Itinerant Spin Research Articles

Effects of ferromagnetic ordering and phase transition on the resistivity of spin current

It has been shown experimentally a long time ago that the magnetic ordering causes an anomalous behavior of the electron resistivity in ferromagnetic crystals. Phenomenological explanations based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. We show by extensive Monte Carlo simulation that this behavior is also observed for 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 ferromagnet. We show, in particular, that across the critical region, the spin resistivity undergoes a huge peak. The origin of this peak is shown to stem from the formation of magnetic domains near the phase transition. The behavior of the resistivity obtained here is compared to experiments and theories. A good agreement is observed.

Phase diagram of the one-dimensional anisotropic Kondo-necklace model

The one-dimensional anisotropic Kondo-necklace model has been studied by several methods. It is shown that a mean field approach fails to gain the correct phase diagram for the Ising-type anisotropy. We then applied the spin wave theory which is justified for the anisotropic case. We have derived the phase diagram between the antiferromagnetic long range order and the Kondo singlet phases. We have found that the exchange interaction $(J)$ between the itinerant spins and local ones enhances the quantum fluctuations around the classical long range antiferromagnetic order and finally destroy the ordered phase at the critical value ${J}_{c}$. Moreover, our results show that the onset of anisotropy in the $XY$ term of the itinerant interactions develops the antiferromagnetic order for $Jl{J}_{c}$. This is in agreement with the qualitative feature which we expect from the symmetry of the anisotropic $XY$ interaction. We have justified our results by the numerical Lanczos method where the structure factor at the antiferromagnetic wave vector diverges as the size of system goes to infinity.

Spin transport in magnetic multilayers

We study by extensive Monte Carlo simulations the transport of itinerant spins travellinginside a multilayer composed of three ferromagnetic films antiferromagneticallycoupled to each other in a sandwich structure. The two exterior films interactwith the middle one through non-magnetic spacers. The spin model is the Isingone and the in-plane transport is considered. Various interactions are taken intoaccount. We show that the current of the itinerant spins going through this systemdepends strongly on the magnetic ordering of the multilayer: at temperaturesT below (above) thetransition temperature Tc, a strong (weak) current is observed. This results in a strong jump of the resistance acrossTc.Moreover, we observe an anomalous variation, namely a peak, of the spin current in the critical region justabove Tc. We show that this peak is due to the formation of domains in the temperature region between thelow-T ordered phase and the true paramagnetic disordered phase. The existence of such domainsis known in the theory of critical phenomena. The behaviour of the resistanceobtained here is compared to a recent experiment. An excellent agreement with ourphysical interpretation is observed. We also show and discuss the effects of variousphysical parameters entering our model, such as interaction range, strength ofelectric and magnetic fields and magnetic film and non-magnetic spacer thicknesses.

Ultrafast demagnetization in thesp−dmodel: A theoretical study

We propose and analyze a theoretical model of ultrafast light-induced magnetization dynamics in systems of localized spins that are coupled to carriers' spins by $sp\text{\ensuremath{-}}d$ exchange interaction. A prominent example of a class of materials falling into this category are ferromagnetic (III, Mn)V semiconductors, in which ultrafast demagnetization has been recently observed. In the proposed model, light excitation heats up the population of carriers, taking it out of equilibrium with the localized spins. This triggers the process of energy and angular momentum exchange between the two spin systems, which lasts for the duration of the energy relaxation of the carriers. We derive the master equation for the density matrix of a localized spin interacting with the hot carriers and couple it with a phenomenological treatment of the carrier dynamics. We develop a general theory within the $sp\text{\ensuremath{-}}d$ model and we apply it to the ferromagnetic semiconductors, taking into account the valence band structure of these materials. We show that the fast spin relaxation of the carriers can sustain the flow of polarization between the localized and itinerant spins leading to significant demagnetization of the localized spin system, observed in (III, Mn)V materials.

Field-induced quantum phase transition in the anisotropic Kondo necklace model

The anisotropic Kondo necklace model in 2D and 3D is treated as a genuine model for magnetic to Kondo singlet quantum phase transitions in the heavy fermion (HF) compounds. The variation of the quantum critical point (QCP) with anisotropy parameters has been investigated previously in the zero field case [1]. Here we extend the treatment to finite fields using a generalised bond operator representation including all triplet states. The variation of critical tc with external field H and the associated phase diagram is derived. The influence of anisotropies and the different g-factors for localised and itinerant spins on tc(H) is also investigated. It is found that three different types of behaviour may appear: (i) Destruction of antiferromangetism and appearance of a singlet state above a critical field. (ii) The inverese behaviour, namely field induced antiferromagnetism out of the Kondo singlet phase. (iii) Reentrance behaviour of the Kondo singlet phase as function of field strength.

