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The mesoscopic tunnelling of magnetization: A historic milestone in twentieth-century science

The tunnel effect of magnetization, a highly unusual property of the world of quantum mechanics discovered by the Magnetism Group of the Department of Fundamental Physics at the University of Barcelona (UB), led by Professor Javier Tejada (Castejón, 1948), in cooperation with groups from the City University of New York and the firm Xerox, has been acknowledged as one of the milestones in the study of spin of the twentieth century in the special collection Milestones in Spin, published by the leading scientific journal Nature.

This is the first time that a Spanish physicist has received credit in this country as the discoverer of a new physical phenomenon: the mesoscopic tunnelling of magnetization in molecular magnets (Physical Review Letters, 1996). This scientific breakthrough has now made its way into textbooks on magnetism.

It explains how the magnetic poles of small magnets, formed by millions of atoms, at very low temperatures, can change orientation due to the tunnel effect and without any energy expenditure. Now, the journal Nature has recognized this discovery as a historic milestone in the science of spin (the property of elementary particles to rotate around their axis in relation to their magnetic field).

Milestones in Spin also records the contributions of great figures in the world of physics, such as Nobel Prize winners Albert Einstein (1921); Paul A. M. Dirac (1933); Otto Stern (1943); Felix Bloch and E. M. Purcell (1952), Douglas Osheroff, Robert Richardson and David Lee (1996); Frank Wilczek, David Gross and David Politzer (2004), and Albert Fert and Peter Grünberg (2007).

The uncertainty principle governs the world of quantum physics: it is impossible to know the position and the momentum of an object at the same time. This is a property of quantum objects and does not depend on the ability to make an exact measurement. This uncertainty, at macroscopic scale, cannot be detected experimentally, and this has generated intense scientific debate in the frontiers of quantum physics and in the mesoscopic world. For the researchers, the tunnel effect is an unexpected consequence of quantum mechanics, and the magnets of mesoscopic size are the best systems for detecting quantum tunnelling phenomena.

The collection Milestones in Spin highlights 23 historic events in the study of spin, from the discovery of the first physical phenomenon in this field (the Zeeman effect, 1896) until the present day. Milestone 22, entitled «Mesoscopic tunneling of magnetization» is the quantum tunnel effect in magnetic poles, a phenomenon discovered by Javier Tejada, J. R. Friedman, M. Sarachik and Ron Ziolo and described in the article «Macroscopic measurement of resonant magnetization tunneling in high-spin molecules» (Physical Review Letters, 1996).

In this study, the scientists showed that the reorientation of the magnetic poles of mesoscopic sized magnets occurs due to quantum tunnelling, a curious property of the quantum world according to which an elementary particle can disappear and reappear outside the space in which it is confined.

In accordance with the theories of the physicist Eugene Chudnovsky on the tunnel effect, Tejada and his co-workers study the magnetism of mesoscopic magnets, and have discovered new fundamental laws of quantum phenomena in magnetism: the first experimental evidence of the tunnel effect of magnetization (1992), the resonant spin tunnelling (1996), quantum spin coherence (1999) and quantum magnetic deflagration (2005). Science, Nature, and Physics Today are some of the international journals that reported the new physical effect, discovered in 1996 by the research team at the UB and in the United States.

In the area of spin physics, the article «Field tuning of thermally activated magnetic quantum tunnelling in Mn12-Ac molecules», was the second to report evidence of the tunnel effect using an independent technique, published in the journal Europhysics Letters (1996), by the researchers Javier Tejada, Joan Manel Hernàndez and X. X. Zhang of the Department of Fundamental Physics at the UB, in conjunction with F. Luis and J. Bartolomé of the Materials Science Institute of Aragon and Ron Ziolo of the Xerox Corporation in New York.

The expectations raised by resonant spin tunnelling in the field of applied and basic physics open up new frontiers for the study of new macroscopic quantum phenomena and the testing of theories.

Winner of the Principe de Viana Prize for Culture in 2006, and recipient of an honorary doctorate from the City University of New York in 1996, Javier Tejada is a renowned expert in the field of magnetism and in the study of quantum effects in magnetism and superconductivity using microwaves and acoustic waves as high frequency. Professor of the Department of Fundamental Physics, he is the director of the UBX and the Magnetism Group at the UB, and is a member of the Spanish Royal Society of Physics, the Catalan Society of Physics, the New York Academy of Sciences and the American Physical Society.

Javier Tejada is the author of more than 280 scientific studies in leading journals such as Science, Physical Review Letters, Physical Review B, Europhysics Letters, Applied Physics Letters and Nature Materials. Tejada, who holds fifteen international patents in cooperation with firms and institutions, has been admitted as a Fellow of the American Physical Society (2000), and has received the Narcis Monturiol Medal from the Catalan government (1994), the International Award of the Xerox Foundation (1998) and the Catalan government distinction for the Promotion of University Research (2001).

Rosa Martínez | alfa
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