Nano Magnetic Tutorial

8/6/2019 Nano Magnetic Tutorial

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NANOMAGNETISM (I)WHAT IS IT????

SO WHAT????

Robert D. Shull

Leader: Magnetic Materials Group,National Institute of Standards and Technology

V. President: The Minerals, Metals, & Materials Society (TMS)

Member: OSTP Nanoscale Science, Engineeringand Technology Subcommittee, NSET

Magnetic Materials Group

OUTLINE

(I).

(II).

(III)

What is Nanotechnology??

Why is it Different???

Nanomagnetism Changes


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O-K

FeL 32O-K

Flame Synthesized SiO 2-Fe 2O3

O-K

FeL 32


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QUANTUM CORRALS (Fe on Cu)


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AFM Tip

Writing direction

Water meniscus

Molecular transport

Substrate

Dip Pen Lithography: Chad Mirkin (Northwestern Univ.)

Plasma or Vapor DepositionMechanical Alloying

PRODUCTION METHODS

PRODUCTION METHODS

Inert Gas Condensation (IGC)Plasma Vapor Deposition (PVD)Condensed Vapor Deposition (CVD)Pulsed Laser Deposition (PLD)Chemical Precipitation from Solution

Solid Solution Precipitation

Ion Replacement & Reduction

Sputtering

Sol Gel Chemistry AFM D. Eigler (IBM)

QUANTUM CORRAL (Fe atoms on Cu)


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THREE REASONS

WHY PROPERTIES ARE DIFFERENT

WHEN MATERIALS POSSESS

SOME NANOSCALE DIMENSION

(1)


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(2)


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Resistivity mean free path

Thermal Conductivity mean free pathStrength dislocation Burgers vector

Transmission & Reflection - wavelengthDiffraction & Scattering - wavelength

Absorption penetration depth

Atomic Transport diffusion lengthSuperconductivity coherence length

Elasticity bond & chain lengthsReaction Rate diffusion length

Boundary Motion radius of curvature

Fluid Flow boundary layer thicknessMagnetism exchange length, domain wall width

CRITICAL LENGTH SCALES(3)


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Spherical Particle Disc-Shaped Particle Rod-Shaped Particle

Fiber Intergranular Film Layered

NANOCOMPOSITE MORPHOLOGIES

Critical Dimensions:Particle Diameter, Separation Distance, Aspect Ratio,Fiber Diameter, Layer Thickness, Grain Diameter,


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= Superparamagnetic Material:

SMALL PARTICLE BEHAVIOR

Assembly of magnetic clusters (each comprised of manyferromagnetically-aligned elemental moments of magnitude )

acting independently.



+M Saturation M Semanent M R

Coercivity

HYSTERESIS LOOP OF A FERROMAGNET


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-9 -7 -5 -3 -1 1 3 5 7 9

60

40

20

0

-20

-40

-60

300 K

Applied Field (10 2 kA/m)

M a g n e

t i z a

t i o n

( A m

2 / k g

)

Applications:

Magnetic InksMagnetic Separation

Vacuum SealingMagnetic Marking

Magnetic RefrigerationMagnetic Resonance Imaging

No Remaining Magnetism Upon Field Removal!!!

(Occurs when Particles are Very Small and Decoupled)

SUPERPARAMAGNETISM


A NewConcern

for Nano-MagneticMaterials

For conventional


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Results in2 Time Constants!

TIME DEPENDENCE


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Kinetically Frozen Nanoparticle Moments

In nanoparticles, theMagnetic momentscan be kinetically frozenif the temperature islow enough. Thattemperature is calledthe blocking

temperature: T B . Thattemperature dependsslightly upon how it ismeasured. It alsodepends on the Volumeof the material and themagnetic anisotropy

of that material.

HIGHER DENSITY

MAGNETIC RECORDING MEDIA

CHANGES WITH SIZE REDUCTION


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Fe3 4O

1 m

20 Angstroms

Ag

(2 nm)

Fe3 4O

NANOCOMPOSITES

Conventional Media High Density Media

IMPROVED MAGNETIC RECORDING MEDIA

Higher Density Achieved by Reduction of Bit Size so there areMore Bits/Unit Area



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-9 -7 -5 -3 -1 1 3 5 7 9

60

40

20

0

-20

-40

-60

300 K

Applied Field (10 2 kA/m)

M a g n e t i z a

t i o n

( A m

2 / k g

)

Applications:

Magnetic InksMagnetic Separation

Vacuum Sealing

Magnetic MarkingMagnetic RefrigerationMagnetic Resonance Imaging

No Remaining Magnetism Upon Field Removal!!!

