Magnetic properties of RCoSi2 compounds (R=rare earth)

New ternary silicides of composition RCoSi₂ (R=rare earth and Y) have been prepared and found to crystallize in the orthorhombic CeNiSi₂-type structure. Their magnetic properties have been studied by means of susceptibility measurements between 2 and 250 K. The Ce and Y compounds show essentially temperature independent Pauli paramagnetism. The compounds with R=Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm show antiferromagnetic ordering below 20 K. The effective rare earh moments in the paramagnetic state agree well with the free ion values, and, for the heavy rare earths, the Néel temperatures vary with the De Gennes factor. There is no indication for a magnetic contribution from the Co sublattice.     

Unconventional temperature enhanced magnetism in Fe1.1Te

Our inelastic neutron scattering study of spin excitations in iron telluride reveals remarkable thermal evolution of the collective magnetism. In the temperature range relevant for the superconductivity in FeTe(1-x)Se(x) materials, where the local-moment behavior is dominated by liquidlike correlations of emergent spin plaquettes, we observe unusual, marked increase of magnetic fluctuations upon heating. The effective spin per Fe at T ≈ 10 K, in the phase with weak antiferromagnetic order, corresponds to S ≈ 1, consistent with the recent analyses that emphasize importance of Hund's coupling [K. Haule and G. Kotliar, New J. Phys. 11, 025021 (2009).]. However, it grows to S ≈ 3/2 in the high-T disordered phase, suggestive of the Kondo-type behavior, where local magnetic moments are entangled with the itinerant electrons.     

Soft mode and magnetic phase transition in PrNi

The spectrum of magnetic excitations in a single crystal of intermetallic compound PrNi was studied by inelastic neutron scattering. Experiment showed the substantial softening of some collective magnetic excitation modes near the ferromagnetic ordering temperature T _c≈20 K. The result is analyzed within the framework of a model that describes the magnetic phase transition in systems with induced magnetic moment.     

Anisotropic magnetic exchange in orthorhombic RCu2 compounds (R = rare earth)

The magnetic excitations in the field induced ferromagnetic phase F3 of a NdCu₂ single crystal were investigated by means of inelastic neutron scattering experiments. A mean field model using the random phase approximation in connection with anisotropic magnetic bilinear R-R (R denotes a rare earth) exchange interactions is proposed to account for the observed dispersion. The relevance of this model to the analysis of the magnetic ordering process in other RCu₂ compounds is discussed. Received 21 April 1999     

Magnetic properties of RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

Quantum critical scaling and the origin of non-fermi-liquid behavior in Sc1-xUxPd3

We used inelastic neutron scattering to study magnetic excitations of Sc1-xUxPd3 for U concentrations (x=0.25, 0.35) near the spin glass quantum critical point (QCP). The excitations are spatially incoherent, broad in energy (E=variant Planck's over 2piomega), and follow omega/T scaling at all wave vectors investigated. Since similar omega/T scaling has been observed for UCu5-xPdx and CeCu6-xAux near the antiferromagnetic QCP, we argue that the observed non-Fermi-liquid behavior in these f-electron materials arises from the critical phenomena near a T=0 K phase transition, irrespective of the nature of the transition.     

Direct evidence for the magnetic ordering of Nd ions in NdMn2Si2 and NdMn2Ge2 by high resolution inelastic neutron scattering

We have investigated the low energy nuclear spin excitations in NdMn₂Si₂ and NdMn₂Ge₂ by high resolution inelastic neutron scattering. Previous neutron diffraction investigations gave ambiguous results about Nd magnetic ordering at low temperatures. The present element-specific technique gave direct evidence for the magnetic ordering of Nd ions. We found considerable difference in the process of the Nd magnetic ordering at low temperature in NdMn₂Si₂ and NdMn₂Ge₂. Our results are consistent with those of magnetization and recent neutron diffraction measurements.     

