Local Spin Dynamics Research Articles

Anomalous Magnetic Phase in an Undistorted Pyrochlore Oxide Cd2Os2O7 Induced by Geometrical Frustration

We report on the muon spin rotation/relaxation study of a pyrochlore oxide, Cd 2 Os 2 O 7 , which exhibits a metal–insulator (MI) transition at T MI ≃225 K without structural phase transition. It reveals strong spin fluctuation (≥10 8 s -1 ) below the MI transition, suggesting a predominant role of geometrical spin frustration amongst Os 5+ ions. Meanwhile, upon further cooling, an incommensurate spin density wave discontinuously develops below T SDW ≃150 K. These observations strongly suggest the occurrence of an anomalous magnetic transition and associated change in the local spin dynamics in undistorted pyrochlore antiferromagnet.

Local spin dynamics in magnetic molecular chains studied by NMR and μSR

We present NMR and μ +SR study of spin dynamics in one-dimensional and quasi-one-dimensional molecular magnets of recent synthesis. In particular, we focus on the so called Gd(hfac) 3NIT-R and CoPhOMe magnetic chains families. For Gd-R helimagnets we show some differences between “weakly frustrated systems” and “fully frustrated systems”. The different behaviour is due to the different radical inserted in the chains (R = Me, Ph for “weakly frustrated systems” and R = iPr, Et for “fully frustrated systems”). The existence of different phase transitions, particularly to 3D long-range magnetic order in Gd-Ph and to chiral order in Gd- iPr, is remarked together with a comparison between results obtained from macroscopic and microscopic investigating techniques. As regards CoPhOMe slowly relaxing chain, the 1H NMR measurements confirm the freezing of the spins at low temperature, which prevents the 3D long-range order, and display the presence of two relaxation mechanisms related to distinct contributions to the local spin relaxation.

Hysteresis behavior of the random-field Ising model with 2-spin-flip dynamics: exact results on a Bethe lattice

We present an exact treatment of the hysteresis behavior of the zero-temperature random-field Ising model on a Bethe lattice when it is driven by an external field and evolved according to a 2-spin-flip dynamics. We focus on lattice connectivities z=2 (the one-dimensional chain) and z=3. For the latter case, we demonstrate the existence of an out-of-equilibrium phase transition, in contrast with the situation found with the standard 1-spin-flip dynamics. We discuss the influence of the degree of cooperativity of the (local) spin dynamics of the nonequilibrium response on the system.

Localization of 4 f State in YbRh2Si2 under Magnetic Field and High Pressure: Comparison with CeRh2Si2

We present detailed measurements of the electrical resistivity and microcalorimetry under magnetic field and high pressure which confirm the proximity of YbRh 2 Si 2 to a quantum critical point at ambient pressure. Our data seems to rule out the divergence of the heavy quasiparticle mass at the magnetic field tuned quantum critical point. Further Mössbauer spectroscopy gives evidence that the observed ESR resonance cannot be assigned directly to the trivalent Yb local spin dynamics. We report for the first time on quantum oscillations in YbRh 2 Si 2 . The obtained angular dependence is compared with band-structure calculation. A significant change of the resistivity and specific heat under high pressure and magnetic field is observed. Above 5 GPa ferromagnetic coupling plays the dominant role. The unusual high pressure phase diagram of YbRh 2 Si 2 cannot be explained in term of the Doniach phase diagram. In Yb heavy fermion case, slow valence fluctuations may occur due to an occupation number of the trivalent state which differs notably from unity. The differences between the Ce and Yb heavy fermion systems are discussed. In particular a comparison is made between CeRh 2 Si 2 and YbRh 2 Si 2 . The observed field and pressure variations of the Hall voltage in CeRh 2 Si 2 are not straightforwardly linked to the change in the Fermi surface topology. A tiny change in the 4 f electron occupation number will have a huge effect on the effective mass of different orbits of the complex band structure and thus on the Hall effect in this multiband electronic structure. A similar scenario must be certainly considered for YbRh 2 Si 2 .

Formula omitted] NMR study of [formula omitted

We have performed 11 B nuclear magnetic resonance (NMR) measurements on the single crystals of GdB 4 to investigate local electronic structure and 4f spin dynamics. 11 B NMR spectrum, Knight shift, spin–lattice and spin–spin relaxation rates, 1 / T 1 and 1 / T 2 , were measured down to 4 K at 8 T . 11 B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Gd. Below T N , the single broad resonance peak of 11 B NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The spin–lattice relaxation rate, independent of temperature above T N , decreases significantly below T N confirming huge suppression of spin fluctuation.

