physics functional material concluding session.pdf

physics functional material concluding session.

1. Physics of Functional Materials and Devices
Prof. Amreesh Chandra
Department of Physics, IIT KHARAGPUR
Module 05: Origin of Magnetism
Lecture 26 : Introduction to magnetism & magnetic properties of solidsNPTEL

2. ❑Briefh ofmagnetism
❑Introduction a
ndoriginofmagnetism
❑Diamagnetism,P
aramagnetism,andFerromagnetism.
❑Pauli e
andHund’srule.
❑Diamagnetisma
nd Larmorprecission.NPTEL

3. Brief history of magnetism
The ancient Greeks were the first known to have used a mineral, which they called a magnet because of its ability
to attract other pieces of the same material and iron.
Figure. Loadstone, anaturalmagnet,
attractingironnails.
⮚Magnetism wasf irstdiscoveredin theancient world when people noticed that
loadstones,naturallymagnetized pieces ofthe mineralcould attractiron.
⮚Thewor
theGreektermmagnetis lithos.NPTEL

4. Fig.AnillustrationfromGilbert's1600De
Magneteshowingoneoftheearliestmethods
of making a magnet.
⮚In1600,W publishedhisDe
Magnete, Magneticisque Corporibus,et
deMagno MagneteTellure(OntheMagnet andMagneticBodies, and ontheGreat
MagnettheEarth).Inthisworkhedescribesmanyofhisexperimentswithhis
model earth calledtheterrela.
⮚Hec
that theEarthwasitself magneticandthat thiswasthereason
compasses pointednorth.
⮚Previously,s
omebelievedthat itwasthepolestarPolarisor a largemagnetic island
onthenorthpolethatattractedthecompass.
What is magnetism?
Magnetismisthe force of repulsion andattractionbetweentwodifferent substances caused by
the motionof charged particles.NPTEL

5. Magneticfield
⮚Magneticf
spacearounda magnet whereits magneticeffect
isfelt.
⮚The ma
represented by magnetic
linesof force emanating fromthenorthpole
and passing throughthesurroundingmedium
andthenre-enteringthesouthpole.
Fig. Magnetic field lines
Fig. Illustration of magnetic forceNPTEL

6. Different types of magnetic materials
Solids are classified according to their magnetic properties:
1.Diamagnet.
2.Paramagnet.
3.Ferromagnet, Antiferromagnet, Ferrimagnet.
Magnetic moment, M = χ H
χ = Magnetic susceptibility
Diamagnetic material Negative value of χ
Paramagnetic material Positive value of χ
In case of group 3 materials, not only the existence of permanent magnetic dipolesisenough but
thereexistsarelativelystronginteractiveforces amongthemagnetic dipoles, namedas
exchangeinteraction.
The value of the magnetic moment of a body i
strength of the magnetism that is present.
Atoms in the various transition elements have
energy levels in which the spins of the electron
giving rise to a net magnetic moment.NPTEi

7. NPTEL

8. ❑Diamagnetism is a property of all materials and always makes a weak contribution to the
m
aterial's response to a magnetic field.
❑Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an
induc
ed magnetic field in them in the opposite direction, causing a repulsive force.
❑Examples of Diamagnetic materials: w
ater, wood, gold, mercury, etc.
What is diamagnetism?NPTEL

9. What is paramagnetism?
❑Paramagnetism is a kindofma
gnetismwhere severalobjects areattractedthroughanexternally
applied magnetic field.
❑Paramagnetic ma
terialshaveumpairedelectrons.They losemagnetismintheabsenceofmagnetic
field.
❑The gr
numberofunpaired electrons,the greater the magneticmomentofthesubstance
andhence greater theparamagnetism.
❑Examples: O
2, CuO,Al,Mn,etc.NPTEL

10. ❑Ferromagnetism is a physical phenomenon (long- range ordering), in which certain materials like iron
strongly attract each other.
❑One of the vital requirements of ferromagnetic material is that ions and atoms should possess permanent
ma
gnetic moments.
❑Examples: Iron, cobalt, nickel, and their alloys.
Wh
at is ferromagnetism?NPTEL

