Introduction to Exotic
Phenomena in New Organicbased Magnetic Materials
Arthur J. Epstein
The Ohio State University
Tutorial, The American Physical Society
March 2, 2003
N
N
-TCNE- TCNETCNETCNE
N
N
V2+
V2+
Outline
• Introduction
– Solid state magnetism – basic concepts
– Organic-based magnets
• Fractal Magnet
– Magnetism in 1.46 dimensions
• Photoinduced magnetism in organic-based magnets
– Mn(TCNE)2 organic-based light-tunable magnet
– PIM long-living, reversible, detected at T > 77 K
• Magnetic Organic Semiconductor V(TCNE)2
– Magnetoresistance
– Spin polarized bands - implications for spintronics
• Summary
Why Study Molecule-Based Magnets ?
• New phenomena observed,
not in conventional magnets
• Tunable properties (‘magnets
by design’)
• Light-weight, bio-compatible
alternative to conventional
magnets
• Low-cost, low-temperature,
flexible syntheses
November 2000
Solid State Magnetism-Basics
• All atoms
 diamagnetism ( < 0, || < 10-5emu/mole)
• Ions with partially filled shells
 uncompensated electronic spins
 net magnetic moment
• Independent (non-interacting) magnetic ions
 paramagnetism ( ~ 10-3 emu/mole at 300 K)
• Interacting magnetic ions  magnetic order
(for strong enough interactions and low enough T )
Curie-Weiss Magnetic Behavior
• Paramagnetic State
Susceptibility = Magnetization/Applied Magnetic Field:  = M/H
Curie-Weiss Law (Susceptibility  Temperature-1)
 C
T
C
Ng 2 2 SS 1
B
3k B
N = Avorgadro's Number = 6.023 x 1023 molecules/mol
µB = Bohr Magneton = 9.274 x 10-24 J/T
kB = Boltzmann's Constant = 1.381 x 10-23 J/K
eff 
3kBT
 2.83 T  B g2SS 1
N
Ordering Temperature, Tc , for 3D System
2JzS(S1)
Tc 
3kB
J = Coupling
z = Number of Nearest Neighbors
Spin Configurations in Solids
t0
t1
Paramagnet (independent ionic
magnetic moments)
Ferromagnet
Antiferromagnet
Ferrimagnet
t0
t1
Spin Glass (spatial disorder,
spins frozen in time)
Cluster Glass (short-range order,
frozen cluster moments)
What Are Organic-Based Magnets ?
• Molecular units play crucial role in magnetic ordering by:
– providing unpaired electronic spins
– mediating exchange interaction
• Spins supplied by electrons in p or s orbitals
A building block for molecule-based magnets:
Tetracyanoethylene (TCNE) anion with spin 1/2 in * molecular orbital
N
N
N
N
N
N
N
N
Tetracyanoethylene
(TCNE)
[TCNE]–
spin density distribution
Schweizer, et al,
JACS 116, 7243 (1994)
Magnetic Interactions
• Orthogonal Orbitals (Intramolecular: Hunds Rule)
• Dipole-dipole interaction
E12 
μ1r12 μ 2r12 
μ1μ 2

