It has recently been shown that giant quantum oscillations above the energy barrier (GQOAB) of a spin $S$ can be coherently induced by the simultaneous application of $2S$ alternating fields (shaped field) associated with successive level separations [Phys. Rev. Lett. 131, 066701 (2023)]. We begin the present paper with a more detailed study of the properties of GQOAB, such as the dependence of their frequency on the value of the anisotropy constant or on that of the amplitude of the alternating fields. We then extend the concept of GQOAB to the case where these ac-fields are applied, not simultaneously, but sequentially, showing that such a protocol also leads to coherent GQOAB but with a number of differences that are discussed in detail. We conclude with a classical approach of this problem showing that ``giant classical oscillations above the barrier (GCOAB)'' can also be designed at zero Kelvin.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY2O4 single crystals doped with the Dy3+ Kramers ions (0.01 at. % and 0.5 at. %) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for Y3+ ions at two nonequivalent crystallographic sites with the same local Cs symmetry but strongly different crystal fields. Well -pronounced double -loop hysteresis is observed at temperatures 2, 4, 5, and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic, and kinetic properties of Dy3+ ions is developed based on the results of electron paramagnetic resonance, site -selective optical spectroscopy, and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time -dependent transition probabilities induced by the electron -phonon interaction, quantum tunneling, and cross relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate, and concentration of paramagnetic ions.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic ${\mathrm{SrY}}_{2}{\mathrm{O}}_{4}$ single crystals doped with the ${\mathrm{Dy}}^{3+}$ Kramers ions (0.01 at. % and 0.5 at. %) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for ${\mathrm{Y}}^{3+}$ ions at two nonequivalent crystallographic sites with the same local ${C}_{s}$ symmetry but strongly different crystal fields. Well-pronounced double-loop hysteresis is observed at temperatures 2, 4, 5, and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic, and kinetic properties of ${\mathrm{Dy}}^{3+}$ ions is developed based on the results of electron paramagnetic resonance, site-selective optical spectroscopy, and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling, and cross relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate, and concentration of paramagnetic ions.

This Letter deals with the broad class of magnetic systems having a single or collective spin S with an energy barrier, such as rare-earth elements and their compounds, single molecule magnets with uniaxial anisotropy, and more generally any other anisotropic quantum system made of single or multiple objects with discrete energy levels. Till now, the reversal of the magnetization of such systems at zero kelvin required making use of quantum tunneling with a significant transverse field or transverse anisotropy term, at resonance. Here, we show that another very simple method exists. It simply consists in the application of a particular sequence of electromagnetic radiations in the ranges of optical or microwave frequencies, depending on the characteristics of the system (spin and anisotropy values for magnetic systems). This produces oscillations of the Rabi type that pass above the barrier, thus extending these oscillations between the two energy wells with mixtures of all the 2S+1 states. In addition to its basic character, this approach opens up new directions of research in quantum information with possible breakthroughs in the current use of multiple quantum bits.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY_2O_4 single-crystals doped with the Dy^3+ Kramers ions (0.01 and 0.5 at. odd Dy isotopes are presented. Impurity ions substitute for Y^3+ ions at two nonequivalent crystallographic sites with the same local C_s symmetry but strongly different crystal fields. Well pronounced double-loop hysteresis is observed at temperatures 2, 4, 5 and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic and kinetic properties of Dy^3+ ions is developed based on the results of EPR, site selective optical spectra and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling and cross-relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate and concentration of paramagnetic ions.

