Neutron diffraction, magnetic, and magnetoelectric studies of phase transitions in multiferroic Mn0.90Co0.10WO4

We have studied various spontaneous and magnetic-field-induced phase transitions in single crystals of multiferroic Mn0.9Co0.1WO4 using magnetic and magnetoelectric measurements and neutron diffraction. Compared to pure MnWO4, our data consistently confirm that the anisotropic Co2+ ions induce reorientation of the spin cycloid structure to the ac plane and reveal Pa and Pc components of spontaneous electric polarization.Field-induced phase transitions accompanied by anomalies of magnetic susceptibility and suppression of both Pa and Pc polarizations have been observed for H∥c (∼3 T) and H∥a (∼8.5 T). Neutron diffraction has revealed that in both cases the spin cycloid plane flops in direction almost perpendicular to H, i.e., close to the ab and bc planes, respectively. Parameters describing the magnetic structures including wave vectors, orientations of the main elliptical axes, etc., have been determined in all spontaneous and field-induced states

Structural, magnetic and electrical properties of single crystalline La_(1-x)Sr_xMnO_3 for 0.4 < x < 0.85

We report on structural, magnetic and electrical properties of Sr-doped LaMnO_3 single crystals for doping levels 0.4 < x < 0.85. The complex structural and magnetic phase diagram can only be explained assuming significant contributions from the orbital degrees of freedom. Close to x = 0.6 a ferromagnetic metal is followed by an antiferromagnetic metallic phase below 200 K. This antiferromagnetic metallic phase exists in a monoclinic crystallographic structure. Following theoretical predictions this metallic antiferromagnet is expected to reveal an (x^2-y^2)-type orbital order. For higher Sr concentrations an antiferromagnetic insulator is established below room temperature.Comment: 8 pages, 7 figure

Orthoferrite Single Crystals Growing by Modified Czochralski Method and their Properties

To obtain bulky large-sized orthoferrite single crystals with different rare-earth elements a new crystal growth technology is proposed including modified Czochralski method and using no metal crucibles for melt suspension. For this purpose two unusual Czochralski method modifications are used. In the first one the melt of crystallized material is obtained at heating the upper surface of polycrystalline block with intensive light radiation focused by 3 powerful single ellipsoidal mirror light concentrators, and in the second one the combination of surface heating with 3 intensive light concentrators and induction of high-frequency heating of the melt placed in cold crucible is used to produce it. In the last case the melt volume is considerably larger and this circumstance allows to obtain large-sized single crystals. For optimum thermal field formation needed for crystal pulling the relation between cold crucible diameter, frequency of inductive coi1 and power of the focused light inputted in the melt plays the main role. Using this technology rather perfect single crystals of yttrium, gadolinium, thulium etc. orthoferrites 25-30 mm in diameter and up to 50 mm long were grown. The maximum crystal diameter depends on the diameter of the cold crucible used and can be really increased up to 50 mm. Yttrium orthoferrite single crystals obtained by this technology have the following perfection characteristics : X-ray rocking curve width is 10" in the central part of the crystal, the dislocation density is 1-10/cm2 in the central part and up to 104 on the periphery, blocks and twins are absent

Single crystals growth of hexaferrits M-type MTixCoxFe₁₂–₂xO₁₉ (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties

Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted hexaferrites BaTixCoxFe₁₂–₂xO₁₉ and SrxTixCoxFe₁₂–₂xO₁₉ (0.8 ≤ x ≤ 2). The dynamics of the growth process is studied and results of the analysis of impurity phases appearing in the initial stages of the crystal growth are presented. Compositions and unit-cell parameters of crystals are determined. Electrical, magnetic and magnetoelectric properties of grown crystals are investigated at temperatures 2–365 K and magnetic fields up to 50 kOe. It is shown that the resistivity of annealed in oxygen crystals at room temperature is ∼10⁶ Ohm·cm while at helium temperatures the crystals become good insulators. Magnetic measurements reveal conical spin structures in the crystals at some concentrations and temperatures. Magnetic field induced electric polarization of the low value (∼0.3 μC/m²) is detected at liquid helium temperatures for compositions with Ti and Co concentrations x = 0.8–0.9