3
r123
r125
Small, usually insignificant
• Exchange interactions – key to magnetic ordering
- Origin: Coulomb interaction + exclusion principle
Direct exchange
Superexchange
e-
RKKY indirect exchange
Intramolecular Species-Based Examples
of Ferromagnetic Exchange (J > 0)
• Orthogonal Orbitals (Hund’s rule)
Intramolecular-High Spin Species
- MnII
(S = 5/2)
- :CH2, :C(CH2)3, O2
(S = 1)
• Exchange Interaction (Configuration Interaction)
Intramolecular - High Spin Molecules
•
•
•
•
•
•
•
•
•
•
(S =5)
[Iwamura et al]]
First Organic-based Magnet:
[Fe(C5Me5)2]•+[TCNE]••••
•••
Fe Cp*
•
•
•
•
•
TCNE
T >> ,  = C/(T- )
 = + 30 K
Jintra = 27 K
T > 16 K
1-D Heisenberg
JCS 1986, PRL 1987
•••
[Fe(C5Me5)2]•+[TCNE]•Specific Heat
• Jinter = 27 K
• Jinter/ Jintra = 0.013
• Tc ~ 1.5(JJ)1/2
•  Very anisotropic
• Specific heat: 4% entropy,
T < Tc = 4.8 K
• 1-D Correlations important,
T > Tc
[Fe(C5Me5)2]•+[TCNE]•Neutron Diffraction
Ferromagnetic Order
Galvinoxyl
• Ferromagnetic coupling; Phase transition at 85K
K. Awaga, T. Sugano, M. Kinoshita, Solid State Communications 57, 453
(1986).
• Small amount of diamagnetic hydroxygalvinoxyl
suppresses phase transition but prevents long range spin
order
:.H
First Nitroxide Organic-based Magnets
O
N•
• N
O
1,3,5,7-tetramethyl-2,6diazaadamantane-N,N’-dioxyl
Tc = 1.48 K
Rassat, et al ~1993
Tc = 0.60 K
Kinoshita, et al,
1991 p-NPNN
Spin Density Map
Schweizer, et al
~1996
Weak Dipolar
Interaction
Contributes to
Low Tc
Examples of Molecule-Based Magnets:
[MnIIITPP]+[TCNE]-
S=2
S=½
S=2
S=½
N
MnTPP
N
• Quasi-1D ferrimagnetic
order along chains (Adv. Mat.
1994)
• Interchain coupling via
magnetic dipolar interactions
(Chem. of Mat. 1997)
N
N
TCNE
TPP = tetraphenylporphyrin
Examples of Molecule-Based Magnets:
[MnIIITPP]+[TCNE]-
S=2
S=½
• Quasi-1D ferrimagnetic
order along chains
• Interchain coupling via
magnetic dipolar interactions
S=2
S=½
MnTPP
• Vary interchain coupling by
varying organic bridges
TPP = tetraphenylporphyrin
Examples of Molecule-Based Magnets:
[MnIIITPP]+[TCNE]-
S=2
S=½
• Quasi-1D ferrimagnetic
order along chains
• Interchain coupling via
magnetic dipolar interactions
S=2
S=½
MnTPP
• Vary intrachain coupling by
varying acceptor molecules
TPP = tetraphenylporphyrin
Spin Glass Properties
8
• AC susceptibility
6
' (emu/mol)
– Peak showing transition
– Broad peak suggesting
complex transition
– Frequency dependence
characteristic of spin glass
• Scaling Analysis
– The scaling form used was
(Phys. Rev. B 41, 4854)
 ‰T /   / z  f ( / 1/ z )
11 Hz
33 Hz
110 Hz
333 Hz
1100 Hz
3330 Hz
11.0 kHz
– At different frequencies, the
function f, should be the
same, “data collapse”.
4
2
Hac= 1 Oe, Hdc= 0 Oe
0
5
10
15
20
25
Temperature (K)
• So, at the peak temperature we
should have
 / z
‰
T