Magnetic properties of some RPd2Si2 compounds (R=Gd,Tb, Dy,Ho and Er)

The magnetic susceptibility of the ternary compounds, RPd₂Si₂ (where R=Gd, Tb, Dy, Ho and Er) has been measured. GdPd₂Si₂ order antiferromagnetically at 13 and 20 K respectively; the rest of the compounds do not show clear ordering down to 4.2 K. Palladium carries no moment in these compounds. The De Gennes formula is not obeyed indicating that the exchange interaction between the 4f moments via conduction electrons is not isotropic     

On the magnetic structure of PrMn2O5: a neutron diffraction study

The long-range magnetic ordering of PrMn(2)O(5) has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at T(N) ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k(1) = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k(2) = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k(1) and k(2). The magnetic structure of the k(1)-domain is given by the basis vectors (C(x),0,0) and (C(x)',0,0) for Mn(4h) and Mn(4f), respectively. In the k(2)-domain, the magnetic structure is defined by the basis vectors (0,0,G(z)) and (F(x)',G(y)',0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k(1), the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μ(B), respectively; for the magnetic phase associated with k(2), the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μ(B), respectively.     

Nuclear magnetic resonance of RCo3(R:Rare earth)

The nuclear magnetic resonance of⁵⁹Co nuclei in magnetic domains of RCo₃(R:Y, Nd, Sm, Gd, Tb, Dy and Ho) has been measured under external fields up to about 50 kOe at 4.2 K. To assign the observed NMR signals to each Co site, the⁵⁹Co nuclear magnetic resonance of R(Co_1−xNi_x)₃ and R(Co_1−xFe_x)₃ has also been measured under the same conditions. The results of NMR studies show that the cobalt atoms at the 6c site in the light R compounds(YCo₃, NdCo₃ and SmCo₃) and these at the 3b site in the heavy R compounds(GdCo₃, TbCo₃ and HoCo₃) have a large orbital contribution.     

Effect of Fermi surface nesting on resonant spin excitations in Ba(1-x)K(x)Fe2As2

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.     

High-energy spin excitations in the electron-doped superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) with T(c) = 21 K

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of the electron-doping superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 21 +/- 1 K) over a wide energy range (4 meV < or = homega < or = 330 meV). The effect of electron doping is to cause a wave vector (Q) broadening in the low-energy (homega < or = 80 meV) commensurate spin fluctuations at (0.5, 0.5) and to suppress the intensity of spin-wave-like excitations at high energies (homega > or = 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility chi'(omega), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O(6.353) (T(c) = 18 K).     

Spin structures with frustrated moments in RNiAl intermetallic compounds

Magnetic order in the ternary equiatomic intermetallic compounds PrNiAl, DyNiAl and HoNiAl is investigated by means of neutron powder diffraction and dc- and ac-susceptibility measurements. DyNiAl shows two magnetic phases: It is ferromagnetic between T₁ =15 K and T_c = 31 K and a tilted antiferromagnet below T₁. HoNiAl is a ferromagnet in a narrow temperature region just below T_c = 14.5 K, but furthermore exhibits two tilted antiferromagnetic phases (T₁ =5.5 K, T₂ = 12.5 K). The corresponding magnetic structures differ in the ordering of some of the magnetic moments which are coupled by frustrated bonds. The existence of those frustrated moments, which is induced by the topology is a common phenomenon in the hexagonal RNiAl series. They are manifested with reduced mean values of their moments in the elastic neutron spectra. Another interesting observation is the scaling of the second temperature T₁ at which the frustrated spins change their order: For all heavy rare earths investigated so far (R = Tb, Dy, Ho) we find T₁: T_N,C ≈ 1: 2. PrNiAl displays an incommensurate sinusoidal (TSW) structure with XY spin behaviour below T_N = 6.9 K, and only one ordered phase is found. The observed ordering temperatures roughly follow the de Gennes scaling factor.     

