Dynamic Spin Fluctuations Research Articles

Low-Temperature Spin Fluctuations beyond Spin Waves

A simple low-temperature dynamic spin-fluctuation theory of ferromagnetic metals is developed. The theory is based on the functional integral formalism for the multiband Hubbard Hamiltonian and takes into account both single-site and nonlocal spin fluctuations. We show that our approach correctly reproduces the T3/2 law at low temperatures. The calculated results of magnetic properties for Fe and Fe0.65Ni0.35 Invar demonstrate that the approach works on a much wider temperature interval than the spin-wave approximation.

Short-range Order Above TC in Ferromagnetic Metals

We study the spin-density correlations in ferromagnetic metals using the dynamic spin-fluctuation theory. We derive computational formulae for the spatial correlation function and use them to calculate the short-range order above TC in bcc Fe. Results of the calculation confirm our theoretical prediction that the inverse correlation radius increases linearly with temperature for T sufficiently large. The calculated short-range order is small but sufficient to correctly describe neutron scattering experiments. We show that considerable amount of the short-range order persists up to temperatures much higher than TC.

Transverse susceptibility and the T3/2 law in the dynamic spin-fluctuation theory

We obtain explicit expressions for elements of the magnetic susceptibility tensor in the dynamic spin-fluctuation theory. Using an analytic continuation of the Green’s functions, we show that the transverse susceptibility has spin-wave poles at low temperatures, yielding the asymptotic T3/2 law for magnetization. We derive an explicit expression for the coefficient in the T3/2 law based on the multiband Hubbard Hamiltonian and real band structure. We demonstrate the correct low-temperature behavior of magnetization in the example of iron.

Low-Temperature Magnetism of Metals in the Dynamic Spin-Fluctuation Theory

The low-temperature behavior of magnetization in ferromagnetic metals is studied by means of the dynamic spin-fluctuation theory. An expression is derived for the dynamic magnetic susceptibility, which extends the one in the random phase approximation. The transverse susceptibility is shown to have the spin-wave poles, which yield the asymptotic T 3/2 law for magnetization. An explicit expression for the coefficient in the T 3/2 law is derived on the basis of the multiband Hubbard Hamiltonian and real band structure. The temperature dependence of magnetization is demonstrated by the example of Fe and disordered Fe-Ni Invar.

Self-Consistent Second-Order Perturbation Theory of Correlation Effect in bcc Iron at Finite Temperatures

The self-consistent second-order perturbation theory is applied to a realistic multiband Hubbard model of bcc iron with realistic local Coulomb interaction terms, and both the single-electron Green's function and the dynamical transverse susceptibility are calculated while preserving the spin rotational symmetry. The overall bandwidth reduction is obtained and is in agreement with experiments and previous theoretical results, but no significant additional mass enhancement near the Fermi surface is found. The spin wave mode is found to evolve in the dynamical spin fluctuation spectrum, whose frequency vanishes correctly in the long wavelength limit. The spin stiffness constant \(D\) is found to be in good agreement with other theoretical results and is smaller than the experimental value.

Finding New Superconductors: The Spin-Fluctuation Gateway to High Tc and Possible Room Temperature Superconductivity

We propose an experiment-based strategy for finding new high transition temperature superconductors that is based on the well-established spin fluctuation magnetic gateway to superconductivity in which the attractive quasiparticle interaction needed for superconductivity comes from their coupling to dynamical spin fluctuations originating in the proximity of the material to an antiferromagnetic state. We show how lessons learned by combining the results of almost three decades of intensive experimental and theoretical study of the cuprates with those found in the decade-long study of a strikingly similar family of unconventional heavy electron superconductors, the 115 materials, can prove helpful in carrying out that search. We conclude that, since Tc in these materials scales approximately with the strength of the interaction, J, between the nearest neighbor local moments in their parent antiferromagnetic state, there may not be a magnetic ceiling that would prevent one from discovering a room temperature superconductor.

Evidence for persistent spin fluctuations in uranium sesquicarbide

The low-temperature magnetic susceptibility, heat capacity, and electrical resistivity of uranium sesquicarbide have been determined down to 2 K by combining measurements carried out on samples of U${}_{2}$C${}_{3}$, containing UC as minority phase, and on pure UC specimens. The presence of spin fluctuations with characteristic temperature ${T}_{SF}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}7$ K is revealed by a nonanalytic contribution to the specific heat behaving (well below ${T}_{SF}$) as ${T}^{3}\text{ln}(T/{T}_{SF})$, and confirmed by a ${T}^{2}$ increase of the electrical resistivity and a $1\ensuremath{-}\ensuremath{\kappa}{T}^{2}$ decrease of the low-temperature magnetic susceptibility. The analysis of the specific-heat data above ${T}_{SF}$ provides a Debye temperature ${\ensuremath{\Theta}}_{D}$ $\ensuremath{\approx}$ 256 K and a moderate high value of the Sommerfeld coefficient, $\ensuremath{\gamma}$ $\ensuremath{\approx}$ 42 mJ mol${}_{U}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$. The zero-temperature many-body enhancement of the electron mass is found to be ${m}^{*}/m$ $\ensuremath{\approx}$ 2.7. Our data rule out the occurrence of magnetic ordering in U${}_{2}$C${}_{3}$. First-principles electronic structure calculations support the absence of a magnetically ordered ground state but suggest that dynamical spin fluctuations are responsible for the electron mass enhancement.