Electric and magnetic properties of titanium-cobalt-oxide single crystals produced by floating zone melting with light heating

Single crystals of spinel Co₂TiO₄, CoTiO₃ of ilmenite structure and pseudobrookite CoTi₂O₅ were grown by means of zone melting equipment URN-2-ZM. The growth processes were performed in an air atmosphere with a speed ∼10 mm/h and a final annealing of the crystal at temperature of 1250 °C. Co₂TiO₄ and CoTi₂O₅ crystals were obtained with a diameter of 12–15 mm and length up to 60 mm, being free of any other phase inclusions and cracks. However, in the CoTiO₃ crystals some controversial features were found: x-ray Laue analysis indicated high-structure perfection while an electronic microscopy revealed small amount of second phase inclusions in contradiction to known phase diagram of this system. Electrical and magnetic properties of grown Co₂TiO₄ and CoTiO₃ single crystals were studied. Semiconducting behavior of the Co₂TiO₄ was established with the energy gap of ∼1.3 eV. No magnetic anisotropy was found in the cubic Co₂TiO₄ single crystals showing a magnetic behavior similar to polycrystals. The rhombohedral CoTiO₃ crystals revealed a magnetic behavior of an easy plane antiferromagnet with a significant anisotropy of the transverse magnetic susceptibilities along and perpendicular to the trigonal c axis

Effect of laser pulse propagation on ultrafast magnetization dynamics in a birefringent medium

Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm 0.5 Pr 0.5 )FeO 3 and (Sm 0.55 Tb 0.45 )FeO 3 , which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium

Low energy Mott Hubbard excitations in LaMnO3 probed by optical ellipsometry

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO 3 crystal in the spectral range 1.2–6.0 eV at temperatures 20≤T≤300 K . The complex dielectric response along b and c axes of the Pbnm orthorhombic unit cell, ε ̃ b (ν) and ε ̃ c (ν) , is highly anisotropic over the spectral range covered in the experiment. The difference between ε ̃ b (ν) and ε ̃ c (ν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A -type antiferromagnetic long-range order at T N =139.6 K . In this study we focus on the analysis of excitations observed at high energy (∼4–5 eV) and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at ∼2 eV . We used a classical dispersion analysis to quantitatively determine the temperature-dependent optical spectral-weights shifts between low- and high-energy optical bands. Based on the observation of a pronounced spectral-weight transfer between both features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d 4 d 4 ⇌d 3 d 5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral-weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e g orbitals in LaMnO 3 , as well as to extract experimental information concerning the type of orbital order in LaMnO 3 . This study identifies the lowest-energy optical transition at ∼2 eV as an intersite d-d transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction

Far-Infrared Study of Field-Induced Phase-Transitions in Y0.5lu0.5feo3 and Hofeo3 Orthoferrites

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Magnetooptical Effects at Submillimetre Wavelengths Near AFMR in YFeO3

Spectra of the transmission t(v), ellipticity ρ(v) and Faraday rotation angle αF(v) have been measured in YFeO3 single crystals near AFMR mode (v1 ≈ 10 cm-1) using the quasi-optical submillimetre backward-wave-oscillator spectrometer. A resonance behaviour of the T(v), ρ(v) and αF(v) spectra was observed near AFMR. The theoretical calculations and computer simulations made it possible to describe the observed T(v), ρ(v) and αF(v) spectra and to determine both diagonal and non-diagonal components of the permittivity and permeability of the YFeO3. An unusual effect of the weak magnetic field (H≈250 Oe) along magnetic moment (m||c-axis) on the line-shape and intensity of the resonance line was revealed and explained by an interference of two normal modes inside the resonance line