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This is the first systematic study of the spin excitations around the MBZ boundary points in -Mn alloy. The magnetic inelastic neutron scattering was measured in the antiferromagnetic Mn(13.7% Ni) alloy in a sample with unequal domain distribution in the temperature range 0.025 TN -1.15 TN . The evidence of the magnetic excitations around the MBZ boundary points equivalent to the MBZ centres in -Mn alloy is presented. At low temperatures these excitations differ from excitations observed around the MBZ centres; they are rather similar to excitations observed above TN . When the temperature increases up to TN the excitations observed around MBZ centres become more similar to those around the MBZ boundary equivalent points. The results of the analysis are consistent with the general features of spin fluctuations.

High-damping alloys with the magnetomechanical damping mechanism, based on the Fe—Cr system, have been studied. It has been shown that the direct investigation of the magnetic domain structure in the bulk materials using the neutron refraction method allows one to obtain important quanti- tative information concermng the formation of the high-damping state. The dependence of the damping capacity on the average magnetic domain size was found to possess a sharp maximum, thus revealing a range of the optimal domain sizes. The results obtained are important for the development of the technology of high-damping materials.

Our studies of the itinerant antiferromagnetic -Mn-based alloys are extended to investigate magnetic excitations in a Mn(13.7%Ni) alloy. The composition of the present sample being close to the part of the phase diagram where a number of structural phase transitions take place, the basic aim of this work is the search for the effect of these transitions on spin dynamics. This is the first study of the Mn - Ni alloy in the fct (c a) and fcc phases. We obtain the concentration dependence of the low-temperature parameters of the spin wave dispersion relation which differs essentially from that found for the Mn - Fe alloys. In particular the change of the spin wave parameters in the vicinity of the phase transitions in Mn - Ni alloys is remarkable.

A systematic study of temperature dependence of magnetic inelastic neutron scattering for itinerant antiferromagnet Mn(12%Ge) is presented. At low temperatures the experimental data are well described in terms of damped spin waves. This description becomes insufficient at least 150 K below TN. In the temperature range 200-400 K the integrated intensity of distribution for energy transfer 13 THz increases by one third and the shape of the deconvoluted neutron scattering cross-section changes. These changes suggest that ca. 200 K below TN an additional contribution to the inelastic neutron scattering starts to appear and that its origin is different from spin waves.

The intermetallic compound MnNi with tetragonal structure, with the highest value, among the 3d metal alloys, of the Neel temperature and a high magnetic moment, presents a unique case for a study of 3d alloy antiferromagnets. Its highly anisotropic arrangement of spins implies a strong anisotropy of various properties. The expected anisotropy of spin-wave velocity is not confirmed. In addition, the very small value of the spin-wave-damping parameter is in strong contrast to the high damping found in all γ-Mn alloys. This difference can be explained by different electron band structures.

Essential differences in spin dynamics between Mn–Ni and Mn–Fe alloys motivated extended studies of these systems. Within these studies the uniaxial anisotropy of the critical scattering found earlier in FCC Mn(38%Ni) alloy suggested the investigation of the anisotropy of the spin-wave (SW) velocity in this alloy. The present data give the first clear evidence of the uniaxial anisotropy of the SW velocity for a 3d metal system with the full cubic symmetry.

Measurements of spin waves in homogeneous antiferromagnetic single domain fct γ-Mn(18%Cu) alloy at 10 K were performed up to energies of 82.7 meV yielding v=224±3 meV Å, Eg=7.8±0.2 meV and damping parameters Γ0=2 meV and Γ1=101.4±2.4 meV Å. There is a evidence on the anisotropy of the spin wave dispersion relation.

The magnetic excitations in antiferromagnetic manganese-copper and manganeseiron alloys are characterized by high spin-wave velocities, an energy gap at long wavelengths and wavelength dependent damping. There are strong elastic constant anomalies above TN as well as magnetoelastic distortions below TN. The theoretical explanation of these effects is reviewed.

