M. Riccò, T. Shiroka, S. Carretta

Dipartimento di Fisica, Università di Parma e INFM, PARMA 

C. Femoni, M.C. Iapalucci, G. Longoni

Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna, BOLOGNA

F. Bolzoni

IMEM-CNR, PARMA

Magnetic PropertiesMagnetic Propertiesof the Carbonyl Cluster of the Carbonyl Cluster [Ni[Ni1616PdPd1616(CO)(CO)4040]]4-4-

Introduction

Only carbonyl clusters with an odd number of valence electrons display a significant magnetic behaviour arising from the presence of a single unpaired electron.

The bimetallic compound [NBu4]4[Ni16Pd16(CO)40] represents an example of a metal-carbonyl cluster which, in spite of an even number of unpaired electrons, shows clear magnetic properties.

In this work we present the results of investigation of magnetism in this cluster using SQUID magnetometry and muon spin spectroscopy (µSR).

Clusters and Crystalline Structure

Cluster symmetry: Ci

Crystalline structure: triclinic

a=15.93 Å, b=15.94 Å, c=17.23 Å

=74.69, =73.65, =79.09

Large distances => Not-interacting clusters

* - Extended Hückel

Molecular Orbital

EHMO*

DC Susceptibility Measurements

T0= - 3.9 K, p = 4.71

J=2 => p=4.9

Ea= 27.5 0.3 meV

Paramagnetic behaviour up to 150 K and activated for higher temperatures.

Magnetization Curves

T=5K

J=2

J=23 !

Fit with one

Brillouin function

HYPOTHESIS: The spin Hamiltonian could include other terms (beside Zeeman) arising from the interaction with the crystal field (due to cluster shape, ligand, etc.)

Crystal-field multiplets (with J=2)

Fitting the magnetization data with the model given by the modified Hamiltonian gives a much better agreement.

D 3 meV

E 0.7 meV

g = 1.8 (isotropic)

Stevens Operators

Predicted Molar Susceptibility

The assumed HJ=2

Hamiltonian can reproduce also the magnetic suscep-tibility data:

= J=2(T) + 0

H = 50 G

Extended Hamiltonian Energy Levels

H

- angle between H and the hard magnetic axis

(unknown)

Interesting level crossings appear at high fields (~

28T)

J = 2

g = 1.8

The µSR Technique (ZF/LF)

Muons (I = ½, = 2.2 µs), localise at an interstitial site and precess at the local field, with = ·Bloc.

ZF - method of detecting weak internal magnetism, that arises due to ordered magnetic moments, or random fields that are static or fluctuating with time.

Applying an external LF field one can “freeze” the muon spin direction, and detect the time evolution of the signal.

Schematic LF (ZF) µSR experiment

µSR Re-polarization Measurements

The LF re-polarisation shows a bi-exponential recovery => two contributes: from muon trapped near the clusters and counter -ions resp. (two magnetically distinct species, see inset).

The high final saturation value does not agree with a simple paramagnetic behaviour.

LF-µSR results with different applied magnetic fields (re-

polarisation)

Conclusions

The molecular [Ni16Pd16(CO) 40]4- cluster shows a high number of unpaired electrons (4 => J = 2).

The magnetometry measurement results, as well as µSR data, cannot be interpreted only on the basis of a simple Zeeman Hamiltonian.

The introduction of an effective crystal field (arising from the cluster’s shape, the ligand, etc.) allows a consistent interpretation of the experimental data and also predicts the cluster behaviour at higher applied fields.