Dynamic magnetic properties of ferromagnetic and antiferromagnetic Heusler alloys

Inelastic neutron scattering measurements on a ferromagnetic Heusler alloy, Pd₂MnIn_1?xSn_x at the composition x = 0.75, have established the spin wave dispersion in the three principal symmetry directions. The results have been interpreted using a simple Heisenberg model in which the exchange constants are of long range, extending beyond 12 Å. The Curie temperature, spin wave stiffness constant and the thermal variation of the magnetisation, calculated using the derived exchange parameters are in close agreement with observation.Anomalies in the spin wave dispersion, which are also present to a lesser degree in Pd₂MnSn, have been interpreted as precursor effects associated with the onset of antiferromagnetic ordering, type AF3A, which is the magnetic structure observed in the range 0.2 ? x ? 0.6.     

Transport properties of the antiferromagnetic systems with fluctuating valence

We present the calculation of the DC resistivity (conductivity) for the antiferromagnetic, two-band, extended s–f model. The influence on the finite bandwidth of the narrow 4f (5f) band on the transport properties of the model is investigated. We notice that the increase of the 4f (5f) bandwidth destroys the antiferromagnetic order and the system becomes paramagnetic in all temperatures. A systematic review of the DC resistivity (conductivity) is presented in the form of the 3D plots including different average occupation numbers of electrons per site (n=0.5, 0.75, 1, 1.25, 1.5, 1.75, 2), different relative positions of the 4f (5f) band and different 4f (5f) bandwidths. The calculated temperature dependence of the DC resistivity (conductivity) shows great similarities to the experimental results for many intermediate-valence rare-earth and actinide-based materials.     

Transport properties of dilute magnetic alloys

The electrical resistivity, thermopower, and the electronic part of the thermal resistivity of dilute magnetic alloys are calculated in the framework of the Suhl-Nagoaka theory. Using Bloomfield's and Hamann's solution of the Nagaoka equations, we derive expressions for the transport quantities in the limitT? ¯ T_K andT?¯ T_K to order (In ¯T _K /T)^?4 where ¯T _K is the Kondo temperature which may depend on the spin independent scattering. We find that the thermopower and deviations from the Wiedemann-Franz law in this limit decrease as ¦In ¯T_K/T¦^?3 if one neglects a trivial temperature dependence of the thermopower due to the electron-phonon interaction.     

Elastic constants of antiferromagnetic chromium-vanadium alloys

Single crystals of chromium alloys containing 0.67 and 1.5% vanadium were prepared by the floating-zone technique. Elastic constants and attenuation were measured by the pulse-echo method between 77 and 300 K. The changes of the elastic properties in the paramagnetic and antiferromagnetic states and in particular at the magnetic transition are described and discussed.     

Modelling magnetic defects in antiferromagnetic alloys

A model for the distribution of magnetic moment in antiferromagnetic alloys is developed from a model for ferromagnetic alloys including a magnetic environment effect which has been successful for Ni alloys [5]. This model is compared with the Marshall model for neutron scattering from antiferromagnetic alloys, and the effect of atomic short range order on both models is discussed.     

Influence of the spin-orbital interaction on the kinetic properties of ferro- and antiferromagnetic alloys

The s-d(f) exchange model is used to estimate the influence of the spin-orbital interaction on the kinetic coefficients of binary ferro- and antiferromagnetic alloys, including the electric and thermal resistivities and the thermo-electromotive force.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 21–28, May, 1972.     

Ultrasonic attenuation in antiferromagnetic chromium - vanadium alloys

Details of the temperature dependence of the attenuation of ultrasonic longitudinal waves in chromium-rich vanadium crystals are presented. The attenuation curves exhibit an oscillatory shape around the Neel temperature in contrast to the sharp peak observed in unalloyed chromium at that temperature.     

BCS-like behaviour in the superconducting state of itinerant antiferromagnetic CrRe alloys

Measurements of the heat capacity and the magnetic susceptibility have revealed BCS-like behaviour in the superconducting state of itinerant antiferromagnetic Cr_1?xRe_x alloys for x = 0.30) and 0.26. The thermodynamic quantities, such as electronic heat capacity and thermodynamic critical field have been reproduced with the BCS theory with the energy gap Δ = (1.76 ± 0.05)k_BT_s, where T_S is the superconducting transition temperature for the corresponding system: T_S = 3.61 K (2.35 K) for x = 0.30 (0.26).     

The variation of sublattice moment with composition for antiferromagnetic nickel-manganese alloys

Neutron diffraction from polycrystalline samples containing from 30 to 90 at% Mn has been used to follow the variation of the sublattice moment with composition. The 30 at% sample is not antiferromagnetic at 7 K but the 35 at% sample is antiferromagnetic with a moment of 0.65μ_B. As the Mn concentration is increased the sublattice moment increases monotonically, and finally linearly, extrapolating through the composition gap at about 50 at% Mn where the face centred cubic phase is not stable.     