(Occurs when Particles are Very Small and Decoupled)

SUPERPARAMAGNETISM SETS A LIMITTO PARTICLE (BIT) SIZE REDUCTION!!!


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MAGNETORESISTANCE

Recorder Heads

MagnetoresistiveRead Head

(e.g., permalloy)

Giant Magnetoresistance

(GMR) Heads

NEED FORHIGHER

SENSITIVITYMAGNETIC

FIELDSENSORS

WITHPARTICLESIZE

REDUCTION(Also true for Biological

Applications of SmallMagnetic Particles)

= 2.5% )( /

> 50% )/


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[Baibich, et. al., Phys. Rev. Lett. 61, 2472 (1988).]


G M R ( % )

TYPICAL PUBLISHED RESULTS

50

60

70

80

90

100

110


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SOFT MAGNETIC

PROPERTIES ARE DIFFERENT

WHEN MATERIALS POSSESS

SOME NANOSCALE DIMENSION

TRANSFORMER

i 2

i 1

115 v AC 20 v AC

+

-

+

-

Ferromagnet


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+HH

-M

+M

Magnetization Reversal byDomain Nucleation & Growth

(Most Common Mechanism)


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But, when the grain size becomes

comparable to the domain wall width,

magnetic coercivity begins dropping with

the grain size and magnetic hysteresis

decreasing grain size.

Normally, coercivity increases with

decreases!!!


Summary

Increased surface/volume ratio of nanomagnetsmakes them more susceptible to interactioneffects with neighboring magnetic materials.

Nanostructuring a material can result in the creationof new magnetic states, like superparamagnetism.


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NANOMAGNETISM (II)

DOMAINSIN NANO-FERROMAGNETS???

Robert D. Shull

Leader: Magnetic Materials Group,National Institute of Standards and Technology

V. President: The Minerals, Metals, & Materials Society (TMS)

Member: OSTP Nanoscale Science, Engineeringand Technology Subcommittee, NSET


OUTLINE

(I).

(II).

(III)

Domains in Nanomaterials?

Size Dependence

Domains in Thin Film Nanocomposites


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+HH

-M

+M

Magnetization Reversal byDomain Nucleation & Growth

(Most Common Mechanism)


DO DOMAINSEXITS IN

NANOCRYSTALLINEFERROMAGNETS???

Domains require ananisotropy to exist. Sincethe magnetocrystalline


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H

(A) (B)

(C) (D)

H=0 H>H A

H>H B H>H C

Domain Patterns in Nanocrystalline (d=18 nm) Ni(Imaged by Magnetic Fluid Method)

Field dependentpatterns in amagnetic fluid ontop of Nano-Nishows domainsexist and growwith the field.

The patterns alsoshow the domainsare much larger than the grains atH=0.

DOMAIN WALLSARE ALSO A FUNCTION OF

SIZE


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Computational Standards


Walls SeparatingMagnetic DomainsChange as the Domain

Dimensions ChangeThis Changes the WayDomains Grow inResponse to a MagneticField Application


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In a Ferromagnet,domains are a wayto reduce themagnetostaticenergy of thematerial.

Sooner or later, itcosts too muchenergy to Createa wall.

Particles less thanthis minimum sizeare a singlemagnetic domain.

CHANGE IN DOMAIN DYNAMICS

IN NANOCOMPOSITE SYSTEMS

EXAMPLE:


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Exchange Bias Hysteresis Loop s

+HH

-M

+M

Coercivity

Saturation M Semanent M R

Coercivity

Remanent M R

0


M i M i l G


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Domain nucleation is controlled by Magnetostatic Energyminimization during the application of a negative field.

But, Exchange Energy minimization controls the

In Antiferromagnetic/Ferromagnetic (AF/FM) bilayers:

When layers become nanometer-thick, interfaceinteractions become predominant.



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Remagnetization with H Oriented Perpendicular to Exchange Bias In Epitaxial NiO/NiFe AF/FM Bilayer

Now, no wallMotion is observedwhile MOIF contrastappears & disappearsas H is reduced andReversed.

This indicatesM reversal occurs byspin rotation and notby domain nucleation& growth.

[V. Nikitenko, V. Gornakov, L. Dedukh, Yu Kabanov, A. Khapikov, A. Shapiro,R. Shull, A. Chaiken, E. Michel, Phys. Rev. B 57, No. 14, R8111 (1998).]

