1 Magnetic Properties of Materials - PowerPoint Presentation

Slide1

1

Magnetic Properties of Materials

… force in

direction

…

sample volume… magnetic susceptibility … magnetic field … gradient of the magnetic field

The magnetic susceptibility characterizes the magnetic properties of materials

 Slide2

2

Other Parameters

… force acting on a material

… permeability

(similar to permittivity:

 = 1 + P/[0E])… magnetic induction… magnetization… magnetic flux (B… magnetic flux density)… magnetization and magnetic momentSlide3

3

Magnetic Properties of Materials

… plus antiferromagnetic and

ferrimagneticSlide4

4

Interaction with an External Magnetic Field

Material

Interaction

Diamagnetic

Is repelled by the applied magnetic fieldParamagneticAre attracted by the applied magnetic field with different forcesFerromagneticAntiferromagneticFerrimagneticSlide5

5

Diamagnetism

Change of the inner or atomic “electrical” current within an external magnetic field

:

Change in angular velocity of strongly bound electrons

Rotation (circular movement) of free (metallic) electronsSlide6

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Diamagnetism

Diamagnetic materials create an induced magnetic field (magnetization

) in a direction opposite to the external magnetic field, therefore the magnetic induction

is small in the material.

 Ideal diamagnetic materials are superconductors in the superconducting state (Meissner effect)

… negative in diamagnetic materialsSlide7

7

Paramagnetism

Without an external magnetic field (

= 0

),

there is no magnetization of the material ( = 0), because the magnetic moments of single atoms (electrons) are oriented randomly.In an external magnetic field (H > 0), the magnetic moments of single atoms (electrons) are oriented in the direction of the external magnetic field  M > 0.Temperature vibrations disturb the orientation of magnetic moments  susceptibility depends on temperature. 

 Slide8

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Paramagnetism

(a) … Curie’s law

(b), (c) … Curie-Weiss law for paramagnetic materials

(d) … diamagnetic material

… Curie

… Curie-WeissSlide9

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Paramagnetism

Meaning of constants

a

nd in Curie’s law and the Curie-Weiss law

 Magnetism of electrons in an atom (orbital electrons)

… number of magnetic moments (atoms)

 Molecular field theory* (Weiss 1907)

* Belongs to the mean field theorySlide10

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Spin Paramagnetism

Additional effect to the orbital magnetism

Elements with 3d electrons (occupation of orbitals is described by

Hund’s

rules):

Fe: 3s

2, 3p6, 3d6

Spin magnetic

Co: 3s

2

, 3p

6

, 3d

7

Spin magnetic

Ni: 3s

2

, 3p

6

, 3d

8

Spin magnetic

Cu: 3s

2

, 3p

6

, 3d

10

Not spin magnetic

Zn: 3s

2

, 3p

6

, 3d

10

Not spin magneticSlide11

11

Elements with 3d ElectronsSlide12

Ferromagnetism

The major characteristics of ferromagnetic materials

Ordering of magnetic moments below

Saturation of magnetization

T

ransition ferromagnetic  paramagnetic at Temperature dependency of  

12Slide13

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Magnetic Properties of Ferromagnetic Materials – Examples

770°C

1131°C

358°C

15.8°CSlide14

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Influence of Real Structure

(Residual Stress)

o

n magnetic properties of ferromagnetic materials

Nickel (fcc)Iron (bcc)Slide15

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Influence of

Real Structure (Crystallite Orientation)

on magnetic properties of ferromagnetic materials

Crystal anisotropy of magnetic properties (magnetization)

The average of physical properties is measuredExample: iron single crystalSlide16

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Permanent

Magnets

Wide hysteresis

curve is neededSlide17

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Materials for Permanent MagnetsSlide18

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Magnetoelastic Effects

Magnetostriction

Change in length

(in the lattice

parameters) of magnetic crystals within a magnetic fieldSpontaneous magnetostriction