Experimental and theoretical studies of the dynamic magnetization in swept magnetic fields of the orthorhombic SrY$_2$O$_4$ single-crystals doped with the Dy$^{3+}$ Kramers ions (0.01 and 0.5 at.%) with natural abundances of even and odd Dy isotopes are presented. Impurity ions substitute for Y$^{3+}$ ions at two nonequivalent crystallographic sites with the same local $C_s$ symmetry but strongly different crystal fields. Well pronounced double-loop hysteresis is observed at temperatures 2, 4, 5 and 6 K for sweeping rates of 5 and 1 mT/s. The microscopic model of spectral, magnetic and kinetic properties of Dy$^{3+}$ ions is developed based on the results of EPR, site selective optical spectra and magnetic relaxation measurements. The derived approach to the dynamic magnetization in the sweeping field based on the numerical solution of generalized master equations with time-dependent transition probabilities induced by the electron-phonon interaction, quantum tunneling and cross-relaxation allowed us to reproduce successfully the evolution of the hysteresis loop shape with temperature, sweeping rate and concentration of paramagnetic ions.

Understanding the damping is an important fundamental problem with widespread implications in magnetic technology. Ferrimagnetic materials offer a rich platform to explore not only the damping of the ferromagnetic mode, but also the damping of the high-frequency exchange mode very promising for ultrafast devices. Here we use time-resolved magneto-optical Kerr effect to investigate the ferromagnetic and exchange resonance modes and their damping in the bismuth-doped gadolinium iron garnet over a broad range of magnetic fields (0-10 T) and temperatures (50-300 K) including the magnetization and angular compensation points. These two resonance modes are excited via the inverse Faraday effect and unambiguously identified by their distinct frequency dependence on temperature and magnetic field. The temperature-dependent measurements in the external magnetic field H-ext = 2 T revealed that the intrinsic damping of the ferromagnetic mode is always smaller than the one of the exchange modes and both have a maximum near the angular compensation point. These results are fully consistent with recent predictions of atomistic simulations and a theory based on two-sublattice Landau-Lifshitz-Bloch equation. We also demonstrate that the damping of these modes varies differently as a function of H-ext. We explain the observed behaviors by considering the different features of the effective fields defining the precession frequencies of the ferromagnetic and exchange modes.

Using magneto-optical Faraday and Kerr measurements, we investigate the magnetic and magnetooptical properties of a thick Bi-substituted gadolinium iron garnet film over a broad range of wavelengths (250-850 nm) and temperatures (150-300 K), including the magnetization compensation point, TM. We observe an exchange-bias-like effect in the vicinity of TM. By slightly changing the sample temperature, we can precisely tune the bias field, which reaches a magnitude 6 times higher than the coercive field. We explain this phenomenon by considering the short-range superexchange interaction and a change in the magnetic behavior when moving from the surface to the bulk of the film. This finding may lead to the development of single-film magneto-optical devices based on the exchange-bias effect.

Louis Neel was a world-renowned scientist who devoted the research part of his multifaceted career to magnetism. Covering roughly the period 1930-1970, his work is explained for a non-specialized audience, with particular attention given to work published in the Comptes rendus hebdomadaires des seances de l'Academie des sciences. (C) 2019 Published by Elsevier Masson SAS on behalf of Academie des sciences.

This paper is a short overview of the works performed with Boris Malkin in a friendly and fruitful collaboration which started fifteen years ago and will, hopefully, be continuing for as many years as possible. Dealing with quantum tunnelling and coherence in non-cooperative magnetic systems such as 3d-based single-molecules or 4f-based diluted alloys, those works also involved our respective students, post-docs and colleagues of the time who all appear in the reference list. If most of those papers were published in usual physics journals, some of them were also published in the proceedings of conferences organized or co-organized by Boris or myself in Kazan, St.Petersburg or Les Houches.

In this work we explore the ultrafast magnetization dynamics induced by femtosecond laser pulses in a doped film of gadolinium iron garnet over a broad temperature range including the magnetization compensation point T-M. By exciting the phonon-assisted S-6 ->(4)G and S-6 -> P-4 electronic d-d transitions simultaneously by one- and two-photon absorption processes, we find out that the transfer of heat energy from the lattice to the spin has, at a temperature slightly below T-M, a large influence on the magnetization dynamics. In particular, we show that the speed and the amplitude of the magnetization dynamics can be strongly increased when increasing either the external magnetic field or the laser energy density. The obtained results are explained by a magnetization reversal process across T-M. Furthermore, we find that the dynamics has unusual characteristics which can be understood by considering the weak spin-phonon coupling in magnetic garnets. These results open new perspectives for controlling the magnetic state of magnetic dielectrics using an ultrashort optically induced heat pulse.