M / M

s

Thick End

II

III

IV

VIII

1 st Reversal of M(FROM the Ground State)

2nd Reversal of M

Asymmetry in Domain Nucleation in AF/FM Bilayer

(Ni 81 Fe 19 )

(Mn 50 Fe 50 )

3 5 n m

3 0n m



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1 s

t R e

v e r s a

l o

f M

( F R O M

t h e

G r o u n

d S t a t e )

2 n

d R e v e r s a

l o

f M

( B A C K t o t h e

G r o u n

d S t a t e )

Asymmetry in Domain Dynamics in AF/FM Bilayer

Note the VerticalDefect does NOTaffect the domainmotion during the1 st Reversal, butDOES affect theMotion during the2 nd reversal.

[V. Nikitenko, V. Gornakov, A. Shapiro, R. Shull, K. Liu, S. Zhou, C. Chien,Phys. Rev. Lett. 84, 765 (2000).]

H


Domain Dynamics in FeMn/NiFe AF/FM Bilayer (AC Demagnetized at T>T Neel of AF)

Field Cooled (Exchange Biased) AC-Demagnetized



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(I) (II)

(III)(IV)

(I) (II)

(III)(IV)

Residual Image at top and bottomof original domain structure showoriginal AF domains have not moved

While the FM domains changed.

Domain Motion in AC Demagnetized FeMn/NiFe AF/FM Bilayer

[Chien, Gornakov, Nikitenko, Shapiro, Shull, IEEE Trans. Magnetics 38, No. 5, 2736 (20002).]


Domain motion in

AC-Demagnetized AF/FMBilayer can be explainedBy the formation of anExchange spring parallelTo the AF/FM interface,


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CHANGE IN DOMAIN DYNAMICS

IN NANOCOMPOSITE SYSTEMS

EXAMPLE:

- IN AN IMPROVED PERMANENT MAGNET


Composites of nm thickHard & Soft Ferromagnetsare thought to be the wayto the future HardestFerromagnets.

This is because themagnetic Exchangeinteraction at the interfacecauses the Magnetizationto change by the moredifficult process of the



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g p

The make the HardestFerromagnetic materials(i.e., for the strongestpermanent magnets),advantage is taken of theInterface effects.

The best Nd-Fe-B magnetsare intentionally madeoff-stoichiometry in order to make them multiplephase so there are manyinterfaces.

The Hard phase is alsomade nanometer in sizein order to increase theinterface area.


Origin of High Coercivity in Hard/SoftFerromagnetic Bilayer

Bottom Layer of the SoftFerromagnet (Fe) is pinnedby the top layer of theHard Ferromagnet (SmCo).

Each successive layer inthe Soft FM is bound to thelayer below it causingi i b



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Since No MOIF imagecontrast was observedon the top of the soft FM(Fe) in a SmCo/Fe Bilayer During field reversal, a

Trick was employed. Asmall hole was drilledthrough the bilayer, andthe magnetization in thearea was determined bythe magnetic poles of opposite sign whichformed on the opposite

inside edges of the hole.

Domain ImagingOf a SmCo/FeHard/Soft FM

Bilayer

Magnetic Materials GroupProof of Rotation Reversal Process in SmCo/Fe Bilayer

Application of areversed field is shownby the rotation of theMOIF image contrastat the hole edges to beaccompanied by arotation of M, when His aligned just slightlyoff the easy axis of Magnetization of the



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New Feature in the Reversal Process in SmCo/Fe Bilayer

When H is alignedalong the easy axis

of magnetization of the Fe, reversal of Hdoes not result in arotation in thecontrast at the holeedge, but it isaccompanied by anoverall reduction in

image contrast.

This effect is causedby a distribution indirections of the easyaxes of magnetizationin the small grains of the material: some

rotating clockwise& some rotating CCWin response to H.


ImprovedPermanentMagnet ByMultilayersof nm-thickSmCo & Fe

Best Nd-Fe-BPermanent

Magnet



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Summary

Increased surface/volume ratio of nanomagnets makesdomain motion more susceptible to interactioneffects with neighboring magnetic materials.

The usefulness of a material for a particular applicationdepends on how easily or hard it is to change itsmagnetization.

AF/FM Exchange Biased films possess a novel asymmetryin their reversal behavior

Nanocomposites provide a means for designing better

Hard ferromagnets (i.e., permanent magnets)

Observation of Domain patterns is critical to understandinghow a material changes its magnetization.


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Robert D. Shull (Group Leader:

Magnetic Materials Group)

National Instituteof Standards and Technology

Gaithersburg, Maryland

USA

MAGNETIC

REFRIGERATION

NANOMAGNETISM (III)

V. President: The Minerals, Metals, &Materials Society (TMS) of AIME

Member: OSTP Nanoscale Science, Engineering,and Technology Subcommittee (NSET)


Dependence on Magnetic StateIn ParamagnetsIn FerromagnetsIn Magnetic Nanocomposites

DefinitionBasic dT and dS Equations

Measurement MethodsDirect MethodIndirect Method

Magnetocaloric Effect (dT)

(III).

dT and dS Data

SummaryVI).

