Low-energy Magnetic Excitations Research Articles

Long Range Order in CeRu2Al10

We studied the macroscopic properties of Ce x La 1- x Ru 2 Al 10 single crystal. From the results of the rapid suppression of the anomaly at T 0 and thermally activated-type behavior and the increase of the γ T term by La doping, we propose that the low-energy magnetic excitation may be induced, which may be related to the randomly suppressed high energy magnetic excitation. We also studied Ce 0.1 Y 0.9 Fe 2 Al 10 single crystal, and found that Ce ion in this compound is nonmagnetic although in Ce 0.1 La 0.9 Ru 2 Al 10 Ce ion is magnetic and a typical impurity Kondo effect is observed.

Magnetic Field-Induced Closure of the Spin Excitation Gap near Optimal Doping in La2-xSrxCuO4

We have investigated how a magnetic field applied perpendicular to the CuO2 planes of the near-optimally hole-doped high-temperature superconductor La1.855Sr0.145CuO4 (Tc ≈36 K) influences the low-energy magnetic excitation spectrum. Our detailed single-crystal neutron scattering experiments reveal that the gap to magnetic excitations falls off linearly with increasing field and reaches zero at the magnetic field µ0Hs=7±1 T required to induce long-range incommensurate magnetic order. A comparison with the electron-doped cuprate Nd1.85Ce0.15CuO4 is made and the possible link between field-induced magnetic order and Fermi surface reconstruction in cuprates is discussed.

Disappearance of static magnetic order and evolution of spin fluctuations inFe1+δSexTe1−x

We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system ${\text{Fe}}_{1+\ensuremath{\delta}}{\text{Se}}_{x}{\text{Te}}_{1\ensuremath{-}x}$ with different Fe and Se compositions. Short-range static magnetic order with an in-plane wave vector near the (0.5,0) (using the two-Fe unit cell), together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave vector (0.5, 0.5), corresponding to ``collinear'' spin correlations. Our results suggest that there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. Excess Fe appears to be important for stabilizing the magnetic order that competes with superconductivity.

Low-energy magnetic excitation spectrum of the unconventional ferromagnetCeRh3B2

The magnetic excitation spectrum of the unconventional ferromagnet CeRh$_{3}$B$_{2}$ was measured by inelastic neutron scattering on single crystal sample in the magnetically ordered and paramagnetic phases. The spin-wave excitation spectrum evidences high exchange interaction along the c-axis about two orders of magnitude higher than the ones in the basal plane of the hexagonal structure. Both strong out of plane and small in plane anisotropies are found. This latter point confirms that considering the $J$=5/2 multiplet alone is not adequate for describing the ground state of CeRh$_{3}$B$_{2}$. Quasielastic scattering measured above $T_{Curie}$ is also strongly anisotropic between the basal plane and the c-axis and suggests localized magnetism.

Spin dynamics inS=12chains with next-nearest-neighbor exchange interactions

Low-energy magnetic excitations in the spin-1/2 chain compound (C$_6$H$_9$N$_2$)CuCl$_3$ [known as (6MAP)CuCl$_3$] are probed by means of tunable-frequency electron spin resonance. Two modes with asymmetric (with respect to the $h\nu=g\mu_B B$ line) frequency-field dependences are resolved, illuminating the striking incompatibility with a simple uniform $S=\frac{1}{2}$ Heisenberg chain model. The unusual ESR spectrum is explained in terms of the recently developed theory for spin-1/2 chains, suggesting the important role of next-nearest-neighbor interactions in this compound. Our conclusion is supported by model calculations for the magnetic susceptibility of (6MAP)CuCl$_3$, revealing a good qualitative agreement with experiment.

Structure factor of low-energy spin excitations in aS=12kagome antiferromagnet

The ground state of the S=1/2 Heisenberg antiferomagnet on kagome can be viewed as a collection of fermionic spinons bound into small, heavy singlet pairs. Low-energy magnetic excitations in this system correspond to breaking the pairs into individual spinons. We calculate the structure factor for inelastic neutron scattering from independent spinon pairs.

