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Frances HELLOGO.GIFellman Group
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Magnetic Molecules

Molecular magnetism has been developed towards the end of the 20th century, with the growing interest in molecular materials as low cost materials whose properties can be tuned by chemical techniques. Nowadays, the continuous trend towards miniaturizing electronic devices makes the device potentials of single (non magnetic and magnetic) molecules of great interest. Also for fundamental research, magnetic molecules are very appealing because in these systems quantum properties coexist with classical ones. 

Between magnetic molecules, compounds of the single molecular magnets (SMM) class are particularly attractive. The first reported and mostly investigated example of SMM is Mn12ac, which is shown in Fig.1a. 

Mn12ac
Fig.1.  (a) Molecular structure of Mn12O12(CH3COO)16(H2O)4), the so called Mn12ac. The molecule is composed by an external ring of eight manganese (III) ions and an internal tetrahedron of four manganese (IV) ions. The organic environment (the ligands) is not shown. In the figure, manganese(III) ions are shown in blu, Manganese(IV) ions in green, oxygen atoms in red, carbon in cyan and hydrogen in white.   (b) Anisotropy barrier (for zero magnetic field) which separates the two favorable states of the molecular spin.

 SMMs are clusters of transition metal ions, characterized by large spin of the ground state (S=10-20). When the spin ground state is associated with easy axis magnetic anisotropy (Ising type), a double well potential with an energy barrier which separates the two favorite orientations of the magnetization is present (see Fig.1b). This barrier is at the origin of extremely long magnetization relaxation times at low temperature. Such an effect is a property of the isolated molecule and not a cooperative feature as in traditional magnetic materials (that is why they are called single molecular magnets!).  Therefore it is in principle possible to store information in one single molecule. This property makes SMM appealing for potential use as magnetic memories with very high storage density. 

We are interested in the magnetic and electronic properties of magnetic molecules and SMM adsorbed in the sub-monolayer regime on surfaces of ultra-thin magnetic and non-magnetic films. We are investigating how the electronic transport of ultra-thin films is influenced by the presence of the molecular magnetic moments adsorbed on their surfaces. 

For transport measurements, we use insulating substrates with pre-fabricated leads which define the electrode geometries (see Fig.2). We evaporate the thin films using shadow masks and measure the conductivity of the films in situ with the four probe method.

SampleResistivity.JPG

Fig.2. Schematic of sample. The insulating substrate is made of 4000 of amorphous SiNx grown on a Si wafer (about 520 μm thick). The leads are gold (yellow) on cromium and platinum (azure), 1000 and 100 respectively. Four different samples (orange) can be grown on such a device. We delimited their geometry during the thin film deposition using a shadow mask. Due to the fixed sample geometry we measure the resistance per square.

For magnetization and magneto-transport measurements we use a SQUID magnetometer (able to apply magnetic fields up to 7T) which is able to detect magnetic moments as low as 10-9 emu.

Our research is now focused on the development of techniques for adsorption of SMM on surfaces, growth and characterization of ultra-thin films whose transport properties change drastically due to the presence of few magnetic moments on their surfaces.


This project is part of an NSF NIRT grant called "Molecular Spin-Active Nanoelectronics".  Member of this grant is an interdisciplinary team that includes members from the Physics and Chemistry of UC Berkeley and Hardvard, as well as the microscopy group at the IBM Almaden Research Center.

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