Molecular Magnetic Materials 2007/Lectures/Michel 0... · 2007. 12. 15. · ICMSICMS--ICMR...
Transcript of Molecular Magnetic Materials 2007/Lectures/Michel 0... · 2007. 12. 15. · ICMSICMS--ICMR...
ICMSICMS--ICMR Winterschool ICMR Winterschool on Chemistry and Physics of Materialson Chemistry and Physics of Materialson Chemistry and Physics of Materials, on Chemistry and Physics of Materials,
November 6November 6--13, 2007, Bangalore13, 2007, Bangalore
Molecular Magnetic MaterialsMolecular Magnetic MaterialsP t II
Mi h l VERDAGUER d
Part II
Michel VERDAGUER andRodrigue LESCOUZEC, Valérie MARVAUD, Cyrille TRAIN, Christophe CARTIER
dit MOULIN, Françoise VILLAIN, Jacqueline VAISSERMANNLaboratory CIM2 CNRS UPMC Paris FranceLaboratory CIM2, CNRS, UPMC, Paris, France
Anne BLEUZEN Laboratory CI, ICMMO, Orsay, France
JNCASR, Bangalore 2 march, 2007
OutlineICMSICMS--ICMR Winterschool on Chemistry and Physics of Materials, Nov. 6ICMR Winterschool on Chemistry and Physics of Materials, Nov. 6--13, 2007, Bangalore13, 2007, Bangalore
1. IntroductionBrief history of molecular magnetism [1-6]
2. Mononuclear species (complexes)2.1. Prequisites : free ion terms, states, orbitals and ligand field. Point Group Symmetry2 2 Metal-ligand interaction to tune electronic structure2.2. Metal ligand interaction to tune electronic structure.2.3. Spin states, spin cross-over, devices [7]3. Magnetism of molecular assemblies in interaction. Polynuclear complexes3.1. Interaction between two electrons : phenomenological and orbital approaches
Localisation and delocalisation conditions, overlap, repulsion, exchangeHeisenberg Hamiltonian ;gHund and Mulliken, Heitler and London approaches
3.2. Interaction Models [1]Kahn ; comparison with Hoffmann (molecules)Comparison with Anderson, Goodenough-Kanamori (solids)
3.3. Case studies :Bi l l hi h i l l [1 5]Binuclear complexes, high spin moelcules [1,5]Ferrimagnetic chains [1,5]Molecule-based magnets ; devices [8]
4. Magnetism of molecules assemblies without interaction :Single-molecule, single-chain magnets [9]M l l "b tt " h f tMolecular, "bottom-up", approach of nanosystemsThe "Mn12" and "Fe8" moleculesLocal anisotropy ; magnetic quantum tunneling effect.
5. ProspectsMagnetism of a single molecule ; Information storage. Electronic quantum computing ;M l if i l i l [10]Multifunctional materials [10]
B f t tiBefore starting …
A warningabout exchangeabout exchange
with (de)localized electrons
Another mechanism for ferromagnetic couplingDelocalized Electrons : double exchangeDelocalized Electrons : double exchange
Delocalized ElectronsLocalized Electrons
NNNN b b
Delocalized Electrons
b b
Localized Electrons
NNNN
NNBr Ni BrNi
a
b
a
b
a
b
a
b
NN
"Polarisation"Ferromagnetism
Overlapantiferromagnetism
Ni(I) Ni(II) Ni(I) Ni(II)
Complex Ni(I)-Ni(II)mixed valency
FerromagnetismSF = 3/2
antiferromagnetismSF = 1/2
Delocalisation of electrons « polarises » : ferromagnetism
Single Molecule MagnetSingle Molecule Magnet
Magnetic moment remains oriented gafter withdrawing of the field(slow relaxation of the magnetisation …)
WITHOUTInteraction between the moleculesInteraction between the molecules
Phenomenon strictly molecular !
