Table of contents

Micro-Raman scattering was used to characterize spinel CoFe₂O₄ at high temperature up to 870 K and under an external magnetic field up to 6.0 kOe. It was found that the rapid increase in the linewidth of the Raman modes was related to the inter-site cation migration starting at ∼390 K. A red-shift of the Raman peaks induced by the magnetic ordering was observed upon applying a magnetic field at room temperature. Phase analysis of CoFe₂O₄ powder was also carried out by means of x-ray diffraction and micro-Raman spectroscopy in this work.

In the first experiment of its kind, we have used x-ray diffraction to study magnetic and charge properties of an antiferromagnet as a function of an applied magnetic field H. The intensity of hard x-rays diffracted at the space-group-forbidden reflection (3, 0, 0) of MnF₂ with H parallel to the easy axis at H = H_c /√2 shows evidence of a spin-flop transition associated with the surface magnetization, and at H_c = 9.3 T a spin-flop transition in the bulk. Above H_c we found intensity much larger than that predicted for purely magnetic scattering. We attribute the unexpected intensity to charge scattering created by a field-induced structural phase transition.

Magnetic oxide superlattices are attractive model systems in which to study coupling and interface effects. After briefly summarizing synthesis and characterization methods, this review describes recent experimental results obtained in investigating epitaxial oxide multilayers comprised primarily of ferromagnetic/paramagnetic, ferromagnetic/antiferromagnetic, and antiferromagnetic/antiferromagnetic materials. The results are discussed in terms of their implications for exchange coupling, exchange biasing, and novel magnetic structures.

Order–disorder transformation triggered by mechanical activation in a perovskite structure was observed in Pb(Sc_1/2Ta_1/2)O₃; it is simulated using a Monte Carlo algorithm, based on the competition between mechanical activation leading to disordering and the thermal diffusion recovering the ordering. The time evolution of the long-range order (LRO) from an initial ordered state shows a steady decrease at the initial stage and then becomes more or less stabilized over a prolonged period; while from the disordered initial state, LRO increases first and then stabilizes at a similar end value. Thermal diffusion is the dominant process at relatively high temperatures, leading to the disorder-to-order transformation. The effect of mechanical activation becomes significant and results in order-to-disorder transformation at relatively low temperatures. Both the mechanical activation intensity and the vacancy migration energy exert an impact on the degree of ordering and the order–disorder transformation temperature at low temperatures. Snapshot images of the simulation demonstrate the competition between thermal diffusion and mechanical activation, which refines the domain size.

The magnetic susceptibility of the prototype of quasi-one-dimensionally conducting arene radical cation salts, the naphthalene salt (C₁₀H₈)₂AsF₆, is analysed. The Peierls transition is characterized by a molecular field temperature T_MF of about 570 K and an actual three-dimensional transition temperature T_P of 234 ± 4 K. The low-temperature gap is extrapolated to 2Δ(0)/k_B = 1950 ± 75 K. The naphthalene salt thus has the highest Peierls transition temperature among the arene radical cation salts analysed so far.

In the most popular approach to the numerical study of the Anderson metal–insulator transition the transfer matrix (TM) method is combined with finite-size scaling ideas. This approach requires large computer resources to overcome the statistical fluctuations and to accumulate data for a sufficient range of different values of disorder or energy. In this paper we present an alternative approach in which the basic TM is extended to calculate the derivative with respect to disorder. By so doing we are able to concentrate on a single value of energy or disorder and, potentially, to calculate the critical behaviour much more efficiently and independently of the assumed range of the critical regime. We present some initial results which illustrate both the advantages and the drawbacks of the method.

The relative intensities in two polarization directions of the incident radiation, and the line strengths of the (⁷F ₀)Γ_1g → (⁵D ₂)Γ_5g, Γ_3g transitions of Eu³⁺ in the cubic crystal host Cs₂NaYF₆ have been calculated using the third-order Judd–Pooler (JP) formalism. The calculated relative intensities are in semiquantitative agreement with experiment, and in good agreement with the results of the direct calculation. The ratio of the line strengths of the Γ_1g → Γ_3g transition from the JP and direct calculations is similar for the cases when the initial and final states are assumed to be pure to that when the multiplets with spin–orbit admixtures are included in the initial and final states. The ratio assumes the correct order of magnitude when the intermediate-state barycentre energy is lowered from the formal Judd–Ofelt–Axe value to a physically intuitive one.

We have studied the superconducting properties of (110) textured Cr/V/Cr trilayers grown on Al₂O₃(1 20), representing an antiferromagnet/superconductor (AF/SC) proximity system. We have prepared several series of samples with either d_V, the thickness of the V layer, or d_Cr, the thickness of the Cr layer being constant, while varying the other thickness. We find an overall good agreement with the classical Werthamer theory of the AF/SC proximity effect, with one noticeable exception. When plotting the superconducting transition temperature as a function of the thickness d_Cr, we find an anomalous drop of T_c(d_Cr) at d_Cr ≈ 6 nm with a sudden quenching of the superconductivity for layer thicknesses d_Cr ≥ 6 nm. We argue that this drop of T_c might be correlated with the onset of an incommensurate spin density wave state in the Cr film.

