PhD students

1
B. Barbara, E. Bonet, W. Wernsdorfer,
Nanomagnetism group, Louis Néel Lab., CNRS,
Grenoble.
Nanomagnetism from atomic clusters to molecules
and ions. First microwave experiments in the
quantum regime.

• PhD students
• L. Thomas (Versailles, IBM), I. Chiorescu (MSU),
• C. Thirion (Durham), R. Giraud (Würzburg), R.
  Tiron (LLN)
• Collaborations with other groups
• D. Mailly (Marcoussis)
• A.M. Tkachuk (St Petersburg)
• H. Suzuki (NIMS, Tsukuba, Japan)
• D. Gatteschi (Florence) A. Müller (Bielefeld)

2

OUTLINE
Some classical and quantum aspects of
nanomagnetism in magnetic nanoparticles and
molecules (Brief introduction to the field)
A new direction
The case of rare-earths ions
Tunneling of the angular momentum J ofHo3 ions
in Y0.998Ho0.002LiF4Example of a metallic
matrix Ho3 ions in Y0.999Ho0.001Ru2Si2
Effects of microwave absorption towards spin
qubits
Conclusions and perspectives
3

Micro-SQUID magnetometry
H Hsw
M - M
DM Large dB/dt
fabricated by electron beam lithography
I Ic Superc. Normal
(D. Mailly, LPM, Paris)
10-4
F0
sensitivity

 
102

µB

  10-18
emu
W. Wernsdorfer, K. Hasselbach, D. Mailly, B.
Barbara, A. Benoit, L. Thomas, JMMM, 145, 33
(1995).
4
Nanometer scale
50
S 10
103
106
Nanoparticle
Cluster
Magnetic Protein
Single Molecule
20 nm
3 nm
1 nm
2 nm
5
Micro-SQUID array
crystal size gt few µm 10-12 to 10-17 emu
temperature 0.03 - 7 K field lt 1.4 T and lt 20
T/s rotation of field transverse field
several SQUIDs at different positions
irradiation with microwaves 0.1 to 345 GHz
6
Evidence of the 2-D Stoner-Wohlfarth
astroid
FeS, filled nanotuble N. Demoncy, H. Pascard, A.
Loiseau
5 nm

W. Wernsforfer, E. Bonnet, B. Barbara, N.
Demoncy, H. Pascard, A. Loiseau, JAP, 81, 5543
(1997).
7
Effect of a transverse field close to the
anisotropy field Telegraph noise

106 spins
Single phonons shots Reversal up, down, up
- W. Wernsdorfer, E. Bonet, K. Hasselbach, A.
Benoit, B. Barbara, N. Demoncy, A. Loiseau, H.
Pascard, D. Mailly, Phys. R.ev. Lett., 78, 1791
(1997) - B. Barbara et al, Proc. Mat. Res. Symp.
475, 265 (1997) Lecture Notes in Physics (2001)
http//www.springer.de
8
Mn12acetate
Mn(III) S2
Mn(IV) S3/2
Total Spin 10
9
Barrier in zero field (symmetrical)H - DSz2 -
BSz4 - E(S2 S-2) - C(S4 S-4)
Landau-Zener Transition at avoided level
crossing (isolated system)
D
If applied field // -M non-symmetrical
barrier New resonances at gmBHn nD
Tunneling probability P1 exp-p(D/h)2/gc
c dH/dt
10
Tunneling of Magnetization in Mn12-ac
Resonant tunneling at Hn 450.n mT (Steps)
ICM94 Barbara et al JMMM (1995) NATO ASI QTM94
ed. Gunther and Barbara Thomas et al Nature
(1996) Friedman et al, PRL (1996) Wernsdorfer
and Sessoli Science (1999) Tupitsyn and Barbara
Magneto Science, Wiley, NY (review, 2001) see
cond/mat. . Slow quantum spin dynamics of
molecule magnets.
11
A new direction Tunneling of the angular
momentum of rare-earths ions
A quasi- infinite number of systems
for the study of mesoscopic quantum dynamics
- different CF and 4f symmetries
- different concentrations
- insulating, metallic, semi-conducting

• Ho3 in Y0.998Ho0.002LiF4

Tetragonal symmetry (Ho in S4) (J
LS 8 gJ5/4)
Dipolar interactions mT ltlt levels
separation
12
CF levels and energy barrier of Ho3 in
Y0.998Ho0.002LiF4
Strong mixing
Barrier short-cuts
Singlet excited state Doublet
ground-state Large t1 (Orbach process)

• R. Giraud, W. Wernsdorfer, D. Mailly, A. Tkachuk,
  and B. Barbara, PRL, 87, 057203-1 (2001)

Energy barrier ( 10 K)
B20 0.606 K, B40 -3.253 mK, B44 - 42.92 mK,
B60 -8.41mK, B64 - 817.3mK Sh. Gifeisman et
al, Opt. Spect. (USSR) 44, 68 (1978) N.I.
Agladze et al, PRL, 66, 477 (1991)
13
Hysteresis loop of Ho3 ions in YLiF4
dH/dt0.55 mT/s
14
Ising CF Ground-state Hyperfine Interactions
H HCF-Z AJzIz (J I- J- I )/2
The ground-state doublet 2(2 x
7/2 1) 16 states
-7/2
-5/2
5/2
7/2
7/2
5/2
3/2
-7/2
gJmBHn n.A/2
A 38.6 mK
Avoided Level Crossings between ??, Iz? and
?, Iz? if DI (Iz -Iz )/2 odd
Co-Tunneling of electronic and nuclear momenta
Electro-nuclear entanglement
15