p p
• Independent determination of
the ratio /z.
30
 T 
‰
p p
 / z
• Able to determine the value of /z
independently
– Results show that
/z = 0.0415 ± 0.0011.
– This value used as restriction in full
scaling plot
 / z
 f ( / 
1/ z
• A full scaling plot allows
determination of Tg and other
exponents
– Tg = 4.1 K ± 0.15
– z = 8.9 ± 0.15
–  = 0.369 ± 0.012
)
1.55
Linear scaling
1.50
1.45
z = 0.0415(0.0011)
1.40
2
25
20
''T//z
 T /
‰
log10[''P()TP()]
Spin Glass Transition: Scaling
3
log10()
4
5
Full scaling plot
11 Hz
33 Hz
110 Hz
333 Hz
1100 Hz
3330 Hz
11.0 kHz
15
10
5
0
0.8
1.2
1.6
/
1/z
2.0
Growth of Fractal Cluster
t vs T
106
t (s)
104
102
100
From
TRM data
10-2
From
AC data
Tg
10-4
10-6
2
4
6
8
10
12
14
16
Temperature (K)
Relaxation: Stretch Exponential
1.8
n, D vs. T
1.6
0.6
1.4
n
D
0.4
1.2
1.0
0.2
n
D
0.8
0.6
0.0
4
5
Temperature (K)
6
Etzkorn, et al, PRL Nov. 2002
Photoinduced Magnetism – a Brief History
Material
Proposed Mechanism for PIM Magnetic Ordering Temp.
Spin-crossover
complexes (1984)
Photoinduced low-spin
to high-spin transition
Paramagnetic
Prussian blue
magnets (1996)
Photoinduced electron transfer
< 25 K
Diluted magnetic
semiconductors
(1997)
Enhancement of RKKY exchange
via photo-generated charge
carriers
< 30 K
Manganite
Pr0.6La0.1Ca0.3MnO3
(1999)
Photoinduced insulator-metal
transition
Mn(TCNE)2 (2001)
Enhancement of kinetic
exchange via lattice distortion
25 K
75 K
PIM in Co-Fe Prussian Blue Magnets
KxCoy[Fe(CN)6]˙zH2O
• Structural disorder dictated by
composition/processing
• PIM initially observed in
K0.2Co1.4[Fe(CN)6]·6.9H2O
O.Sato et al., Science 272, 704 (1996)
Defect
(missing Fe)
Fe
Co
C
N
K+
is interstitial
- Ferrimagnetic ordering below ~16 K
- Magnetization increase obtained by red light
- Photoinduced state has lifetime >10 5 s at low T
- Effect reversed by blue light, heating
PIM in Co-Fe Prussian Blue Magnets
• Microscopic origin of PIM: Photoinduced electron transfer
K.Yoshizawa et al., J. Phys Chem. B 102, 5432 (1998)
Step 1: photoinduced charge transfer from Fe to Co
Intermediate state has spins of 1/2 on both ions
PIM in Co-Fe Prussian Blue Magnets
• Microscopic origin of PIM: Photoinduced electron transfer
K.Yoshizawa et al., J. Phys Chem. B 102, 5432 (1998)
Step 2: intersystem crossing (spin flip on Co site),
lattice distortion (extension of the N-Co
bond)
PIM in Co-Fe Prussian Blue Magnets
7 .3 0
C o o le d in fie ld o f 5 0 G
Illu m in a te d a t T = 5 K , H = 5 0 G
• Basic PIM phenomena:
M (e m u / g )
7 .2 5
7 .2 0
– Magnetization increased by red light
7 .1 5
– Effect reversed by blue light
7 .1 0
R e d lig h t (  = 6 5 0 n m ,F W H M = 9 0 n m )
7 .0 5
B lu e lig h t (  = 4 7 0 n m ,F W H M = 7 0 n m )
7 .0 0
0
20
40
60
80
tim e (m in )
M (em u G / g)
60
before illumination
after illumination
40
20
– Changes in hysteresis:
T=5K
0
increased coercivity, remanence
-20
-40
-60
-4000
-2000
0
H (G)
2000
4000
M (emu / mol)
M (emu / mol)
PIM in Co-Fe Prussian Blue Magnets
 DC Magnetization data
Hdc = 500 Oe
1500
h
1000
•Indications of ‘cluster glass’ behavior:
500
0
1000
800
- Strong MFC / MZFC irreversibility
Hdc = 100 Oe
h
- Bifurcation T decreases with increased H
600
400
- Remanence higher than in spin glasses
200
M (emu / mol)
0
400
Hdc = 10 Oe
300
• M increased by illumination
h
200
• Tc increased by ~2.5 K
100
0
5
10
15
T (K)
20
PIM in Co-Fe Prussian Blue Magnets
4
f

f
f
 AC susceptiblility
3
ac( emu / mol )
3
2

f
f
f
2
0.5
0.4
0.3
0.2
10
1
12
14
16
T (K)

10

 ´ and  peaks increased
 Peak T increased by ~2 K
 ´ and  are f-dependent
 long relaxation times
• Small shift of the ´ peak
0
5
• Effects of illumination:
15
20
T (K)
25
f = 11, 33, 110, 333, 1100 Hz
30
(Tp  Tp / Tp  log f  ~ 0.01 )
 cooperative freezing of spins
PIM in Co-Fe Prussian Blue Magnets
K0.6Co1.2[Fe(CN)6]·4.9H2O
7
33 Hz
110 Hz
333 Hz
1.1 kHz
3.33 kHz
' (10-6 emu)
6
5
4
3
h
1
0
8
" (10-7 emu)
 rapid relaxation of spins
• Frequency-dependent
response after illumination
2
110 Hz
333 Hz
1.1 kHz
6
 slow relaxation of spins
 Direct observation of
slowing down of spin dynamics
h
4
• No frequency dependence
before illumination
2
0
4
6
8
10
12
14
T (K)
16
18
20
22
24
D. A. Pejaković et al., Phys.
Rev. Lett. 85, 1994 (2000)
Model for PIM in Prussian Blue Magnets
• Quantities characterizing cluster glass freezing:
ns - density of spins
- size of spin clusters
 - relaxation time (larger for larger  
M
Tc
Tc - quasicritical temperature
ns