Numerical simulations of magnetization reversal of a quantum uniaxial magnet under a swept magnetic field [Hatomura et al., Phys. Rev. Lett. 116, 037203 (2016)] are extended. In particular, how the "wave packet" describing the time evolution of the system is scattered in the successive avoided level crossings is investigated from the viewpoint of the distribution of the eigenstate populations. It is found that the peak of the distribution as a function of the magnetic field does not depend on spin-size S, which indicates that the delay of magnetization reversal due to the finite sweeping rate is the same in both the quantum and classical cases. The peculiar synchronized oscillations of all the spin components result in the beating of the spin length. Here, dissipative effects on this beating are studied by making use of the generalized Lindblad-type master equation. The corresponding experimental situations are also discussed in order to find conditions for experimental observations.

We report the first observation of Rabi oscillations in the spin-7/2 ensemble of trivalent gadolinium ions hosted in CaWO$_4$ single crystal. A number of transitions within the lowest electronic multiplet $^8S_{7/2}$ of Gd$^{3+}$ ion are studied using a combination of continuous-wave and pulsed electron paramagnetic resonance spectroscopy. The corresponding Rabi damping curves and the spin coherence times are detected at varying strengths of the microwave field. These data are well reproduced by a theoretical model which accounts for the intrinsic inhomogeneity of the microwave field within the microwave resonator and the magnetic dipole interactions in the diluted spin ensemble. The results indicate that the studied 8-level ground manifold of Gd$^{3+}$ ion can represent an effective three qubit quantum system.

Femtosecond optical transients are extensively used to study electronic coherence and population relaxation in condensed matter. In this work we show that it is possible to determine the spin coherence and population dynamics in magnetic materials using magneto-optical (MO) four-wave mixing transients. The methodology is applied to Bi-doped garnet films where an ultrashort coherent MO response is observed during the laser pulse and attributed to the spin-orbit interaction. In addition, long-lived spin populations (T-1 = 1.35 ps) are shown to depend on the laser frequency detuning with respect to Fe3+ electronic resonance. Otherwise, our approach paves the way for realizing diffractive MO systems that can be controlled with external magnetic fields. (C) 2017 Optical Society of America

In magnetic materials, the exchange is the strongest quantum interaction due to the Pauli exclusion principle. For that reason it can induce high-frequency modes f(exch) of the magnetization precession. In this work we investigate these modes over a wide range of temperatures (50-300 K) and magnetic fields up to 10T in a bismuth-doped garnet with perpendicular magnetic anisotropy by performing femtosecond magneto-optical pump-probe experiments. Near the compensation temperature T-M the divergence of 1/f(exch)(T) allows identifying unambiguously f(exch) with the rare-earth <-> iron exchange mode. In addition, at low temperature f(exch) is independent of the field as usually observed. In contrast, we find that near T-M, f(exch) decreases linearly with an increasing magnetic field. This behavior is explained in the context of the ferromagnetic resonance theory by including the perturbation term linear in the external applied field H-ext.

The quantum mechanical counterpart of the famous Stoner-Wohlfarth model-an easy-axis magnet in a tilted magnetic field-is studied theoretically and through simulations as a function of the spin size S in a sweeping longitudinal field. Beyond the classical Stoner-Wohlfarth transition, the sweeping field-induced adiabatic change of states slows down as S increases, leading to a dynamical quantum phase transition. This result gives us new insights to describe the collapse of the metastability from the viewpoint of a critical phenomenon associated with the Landau-Zener tunneling gaps. Furthermore, a beating of the amplitude of the magnetization (the spin-length fidelity) is discovered after the Stoner-Wohlfarth transition. The period of the beating, confirmed analytically, arises from a new type of quantum phase factor.

Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope (STM) tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au(111) at a temperature of 4.6 K in the presence of H2. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H2 pinning at an atomic cluster.