11. The magnetic induction B
B = H + 4π M = μ’ H
Relative permeability (µ
r) of a medium is defined as the ratio between the absolute permeability (µ) to the permeability of
the free space (µ
0).
Relative permeability (µ
r)
It is defined as the ratio of the intensity of magnetization M to the magnetic field strength H.
Magnetic susceptibility (????????????)
It is defined as the ratio of magnetic induction B to the applied field H
The permeability of vacuum of free space is µ
0=4 x 10
-7
H/m.
Different important parameters of magnetismNPTEi

12. Origin of Magnetism
Magnetism arises dueto thepresence of an effectivemagnetic moment in theatom.
The magnetic momentarises fromthreeprincipal sources:
1.The spin withwhich electronsare endowed.
2.Theirorbital angularmomentumaboutthenucleus.
3.Thechangein theorbitalmomentinduced by an applied magneticfield.
Fig. Illustration of the spin of electron, producing magnetic field
Fig. Illustration of the orbital motion of
electron, producing magnetic fieldNPTEL

13. Origin of permanent magnetic dipoles
✔Accordingto th ehypothesis of Ampere,magneticdipoles havetheir origin intheflow of
electriccurrents.
✔As
loopcurrentflowing ina plane produces a magneticfield thatatlarge
distancesmaybe described asresultingfrom amagneticdipole.
Fig. Ampere’s hypothesis
❑The minus sign indicates that the dipole moment points in the direction opposite to the
v
ector representing the angular momentum.
❑The relation is valid for any electron orbit.
Magnetic dipole
moment
Angular frequencyNPTEL

14. Angular momentum of the orbit
❑Thep components ofthe angular momentalonganyspecified directionare
determined bythemagneticquantum numberm
l
❑m
l=l,l-1, 0, -l(l-1), -l, for aparticularl.
❑Hence,t
possiblemagneticmomentcomponents alongthepossibledirection of an applied
magneticfieldare:
-l.eħ/2m,….0, …..l. eħ/2m
❑Theel
itselfhasan angularmomentumknown asthespin.
Fig.Componentsof possibleorbital angular momentum
Fig. Two allowed orientation of the s = 1/2
❑Theangul theorbitisdetermined bythequantum numberl.l= 0 to n-1,insteps of1.
❑Them
the totalangular momentum=ħ[l(l+1)]
1/2
Major points:NPTEL

15. Spectroscopic splitting factor
In this case, according to the Lande formula:NPTEL

16. Pauli exclusion principle and Hund’s rule
⮚Theal
discussedphenomenonmustbe combinedwith PauliexclusionprincipleandHund’sruletopredictthe
magneticdipolemomentassociatedwith theelectrons. ⮚AccordingtoP auliexclusion principle,oneelectron can occupyastatedefined bytheset
ofquantum numbers n,l,m
lands.
⮚Magneticmo mentresult fromincompletelyfilled shells.
Hund’srules:
⮚The magnetic momentaddstogivethemaximum possibleSconsistentwith the Pauli
principle.
⮚Theorbitalmomentacombinetogivethe maximumvalue of L.
⮚For anincompletely filled shell,we have
J = L –S,for ashellless than halfoccupied,
J = L + S for ashellmore than halfoccupied.
Electron configuration of CrNPTEL

17. Example of Cr
⮚Thereist presence of4unpaired electrons.Only3dshellis unfilled.
dshell,l = 2.Hencenumberof possiblem
lvalues= 2l + 1 = 5
Totalno.of possible electrons 10.
As the atom is less than half occupied. Hence, J = L – S = 0
Nuclear magnetic momentNPTEL

18. ❑The g theoryrelatedtothe originofmagnetism has beendiscussed.
❑Theide
abouttheorbitalangularmomentumandspinangularmomentumhas been
highlighted.
❑Pauli’se
principle andHund’sruleare employedtoevaluate the angularmomentum.NPTEL

19. ❑Physicso byHasse Fredriksson &Ulla Akerlind.
❑SolidS
PhysicsbyA J Dekker.
❑Introduction toS
olidStatePhysicsbyCharles Kittel.NPTEL

20. Thank you…NPTEL

21. Physics of Functional Materials & Devices
Prof. Amreesh Chandra
Department of Physics, IIT KHARAGPUR
Module 05: Magnetism and magnetic properties of solids
Lecture 27 : From magnetic to multiferroic materialsNPTEL

