Molecular magnets contain a large number of (nearly) identical magnetic clusters of total spin on the order of 10. Borderline between classical and quantum magnetism, these nanomagnets hold promise for high-density information storage and, possibly, quantum computation. The goal of the present project is to gain a deeper understanding of Mn-12, a prototypical member of the class. (1) High resolution spectroscopic measurements will be performed using synchrotron radiation over a very broad range of frequencies; the aim is to investigate the internal degrees of freedom of the magnetic clusters and to study the line-widths and line shapes which will yield information regarding disorder, internal magnetic fields and electron-phonon coupling strengths; magnetic avalanches will be studied to determine whether they are accompanied by coherent super-radiant emission, as has been recently suggested; (2) Micro-Hall bar detectors will be used to investigate the local spatial and temporal evolution of spin reversal inside the material as the magnetization relaxes toward equilibrium under controlled conditions and during magnetic avalanches. Participation in forefront research in the laboratory provides high-level education and training for both graduate and undergraduate students. It should be noted that City College draws its undergraduate student population from every economic stratum and from diverse ethnic and racial backgrounds. %%%

Molecular nanomagnets, sometimes referred to as "single molecule magnets" are potential candidates for high-density information storage and quantum computation. Quantum computation is a new and entirely different computing paradigm based on quantum phenomena. Rather than having two possible "classical" values, 0 or 1, the quantum mechanical elements of quantum computers, called "qubits", represent a far broader set of possibilities, enabling much greater computing power. The goal of the current proposal is to gain a deeper understanding of a prototypical member of this class of materials, Mn12-acetate, which consists of a very large number of identical clusters of 12 manganese atoms that form magnetic molecules regularly arranged in an organic crystal; each molecule is a little magnet that is equivalent to 20 times the magnetism of a single electron. Spectroscopic studies will be performed at low temperatures at the synchrotron light source at Brookhaven, and measurements will be taken at City College of New York of the local magnetization at low temperatures on a length scale of microns using very small Hall bars. Participation in forefront research in the laboratory provides high-level education and training for both graduate and undergraduate students. It should be noted that City College draws its undergraduate student population from every economic stratum and from very diverse ethnic and racial backgrounds

Project Report

INTELLECTUAL MERIT: A crystal of molecular magnets consists of a regular array of (nearly) identical molecules, each of which is a magnet that is large by atomic/molecular standards. These materials are exceptionally interesting for their fundamental physical properties and have potential applications for high-density storage of information and for possible use as qubits for quantum information, quantum computation and quantum cryptography. Our research has focused on Mn_12-acetate, a relatively simple and highly symmetric prototype. Quantum tunneling, the central feature that makes Mn-12 interesting, was discovered in our laboratory during an earlier period. During the last grant period: (1) we performed a series of detailed studies to better understand the behavior of Mn-12; (2) we investigated the effect of randomness on the magnetic response; and (3) we discovered that the reversal of the magnetic moment (north pole/south pole) that often occurs as a sudden magnetic "avalanche" proceeds along a traveling front in a process that is analogous to chemical combustion . (1) There is currently a widespread search for methods to deposit individual Mn-12-ac molecules in a two-dimensional array so that they can be addressed and controlled individually for use as computer elements. Understanding the effect of randomness and disorder is clearly central to this quest. Our studies of the effect of crystalline randomness constitute an important step toward this goal. Moreover, the "Random Field Ising Ferromagnet" has been a fundamentally interesting problem for many years: Mn-12 is only the second realization of the model, providing new opportunities for study. (2) We have discovered that the occasional, abrupt switch of the polarity (referred to as a magnetic avalanche) of a fully magnetized Mn-12-ac crystal proceeds as a "deflagration" front along which the individual magnetic molecules reverse their magnetic polarity. Deflagration is a reaction-diffusion process in which a locally burning substance increases the temperature of adjacent unburned substance and ignites it giving rise to a combustion front (as in burning paper) that propagates at subsonic speed via thermal conduction. We discovered and investigated the magnetic analog, in which the reaction is the reversal of the magnetic direction of a particular molecule which locally introduces Zeeman energy that diffuses and heats the adjacent layer. It is important to note that unlike chemical deflagration, magnetic deflagration is nondestructive and reversible, allowing a controlled investigation of this process measurements can be repeated on a given sample and parameters can be controllably tuned. BROADER IMPACT: A major goal of our efforts has been to inspire and train the scientific workforce of the future, and to stimulate youngsters’ interest in the sciences; we targeted students at all levels. At the top end, graduate students executed forefront experiments in fulfillment of the requirements for a Ph. D. degree. The many undergraduates who have spent time working in our laboratory comprise a diverse group that has included women, African American, Hispanic, Arabic students, Chinese students, others. A few did special "honors" projects for which they received academic credit, some received stipends from the grant or through special programs, and some did this for their own pleasure and gratification. A couple of examples: Kurt James, an exceptionally bright African-American undergraduate student carried out a sophisticated analysis that compared our data with theory; he was a coauthor of the paper that reported these results. He continued his education as a graduate student at Cornell University. Ricardo Gonzalez from the Dominican Republic, one of the best physics majors we’ve had in recent years, continued his education as a graduate student at MIT. The lab has hosted a number of high school students. For example, during one summer a student from a nearby high school measured and analyzed the resistivity of a series of samples between 1.8 K and room temperature. Her project was submitted to the Intel talent search, and the NYC EXPO Science Fair; a Stuyvesant High School student spent a very productive summer last year working with members of the group – he is currently working toward his Ph. D. in Physics at Princeton University. We mentored Harlem middle school students (Waciuma Maina, Letisia Quezada, Johnnery De Jesus, Omar Faueras, Nicholas Medina, Wacira Maina) through the STARS program, a collaboration between the neighboring Mott Hall School and City College. The goal of STARS is to empower motivated minority students to pursue careers in STEM. This is a one-to-one mentoring exercise where students spent three hours a week working in the lab from October through May. One of these students, Waciuma, is currently enrolled at CCNY working toward his MS degree in Biology. Graduate students took the lead in this very important activity – which benefited the mentors as well as the mentees. The graduate students gained teaching experience and an interest in expending some of their energy to activities beyond their own personal goals and careers.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Daniele Finotello
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CUNY City College
New York
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