The magnetic excitations in antiferromagnetic manganese-copper and manganeseiron alloys are characterized by high spin-wave velocities, an energy gap at long wavelengths and wavelength dependent damping. There are strong elastic constant anomalies above TN as well as magnetoelastic distortions below TN. The theoretical explanation of these effects is reviewed.

Measurements of spin waves in antiferromagnetic single domain FCT γ-Mn(10% Fe, 3% Cu) alloy at a temperature of 4.2 K and at energies up to 103meV have a given velocity, v= 290 ± 15meV A , energy gap, Eg=12+0.5meV, and damping parameters, Γ 0 =1meV, and Γ 1 =100 meV A . The intrinsic linewidths are finite and there is no evidence for any anisotropy of the spin-wave dispersion relation.

The temperature dependence of the spin wave parameters was studied in the range from 130 K to 433 K. The temperature dependence of the energy gap and sublattice magnetization follows the 3/2 power law, while damping parameters are temperature independent, except in the vicinity of TN.

The spin waves in a single-domain fct γ-Mn alloy have been measured by inelastic neutron scattering. The measurements were performed at 4.2 K for [001], [110] and [111] directions and energies up to 100 meV. Data analysis gives a spin-wave velocity, v = (290±15) meV A, an energy gap, E g = (12±0.5) meV, and a damping parameter, Γ, having about 30% of the excitation energy. Within the experimental accuracy the spin-wave velocity was independent of direction.

The critical inelastic neutron scattering was investigated for the first time in the Mn(38%Ni) alloy. The intensity distribution determined in the (001) plane around the (100) rlp confirmed the large, independent of temperature, anisotropy of the transverse component of the static generalized susceptibility. The spectral width of the dynamic part of generalized susceptibility is anisotropic in q - space. The temperature dependence of the spectral width at q = 0 is characteristic for the continuous phase transitions.

Spin waves were studied by inelastic neutron scattering in a fct (c > a) Mn(17%Ni) alloy and compared with those obtained for the fcc Mn(18%Ni) alloy. The very small difference in magnon spectra for both phases indicates their insensitivity to substantial changes of the crystal lattice at the fcc-fct (c > a) transition.

The influence of atomic ordering on the spin waves in Ni0.75Fe0.25 alloy was investigated by inelastic neutron scattering. The spin wave stiffness constant D for the annealed, quenched and 'as grown' samples was measured. Surprisingly high sensitivity of D to atomic ordering, amounting to a 30% increase, is demonstrated.

Measurements of magnetic excitations are reported for a wide range of temperature of Cr-Re alloy in the incommensurate spin density wave region. It is shown that the rapid decrease of the excitation intensity with falling temperature below the Neel temperature arises mainly not from the spin waves but from other types of scattering smeared out around the (100) point.

Etude par diffusion de neutrons de l'aimantation du sous-reseau au voisinage du point de Neel dans l'alliage Mn 0,6 Fe 0,37 Cu 0,03 -γ

Detailed neutron diffraction studies both by double axis and triple axis spectrometer are reported for the first time for fct single-domain single crystals of γ-Mn-Cu and γ-Mn-Fe. The main purpose of these studies was to elucidate further the nature of the diffuse (001) peak, forbidden by the AF1 magnetic structure, established earlier in this group of γ-Mn alloys. Line shapes and their temperature dependence were measured for (001), (100) and (010) reflections in Mn-Fe and in Mn—Cu, in the latter case both for homogeneous (quenched) and chemically decomposed (aged) samples. Only (001) reflection was found in Mn-Fe, while in Mn-Cu samples diffuse peaks were also observed at (100) and (010). Together with their temperature dependence and the results of energy analysis the data suggest a new model of the magnetic structure of these alloys. In Mn-Fe the spins deviate by a fixed angle from the c axis and this perturbation has an infinite correlation range and life time. In Mn-Cu a more complex set of perturbations is required to interpret the data.

