Principal Crystallographic Axes Research Articles

Spin dynamics in the magnetoelectric effect compoundLiCoPO4

Inelastic neutron scattering (INS) experiments were performed to investigate the spin dynamics in magnetoelectric effect (ME) LiCoPO$_4$ single crystals. Weak dispersion was detected in the magnetic excitation spectra along the three principal crystallographic axes measured around the (0 1 0) magnetic reflection. Analysis of the data using linear spin-wave theory indicate that single-ion anisotropy in LiCoPO$_4$ is as important as the strongest nearest-neighbor exchange coupling. Our results suggest that Co$^{2+}$ single-ion anisotropy plays an important role in the spin dynamics of LiCoPO$_4$ and must be taken into account in understanding its physical properties. High resolution INS measurements reveal an anomalous low energy excitation that we hypothesize may be related to the magnetoelectric effect of LiCoPO$_4$.

Anisotropy of the electron g factor in semiconductors with cubic symmetry

The spin-orbit corrections to electronic states in bulk cubic semiconductors without the center of inversion in an ultraquantum magnetic field are investigated. It is shown that the spin-orbit interaction results in a shift of the Landau levels and the appearance of additional terms in the relationship for the electron g factor. The corrections to the g factor lead to a deviation of the macroscopic magnetization from the direction of the magnetic field, the dependence of the spin resonance frequency on the magnetic field orientation with respect to the principal crystallographic axes, and anisotropy of the spin relaxation through the D’yakonov-Perel’ mechanism.

Spontaneous generation of voltage in the magnetocaloric compound Tb 5Si 2.2Ge 1.8 and elemental Gd

The spontaneous generation of voltage (SGV) in single crystalline Tb 5Si 2.2Ge 1.8 and Gd has been studied. Temperature-induced SGVs were observed along the three principal crystallographic axes of Tb 5Si 2.2Ge 1.8, but not in Gd. Field-induced SGVs were observed with magnetic fields less than 40 kOe applied along the a-axis of Tb 5Si 2.2Ge 1.8, and the c-axis of Gd. The absence of the temperature-induced SGV in Gd indicates the key role first-order phase transformations play in the appearance of the effect when temperature varies. The anisotropy of the magnetic field-induced SGV in Tb 5Si 2.2Ge 1.8 and the existence of the field-induced SGV in Gd, highlight the importance of the magnetocaloric effect in bringing about the SGV.

Spatial anisotropy of linear electro-optic effect in crystal materials: II. Indicative surfaces as efficient tool for electro-optic coupling optimization in LiNbO 3

The electro-optic (EO) properties of pure and MgO-doped LiNbO 3 crystals are characterized by indicative surfaces (IS) describing the spatial anisotropy of the longitudinal and transverse linear EO effects. By analyzing the IS and their stereographic projections, we have determined the geometries with maximum EO coupling. The maximum magnitudes of the linear EO effect are revealed in the longitudinal and transverse geometries along the directions that substantially deviate from the principal crystallographic axes of both crystals. Spatial analysis of the IS thus yields a set of the optimized sample geometries recommended for designers who develop EO modulators or deflectors. Just by switching from the standard cell geometry to the optimized cell geometry, as determined in the present work, one may improve almost three times the modulation efficiency of EO devices based on lithium niobate crystals. This results in a corresponding reduction of their driving voltages being evidently of great practical importance for many applications.

Crystallographic and dynamic characteristics of K1 − x Rb x TiOPO4 solid solutions in the low-temperature ranges

The unit cell parameters a and c of K1 − x Rb x TiOPO4 (x= 0, 0.3, and 0.5) crystals are investigated using x-ray diffraction in the temperature range 90–320 K. The thermal expansion coefficients along the principal crystallographic axes of the crystals are determined from the obtained temperature dependences of the unit cell parameters. It is found that the parameter a increases with increasing temperature, while the parameter c decreases. For the crystals under investigation, the elastic moduli c 11 and c 33 along the [100] and [001] directions are determined by the ultrasonic pulse-echo technique in the temperature range 100–350 K. It is shown that c 33 > c 11. The anomalies revealed in the temperature dependences of the crystallographic and dynamic characteristics of the samples at a temperature T ≈ 280 K indicate the occurrence of a phase transition. The temperature of the phase transition is found to increase with increasing rubidium content x.

Surface polaritons in a thin layer of an anisotropic superconductor

An expression for the permittivity tensor of a superconductor is derived within the framework of a two-fluid model of the electron subsystem. For the temperature range T ≪ T c , the dispersion relations for surface polaritons that propagate in the direction of the three principal crystallographic axes of a thin superconducting layer along its two interfaces with isotropic dielectrics are found. A significant effect of the superconductor anisotropy on the dispersion relations is revealed. The polarization structure of the wave field inside and outside the superconducting layer is determined.

