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Single Molecule Magnets

Introduction

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SMMs consist of a core of strongly exchange-coupled transition metal ions that collectively have a large magnetic moment per molecule. SMM crystals have a number of advantages over other types of magnetic nanostructures. Most importantly, they are monodisperse—each molecule in the crystal has the same spin, orientation, magnetic anisotropy and atomic structure. They thus enable fundamental studies of properties intrinsic to magnetic nanostructures that have previously been inaccessible. One of the most important results obtained with these compounds is the observation of quantum tunneling between different states of the magnetization of the molecule, leading to a step-wise hysteresis magnetization curve with acceleration of the magnetic relaxation for fields where quantum tunneling is turned on [1,2]. Such quantum effects are also widely studied in superconducting quantum interference devices (SQUIDs) [3] and quantum dots [4]. Condensed matter systems like these, with discrete energy levels coupled to the environment in the solid, are important to understand the border between quantum and classical physics, and the decoherence in quantum systems [5,6]. From a technological point of view, these systems have several new potential applications. For example, SMMs have recently been proposed for use in quantum computing and quantum information storage [7]. They also represent the ultimate limit to classical magnetic information storage, with one bit per molecule and possible storage densities many orders of magnitude greater than present day magnetic media. Understanding the quantum properties of these materials and how these relate to their structural characteristics will enable the creation of new and more adequate molecular compounds to study fundamental aspects of nanoscale physics and prototypes for quantum information technology.

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Magnetization and EPR studies of the single molecule magnet Ni4 with integrated sensors

Integrated magnetic sensors that allow simultaneous EPR and magnetization measurements have been developed to study single molecule magnets. A high frequency microstrip resonator has been integrated with a micro-Hall effect magnetometer. EPR spectroscopy is used to determine the energy splitting between the low lying spin states of a Ni4 single crystal, with an S=4 ground state, as a function of applied fields, both longitudinal and transverse to the easy axis at 0.4K. Concurrent magnetization measurements show changes in spin population associated with microwave absorption. Such studies enable determination of the energy relaxation time of the spin system.

J. Appl. Phys. 101, 09E104 (2007). pdf_logo04

Interdisciplinary research

Research on SMMs is a multidisciplinary field, which involves close collaboration between Physics and Chemistry. The research objectives of this proposal are focused on the understanding of fundamental aspects of the physicals of these materials, as well as possible applications in information technology.

This is a rapidly advancing field with many recent fundamental discoveries. For example, recently a crossover between quantum and thermal regimes of the magnetic relaxation has been observed and studied [9]. Berry phase interference between different quantum tunneling trajectories has also been found [10], with minima in the tunnel rates for discrete values of an external transverse magnetic field. This allows turning on and off the quantum tunneling by slightly varying the magnitude of the applied magnetic field. Moreover, a very recent observation established that quantum magnetic states of neighboring molecules can be quantum entangled [11]. Also, the observation of quantum coherence between superposition states of opposite spin projections has been achieved recently by the PI and collaborators at NYU [12]. These findings led to an extension of the field since 1996, involving more than twenty experimental groups around the world, working with many different measurement techniques. These include dc and ac magnetometry, high-frequency electron paramagnetic resonance (EPR), neutron scattering or calorimetric experiments as well as some new high sensitivity measurement techniques that have been developed in the field (i.e. micro-Hall effect and micro-SQUID magnetometries). Moreover, magnetic fields are always necessary in the study of magnetic systems. Particularly in these materials, the ability to apply high magnetic fields is a precious tool that can be used to tune the energy scale of the spin system and allows the use of experimental techniques with very different characteristic measure-times, ranging from d.c. magnetometry to high frequency EPR, NMR or infrared spectroscopy.

All these techniques of measurement have led to an increasing number of fundamental physical and chemical properties of SMMs to be examined, providing a better understanding of the principles that govern the quantum behavior of nanoscale magnetic systems. For example, the PI has recently found that the causes and origin of MQT in SMMs are due to changes (disorder) in the symmetry of the molecules and molecular constituents (solvent molecules) of these materials [13,14]. This research answered fundamental questions about the nature of MQT in SMMs that, which remained poorly understood from the beginning of the field, and established that the structural composition and symmetry of SMMs determine the quantum behavior of the magnetization. Therefore, structural characteristics such as symmetry, composition, spin, anisotropy or inter- and intra-molecule interactions of SMMs can be chemically engineered in order to obtain more appropriate materials for a specific purpose.

In general, SMMs allow the study of a number of fundamental phenomena of quantum mechanics in the magnetism of nanostructures and constitutes a bottom-up approach to nanomagnetism molecular engineering.

