Molecular Nanomagnets

Dr. Glenn Agnolet, Dr. Kim R. DunbarDr. Donald G. Naugle, Dr. Joseph H. Ross, Jr.,and

Dr. Winfried Teizer

 

Objectives

Advances in chemical synthesis[i] have opened a new field by allowing the production of single molecules with large magnetic moments (Fig. 1). These molecules exhibit fascinating fundamental properties as a result of a large number of coupled magnetic moments in a single molecule (e.g. macroscopic quantum tunneling).[ii],[iii] At the same time the possible integration of Large Magnetic Moment Molecules (LMMMs) into materials devices accounts for significant technological potential in spintronics, quantum computing[iv] and high-density data storage. The rising interest in LMMMs is based on the expectation that they may be core materials for future technologies (similar to fullerenes).

Figure 1: Schematic diagram of Mn-12 (left) and Mn-6 (right) molecules. The large (green) spheres symbolize the Mn atoms, which lead to the magnetic moment to the molecule. The Mn atoms are bound to each other via organic ligands.

 

The particular objectives of this study are:  1. To investigate the magnetic properties of small crystals of LMMMs. 2. To reduce the crystal size until ultimately only one LMMM is measured, thereby eliminating any interaction effects between different molecules. This can in principal be accomplished by using miniaturized Superconducting Quantum Interference Devices (SQUIDs), produced by nanolithography (see methods). 3. To study derivative compounds, which are currently being synthesized. The magnetic properties of LMMMs have shown remarkable sensitivity to seemingly small functional changes. There are also theoretical studies at Texas A&M, which aim at understanding this sensitivity.[v] 4. To produce films of LMMMs (see Methods section). 5. To utilize the film growth techniques to produce devices, where patterned LMMM films are integrated into conventional thin film structures.

 

This project builds on a combination of local resources at the chemistry and physics departments at Texas A&M University, which, to our knowledge, is unique to the country. In the chemistry department, a large and active research group is synthesizing LMMMs and is led in this effort by an expert in the field (Dunbar). This group has synthesized various known and novel LMMMs, which are characterized on-site. In a prior Post-Doc, Teizer fabricated and tested MicroSQUIDs, a vehicle of choice for a magnetic study of small particles (see Methods section). Current work aims at the further miniaturization of MicroSQUIDs, towards the length scales of individual molecules, and on the development of thin film growth techniques for LMMMs. The combination of newly synthesized LMMMs by expert chemist and the expertise in magnetic measurements and thin film depositions within walking distance is expected to provide for significant synergy and a highly efficient feedback between chemists and physicists. The research environment is further strengthened by an NSF Nanoscience Interdisciplinary Research Team of Chemists (Cotton, Dunbar) and Physicists (Agnolet, Naugle (PI), Pokrovsky, Ross), which has been funded to synthesize and investigate LMMMs. Pushing the boundaries of SQUID miniaturization is also of industrial interest, as indicated by a letter from Quantum Design, the leading manufacturer of turn-key SQUID systems for the research environment.[vi]

 

Methods

A well-established and powerful magnetic characterization technique is that of Superconducting Quantum Interference Devices (SQUIDs). Commercial SQUIDs typically measure the magnetic moment of a large ensemble of particles, in form of a bulk or film medium. To use this technique to its full potential in measuring the magnetic moment of molecules, a miniaturization of SQUIDs towards the dimensions of the molecules is beneficial. Such a miniaturization allows for the precise location of the particle to be measured with respect to the SQUID and therefore for an optimization of the magnetic particle flux that penetrates the SQUID loop.[vii] Currently the smallest SQUIDs are ~2 mm x 2 mm in size (Fig. 2, left). They have been tested to show the characteristic modulation of the critical current IC with changing magnetic field H (Fig. 2, right).

       


 

 

 

                                                                                   

      

Figure 2: Left - SEM picture of a 2mm x 2mm Al-MicroSQUID (Al thickness: 30nm), produced by Teizer and Dynes. The width of the wires is 200nm, weak links on the left side of the loop are ~30nm wide. Right - Critical current Ic versus magnetic field H for this Al-MicroSQUID. IC shows a periodic modulation with a period of H=5Oe, the field required to change the flux inside the 4mm2 SQUID loop by one flux quantum.

To control the location of the LMMM with respect to the MicroSQUID, Teizer and the NIRT team are planning to use two methods. The first method, which Teizer’s group is focusing on, relies on patterning of a LMMM film by nanolithography. The production of films of MNs is complicated by the fact that the compounds decompose at quite low temperatures (typically ~100oC), a common feature of organic molecular solids. This precludes thermal evaporation, sputtering, e-beam evaporation etc. from being employed. Several novel techniques have been used to gently deposit films of easily decomposing organic compounds, most promisingly Matrix Assisted Pulsed Laser Evaporation (MAPLE).[viii] MAPLE has been successfully employed to deposit films of various organic materials.[ix],[x],[xi],[xii],[xiii],[xiv],[xv],[xvi] Preliminary characterization of the deposited films by electron spray ionization mass spectrometry, X-ray photoelectron spectroscopy and infrared spectroscopy indicate that the properties of the deposited film are identical to the as-produced Mn12. We need nanolithography to pattern these films into devices.

 In parallel work to this approach, one of the thrusts in the chemical synthesis of Dunbar’s group is currently to chemically functionalize the LMMMs to introduce preferential adsorption to gold. If successful, this will allow control of the LMMM location by immersing a substrate with pre-patterned gold islands (using nanolithography) into an LMMM solution.