22. ❑Brief h ofmutiferroics
❑Couplingb
theorderparameters
❑Synthesis and c
haracterization
❑ApplicationsNPTEL

23. 1.Thebas ideaofpossessingsimultaneousferromagnetism and ferroelectricityina crystal was discovered by P.Curie
in19
th
century.
2.Valasekin1920 s
howed ferroelectricityinRochellesalt.
3.Firstmu
be discovered was weakly ferromagnetic ‘nickeliodineboracite (Ni
3B
7O
13I).
4.Search for ot
hermultiferroicsbeganinRussiaas early asthe1950s.
5.Smolenskii’sgr
oupinLeningard pioneeredthestudyofbismuth ferrite(BiFeO
3)in1958-59.
6.Thet
understandingof magnetoelectricityin termsof free energy was postulated byDzyaloshinskiiin1959.
7.The ter
wasfirst usedby N-Schmid in 1994.NPTEL

24. Introduction
Multiferroicsarethose which possesstwoormorethantwo ferroicproperties, which areferroelectricity,ferromagnetism,
ferroelasticityandferrotorroidal,inasinglephase. Examples of suchmaterials:BiFeO
3,BiMnO
3,etc.NPTEL

25. Some important multiferroic materials and composites
✔BiFeO
3,BiMnO
3,YMnO
3,HoMnO
3,YCrO
3, RMn
2O
5,(R=Y,Tb,Dy,..),LuFe
2O
4
✔BFO-CT, BFO-BT, BFO-CFO…etc.NPTEL

26. Materials PreparationNPTEL

27. 7
Characterization
Stoichiometric amount of all
ingredients (in nitrate forms)
Mixing in the
distilled water. As
well as small
amount of HNO
3
added
Stirring at
constant
temperature
100°C for 4-6 h
Calcinations, Grinding
and Re-calcination,
Sintering
Sample Bi
1-xSr
xFeO
3(x=0.0, 0.05,
0.10, 0.15, 0.20, 0.25, 0.60)
Material synthesis:NPTEL

28. Stiochiometric amount of
all ingredients (in nitrate
forms)
Mixing in the
distilled water.
As well as small
amount of
HNO
3added
Stirring at
constant
temperature
100°C for 4-6 h
Calcinations,
Grinding and Re-
calcination,
Sintering
Characterization
Sample Bi
1-xCa
xFeO
3(x=0.0,
0.05, 0.10, 0.15, 0.20, 0.25, 0.50)
8
Material synthesis:NPTEL

29. Majorinferencesdrawn:
✔Single phas formationofBi
1-xSr
xFeO
3(0.00<x<0.50).
✔All Bi
1-xSr
xFeO
3(0.00<x<0.50) samples were perovskiteinnature.
✔XRDpat
itsnatureinsamples, which suggestssome
compositional dependent phasetransition.
Figure showing XRD pattern of Bi
1-xSr
xFeO
3
(0.0≤x≤0.50)
Structural analysis Bi
1-xSr
xFeO
3 (0.0<x<0.60):NPTEL

30. Major inferences drawn:
✔SEMr themorphological study ofthe
Bi
1-xSr
xFeO
3(0.0≤x≤0.30).
✔The par
seemstobe morphologically
uniform and sphericalinshape.
a b c
fed
Fig.showingscanning electron micrograph(SEM)ofBi
1-xSr
xFeO
3(a)x=0.00
(b)x=0.05(c)x=0.10(d)x=0.15(e)x=0.20(f)x=0.25
Morphological analysis:NPTEL

31. ❖Evolution of X- ray diffraction (XRD) pattern of Bi
0.95Sr
0.05FeO
3samples prepared at different temperatures.
Major inferences drawn:
✔Single phase formation of Bi
1-xSr
xFeO
3(x=0.05)
✔The particle size is found to be vary from 70-85 nm.
Effect of calcination temperature –
Using Bi
0.95Sr
0.05FeO
3as an exampleNPTEL

