Itinerant Spin Research Articles

Fermi Surface Properties of Ferromagnet UCu2Si2

We have succeeded in growing a high-quality single crystal of UCu 2 Si 2 with a tetragonal structure by the Sn-flux method and measured the de Haas–van Alphen effect. UCu 2 Si 2 orders antiferromagnetically below T N =106 K and follows a successive ferromagnetic ordering at T C =100 K. Several de Haas–van Alphen branches are observed in the ferromagnetic state, ranging from 5×10 6 to 7×10 7 Oe. The corresponding cyclotron effective masses are moderately large, ranging from 1.4 to 4.2 m 0 , consistent with the electronic specific heat coefficient γ=20 mJ/(K 2 ·mol). These experimental results are approximately explained by the 5 f -itinerant spin- and orbital-polarized energy band model.

Self-consistent treatment of spin and magnetization dynamic effect in spin transfer switching

The effect of itinerant spin moment ( m) dynamic in spin transfer switching has been ignored in most previous theoretical studies of the magnetization ( M) dynamics. Thus in this paper, we proposed a more refined micromagnetic model of spin transfer switching that takes into account in a self-consistent manner of the coupled m and M dynamics. The numerical results obtained from this model further shed insight on the switching profiles of m and M, both of which show particular sensitivity to parameters such as the anisotropy field, the spin torque field, and the initial deviation between m and M.

Sliding conductivity of a magnetic kink crystal in a chiral helimagnet

We derive a current-driven sliding conductivity of the magnetic kink crystal (MKC) in chiral helimagnet under weak magnetic field applied perpendicular to the helical axis. For this purpose, we discuss the correlated dynamics of quantum-mechanical itinerant spins and the MKC which are coupled via the sd exchange interaction. The itinerant spins are treated as fully quantum-mechanical operators whereas the dynamics of the MKC is considered within classical Lagrangian formalism. By appropriately treating elementary excitations around the MKC state, we construct coupled equations of motion for the collective coordinates (the center-of-mass position and quasi-zero-mode coordinate) associated with the sliding motion of the MKC. By solving them, we demonstrate that the correlated dynamics is understood through a hierarchy of two time scales: Boltzmann relaxation time ${\ensuremath{\tau}}_{\text{el}}$, when a nonadiabatic spin-transfer torque appears, and Gilbert damping time ${\ensuremath{\tau}}_{\text{MKC}}$, when adiabatic spin-transfer torque comes up. As a notable consequence, we found that the terminal velocity of the sliding motion reverses its sign depending on the band-filling ratio of the conduction electron system.

Mass modification of itinerant carriers in RKKY oscillations induced by finite-range exchange interactions

We investigate the Ruderman-Kittel-Kasuya-Yosida oscillations of the itinerant carrier spin density in a system where those oscillations appear only due to a finite distribution of a localized spin. The system represents a half-infinite one-dimensional quantum wire with a magnetic impurity located at its edge. In contrast to the conventional model of a pointlike exchange interaction the itinerant carrier spin-density oscillations in this system exist. We analytically demonstrate that when the radius of the exchange interaction is less than the wavelength of the itinerant carriers living on the Fermi surface, the long-range behavior of the oscillations is identical to the one taking place in the zero-radius limit of the same exchange interaction but for an infinite one-dimensional quantum wire where, in comparison with the original half-infinite system, the mass of the itinerant carriers is strongly modified by the exchange interaction radius. On the basis of our analysis we make a suggestion on directionality of surface Ruderman-Kittel-Kasuya-Yosida interaction shown in recent experiments: we believe that in general the anisotropy of the Ruderman-Kittel-Kasuya-Yosida interaction could result not only from the anisotropy of the Fermi surface of itinerant carriers but also from the anisotropy of the spin carrying atomic orbitals of magnetic impurities.