Unified Origin for the 3D Magnetism and Superconductivity inNaxCoO2

We analyze the origin of the three-dimensional (3D) magnetism observed in nonhydrated Na-rich Na(x)CoO2 within an itinerant spin picture using a 3D Hubbard model. The origin is identified as the 3D nesting between the inner and outer portions of the Fermi surface, which arise due to the local minimum structure of the a(1g) band at the Gamma-A line. The calculated spin wave dispersion strikingly resembles the neutron scattering result. We argue that this 3D magnetism and the spin fluctuations responsible for superconductivity in the hydrated systems share essentially the same origin.

Nonlinear Damping Generated by Spin Injection in a Pseudo-spin-valve Structure

ABSTRACTThe current induced magnetic dynamics of a nano-scale pseudo-spin-valve (PSV) structure was theoretically studied. The spin relaxation mechanisms and the influence of ferromagnetic/nonmagnetic (FM/NM) interfaces on the current polarization were investigated, and a modified magnetic dynamic equation was developed. Both the free layer's local magnetic moments and itinerant electrons' spins were regarded as a macro-spin, whose movement was resulted from two items: spin relaxation due to the magnetic damping and spin accumulation due to the polarized current. The injected current not only produces a spin transfer torque, but also alters the effective magnetic field, and thus affects the damping. Therefore, the damping is nonlinear and correlated to the current. Based on the analysis of the competition between magnetic damping and spin accumulation, the dynamic behaviors of magnetization switching and oscillation can be explained.

Electron Spin Resonance of Proton-Irradiated Graphite

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.

Mean-field theory for double perovskites: Coupling between itinerant electron spins and localized spins

A mean field approximation of a model for double perovskites that takes into account the coupling between itinerant electron spins and localized spins is developed. As in previously reported theoretical results, and contrary to experimental observation, the critical temperature is suppressed for large electron density. An effective Heisenberg model reveals the cause of this discrepancy: the competition between degenerate antiferromagnetic and ferromagnetic channels. This degeneracy can be broken by the inclusion of a Hubbard-type U term. It is therefore suggested that electron correlation effects need to be incorporated in the minimal model of double perovskites in order to explain the experimental observation of increasing ferromagnetic critical temperature with increasing electron doping.

Spin-wave relaxation in diluted magnetic semiconductors within the self-consistent Green’s function approach

The authors employ a self-consistent Green’s function approach to investigate the spin-wave relaxation Γ(p) in diluted magnetic semiconductors. They find that the trend of the spin-wave relaxation strongly depends on the ratio of the itinerant and impurity spin densities. For density ratios in the Ruderman-Kittel-Kasuya-Yosida phase, Γ(p) decreases even though thermal fluctuations increase. On the other hand, in the strong coupling phase, an interesting peak structure appears. They discuss the implications of their numerical results for experiments.

A [formula omitted] underscreened Kondo lattice model

The underscreened Kondo lattice model presented here includes both an intra-site Kondo exchange interaction J K between the conduction band and localized 5 f electrons described by S = 1 spins, and an inter-site exchange f – f interaction J H . We write both localized and itinerant spins in a Fermionic representation, and then use a mean-field approximation. We obtain a coexistence of Kondo effect and magnetism which can account for the behavior of some Uranium compounds.

Deviation from Matthiessen rule in magnetic disordered alloys–theoretical investigation

The model of the magnetic disordered alloys, including the itinerant electrons, localized spins and phonons, was elaborated in details in the paramagnetic region with the use of the Coherent Potential Approximation (CPA) approach. The expression for the DC electrical conductivity was taken from the Kubo-Greenwood formula for an alloys. The separate (residual, phonon and magnetic) contributions to the resistivity for the temperatures above Curie transition temperature, different sets of the model parameters were calculated. The linear temperature dependence of the phonon contribution to the resistivity was reproduced except from the anomalous behavior observed for the parameters sets for which the Fermi level locates near the band's edges. For the same parameters sets our calculations showed the largest percentage deviations from the Matthiessen's rule. Our model calculations confirmed the Matthiessen's rule generalized to the case of the three, residual, phonon and magnetic contributions to the resistivity of the magnetic disordered alloys with the partially filled band when the Fermi level locates far from the bands edges. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SPIN LIQUID STATE AND THE TIME-REVERSAL SYMMETRY BREAKING EFFECT OBSERVED IN Bi2Sr2CaCu2O8+δ

The unusual properties of a doped high-Tc oxide can be attributed to its unique electronic structure, here we put emphasis on importance of the O2p band for the origin of magnetism and superconductivity. Antiferromagnetism of a charge transfer type insulator should be described by the Kramers-Anderson theory, the superexchange between localized Cu3d spins is mediated by O2p electrons. A doped hole resided in the O2p orbital in a Cu - O - Cu bond effectively suppresses the superexchange between Cu3d spins creating a local moment or a "Kondo impurity" in the magnetically ordered CuO 2 layer. The interaction between a local moment and O2p electrons yields an effective spin coupling between O2p electrons and pairing. Accordingly, itinerant O2p spins may condense into a paired state or a spin liquid state via local exchange interactions. On the contrary, Cu3d spins merely provide a magnetic background, the existing local field breaks the time-reversal symmetry in a photoemission process. We use a semiclassical approach to estimate the difference between the intensities of photocurrents generated by the left and the right circularly polarized photons observed in Bi 2 Sr 2 CaCu 2 O 8+δ by Kaminski et al.