32. F T=700°C
B T=500°C
C T=550°C
D T=600°C
E T=650°C
A BFO
Fig. SEM picture of (a)BFO and (b)-(f)BSFO (x=0.05)
Scanning electron microscopy (SEM):
Major inferences drawn:
✔Atf themorphology looks sphericalinshape
which gradually transformedinto pillar likestructure.
✔Afterr
particlesizethevolume
energy predominates overthesurfaceenergyterm
resulting pillar likestructure.
✔Thecr
sizeoftheparticleisfoundtobein
between 75to85nm.NPTEL

33. Majorfindings:
✔
Throughsol gel we canalso tailortheparticle
morphology.
✔Homogenous n
ucleationresults in pillar like
structure.
✔Volumee
termispredominating overthe
surface energytermafter reachingcriticalradii.
❖The c to
pillarstructureisbased onminimizationofthe
Gibb'sfreeenergy.
Crystal growth mechanism:NPTEL

34. Orthorhombic unit unit cell of Bi
0.5Ca
0.5FeO
3
14
Similar studies on Bi
1-xCa
xFeO
3
Fig. showing variation in the unit cell volume and lattice parameter as
function of Ca
2+
concentration in Bi
1-xCa
xFeO
3. (b) Hexagonal unit cell
of Bi
0.85Ca
0.15FeO
3.(c) & (d) Shows the distortion in the magnetic ion
(Fe
3+
) in pure BFO and doped Bi
0.85Ca
0.15FeO
3ceramic.NPTEL

35. Fig (a-d) Ferroelectric hysteresis loops for Bi
1-xCa
xFeO
3 (a)
x=0, (b) x=0.10, (c) x=0.15 and (d) x=0.20) before and after
magnetic poling in 2T filed in 2hours.
NatureofHysteresisLoops:P-EandM-H:
P-E loops
Fig SQUID- VSM magnetic hysteresis loops for Bi
1-xCa
xFeO
3
(x=0.0 0.10, 0.15, and 0.20).
M-H loops
1.Inonl electricallypolingsamples,nosaturation polarizationwas observedwhileremnant
polarization(P
r)values werefoundtodecreasewith increasingCa
2+
concentration.
2.Theo
P
rvalue for x=0.0was0.26µC/cm
2
whilefor x=0.20,itwasnoted as0.04µC/cm
2
.
3.Inca
ofmagneticallypoledsamples, therewas clearchangeinareaundertheP-Eloops.
Major inferences
1.Withi Ca
2+
concentration,the
hysteresisloopsstarts toopenand, for
x>0.10,loop typicalofweakFMbehavioris
observed.
2.This w
FMbehaviorisdueto fractionof
uncompensatedspinsFe
3+
/Fe
2+
and/or
oxygenvacancies,suppressthecantingspin
structure.
15
Major inferencesNPTEL

36. High temperature dielectric studies:
1.Occurrenceofadi electric anomaly nearthemagnetic
phasetransitiontemperature.
1.Theo
of frequencyindependent(T
*
)confirms
theoccurrence ofphasetransition.Butthepeakis
prominent atlowfrequencywhileitisabsent at higher
frequencies.
1.Ast
lowfrequency data hasits origin intheinterfacial
capacitance,thephasetransitionseemstobe
predominantly confinedtothesurfaceofthematerial.
Major inferences:
16
Fig. (a-d) Temperature and frequency dependent variation of real and
imaginary part of dielectric constant in Bi
1-xCa
xFeO
3samples with x= (a) 0.0,
(b) 0.05, (c) 0.10 and (d) 0.15.NPTEL

37. Physical properties variation –
Defining applicationsNPTEL

38. Majorinferences:
✔We got opt ical
luminescenceinthenear IR
region
✔The m

increaseswithcalcination
temperature
✔λ~850 nmisdom
inating at
the final calcination
temperature.
Fig. PLofBi
1-xSr
xFeO
3(a)x=0.05(b)Zoomed picture ofthePLspectrain theUVandIRregion
Aim:
✔O- mightproduce
emission in thenear IR region
similar to ZnO
Corresponding PL spectrum in IR region
PhotoluminescencestudyofBi
0.95Sr
0.05FeO
3samplesNPTEL