Applications: Quantum information processes

Among the proposed applications of SMMs in technology (i.e. magnetic cooling [15], microwave lasers [16], etc…), some suggest that it is possible to use them as magnetic molecular qubits for quantum information and computation processes [7]. For an isolated spin, e.g. initially up, the transverse terms in the Hamiltonian that give rise to QTM would lead to coherent oscillations between the unperturbed up and down spin states at a frequency proportional to the tunnel splitting—this is known as quantum coherence. Coherent oscillations of many periods are necessary for quantum computing with SMMs. However, the electronic spins in SMMs interact with their environment which includes nuclear spins, lattice vibrations and other SMMs in the crystal that introduce decoherence processes. Decoherence is a general problem for quantum computation and must be reduced as much as possible. In SMMs, the degree of decoherence is unknown and has only been indirectly estimated [11,12]. Via the synthesis of new materials and the use of new measurement methods that will be developed in our group, it will be possible to study how decoherence appears and what are the causes of its appearance and its relation to the internal structure and composition of a SMM.

An understanding of the dynamics of MQT in SMMs is indispensable to the application of these systems for quantum computing and quantum information process.

References

[1]. J. R. Friedman, M. P. Sarachik, J. Tejada and R. Ziolo, Phys. Rev. Lett.76, 3830 (1996); J. M. Hernandez et al., Europhys. Lett. 35, 301 (1996); L. Thomas, F. Lionti, R. Ballou, D. Gatteschi, R. Sessoli and B. Barbara, Nature 383,145 (1996).

[2] E. M. Chudnovsky and J. Tejada, Macroscopic Quantum Tunneling of Magnetic Moment. Cambridge University Press, Cambridge, England, (1998)

[3] C. H. Van der Val, A. C. J. ter Haar, F. K. Wilhelm, R. N. Schouten, C. J. P. M. Harmans, T. P. Orlando, Seth Lloyd, and J. E. Mooij, Science 290, 773 (2000); I. Chiorescu, Y. Nakamura, C. J. P. Harmans and J. E. Mooij, Science 299, 1869 (2003).

[4] Y. Nakamura, Yu. A. Pashkin, J. S. Tsai, Nature 398, 786 (1999); Y. A. Pashkin, T. Yamamoto, O. Astafiev, Y. Nakamura, D. V. Averin, J. S. Tsai, Nature 421, 823 (2003).

[5] A. J. Leggett, Prog. Theor. Phys. Suppl. 69, 80 (1980).

[6] See, for example, E. M. Chudnovsky, Phys. Rev. Lett. 92, 120405 (2004), and references herein.

[7] M. N. Leuenberger and D. Loss, Nature 410, 789 (2001); J. Tejada, E. M. Chudnovsky, E. del Barco and J. M. Hernandez, Nanotechnology 12, 181 (2001).

[8] A. J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboud and G. Christou, arxiv:cond-mat/0404625 (2004).

[9] L. Bokacheva, A. D. Kent, and M. A. Walters, Phys. Rev. Lett. 85, 4803 (2000); A. D. Kent, Y. Zhong, L. Bokacheva, D. Ruiz, D. N. Hendrickson and M. P. Sarachik, Europhys. Lett. 49, 521 (2000).

[10] W. Wernsdorfer and R. Sessoli, Science 284, 133 (1999).

[11] W. Wernsdorfer, N. Aliaga-Alcalde, D. N. Hendrickson, G. Christou, Nature 416, 406 (2002); S. Hill, R. S. Edwards, N. Aliaga-Alcalde and G. Christou, Science 302, 1015 (2003).

[12] E. del Barco, A. D. Kent, E. C. Yang and D. N. Hendrickson, arXiv:cond-mat/0405331 (2004).

[13] E. del Barco, A. D. Kent, E. M. Lumberger, D. N. Hendrikson and G. Christou, Phys. Rev. Lett. 91, 047203 (2003).

[14] E. del Barco, A. D. Kent, S. Hill, J. M. North, N.S. Dalal, E. M. Rumberger, D. N. Hendrickson, N. Chakov and G. Christou, arXiv:cond-mat/0404390 (2004).

[15] M. Sales, J. M. Hernandez, J. Tejada, and J. L. Martínez, Phys. Rev. B60, 14557 (1999).

[16] E. M. Chudnovsky and D. A. Garanin, Phys. Rev. Lett. 89, 157201 (2002).

[17] R. S. Edwards, S. Maccagnano, E. C. Yang, S. Hill, W. Wernsdorfer, D. Hendrickson and G. Christou, J. App. Phys. 93, 7807 (2003)..

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