Tf

(finite range order/cluster formation)
Tf - freezing temperature (clusters freezing)
Model for PIM in Prussian Blue Magnets
• Quantities characterizing cluster glass freezing:
M
M
ns - density of spins
- size of spin clusters
 - relaxation time (larger for larger  
T
T
c
c
Tc - quasicritical temperature

ns
ns
h
TT f
f


(finite range order/cluster formation)
Tf - freezing temperature (clusters freezing)
••The
•Due
Spin
Magnetization
Sizes
entire
toconcentration
of
slower
spin
dynamics
clusters
dynamics,
and of
T
increases
magnetic
increase
freezing
upon
due
ordering
of
cincrease,
shifts
clusters
illumination
to increased
their
torelaxation
higher
occurs
via
#temperatures
at
ofphotoinduced
slows
higher
magnetic
down
temperature
neighbors
due to the
photoinduced
charge transfer
increase in ns
Photo-Induced Magnetism
(PIM) in Mn(TCNE)2
• High-Tc molecule-based
magnets: M(TCNE)x (M = Mn,
Fe, Co, Ni, V)
• Synthesis: Adv. Mater. 12, 410
(2000),
Angew. Chem. Int. Ed. 37, 657
(1998)
[TCNE]– spin density distribution
(J. Am. Chem. Soc. 116,7243 (1994)
• Mn2+ (S = 5/2)
• [TCNE]– : spin S = 1/2 in *
orbital
• Ferrimagnetic ordering, Tc =
75 K
C. M. Wynn et al., PRB 58,
8508 (1998) M. A. Gîrţu et al.,
PRB 61, 492 (2000)
Photoinduced Magnetization (PIM) in
Mn(TCNE)2
• Effects of illumination on
the magnetization:
– Mfc increased by 25%
– PIM persists for several
days at T < 50 K
– PIM partially reversed
by lower energy light
Photoinduced Magnetization
(PIM) in Mn(TCNE)2
• Effects of blue light
excitation on the ac
susceptibility:
 ’ increased up to 50 %
 ” increased up to 3 times
– PIM observed at up to 80 K
' (10
-6
emu)
4

3
90 K
2
h
1
0
Tc
" (10 - 7 emu)
3
2
• First organic-based
light-tunable magnet
Before illumination
After illumination

D. A. Pejaković et al., Phys. Rev.
Lett. 88, 057202 (2002)
1
0
0
20
40
60
T (K)
80
100
120
UV/Vis Photoinduced Absorption (PA)
in Mn(TCNE)2
1.0
[TCNE]–
A (a.u.)
A (a.u.)
• Assignment of absorption
bands:
PIM
1.5
0.5
PIM reversal
0.0
2
3
E (eV)
4
0.02
A (a.u.)
– ~2.5-3.5 eV – [TCNE]–   *
– ~1.8-2.5 eV – charge-transfer
• Long-living PA after excitation
with blue light
• Increased oscillator strength
of the CT transition
h
2.54 eV
0.01
0.00
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
E (eV)
UV/Vis Photoinduced Absorption (PA)
in Mn(TCNE)2
1.0
0.5
PIM reversal
2
0.0
0.02
A (a.u.)
[TCNE]–
A (a.u.)
A (a.u.)
• Assignment of absorption
bands:
PIM
1.5
h
2.54 eV
3
E (eV)
4
h
2.41 eV
0.01
0.00
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
E (eV)
– ~2.5-3.5 eV – [TCNE]–   *
– ~1.8-2.5 eV – charge-transfer
• Long-living PA after excitation
with blue light
• Effect partially reversed by
green light
 Formation of a metastable
electronic state
Infrared Photoinduced Absorption (PA)
in Mn(TCNE)2
CN
A (a.u.)
A (a.u.)
0.80
0.60
T = 13 K
0.40
0.20
C=C
0.02
2.41eV
• PA in the region of CN and CC
stretching modes of [TCNE]–
 Lattice distortion accompanies PIM
h
2.54 eV
0.00
-0.02
1350
1400
2150
2200
-1
Wavenumber (cm )
2250
Proposed Model for PIM in
Mn(TCNE)2
Potential energy
Photoinduced state
h
spins
Ground state
Nuclear configuration
• * transition induced
by blue light
Proposed Model for PIM in Mn(TCNE)2
Potential energy
Photoinduced state
spins
Ground state
Nuclear configuration
• Vibrational relaxation
Potential energy
Proposed Model for PIM in
Mn(TCNE)2
Metastable state
Ground state
Nuclear configuration
spins
• Relaxation
Enhanced overlap
into metastable
of magnetic
state
orbitals
• Changed system geometry
 better alignment of spins
 Enhanced metal-ligand orbitals
overlap