39. Major inferences:
✔The loop is not saturated
✔H
c~2.4 kV/cm and remnant polarization (P
r)
is found to be 0.023 μC/cm
2
and saturation
polarization (P
s) is ~ 0.243 μC/cm
2
.
Figure showingroomtemperature
dielectricplot ofBSFOsample
calcined atT=550°C
Aim:
✔Toin whether polarization value has been
improved alongwith themagnetization property
✔Arewe abl
toreduce the E
cvalues and improved P
r
value
Polarization study of Bi
1-xSr
xFeO
3NPTEL

40. 1)Introduction to multiferroics
2)One can tune the properties of the materials by changing the synthesis parameters.
3)Multiferroics have a large number of applications.NPTEL

41. ❑IntroductiontoN anotechnology, CharlesP.Poole,Jr.andFrank J.Owens,
wiley-interscience.
❑PhysicsofFunc
tionalMaterialsby Hasse Fredriksson andUlaAkerlind.
❑IntroductiontoSo
lidStatePhysics byCharlesKittel.
❑PhDT
Dr. PTitupathi
❑PhDT
Dr. S KMandalNPTEL

42. Thank you…NPTEL

43. Physics of Functional Materials & Devices
Prof. Amreesh Chandra
Department of Physics, IIT KHARAGPUR
Module 07: Magnetism and magnetic properties of solids
Lecture 28 : Magnetic materials and their applications NPTEL

44. ❖Magnetism
❖Ferromagnetism, Ferrimagnetism, and Antiferromagnetism
❖Exchange interaction
❖Soft and Hard magnetic materials
❖Application of magnetic materialsNPTEL

45. Magnetism
•Atomsint hevarious transition seriesoftheperiodictablehaveunfilled inner energylevelsin
which thespinsoftheelectrons are unpaired,givingtheatoma net magnetic moment.
Example:
•The iron atom has 26 electrons circulating about the nucleus.
•Eighteen of these electrons are in filled energy levels that constitute the argon atom inner core of the
e
lectron configuration.
•Thedlev n=3orbitcontainsonly6of the possible 10 electronsthat would fill it.
•Itis incomplete totheextentoffourelectrons.
This incompletely filled electron d shell causes the iron atom to
have a strong magnetic moment.NPTEL

46. •Whenc asbulkiron are formedfromatomshaving anet magneticmoment a numberof
differentsituations can occurrelatingtohow the magnetic momentsoftheindividualatomsarealigned
with respect to each other.
Some of the possible arrangements that can occur in two dimensions:
Paramagnetic Ferromagnetic Ferrimagnetic Antiferromagnetic
Possible arrangements of spin:NPTEL

47. ❖In a p themagnetic momentsarerandomly arrangedwithrespecttoeach
other, then thecrystalhasazeronet magnetic moment.
❖The a
ofaDCmagneticfield alignssomeofthemoments,givingthecrystal a small net
moment.
Paramagnetic:
Ferromagnetic:
❖In a f thesemoments
all point inthesame direction,evenwhenno
DCmagneticfieldisapplied,sothe whole
crystalhasa magnetic moment and behaves
like a barmagnetproducing a magneticfield
outsideofit.
Various kind of magnetism:NPTEL

48. Applications:
Paramagnetic Ferromagnetic
FerrimagneticAntiferromagnetic
✔Magnetic Resonance Imaging (MRI)
✔Cryogenics and Refrigeration
✔Memory storage devices
✔Permanent magnets
✔Transformer core
✔Spintronic de
vices
✔Magneticf
ieldsensors
✔Magneticsh
ielding
✔Spin v

✔Useda
a coreofcoilsinmicrowave
frequency systems.
✔Used in e
lectrical circuitsas
ferromagnetic insulators.NPTEL

49. Why the individual atomic magnets align in some materials and not in
others?
✔WhenaD magneticfieldisapplied toa barmagnet, the magneticmomenttends toalignwith the
directionofthe applied field.
✔In a c
each atomhaving amagneticmomenthasamagnetic fieldabout it. Ifthe magneticmomentis
large enough, the resulting largeDCmagnetic fieldcan force a nearest neighborto aligninthesame
direction provided theinteractionenergyislargerthan the thermalvibrationalenergyk
BToftheatomsin
the lattice.
✔The i
atomicmagneticmoments isoftwo types:theso-
calledexchange
interactionandthedipolarinteraction.
✔Thee
purelyquantum mechanicaleffect,andis
generally the strongerofthe twointeractions.
Ferromagnetism:NPTEL