Etude realisee par diffraction de neutrons. On montre que les moments magnetiques des atomes de manganese sont dans les plans (001) et orientes le long des directions [110]

The temperature dependence of spin-waves in Ni0.5Fe0.5 alloy was investigated. The integrated intensity measured at 530 degrees C as a function of the spin-wave energy decreased suddenly in the vicinity of 24 meV.

The concentration dependence of the exchange stiffness constant was investigated over the Fe concentration range 0 to 0.61. The influence of the ordering of Ni3Fe and NiFe alloys is discussed.

Neutron diffraction studies have been performed on five Cr-Re single crystal alloys with a Re content from 0 to 0·8 at. %. It was found that the wave vector of the sinusoidally modulated spin arrangement increases uniformly with temperature and concentration until a critical value of about 0·97. (2π/ a ) when it changes abruptly to 2π/ a . This agrees with earlier experiments on Cr-Mn alloys. For dilute Cr alloys there is thus no essential difference between the addition of 5 d -electrons and 3 d -electrons. It was furthermore found that the most dilute alloys undergo a first order transition at the Neel temperature in agreement with earlier observations on pure Cr. Due to experimental difficulties the order of the transition at the Neel temperature cannot be determined unambiguously for the more concentrated alloys.

Different experiments for determining the slow-neutron wavelength distribution in the region 227-3 meV have been carried out, and the results compared. It is concluded that the slow-neutron wave-length distribution can be determined accurately by elastic scattering on a pure incoherent or a pure coherent scatterer.

A new method is described for neutron diffraction structure investigations of single crystals using the time-of-flight technique. A pulsed incident neutron beam is used, and the neutrons scattered from a single-crystal sample within an angular range of Δθ are counted by means of a multi-time-channel analysing system. In this way the integrated intensities are measured for several reciprocal lattice points simultaneously. Two cases are discussed, one involving a large and the other a small range of scattering angles. The feasibility of the method has been demonstrated for single crystals of aluminium and chromium at the DR 3 reactor at Riso and at the EWA reactor at świerk.

Detailed neutron diffraction investigations have been performed on Cr-Re alloys in order to explain the several observations in Cr alloys of the coexistence of a commensurable and an oscillatory magnetic phase. It is concluded that the individual magnetic phases probably occur in separate domains.

Spin waves were measured over a wide energy range in single-domain γ-Mn-Fe fct antiferromagnetic alloys. Both the energy gap in the spin wave spectrum and the magnon velocity are not much different form those in fcc γ-Mn-Fe alloys.

The experience with a metallic, multicrystal focusing monochromator system with an adjustable radius of curvature used in neutron scattering experiments is described. The system was used both as monochromator and analyser. Easily and quickly tuned to given experimental conditions the system assured flux gains for magnons and phonons up to 2.7 accompanied by a marked improvement of the peak to background ratio and without noticeable changes in resolution.

Neutron scattering studies of a diffuse peak at the (100) and (001) r.l. points ware performed in Mn-Fe and Mn-Cu fct single-crystal single-domain samples. Temperature dependence of integrated intensity of (001) peak in Mn-Fe follows closely the sublattice magnetization, while in Mn-Cu it shows a maximum at T N . The origin of the scattering present at (100) — Z.B. point in Mn-Cu remains unknown.

Inelastic neutron scattering measurements performed for Mn(38% Ni) alloy yielded the following values of the spin wave spectrum: the velocity v = 140 ± 20 meV A and the energy gap E g ⋍ 3.5 meV . A considerable reduction of the SW lifetimes was observed. The SW parameters in Mn-Ni alloys are compared to those obtained for the Mn-Fe system.

The neutron scattering studies of the sublattice magnetization in the vicinity of the Neel temperature in γ-Mn 0.6 Fe 0.37 Cu 0.03 and Mn 0.62 Ni 0.38 alloys reveal marked critical behaviour. The magnetization critical index β is approximately equal to 0.4 for both alloys. The transverse spin correlations are anisotropic, showing weaker interactions in the direction perpendicular to the magnetic sheets than within them.