enhanced magnetic response
Potential energy
Proposed Model for PIM in Mn(TCNE)2
Charge transfer
state
h
spins
Ground state
• Inverse transition induced by
Nuclear configuration
green light (charge transfer)
Potential energy
Proposed Model for PIM in
Mn(TCNE)2
Charge transfer
state
spins
Ground state
Nuclear configuration
• •Decay
into the
ground state
Vibrational
relaxation
Optimizing PIM in Mn(TCNE)2 Through
Improved
Sample Preparation
Mn(TCNE)
(sample JR2-79)
2
3.5
' (10-6 emu)
• Polycrystalline sample
filtered, dispersed in a
transparent nonmagnetic
host (oil)
 Allows for more efficient
photoinduced transition in
the bulk of material
before illumination
after illumination
(488 nm, 5 days)
3.0
2.5
2.0
1.5
h
'
1.0
0.5
0.0
" (10-7 emu)
1.5
• Dramatic effects of blue light
excitation:
 ’ increased up to 170%
1.0
"
h
0.5
0.0
0
20
40
60
T (K)
80
100
120
  ” increased upto 25 times
– PIM observed T up to 80 K
Photoinduced Magnetization (PIM) in
Mn(TCNE)2
1.6
' (10-6 emu)
 Susceptibility measured at
20 K
Excitation by 2.54 eV laser
line
100 K
1.5
h
1.4
1.3
h
210 K
157 K
h
1.2
1.1

1.0
303 K
250 K
0.9
0
2
4
6
Time (a. u.)
8
• PIM persists after warming
above 200 K
• PIM fully erased after
warming above ~250 K
Photoinduced Magnetism
Summary
•
•
•
•
Mn(TCNE)2 - New class of light-tunable magnets
PIM stabilized by metastable lattice distortion
High operating temperature
PIM in an organic-based material
 tuning of PIM by versatile organic chemistry methods
Pejaković et al., PRL 88, 057202 (2002)
• Prussian blue magnets – coexistence of PIM and
unusual “cluster glass” magnetic order
• PIM due to photoinduced charge transfer between sites, stabilized
by lattice distortion
O.Sato et al., Science 272, 704 (1996)
Ohkoshi et al., Phys. Rev. B 56, 11642 (1997)
Pejaković et al., PRL 85, 1994 (2000)
High Tc (> 350 K)
Organic-based Magnet
Low temperature (40 oC) chemical
vapor deposition (CVD) setup
Heater
Valve
Ar
Gauge
Reaction
zone
TCNE
Valve
V(TCNE)x
film
xTCNE + V(CO)6
Vacuum V(CO)6
Ar
K. I. Pokhodnya, A. I. Epstein, and J. S. Miller,
Adv. Mater. 2000, 12, 410.
—› V(TCNE)x + 6 CO
Pokhodnya et al., Adv. Mater. 12, 16410 (2000)
Increased air stability
Electron transfer salt:
S = 3/2, donor: [V]++
S = ½, acceptor: [TCNE]-
Controlling Magnetic Fields
Conventional magnet
Organic-based magnet
guides magnetic fields
Possible Future: lightweight “plastic” electric
generators and transformers
Solution made V[TCNE]x:Manriquez et al Science 252, 1415(1991)
Shielding, Inductor: Morin et al, J Appl. Phys. 75, 5782 (1994)
Spin States
[TCNE]–
Octahedral coordination
of V with Ns splits
3d-level of V2+
(EXAFS, ANL)
: S = 1/2
unpaired electron in * state
4.426 Å
eg
3.959 Å
3d
t2
V2+
g
Spin density distribution in [TCNE]–
J. Am. Chem. Soc. 116,7243 (1994)
Large Hund’s pairing energy
keeps all three spins parallel
providing high spin state
V2+: S = 3/2
Magnetic Order
Magnetic order is due to
antiferromagnetic coupling
spins of V2+ s and [TCNE]s.
The net spin per “repeat”
cell is 3/2 - 2(1/2) = 1/2.
[TCNE] -
V2+
150
100
50
0
Strong exchange J is due level
hybridization of V2+ and [TCNE]
Adv. Mater. (2000)
H = 3 Oe
0
100
200
300
Temperature, K
400
J=
t
t2
~120 K
E
*
[TCNE] -
V2+
3d
t and E = E * - E 3d are small
EPR Spectra
T= 220 K
30
20
3510
10
0
-10
-20
3490
-30
3470
3000
ivative of Absorption (Arb. Units)
Angular Dependence of
Resonance Field
Hr(G)
Derivative of Absorption (Arb. Units)
Ferrimagnetic Resonance
2000
1000
0
-1000
-2000
B
M
B
M
T= 100 K
B
M
90
180

