Magnetic Spin Correlations Research Articles

Magnetic short-range order in polycrystalline SrGd2O4 and SrNd2O4 studied by reverse Monte Carlo simulations and magnetic pair-distribution function analysis

We present a study combining total-scattering powder neutron diffraction, reverse Monte Carlo simulations, and magnetic pair-distribution function analysis to deduce both the static and dynamic short-range magnetic spin correlations in two compounds of the rare-earth strontium oxides: ${\mathrm{SrGd}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{SrNd}}_{2}{\mathrm{O}}_{4}$. Both compounds exhibit a distorted honeycomb lattice which forms a set of zigzag ladders along the crystallographic $c$ axis of space group $Pnam$. Each set consists of two one-dimensional chains separated by diagonal rungs forming triangles, thus inducing a large degree of geometrical frustration due to the antiferromagnetic exchange between the magnetic ions. Significant magnetic diffuse scattering was observed well above the respective N\'eel temperatures and analyzed with reverse Monte Carlo techniques in reciprocal space. As a complementary analysis in real space we have derived the magnetic pair-distribution function for both compounds as a function of temperature. Our results clearly indicate that the individual zigzag ladders begin ordering well above ${T}_{\mathrm{N}}$ due to the dominating nearest and next-nearest interactions, long before interladder spin correlations become significant only slightly above ${T}_{\mathrm{N}}$. Additionally, for ${\mathrm{SrNd}}_{2}{\mathrm{O}}_{4}$ we find some qualitative differences between the short-range spin correlations above ${T}_{\mathrm{N}}$ and those observed in the ordered state.

Magnon specific heat and free energy of Heisenberg ferromagnetic single-walled nanotubes: Green's function approach

The effect of magnetic spin correlation on the thermodynamic properties of Heisenberg ferromagnetic single-walled nanotubes are comprehensively investigated by use of the double-time Green's function method. The influence of temperature, spin quantum number, diameter of the tube, anisotropy strength and external magnetic field to internal energy, free energy, and magnon specific heat are carefully calculated. Compared to the mean field approximation, the consideration of the magnetic correlation effect significantly improves the internal energy values at finite temperature, while it does not so near zero temperature, and this effect is related to the diameter of the tube, anisotropy strength, and spin quantum number. The magnetic correlation effect lowers the internal energy at finite temperature. As a natural consequence of the reduction of the internal energy, the specific heat is reduced, and the free energy is elevated.

Theoretical study of magnetic spin correlations and the magnetocapacitance effect in BiMnO3

The magnetic correlations between Mn ion spins and their effects on the magnetocapacitance in BiMnO3 are studied based on a microscopic model. Above the phase transition temperature of the magnetic subsystem, the magnetic correlations including the longitudinal and transverse parts are nonzero, and they lead to a nonzero specific heat of magnetic subsystem. We show that the magnetocapacitance effect is ascribed to the fluctuation of magnetic correlation. The linear relation between electric susceptibility and the magnetic correlation is obtained and the relevant experiments are explained.

FMR evidence of ferromagnetic correlations at zigzag edge states in single-layer graphene

Magnetic properties of zigzag edges of single-layer graphene were investigated by ferromagnetic resonance (FMR)/electron spin resonance (ESR) technique. We observed the FMR lines in single-layer graphene, which provide the most direct experimental proof of the theoretical prediction of ferromagnetic correlations at zigzag edges. To explain the observed temperature dependence of the FMR lines we analyzed the magnetic spin correlation length, which is responsible for magnetic ordering in these quasi-1D ferromagnetic nanodomains formed by zigzag edges. ESR spectra of conduction electrons were also studied.

Neutron polarization analysis study of the frustrated magnetic ground state ofβ-Mn1−xAlx

We have performed a neutron polarization analysis study of the short-range nuclear and magnetic correlations present in the dilute alloy, $\ensuremath{\beta}{\text{-Mn}}_{1\ensuremath{-}x}{\text{Al}}_{x}$ with $0.03\ensuremath{\le}x\ensuremath{\le}0.16$, in order to study the evolution of the magnetic ground state of this system as it achieves static spin-glass order at concentrations $x>0.09$. To this end we have developed a reverse--Monte Carlo algorithm which has enabled us to extract Warren-Cowley nuclear short-range order parameters and magnetic spin correlations. Using conventional neutron powder diffraction, we show that the nonmagnetic Al substituents preferentially occupy the magnetic site II Wyckoff positions in the $\ensuremath{\beta}\text{-Mn}$ structure---resulting in a reduction of the magnetic topological frustration of the Mn atoms. These Al impurities are found to display strong anticlustering behavior. The magnetic spin correlations are predominantly antiferromagnetic, persisting over a short range which is similar for all the samples studied---above and below the spin-liquid-spin-glass boundary---while the observed static (disordered) moment is shown to increase with increasing Al concentration.

Stripes and dimensional crossovers in high- Tc cuprates

The key problem in high T c physics is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T o shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates (including high field transport data in fields up to 60 T) can be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Close to the insulator - metal transition disorder strongly influences the formation of stripes. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data. The possible dimensional crossovers observed with decreasing temperature at T o(x) (2D-1D) and T c(x) (1D–2D) are discussed and the main unsolved problems of the evolution of the normal state properties with doping are outlined.

Doped CuO 2 Planes in High T c Cuprates: 2D Or Not 2D ?

The key problem in physics of underdoped cuprates is whether the doping is inhomogeneous and holes are expelled into the 1D domains (“stripes”). Direct comparison of the resistivity vs T curves of the recently discovered 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T* shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates can then be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data.

Observation of magnetic spin correlations in the LuFe10Mo2 alloy

Results of magnetic, medium-resolution neutron diffraction and small-angle neutron scattering (SANS) measurements on a polycrystalline LuFe10Mo2 intermetallic alloy show that ferromagnetic (FM) interactions exist between 5 and 325 K, but finite magnetic spin correlations emerge in the examined temperature range. It is observed that in zero applied magnetic field there is no well defined critical point Tc indicating a transition from a paramagnetic to a long-range ordered magnetic phase. A linear variation of magnetization as a function of T3/2 remarkably persists up to a temperature that is a large fraction of the Tc for the LuFe12-xMox alloys.

Percolation, fractals, and anomalous diffusion

Both the infinite cluster and its backbone are self-similar at the percolation threshold,pc. This self-similarity also holds at concentrationsp nearpc, for length scalesL which are smaller than the percolation connectedness length,ξ. ForL<ξ, the number of bonds on the infinite cluster scales asLD, where the fractal dimensionalityD is equal to(d-β/v). Geometrical fractal models, which imitate the backbone and on which physical models are exactly solvable, are presented. Above six dimensions, one has D=4 and an additional scaling length must be included. The effects of the geometrical structure of the backbone on magnetic spin correlations and on diffusion at percolation are also discussed.