Detailed investigations of magnetic excitations by neutron spectrometry are reported for the Cr(0.8% Re) crystal. The integrated intensity of spin waves decreases slowly with falling temperature contrary to the rapid decrease found in the incommensurate spin density wave (SDW) phase and also quite different from the rapid increase found in the commensurate SDW phase of the Cr-Mn alloy.

Magnetic excitations in a Co-Fe alloy were investigated by inelastic neutron scattering in a wide temperature range. The observed peaks remained fairly intense and well defined close to and above Tc even in the low momentum transfer region where no spin wave peaks could be observed in pure nickel and iron.

Neutron diffraction studies of single‐domain single crystals of fct γ‐Mn‐Cu and γ‐Mn‐Fe alloys have shown the presence of a small deviation of magnetic moment direction from c axis in both alloys and existence of additional perturbations in Mn‐Cu. When compared with existing results for fcc γ‐Mn alloys our data show no sensitivity of the SW dispersion relation to tetragonal distortion.

We have confirmed the significant anisotropy of the generalised magnetic susceptibility in the paramagnetic and antiferromagnetic phase of the fcc Mn–Ni alloys. In this paper the detailed account of the experiments performed on the Mn0.71Ni0.29 alloy is given. Our main result for the paramagnetic phase is that the correlation length is larger for the direction parallel to the anisotropy axis than for the perpendicular one by a factor of 1.6. The spin-wave velocity observed at 15 K is higher for the direction parallel to the anisotropy axis than that for the perpendicular direction by a factor of 1.2. These results are similar to that for the Mn0.62Ni0.38 alloy. Our results indicate that the magnetic carriers are more localized and the magnetic interactions are less extended in alloys with higher Ni concentration.

The spin wave stiffness for NiCo alloys was investigated by inelastic neutron scattering. Contrary to spin wave resonance data and the rigid band model calculations within RPA, the stiffness constant D does not decrease with Co concentration.

Spin wave dispersion relations were measured in Ni-Cu alloys by triple axis neutron spectrometry. The results were compared with theoretical calculations of the composition dependence of the spin wave stiffness constant D . The estimates were made in the Heisenberg model on the effect of clustering in Ni-Cu alloys on the value of D .

Spin waves in Fe(30% Cr) and Fe(50% Cr) alloys were investigated by triple axis neutron spectrometry. The values obtained for the spin wave stiffness constant D are: for Fe(50% Cr): (150 ± 20) meV A 2 at 295 K and (200 ± 20) meV A 2 at 104 K, and for Fe(30% Cr): (220 ± 20) meV A 2 at both temperatures.

Magnetic excitations were investigated in a homogeneous single-domain Mn(18% Cu) alloy over a wide energy transfer and temperature range and at different reciprocal lattice points. Inelastic scattering around the (100), clearly established, is a puzzling phenomenon as is also the serious disturbance of spin wave compared to highly non-homogeneous samples.

The angular distributions of the prompt neutrons of specified mean energies emitted in the thermal neutron fission of U 235 have been measured with respect to the direction of motion of light fragments. A combination of a gridded ionisation chamber and a gas scintillation counter system using Xenon gas was used to measure the energy of the fission fragments as well as their direction of motion relative to the electric field direction of the ionisation chamber. A fast neutron detector was placed along the electric field direction at a distance of one meter to measure the energy of the prompt neutrons by time of flight technique. The distributions obtained are compared with the distributions computed on the assumption of isotropic emission of neutrons with a Maxwellian emission spectrum. It is shown that the laboratory anisotropy N (0°)/ N (90°) is an increasing function of the neutron energy in the laboratory system. The laboratory angular distributions are transformed to obtain the centre-of-mass angular distributions for two groups of prompt neutron emission energies. Anisotropy of the form ( a + b cos 2 θ) is indicated for prompt neutrons with emission energies greater than 1 MeV. Anisotropy of emission is discussed in terms of the possibility of the fragments acquiring a large angular momentum at the instant of scission.