50. Inthecaseofasmallparticlesuchas anelectron that hasamagneticmoment,the applicationofaDC
magnetic fieldforcesitsspinvector toalignsuchthatit canhaveonly two projectionsin thedirectionof
theDCmagnetic field:
Designated as A Designated as B
❖Quantum me chanicsdeals with thissituation by requiringthat the
wavefunctionoftheelectronsbe antisymmetric,thatis,changesign ifthe
twoelectrons are interchanged.
Ferromagnetism:NPTEL

51. The form of the wavefunction that meets this condition is:
The electrostatic energy for this case is:
which involves carrying out a mathematical operation from the calculus called integration. Expanding the
square of the wave functions gives two terms:
✔Thef termisthenormalCoulombinteraction between thetwocharged
particles.
✔Thesecond term,called the exchangeinteraction,represents thedifference
intheCoulombenergy betweentwoelectrons withspinsthatareparalleland
antiparallel.NPTEL

52. The otherinteraction,whichcan occurinalatticeofmagnetic ions, iscalledthe
dipole-dipoleinteraction, andhas theform:NPTEL

53. Magnetization:
✔The m Mofa bulksampleisdefinedasthe total magneticmomentperunit volume.
✔It isthe v
sumofallthe magneticmomentsofthe magnetic atomsinthebulksample dividedby
thevolumeofthe sample.
✔It inc
attheCurietemperatureT
c,thetemperatureatwhich the sample becomes
ferromagnetic, andthe magnetizationcontinuestoincreaseasthetemperatureislowered further
belowT.
The magnetization depends on temperature
where M(0) is the magnetization at zero degrees Kelvin and c is a constant.
❑Thesusc
χofasampleisdefinedasthe ratioofthe magnetizationat
agiven temperature to the applied fieldH.NPTEL

54. Hard magnetic materials:
Soft magnetic materials:
❖Thes
andsizeofthehysteresis loopof a ferromagneticmaterial determine the
suitabilityofthe material inatechnical application.
Materialshavingsmallcoercive forcesarecalledsoftmagneticmaterials.
Theycan be easilymagnetizedanddemagnetized.
Materialshavinglargecoercive forcesarecalled hardmagneticmaterials.
Theycan not be easilymagnetizedanddemagnetized.
Soft and hard magnetic materials:NPTEL

55. Properties of soft magnetic materials:
✔Greater p
✔Greater r

✔Smallerh
looparea
✔Smaller en
during cyclic
magnetization
Properties of hard magnetic materials:
✔Smaller p
✔Smaller r

✔Largerh
looparea
✔Greateren
ergylossduring
cyclicmagnetization
Example:Soft iron
Example:Steel
Properties of soft & hard magnetic materials :NPTEL

56. Comparison between soft iron and steel:
✔The retentivity of soft iron is greater than that of steel.
✔The coercivity of soft iron is smaller than that of steel.
✔Area of B-
H loop and hence hysteresis loss for soft iron is smaller than that of
steel.
Hysteresis curve of steel Hysteresis curve of soft ironNPTEL

57. Applications of soft and hard magnetic materials:
Permanent magnets:
✔The m
construction of apermanentmagnetshouldhavehigh
retentivity and highcoercivity, sothatthemagnetbecomes strongand it becomes ableto
resist loss ofmagnetismdue to roughhandlingor temperaturechanges.
✔Hysteresis los
sisunimportanthere becauseapermanentmagnetis neversubjectedto
cyclesofmagnetization.
✔All t
steelismore suitablethansoftironfor the
construction ofpermanent magnet.NPTEL

58. Electromagnet:
✔The m
usedfor theconstructionofanelectromagnet shouldhave high magnetization at
low magnetizing field, high initial permeability, and low hysteresisloss.
✔Soft i
possessesall thesepropertiesand henceisbestsuitedfortheconstruction of
electromagnet.
Applications of soft and hard magnetic materials:NPTEL

59. Cores of transformer, armatures of dynamos and motors:
✔Here t magnetic material isusedtoincreasetheeffectofinduction. Sothematerial
selected should have high permeabilityatlowmagnetizingfield.
✔As the m
used inthesecases is subjectedtocyclesofchanges the
hysteresislossandhenceB-Hloopareamust be small.
✔Softiro
is foundtobebetter than steelinthese applications.
Applications of soft and hard magnetic materials:NPTEL