ChemInform Abstract: Effects of Impurities and Vortices on the Low‐energy Spin Excitations in High‐TC Materials

AbstractReview: 56 refs.

Noise studies of magnetization dynamics in dilute magnetic semiconductor heterostructures

We study theoretically and experimentally the frequency and temperature dependences of resistivity noise in semiconductor heterostructures $\ensuremath{\delta}$-doped by Mn. The resistivity noise is observed to be nonmonotonous as a function of frequency. As a function of temperature, the noise increases by two orders of magnitude for a resistivity increase of about 50%. We study two possible sources of resistivity noise, dynamic spin fluctuations and charge fluctuations, and find that dynamic spin fluctuations are more relevant for the observed noise data. The frequency and temperature dependences of resistivity noise provide important information on the nature of the magnetic interactions. In particular, we show how noise measurements can help resolve a long-standing debate on whether the Mn-doped GaAs is a $p$-$d$ Zener/Ruderman-Kittel-Kasuya-Yosida (RKKY) or double-exchange ferromagnet. Our analysis includes the effect of different kinds of disorder such as spin-glass type of interactions and a site-dilution type of disorder. We find that the resistivity noise in these structures is well described by a disordered RKKY ferromagnet model dynamics with a conserved order parameter.

Extended Dynamic Spin-Fluctuation Theory with Application to Iron

The problem of discontinuous phase transition in the dynamic spin-fluctuation theory is resolved by taking into account large anharmonic spin fluctuations and nonlocality of the mean Green function. The extended theory is applied to the calculation of magnetic properties of iron.

Local Moments in the Dynamic Spin-Fluctuation Theory of Metallic Magnetism

In the dynamic spin-fluctuation theory extended by taking into account higher-order terms of the free energy and uniform fluctuations along with the local ones, simple computational formulae for local magnetic moment are obtained. The temperature dependence of local magnetic moment in the Fe0.65Ni0.35 Invar is calculated. Good agreement with the experimentally estimated value of the magnetic volume change is achieved.

Extended dynamic spin-fluctuation theory of metallic magnetism

A dynamic spin-fluctuation theory that directly takes into account nonlocality of thermalspin fluctuations and their mode–mode interactions is developed. The Gaussianapproximation in the theory is improved by a self-consistent renormalization of the meanfield and spin susceptibility due to the third- and fourth-order terms of the free energy,respectively. This eliminates the fictitious first-order phase transition, which is typical forthe Gaussian approximation, and yields a proper second-order phase transition. The effectof nonlocal spin correlations is enhanced by taking into account uniform fluctuations in thesingle-site mean Green function. Explicit computational formulae for basic magneticcharacteristics are obtained. The extended theory is applied to the calculation of magneticproperties of Fe–Ni Invar. Almost full agreement with experiment is achieved for themagnetization, Curie temperature, and local and effective magnetic moments.

Effects of impurities and vortices on the low-energy spin excitations in high-Tc materials

We review a theoretical scenario for the origin of the spin-glass phase of underdoped cuprate materials. In particular it is shown how disorder in a correlated d-wave superconductor generates a magnetic phase by inducing local droplets of antiferromagnetic order which eventually merge and form a quasi-long range ordered state. When correlations are sufficiently strong, disorder is unimportant for the generation of static magnetism but plays an additional role of pinning disordered stripe configurations. We calculate the spin excitations in a disordered spin-density wave phase, and show how disorder and/or applied magnetic fields lead to a slowing down of the dynamical spin fluctuations in agreement with neutron scattering and muon spin rotation (μSR) experiments.

Disorder-Induced Freezing of Dynamical Spin Fluctuations in Underdoped Cuprate Superconductors

We study the dynamical spin susceptibility of a correlated d-wave superconductor (dSC) in the presence of nonmagnetic disorder, using an unrestricted Hartree-Fock approach. This model provides a concrete realization of the notion that disorder slows down spin fluctuations, which eventually "freeze out." The evolution of disorder-induced spectral weight transfer agrees qualitatively with experimental observations on underdoped cuprate superconductors. For sufficiently large disorder concentrations, static spin density wave (SDW) order is created when droplets of magnetism nucleated by impurities overlap. We also study the disordered stripe state coexisting with a dSC and compare its magnetic fluctuation spectrum to that of the disorder-generated SDW phase.