Top down3D
Fragments Threads
Dots
MetalsOxydes
• Nice Chemistry• New Physics• Q t / Cl i l Nanosystems • Single molecule magnets• Quantum / Classical• Quantum tunneling
Giant Molecular Clusters
• Applications (far …)• Recording0D, Molecules• Quantum computing
Bottom up
MMn4
Mn30Mn12 Mn84
1 10 100 1000N
Quantum worldMolecular (bottom-up) approach
Classical worldClassical (top-down) approach
I. Tasiopoulos, G. Christou, W. Wernsdorfer et al. Angew. Chem, 43, 2117, 2004
Nanomagnets throughg gMolecular Clusters
• No dispersion in size, in shape and in orientation
• Solubility• Biocompatibility
• Systems well characterised : structure,
ti t
p y
magnetic parameters• Control of parameters by
th isynthesis
Interest of magnetic gmolecular clusters
• Synthetic strategies to get large size molecules andSynthetic strategies to get large size molecules and supramolecular objects
• Creation of clusters analogues of biological systems• Exploration of mesoscopic region, where quantum and
classical effects are coexisting.Devel pment f materials f r the future• Development of materials for the future …
Magnetization : how objects behave in a magnetic field ?
Magnetization M(how they become « magnetized »)
A li d i
magnet paramagnetic M = χ H, χ > 0
Applied magnetic field H
diamagnetic M = χ H χ < 0diamagnetic M = χ H, χ < 0
Magnetization« Soft » Magnet
Magnetization M
A li d i
Remnant Magnetization
CoerciveApplied magneticField H
Field
Magnetization« Hard »Magnet
Remnant Magnetization Magnetization M
A li d iCoercive
Applied magneticField H
Field
Mesoscopic PhysicsMacroscopic Nanoscopic
Clusters IndividualSpins
MolecularClusters
NanoparticlesMicronic Particles
PermanentMagnets
M p N p
S = 10 20 10 10 10 8 10 6 10 5 10 4 10 3 10 2 10 1
Multi - domains Unique Domain Magnetic MomentNucleation, propagation
and annihilation ofdomains’ walls
Unique DomainUniform Rotation
Curling
Magnetic MomentQuantum Tunneling Effect
QuantisationInterference Quantum
1 1 1Fe 8
1
0
M/M
S
0
M/M
S
0
M/M
S
1K 0.1K
0.7K
-1
-40 -20 0 20 40µ0H(mT)
-1
-100 0 100
M
µ0H(mT)
-1
-1 0 1
M
µ 0H(T)From W. Wernsdorfer, Grenoble
UnderstandingUnderstanding……
What is namedWhat is named Single Molecule Magnet ?
=
Hi h S iHigh SpinAnisotropic
High Spin Paramagnetic MoleculeHigh Spin Paramagnetic Molecule Paramagnetic
hphase
H
Single Molecule MagnetSingle Molecule Magnet
H
Single Molecule MagnetSingle Molecule Magnet
Below T < TBlocking
Slowrelaxation
f hof the magnetization
Towards information storage at the molecular level ?
[Mn[Mn1212OO1212(CH(CH33COO)COO)1616(H(H22O)O)44].2CH].2CH33COOH.4HCOOH.4H22OO
or : Mnor : Mn1212
M (IV)
Mn(III) S=2
S=3/2Mn(IV)
Ion Oxyde
S=3/2
Ion Oxyde
Carbone
S=10S =8x2 -4x3/2 =
From D.Gatteschi and R. Sessoli
Mn12 is a hard magnetg
Bi t bilit i
Remnant Magnetisation
10
20T=2.1K
µ B)
Bistability : in zero field the magnetisation can
Magnetisation
n /
µ B
0
10
ZATI
ON
(µ
gbe positive or negative depending of the story of theet
izat
io
-20
-10
MA
GN
ETI of the story of the
sample
C i Fi ld
Mag
ne
-3 -2 -1 0 1 2 3
20M
M A G N ETIC F IELD (T )
Coercive Field
Mn12 Magnetisation relaxes slowly at l t tlow temperature
25
τ=τ0exp(∆/kT)τ0 is much larger than a bulk
15 τ0=2x10-7s;∆/k=62 K
1 yog(τ
)
magnet.
The height of the b i i l t dLo
g τ
5
1 y
1 h
1 s
lo barrier is related to zfs, D, of the S = 10 ground
-52.5 5.0 7.5 10.0
gstate.
T(K)
Magnetic Circular Dichroïsm (MCD) Fe8Hysteresis in solution !y
MCD : Magnetic Circular Dichroïsm
Different absorption for light right or left circularly polarised underDifferent absorption for light, right or left circularly polarised, under applied magnetic field (Cotton Effect) D. Gatteschi et al.
