B. Barbara

Quantum oscillations in a molecular magnet

The term ‘molecular magnet’ generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field. There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits—‘qubits’, the basic building blocks of a quantum computer—based on molecular magnets. But strong decoherence must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/2 spins, leading to an S = 1/2 collective ground state. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 μs (at liquid helium temperatures) are a realistic prospect.

Quantum tunneling of magnetization - QTM '94

Preface. I: General Introduction. Macroscopic Quantum Effects in Magnetic Systems: An Overview A.J. Leggett. II: Particles (Theory). Theory of Mesoscopic Quantum Tunneling in Magnetism: A WKB Approach J.L. van Hemmen, A. Suto. Quantum Spin-Tunneling: A Path Integral Approach R. Schilling. Quantum Tunneling in Small Particles E.M. Chudnovsky. Macroscopic Quantum Tunneling in Ferromagnets and Antiferromagnets H. Simanjuntak. Monte-Carlo Simulations on Reversal Magnetization of Small Clusters P.A. Serena, N. Garcia. III: QTM in Magnetic Relaxation (Experiment). Quantum Tunneling of Magnetization J. Tejada, X. Zhang. Relaxation and Mesoscopic Quantum Tunneling of Magnetization in Amorphous Rare-Earth Alloys J.I. Arnaudas, et. al. Linear Response and Thermal Equilibrium Noise of Magnetic Materials at Low Temperature: Logarithmic Relaxation, VF Noise, Activation and Tunnelling S. Vitale, et al. AC Susceptibility Relaxation Studies on a Manganese Organic Cluster Compound: MN12Ac M.A. Novak, R. Sessoli. Evidence for Quantum Tunnelling of the Magnetization in Mn12Ac C. Paulsen, J.-G. Park. Acoustic and Magnetic Properties of Rare-Earth-Ion-Doped Glasses: Elastic and Magnetic Tunnelling States G. Bellessa, et al. DC- Squid Magnetization Measurements of Single Magnetic Particles W. Wernsdorfer, et al. IV: Macroscopic Coherence of Magnetization (Experiment). Macroscopic Quantum Tunnelling of Magnetization in Natural and Artificially Engineered Ferritin S. Gider, D.D. Awschalom. V: Quantum Tunnelling of Domain Walls (Experiment). Domain Wall Tunnelling in a One Dimensional Ferromagnet K. Hong, N. Giordano. VI: Dissipation inQTM (Theory). Nuclear Spin Dissipation in Magnetic Macroscopic Quantum Phenomena A. Garg. Macroscopic Quantum Tunnelling and Dissipation of Domain Wall in Ferromagnetic Metals G. Tatara, H. Fukuyama. The Collective Coordinate Method and Bloch Wall Motion A.O. Caldeira. VII: Spin Parity Effects in QTM (Theory). Spin Parity Effects and Macroscopic Quantum Coherence of Bloch Walls H.-B. Braun, D. Loss. Spin Environments and the Suppression of Quantum Coherence N.V. Prokofev, P.C.E. Stamp. Unconventional Environments P.C.E. Stamp. VIII: QTM & Electron-Electron Interactions (Theory). Particle Tunneling and Magnetization Tunneling in the Presence of Electron-Electron Interaction J.M.P. Carmelo, F. Guinea. IX: Comments on Theory and Experiment. On the Search for Quantum Tunneling of Magnetization L. Gunther. X: Macroscopic Quantum Tunneling in Superconductors (Experiment). Quantum Tunneling of Vortices in High-Tc Superconductors: Magnetic Relaxation Experiments in T1BaCACuO Compounds D. Fiorani, et al. Crossover from Thermal to Quantum Regime in Vortex Motion in Conventional Type II Superconductors: Slow Magnetic Relaxation and Abrupt Flux Jumps M. Uehara. Flux Motion by Quantum Tunneling A.C. Mota. Macroscopic Quantum Tunneling in Long Josephson Junctions O.G. Symko. XI: Quantum Computers (Theory). Quantum Computing and Spin Physics D.P. DiVincenzo. XII: Scientific Summary of Workshop. B. Barbara, L. Gunther, A.J. Leggett. Index.