(I).

(II).

(IV).

(V).

OUTLINE

Garnet Nanocomposites11%Fe+Silica Gel Nanocomposite


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MAGNETOCALORIC EFFECT

SYSTEM = SPIN + LATTICE

at H = 0 T = T 0

at H = H Appl T = T 1

spins lattice

Total entropy change of the (Spin +Lattice) system upon application of amagnetic field, H Appl , (reversibly) isZERO.

Decrease in spin entropy causes anincrease in lattice entropy, C HdT/T.

Magnetocaloric effect = dT = (T 1-T0).


I

II

III

IV

T2

T3

T1

T4

T e m p e r a t u r e

Entropy

H = 0

a

b

c

d H = H o F

i e l d

Steps , : AdiabaticI III

i

MagnetWindings

Heat Exchanger: A

Heat Exchanger: B

Tube filled withheat exchange fluid

NanocompositeRefrigerant

Magnetic

Cyclic motion of therefrigerant in and out of the field

(Heat)

(Heat)

T1

T3

Shutter

MAGNETIC REFRIGERATION

Refrigeration Cycle

Refrigerator


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WHY

MAGNETIC

REFRIGERATORS

?????

Large Entropy Change in Ordering

Based on a REVERSIBLE Process

Refrigerant and Heat Transfer Media

(40-200 times that of a gas)

(Carnot efficiencies conceivable)

are DIFFERENT

(No chlorofluorocarbons)

No Compressor & Few Moving Parts

(Low vibration, High durability)



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Magnetic Materials GroupMagnetic Materials Group

Domain 1 Domain 2

Paramagnetic Material:

Ferromagnetic Material:

Superparamagnetic Material:

Assembly of elemental moments,each of magnitude actingindependently

Assembly of magnetic domains(each comprised of many elementalmoments of magnitude alignedin the field direction in concert

with the other domains

Assembly of magnetic clusters(each comprised of manyferromagnetically-aligned elementamoments of magnitude actingindependently.


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VERY SMALL PARTICLE LIMIT

Paramagnetic Material:

Assembly of elemental moments,each of magnitude , actingindependently



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0 20 40 60 80 100

1.0

0.8

0.6

0.4

0.2

0

spin 7/2 atoms10 atom clusters30 atom clusters100 atom clusters

Temperature (K)

S ( J / K - m o l

)

Entropy Enhancement in Nanocomposites

H=800 kA/m

Grouping the atoms together in clusters giveslarger entropy changes than single atoms inthe H,T regiemes accessible by refrigerators.


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"Universal" Entropy Change Curve

0 2 4 6 8 10

H/kT

0.28

0.24

0.20

0.16

0.12

0.080.04

0

TSMoH

Maximum at

S = .271 M oH/T

H/k BT = 3.5

[R. McMichael, R. Shull, L. Swartzendruber, L. Bennett,R.E. Watson, J. Mag. & Magn. Mat. 111, 29 (1992)]


SampleThermocouple

Vacuum

Chamber

Superconducting

MagnetWindings

MagnetBore

InsulatingSupport

He gas

Sample motion

CALORIMETER

Measurement of theMagnetocaloric Effect (MCE)


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0 20 40 60 80 100 120 140 160

Time (sec)

T e m p e r a

t u r e

( K )

32

30

28

26

24

22

20

50,000 GaussApplied

50,000 Gauss

Removed

GGG

Magnetocaloric Effectin

Gadolinium Gallium Garnet

(This is the material with the largestmagnetocaloric effect at low temperaturesand is useful as a magnetic refrigerant up to15 K)



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Magnetization Isotherms(for Entropy Calculations)

Gd Ga O5 12

0 200 400 600

Applied Field (kA/m)

28

24

20

16

12

8

4

0

GGG 6 K20 K

100 K

Ma

zao

(Am2k


0 20 40 60 80 100

Temperature (K)

1.0

0.8

0.6

0.4

0.2

0

GGG

(JKk

Experiment

Calculated

(H=800 kA/m)

Magnetocaloric Effect

Gd Ga O3 5 12

Calculation of the MCE values from magnetizationdata is a good way to determine MCE (& it is muchfaster).