11B NMR study of [formula omitted

We have performed 11B nuclear magnetic resonance (NMR) measurements on a single crystal of TbB 4 to investigate local electronic structure and 4f spin dynamics. 11B NMR spectrum, Knight shift, spin–lattice and spin–spin relaxation rates, 1 / T 1 and 1 / T 2 , were measured down to 5 K at 8 T. 11B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. Below T N , the single broad resonance peak of 11B NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The spin–lattice relaxation rate, independent of temperature above T N , decreases significantly below T N confirming huge suppression of spin dynamics.

Magnetic microstructures and their dynamics studied by X-ray microscopy

Full-field soft X-ray microscopy in combination with X-ray magnetic circular dichroism as contrast mechanism is a powerful technique to image with elemental specificity magnetic nanostructures and multilayered thin films at high lateral resolution down to 15 nm by using Fresnel zone plates as X-ray optical elements. Magnetization reversal phenomena on a microscopic level are studied by recording the images in varying external magnetic fields. Local spin dynamics at a time resolution below 100 ps can be addressed by engaging a stroboscopic pump-and-probe scheme taking into account the time pattern of synchrotron storage rings. Characteristic features of magnetic soft X-ray microscopy are reviewed and an outlook into future perspectives with regard to increased lateral and temporal resolution is given.

Qubit coherence control in a nuclear spin bath

Coherent dynamics of localized spins in semiconductors is limited by spectral diffusion arising from dipolar fluctuation of lattice nuclear spins. Here we extend the semiclassical theory of spectral diffusion for nuclear spins $I=1∕2$ to the high nuclear spins relevant to the III-V materials and show that applying successive qubit $\ensuremath{\pi}$ rotations at a rate approximately proportional to the nuclear spin quantum number squared $({I}^{2})$ provides an efficient method for coherence enhancement. Hence robust coherent manipulation in the large spin environments characteristic of the III-V compounds is possible without resorting to nuclear spin polarization, provided that the $\ensuremath{\pi}$ pulses can be generated at intervals scaling as ${I}^{\ensuremath{-}2}$.

Spin Wave Spectrum of a Disordered Double-Exchange Model

A double-exchange model with quenched disorder for conduction electrons is studied by field theoretical methods. Using a path integral formalism and replica techniques based on it, an ensemble-averaged spin wave dispersion of localized spins is derived. It is shown that the spectrum of the spin wave has gaps at the multiple of the Fermi wave number of the conduction electrons in the presence of disorder, and hence, quenched disorder for electrons adds a striking effect to the dynamics of localized spins. In the strong disorder limit, the present results suggest a spin-glass-like behavior due to the frustration of the exchange coupling.

Dynamics of localized Mn spins in diluted‐magnetic‐semiconductor nanostructures with quantum dots

AbstractDynamics of spin–lattice relaxation of the magnetic Mn‐ion system is investigated for nanostructures with (Cd,Mn)Te and (Cd,Mn)Se diluted‐magnetic‐semiconductor (DMS) quantum dots. A nonequilibrium phonon technique with optical detection of the induced heating of the magnetic ion system was exploited. A strongly nonexponential decay of the spin dynamics is found to be specific for the DMS quantum dots. It is explained in the frame of a model suggesting that the spin dynamics in quantum dots is strongly affected by a spin diffusion into/from a DMS wetting layer, where the spin–lattice relaxation rate differs from that in the quantum dots. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Local spin dynamics of magnetic moments on metal surfaces

We present a theory of the local spin dynamics of an isolated magnetic moment either embedded within or adsorbed on a metal surface, within the framework of an itinerant electron description of the combined system. We explore issues that are relevant to probes of local spin dynamics via scanning tunneling microscope (STM) tips. Explicit calculations are presented for Co and Mn moments on the Cu(100) surface. We describe the system via a mean-field description of the ground state, and a treatment of the spin dynamics via the random-phase approximation. Thus, the discussion is appropriate to a system at temperatures above the Kondo temperature, and in a magnetic field sufficiently high the moment is pinned parallel to the field. Attention is directed to the following issue. If, in the absence of spin-orbit coupling or other terms in the Hamiltonian that breaks spin rotation invariance, any experiment (such as electron-spin resonance) which measures the total magnetic moment of the system necessarily sees a response with zero linewidth, at precisely $g=2.$ However, we show that a local probe such as a STM tip will see a broad structure with origin in decay of the moment's precession to particle-hole pairs, whose peak displays an appreciable g shift. We illustrate with explicit calculations directed to the systems mentioned above.

A new mean field theory for Ising spin-fermion model: application to diluted magnetic semiconductors

We have investigated the ferromagnetic transition of an Ising spins-fermions model, where Ising spins are located randomly and tight-binding fermions interact with the Ising spins via the exchange interaction. We extended the inhomogeneous mean field theory to include the local dynamics of localized spins and applied to this model. We have found that the ferromagnetic moment is not saturated because of localization of fermions due to the randomness.

