie temperature is also an intrinsic The Curie temperature is also an intrinsic property and is a diagnostic parameter property and is a diagnostic parameter that can be used for mineral identification.that can be used for mineral identification.

However, it is not foolproof because However, it is not foolproof because different magnetic minerals, in principle, different magnetic minerals, in principle, can have the same Curie temperature can have the same Curie temperature

Hysteresis:Hysteresis:

In addition to the Curie temperature and In addition to the Curie temperature and saturation magnetization, Ferro magnets saturation magnetization, Ferro magnets can retain a memory of an applied field can retain a memory of an applied field once it is removed. This behavior is called once it is removed. This behavior is called hysteresis and a plot of the variation of hysteresis and a plot of the variation of magnetization with magnetic field is called magnetization with magnetic field is called a hysteresis loop.a hysteresis loop.

Ferrimagnetisms:Ferrimagnetisms:

In ionic compounds, such as oxides, more In ionic compounds, such as oxides, more complex forms of magnetic ordering can complex forms of magnetic ordering can occur as a result of the crystal structure. occur as a result of the crystal structure. One type of magnetic ordering is call One type of magnetic ordering is call ferrimagnetisms. A simple representation ferrimagnetisms. A simple representation of the magnetic spins in a ferromagnetic of the magnetic spins in a ferromagnetic oxide is shown here.oxide is shown here.

The magnetic structure is composed of The magnetic structure is composed of two magnetic sub lattices (called A and B) two magnetic sub lattices (called A and B) separated by oxygen. The exchange separated by oxygen. The exchange interactions are mediated by the oxygen interactions are mediated by the oxygen anions. When this happens, the anions. When this happens, the interactions are called indirect or super interactions are called indirect or super exchange interactions. The strongest exchange interactions. The strongest super exchange interactions result in an super exchange interactions result in an ant parallel alignment of spins between the ant parallel alignment of spins between the A and B sub lattice.A and B sub lattice.

Crystal Structure of MagnetiteCrystal Structure of Magnetite

Magnetite, Fe3O4 crystallizes with the Magnetite, Fe3O4 crystallizes with the spinal structure. The large oxygen ions are spinal structure. The large oxygen ions are close packed in a cubic arrangement and close packed in a cubic arrangement and the smaller Fe ions fill in the gaps. The the smaller Fe ions fill in the gaps. The gaps come in two flavors gaps come in two flavors

Tetrahedral site:Tetrahedral site:

Fe ion is surrounded by four oxygen Fe ion is surrounded by four oxygen

Octahedral siteOctahedral site:: Fe ion is surrounded by six oxygen Fe ion is surrounded by six oxygen

The tetrahedral and octahedral sites form The tetrahedral and octahedral sites form the two magnetic sub lattices, A and B the two magnetic sub lattices, A and B respectively. The spins on the A sub lattice respectively. The spins on the A sub lattice are anti parallel to those on the B sub are anti parallel to those on the B sub lattice. The two crystal sites are very lattice. The two crystal sites are very different and result in complex forms of different and result in complex forms of exchange interactions of the iron ions exchange interactions of the iron ions between and within the two types of sites. between and within the two types of sites.

The structural formula for The structural formula for magnetite ismagnetite is

[Fe3+]A [Fe3+,Fe2+]B O4[Fe3+]A [Fe3+,Fe2+]B O4This particular arrangement of cations on This particular arrangement of cations on

the A and B sub lattice is called an inverse the A and B sub lattice is called an inverse spinel structure. With negative AB spinel structure. With negative AB exchange interactions, the net magnetic exchange interactions, the net magnetic moment of magnetite is due to the B-site moment of magnetite is due to the B-site Fe2+. Fe2+.

Antiferromagnetism:Antiferromagnetism:

If the A and B sub lattice moments are exactly If the A and B sub lattice moments are exactly equal but opposite, the net moment is zero. This equal but opposite, the net moment is zero. This type of magnetic ordering is called type of magnetic ordering is called antiferromagnetism.antiferromagnetism.

The clue to antiferromagnetism is the behavior The clue to antiferromagnetism is the behavior of susceptibility above a critical temperature, of susceptibility above a critical temperature, called the Néel temperature (TN). Above TN, the called the Néel temperature (TN). Above TN, the susceptibility obeys the Curie-Weiss law for susceptibility obeys the Curie-Weiss law for paramagnets but with a negative intercept paramagnets but with a negative intercept indicating negative exchange interactions.indicating negative exchange interactions.

Crystal Structure of Hematite:Crystal Structure of Hematite: Hematite crystallizes in the corundum structure with Hematite crystallizes in the corundum structure with

oxygen ions in an hexagonal close packed framework. oxygen ions in an hexagonal close packed framework. The magnetic moments of the Fe3+ ions are Ferro The magnetic moments of the Fe3+ ions are Ferro magnetically coupled within specific c-planes, but magnetically coupled within specific c-planes, but antiferromagnetically coupled between the planes. antiferromagnetically coupled between the planes.

Above -10°C, the spin moments lie in the c-plan but are Above -10°C, the spin moments lie in the c-plan but are slightly canted. This produces a weak spontaneous slightly canted. This produces a weak spontaneous magnetization within the c-plan (ss = 0.4 Am2/kg). magnetization within the c-plan (ss = 0.4 Am2/kg).

Below -10°C, the direction of the antiferromagnetism Below -10°C, the direction of the antiferromagnetism changes and becomes parallel to the c-axis; there is no changes and becomes parallel to the c-axis; there is no spin canting and hematite becomes a perfect spin canting and hematite becomes a perfect antiferromagnet. antiferromagnet.