60. •Various ki of magnetizationsuchas Paramagnetism,Ferromagnetism, Ferrimagnetism and
Antiferromagnetismhasbeendiscussed.
•The a
ofsoftandhardmagnetic materialshasbeenhighlighted.NPTEL

61. ❑IntroductiontoN anotechnology, CharlesP.Poole,Jr.andFrank J.Owens,
Wiley-interscience.
❑PhysicsofFunc
tionalMaterialsby Hasse Fredriksson andUlaAkerlind.
❑IntroductiontoSo
lidStatePhysics byCharlesKittel.NPTEL

62. Thank you…NPTEL

63. Physics of Functional Materials & Devices
Prof. Amreesh Chandra
Department of Physics, IIT KHARAGPUR
Module 07: Magnetism and magnetic properties of solids
Lecture 29 : Magnetism at nanoscaleNPTEL

64. ❖Magnetism in nanostructured materials
❖Concept of elongated grain
❖Stone-Wohlfarth (SW) model
❖Superparamegntism
❖ApplicationsNPTEL

65. Effect of nanostructuring on magnetic properties:
•The strongest known permanent magnets are made of neodymium, iron, and boron.
Nd
2Fe
14B
Remnant magnetizations 1.3 T
Coercive fields 1.2 T
Coercive field decreases significantly below 40 nm and the remnant magnetization increases.
Size
Remnant magnetization
Coercive field
Can be tuned by changing the size of
the particles.NPTEL

66. Synthesis of Nd
2Fe
14B
Soft chemistry method of thesynthesisofNd
2Fe
14B
Step 1:
❑Them involves two stepsof thesynthesisofNd
2Fe
14B.
❑Step1:Th
stepinvolvesthepreparationofNd-Fe-Boxide powder.
❑Step2:Th
isisthefinalstepwhichinvolvesthereductive annealingprocess of theas-synthesizedNd–Fe–B
oxide powder.
Nd(acac)
3.xH
2O (Neodymium acetylacetonate) + Fe(acac)
3(Iron acetylacetonate)(dissolved in Oleylamine)
Inject (C
2H
5)
3.NBH
3into upper solution
Precipitated by hexane/ethanol and separated by centrifugation
Nd-Fe-B oxide powders
Degassed at 120 °C for 1 hour under vacuum condition
300 °C for 1 hour under Ar gas
Ref: New J. Chem., 2016,40, 10181- 10186NPTEL

67. Step 2: Reductive annealing process
Production of α-Fe, Nd
2O
3, Fe
3B, NdBO
3.
Powders grounded and mixed with CaH
2in a glove box
Nd-Fe-B oxide powders
Reduction treatment: 800 ° C for 2 hours
under Ar + 5% H
2gas condition
Reduction treatment: 900 ° C for 2 hours
under highly pure Ar gas
Nd
2Fe
14B powder + CaO + Ca
Wash and dry
Nd
2Fe
14B nanoparticles
Ref: New J. Chem., 2016,40, 10181- 10186NPTEL

68. Figure:Dependence oftheremnant
magnetization ontheparticlesizedofthe
grains thatform thestructureofNd-Fe-B.
Figure:Dependence ofthecoercivefieldon
thegranularparticle sizedofaNd-Fe-B
permanent magnet.
Effect of nanostructuring on magnetic properties:
REMANANT MAGNETIZATION (Tesla)
d, NANOMETERSNPTEL

69. Superparamagnetism:
❖Superparamagnetism is a ty peofmagnetismthatoccursinsmall ferromagneticor
ferrimagnetic nanoparticles.
❖Thisi
sizesaround afew
nanometerstoacoupleoftenthofnanometers, dependingon
thematerial.
❖Thesena
particles.
❖In as
approximation,the totalmagnetic momentofthenanoparticle
canberegardedasone giant magnetic moment, composed ofallthe
individual magnetic momentsoftheatomswhichformthenanoparticle.
Effect of nanostructuring on magnetic properties:NPTEL