Magnetic quantum tunneling inFe8with excited nuclei

We investigate the effect of dynamic nuclear spin fluctuation on quantum tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing the nuclei temperature using radio frequency (RF) pulses before the hysteresis loop measurements. The RF pulses do not change the electrons spin temperature. Independently we show that the nuclear spin-spin relaxation time T2 has strong temperature dependence. Nevertheless, we found no effect of the nuclear spin temperature on the tunneling probability. This suggests that in our experimental conditions only the hyperfine field strength is relevant for QTM. We demonstrate theoretically how this can occur.

μSR study on the spin singlet state in the S = 1/2 cluster magnet GaNb4S8

GaNb4S8 is a S = 1/2 Nb4-cluster magnet exhibiting a structural phase transition at Ts = 32 K. We have investigated the magnetic ground state of the compound by means of the zero-field (ZF) and the longitudinal-field (LF) μSR techniques. In the ZF-μSR spectra, no muon spin precession was observed below Ts, revealing an absence of magnetic long-range orders. Instead, an stretched exponential relaxation was observed, which remains under the LF up to 3950 G. The field dependence of the relaxation rate obeys the Redfield's equation, indicating that a dynamic component of electronic spin fluctuations is dominant below Ts. These μSR results can be understand as sporadic dynamics of spin fluctuations in a spin singlet sea and support the spin singlet state of GaNb4S8 due to the formation of Nb8 octamer from two Nb4 tetramers.

First-principles dynamical CPA to finite-temperature magnetism of transition metals

We present here the first-principles dynamical CPA (coherent potential approximation) combined with the tight-binding LMTO LDA+U method towards quantitative calculations of the electronic structure and magnetism at finite temperatures in transition metals and compounds. The theory takes into account the single-site dynamical charge and spin fluctuations using the functional integral technique as well as an effective medium. Numerical results for Fe, Co, and Ni show that the theory explains quantitatively the high-temperature properties such as the effective Bohr magneton numbers and the excitation spectra in the paramagnetic state, and describes the Curie temperatures semiquantitatively.

Spin fluctuation renormalizations of normal and superconducting state properties in t − J*-model

The effect of spin-fluctuation scattering processes on the region of the superconducting phase in the strongly correlated electrons (Hubbard fermions) has been investigated by the diagram technique for Hubbard operators. It is shown that spin fluctuations in the one-loop approximation for the t − t′ − t" − J*- model taking into account long-range hoppings and three-center interactions are reflected by components of the strength operator. In this approximation for the d-type symmetry of the order parameter for the superconducting phase a system of infinite integral equations has been derived. Obtained dependencies of critical temperature on the electron concentrations show that joint effect of long-range hoppings, three-center interactions, and dynamical spin-fluctuation processes leads to strong renormalization of the superconducting phase region. It was shown that these processes essentially modify distribution functions of Hubbard fermions in normal phase.

Spin freezing and dynamics inCa3Co2−xMnxO6(x≈0.95)investigated with implanted muons: Disorder in the anisotropic next-nearest-neighbor Ising model

We present a muon-spin relaxation investigation of the Ising chain magnet ${\text{Ca}}_{3}{\text{Co}}_{2\ensuremath{-}x}{\text{Mn}}_{x}{\text{O}}_{6}$ $(x\ensuremath{\approx}0.95)$. We find dynamic spin fluctuations persisting down to the lowest measured temperature of 1.6 K. The previously observed transition at around $18\text{ }\text{K}$ is interpreted as a subtle change in dynamics for a minority of the spins coupling to the muon that we interpret as spins locking into clusters. The dynamics of this spin fraction freeze below a temperature ${T}_{\text{SF}}\ensuremath{\approx}8\text{ }\text{K}$, while a majority of spins continue to fluctuate. An explanation of the low-temperature behavior is suggested in terms of the predictions of the anisotropic next-nearest-neighbor Ising model.

Antiferromagnetism of SrFe2As2 Studied by Single-Crystal 75As-NMR

We report results of 75 As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of SrFe 2 As 2 . The NMR spectra clearly show sharp first-order antiferromagnetic (AF) and structural transitions occurring simultaneously. The behavior in the vicinity of the transition is compared with our previous study on BaFe 2 As 2 . No significant difference was observed in the temperature dependence of the static quantities such as the AF splitting and electric quadrupole splitting. However, the results of the NMR relaxation rate revealed difference in the dynamical spin fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear to be more anisotropic in BaFe 2 As 2 than in SrFe 2 As 2 .