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 Properties of the Carbonyl Cluster [Ni 16 Pd 16 (CO) 40 ] 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 [NBu 4 ] 4 [Ni 16 Pd 16 (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: C i 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 T 0 = K, p = 4.71 J=2 => p=4.9 E a = 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 H J=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  =  ·B loc.   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 [Ni 16 Pd 16 (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.