toolkit examples - magician.ucsd.edumagician.ucsd.edu/sio190/lectures/archaeomag_l12.pdf · toolkit...

Toolkit

Examples

1

Environmental magnetism

Thursday, May 19, 2011

ToolkitImages

Critical temperatures, e.g., Curie T

magnetic susceptibility

magnetizations

grain size dependence

useful ratios

2


highT

windblown

secondaryalteration

industrialpollution

cosmic

(300 um)

Images

Chapter 8


4Chapter 6


5

Chapter 6


6

Chapter 6

Curie T and composition


magnetic susceptibility

reflects concentration, magnetic grain size and magnetic mineralogy

is quick (cheap) to measure

when calibrated, can be used as proxy for more expensive measurements

7


8

Chapter 8

pollution detection


Magnetic remanences

Isothermal remanence (IRM)

Anhysteretic remanence (ARM)

Gyromagnetic remancence (GRM)


But first

a tutorial on hysteresis loops


Lots of math in Chapter 5 shows that there will be a field of sufficient energy to flip the moment of a particle with easy axis aligned at an arbitrary angle to the applied field.

called the flipping field. the maximum flipping field is the intrinsic coercivity discussed before


B (mT)

! 1

2 3

4

m

B1

m

B2

m

B3

m

B4

a)

µ Hf"

B

Chapter 5


5

m

B5

B (mT)

!b)

µ Hf"

Chapter 5


State 2

State 3

B

B = 0

State 1 B = 0 M=0

State 4B

M=0

M=Ms

M=Mr

Mr

Ms

IRM

!lf

2

3

14

µ Hc"

descend

ing

ascending

µcr"

a)

B (mT)

Assemblages of (SD) particles

Note - figure in book is wrong - see errataChapter 5


So far, we have only considered uniformly magnetized particles

But big particles have BIG self energies

for highly magnetic particles like magnetite, self energy quickly exceeds other energies keeping things mutually parallel (exchange) or aligned in particular crystal directions (anisotropy)

so spins in larger particles begin to have more complex structures


SP

“vortex”“flower”dom

ain walls

Particle width (nm)

Mr/

Ms

1.0

0.8

0.6

0.4

0.2

0.0

0 40 80 120 160 200

Can model spin alignment - need big computers with lots of memory, called micromagnetic

modelling


• Even bigger particles develop regions of uniform magnetization separated by domain walls


b) a) W

W


40 µm

20 µma)

domains can be imaged


-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 B (T)

-3

-2

-1

0

1

2

3

!lf

!hf

Ms Mr

µ Hc"

µ H!cr"M/M

s

Chapter 5

Some definitions


Chapter 8

hysteresis loops of mixtures


Mr

Ms

IRM

B (mT) Pulse Field (Tesla)

IR

M I

ntensit

y (

Am

2)

Mr

µoH

cr

Two ways to give IRMs

Chapter 5 Chapter 7


Neat application

Coercivity (mT)

Applied Field (mT)

µ H1/2

DP

LAP

GAP

Isot

herm

al R

eman

ent

Mag

neti

zati

on

o

Chapter 8Thursday, May 19, 2011

ARM - what’s it good for

Strong function of concentration (particle interaction)

Strong function of grain size


ARM

/sIR

M

Bdc (mT)

increasing concentration


0.01 0.1 1 10 100 1000Grain size (µm)

10

1

0.1

0.01

! ARM

Chapter 8Thursday, May 19, 2011

Ratios (and differences)

Mr/Ms versus Hcr/Hc

ARM versus magnetic susceptibility

S (IRMx/sIRM or fraction that is “soft”, x) and HIRM (sIRM-IRMx or fraction that is “hard”)

see Table 8.2 in the book for more....


0 5 10 15 20 25

Hcr/Hc

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7M

r/M

s

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

B (T)

SD

“PSD”

MD and SP

Hysteresis ratios


Examples

detection of environmental change

detection of biogenic magnetite


Magnetic Susceptibility

Anh

yste

reti

c Re

man

ence

onset of Agriculture

baseline

decreasing grain size

little ice age

Banerjee et al. 1981 Chapter 8Thursday, May 19, 2011

60 100 140 180

Zr (ppm)0.40.2 0.6 0.8 1.0

Ti (Wt %)10-2 10-1 100

mSI

20

18

16

14

12

10

8

6

10-1 100

HIRM A/m

Magnetite Content

HematiteContent

Detrital Input

Pollen Zone

D(Warm)

C(Cold)

B(Warm)

A(Cold)

!

Dept

h (m

)


chains of biogenic magnetite have aligned

[111] axes

get enhanced FC IRMs

because choice of [100] axis on cooling is not random


sIRM

(nA

m /

l)

0 50 100 150 200 250 300Temperature (K)

800

900

1000

1100

1200

1300

1400

2

FCZFC

is:

0 50 100 150 200 250 300Temperature (K)

180

200

220

240

260

280

300

320

340


1.0 1.5 2.0 2.5

FC ZFC

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0W

ater

Dep

th (m

)

! /!

sr

sr

sr

oxic

anoxic

OAIintact chains of SD

magnetite (biogenic)


Take home message

Magnetic measurements can be quick and cheap to make

Can be used as a proxy for detecting, e.g., industrial pollution, environmental change, etc.

36


toolkit examples - magician.ucsd.edumagician.ucsd.edu/sio190/lectures/archaeomag_l12.pdf · toolkit...

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