Neutron scattering study of spin waves in an antiferromagnet FePt3

Spin wave dispersion relations in an antiferromagnetic ordered alloy of FePt₃ were determined along three crystallographical axes by neutron scattering. Well defined magnon groups could be observed up to the magnetic zone boundary. The dispersion relation could be interpreted in terms of the Heisenberg model using more than four exchange parameters.     

Neutron scattering study of anisotropic spin correlations in quasi two-dimensional antiferromagnet MnTiO3

An anisotropic three-dimensional neutron critical scattering has been observed from a “quasi” two-dimensional antiferromagnet MnTiO₃ in the vicinity of the Néel temperature. When temperature increases, the scattering profile transforms gradually to a ridge-like form, indicating that “cross-over” from three-dimensional to two-dimensional character appears at about 16 degrees above T_N.     

Inelastic neutron scattering study of multimagnon states in the 1-D-ferromagnet CsNiF3

Two theoretical propositions concerning scattering from higher-order-spin-wave processes have been tested by means of neutron scattering. The so-called “second magnon” scattering could not be observed as such, and an upper limit for its intensity relative to the single-spin-wave scattering is given. Experimental evidence for the scattering from the simultaneous creation of two spin waves has been observed. The results are compared with the corresponding theories.     

Neutron inelastic scattering study of exchange interactions in a ruthenium V dimer a3CaRu2O9

Inelastic neutron scattering measurements have been performed on the binuclear Ru(V) cluster compound Ba₃CaRu₂O₉. The S = 0 →S = 1 transition split levels of the electronic ground state has been observed. The value for the exchange parameter is very close to that obtained from magnetic susceptibility measurements. The observed intensity ratios of the neutron inelastic scattering transition agree well with those calculated from a simple spin-spin coupling model for the ruthenium pair.     

Ultrasonic propagation and Brillouin scattering studies in the 1D-ferromagnet CsNiF3

The elastic constants of the ID-ferromagnet CsNiF₃ and their temperature dependence in the 10 K – 300 K range have been obtained using Brillouin scattering and ultrasonic propagation measurements. The results show a marked anisotropy. A small softening of C₄₄ is observed when T decreases : it could arise from magnetic correlations. Comparison with previous experimental data derived from other techniques is discussed.     

Neutron scattering study on CeAgAs2

CeAgAs₂, an HfCuSi₂ like layered pnictide, orders antiferromagnetically at T_N=6.2(1) K. The ordering process was monitored in neutron diffraction experiments in the temperature range 10 K≥T≥3.5 K. At T=4 K the lattice parameters are a=5.7438(1) Å, b=5.7696(1) Å and c=21.0067(2) Å. The diffraction pattern of the antiferromagnetic phase with a propagation vector k=[0,0,0] point towards ferromagnetically ordered moments in Ce layers stacked along [001], the individual layers are coupled antiferromagnetically with a +− −+type sequence. The alignment of moments within the Ce layers cannot be determined reliably from the experimental data so that two different structure models are discussed. The proposed metamagnetic transition was confirmed by diffraction experiments applying an external magnetic field at T<T_N. In the interval 4 K≤T≤6 K, a relatively small field of μ₀H≈0.3 T suffices to fully suppress the antiferromagnetic ordering. The effect is completely reversible yet subject to hysteresis: After switching off the external field at any T<T_N the magnetic reflections gain their original intensity within several 10 min indicating the restoring of the antiferromagnetic phase.     

Dynamical critical slowing down in CsNiF3

Dynamical critical slowing down in CsNiF₃ is studied using the a.c. susceptibility measurements at 9.5 GHz in zero external static magnetic field. The dynamical critical exponent for the relaxation time of the in-plane spin fluctuations is obtained for the temperature interval 6 K < T < 20 K. For this temperature interval where one-dimensional spin fluctuations are dominant, very good qualitative and quantitative agreement with the spinwave calculation of the dynamical response is obtained at long wavelength and low frequency. The dynamical critical exponent for the relaxation time is measured to be 0.96 ± 0.6. At higher temperature, a gradual crossover to an isotropic Heisenberg chain behaviour is observed. For temperatures close to the 3-d antiferromagnetic ordering temperature T_N, a crossover to 3-d fluctuation regime gives rise to a speeding-up of the spin relaxation rate.     

Neutron scattering by spin fluctuations in dilute Al-Mn alloy

Neutron scattering experiments were carried out on Al(0.5%Mn) alloy at T = 80°K and 300°K. Effects of spin fluctuation and conduction electron polarization on the incoherent magnetic scattering cross section were observed.     

Hydrodynamic theory of central peak in SrTiO3

It is proposed that above displacive phase transitions driven by a soft transverse R-corner mode a central peak occurs as a consequence of the temperature dependence of the soft mode frequency. This temperature dependence gives rise to a coupling between staggered displacement and temperature variations. This proposal is supported by detailed comparison with neutron scattering and paramagnetic resonance data.     

