Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

Anomalous ferromagnetism in manganites by finite block spin phenomenology

We investigate the relation between ferromagnetism and spin glasses which have been observed in manganese oxides.We want to explain the spin-glass phase on the basis of finite-sized block spin concepts.Then the phase of colossal ferromagnetism in manganites may be considered as the ferromagnetic ordering between block spins comprised of many random spins with respective majority spin directions. Using the Curie law for block spins, the magnetization and susceptibility are obtained in the lower-temperature and higher-temperature approximations of Brillouin function. The resistivity is also obtained from the electric susceptibility.     

Electron correlations, spontaneous magnetization and momentum density in quantum dots

The magnetization of quantum dots (QDs) is discussed in terms of a relatively simple but exactly solvable model Hamiltonian. The model predicts oscillations in spin polarization as a function of dot radius for a fixed electron density. These oscillations in magnetization are shown to yield distinct signature in the momentum density of the electron gas, suggesting the usefulness of momentum resolved spectroscopies for investigating the magnetization of dot systems. We also present variational quantum Monte Carlo calculations on a square dot containing 12 electrons in order to gain insight into correlation effects on the interactions between like and unlike spins in a QD.     

Roles of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins in a spatial and time varying magnetization M(r,t) in the time-dependent semiclassical transport theory. We show that the induced nonequilibrium conduction spin density in turn generates four spin torques acting on the magnetization-with each torque playing a different role in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques which has not been previously identified is crucial to consistently interpreting experimental data on domain wall motion.     

Optical control of coherent interactions between electron spins in InGaAs quantum dots

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with μeV strength.     

Effect of optical spin injection on ferromagnetically coupled Mn spins in the III-V magnetic alloy semiconductor (Ga,Mn)As

We report on the new type of photoinduced magnetization in ferromagnetic (Ga,Mn)As thin films. Optically generated spin-polarized holes change the orientation of ferromagnetically coupled Mn spins and cause a large change in magnetization, being 15% of the saturation magnetization, without the application of a magnetic field. The memorization effect has also been found as a trace after the photoinduced magnetization. The observed results suggest that a small amount of nonequilibrium carrier spins can cause collective rotation of Mn spins presumably through the p-d exchange interaction.     

Vector model of electron spin echo envelope modulation due to nuclear hyperfine and zeeman interactions

The transverse electron spin magnetization of a paramagnetic center with effective spinS=1/2 interacting with nonquadrupolar nuclei may be presented as a function of pairs of nuclei magnetization vectors which precess around the effective magnetic field directions. Each vector of the pair starts its precession perpendicular to both effective fields. The free induction decay (FID) signal is proportional to the scalar product of the vectors for nuclear spinI=1/2. The electron spin echo (ESE) signal can be described with two pairs of magnetization vectors. The ESE shape is not equal to two back-to-back FID signals except in the absence of ESE envelope modulation. A recursion relation is obtained which allows calculation of ESE signals for larger nuclear spins in the absence of nuclear quadrupole interaction. This relation can be used to calculate the time course of the ESE signal for arbitrary nuclear spins as a function of the nuclear magnetization vectors. While this formalism allows quantitative calculation of modulation from nuclei, it also provides a qualitative means of visualizing the modulation based on simple magnetization vectors.     

Skyrmion strings and the anomalous Hall effect in CrO2

Topological (or singularity point) defects are thought to play a crucial role in the phase transitions of 3D spin systems, as they do in such 2D systems as the XY model. In double-exchange ferromagnets the conduction electrons are strongly coupled with core spins through Hund's rule, and, in the presence of a nontrivial spin texture, acquire a Berry phase contribution to the anomalous Hall effect. We combine Hall effect and magnetization data on CrO2 with a thermodynamical scaling hypothesis to confirm that the critical behavior of the topological-spin-defect density is consistent with that of the heat capacity. This analysis is the first experimental confirmation of the topological character of critical fluctuations.     

Spectrophotometric and E.P.R. study of TCNE charge-transfer complexes

Liquid state charge-transfer complexes formed from the strong acceptor tetracyanoethylene (TCNE) and four donors of like donor properties are studied by spectrophotometric and E.P.R. techniques. If the enthalpy of complex formation ΔH is sufficiently large, the triplet state of the complex ³C will be appreciably populated thermally during complex formation. The E.P.R. signal of the TCNE anion promptly arises because of a dissociation of ³C, reaches a maximum, and then follows second-order decay kinetics. The higher the dielectric constant of the donor, the slower the decay rate. The initial intensity of the E.P.R. signal is proportional to the original TCNE concentration in the donor-solvent. Reversible photoinduced TCNE anions are formed by irradiating the complexes in the charge-transfer band. The relationship between the prompt and photoinduced E.P.R. signals is shown. Singlet-triplet separation energies are estimated.     

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100 mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.     

Viscous behavior in a quasi-1D fractal cluster glass

The spin glass transition of a quasi-1D organic-based magnet ([MnTPP][TCNE]) is explored using both ac and dc measurements. A scaling analysis of the ac susceptibility shows a spin glass transition near 4 K, with a viscous decay of the thermoremanent magnetization recorded above 4 K. We propose an extension to a fractal cluster model of spin glasses that determines the dimension of the spin clusters (D) ranging from approximately 0.8 to over 1.5 as the glass transition is approached. Long-range dipolar interactions are suggested as the origin of this low value for the apparent lower critical dimension.     