Unusual magneto-optical phenomenon reveals low energy spin dispersion in the spin-1 anisotropic Heisenberg antiferromagnetic chain system NiCl2-4SC(NH2)_{2}

Electron paramagnetic resonance measurements of NiCl2-4SC(NH2)_{2} reveal the low-energy spin dispersion, including a magnetic-field interval in which the two-magnon continuum is within k_{B}T of the ground state, allowing a continuum of excitations over a range of k states, rather than only the k=0 single-magnon excitations. This produces a novel Y shape in the frequency-field EPR spectrum measured at T > or = 1.5 K. Since the interchain coupling J_{ perpendicular}相似文献   

Hyperfine interaction in Co2SiO4 investigated by high resolution neutron spectroscopy

We have investigated the hyperfine interaction in Co₂SiO₄ by inelastic neutron scattering with a high resolution back-scattering neutron spectrometer. The energy spectrum measured from a Co₂SiO₄ powder sample revealed inelastic peaks at at T=3.5 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature . The inelastic peaks have been interpreted to be due to the transition between hyperfine-split nuclear level of the ⁵⁹Co isotopes with spin . The temperature dependence of the energy of the inelastic peak in Co₂SiO₄ showed that this energy can be considered to be the order parameter of the antiferromagnetic phase transition. The determined hyperfine splitting in Co₂SiO₄ deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co-P amorphous alloys and CoO presumably due to the presence of unquenched orbital moment. These results are very similar to those of CoF₂ recently reported by Chatterji and Schneider [7].     

Ferroelectricity in a diatomic Ising chain as investigated by the elastic Ising model

An elastic Ising model for a one-dimensional diatomic spin chain is proposed to explain the ferroelectricity induced by the collinear magnetic order with a low-excited energy state. A statistical theory based on this model is developed to calculate the electrical and magnetic properties of Ca₃CoMnO₆, a typical quasi-one-dimensional diatomic spin chain system. The calculated ferroelectric polarization and dielectric susceptibility show a good agreement with recently reported data on Ca₃Co_2-xMn_xO₆ (x ≈0.96) (Phys. Rev. Lett. 100 047601 (2008)), although the predicted magnetic susceptibility does not coincide well with experiment. We also address the rationality and deficiency of this model by including a first-order correction which improves the consistency between the model and experiment.     

Structural and magnetic studies on RPtIn deuterides (R=Tb,Er, Tm)

In this communication, structural and magnetic properties of RPtInD_1.3 (R=Tb, Er, Tm) deuterides are reported. For the first time deuterium-rich compounds were synthesized for the RPtIn family. The investigated deuterides crystallize with the hexagonal ZrNiAl-type crystal structure, with slightly different lattice constants with respect to the basic compounds. In general, the a-lattice constant exhibit contraction, while the c-lattice constant tends to increase upon introducing deuterium. The compounds with Tb and Er shows magnetic ordering at 95 K and 15.5 K, respectively. On the other hand, for Tm based sample no magnetic ordering was evidenced down to 2 K.     

Low energy spin waves and magnetic interactions in SrFe2As2

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe2As2 (T_{N}=200-220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,Planck's omega) consists of a Bragg peak at the elastic position (Planck's omega=0 meV), a spin gap (Delta< or =6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_{N}, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe2As2 is therefore consistent with a first order phase transition, similar to the structural lattice distortion.     

Field-induced order and spin waves in the pyrochlore antiferromagnet Tb2Ti207

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.     

Magnetic interactions between Copper and RE in RE Ba2Cu3O7-δ

The antiferromagnetic ordering temperatures of the rare earth (RE) moments in RE Ba₂ Cu₃O_7-gd, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the ordering mechanism can be mainly seen as an indirect spin-spin exchange. However, the oxygen dependence of T _N is found to be reversed for the light rare earths compared to the heavy rare earths. As origin for this systematic observation an indirect interaction between the 3d-moments of copper and the 4f-moments of rare earths is discussed as a second order effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1: 2: 3 cuprates by inelastic magnetic neutron scattering. Here, the usual thermally driven increase of the magnetic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat- Mignod in YBa₂Cu₃O₇ and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO₂ planes. Furthermore, the quasielastic magnetic response has a Gaussian contribution at temperatures below 100 K, i.e., much above the long ranged ordering temperatures T _N. Magnon-like excitations appear already at slightly larger temperatures than T _N. In addition the paramagnetic inelastic spectra show only little dependence of the crystal field scheme on the oxygen concentration.