70. What are the implications of such superparamagnetic states?
Effect of nanostructuring on magnetic properties:
❖Withoute field, thenet moment iszero.
❖Ass
anexternal magneticfieldisapplied, thenanoparticles react similar
toa
paramagnetwith theone exceptionthat theirmagnetic susceptibilityis muchlarger.
Hence the “paramagnetism” in
the name
Hence the “super” in
the nameNPTEL

71. Effect of nanostructuring on magnetic properties:
M-H curve of superparamagnetic material:
Superparamagnetism:
•High saturation magnetization
•No remanence
•Zero coercivityNPTEL

72. Comparison between paramagnet, ferromagnet, and superparamagnet:
Magnetic Material Zero Magnetic Field Magnetic Field Applied
Paramagnet
Domains moments align
randomly – nonetmoment
Net moment appears;the
applied magnetic field helps
thedomains “find”each
other tobecomecoupled.
Ferromagnet
Domain moments coupled
(belowCurietemperature)to
producestrong,permanent
moment
Even higher magnetic
moment.
Superparamagnet
Domainmomentsthatwould
coupleasin Ferromagnetdo
notdoso becauseofsmallsize
– boundaryeffect.
Domains “find”each other
andnowitgeneratesa
moment comparable to
Ferromagnet.NPTEL

73. Superparamagnetism:
Ref: http://dx.doi.org/10.1016/B978-0- 12-415769-0.00011-
XNPTEL

74. Applications:
❖Targeted drug delivery
❖Magnetic data storage: Hard disks, tape in video and audio recorders
❖Radionuclide and gene delivery
❖Magnetic resonance imaging
❖Biosensing
Sup
erparamagnetismisoneoftheusefulsize-dependentpropertiesbroughtaboutwith the
introductionofnanomagneticparticlesproviding promisingapplicationssuchas:NPTEL

75. Concept of elongated grains:NPTEL

76. Dynamics of nanomagnets:
✔When len ofmagnetic nanoparticlesbecome small,
the magneticvectors becomealignedintheorderedpattern
ofasingledomain inthepresenceofaDCmagnetic field,
eliminatingthecomplicationofdomainwallsandregionshaving
the magnetizationindifferentdirections.
✔TheS
Wohlfarth (SW)modelhasbeenusedtoaccount
forthedynamical behaviorofsmall nanosizedelongated
magneticgrains.
✔Elongatedg
rainsaregenerally the typeused inmagnetic
storage devices.
✔TheSWm
odel postulates thatintheabsenceofaDC
magnetic field, ellipsoidal magnetic particlescanhave only two
stableorientationsfortheir magnetization, eitherupordown
with respect to the longaxisofthe magnetic particles.NPTEL

77. The particlemayflipitsorientation bythermalactivation,due
toanArrhenius process,where the probability Pfor
reorientation isgivenby:
where E is the height of the energy barrier between the two orientations.
✔The p
canalsoflipitsorientation by a muchlowerprobabilityprocess
calledquantum mechanical tunneling.
✔Thiscanoccurwhen the thermal energyk
BTofthe particleismuchless than
thebarrierheight.
✔If amagnetic fieldisapplied, the shapeofthe potential changes.NPTEL

78. Limitations of SW model:
✔It o the strengthofthecoercivefieldbecauseitallowsonlyonepathfor
reorientation.
✔Them
assumesthat themagnetic energyofthe particledependsonitsvolume.However,
when particlesareintheorder of6nmin size,mostoftheiratomsareonthesurface,which
meansthat theycanhavevery different magneticproperties than largergrainparticles.
Thusthedynamical behaviorofvery small magneticparticles
issomewhatmore complicatedthanpredicted bytheSW
model,NPTEL

79. •Magnetismatna noscalehasbeendiscussed briefly.
•TheSt
Wohlfarth(SW) modelhasbeen usedtoaccountforthedynamicalbehavior of
small nanosizedparticles.NPTEL

80. ❑IntroductiontoN anotechnology, CharlesP.Poole,Jr. and Frank J. Owens,Wiley-interscience.
❑PhysicsofFunc
tionalMaterialsby Hasse Fredriksson andUlaAkerlind.
❑IntroductiontoSo
lidStatePhysics byCharlesKittel.NPTEL

81. Thank you…NPTEL