Photoinduced magnetism and random magnetic anisotropy in organic-based magnetic semiconductor V(TCNE){x} films, for x approximately 2

The V(TCNE){x}, x approximately 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE){x} film. Upon optical excitation (lambda approximately 457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE){x} induced by the increased structural disorder in the system.     

One-Step Implementation of Single Spin Measurement in Spin Star Network

We present an efficient one-step scheme for a single spin measurement based on nuclear magnetic resonance （NMR） techniques. This scheme considerably reduces the time of operation using a spin star network where a target spin and an ancillary spin are coupled to a ring of N spins. As opposed to the proposal in [Phys. Rev. Lett. 97 （2006） 100501] using a cubic lattice crystal to achieve a cubic speedup, the distinct advantage of this scheme is that under ideal conditions it requires the application of only one step to create a system of N correlated spins. In the process of single spin measurement, the total macroscopic magnetization, the individual magnetization and the transfer fidelity are calculated analytically as simple cosine functions of time and the amplitude of irradiation.     

Electrical spin injection from an organic-based ferrimagnet in a hybrid organic-inorganic heterostructure

We report the successful extraction of spin-polarized current from the organic-based room temperature ferrimagnetic semiconductor V[TCNE](x) (x～2, TCNE: tetracyanoethylene; T(C)～400 K, E(G)～0.5 eV, σ(300?K)～10(-2) S/cm) and its subsequent injection into a GaAs/AlGaAs light-emitting diode. The spin current tracks the magnetization of V[TCNE](x～2), is weakly temperature dependent, and exhibits heavy-hole-light-hole asymmetry. This result has implications for room temperature spintronics and the use of inorganic materials to probe spin physics in organic and molecular systems.     

Electron spin precession upon reflecting from ferromagnetic surfaces

Electrons with the polarization vector perpendicular to the magnetization of Fe, Co, and Ni films are spin analyzed after reflecting from the ferromagnet. At low primary electron energies a strong spin motion is found, namely, a precession of the polarization around the magnetization and a change of the angle between the polarization and the magnetization. This observation can be accounted for by the existence of spin-dependent gaps in the electronic band structure of the ferromagnets.     

Current-induced spin polarization in strained semiconductors

The polarization of conduction electron spins due to an electrical current is observed in strained nonmagnetic semiconductors using static and time-resolved Faraday rotation. The density, lifetime, and orientation rate of the electrically polarized spins are characterized by a combination of optical and electrical methods. In addition, the dynamics of the current-induced spins are investigated by utilizing electrical pulses generated from a photoconductive switch. These results demonstrate the possibility of a spin source for semiconductor spintronic devices without the use of magnetic materials.     

Magnetization of Paramagnetic Ions by Phase-Modulated Microwave Pulses in a Zero Magnetic Field

Physics of the Solid State - The effect of magnetization formation for paramagnetic ions with spin S = 1 in a solid under a zero magnetic field by pulsed manipulations with electron spins has been...     

Motional-narrowing-type dephasing of electron and hole spins of itinerant excitons in magnetically doped II-VI bulk semiconductors

Time-resolved optical spin-quantum-beat measurements performed on magnetically doped II-VI bulk semiconductors reveal an increase of the electron spin dephasing time with rising temperature typical for motional narrowing. With the dephasing being notably faster than in undoped II-VI semiconductors, the magnetic dopants must play a key role, modifying the known dephasing mechanisms and introducing new ones. Focusing on the latter, we theoretically explore the spin dephasing channel arising from magnetization fluctuations sampled by the itinerant excitons. This mechanism suffices to explain quantitatively the results of our time-resolved Faraday-rotation experiments on optically excited Cd(1-x)Mn(x)Te which we present here as a function of magnetic field, temperature and manganese dopant density. In addition to electron spin-quantum beats, some of our experiments reveal hole spin beats as well.     

Spherical spin waves

The spin waves in a two-sublattice ferrimagnetic sphere are derived from the equation of motion for the sublattice magnetization. The pinning conditions are discussed and the results are compared with those obtained by numerical diagonalization of corresponding Heisenberg hamiltonian. For a sphere with the diameter of about 40 lattice constants the continuum approximation is fully applicable for estimation of some (long wavelength) spin wave energies and angle of the core spins. The transition probabilities seem to be considerably overestimated by this method.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.The author wishes to thank Dr. V. Kamberský for many valuable discussions which enabled him to improve the text.     

Ergodicity and efficiency of cross-polarization in NMR of static solids

Cross-polarization transfer is employed in virtually every solid-state NMR experiment to enhance magnetization of low-gamma spins. Theory and experiment is used to assess the magnitude of the final quasistationary magnetization amplitude. The many-body density matrix equation is solved for relatively large (up to N=14) spin systems without the spin-temperature assumption for the final spin states. Simulations show that about 13% of the thermodynamic limit is still retained within the proton bath. To test this theoretical prediction, a combination of a reverse cross-polarization experiment and multiple contacts is employed to show that the thermodynamic limit of magnetization cannot be transferred from high- to low-gamma nuclei in a single contact. Multiple contacts, however, fully transfer the maximum magnetization. A simple diffusion on a cone model shows that slow dynamics can affect the build up profile for the transferred magnetization.