Magnetization of Mn_1−xZn_xPS₃ in fields up to 35 T is presented. Small amounts of weakly bound moments have been observed for the more diluted compounds with a preferential orientation equal to that of the long-range antiferromagnetic order. The width of the spin-flop transition can be described by static domain wall broadening which is limited by the magnetic domain size. We estimate an average domain size of 200–250 nm.

Polarized neutron diffraction has been used to determine the manganese magnetic moments aligned by magnetic fields applied parallel to the [001] axis in the antiferromagnetic phases of Mn₅Si₃. It has been found that in the two antiferromagnetic phases which exist below T_N the magnetic moments on the manganese sites which can be aligned by a magnetic field all have the same order of magnitude, although in the antiferromagnetic structures the ordered moments are very different. The transition, on cooling, from the AF2 to the AF1 phase is accompanied by a sharp decrease in the susceptibilities of all the sites. This uniform decrease is not consistent with disordered local moments on the sites which do not have ordered moments, but can be understood using the same criteria for stability of Mn moments as have been invoked for manganese rare-earth compounds (Ballou R, Lacroix C and Nunez Regueiro M D 1991 Phys. Rev. Lett.66 1910).

It is shown theoretically that by the use of two radio-frequency fields of the same resonance frequency but with the different phases and directions the degeneracy of the energy spectrum of a spin system with I = 3/2 is removed. This leads to four non-degenerate spin states which can be used as a platform for quantum computing. The feasibility of quantum computing based on a pure (without DC magnetic fields) nuclear quadrupole resonance technique is investigated in detail. Various quantum logic gates can be constructed by using different excitation techniques allowing different manipulations with the spin system states. Three realizations of quantum logic gates are considered: the application of an additional magnetic field with the resonance frequency, the amplitude modulation of one of the applied RF fields by the resonance frequency field, and the level-crossing method. It is shown that the probabilities of the resonance transitions depend on the method of excitation and on the direction of the excitation field. Feasibility of quantum computing is demonstrated with the examples of constructing a controlled-NOT logic gate using the resonance excitation technique and SWAP and NOT2 logic gates using the level-crossing method.

Raman scattering experiments have been performed on orientationally disordered crystal chloroadamantane under pressure. Investigations carried out in both the lattice-mode and the internal-mode regions have shown that this plastic crystal exhibits spectroscopic features common to most molecular solids. The pressure-induced disorder–order phase transition around 0.5 GPa has been analysed using crystallographic and Raman data in the low-temperature phase. Additional features were observed with further compression above 8 GPa. Two different hypotheses are proposed to explain these features.

The new compounds Ln[M(OH)₆Mo₆O₁₈]·11H₂O (Ln = Tb, Eu and M = Al, Cr) were synthesized and their luminescence properties analysed and discussed. Results show that there is a very efficient energy transfer from Eu³⁺ (Tb³⁺) to Cr³⁺ ions in Eu[Cr(OH)₆Mo₆O₁₈]·11H₂O and Tb[Cr(OH)₆Mo₆O₁₈]·11H₂O. The transfer strongly quenches Eu³⁺ (Tb³⁺) luminescence and greatly shortens the lifetimes of the radiative levels. By using selective excitation at low temperature, the transfer mechanisms are investigated and models are presented. Based on the above model, a rate of ET k_C = 1.33 × 10⁶ s⁻¹ from Tb³⁺ to Cr³⁺ ions in Tb[Cr(OH)₆Mo₆O₁₈]·11H₂O at 12 K is obtained.

In a recent paper Michalopoulou et al(Michalopolou A, Syskakis E and Papastaikoudis C 2001 J. Phys.: Condens. Matter13 11 615) reported the measurements of electrical resistivity and specific heat at zero magnetic field carried out on polycrystalline non-stoichiometric La_{0.95 −x}Sr_xMnO₃ manganites. In particular, the authors attributed the low temperature (LT) behaviour of resistivity (shallow minimum and slight upturn at lowest temperatures) to three-dimensional electron–electron interaction enhanced by disorder, using results of numerical fittings of resistivity versus temperature dependences in the interval 4.2–40 K. It is shown in this comment that such analysis may be not valid for polycrystalline manganites where relatively strong grain boundary effects might mask a weak contribution of quantum effects to the LT resistivity. The crucial test of applicability of the theory of quantum corrections to conductivity in this case is the resistive measurements under non-zero magnetic field.

The authors of the comment dispute that the shallow minimum, observed at low temperatures in different manganates, has the origin to the 3D enhanced electron–electron interaction. In this reply various arguments are used to show that the cause of this phenomenon is more probably the electron–electron interaction.