Proton NMR study of the central peak in ferroelastic KH3 (SeO3)2

Measurements of the temperature and the magnetic field dependence of the proton spin-lattice relaxation time T₁ in KH₃ (SeO₃)₂ show the presence of a dynamic narrow central peak in the local proton fluctuation spectrum the width of which is less than 0.1 MHz.     

A microscopic theory for the central peak in SrTiO3

A microscopic theory for the central peak at the R point of SrTiO₃ and similar systems is proposed. A phonon resonance is formed by two phonons near the R point. The central resonance arises from the decay of the soft zone boundary phonon into the resonance and a zone boundary phonon. The theory applies also to other non piezoelectric and non pyroelectric soft mode systems.     

CsNiF3: Ferromagnetic chains with X-Y like anisotropy

We have measured the specific heat and the susceptibility of powdered samples of CsNiF₃, a compound which can be considered as consisting of nearly isolated ferromagnetic chains with a large uniaxial anisotropy, which favours the plane perpendicular to the chain-axis. For the intrachain exchange constant a value J/k=(8.3±0·8)K has been deduced from the susceptibility. Using this we were able to separate the magnetic contribution from the measured specific heat. The magnetic part shows a broad maximum at kT_max/J=1.8 and a small anomaly at kT_c/J=0.32, the latter reflecting the transition to 3-dimensional order. Only about 13 per cent of the magnetic entropy is gained below T_c.     

Anisotropic spin fluctuations in CeCoIn5

CeCoIn₅ is an unconventional heavy fermion superconductor with a relatively large transition temperature . NMR measurements of the spin lattice relaxation rate at the In(1) site reveal a significant anisotropy of the fluctuating hyperfine fields, which reverses below 30 K. These results suggest that two-dimensional fluctuations of the Ce 4f moments are relevant for the superconducting pairing.     

Magnetic form factor of Ho3+ in HoCrO3, an inelastic neutron scattering study

The Q dependence of the magnetic form factor of Ho³⁺ in HoCrO₃ was determined by inelastic neutron scattering. The method is compared with other techniques and the results are compared with those of other techniques and with theoretical calculations.     

Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies ( approximately 10 meV), which is localized in wave vector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.     

Neutron diffraction study of β-UD3 and β-UH3

The results of a neutron powder diffraction study on β-UD₃ and β-UH₃ are reported. Diffraction patterns have been obtained both above (220 K) and below (10 K) the Curie temperature, in order to refine the crystal structure on the basis of a large number of resolved Bragg peaks and to obtain the ordered magnetic moment. The two kinds of uranium atoms present in the structure appear to be magnetically equivalent. The observed magnetic moment for β-UD₃ at T = 10 K is μ_ord = (1.45 ? 0.11) μ_B/U_atom     

Neutron diffraction study of antiferromagnetic TbIn3

Observations were on a polycrystalline sample using unpolarised neutrons, at temperatures of 4.2 K and 100 K. There is 1 1 0 antiferromagnetic ordering. The magnetic moment is 7.2±0.4 μ_B, with its direction close to that of the c-axis.     

Neutron diffuse scattering in Fe3O4 due to molecular polarons

The results of a precise re-examination of neutron diffuse scattering above the Verwey transition of Fe₃O₄ are reported. A model to describe the properties of valence fluctuations at the B-site Fe-ions are proposed, which assumes the existance of “molecular polarons”. The experimental results of anisotropic distribution of diffuse scattering seem to give an evidence for the feasibility of the proposed model.     

On the width of the central peak in the critical scattering of γ-rays by KMnF3

The energy distribution of 14.4 keV Mössbauer γ-rays scattered from the ($\text{1}\text{2}\text{1}\text{2}\text{3}\text{2}$) reflection of KMnF₃ was measured. The result limits the energy width of the central mode at 189 K (3 degrees above the transition temperature) to less than 10^-8 eV.     

Neutron scattering study of the phase transitions in a doped polycrystalline Fe3O4: Zn0.005Fe2.995O4

Temperature dependent neutron elastic scattering and resistivity measurements have been made on a polycrystalline sample of Zn_0.005Fe_2.995O₄, which displayed a well-separated, bifurcated, heat-capacity anomaly. The lower-temperature anomaly (near 110 K) was found to be associated with a lattice distortion, but no change in neutron intensity was observed at the higher-temperature anomaly (near 119 K). The electrical resistivity of this sample also exhibited an abrupt change only at the lower-temperature anomaly. The downward shift of the lower-temperature, λ-type anomaly as a result of the very low Zn impurity is, however, confirmed by the neutron scattering results and demonstrates the extreme sensitivity of the structural transition to impurities.