Spin-wave dispersion softening in the ferromagnetic Kondo lattice model for manganites

Spin dynamics is calculated in the ferromagnetic (FM) state of the generalized Kondo lattice model taking into account strong on-site correlations between e_g electrons and antiferromagnetic (AFM) exchange among t_2g spins. Our study suggests that competing FM double-exchange and AFM super-exchange interaction lead to a rather nontrivial spin-wave spectrum. While spin excitations have a conventional Dq² spectrum in the long-wavelength limit, there is a strong deviation from the spin-wave spectrum of the isotropic Heisenberg model close to the zone boundary. The relevance of our results to the experimental data are discussed.     

Correlation between vortex lattice and crystal lattice in type II superconductors due to elastic interaction

In an elastic anisotropic crystal certain orientations of the vortex lattice with respect to the crystal lattice are favoured due to the magneto-elastic strain fields of the vortex lattice.     

Relation between giant volume magnetostriction and colossal magnetoresistance near the Curie temperature of Sm0.55Sr0.45MnO3

The magnetic, transport, and elastic properties of Sm_0.55Sr_0.45MnO₃ have been established to be interrelated. At the Curie point, one observes a large volume compression ΔV/V≈0.1%, a sharp minimum in the temperature dependence of negative volume magnetostriction ω(T), and a maximum in the temperature dependence of the electrical resistivity. Giant negative volume magnetostriction ω=?5×10^?4 has been found in a magnetic field H=0.9 T, which is accompanied by a colossal negative magnetoresistance of 44% in the same field. The results obtained are discussed in terms of a model of electronic phase separation.     

High-pressure effect on the crystal and magnetic structure of LuMnO3: Correlation between a distortion of the triangular lattice and the symmetry of the magnetic state in hexagonal frustrated manganites

The effect of a high pressure (up to 6 GPa) on the crystal and magnetic structure of the hexagonal manganite LuMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. It is found that, as the pressure increases, the ordered magnetic moment of Mn ions at T = 10 K decreases noticeably from 2.48 (0 GPa) to 1.98 μ_B (6 GPa). This decrease is due to an enhancement of the geometrical frustration effects on the triangular lattice. At the same time, the symmetry of the triangular antiferromagnetic state (the irreducible representation Γ₂) remains unchanged. A correlation is revealed between the distortion parameter of the triangular lattice formed by Mn ions and the symmetry of the antiferromagnetic state of hexagonal manganites RMnO₃. Based on this correlation, a generalized magnetic phase diagram of these compounds is constructed. The obtained phase diagram provides an explanation for the changes observed in the magnetic state of hexagonal manganites caused by high pressure and chemical substitution.     

On the correlation between flux line lattice and crystal lattice

For pure type-II superconductors with κ slightly above 1/√2 it is shown that the preferential orientations of the flux-line lattice relative to the crystal lattice are related to the anisotropy of the coherence length.     

Correlation between laminar structure and crystal lattice in type-I superconductors due to elastic interaction

The effect of the anisotropic magnetoelastic energy on the correlation between the laminar structure and the crystal lattice in the intermediate state of the type-I superconductors is investigated.     

Fe-Mn-C alloys with anomalous volume of crystal lattice

The structure of intergranular interlayers in the 110G13L steel have been investigated. An increase in the lattice volume and occurrence of magnetization under dynamic load of samples quenched to austenite have been revealed.     

Relation between charge ordering and local lattice disorder in manganites studied by EXAFS

We report extended X-ray absorption fine structure measurements at the Mn K absorption edge and in a wide temperature range on La_0.25Ca_0.75MnO₃ samples with a charge ordering transition temperature, T_CO of the Mn³⁺ and Mn⁴⁺ ions around 225 K. The mean Mn-O distances do not show major changes at the transition; however, our results show the presence of an anomalous local lattice disorder and distortion below T_CO, possibly determined by the breaking of the symmetries of the high temperature state, that is the splitting of the Mn-O distances.     

Correlation between phase diagrams and spontaneous magnetization jumps in narrow-band manganites

The correlation between the phase diagrams of narrow-band manganites and the abrupt magnetization jumps observed previously at a magnetic-field-induced transition from the antiferromagnetic to ferromagnetic state has been studied. This transition occurs during several milliseconds and is related to a spontaneous thermal avalanche; however, the mechanisms of the avalanche formation and development have not been conclusively established. It has been shown that different compositions (Pr_0.67Ca_0.33MnO₃, Sm_0.55Sr_0.45Mn¹⁸O₃, and Eu_0.58Sr_0.42MnO₃) are qualitatively characterized by analogous phase diagrams with a negative incline of the antiferromagnet/ferromagnet interface in the H-T plane; apparently, this analogy determines the avalanche-like character of the transition.     