Photoemission Spectroscopy Study of Cu/CeO2 Systems: Cu/CeO2 Nanosized Catalyst and CeO2(111)/Cu(111) Inverse Model Catalyst

Cerium oxide films equivalent to 2 ML of CeO2 were grown at 520 K in an oxygen atmosphere on a clean Cu(111) substrate in order to prepare a model catalytic system. This “inverse model catalyst” was characterized by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) of core levels, and resonant photoelectron spectroscopy (RPES) of the valence band. Samples annealed at 770 K exhibited a LEED pattern corresponding to the (1.5 × 1.5) CeO2(111)/Cu(111) structure that can be interpreted as formation of a flat, well-ordered cerium oxide overlayer aligned with the principal crystallographic axes of the substrate. The model catalytic system corresponds well to a copper-loaded ceria nanopowder catalyst that exhibits growth of Cu(111) film structure on CeO2(111) planes. Lowering of the CO oxidation temperature due to the Cu loading is explained by CO adsorption on copper in the vicinity of highly active ceria planes providing oxygen for the reaction.

Orientation dependence of the anomalous Hall resistivity in single crystals ofYb14MnSb11

The Hall resistivity, electrical resistivity and magnetization of single crystals of the tetragonal ferromagnet Yb14MnSb11 are reported as a function of the direction of the current, I, and magnetic field, H with respect to the principal crystallographic axes. With I along the unique c direction and H in the a-b plane, the anomalous Hall resistivity in the limit of zero applied field is negative for all temperatures T< Tc= 53 K. An intrinsic Hall conductivity , !xy 0, of -32 -1 cm-1 is found for T << Tc, which decreases linearly with the spontaneous magnetization (order parameter) as the temperature is increased to Tc . In this direction, the anomalous Hall effect behaves in a manner similar to that observed in other ferromagnets such as Fe, Co, Mn5Ge3, and EuFe4Sb12. However, with I in the a-b plane and H along the c direction, the anomalous Hall behavior is completely different. The anomalous Hall resistivity data are positive for all T < Tc and a similar analysis of these data fails. In this direction, the anomalous response is not a simple linear function of the magnetization order parameter, and for a fixed temperature (T < Tc) does not depend on themore » magnitude of the magnetization perpendicular to the current in the a-b plane. That is, when the magnetization and applied field are rotated away from the c direction, the anomalous Hall resistivity does not change. In all other soft ferromagnets that we have examined (including La doped crystals of Yb14MnSb11, i.e. 2 Yb13.3La0.7MnSb11) rotation of the magnetization and magnetic field by an angle # away from a direction perpendicular to I results in a decrease in both the anomalous and normal portions of the Hall resistivity that approximately scales as cos(#). We suggest that the unique response exhibited by Yb14MnSb11 is a direct reflection of the delicate balance between a ferromagnetic and a non-magnetic Kondo lattice ground state« less

ANISOTROPIC ASSEMBLY OF COLLOIDAL NANOPARTICLES: EXPLOITING SUBSTRATE CRYSTALLINITY

We show that the crystal structure of a substrate can be exploited to drive the anisotropic assembly of colloidal nanoparticles. Pentanethiol-passivated Au particles of ~ 2 nm diameter deposited from toluene onto hydrogen-passivated Si (111) surfaces form linear assemblies (rods) with a narrow width distribution. The rod orientations mirror the substrate symmetry, with a high degree of alignment along principal crystallographic axes of the Si (111) surface. There is a strong preference for anisotropic growth with rod widths substantially more tightly distributed than lengths. Entropic trapping of nanoparticles provides a plausible explanation for the formation of the anisotropic assemblies we observe.

Heat capacity and magnetic properties of aEuVO3single crystal

We have studied the heat capacity and magnetic properties of a $\mathrm{Eu}\mathrm{V}{\mathrm{O}}_{3}$ single crystal. The compound undergoes an orbital ordering transition at ${T}_{\mathrm{O}\mathrm{O}}=204\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is then followed by a spin ordering transition at ${T}_{\mathrm{S}\mathrm{O}}=131\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In the spin ordered region, we have found that the compound is very sensitive to small trapped fields present in the superconducting solenoid of a magnetometer during cooling. Large negative Weiss temperatures are seen along all principal crystallographic axes indicating that strong antiferromagnetic interactions are present in this compound. Nevertheless, at low temperatures, we observed open hysteresis loops with a relatively high remanent magnetization and coercivity. We discuss the results for $\mathrm{Eu}\mathrm{V}{\mathrm{O}}_{3}$ in terms of an inhomogeneity with random fields.

