Magnetic excitations in neodymium,a sinusoidally modulated antiferromagnet

We report the first unambiguous observations of the magnetic excitations in neodymium metal, which exhibits complex antiferromagnetic structures below T_N = 20 K. The wavevector and temperature dependence of the excitations have been determined from neutron inelastic scattering measurements made at the Institut Laue-Langevin. Two district, low-energy, magnetic modes are observed for the [100] basal plane direction (TM) whilst there is only a single mode in the c-direction (TA) These modes are only observed clearly at low temperatures (T?8 K): they broaden and soften as the temperature is raised through T_N. Measurements were also made of the longitudinal and transverse acoustic phonon spectra in neodymium. The predictions of current theories of the excitations in sinusoidally modulated magnetic structures are discussed and compared with the results of our observations.     

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.     

Long-wavelength magnetic excitations in the Fe<Subscript>65</Subscript>Ni<Subscript>35</Subscript> alloy

A theoretical analysis of small-angle neutron scattering is performed for the Fe₆₅Ni₃₅ alloy at a temperature T=0.9T _C =450 K. The results obtained indicate the existence of long-wavelength magnetic excitations that do not correspond to spin waves of the Holstein-Primakoff type. The possible nature of these excitations is discussed.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.     

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn_0.75Fe_0.25Ge, the magnetic-ordering temperature T _N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T _N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.     

Effect of neutron irradiation on the long-wavelength magnetic excitations in the Zn0.60Ni0.40Fe2O4 ferrite

An energy analysis of small-angle neutron scattering in the Zn_0.60Ni_0.40Fe₂O₄ ferrite nonirradiated and irradiated by fast neutrons has been performed. It has been revealed that the inelastic neutron scattering in the initial ferrite below T _C is due to not only the intradomain spin waves but also the significant contribution of the intraboundary long-wavelength magnetic excitations (w) and those localized near the domain boundaries (nw). The number of these magnetic excitations in the irradiated sample is significantly smaller. The elevated density of states of the w and nw magnetic excitations is assumed to be related to the competition between the intersublattice and intrasublattice exchange interactions, which is significantly weakened in the irradiated ferrite, where the corresponding exchange integrals are related by the inequality J _AB ? J _BB .     

Magnetic excitations in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

Magnetic excitations in the antiferromagnetic Bi₂CuO₄ (T _N =42K) are investigated on the basis of anisotropic exchange interaction between spins of Cu²⁺ ions. We calculate the dispersion curves and evaluate the intensity of the inelastic neutron scattering by spin wave excitations. Spin contraction at OK and the effect of spin wave interaction are studied.     

Magnetic phase transition in MnTe2

The magnetic phase transition in MnTe₂ has been investigated by neutron powder diffraction. MnTe₂ orders at T_N=86.6 K with a second-order transition. Strong critical scattering has been observed both below and above T_N and the critical scattering seems to be centered at a slightly higher angle than the magnetic 100 Bragg peak.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

Spin glass state in crystals of barium ferrigermanate Ba2Fe2GeO7

The temperature dependence of the magnetization and elastic neutron scattering spectra of Ba₂Fe₂GeO₇ barium ferrigermanate polycrystals are studied. The magnetization is found to depend on the magnetic prehistory of a Ba₂Fe₂GeO₇ sample below T = 8 K. Analysis of the neutron scattering spectra does not reveal long-range magnetic order down to 2 K. Our experimental data indicate the existence of a spin glass state in Ba₂Fe₂GeO₇ polycrystals.     

Magnetic excitations in the quasi one-dimensional antiferromagnetic singlet groundstate system CsFeBr3

We have measured the magnetic excitations in CsFeBr₃ along the chain direction (z-axis) and perpendicular to it by inelastic neutron scattering. The measured dispersion curves can be reproduced by the formula $$\omega ^2 (q) = A^2 - 8A[J\cos (\pi q_c ) + J'\gamma (2\pi q_ \bot )]R(T)$$ which is very different from dispersion relations for usual 1 D antiferromagnets, because of the singlet groundstate of this system. The large value of the anisotropy energyA/k=29.8±0.5 K, which is independent of temperature, causes the singlet groundstate. The intra-and inter-chain exchange parameters areJ/k=?3.2±0.15K andJ'/k=?0.32±0.02 K determined atT=1.4 K. At 1.4 K the renormalisation factor is taken to beR(T)=1. AtT=30 K the value forR(T) was found to be 0.5±0.1 keepingJ andJ' unchanged. The excitations at (1/3 1/31) show soft mode behaviour but no phase transition. The observed intensities are interpreted by a heuristic model for the eigenvectors in the excited state including antiferro-as well as ferromagnetic configurations of the fluctuations.     