 

Impact

This project will have several direct and indirect benefits. It will further the fundamental understanding of Quantum Magnetism by providing a systematic study of various LMMMs and derivatives. The replacement of organic ligands in LMMMs by other functional groups is already being pursued. Such a systematic study of the magnetic properties depending on the particular ligands is necessary to allow for any further technological use of this important class of novel materials. A second impact will be the development of thin film deposition techniques. While the availability of film samples allows various, hereto-impossible fundamental experiments, the greatest appeal lies in the possibility to integrate LMMMs into devices. This may set the stage for the creation of functional materials for use in spintronics, quantum computing4 and high-density data storage.

 

In summary, a systematic investigation of the magnetic properties of LMMMs is under way at TAMU. While the initial thrust will be on understanding quantum magnetism and improving the synthesis, there will also be a technological impact based on nanopatterned thin film devices containing LMMMs. In order to allow for the direct magnetic moment measurement of individual LMMMs, miniaturized SQUIDs, produced by nanolithography, are necessary.


 

[i] S.M.J. Aubin et al., Distorted MnIVMnIII Cubane Complexes as Single-Molecule Magnets, J. Am. Chem. Soc., 118, 7746 (1996) and references therein.

[ii] W. Wernsdorfer et al., Measurements of magnetization Switching in Individual Nickel Nanowires, Phys. Rev. B 55, 11552 (1997). W. Wernsdorfer et al., Experimental Evidence of the Néel-Brown Model of Magnetization Reversal, Phys. Rev. Lett. 78, 1791 (1997). And further work by same group of authors.

[iii] W. Wernsdorfer and R. Sessoli, Quantum Phase Interference and parity Effects in Magnetic molecular Clusters, Science 284, 133 (1999). W. Wernsdorfer et al., Observation of the Distribution of Molecular Spin States by Resonant Quantum Tunneling of the Magnetization, Phys. Rev. Lett. 82, 3903 (1999). And further work by same group of authors.

[iv] J. Tejada et al., cond-mat/0009432, M. Leuenberger et al., cond-mat/0011415.

[v] V.A. Kalatsky et al., Berry’s Phase for Large Spins in External Fields, Phys. Rev. Lett., 80, 1304 (1998). V.A. Kalatsky et al., Europhys. Lett. 49, 539 (1998). V.A. Kalatsky et al., Spectra and Magnetic Properties of Large Spins in External Fields, Phys. Rev. A 60, 1824 (1999).

[vi] See supporting documentation.

[vii] C. Chapelier et al., DC-SQUID with Aluminum Microbridges far from Tc, p. 286 in Superconducting Devices and Their Applications, eds. H. Koch, H. Lubbig, Springer (1992).

[viii] A. Pique, R. A. McGill, D. B. Chrisey, D. Leonhardt, T. E. Mslna, B. J. Spargo, J. H. Callahan, R. W. Vachet, R. Chung, M. A. Bucaro, Growth of organic thin films by matrix assisted pulsed laser evaporation technique, Thin Solid Films 355, 536 (1999).

[ix] P. K. Wu, J. Fitz-Gerald, A. Pique, D. B. Chrisey, R. A. McGill, Deposition of nanotubes and nanotube composites using matrix assisted pulsed laser evaporation, Mater. Res. Soc. Symp. Proc. 617, J2.3 (2001).

[x] D. M. Bubb, B. R. Ringeisen, J. H. Callahan, M. Galicia, A. Vertes, J. S. Horwitz, R. A. McGill, E. J. Houser, P. K. Wu, A. Pique, D. B. Chrisey, Vapor deposition of intact polyethylene glycol thin films, Appl. Phys. A 73, 121 (2001).

[xi] A. Pique, R. A. McGill, D. B. Chrisey, J. Callahan, T. E. Mslna, Matrix Assisted Pulsed Laser Evaporation (MAPLE) of Polymeric Materials: Methodology and Mechanistic Studies, Mater. Res. Soc. Symp. Proc. 526, 375 (1998).

[xii] A. Pique, P. Wu, B. R. Ringeisen, D. M. Bubb, J. S. Melinger, R. A. McGill, D. B. Chrisey, Processing of functional polymers and organic thin films by the matrix assisted pulsed laser evaporation, Appl. Surf. Sci. 186, 408 (2002).

[xiii] A. Pique, D. B. Chrisey, B. J. Spargo, M. A. Bucaro, R. W. Vachet, J. H. Callahan, R. A. McGill, D. Leonhardt, T. E. Mslna, Use of Matrix Assisted Pulsed Laser Evaporation (MAPLE) for the Growth of Organic Thin Films, Mater. Res. Soc. Symp. Proc. 526, 421 (1998).

[xiv] M. K. Nishimura, F. Tolanai, Y. Matsuo, T. Koboyashi, J. Kawai, K. Midorikawa, I. Tanihata, Y. Hayahizaki, Femtosecond laser ablation for simultaneous atomization and ionization (fs-SAI) of large organic molecules, RIKEN Review, 33, 18 (2001).

[xv] Y. Hosokawa, M. Yashiro, T. Asahi, H. Masuhara, Femtosecond laser ablation dynamics of amorphous film of a substituted Cu¯phthalocyanine, Appl. Surf. Sci. 154, 192 (2000).

[xvi] K. Hatanaka, Y. Tsuboi, H. Fukumura and H. Masuhara, Nanosecond and femtosecond laser photochemistry and ablation dynamics of neat liquid benzenes, J. Phys. Chem. B, 106, 3049 (2002).