Spin Dynamics

This thesis reports on electron transport and the magnetization dynamics of crystalline multilayers grown on Fe-whiskers(001) and clean GaAs(001) wafers by means of molecular beam epitaxy (MBE). The high quality magnetic multilayers with well defined interfaces are required to allow one to compare quantitatively the experimental results with the theoretical predictions. The electrical properties of crystalline Fe/MgO/Fe-whisker structures were characterized by in-situ scanning tunneling spectroscopy. Far mast of the scanned area, the tunneling I-V characteristics have revealed a tunneling barrier of 3.6 V which corresponds to the perfect MgO layer. At negative bias voltages, the localized spikes in the tunneling current have been observed indicating ballistic transport in crystalline tunnel junctions. Kerr microscopy has shown, that the magnetization of Fe-whisker and Fe film are coupled via stray field of the Fe-whisker domain wall. Atom force microscope (AFM) operating in an exter...

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic ...

The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.

Copper metaborate CuB 2O 4 has been payed attention in the point that the magnetic soliton lattice appears. With decreasing temperature this material shows two step successive phase transition. Around the second phase transition temperature T ∗˜10 K the magnetic soliton lattice has been reported to appear from the result of the higher order magnetic satellites observed in the neutron diffraction experiment. In order to study the spin dynamics of the soliton lattice a nuclear magnetic resonance experiment on 11B in CuB 2O 4 has been performed. A preliminary result on the powder sample is reported.

The results of two techniques of dipolar recoupling, REDOR and CPMAS, are compared in the case of a coupled multiple-spin system. A fundamentally different behavior is observed for these two techniques. In REDOR, the terms associated with each interaction S-I(k) commute with each other and no truncation takes place so that each addition of spin I(k) causes a splitting with its dipolar frequency. In CPMAS, the flip-flop terms of the dipolar Hamiltonian do not commute with the dominant term from the strongly coupled spin pair so that the weak coupling terms from the neighboring spin I(k) are effectively truncated by the dominant pair interaction. Spin dynamics calculations are in agreement with the experimental data in a cubane shaped cluster.

We present certain exact analytical results for dynamical spin correlation functions in the Kitaev Model. It is the first result of its kind in non-trivial quantum spin models. The result is also novel: in spite of presence of gapless propagating Majorana fermion excitations, dynamical two spin correlation functions are identically zero beyond nearest neighbor separation, showing existence of a gapless but short range spin liquid. An unusual, \emph{all energy scale fractionization}of a spin -flip quanta, into two infinitely massive $\pi$-fluxes and a dynamical Majorana fermion, is shown to occur. As the Kitaev Model exemplifies topological quantum computation, our result presents new insights into qubit dynamics and generation of topological excitations.

We present here the results of a detailed investigation on magnetic and transport properties of La 0.5Sr 0.5Co 1- xFe xO 3 ( x=0, 0.1) compounds. It is shown that the Curie temperature ( TC) decreases slightly and the resistivity increases, even with a very small extent of Fe substitution. There was an evident coexistence of cluster glass (CG) and spin glass (SG) behaviors for x=0 composition. The Fe substitution induced additional antiferromagnetic Fe-O-Fe interactions that suppress strongly the ferromagnetism and conductivity, thus causing a phase separation for x=0.1 composition. We also figure out the nature of CG and SG behaviors and the influence of Fe doping on magnetic and transport properties of such a cobaltite system relying on electron paramagnetic resonance spectra.

Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La 1-x Ca x MnO 3 in the ferromagnetic regime (0≤x<1 / 2). For x = 1 / 3 (T C = 250 K) where the magnetoresistance effects are largest the system behaves as an ideal ...

We discuss the quantum and classical dynamics of a particle with spin in the gravitational field of a rotating source. A relativistic equation describing the motion of classical spin in curved spacetimes is obtained. We demonstrate that the precession of the classical spin is in a perfect agreement with the motion of the quantum spin derived from the Foldy-Wouthuysen approach for the Dirac particle in a curved spacetime. We show that the precession effect depends crucially on the choice of a tetrad. The results obtained are compared to the earlier computations for different tetrad gauges.

The networks formed from the links between telephones observed in a month's call detail records (CDRs) in the UK are analyzed, looking for the characteristics thought to identify a communications network or a social network. Some novel methods are employed. We find similarities to both types of network. We conclude that, just as analogies to spin glasses have proved fruitful for optimization of large-scale practical problems, there will be opportunities to exploit the statistical mechanics of the formation and dynamics of social networks in today's electronically connected world.

Inelastic neutron scattering measurements on the low energy spin waves in CaFe2As2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data.

We show a method to accelerate quantum adiabatic dynamics of wavefunctions under electro-magnetic field by developing the previous theory (Masuda & Nakamura 2008 and 2010). Firstly we investigate the orbital dynamics of a charged particle. We derive the driving field which accelerates quantum adiabatic dynamics in order to obtain the final adiabatic states except for the spatially uniform phase such as the adiabatic phase in any desired short time. Fast-forward of adiabatic squeezing and transport in the electro-magnetic field is exhibited. Secondly we investigate spin dynamics under the magnetic field, showing the fast-forward of adiabatic spin inversion and of adiabatic dynamics in Landau-Zener model. The connection of the present framework with Kato-Berry's transitionless quantum driving is elucidated in Appendix.

A magnetic susceptibility which decreases with decreasing temperature is observed in all $CuO_2$ based superconductors with less than optimal doping. We propose that in $La_{2-x} Sr_x CuO_4$ this is due to antiferromagnetic ordering which is prevented by the low spatial dimensionality while in $YBa_2 Cu_3 O_{6.6}$ it is due to the interplay between antiferromagnetic fluctuations within a plane and singlet pairing of electrons between nearest neighbor planes.

We present measurements of resonant tunneling through discrete energy levels of a silicon double quantum dot formed in a thin silicon-on-insulator layer. In the absence of piezoelectric phonon coupling, spontaneous phonon emission with deformation-potential coupling accounts for inelastic tunneling through the ground states of the two dots. Such transport measurements enable us to observe a Pauli spin blockade due to effective two-electron spin-triplet correlations, evident in a distinct bias-polarity dependence of resonant tunneling through the ground states. The blockade is lifted by the excited-state resonance by virtue of efficient phonon emission between the ground states. Our experiment demonstrates considerable potential for investigating silicon-based spin dynamics and spin-based quantum information processing.