Investigations of magnetic excitations in a La2CuO4 single crystal

A detailed neutron scattering study is carried out of magnetic excitations in pure La₂CuO₄ in the low energy region up to 15 meV, at temperatures in the vicinity of the transition temperatureT _N to the paramagnetic state. In particular the question of existence of propagating spin waves in this energy region is addressed. The experimental results show that the energy region studied (up to 15 meV) is characterized by aquasielastic peak of the width of order 4 meV (at 300 K). Thus the magnetic correlation function has relaxational behavior in the studied energy region, no indications for propagating spin waves are obtained. The measured quasielastic width agrees well with existing theories. Polarized neutrons and polarization analysis are also used to identify the magnetic origin of the observed scattering.     

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.     

Crystal electric field excitations in ferromagnetic CeTX compounds

A ferromagnetic ground state was identified for the compounds CeCuGe (T_C=10 K), CeCuSi (T_C=15 K) [F. Yang, et al., J. Appl. Phys. 69 (1991) 4705] and CeAuGe (T_C=10 K) [R. Pottgen, J. Magn. Magn. Mater. 152 (1996) 196]. The observed saturation magnetic moment values at low temperatures for all three compounds are considerably less than the theoretically expected value g_JJ=2.14μ_B for the free Ce³⁺ ion involving the entire six-fold J=5/2 multiplet, and thus provide a first indication of partial lifting of the f-electron level degeneracy in these compounds. Specific heat data yield crystal electric field (CEF) excitation energies (Δ_Sch) equivalent to 140 K for CeCuGe, 110 K for CeCuSi and 280 K for CeAuGe. To confirm the presence of CEF excitations directly, we have carried out inelastic neutron scattering (INS) measurements on all three compounds, using the HET spectrometer at ISIS Facility. Here, we present a detailed analysis of the INS spectra of CeCuSi on the basis of a CEF model and the detailed analysis of the INS of the other two compounds will be reported elsewhere.     

On the nonlinear dynamics of the easy-plane antiferromagnetic chain in an external magnetic field

We present a theoretical investigation of soliton excitations in an easy-plane 1d antiferromagnet in an external magnetic field beyond the Sine-Gordon approximation. We find that there exist two types of solitons. One of them depends strongly on the magnetic field, becoming unstable at a critical magnetic field B_c. The other one is stable for all magnetic fields and can be described by a Sine-Gordon model. We discuss the relevance of these excitations for neutron scattering experiments.     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

Neutron scattering study of the 1-D short range and 3-D long range magnetic order in TMNB

Quasielastic neutron scattering experiments on TMNB show for T > 2.8 K a diffraction pattern characteristic for a 1-D ferromagnet with an exchange energy along the chain J/k_B ? 10 K. For T < T_N =2.7 K the 3-D magnetic ordering was found to be of a simple antiferromagnetic type. A lattice distortion was observed as well below 200 K.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Magnetic-field-induced slowing-down of molecular rotation in C<Subscript>60</Subscript> crystals

The molecular dynamics of C₆₀ crystals was studied by inelastic neutron scattering at T=290 K, i.e., above the first-order phase transition temperature (T_C≈260 K), in the region of free C₆₀-spheroid rotation in the lattice. The energy broadening of the original neutron spectrum 2Γ₀≈0.1 meV for a momentum transfer q=2 Å^?1 is in agreement with NMR data on the rotational relaxation time of the molecule τ～10^?11 s～ ?Γ₀. This effect was observed to decrease in magnetic fields H=2.5–4.5 kOe applied along the scattering vector: Γ_H=0.7Γ₀. The slowing-down of the molecular rotation is discussed in connection with the interaction of a magnetic field with the molecular currents, which fluctuate when the C₆₀ cage rotates.