Magnetochemistry Part I: Study of Magnetic Properties of Materials

Magnetochemistry Part I
Lecture by Prof. G.M.Dongare
Dept. of Chemistry,
Shri Shivaji College Of Arts, Commerce and
Science, Akola (Maharashtra) India
Academic year 2021-22
Class B.Sc and M.Sc I

Magnetism-Background
• Magnetochemistry is the study of the magnetic properties of
materials. By "magnetic properties" we mean not only whether a
material will make a good bar magnet, but whether it will be
attracted or repelled by a magnet. This includes synthesis, analysis
and understanding. This short description is meant to give a basic
understanding before you delve into a more complex treatment.
• Magnetism arises from moving charges, such as an electric current
in a coil of wire. In a material which does not have a current present,
there are still magnetic interactions. Atoms are made of charged
particles (protons and electrons) which are moving constantly.
• Each spinning electron causes a magnetic field to form around it. In
most materials, the magnetic field of one electron is cancelled by an
opposite magnetic field produced by the other electron in the pair.
• The atoms in materials such as iron, cobalt and nickel have unpaired
electrons, so they don't cancel the electron’s magnetic fields.
• As a result, each atom of these elements acts like a very small
magnet.

The processes which create magnetic fields in an atom
• Nuclear spin. Some nuclei, such as a hydrogen atom,
have a net spin, which creates a magnetic field.
• Electron spin. An electron has two intrinsic spin
states (similar to a top spinning) which we call up and
down or alpha and beta.
• Electron orbital motion. There is a magnetic field due
to the electron moving around the nucleus.
• Each of these magnetic fields interact with one
another and with external magnetic fields. However,
some of these interactions are strong and others are
negligible.

The magnetic moment of a single atom(μ)
• μ is a vector
• μ= i F [Am2], circular current i, area F
μB= eh/4πme= 0.9274 10-27Am2
(h: Planck constant, me: electron mass)
μB: “Bohr magneton”
(smallest quantity of a magnetic moment)
• For one unpaired electron in an atom
μs = 1.73 μB; μs = spin only magnetic moment
• The magnetic moment of an atom has two components
a spin component(“spin moment” ) and
an orbital component(“orbital moment”).
→Frequently the orbital moment is suppressed (“spin-only-
magnetism”, e.g. coordination compounds of 3d elements)

Magnetism
• when a substance is placed within a magnetic field, H, the field
within the substance, B, differs from H by the induced field, 4πI,
which is proportional to the intensity of magnetization, I.
That is; B = H + 4πI
where B is the magnetic field within the substance
H is the applied magnetic field and
I is the intensity of magnetisation
This can also be written as
B/H = 1 + 4π I/H, or B/H = 1 + 4 π 
Where B/H is called the magnetic permeability of the material and
 is the magnetic susceptibility per unit volume, (I/H)
The classical theory of magnetism was well developed before quantum
mechanics. Lenz's Law (~1834), states that:

Magnetism
By definition,  in a vacuum is zero, so that B=H.
It is usually more convenient to measure mass (gram)
susceptibility, χg,
which is related to the volume susceptibility through the density.
χg =  /ρ
where ρ is the density.
Finally to get our measured quantity on a basis that can be related
to atomic properties, we convert to molar susceptibility
χm = χg x MW ; (MW = molecular weight of the sample)
χm = Nμ2 / 3kT
where N is Avogadro's Number; k is the Boltzmann constant
and T the absolute temperature
Rewriting this gives the magnetic moment as
μ = 2.828 χmT = 2.828(χmT)1/2

Magnetic States of Matter
Diamagnet - A diamagnetic compound has all of it's electron spins
paired giving a net spin of zero. Diamagnetic compounds are weakly
repelled by a magnet.
Paramagnet - A paramagnetic compound will have some electrons with
unpaired spins.
Paramagnetic compounds are attracted by a magnet.
Paramagnetism derives from the spin and orbital angular momenta of
electrons. This type of magnetism occurs only in compounds containing
unpaired electrons, as the spin and orbital angular momenta is cancelled
out when the electrons exist in pairs.
Compounds in which the paramagnetic centers are separated by diamagnetic
atoms within the sample are said to be magnetically dilute.
If the diamagnetic atoms are removed from the system then the
paramagnetic centres interact with each other. This interaction leads to
ferromagnetism (in the case where the neighboring magnetic dipoles
are aligned in the same direction) and antiferromagnetism (where the
neighboring magnetic dipoles are aligned in alternate directions).

Different types of collective magnetism in a solid due to coupling
of magnetic moments

Curie-und Curie-Weiss law for paramagnetic samples

Curie-und Curie-Weiss law for paramagnetic samples
Curie: 1/χ= C•T; Curie-Weiss: 1/χ= C•(T-Θ)

Magnetism in solids (cooperative magnetism)
Diamagnetism and paramagnetism are characteristic of
compounds with individual atoms which do not interact
magnetically (e.g. classical complex compounds)
Ferromagnetism, anti-ferromagnetism and other types of
cooperative magnetism originate from an intense magnetical
interaction between electron spins of many atoms

Magnetism
Diamagnetism
• External field is weakened
• Atoms/ions/molecules with closed shells
-10-4 < χm < -10-2 cm3/mol (negative sign)
Paramagnetism (van Vleck)
• External field is strengthened
• Atoms/ions/molecules with openshells/unpairedelectrons
10-4 < χm < 10-1 cm3/mol
diamagnetism(core electrons)
+ paramagnetism (valence electrons)
Pauli-Paramagnetism:
→special type of magnetism of the conduction electrons in
metals
→refers only to the free electrons in the electron gas of a
metallic solid)
10-6 < χm < 10-5 cm3/mol

Magnetochemistry Part I: Study of Magnetic Properties of Materials

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Magnetochemistry Part I: Study of Magnetic Properties of Materials