Outline

• How is ferromagnetism manifested?

• What are the types of magnetism?

• What is Fe3O4 – spinel?

• What is nanoscience?

• How do we make ferrofluids?

We will have a Monday class next week

• Turn in extra credit• Writing exercises will be returned• Possible chance for regaining lost points• SRTI evaluations

S S

SSN

N N

N

The field of a force – a property of the space in which the force acts

Magnetic field

attraction

repulsion

http://www.trincoll.edu/~cgeiss/GEOS_312/GEOS_312.htm

θ

Interaction with magnetic field

m = AInm = pd

+p

-p

dτ = m B sinθ

B

θ

aligning torque:

http://www.trincoll.edu/~cgeiss/GEOS_312/GEOS_312.htm

Magnetic field (force lines)

Magnetic field is not a central field (no free magnetic charges)

SN

F

http://www.trincoll.edu/~cgeiss/GEOS_312/GEOS_312.htm

Behavior of magnetic materials

TNeel TCurie

Ferromagnet, Ferrimagnet

Antiferromagnet

Paramagnet

Magnetization

(m or B or M)

Temperature

Types of bulk magnetism

Ferromagnetism Antiferromagnetism Ferrimagnetism

Large M

(1-5 B / atom)

Small M

(10-3 B / atom)

Large M

(1-5 B / atom)

H

Small M

(10-3 B / atom)

Paramagnetism

Development of permanent (hard) magnets

M M

http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_4/backbone/r4_3_6.html

Hard magnets Soft magnets

What is nanoscience?

Contacts on a 60nm bismuth wire to study motion of single defects (kmf.pa.msu.edu/Research/resrch04.asp )

ZnO nanowire UV lasers of about 100nm diameter and 10mm length synthesized at Berkeley. (Yang et al, Science, 292, p. 1897, 2001).

ZnO nanowire UV lasers of about 100 nm diameter and 10 m length synthesized at Berkeley.(Yang et al, Science, 292, p. 1897, 2001)

Radius rules

CNRelative

radiusVoid geometry Polyhedron name

2 <15.5% Linear Line

3 15.5% Triangular Triangle

4 25.5% Tetrahedral Tetrahedron

6 41.4% Octahedral Octahedron

8 73.2% Cubic (BCC) Cube

12 100% Cuboctahedral (HCP, CCP) Cuboctahedron

• A sphere of this size (relative to the lattice of size of its neighbors) is just able to touch all off its neighbors for the void geometries below.

• Similar considerations govern the formation of more complex structural arrangements

Arrangements of nanoparticles mimics arrangements of atoms

Some arrangements are very complex

Electronic and magnetic materials can be combined into sophisticated devices

Magnetite

• Magnetite vs. lodestone

• General spinel formula: AB2O4

A = 2+ metal, B = 3+ metal

• 1/2 of octahedral holes, 1/8 of tetrahedral holes filled on an approximate FCC oxygen lattice

• Fe3O4 1 Fe2+ + 2 Fe3+

• Inverse spinel B(AB)O4

• Ferrimagnetic ordering at ~850K

Fe3O4

• Synthesis: 2 FeCl3 + FeCl2 + 8 NH3 + 4H2O --> Fe3O4 + 8 NH4Cl

Magnetite (Fe3O4)

Unit cell:

A-sites (8 Fe3+)

B-sites (8 Fe3+ and 8 Fe2+)

Ferrimagnetism

Normal Spinel (ZnFe2O4) Inverse Spinel (Fe3O4)

A B

Zn2+ Fe3+ Fe3+

A B

Fe3+ Fe3+ Fe2+=>

5µB 5µB 4µB

Development of permanent (hard) magnetsM

agne

tic

ener

gy (

Gau

ss /

m3 )

Steel

Mag

neti

c en

ergy

(G

auss

/ m

3 )

Steel

Nd2Fe14B

M

M

http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_4/backbone/r4_3_6.html

Ferrofluids of ~10nm ferrite particles

Ferrofluids

Types of bulk magnetism

Ferromagnetism Antiferromagnetism Ferrimagnetism

Large M

(1-5 B / atom)

Small M

(10-3 B / atom)

Large M

(1-5 B / atom)

H

Small M

(10-3 B / atom)

Paramagnetism

Ferromagnetism Antiferromagnetism Ferrimagnetism

H

Canted Antiferromagnetism

Ferromagnetism Antiferromagnetism Ferrimagnetism

H

Canted Antiferromagnetism

Ferromagnetism Antiferromagnetism

H

Ferromagnetism Antiferromagnetism

H

Ferrimagnetism

Ferrimagnetism

Paramagnetism

Paramagnetism

Ferromagnetism Antiferromagnetism Ferrimagnetism

Large M

(1-5 B / atom)

Small M

(10-3 B / atom)

Large M

(1-5 B / atom)

H

Small M

(10-3 B / atom)

Paramagnetism

Types of magnetism

Ferrofluid topics

• Magnetic dipoles, not monopoles like charges

• Field gradient - emphasized by magnetic field lines

• A “test dipole” will aligns itself parallel to magnetic field lines

Development of permanent (hard) magnets

MM