UnderstandingUnderstanding……
ActingActing……
Ground State Energy LevelsE gy L
M=±8
H = 0M=±9
H = 0 M=±10
Energy Levels in a Magnetic FieldE gy L g F
S
M=S-S
M S
H≠0l i fi ld l l
M=-SH≠0At low temperature, a magnetic field populates only
the M = -S state
Going back to equilibrium :Th l i i i i lThermal activation :trivial
Axial symmetry
E(M) = DM2
H=0 M SM SH 0∆E=DS2
M=-SM=S
( E/k T)τ = τ0 exp(∆E/kBT)
T li ff t : !Towards equilibrium :
Tunneling effect : new !
H=0 M=S M=-S
Mn12 is a Hard MagnetM
MsaturationMremnant
HH
HcoerciveHcoercive
St i th ti ti+ Steps in the magnetisation curve
Resonnant Tunneling Effect f H D/for H = nD/gµB
M=SM S
H = nD/gµB
M=-S
H = nD/gµB
Conditions to observe tunnelling effect
• Degenerated wave functions must superpose• A transversal field must couple the two wave p
functions • Coupling splits the two levels : “tunnel
splitting”• Tunnelling Effect Probability increases with
tunnel splitting”
From D. Gatteschi
MM
Tunneling EffectH
No resonnant Tunneling Effectgwith a magnetic field parallel to z
M = S
H ≠ nD/gµM = -S
H ≠ nD/gµB
MM
N t li ff tH No tunneling effect
Relaxation time of magnetization independent of temperature below 300 mKindependent of temperature below 300 mK
0.6
0.8a.
u.)
on
0 2
0.4
etiz
atio
n (a
agne
tizat
io
0.2
Mag
ne Ma
Change with temperature of l Frelaxation time τ in Fe8
Quantum l
8
12
QTunneling Effect4
8
/sec
)Lo
g τ
Thermal -4
0
ln (τ
/
Activation-12
-8
0 2 4 6
1/T (1/K)
τ(s ) Fe 8 w ith D
100
1000 Fe 8 standard
Fe 8 w ith 57 Fe
10
100
1
0.10 2 4 6 8 10
1/T (1/K )
Time to relax 1% of Msat
1/T (1 /K )
f satFe8
D > Fe8st > 57Fe8
Polarised Neutrons Diffraction Polarised Neutrons Diffraction Spin density Map in FeSpin density Map in FeSpin density Map in FeSpin density Map in Fe88
10µB
0µB
-8µBµB
Polarised Neutrons Diffraction Polarised Neutrons Diffraction Spin density Map in FeSpin density Map in FeSpin density Map in FeSpin density Map in Fe88
10µB
0µB
-8µBµB
Y. Pontillon et al. J. Am. Chem. Soc. 1999, 121, 5342.
10 99 88 77 66 55 44 33 22 11 010→99→88→77→66→55→44→33→22→11→0
Neutrons Inelastic scatteringNeutrons Inelastic scattering
UnderstandingUnderstanding……
Single molecule magnetsg gwithout interaction between the molecules !
High Spin Anisotropic MoleculesHigh Spin Anisotropic Molecules
zE Thermal
yx
E0
DS z2
Anisotropy Barrier
hermalActivation
SSz
S024 +2 +4
Anisotropy Barrier
Tunneling
Magnetisation reversal
- Sz +Sz0-2-4 +2 +4
Anisotropy Barrier
DSz2
DSz2 = 400K ?