o
270
R/R295 K
Conductivity Activation Energy Gap
10
4
10
3
10
2
10
1
10
0
 Eg
R  R0 exp 
 2k BT
V(TCNE)x
Eg ~ 0.5 eV
*+Uc



C.B.
Eg
*
Sample #1
Sample #2
3
4
5
6
7
-1
1000/T (K )
8
V.B.
TCNE Energy Diagram
Eg Due to Coulomb
Repulsion Between
Electrons in * Orbital
Spintronics
Prinz (1995), Wolf (2000)
Microelectronics
Charge Control
Spin + Charge
Magnetics
Spin Control
Phenomena:
Applications:
GMR/TMR
Spin Injection
Magnetic Semicon.
Spin Relaxation
Read Head, Sensors
MRAM
Spin-FET, Spin-LED
Logic Device
Capable of much
larger functionality
higher speed
at very low power
Devices
MRAM Cell
Spin Valve
He
Hard
Magnet
Write
Line
Magnetic
Memory Cell
Size < 1m
Spin
Spacer
Current
Aligner
Read
Line
Resistance is minimal
for parallel orientation
Performance of MRAM:
Recording time: < 10 ns (50 ns for DRAM)
Power Cons.: 1~10 mW (400 mW for DRAM)
Nonvolatile Memory
Non-Magnetic Junction
E
E
F
N(E)
N(E)
N↓
N↑
Tunneling barrier
or
layer with
thickness less than
spin-coherence
length
F
N(E)
N(E)
N↓
G0 = (e2/h) T [N↑N↑ + N↓N↓ ] = (1/2) (e2/h) T N2(F)
N↑ = N↓ = (1/2)N(F)
N↑
Spin-Valve Effect
E
E
F
N(E)
N(E)
Nm
Nn
G = G0[1 + (S/N)2];
Tunneling barrier
or
layer with thickness
less than spincoherence length
e
F
N(E)
N(E)
Nm
N = Nm+Nn = N(F);
S = Nm- Nn;
E
M= BS
E
F
N(E)
N(E)
Nm
Nn
Nn
Tunneling barrier
or
layer with thickness
less than spincoherence length
e
F
N(E)
N(E)
Nm
Nn
G = G0[1 - (S/N)2 ]
Variation of MR with Field