Acceleration of quantum dynamicsin a transverse
field
. slow sweeping field tmeas gtgt tbott gt
t1 Near thermodynamical equilibrium at the
cryostat temperature
16
Additional steps at fields Hn (23/2).n
(mT) single Ho3 tunneling being at avoided level
crossings at Hn 23.n (mT)

50 mK 0.3 T/s
50 mK 0.3 T/s

Simultaneous tunneling of Ho3 pairs
(4-bodies entanglement) Two Ho3
Hamiltonian avoided level
crossings at Hn (23/2).n
Giraud et al, PRL 87, 057203 1 (2001)

17
R. Giraud, A.
Tkachuk, and B. Barbara, PRL (2003).
Single-ion level structure En DE ?
gmBHn Tunneling gmBHn (n-n)DE/2 Co-tunnelin
g gmBHn(n-n1/2)DE/2 (DE
A)
Two-ions Level structure Co-tunneling Biais
tunneling Diffusive tunneling
18
Model of two coupled effective spins

• H/J ?ijSizSjz ??ij(SiSj- SjSi-)/2
  b?ij (SiSj Sj-Si-)
• 
  
• (A/J)?iIizSiz
  1/2(IiSi- Ii-Si)
• with
• a (Jx Jy)/2J
  b (Jx - Jy)/4J

This term becomes negligible at Tgtgt2K
Co-tunneling
Diffusive tunneling
This is why dipolar interactions induce
multi-tunneling effects B. Barbara et al,
ICM03, JMMM to appear
19
Case of a metallic matrix Ho3 ions in
Y0.999Ho0.001Ru2Si2

n2
n0
n1
These steps come from tunneling transitions of
JI of single Ho3 ions, In a sea of free
electrons.
20
Spin tunneling assisted by photons Irradiation
of a single crystal of Fe8 by circularly
polarized electromagnetic radiations

DM1
Effects of photons and of phonons can be
differenciated
21
Absorption of circular polarized microwaves(115
GHz)
22
Absorption of circularly polarized
microwaves(115 GHz)
23
Absorption of circularly polarized microwaves(95
GHz)
24
Photon induced tunnel probabilityPassisted P -
n10P10
0.8
Ts
n0
n1
0.12
0

25
Photon induced line-width broadening
Multitunneling transitions R. Giraud, W.
Wernsdorfer, D. Mailly, A.Tkachuk, and B.
Barbara, PRL, 2001
L. Sorace, W. Wernsdorfer, C. Thirion, A-L.
Barra, M. Pacchioni, D. Mailly, and B. Barbara ,
Phys. Rev. B 68, 220407 (2003)
26
V15 a spin 1/2 molecule with adiabatic LZ
transition
Absorption of sub-centimetric waves
G Max 5 s-1
I. Chiorescu, W. Wernsdorfer, A. Müller, H.
Boggë, and B. Barbara et al, PRL (2000) W.
Wernsdorfer, D.Mailly, A. Müller, and B. Barbara,
EPL, to appear.
.
27
Frequency dependence of microwaves absorption in
V15
28
Resonant absorption at n gB
g 0. 97
29
Resonant absorption Field-independent
absorption
DM / Ms
30
Gaussian absorption lines

• Important broadening by nuclear spins
  Loss of coherence
• WR gb 30 kHz ltlt 1/t2 gs 0.2 GHz
• Rabi oscillations, require larger b.

N BMax/2ps gBt2/2p 20 Precession 20 turns
31


Another example substituted magnetic wheels Fe5Ga
A. Cornia, Modena
Relatively narrow Resonant absorption 7 mT
(15 times smaller) Still 20 precession
turns, and
WR gb 30 kHz ltlt 1/t2 gs 10 MHz
32
Multi-photonabsorption
Cr7Ni S 1/2
G. A. Timco and R. E. P. Winpenny
33

Quantum computing in molecular magnetsSeveral
ways
Leuenberger Loss, NATURE, 410, 791 (2001)
implementation of Grover's algorithm storage
unit of a dynamic random access memory device.
fast electron spin resonance pulses can be used
to decode and read out stored numbers of up to
105 with access times as short as 0.1 nanoseconds.
34
CONCLUSION Ho3 in LiYF4 Evidence for
tunneling of the total angular momentum J
Quasi-isolated Ho3 ions (J and I tunnel
simultaneously co-tunneling) Pairs of Ho3
ions (four-body entanglement) Relevant quantum
number (Kramers,..) IJ at T lt 2K Crucial role
of the anisotropic character of dipolar
interactions Metals spin tunneling in the
presence free carriers Molecular magnets
Hidden multi-tunneling effects Tunneling
assisted by photons Highly non-linear effects
(Fe8) Evaluation of coherent precessional time
in molecular magnets Most important requirement
to observe Rabi oscillations Radiation
Field x 104 because spins are small !!
Absorption width 102 because of
the spin-bath
(Stamp, Prokfiev and Tupitsyn,
1996-2004)

35
Some perspectives Dissipation and
decoherence by free carriers on spin tunneling in
metals (Kondo, heavy fermions, spintronics)
Higher order many-body tunneling and decoherence
by the environment (quantum phase
transitions) Rabi oscillations and spin-echo
experiment on electronic states of -
Molecular magnets (intra-molecules hyperfine
interactions 10 mK) - Entangled E-N pairs
of Ho3 (dipolar interactions, hyperfine
interactions 1 mK) Spin Qubits manipulated by
photons in new molecular and
systems.