Change in length (lattice parameters) of magnetic crystals in the own magnetic fieldObserved in some materials below – at the ordering of magnetic moments

 Slide19

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Spontaneous Magnetostriction

ErCo

2

RT: Fd-3mLT: R-3m

 = 90°    90° Slide20

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Spontaneous Magnetostriction

Separation of

crystallographically

non-equivalent

diffraction linesSlide21

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Magnetostriction

Coefficients of magnetostriction in

Er

(Co,Ge)2 and

Er(Co,Si)2Slide22

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Er

(Co

1-x

Six)2Increase of lattice parameters (volume of unit cell) at low temperatures

Ordering of magnetic moments  magnetic interactions between single atoms  Change of the crystal structureSlide23

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Antiferromagnetism

Ordering of magnetic moments below

(

…

Néel temperature) Example: MnO, UN (fcc

, Fm3m, NaCl structure), MnF2Antiparallel ordering of magnetic moments

Negative critical temperature:

Susceptibility in paramagnetic stateSlide24

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Experimental Methods to Investigate the Orientation of

Magnetic Moments

Neutron diffraction

Elastic scattering of neutrons on atomic nuclei

 Information about the crystal structure (similar to x-ray diffraction)Interaction between the magnetic moments of the neutrons and the magnetic moments of atoms  information about the magnetic structureSlide25

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Magnetic Properties of Antiferromagnetic Materials – Examples

UN

=

53 K

= 247 KCrN = 273-286 K Slide26

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Influence of Real Structure

on magnetic properties of

antiferromagnetic materials

Thin layers of UN

Different temperature of coating  different residual stress, crystallite sizes and density of defectsFormation of an apparent ferromagnetic component at low temperatures  unbalanced magnetic moments

UN = 53 K = 247

K Slide27

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Ferrimagnetism

Spontaneous ordering of magnetic moments and hysteresis below the Curie temperature as in ferromagnetic materials

A

ferrimagnetic

compound is typically a ceramic material (ferrite – FeO.Fe2O3, NiO.Fe2O3, CuO.Fe2O3, …) with spinel structure.Slide28

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Susceptibility and Magnetization of

Ferrimagnetic Materials

NiO.Fe

2

O3Slide29

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GMR Effect

Giant Magnetoresistance in Multilayers

dia

ferro

diaferroH = 0diaferrodiaferro

H > 0

Diamagnetic material: Cu, Ag, AuFerromagnetic material: Fe, Co, Ni

I



I

Slide30

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Physical Principle of GMR

Scattering depends on the relative orientations of the electron spins and

the

magnetic moments of atoms.Parallel: weakest scattering Antiparallel: strongest scattering

Antiferromagnetic coupling of two ferromagnetic layers above a diamagnetic layerSlide31

Nobel prize in physics 200731Peter Andreas Grünberg

Albert Louis François Fert

For discovery of the giant magneto-resistance effectSlide32

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Change of the Electrical Resistance

in an External Magnetic Field

Definition of GMR:Slide33

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Change of Electrical Resistance in an External Magnetic Field

System: Co/CuSlide34

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Important Parameters of Magnetic Multilayers

Selection of materials (diamagnetic, ferromagnetic)Thickness of layersRoughness and morphology of the interfaces

Methods for investigationMeasurement of the resistance within a variable magnetic fieldXRD, neutron diffractionTEMApplications

Magnetic field sensors (reading heads for hard disks)Solenoid valves (Spin valves)

10 nmSlide35

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Influence of Thickness of “Spacers”

Co

Cu

.

.

. . .CoCu

50xon magnetic properties of multilayersSlide36

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Reading Head in a Hard Disk

Pros:

Very small dimensions

[(Co 11Å/ Cu 22 Å) x 50] =

= 1650 Å = 165 nm = 0.165 mSlide37

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Storage capacitySlide38

Storage capacity38

Inductive reading heads

Magneto-resistive

reading heads

Reading heads

with GMR effect