Magnetic and orbital ordering ofRuO2planes inRuSr2(Eu,Gd)Cu2O8

We start from an effective Hamiltonian for $\mathrm{Ru}$ ions in a square lattice, which includes the on-site interactions between ${t}_{2g}$ orbitals derived from Coulomb repulsion, and a tetragonal crystal-field splitting. Using perturbation theory in the hopping terms, we derive effective Hamiltonians to describe the ${\mathrm{RuO}}_{2}$ planes of ${\mathrm{RuSr}}_{2}(\mathrm{Eu},\mathrm{Gd}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$. For undoped planes (formal valence ${\mathrm{Ru}}^{+5}$), depending on the parameters we find three possible orderings of spin and orbitals, and construct a phase diagram. This allows us to put constraints on the parameters based on experimental data. When electron doping consistent with the hole doping of the superconducting ${\mathrm{RuO}}_{2}$ planes is included, we obtain (for reasonable parameters) a double-exchange model with infinite antiferromagnetic coupling between itinerant electrons and localized spins. This model is equivalent to one used before [H. Aliaga and A. A. Aligia, Physica B 320, 34 (2002)], which consistently explains the seemingly contradictory magnetic properties of ${\mathrm{RuSr}}_{2}(\mathrm{Eu},\mathrm{Gd}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$.

Critical behavior of a photo-disordered spin system in epitaxial CrO 2 films

We have investigated dynamical properties of the spin system in a ferromagnetic CrO 2 thin film by means of a nanosecond pump–probe method. The absorption spectrum consists of both temperature-dependent and -independent components, which indicates a characteristic electronic structure with localized and itinerant spins in a double-exchange system. With strong pulse excitations we have observed absorption changes reflecting photo-induced spin disorder in the ferromagnetic phase. The temperature variation of the relaxation time near the Curie temperature shows a critical slowing down of the relaxation time near the phase transition. The critical exponent obtained is 0.18±0.01.

Unconventional metallic magnetism in LaCrSb3

Abstract The compound LaCrSb3 is a non-collinear ferromagnet with a spin reorientation at T sr ≈95 K . Coexistence of itinerant and localized spins at one and the same Cr site was suggested to explain the experimental findings. Here, arguments against this model are provided on the basis of electronic structure calculations and re-considered experimental information. An alternative model is proposed.

Specific heat of amorphous Mn 100− xCe x alloys

We have measured the specific heat C p on sputtered amorphous Mn 100− x Ce x alloys for 25⩽ x⩽79 from 4.2 to 300 K using a light-chopped AC method. The C p / T follows γ+ AT 2 law below 15 K in each Ce concentration. The γ value increases systematically as a function of x and reaches 0.124 J/mol K 2 for x=79, suggesting formation of a dense-Kondo and/or heavy-fermion state at low temperature for the Ce-rich side. Large magnetic contribution in C p has been observed at higher temperature, suggesting existence of large itinerant spin fluctuations.

Origin of electron spin resonance in La0.7Ca0.3MnO3

AbstractAn electron spin resonance study was carried out for a La0.7Ca0.3MnO3 bulk sample above the ferromagnetic ordering temperature. From careful analysis on the ESR parameters, it is shown that itinerant spins interacting with magnetic spin clusters in bottleneck regime are responsible for the ESR signals.

Magnetic phase diagram of metallic pyrochlore lattice in the double-exchange model

The magnetic phase diagram of metallic kagomeand pyrochlore lattices at the ground state have been studied theoretically in the double-exchange model using a mean-field approximation. The model includes the hopping of itinerant electrons, which is expressed by a single tight-binding band, antiferromagnetic coupling between localized spins, and ferromagnetic exchange interaction between the localized and itinerant electron spins. The phase diagram was found to be rather insensitive to the ferromagnetic exchange interaction but to show a rich variety in magnetic structure with doping. A crossover between ferromagnetic and spin glass phases observed in RE2Mo2O7 pyrochlore lattices, where RE denotes rare-earth ions, is discussed in view of the calculated results.

TRANSPORT PROPERTIES IN A SIMPLIFIED DOUBLE-EXCHANGE MODEL

Transport properties of the manganites by the double-exchange mechanism are considered. The system is modeled by a simplified double-exchange model, i.e. the Hund coupling of the itinerant electron spins and local spins is simplified to the Ising-type one. The transport quantities such as the electronic resistivity, the thermal conductivity, and the thermal power are calculated by using the dynamical mean-field theory. The transport quantities obtained qualitatively reproduce the ones observed in the manganites. The results suggest that the simplified double exchange model underlies the key properties of the manganites.