MR %  R R
R
H
Tc ~ 235 K
H 0
0.10
100
0.08
Quadratic behavior at T > Tc
MR %
H 0
Linear behavior at T  Tc
V(TCNE)x Sample C
T = 297 K
0.06
0.04
0.02
0.00
0.7
 In non-magnetic heavily doped
semi-conductors:
— MR  H 2
%
T = 225 K
0.5
MR %
— Typical MR at RT ~
10-4
0.6
0.4
0.3
0.2
0.1
See Paper P10.3 N.P. Raju
(Thursday, 4:00pm)
0.0
-0.6 -0.4 -0.2 0.0
H (T)
0.2
0.4
0.6
Linear MR vs Field up to 32 T
12
V(TCNE)x 288 K
Batch Dec21-02
Sample #1
10
MR %
8
6
4
2
0
-30
-20
-10
0
10
20
30
Field (T)
• MR Linear to 0.32 Megagauss, T < Tc
Non-linear MR for T >> Tc
14
V(TCNE)x film
batch dec-20-02
sample #2
(Tc ~ 275 K from MR vs T data)
12
MR%
10
8
T = 350 K
6
4
2
0
0
5
10
15
20
25
30
35
Field (T)
• MR ~ H2 observed for samples for T >> Tc
Temperature Variation of MR
0.4
25
20
V(TCNE)x
a)
0.3
MR % at 0.6 T
Sample A; Tc > 350 K
Sample B; ~ 275 K
Sample C; ~ 235 K
0.2
0.1
Sample A
0.0
15
0.6
10
MR % at 0.6 T
V(TCNE)x films
5
0
b)
0.4
Sample B
0.2
0.0
0.8
0
50
100
150
200
250
300
Temperature (K)
• MR peaks at the corresponding FM
ordering temperature
0.6
MR % at 0.6 T
FMR Intensity (arb. units)
30
0.4
Sample C
c)
0.2
0.0
150
175
200
225
250
Temperature (K)
275
300
Model of Half Semiconductor
•Effect of Coulomb repulsion on charge transport
Hubbard Model:
He =  i nis+ Uc  nisnis- t  a+isajs
i,s
i,s
<i,j>,s
At half-filling for strong Uc the *-band is split into two subbands
*-band
of TCNE-
C.B. (empty)
Eg = Uc
V.B. (filled)
Antiferromagnetic insulator with exchange constant
J** = 2t2/Uc
… Model of Half Semiconductor
•Effect of antiferromagnetic exchange with V2+ spins
Hm= J** (sisj2J  (siSa
Heisenberg Model:
<i,a>
<i,j>
At J >> J**ferrimagnetic
half semiconductor:
Conduction
and
valence
bands are
oppositely
spin polarized
E
C.B.
Eg
V.B.
TCNE- TCNE-
TCNE-
TCNE-
3d-level
V2+
V2+
N(E)
UPS: Linkoping Univ.
Eg = Uc ~ 0.5 eV
Mean Field Theory of
Magnetoresistance (MR)
•Paramagnetic phase, T > Tc :
<S> ~ - <s> ~ h;
 ~ 1/
MR ~ <s> <S> ~ (h)2 ~ (h/)2
•Critical Regime, T ~ Tc :
<S> ~ - <s> ~ h1/3;
MR
h2/3
T < Tc
T > Tc
h/|1/2
(h/)2
MR ~ h2/3
•Magnetic Phase, T < Tc :
<S> ~ - <s> ~ |12 + h;  ~ 1/|
MR ~ |h ~ h/|1/2
-h2/3 h2/3
T- T
 ——c << 1
Tc
gBH <
-3
h = ——
~ 10 << 1
kBTc

Comparison with Experiment
Experiment
Predictions of
for
Conventional Model of
Disordered Semiconductor V[TCNE] x
VRH law: R(T) ~exp(T0 /T) 1/4
but T 0 is too high ~ 10 9 K
Positive MR
but MR is too small ~10 -6
Yes
?
Yes
?
T-3/4 -dependence of MR
No
H 2 -dependence of MR
everywhere
No
Total:
-2
Experiment
Predictions of
for
the Present Model of
Half-Semiconductor V[TCNE] x
Yes
Arrhenius law for R(T)
Positive MR~10 -2
MR(T)~(T) has
maximum near T c
H 2 -dependence of MR
at T>T c
H-dependence of MR
at T<T c
Total:
Yes
Yes
Yes
Yes
5
Summary
• Organic-based Magnets
– Magnets from unpaired s and p electrons
– Magnets with ‘conventional’ phenomena typical of
‘conventional’ magnets and
• New Phenomena such as
–
–
–
–
–
–
–
–
–
Dipole-dipole interaction controlled magnets
Fractal magnets
2D Triangular spin glass
Photoinduced magnetism
Light weight magnets for shielding and induction
Magnetic organic semiconductors
Spin tunneling (M. Sarachik)
Spin ladders (C. Landee)
…
Acknowledgments
Yurii Bataiev, Will Brinckerhoff,
Animesh Chakraborty, Sailesh Chittipeddi,
Gang Du, Stephen Etzkorn,
Mihai Girtu, Carmen Kmety,
Steven Long, Brian Morin,
Raju Nandyala, K.S. Narayan,
Dušan A. Pejaković, Kostia Pokhodnya
Vladimir Prigodin, Chuck Wynn,
Ping Zhou, Fulin Zuo
Additional Graduate Students, Postdocs
The Ohio State University
Joel S. Miller
Many Graduate Students, Postdocs
University of Utah
Many,Many More
ANL, NHMFL, Linkoping U., Grenoble,
Columbia U., NIST, BNL, DuPont, …
Supported by DOE, AFOSR, ARO, DARPA, NSF