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.     

Critical behavior of the E.S.R. line widths in a quasi two-dimensional antiferromagnet MnTiO3

The critical behavior of the electron paramagnetic resonance line widths have been studied in the quasi two-dimensional antiferromagnet MnTiO₃. Line widths diverge like ΔH∝(T?T_N)^?0.49, which is in good agreement with the theoretical value predicted by Kawasaki.     

Magnon Raman scattering in the quasi 1-D antiferromagnet CsCoBr3

Raman scattering from magnons has been observed in the three magnetic phases of CsCoBr₃. In the 1-D Ising phase T > 28K a broad band at 96 cm^?1 is observed. This band grows in intensity but shows little renormalisation (100.5 cm^?1 at 14K) in the partially disordered antiferromagnetic phase 14K < T < 28K. For T < 14K additional structure at 111.5, 123.5, 133, and 141 cm^?1 is attributed to magnon-magnon combination bands. Two extra magnon branches are expected for this ferrimagnetic phase. One of these has an energy of ≈ 11 cm^?1.     

Two-magnon Raman scattering in the two-dimensional antiferromagnet K2FeF4

Two-magnon Raman scattering in the planar quadratic antiferromagnet K₂FeF₄ is investigated. The temperature dependence of the energy shift is in good agreement with second-order Green-function theory, as is the linewidth at low temperature. Numerical results, including renormalization, are the Heisenberg exchange $\text{J}\text{k}{}_{\mathrm{B}}\text{= ?14.5 ± 0.7}\text{K}$ and the anisotropy $\text{Δ(T = 0) = gμ}{}_{\mathrm{B}}\text{H}{}_{\mathrm{A}}\text{4|J|S = 0.18 ± 0.05}$, but with $\text{J[1 + Δ(T = 0)]}\text{k}{}_{\mathrm{B}}\text{= ?17.06 ± 0.10}\text{K}$.     

Neutron scattering study of two-dimensional Ising nature of K2CoF4

By neutron elastic and quasi-elastic scattering from K₂CoF₄, sublattice magnetization and spin correlations are examined quantitatively. The obtained critical exponents β, ν, γ, and η coincide precisely with those from Onsager's exact solution for the two-dimensional Ising model.     

Neutron inelastic scattering study of transverse spin fluctuations in CsNiF3: A soliton-only central peak

We have observed a quasi-elastic contribution to the spectrum of the transverse spin fluctuations S^perp;(Q, ω), perpendicular to an applied magnetic field in the easy plane of the one-dimensional ferromagnet CsNiF₃. According to the present theoretical understanding this contribution is due solely to soliton quasi-particles and it should not contain two-magnon scattering. The observed dependence on momentum transfer is as expected for soliton scattering with zero intensity at q_c = 0 rising through a maximum with increasing q_c.     

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.     

Spin-wave response in the one dimensional anisotropic antiferromagnet CsCoCl3

The magnetic excitation spectrum of CsCoCl₃ has been studied by neutron scattering. Evidence is presented that the long wavelength spectrum consists of a sharp spin-wave-like peak superposed on a broader band of excitations that extends up to the Ising-limit zone-boundary frequency. In contrast, the zone boundary response is sharp, suggesting that the continuum has a greater breadth at the zone centre. Further, it is found that the spectral weight of longitudinal scattering within the spin-wave band is small.     

Nuclear spin lattice relaxation in the linear antiferromagnet CsNiCl3

Anisotropic effects have been found in the low temperature (< 10 K) nuclear spin lattice relaxation rates in the linear paramagnet CsNiCl₃ in a strong magnetic field (～ 60 kOe). The existence of a small single ion anisotropy term may be the cause of these effects.     

Irradiation- and substitution- induced disorder in the quasi one-dimensional antiferromagnet Ag0.50Cr0.50PS3

The deviation from the 1D antiferromagnetic behaviour in Ag_0.50Cr_0.50PS₃ is greatly enhanced by substitutional or electron - irradiation - induced disorder. Accordingly, we interpret this deviation as a contribution of unpaired spins related to finite length of antiferromagnetic segments and to isolated magnetic defects. In the case of short segments, a strongly field dependent susceptibility is observed in the temperature range of 1D interactions, suggesting a possibility of 1D spin glass behaviour.     