Spin dynamics and spin disorder in frustrated TbCo xNi 1− xC 2

The orthogonal dicarbides RTC 2 ( R= rare earth , T= transition metal ) show ferromagnetic (fm) order of the R ions below T C ≈30 K for T=Co with a being the easy axis and antiferromagnetic (afm) order (with about the same transition temperature) for T=Ni with c as easy axis. The spin turning is caused by changes in the crystalline electric field (CEF) interaction. In the mixed TbCo x Ni 1− x C 2 alloys, fm and afm exchange on the one side and different CEF interactions on the other compete as a function of x. These competing interactions are a source of magnetic frustration. They also cause the materials to undergo several magnetic transitions into fm, afm or spin modulated structures which are well documented by neutron diffraction. Zero, longitudinal and transverse field μSR was carried out on TbCo x Ni 1− x C 2 with x=0.3, 0.5 0.7 and 0.85 using the same samples as in the neutron studies. Marked local spin disorder and persistent spin dynamics were seen in all magnetically ordered states as a consequence of frustration, but distinct differences exist for different values of x. Not all reported phase transition show clearly in the μSR data. In TbCo 0.85Ni 0.15C 2, a short-range dynamically correlated spin state having a strong fm polarizability exists above T C =30 K extending for about 50 K .

Spin freezing and magnetic inhomogeneities in bilayer manganites.

We have performed a muon spin rotation study on polycrystalline samples of electron-doped layered manganites, La2-2xSr1+2xMn2O7 (0.4< or =x<1), in order to investigate the local magnetic structure and spin dynamics. Our results provide evidence for phase separation into A-type antiferromagnetic and charge-ordered phases for x=0.52 and spin freezing at low temperatures (T<100 K) for 0.52< or =x<0.75. A new phase diagram which includes this spin-freezing region is proposed.

Combined k-Space q-Space Pulsed ESR Imaging: Mapping of Restricted Diffusion in (FA) 2PF 6

A (FA) 2PF 6 crystal from the family of the quasi-one-dimensional organic conductors was selectively damaged by a beam of Helium ions with a slitted mask placed in the beam's trajectory. Pulsed ESR density weighted imaging of the damaged crystal revealed the appearance of regions where the ESR signal was absent. The one-dimensional motion of the charge carriers was thus restricted to the undamaged sections. The local charge carrier spin dynamics in these restricted areas was probed by combined k-space q-space pulsed ESR imaging. The local expected appearance of the restricted pulsed gradient spin echo (PGSE) “diffusive diffraction” effect is shown. The position of the diffraction minima is compatible with the density imaging results.

Glassy spin dynamics in non-fermi-liquid UCu5-xPdx, x = 1.0 and 1.5.

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.

11B NMR study of TbNi 2B 2C

11B pulsed NMR measurements have been performed to investigate local electronic structure and 4f spin dynamics for TbNi 2B 2C single crystal. 11B NMR shift and linewidth are huge and strongly temperature dependent. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. The shift and the spin-lattice relaxation rate show high anisotropy for field parallel and perpendicular to the c-axis. Anisotropy of the shift and the relaxation rates suggests that the hyperfine field perpendicular to the c-axis is larger.

Charge localization and magnetic dynamics in manganites

The local spin dynamics in the ‘ paramagnetic’ phase of ferromagnetic and charge-ordered Mn perovskites has been investigated by μ +SR. A glassy like spin dynamics is found in the first which has been completely suppressed in samples with dominant AF exchange interactions. The observation of a fall-off in the relaxation rate at T CO is also discussed in terms of the spin and charge dynamics.

Charge localization and magnetic dynamics in ferromagnetic and charge-ordered manganites

The local spin dynamics in the paramagnetic phase of ferromagnetic and charge-ordered Mn perovskites has been investigated by muon spin relaxation. Significant differences are observed in the muon relaxation in ferromagnetic (F) and antiferromagnetic (AF) Ln1−xCaxMnO3 samples with x=1/3 and 1/2. A glassy like spin dynamics is found close to TC in the former which is completely suppressed in samples with dominant AF exchange interactions. The observation of a falloff in the muon relaxation rate at TCO in Ln1/2Ca1/2MnO3 is also discussed.

Coherent dynamics of localized spins coupled with a two-dimensional hole gas in diluted-magnetic quantum wells

Spin dynamics in a quantum well made of diluted-magnetic semiconductor is studied theoretically. The exchange interaction of the ensemble of localized spins with two-dimensional heavy-hole gas is shown to affect time evolution of the spin system in an in-plane magnetic field. The Larmor frequency of localized spins is reduced under influence of an oscillating effective field of holes.