Adiabatic and nonadiabatic spin-transfer torques in the current-driven magnetic domain wall motion

A consistent theory to describe the correlated dynamics of quantum mechanical itinerant spins and semiclassical local magnetization is given. We consider the itinerant spins as quantum mechanical operators, whereas local moments are considered within classical Lagrangian formalism. By appropriately treating fluctuation space spanned by basis functions, including a zero-mode wave function, we construct coupled equations of motion for the collective coordinate of the center-of-mass motion and the localized zero-mode coordinate perpendicular to the domain wall plane. By solving them, we demonstrate that the correlated dynamics is understood through a hierarchy of two time scales: Boltzmann relaxation time when a non-adiabatic part of the spin-transfer torque appears, and Gilbert damping time when adiabatic part comes up.

Monte Carlo study of the spin transport in magnetic materials

The resistivity in magnetic materials has been theoretically shown to depend on the spin–spin correlation function which in turn depends on the magnetic field, the density of conduction electron, the magnetic ordering stability, etc. However, these theories involved a lot of approximations, so their validity remained to be confirmed. The purpose of this work is to show by newly improved extensive Monte Carlo (MC) simulation the resistivity of the spin resistivity from low- T ordered phase to high- T paramagnetic phase in ferromagnetic and antiferromagnetic films. We take into account the interaction between the itinerant spins and the localized lattice spins as well as the interaction between itinerant spins themselves. We show that in ferromagnets the resistivity shows a sharp peak at the magnetic phase transition in agreement with previous theories in spite of their numerous approximations. Resistivity in antiferromagnets on the other hand shows no peak for the SC, BCC and diamond lattices. Discussion on the origin of these resistivity behaviors is given.

Model of magnetic ordering in coupled electron and spin systems

We present a simple model for a description of ground-state properties of itinerant electrons interacting with localized spins via an anisotropic spin-dependent local interaction (Jz) supplemented by the Ising interaction (J1) between localized spins. The magnetic phase diagrams for both the itinerant electrons and the localized spins are analysed as functions of the spin-dependent interactions Jz and J1 as well as the total number of itinerant electrons (Nd). It is found that for both the ferromagnetic as well as antiferromagnetic Ising coupling J1 various new types of spin orderings exhibiting a domain structure are generated. The strong correlations between magnetic phase diagrams of itinerant electron and spin subsystems are observed for J1 > 0, while the coincidence is broken for J1 < 0.

Multi-Frequency EPR and DC Conductivity of Itinerant Spins in Single-Wall Carbon Nanotubes

Sealed, deoxygenated single-wall carbon nanotubes show two characteristic electron paramagnetic resonance (EPR) signals at g = 2.07 and g = 2.00 in the temperature range from 300 to 50 K. Reversible interconversion between both components was observed. The large g-shift and the temperature dependence of the EPR susceptibility of the g = 2.07 signal indicate that this signal can be attributed to itinerant spins. At low temperatures only the g = 2.00 signal remained, which could be further characterized using microwave frequencies up to 320 GHz. The direct current conductivity of a partially aligned sample was also measured. The room temperature value was estimated as 0.7 (Ωcm)−1. The observed temperature dependence can be described by assuming temperature-activated hopping in a small-gap semiconductor with an activation energy of 3.5 meV, similar to the characteristics of the previously measured 9.4 GHz microwave conductivity.

Emerging nonequilibrium bound state in spin-current–local-spin scattering

Department of Physics, University of Houston, Houston, Texas(Dated: November 5, 2009)Magnetization reversal is a well-studied problem with obvious applicability in computer hard-drives. One can accomplish a magnetization reversal in at least one of two ways: applicationof a magnetic field, or through a spin current. The latter is more amenable to a fully quantummechanical analysis. We formulate and solve the problem whereby a spin current interacts with aferromagnetic Heisenberg spin chain, to eventually reverse the magnetization of the chain. Spin-flips are accomplished through both elastic and inelastic scattering. A consequence of the inelasticscattering channel, when it is no longer energetically possible, is the occurrence of a new entity:a non-equilibrium bound state (NEBS), which is an emergent property of the coupled local plusitinerant spin system. For certain definite parameter values the itinerant spin lingers near the localspins for some time, before eventually leaking out as an outwardly diffusing state. This phenomenonresults in novel spin-flip dynamics and filtering properties for this type of system.INTRODUCTION

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.