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Prepared nanocomposites by addingaqueous solutions of the mixed metalnitrates to excess tartaric acid (for

Air dried (200-325 C)

Samples characterizedX-ray diffractionMossbauer EffectFerromagnetic ResonanceMagnetic susceptibility

Formed Garnet Structure(1) Air (950 C)

complexing)

Gd Ga Fe O3 5-x x 12

Bulk

Nanocomposites


Schematic Picture of

Magnetic Nanocomposites

Gd Fe Ga

GarnetGd Ga Fe O5-x x 123-y

Fe spins create magnetic clusters (dashed areas)


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Gd Ga O5 12

0 200 400 600


28

24

20

16

12

8

4

0

GGG 6 K20 K

100 K

Ma

zao

(Am2k

Straight line isotherms show paramagnetic behavior & therefore no interaction between magnetic spins


0 200 400 600

60

50

40

30

20

10

0


Ma

zao

(Am2k 6 K20 K

50 K


Gd Ga Fe O3.25 1.75 12

Curved isotherms show superparamagnetic behavior & therefore the presence of magnetic clusters


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Nanocomposites show one way to move thetechnology up in temperature at reduced fields

Magnetocaloric Effect EntropyChanges for

Gd 3 Ga 5-X Fe XO 12 Nanocomposites

( H=800 kA/m)

Minimum S Needed

0 20 40 60 80 100

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Temperature (K)

x = 2.5

x = 1.75

x = 0

S ( J / K - k g

)

[R.D. McMichael, J.J. Ritter, and R.D. Shull,J. Appl. Phys. 73(10), 6946 (1993)]


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80

Temperature (K)

-

S

J / K

- K

Gd3Ga2.5Fe2.5O12Ho1Gd2Ga2.5Fe2.5O12Gd2.25Dy0.75Ga2.5Fe2.5O12Dy3Ga2.5Fe2.5O12Gd3Ga5O12

- S vs T for GGI, HGGI, GDGI, DGI GarnetNanocomposites and GGG

Heat Treated for 15 h at 1200 oC ( H=1T)

[V. Provenzano, J. Li, T. King, E. Canavan, P. Shirron,M. DiPirro, and R.D. Shull, JMMM 2 66 , 185 (2003)]


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LARGE PARTICLE LIMIT

Domain 1 Domain 2

Ferromagnetic Material:

Assembly of magnetic domains(each comprised of many elemental

in the field direction in concert

with the other domains

) alignedoments of magnitude



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0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300

Temperature (K)

-

S

( J / K g -

Heat Treated 3 THeat Treated 2 THeat Treated 1 TAs Cast 3 TAs Cast 2 TAs Cast 1 T

S vs T for Gd 5Ge 2Si2 : As-Cast andHeat Treated (1100 oC, 1 h + 1400 oC, .2 h)

[V. Provenzano, A.J. Shapiro, and R.D. Shull,Nature 4 29 , 853 (2004)]


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0 200 400 600

0.5

0.4

0.3

0.2

0.1

0

single atoms

10 atom clusters

100 atom clusters

Temperature (K)

S(JKmo)

for Interacting Clustersof Spin 7/2 Atoms

( H=1T)

Entropy Change

Interacting clusters of atoms possess smaller MCEvalues at the critical temperature, but larger MCEvalues at all higher temperatures.

[R.D. Shull, IEEE Trans. on Magnetics 29, 2614 (1993)]


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WHAT ABOUT MCE CALCULATIONFOR A FERROMAGNET???

M is NOT a Single Valued Function!

M is No Longer a Good Order Parameter !!!

Hysteresis Loss NEEDS to be subtractedfrom the MCE!!!

[V. Provenzano, A.J. Shapiro, and R.D. Shull,Nature 4 29 , 853 (2004); V. Provenzano, B. Baumgold,

R.D. Shull, A.J. Shapiro, K. Koyama, K. Watanabe,N.K. Singh, K.G. Suresh, A.K. Nigam, and S.K. Malik,

J. Appl. Phys. 9 9 , 08K906 (2006)]


This is a good way to vary the magnetic field froma permanent magnet in a compact volume.Perhaps for a magnetic refrigerator???


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nanocomposites:

- show predicted systematics

Nd Fe Al B1-X X )13.75 80.25 6SP:

- are potentially usable near 300 K

Conclusions

particle size and distributiondependent onagnetocaloric effects are

(e.g., in magnetic nanocomposites)

Enhanced Magnetocaloric Effects can be

small particle materialsbtained in

be usable now as a magnetic refrigerant

at low temperatures

nanocomposites mayd Ga Fe O3 5-x x 12

Their maximum enhancements are atLOW fields and/or HIGH temperatures

For ferromagnets, hysteresis loops shouldAlways be shown if magnetic data is usedto calculate the MCE

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