STATIC AND DYNAMIC
PROPERTIES OF MOLECULAR
NANOMAGNETS
INVESTIGATED BY NMR
M.Mariani1,2, M. Belesi1, H.Amiri1, A.Lascialfari1,2,3, F.Borsa1, P.Khuntia1, F. Orsini2,3, P. Arosio3,
A. Cornia4, R. Winpenny5, E. Mc Innes5, M.Andruh6, P. Santini2,7, S. Carretta2,7, G. Amoretti2,7
1Dipartimento di Fisica “A.Volta” e Unita’ CNISM , Universita’ di Pavia, Pavia, Italy
2 S3-CNR-INFM, Modena, Italy
3DISMAB, Universita` degli studi di Milano, Milano, Italy
4 Dipartimento di Chimica e INSTM, Università di Modena, Modena, Italy
5 Department of Chemistry, University of Manchester, Manchester, UK
6 Inorg. Chemistry Laboratory, University of Bucharest, Bucharest, Romania
7 Dipartimento di Fisica, Università degli studi di Parma, Parma, Italy
The subject
Molecular nanomagnets are organic molecules with a
limited number of magnetic ions where the
intramolecular magnetic interactions are dominant
over the intermolecular interaction .
IDEAL MODEL SYSTEMS TO INVESTIGATE
NANOMAGNETISM
We show here three broad classes of
nanomagnets showing interesting properties
and where NMR has given relevant information
Phonon trapping in Ni10 [1]
.
The superparamagnetic
slowing down of the
magnetization in Ni10
nanomagnet is not due to an
anisotropy barrier ( as e.g. in
Fe8) but rather to a novel
phonon trapping
phenomenon as
demonstrated by the
persistence of a narrow
proton NMR line even at low
T
Heterometallic rings
Static effects [2]
The redistribution of the local spin
53
density can be probed by Cr NMR
in a Cr8 ring where one Cr ion is
replaced by a diamagnetic Cd ion
Heterometallic rings
Dynamic effects [3,4]
6
5
Cr7Fe
Cr7Ni
The shift in the NMR
relaxation rate between
the Cr7Fe and Cr7Ni is
related to the different
perturbation effects of
the Fe and Ni ions on
the spin dynamics
-1
1/T1(ms )
4
3
2
1
0
1
10
T(K)
100
Single molecule paramagnets [5]
3.0
Cu6Fe
Exp.
Fit
T ( emu K /mol)
2.9
2.8
2.7
2.6
2.5
0
50
100
150
T(K)
200
250
T ( emu K /mole)
2.4
2.2
2.0
Cu6Co
Exp.
Fit
1.8
1.6
0
40
80
120
T (K)
160
The susceptibility is Curie- like down
to very low temperature with
J =+0.14K in Cu6Fe
and J = -1.12K in Cu6Co
Single molecule paramagnets [5]
3.0
-1
T1 _FeCu6 at 1.5T
1.8
1.2
T1 (ms )
-1
2.4
-1
The proton NMR
spin-lattice relaxation
rate is weakly T
dependent down to 4K
as in bulk paramagnets
-1
T1 _CoCu6 at 0.5T
-1
T1 _CoCu6 at 1.5T
0.6
0.0
1
10
T(K)
100
References
[1]
Belesi
et.
al,
PRL
102,177201(2009)
[2] Micoti et al., PRL 97, 267204
(2006)
[3] H.Amiri et al., umpublished
[4] S.Carretta et al,. umpublished
[5] P.Khuntia et al,.PRB 80,094413
(2009)