Green’s function approach to the thermodynamic properties of the anisotropic Kondo necklace model

We have studied the two-dimensional anisotropic Kondo necklace model with antiferromagnetic (AF) Kondo coupling ${J}_{\ensuremath{\perp}}$ and exchange coupling between ``itinerant'' spins $J$ on the square lattice. The bond operator formalism is used to transform the spin model to a hard-core bosonic gas. We have used the Green's function approach to obtain the temperature dependence of spin excitation spectrum (triplet gap). We have also found the temperature dependence of the specific heat and the local spin-correlation function between localized and itinerant spins for various Kondo couplings ${J}_{\ensuremath{\perp}}/J$ and anisotropies in both coupling strengths. Furthermore we studied the temperature dependence of the structure factor for localized spins which is determined by effective interactions via itinerant spins. For low temperature and close to the quantum critical point we have obtained an analytical formula for temperature dependence of the energy gap and specific heat. Finally we compared our results with those of previous mean-field treatments.

Coexistence of static magnetism and superconductivity in SmFeAsO1−xFx as revealed by muon spin rotation

The recent observation of superconductivity with critical temperatures (Tc) up to 55 K in the pnictide RFeAsO(1-x)F(x), where R is a lanthanide, marks the first discovery of a non-copper-oxide-based layered high-Tc superconductor. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprate superconductors, as both families exhibit superconductivity following charge doping of a magnetic parent material. In this context, it is important to follow the evolution of the microscopic magnetic properties of the pnictides with doping and hence to determine whether magnetic correlations coexist with superconductivity. Here, we present a muon spin rotation study on SmFeAsO(1-x)F(x), with x=0-0.30 that shows that, as in the cuprates, static magnetism persists well into the superconducting regime. This analogy is quite surprising as the parent compounds of the two families have rather different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings therefore suggest that the proximity to magnetic order and associated soft magnetic fluctuations, rather than strong electronic correlations in the vicinity of a Mott-Hubbard transition, may be the key ingredients of high-Tc superconductors.

Rational Approaches to Organic Ferromagnets Based on Neutral Radicals Appended to Organosulfur Donors

The science of molecule-based materials is an exciting subject in light of its fundamental scientific interest and potential appli- cations. Considerable efforts are now being invested in developing strategies for the design and preparation of new materials involving an interplay or synergy between electrical conductivity and magnetic properties. This review examines approaches for the preparation of or- ganic conducting ferromagnets from organic spin systems in the form of stable organic radicals that are covalently appended to organo- sulfur donors. The concept of a spin-polarized donor and spin alignment via itinerant electrons is addressed, together with an overview of the most important classes of stable organic radicals. A full comprehensive discussion of progress and achievements in this area spanning the last 15 years is presented. in exceptional cases order ferromagnetically (12). In general, this approach has afforded materials in which the interactions between the two systems are weak. In contrast to the  -d model, there are remarkably few examples of rational synthetic strategies for the realization of purely organic conducting and magnetic materials (13). In this review we summarize the approaches that have been developed for the realization of organic spin systems that carry both localized and itinerant electron spins.

Evidence for Strong Itinerant Spin Fluctuations in the Normal State ofCeFeAsO0.89F0.11Iron-Oxypnictide Superconductors

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All of the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

Nanomechanical detection of itinerant electron spin flip

Electrons and other fundamental particles have an intrinsic angular momentum called spin. A change in the spin state of such a particle is therefore equivalent to a mechanical torque. This spin-induced torque is central to our understanding of experiments ranging from the measurement of the angular momentum of photons and the g-factor of metals to magnetic resonance and magnetization reversal in magnetic multilayers. When a spin-polarized current passes through a metallic nanowire in which one half is ferromagnetic and the other half is nonmagnetic, the spins of the itinerant electrons are 'flipped' at the interface between the two regions to produces a torque. Here, we report direct measurement of this mechanical torque in an integrated nanoscale torsion oscillator, and measurements of the itinerant electron spin polarization that could yield new information on the itinerancy of the d-band electrons. The unprecedented torque sensitivity of 1 x 10(-22) N-m Hz(-1/2) may have applications in spintronics and precision measurements of charge-parity-violating forces, and might also enable experiments on the untwisting of DNA and torque-generating molecules.