Temperature dependence of the giant magnetostriction of submicrocrystalline dysprosium

The temperature dependence of the magnetostriction of Dy was measured in various structural states. A submicrocrystalline state was created by intense plastic deformation on Bridgman anvils. Intermediate-and large-grain states were obtained by annealing samples in a vacuum at 573 and 873 K respectively. In the large-grain state, plots of the magnetostriction versus temperature exhibit a kink at around 85 K, which is evidence of a magnetic phase transition from ferromagnetism to helical antiferromagnetism. In the submicrocrystalline state these curves have no kinks and decrease smoothly to zero at high temperatures. This is interpreted as the absence of helical antiferromagnetic order in the sample. Fiz. Tverd. Tela (St. Petersburg) 41, 1665–1667 (September 1999)     

Spin glass-like behavior,giant magnetocaloric and giant magnetoresistance effect in PrPb manganites

The Pr_1−xPb_xMnO₃ (x=0.1–0.5) perovskites have been fabricated by solid-state reaction. The X-ray diffraction patterns show that the samples are of single phase with orthorhombic structure. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit the spin glass-like state. The Curie temperature of samples increased with increase in Pb content. The maximum magnetic entropy change |ΔS_m|_max reaches the giant values of 3.91 and 3.68 J/kg K for quite low magnetic field change of 1.35 T for the samples x=0.1 and 0.4, respectively. The resistance measurements show that there is insulator–metal phase transition on the R(T) curves for samples with x?0.3. The giant magnetoresistance effect is also observed for all samples studied.     

Phase separation, percolation and giant isotope effect in manganites

Phase separation and a tendency to form inhomogeneous structures seems to be a generic property of systems with strongly correlated electrons. After shortly summarising the existing theoretical results in this direction, I concentrate on the phenomena in doped manganites. I discuss general theoretical results on the phase separation at small doping and close to the doping x=0.5. The “global” phase diagram in this region is constructed. These general results are illustrated on the example of the particular system with rich and complicated properties — (LaPr)_1−xCa_xMnO₃, in which there exist a ferromagnetic metallic (FM) phase and a charge ordered (CO) insulating one. The experimental situation in this system is discussed and the interpretation is given in the framework of the model with competition of FM and CO, and the indications of phase separation and percolative nature of this system are given. The giant isotope effect observed in this situation is shortly discussed.     

Correlation of temperature anomalies of the hyperfine field and thermal expansion of the crystal lattice

Consideration is given to the effect of thermal expansion of crystal lattice upon the partial contributions to the magnetic hyperfine field for non-magnetic atoms in metallic ferromagnets. The existence of correlation between the temperature anomalies of a hyperfine field and the thermal expansion of the crystal lattice is shown in a simple and illustrative way.     

Theory of lattice softening and spin-susceptibility in A15-compounds

The one-dimensional model of A15-compounds is used to calculate phonon frequencies Ω_λ (q) from temperature dependent screening properties of the electron system. The interrelationship between the magnetic susceptibility χ(q) and Ω_λ (q) is derived and both quantities are studied in the longwavelength limit ¦q¦=0 and for ¦q¦=2 ·k _itF. Numerical values are obtained for V₃Si and Nb₃Sn.     

Structural effects in the growth of giant magnetoresistance (GMR) spin valves

An investigation has been made of the thin-film structure and interface morphology of giant magnetoresistance (GMR) spin valves of the cobalt/copper/cobalt (Co/Cu/Co) type that were grown on polycrystalline NiO substrates at three different temperatures (150, 300 and 450 K). Sputter-depth-profile analyses indicate that the quality of the layering in the Co/Cu/Co structure was only slightly better for the 150 K sample than for the 300 K sample. For the 450 K sample, however, the Co/Cu/Co structure showed extensive disruption. The similarity in the depth-profiles for the 150 and 300 K samples indicates the sensitivity of the GMR to subtle structural differences.     

Temperature dependence of forced volume magnetostriction in fcc Fe-Ni alloys

The forced volume magnetostriction of the fcc Fe-Ni Invar alloys has been measured in the temperature range from 77 to 900 K to elucidate the magnetovolume effect at finite temperatures. The temperature dependence of (θ ω/θ(H/M) is evaluated as well as (θ ω/θH), where ω is the volume expansion. These results are discussed using a phenomenological model for the magnetovolume effect which takes into account the contribution of the correlation between local magnetic moments.     

Effects of cation size disorder and lattice distortion on metamagnetism in phase-separated manganites

The effects of A-site cation size disorder in ABO₃ type charge-ordered and antiferromagnetic Pr_0.5Ca_0.5MnO₃ system have been studied by substituting La³⁺, Sr²⁺ or Ba²⁺, while keeping the valency of Mn ions and the tolerance factor (t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO₆ octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO₆ octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites.     

Correlation between the birefringence and the structural parameter in photonic crystal fiber

In order to simply design a highly birefringent photonic crystal fiber (HB-PCF), we numerically simulated the correlation between the birefringence and the structural parameter of photonic crystal fiber with square-lattice or triangle-lattice air-holes by using multipole method. It is shown that the phase birefringence B(λ) and the group birefringence G(λ) can be modulated by the structure parameter of normalized wavelength λ/Λ and the relative air-hole size d/Λ. Numerical results show very high phase and group birefringence of the order of 10⁻². The group birefringence becomes negative in the region where phase birefringence increases with an increase in normalized wavelength that does not appear in traditional highly birefringent fibers.     

Saturation and forced volume magnetostriction of Fe-rich FeZr amorphous alloys

AC susceptibility, saturation and forced volume magnetostriction were studied on iron-rich zirconium amorphous alloys a-Fe_100-xZr_x (8 at% ≤ x ≤ 12 at%). The experimental results are discussed in relation to the re-entrant spin glass state. It is found that the transverse magnetostriction accompanies a remarkable magnetic aftereffect in the spin glass phase, and that the forced volume magnetostriction shows an apparent peak at the ferromagnet to spin glass transition where the ac susceptibility has a cusp.