D = 1KS = 20
For the chemistParameters to ControlParameters to Control
E0
DS 2
ThermalActivation
S = SpinD = Anisotropy
J = Exchange ConstantIntramolecular interaction
zJ’ = Intermolecular interactionSz
DS zAnisotropy Barrier
Tunneling
- Sz +Sz0-2-4 +2 +4
J’J
Remark : when DGS > 0GS
- Sz Sz+Sz0-2-4 +2 +4
D>0
z zz
DSz2
Sz=0Sz 0
In the ground state : f M 0 h lif MS = 0 at the lower energy
No more SMM behaviourD <0 is compulsoryDGS<0 is compulsory …
O i i f th b iOrigin of the barrier
• Isolated Ion Anisotropy Di
Dip l Int ti n• Dipolar Interaction• Anisotropic Exchange Di,j
D = ∑i ci Di + ∑ ci,j D i,j
Zero field splittingp g• Uniaxial Anisotropy D• Rhombic Anisotropy E (0 ≤ |E/D| ≤ 1/3)• Rhombic Anisotropy E (0 ≤ |E/D| ≤ 1/3)• Higher terms A, B
H = D [Sz2 -S(S+1)/3] + E[Sx
2-Sy2] + A Sz
4+ B (S+4+ S-
4)[ z ( ) ] [ x y ] z ( + - )
Diagonal Term Couples states differing
by ± 2 in M
Couples states differing
by ± 4 in M
Conditions to observe tunnelling effect
• Degenerated wave functions must superpose• A transversal field must couple the two wave p
functions • Coupling splits the two levels : “tunnel
splitting”• Tunnelling Effect Probability increases with
tunnel splitting”
From D. Gatteschi
Anisotropie Transversale et mélange Anisotropie Transversale et mélange d i Md i Mdes niveaux Mdes niveaux M
E/K
… « Single molecule magnets» Giant Molecular Clusters
Idée Mn4And many others
High spin + Anisotropy∆E = DSz
2 Mn12Fe8
SynthesisTheory
New
NewConcepts
y
Properties
NewMaterials
NewNewFunctions
A new comer : Mn6
Mn12,S=10,DSz2 ≈ 70K
Mn6,S=12, DSz2 ≈ 90K
R. TomsaP. Gouzerh
J.Milios et al. Angew, Chem. 2004,43, 210 ; E. Brechin et al., W. Wernsdorfer, J. Am. Chem. Soc. 2007, 129,8-9 & 2754-2755 & Chem Comm. 2007, 3476
Anisotropy A rational control is more difficult !A rational control is more difficult !
Two aspects :
- Structural- low symmetry of the clustery y- one anisotropy axis : Cnv, Dnh,…
- ElectronicElectronic- local anisotropy of the magnetic ions- exchange anisotropy
Single Chain MagnetsSingle Chain Magnets ……
Trinuclear speciesDouble
i h izig-zag chainsBis double
zig-zag chains
[{FeIII(L)(CN)4}2CoII(H2O)4] .4H2O
[{FeIII(L)(CN)4}2MII(H2O)4].4H2O [L = 2,2’-bipy and 1, 10-phen), M = Mn, Co, Cu, Zn]
[{FeIII(bpy)(CN)4}2MII(H2O)] .CH3CN . 1/2H2O [M = Mn, Co, Cu]
R. Lescouezec, PhD Thesis
Feasibility of « Molecular nanowires » ?
Anisotropic precursor
[F (III)(bi )(CN) ][Fe(III)(bipy)(CN)4]-
l lR. Lescouëzec, M. Julve, Valencia, Spain D. Gatteschi, W. WernsdorferAngewandte Chem. 2003, 142, 1483-6
2 [FeIII(bipy)(CN) ]- + [CoII(H O) ]2+2 [FeIII(bipy)(CN)4]- + [CoII(H2O)6]2+
Anisotropic precursor(Structure)
Anisotropic assembler(Electronic Structure)
FeIII, d5 CoII, d7
bas spinS = 1/2
haut spinS = 3/2
Chain [{FeIII(bipy)(CN)4 }2CoII(H2O)2•�4H2O]Chain [{Fe (bipy)(CN)4 }2Co (H2O)2 4H2O]nPerspective View
R. Lescouezec, M. Julve, F. Lloret (Valencia), Angewandte Chemie, march 2003
Magnetic Orbitals Orthogonality Ferromagnetism
{[FeIII(bpy)(CN)4]2CoII(H2O)2} ⋅ 6H2O
FF
F
AFN
N
N
N
FeCCN
NM
N
N
N
N
AF
F
AFC
CNN
MAF
Fx
y
[FeIII(AA)(CN)4]-
AA = bpy, phen, bpym
5 C II d7 hi h id5, low spin CoII, d7, high spin
Chain catena µ-[{FeIII(bipy)(CN)4 }2CoII(H2O)2•�4H2O]Chain catena µ [{Fe (bipy)(CN)4 }2Co (H2O)2 4H2O]n
Single crystal2.0
1000100
1010 1
Single crystal ac susceptibilityMeasurements
1.50.1(SQUID)
0 5
1.0
χ" / a. u.
0.0
0.5Slow relaxation of the magnetisation !