Magnetization relaxation and collective spin excitations in correlated double-exchange ferromagnets

We study spin relaxation and dynamics of collective spin excitations in correlated double-exchange ferromagnets. For this, we introduce an expansion of the Green's functions equations of motion that treats nonperturbatively all correlations between a given number of spin and charge excitations and becomes exact within a subspace of states. Our method treats relaxation beyond Fermi's golden rule while recovering previous variational results for the spin-wave dispersion. We find that the momentum dependence of the spin-wave dephasing rate changes qualitatively due to the on-site Coulomb interaction, in a way that resembles experiment, and depends on its interplay with the magnetic exchange interaction and itinerant spin lifetime. We show that the collective spin relaxation and its dependence on the carrier concentration depend sensitively on three-body correlations between a spin excitation and a Fermi sea electron and hole. The above spin dynamics can be controlled via the itinerant carrier population.

SWNT probed by multi‐frequency EPR and nonresonant microwave absorption

AbstractIn addition to g = 2.00 signals seen frequently in EPR spectra of SWNT, signals at g = 2.07 of SWNT prepared by CVD were detected, exhibiting a Pauli susceptibility temperature dependence. This Pauli magnetism in combination with the large g shift is taken as evidence that these signals originate from itinerant electrons of metallic nanotubes. At temperatures below 150 K, a dominant narrow signal develops at g = 2.00. By applying multifrequency EPR up to 319 GHz, its inhomogeneous nature was confirmed. This signal is assigned to defects of the carbon network of the tubes.Comparing room temperature EPR spectra of CVD and arc‐grown SWNT, we found a much lower concentration of metallic tubes in arc material. No g = 2.07 signals of itinerant spins could be observed, which might be also caused by the high amount of residing catalyst. A drastic increase in nonresonant microwave absorption is observed below 10 K for both types of samples, if a threshold microwave power level is passed. In the same temperature range a drop in EPR intensity is also detected. These observations are taken as evidence for a transition into a superconducting phase of part of the sample. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spin Dynamics in (III,Mn)V Ferromagnetic Semiconductors: The Role of Correlations

We address the role of correlations between spin and charge degrees of freedom on the dynamical properties of ferromagnetic systems governed by the magnetic exchange interaction between itinerant and localized spins. For this we introduce a general theory that treats quantum fluctuations beyond the random phase approximation based on a correlation expansion of the Green's function equations of motion. We calculate the spin susceptibility, spin-wave excitation spectrum, and magnetization precession damping. We find that correlations strongly affect the magnitude and carrier concentration dependence of the spin stiffness and magnetization Gilbert damping.

Temperature dependence of the spin resistivity in ferromagnetic thin films: Monte Carlo simulations

The magnetic phase transition is experimentally known to give rise to an anomalous temperature-dependence of the electron resistivity in ferromagnetic crystals. Phenomenological theories based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. In this paper, we show by extensive Monte Carlo (MC) simulation the behavior of the resistivity of the spin current calculated as a function of temperature ($T$) from low-$T$ ordered phase to high-$T$ paramagnetic phase in a ferromagnetic film. We analyze in particular effects of film thickness, surface interactions and different kinds of impurities on the spin resistivity across the critical region. The origin of the resistivity peak near the phase transition is shown to stem from the existence of magnetic domains in the critical region. We also formulate in this paper a theory based on the Boltzmann's equation in the relaxation-time approximation. This equation can be solved using numerical data obtained by our simulations. We show that our theory is in a good agreement with our MC results. Comparison with experiments is discussed.

Electron paramagnetic resonance investigation of metalloendofullerene derived carbon nanotube peapods

Single Wall Carbon Nanotubes (SWCNT) prepared by the "super growth" method and arc-grown material were used as templates for peapod preparation with La@C(82). A qualitative change of the electron paramagnetic resonance (EPR) properties of La@C(82) is observed after incorporation into SWNT. The loss of lanthanum hyperfine interaction in combination with the observed increase of EPR susceptibility by two orders of magnitude after peapod preparation when comparing with signals from "empty" tubes is indicative for the generation of itinerant spins by charge and spin transfer from La@C(82) to the tubes. This interpretation is supported by the observation of fast spin dephasing, detected with pulsed EPR techniques.