Coupling of spin and orbital excitations in the iron-based superconductorFeSe0.5Te0.5

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe(0.5)Te(0.5). The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0), whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

Unusual Relationship between Magnetism and Superconductivity inFeTe0.5Se0.5

We use neutron scattering to study magnetic excitations in crystals near the ideal superconducting composition of FeTe(0.5)Se(0.5). Two types of excitations are found, a resonance at (0.5,0.5,0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

10.1007/s11451-008-1010-7

The small-angle neutron scattering energy spectra of the Zn0.55Mn0.45Fe2O4 ferrite are analyzed at different temperatures (both below and above T C ≃ 390 K) and scattering angles. The thermal expansion coefficient α(T) is measured in the temperature range 80–600 K. It is revealed that inelastic neutron scattering is governed not only by spin waves of the Holstein-Primakoff type but also by the substantial contribution of additional long-wavelength magnetic excitations. The physical nature of these low-energy magnetic excitations is discussed.

Spin-wave velocities, density of magnetic excitations, and NMR relaxation in iron pnictides

We perform an analysis of the experimentally known temperature dependence of the staggered magnetization in the antiferromagnetic phase. This analysis allows us to put an upper limit on the unknown value of the spin-wave velocity along the stripes of equal spin direction (spin stripes). The velocity is about ten times smaller than the velocity perpendicular to the spin stripes. The strongly anisotropic spin-wave dispersion implies a high density of low-energy magnetic excitations. We demonstrate that this high density strongly enhances the $^{75}\text{A}\text{s}$ NMR spin-lattice relaxation via the Raman scattering of magnons. We derive the polarization dependence of this relaxation channel and find very good agreement with experimental data. The high density of low-energy magnetic excitations deduced from our phenomenological analysis supports the scenario that ferropnictides are close to a quantum phase transition.

Dimensional Crossover of Low-Energy Magnetic Excitation for Delafossite Oxide Cu1-xAgxCrO2 with a Spin-3/2 Antiferromagnetic Triangular Sublattice

Dimensional crossover from anisotropic-three-dimensional (3D) to 2D low-energy magnetic excitations was found in delafossite oxide Cu 1- x Ag x CrO 2 having a spin-3/2 antiferromagnetic (AF) triangular lattice. The crossover is evidenced by a decrease in AF transition temperature ( T N ), distinct 2D magnetic excitations above T N for Cu 0.85 Ag 0.15 CrO 2 , a disappearance of a peak of magnetic specific heat ( C mag ) around T N , and a successive change from T 3 to T 2 dependence of low- T C mag accompanying a low- T shift of magnetic entropy. These results may suggest a gradual change from AF to spin-liquid like states with an increase in x .

Anomalous frequency and intensity scaling of collective and local modes in a coupled-spin tetrahedral systemCu2Te2O5Cl2

We report on the magnetic excitation spectrum of the coupled-spin tetrahedral system ${\text{Cu}}_{2}{\text{Te}}_{2}{\text{O}}_{5}{\text{Cl}}_{2}$ using Raman scattering on single crystals. The transition to an ordered state at ${T}_{N}^{\text{Cl}}=18.2\text{ }\text{K}$ evidenced from thermodynamic data leads to the evolution of distinct low-energy magnetic excitations superimposed by a broad maximum. These modes are ascribed to magnons with different degree of localization and a two-magnon continuum. Two of the modes develop a substantial energy shift with decreasing temperature similar to the order parameter of other N\'eel ordered systems. The other two modes show only a negligible temperature dependence and dissolve above the ordering temperature in a continuum of excitations at finite energies. These observations point to a delicate interplay of magnetic intertetrahedra and intratetrahedra degrees of freedoms as expected for a system in the proximity to quantum criticality.

Magnetic field induced enhancement of spin-order peak intensity inLa1.875Ba0.125CuO4

We report on neutron-scattering results on the impact of a magnetic field on stripe order in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$. It is found that a 7 T magnetic field applied along the {\it c} axis causes a small but finite enhancement of the spin-order peak intensity and has no observable effect on the peak width. Inelastic neutron-scattering measurements indicate that the low-energy magnetic excitations are not affected by the field, within experimental error. In particular, the small energy gap that was recently reported is still present at low temperature in the applied field. In addition, we find that the spin-correlation length along the antiferromagnetic stripes is greater than that perpendicular to them.

Low-energy magnetic response of the noncentrosymmetric heavy-fermion superconductorCePt3Sistudied via inelastic neutron scattering

The low-energy magnetic excitations of the noncentrosymmetric heavy-fermion superconductor ${\text{CePt}}_{3}\text{Si}$ have been measured with inelastic neutron scattering on a single crystal. Kondo-type spin fluctuations with an anisotropic wave-vector dependence are observed in the paramagnetic state. These fluctuations do not survive in the antiferromagnetically ordered state below ${T}_{N}=2.2\text{ }\text{K}$ but are replaced by damped spin waves, whose dispersion is much stronger along the $c$ axis than in other directions. No change is observed in the excitation spectrum or the magnetic order as the system enters the superconducting state below ${T}_{c}\ensuremath{\approx}0.7\text{ }\text{K}$.

