SEM 2 PT3

Question 1

What is a magnetic moment

Answer 1

Measure of an object’s tendacy to align with a magnetic field

Question 1

What are diamagnets

Answer 1

Materials with no pre-existing magnetic moments

Question 2

Why are superconductors perfect diamagnetic materials?

Answer 2

Induced magnetisation cancels applied B-field leading to 0 B-fields in bulk superconductor

Question 3

What is paramagnetic behaviour?

Answer 3

B-field applied to material with pre-existing magnetic moment and induces a net alignment of magnetic moments in the direction of applied field.

Question 4

What is ferromagnetism?

Answer 4

Magnetic moments tend to align parallel to each other to produce a strong magnetisation even in the absence of applied magnetic field

Question 5

What is anti-ferromagnetism?

Answer 5

Materials the moment directions alternate giving zero net magnetisation

Question 6

What is magnetic moments due to

Answer 6

unpaired electron spins or orbital electron motion

Question 7

Is magnetic susceptability positive or negative for paramagnets and diamagnets

Answer 7

Paramagnets are positive
Diamagnets are negative

Question 8

Why do diamagnets occur in all materials

Answer 8

Due to magnetic moments induced in a material due to applied B-fields

Applied B-Fields induces orbital moments in atoms of material

and material is conducting thenthe
Net orbital moments in conducting electrons

Induced moments and magnetisation is in opposite direction to applied magnetic field therefore magnetisation is negayive

Question 9

How paramagnetism words in materials with pre-exisitng magnetic moments

Answer 9

W/O an applied B-fields, moments are oriented randomly giving 0 magnetisation

When magnetic field is applied moments align in direction of magnetic field - giving net magnetisation which increases applied field

At Room temp - magnetisation increases linearly w applied b-fields

Question 10

For material where interaction between magnetic moment is weal what is the low magnetic field susceptibility dependant on?

Answer 10

inversely on temperature

At very low
temperatures and very high magnetic fields M(H) becomes
non-linear and M ultimately reaches a maximum value,
the saturation magnetisation, Msat, when all the moments
are fully aligned with the applied field.

Question 11

How do ordered magnetic states arise

Answer 11

interactions between individual magnetic magnetic moments in a material

Question 12

How ferromagnetis work

Answer 12

Spotaneous parallel alignments of moments with M not equal to 0 and H=0

Question 13

How do materials induce an EMF

Answer 13

Increasingly applying B-field from 0

Question 14

What is Lenz law

Answer 14

Resulting electric currents produce a B-fields of opposite sign to applied field.

Question 15

Origin of diamagnetism at atomic level

What is magnetic moment due to

Answer 15

orbital motion is induced the resulting magnetic moments persist even when applied field is constant.

Magnetic moment due yo planar conducting loop carrying current

Question 16

How to calculate susceptibility

Answer 16

Calculate mean square distance of an electron from the z- axis of a particular atom

Question 17

How many energy levels does an applied magnetic field produce

Answer 17

(2𝐽 + 1)

Question 18

What is the Curie temperature

Answer 18

temperature above which the spontaneous magnetization vanishes

Question 19

in ferro magnetisx,when are the energies of the neighbouring atoms lower

Answer 19

When in parallel

Question 20

What are exchange interactions

Answer 20

dependance of total energy on relative orientation of spins

Question 21

What are magnons

Answer 21

vibrational modes of magnetic moments

Question 22

In the absence of a magnetic field, what determines the direction of moments points

Answer 22

Magnetic anisotropy - favouring onr or a number of lower energy orientations of magentisation

Magneto-crystalline anisotropy- specific crystal structures

shape anisotropy- non-spherical samples

strain anisotropy - samples with uniaxial strain

Question 23

What is Strain Anisotropy

Answer 23

atoms of a solid being closed together along specific direction - leading to anisotropic exchange and spin-orbit coupling