3 4 5 6 7 8T / Kχ‘‘ vs. T plots along the b axis R. Lescouezec, F. Lloret
Chain catena µ-[{FeIII(bipy)(CN)4 }2CoII(H2O)2•�4H2O]Chain catena µ [{Fe (bipy)(CN)4 }2Co (H2O)2 4H2O]n
Magnetisation on microSQUID (microcrystal)Magnetisation on microSQUID (microcrystal)
W. Wernsdorfer, LLN Grenoble
Coll. A. R. Lescouezec, M. Julve, F. Lloret P. Herson, Y. Dromzée
Slow relaxation of the magnetisation …
1Magnetization as a function f tim
0.6
0.8
M/M
s
1.5 K
1.6 K
2 6 K2.5 K 2.3 K
2.4 K
of time
0 2
0.4
M
1.7 K
1.8 K
1.9 K
2.8 K
2.7 K
2.6 K2.2 K
2.1 K
2.0 KThermally activated l i f h
0
0.2
0.01 0.1 1 10 100 1000t (s)
relaxation of the magnetization
t (s)
M vs. t plots along the b axis. ac: Ea= 142 K, τ0 = 6.10-11 s
W. Wernsdorfer, Grenoble
Scaling plot of the f ld1 2
1.60.04 K
0.6 K 0.8 K0.2 K
0.5 K
0.4 K
0.3 K 0.7 K 0.9 K
1.0 K 1.1 K1.2 K
1.3 K1.4 K
1.5 K
coercive fieldof the Fe2Co(bipy)
chain0.8
1.2
µ 0H n
(T) 1.6 K
1.7 K
1.8 K1.9 K2.0 K
2.2 K
Hn0 = 5.8 T
E0 = 54 K chain
0
0.4
0 1 2 3 4 5 6 7 8
2.6 K
2.4 K
2.8 K
E0 54 K
τ0 = 1.3 x 10 -8 s
0 1 2 3 4 5 6 7 8[Tln(c/v)] 1/2
0 04 1 0 K
1.2
1.6
(T)
0.04 - 1.0 K 1.1 K1.2 K
1.3 K1.4 K
1.5 K
1.6 K1.7 K
T* (K)
Tc = 1.0 KH 0 = 5 8 T
0.4
0.8µ 0H n
2.4 K
1.8 K1.9 K2.0 K
2.2 K
0.5
1
1.5
2 T* (K)
T (K)
Tc
Hn0 = 5.8 T
E0 = 54 Kτ0 = 1.3 x 10-8 s
00 1 2 3 4 5 6 7 8
[Tln(c/v)] 1/2
2.6 K
2.8 K0
0 0.5 1 1.5 2
T (K)
Low temperature relaxation at H = 1.5 T
11.6 K
0.51.5 K
1.4 K
1.3 K
0M/M
s
1.2 K1.1 K
1.0 K0.9 K
0.8 K0 7 K
-0.5
0.7 K0.6 - 0.04 K
-1
1.5 T
1 10 100 1000 10 4
t (s)
Devices ? To come …m
Two kinds :Two kinds :• Information storage at the molecular level
I l d R b A i iIsolated Robust AnisotropicMagnetic Molecular Dots on a Surface
• Electronic Quantum bitsQFine tuning of magnetic tunnelingTunneling : information change No Tunneling : storageNo Tunneling : storage
Control of the ground spin state• Nuclearity
• Exchange interaction J (F or AF)• Nature of the paramagnetic ions
Control of the anisotropy• Molecular (and Crystal) Structure
Control of the intermolecular interaction J ’• Electronic anisotropy (nature of the ions)
• Bulky ligands• Charged complexes and counterions
• Dilution in an diamagnetic matrix
Prospects (short term)• New chemical systems with larger ∆E
• Improved Instrumentation (microSQUID and nanoSQUID)
New chemical systems with larger ∆E
Prospects (long term)• Magnetic storage on ONE single molecule• Quantum computing
Next « device » ?Recording on one molecule !
Magnetic TipMagnetic TipHSM "up" HSM "down"
SurfaceSurface
Quantum Computing : principleQ m mp g p p
M = ± 8hν
M 10
M = ± 9hν
∆M = + 2(left polarized)
H 0
M = ± 10
H = 0See D. Loss et al., Nature, 2000
Quantum Computing : principlei h (E H ) dto mix the states (E, Hy) and …
1 0 10TunnelingH = 0
M 8 M 8
1 0
M = - 8 M = + 8
10
M = - 8 M = + 8 M = - 8 M = + 8
COMPUTING !
H ≠ 0No
10
10No
TunnelingM = - 8M = + 8
M = - 8M = + 8
0
M 8
STORAGE !
M + 8
Molecular information processing : Will it h ?Will it happen ?
Please, look atthe answer of
P. DayProceedings of the Royal Institution of Great BritainProceedings of the Royal Institution of Great Britain
Oxford, 1998, 85-106