Effect of Zn impurities on the electronic structure ofLa1.85Sr0.15CuO4studied by high-resolution angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Zn-substituted ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ to study the impurity effect on the electronic structure near the Fermi level. Upon Zn substitution, the kink in the band dispersion at $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and the coherence peak in the off-nodal region in the superconducting state are significantly weakened and, at the same time, additional electronic states systematically evolve inside the superconducting gap. The suppression of the kink with Zn substitution shows a good correspondence to the low-energy incommensurate magnetic excitations.

Long-wavelength magnetic excitations in the Zn0.55Mn0.45Fe2O4 ferrite

The small-angle neutron scattering energy spectra of the Zn0.55Mn0.45Fe2O4 ferrite are analyzed at different temperatures (both below and above TC ≃ 390 K) and scattering angles. The thermal expansion coefficient α(T) is measured in the temperature range 80–600 K. It is revealed that inelastic neutron scattering is governed not only by spin waves of the Holstein-Primakoff type but also by the substantial contribution of additional long-wavelength magnetic excitations. The physical nature of these low-energy magnetic excitations is discussed.

Evidence for Gapped Spin-Wave Excitations in the FrustratedGd2Sn2O7Pyrochlore Antiferromagnet from Low-Temperature Specific Heat Measurements

We have measured the low-temperature specific heat of the geometrically frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic field. The specific heat is found to drop exponentially below approximately 350 mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy resulting from single-ion effects and long-range dipolar interactions. The data are well fitted by linear spin-wave theory, ruling out unconventional low-energy magnetic excitations in this system, and allowing a determination of the pertinent exchange interactions in this material.

Spin dynamics in the diluted ferromagnetic Kondo lattice model

The interplay of disorder and competing interactions is investigated in the carrier-inducedferromagnetic state of the Kondo lattice model within a numerical finite-size study in whichimpurity positional disorder is treated exactly. Competition between impurityspin couplings with increasing hole doping results in a strong optimization ofTc at , highlighting the importance of compensation in diluted magnetic semiconductors. The estimatedTc is in good agreement with experimental results forGa1−xMnxAs for corresponding impurity concentration, hole bandwidth, and compensation. Dilution isshown to result in a sharply enhanced density of low-energy magnon modes extended overthe majority bulk spins and high-energy magnon modes localized over impurity spinclusters. The effects of these magnon properties on finite-temperature spin dynamics arestudied quantitatively using a locally self-consistent magnon renormalization scheme.The large enhancement in the density of low-energy magnetic excitations resultsin a rapid thermal decay of magnetization, which fits well with the Bloch formM0(1−BT3/2) at lowtemperature, with B of the same order of magnitude as obtained in recent superconductingquantum interference device (SQUID) magnetization measurements onGa1−xMnxAs samples. Furthermore, strong cluster couplings and correlations are shownto slow down the magnetization decay and prolong magnetic ordering nearTc, as is also observed in magnetization measurements.

HoTbTi 2O 7, the mixtures of spin ice and spin liquid

Polycrystalline samples of Ho 2− x Tb x Ti 2O 7 ( x=0.5, 1, and 1.5) have been prepared and characterized. No long-range order is observed for HoTbTi 2O 7 in magnetization and specific heat measurements down to 2 K. The low-energy magnetic excitation measurements suggests that HoTbTi 2O 7 possesses both characteristics of spin ice and spin liquid in the ground state.

Correlation between long-wavelength magnetic excitations and an inhomogeneous magnetic structure in γ-FeNi alloys

An energy analysis of small-angle neutron scattering in FexNi1−x (x = 0.50, 0.57, 0.65 at %) alloys is performed at various temperatures and wavevectors. Low-energy long-wavelength magnetic excitations other than spin waves of the Holstein-Primakoff type are detected. These data and the data obtained earlier for the Fe0.65Ni0.35 alloy indicate that the density of states of the low-energy long-wavelength magnetic excitations correlates with the inhomogeneity of the magnetic structure of the alloys, which is caused by the iron concentration and a competing exchange interaction (JNiNi > 0, JFeNi > 0, JFeFe < 0).