Optical properties of antiferromagnetic/ion-crystal superlattices

Transmission, refraction and absorption properties of an antiferromagnetic/ion-crystal superlattice are investigated. The transmission spectra based on FeF₂/TlBr superlattices reveal that there exist two intriguing guided modes in a wide stop band. Additionally, FeF₂/TlBr superlattices possess either the negative refraction or the quasi left-handedness, or even simultaneously hold them at certain frequencies of two guided modes, which require both negative magnetic permeability of antiferromagnetic layers and negative permittivity of ion-crystal layers. Frequency regimes of the guided modes will be dependent on the magnitude of the external magnetic field. Therefore, handedness and refraction properties of the system can be manipulated by modifying the external magnetic field. Absorption spectra exhibit that absorption corresponding to guided modes is noticeable.     

A neutron study of the antiferromagnetic order in CrSi alloys

Neutron diffraction measurements were made on polycrystalline CrSi alloys containing 0.90, 1.37, 1.85 and 2.74 at.% Si to establish the types of antiferromagnetic order in this system. The results show that three types of antiferromagnetic order occur in this concentration range. Below 1 at.% Si the behavior is similar to that of pure Cr with a transversely polarized spin density wave at high temperatures and a spin-flip to longitudinal polarization at low temperatures. Above 1.8 at.% Si only the commensurate structure develops while both the commensurate and incommensurate structures occur in the intermediate concentrations. The magnitude of the modulation wave vectors and also the average moments increase with increasing Si content. Within the rigid-band extension of the Fermi surface “nesting” model of Cr this implies that Si behaves as an electron donor in Cr.     

Magnetic structure of single-domain antiferromagnetic FCT γ-Mn-Cu and γ-Mn-Fe alloys

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.     

Thermal and magnetic properties of degenerate antiferromagnetic semiconductors

Based on the model presented by Umehara and Kasuya, specific heat, susceptibility and magnetization of degenerate antiferromagnetic semiconductors are investigated theoretically. It is suggested that the model explains very well the qualitative behaviour of the thermal and magnetic properties of non-stoichiometric gadolinium-chalcogenides of Th₃P₄ type crystal structure with small number of conduction electrons.     

Anomalous magnetic properties of electron-irradiated antiferromagnetic copper monoxide

The influence of radiation defects on the magnetic properties of polycrystalline CuO and a high density nanoceramic with crystallite sizes d = 5 and 15 nm has been studied in the temperature range T = 77–300 K. Electron irradiation at fluences Φ = 5 × 10¹⁸ cm^?2 initiated an increase in the susceptibility χ ～ 1/T below 150 K, a feature anomalous for 3D antiferromagnets. The nonlinear behavior of magnetization in weak fields, the increase in the magnetic moment with decreasing temperature, and observation of the spontaneous magnetic moment at temperatures T < 150 K can be attributed to local changes in the exchange parameters and the formation of clusters with uncompensated magnetic moments in the antiferromagnetic matrix near point defects.     

Mössbauer effect study of antiferromagnetic Fe–Mn–Si alloys

Antiferromagnetic Fe–30Mn–Si alloys containing 2.0–8.7 at.% Si are known to exhibit several attractive physical properties at Néel temperatures which render them candidate materials for functional alloys applications. The Néel transitions and anomalous transport phenomena have been studied extensively in a wide temperature range. In the present work, the hyperfine interactions are studied by Mössbauer spectroscopy measured at temperatures 95–623 K. It is found that the Mössbauer spectra are singlets at temperatures above the Néel temperature and doublets below the Néel temperature. The alloys have a small hyperfine field around the Fe nuclei below the Néel temperature and the hyperfine field increases linearly with increasing silicon concentration. This can be explained by the presence of a localised net magnetic moment on the Fe nuclei which is induced by the silicon atoms. A decrease in isomer shift with increasing silicon concentration is observed and this can be accounted for by the change in the occupation of the Fe 3d shell. There is a small quadrupole splitting, it increases with increasing silicon concentration, and is consistent with the lattice shrinking and magnetostriction.     

Transport properties of SmO

The electrical resistivity and thermoelectric power of SmO are measured between 4.2 and 300 K. The existence of a low temperature T² law in the resistivity and the large and roughly constant magnetic susceptibility suggest that SmO presents large spin fluctuations.     

Transport properties of silicon

Electrical conductivity, thermal conductivity, and thermoelectric power of single-crystalline silicon are investigated at temperatures between 2 and 300 K. From the measured data we calculate the mean free path of electrons and phonons and separate diffusion part and phonon-drag part of the thermoelectric power. Using a new method, we evaluate the mean free path of those phonons which are responsible for the phonon drag effect.Dedicated to H.-J. Queisser on the occasion of his 60th birthday