Magnetic behavior of the two-dimensional antiferromagnet RbFeF4

RbFeF₄ shows anomalous magnetic susceptibility behavior rather similar to that of KFeF₄, in which the existence of two-dimensional antiferromagnetic ordering has recently been clearly demonstrated. A neutron diffraction study of powder samples reveals that RbFeF₄ orders antiferromagnetically in three dimensions at T_N = 134 K. In addition around T_N, the diffraction patterns indicate a magnetic ridge, thus giving, in accordance with KFeF₄, the evidence of the two-dimensional nature of the magnetism in RbFeF₄.     

Surface-layer effect in MnTiO3 ceramics at low temperatures

MnTiO₃ ceramics were prepared via the traditional solid-state reaction route. The low-temperature (100–330 K) dielectric properties of MnTiO₃ have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Our results showed that MnTiO₃ exhibits intrinsic dilectric response in a wide temperature range up to 200 K. A relaxation appears near room temperature. This relaxation can be enhanced by annealing treatment in oxygen and weakened in nitrogen. Reducing the sample thickness gradually leads to the disappearance of the relaxation. Based on these experimental facts, the relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.     

Neutron diffraction study on the hyperfine-induced nuclear spin order in the antiferromagnet PrSn3

At several $\text{(h+}\text{1}\text{2}\text{0 0)}$ reciprocal lattice positions of PrSn₃, we observed Bragg scattering of neutrons which is purely due to the nuclear spin polarization of ¹⁴¹Pr, and could measure the temperature dependence of the polarization between 10 mK and 4.2 K. From the perfectly polarized state of the nuclear spins, the difference between the spin-dependent nuclear scattering lengths b⁺ and b^- has been determined to be b⁺ - b^- = -0.110 ± 0.006 × 10^-12 cm, which is significantly larger in magnitude than the previously reported value. An additional electronic polarization which is linearly proportional to the nuclear spin polarization has also been observed.     

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.     

Zero-point spin reduction in a two-dimensional Heisenberg antiferromagnet Cu(HCOO)2 · 4H2O

The copper NMR in a two-dimensional Heisenberg antiferromagnet Cu(HCOO)₂ ·4H₂O (T_N = 17 K) was observed using a standard spin- echo method at liquid He temperatures. The field-dependence of the ⁶³Cu NMR frequency was measured for H₀ (? 56 kOe) | b-axis. The result was interpreted by taking into account a zero-point spin reduction of about 35% in addition to the rotation of the antiferromagnetic spin axis. The amount of the observed spin reduction is in good agreement with the prediction of spin-wave theory.     

Neutron diffraction study of the layered compounds MnPSe3 and FePSe3

In the insulating compounds MnPSe₃ (1) and FePSe₃ (2) the divalent transition metal ions form planar honeycomb lattices. A neutron diffraction study revealed a collinear antiferromagnetic order below T_N = 74 ± 2 K (1) and T_N = 119 ± 1 K (2) with the corresponding wavevectors k = [000] (1) and $\text{k = [}\text{1}\text{2}\text{0}\text{1}\text{2}\text{]}$ (2). In MnPSe₃ the magnetic moments (m₀ = 4.74 μ_B) lie within the basal plane and in FePSe₃ (m₀ = 4.9 μ_B) they are pointing along the c-axis. The collinear structures are determined by the dominating intralayer interactions between first (J₁), second (J₂) and third neighbours (J₃) which in MnPSe₃ are all antiferromagnetic whereas in FePSe₃J₁ is ferromagnetic and J₂ and J₃ are antiferromagnetic.     

Neutron scattering study of the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2

We have carried out a neutron scattering investigation of the static structure factorS(q _2D ) (q _2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr₂CuO₂Cl₂. For the spin correlation length we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr₂CuO₂Cl₂-Monte Carlo data, which cover the length scale from 1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0) ² for the reduced temperature range 0.16<T/2 _s <0.36, whereas current theories predict that at low temperaturesS(0)T ^{2 2}. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ _XY /J=1.4×10^–4. From measurements of the sublattice mangetization we obtain =0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La₂CuO₄, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K₂NiF₄, the gap energy in Sr₂CuO₂Cl₂ scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K₂NiF₄ using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffness _s reduced by 20% from the spin-wave value. As in Sr₂CuO₂Cl₂ we find thatS(0) ² for the reduced temperature range 0.22<T/2 _s <0.47.     

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.