Carrier-induced ferromagnetism in p − Zn 1 − x Mn x Te

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped

DC-SQUID magnetization measurements of single magnetic particles

{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}

Macroscopic quantum tunneling in antiferromagnets

epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

QUANTUM TUNNELING OF MAGNETIZATION IN SOLIDS

We present the first magnetization measurements of single submicronic particles at very low temperature made of either Ni, Co, CoZrMoNi or TbFe. As detector we use a micro-bridge-DC-SQUID deposited onto or next to the sample. Sample and detector are patterned by electron-beam lithography. The dynamics and the temperature dependence of the magnetization reversal is studied in view of Macroscopic Quantum Tunneling (MQT).

Macroscopic quantum tunneling in molecular magnets

Abstract Starting from the Neel state of a uniaxial antiferromagnetic particle, we show that, due to the tunneling of the Neel vector between easy directions, the ground state of a sufficiently small particle is a quantum superposition of two equivalent Neel states. A certain orientation of the Neel vector becomes frozen as the volume of the particle grows, or the dissipation due to the interaction of the Neel vector with microscopic degrees of freedom increases. For the weak dissipation, which is mostly the case, the crossover from classical to quantum regime occurs at temperature T∗∼(ϵaϵe) 1 2 TN, where ϵa and ϵ e are anisotropy and exchange constants, TN is the Neel temperature.

Nuclear Spin Driven Quantum Relaxation in LiY0.998Ho0.002F4

Magnetic solids should, under certain circumstances, show macroscopic quantum behavior, in which coherence exists between completely distinct magnetization states, each involving a very large number of spins (~1012 spins). This article reviews the recent work in this field, concentrating particularly on macroscopic quantum tunneling (MQT) of magnetization. The two main phenomena discussed are (a) the tunneling of magnetization in singledomain particles or grains (in which some 103−104 spins rotate together through an energy barrier), and (b) the tunneling of domain walls in films or in bulk magnets; where walls containing ~ 1010 spins may tunnel off a pinning potential, or from one pinning centre to another. Some attention is also given to the quantum nucleation of magnetization reversal in a bulk magnet, and to the quantum motion of other magnetic solitons (such as vortices). After a thorough analysis of the basic grain and wall tunneling phenomena, we continue on to a discussion of the various dissipative or “decoherence” mechanisms, which destroy the phase correlations involved in tunneling. The coupling of grain magnetization to phonons, photons, and electrons is shown to have little consequence for weaklyconducting or insulating grains. Domain walls couple to these and also to magnons and impurities or defects; the 3rd order coupling to magnons can have serious effects, but if one uses pure insulators at low temperatures, these can also be ignored. As a result, theory indicates that MQT should be visible in both grains and bulk magnets at low temperatures (at least below ~1 K). The present experimental evidence for such behavior is inconclusive, partly because few experiments have been done. We discuss these experiments, and make some suggestions for future work. It is hoped this review will stimulate such work, not only because of the fundamental interest in macroscopic quantum phenomena, but also because of the considerable scope for technological innovation.

Magnetic ordering and anomalous ground state in CeAl2

Abstract Our present understanding of the phenomenon of quantum tunneling of magnetization (QTM), is reviewed in the light of the experiments performed on the molecular complex Mn 12 -ac. This system, in which QTM was clearly shown for the first time, consists of molecules with mesoscopic spins in dipolar interactions. Both single and many-molecule effects are essential for the observation of QTM (crystal field, hyperfine and dipolar interactions), which allows one to make a link between mesoscopic physics and magnetism.

Results from the OSQAR photon-regeneration experiment : No light shining through a wall

Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.