He Direction of Spontaneous Magnetisation of Lanthanide Ions at a Site of Cubic Symmetry

The zero temperature direction of spontaneous magnetisation of lanthanide ions at a site of cubic symmetry is investigated as a function of the electrostatic and magnetic interactions. For all the values of J between J = 3 and J = 8, two-dimensional diagrams giving the orientation of spontaneous magnetisation are obtained as a function of the parameters R representing the relative strength of the magnetic to electrostatic interaction and the parameter x representing the relative strength of the fourth to the sixth order terms in the crystal field. The boundaries between regions of the parameter space with different directions of spontaneous magnetisation are investigated. It is found that at some boundaries there is a gradual rotation of the direction of spontaneous magnetisation and that at other boundaries there is a sudden change of orientation of spontaneous magnetisation at a critical value of the (R,x) parameters. Two types of behaviour are observed when there is a critical value of (R,x). There are boundaries where there is at the critical value a degenerate plane in which all the orientations can be direction of spontaneous magnetisation and some boundaries where two different principal crystallographic axes can be direction of spontaneous magnetisation at the critical value. In the latter case there is a region near the boundary where an unstable equilibrium orientation for the magnetisation can be found.

Effect of polarization and uniaxial stress on the R-line luminescence of single crystal sapphire

The polarization dependence of the optical absorption and R-line emission of a Cr3+-doped sapphire single crystal have been characterized. The absorption bands, U(T42) and Y(T41), as well as the R-line emission are determined to be dipole-like transitions associated with three incoherent dipoles oriented along the sapphire principal crystallographic axes (a,m,c). From measurements using the Muller method, it is found that the polarization of the R lines does not depend on either the polarization of the incident excitation or its wavelength. The σ emission is also an order of magnitude stronger than π emission. The R2-R1 peak separation and the ratio of the R2∕R1 peak areas are found to be dependent on both the polarization and the stress directions. Stress in the basal plane breaks the symmetry of α polarization with the R2∕R1 ratio increasing along the stress direction and decreasing in the perpendicular direction in the basal plane. The π emission also decreases with stress in the basal plane. However, in both polarizations the ratio R2∕R1 does not depend on stress applied along the c axis. The value of the R2∕R1 peak area ratio can be understood in terms of a piezoelectric-like effect on the dipole transition.

Electrochemical Growth and Control of ZnO Dendritic Structures

The dendritic crystal growth patterns that typically grow along principal crystallographic axes and have the hierarchical structure have been attracting much attention from scientists for several centuries. Here we report that the ZnO dendritic nanostructure as a new member of the ZnO family could be successfully prepared on Cu substrates by electrochemical deposition in the solution of ZnCl2 + citric acid at a temperature of 90 °C. Furthermore, our synthetic parameters allow further structural manipulation. The morphology evolvement from dendritic structures to nanorods could be successfully realized when KCl as supporting electrolyte was added to the deposition solution. The green light emission band of the ZnO dendritic structure prepared in 0.05 M ZnCl2 + 0.05 M citric acid is almost negligible, indicating that these ZnO deposits are highly crystallized and of excellent optical quality. The PL spectra of the as-grown ZnO nanorods show they possess many oxygen vacancies, and the acquired ZnO nanorods h...

Electronic band structure of the borocarbide superconductor LuNi2B2C

We present de Haas–van Alphen (dHvA) measurements of the nonmagnetic rareearth borocarbide superconductor LuNi2B2C which have been performed by using a torque cantilever for temperatures down to 0.45K and in magnetic fields up to 32.5T. We mapped the dHvA oscillations of a high-quality single crystal by rotating between all three principal crystallographic axes [100], [001], and [110]. This set of data gives a complete overview of the electronic band structure of LuNi2B2C. A comparison with band-structure calculations allowed us to assign the dHvA frequencies to individual bands. Overall positions and band dispersions resemble those observed in the isostructural compound YNi2B2C, which is another nonmagnetic member in the family of the borocarbide superconductors.

Orientation selection in dendritic evolution

Dendritic crystal growth patterns have fascinated scientists for several centuries. Much of the aesthetic appeal of these patterns stems from the hierarchical structure of primary-, secondary-, and higher-order branches, which typically grow along principal crystallographic axes. Atypical growth directions have also been observed. Here, we demonstrate both computationally and experimentally that the range of possible dendrite growth directions, and hence the morphological diversity of the resulting dendritic structures, is much richer than previously anticipated. In particular, we show that primary dendrite growth directions can vary continuously between different crystallographic directions as a function of the composition-dependent anisotropy parameters. The study combines phase-field simulations of equiaxed dendritic growth and directional freezing of Al-Zn alloys. Both simulations and experiments exhibit continuous changes of direction from <100> to <110> for an underlying cubic symmetry. These results have important implications for controlling the microstructure of a wide range of cast alloys that solidify dendritically.

Evidence for strong spin-lattice coupling in multiferroic RMn2O5 (R=Tb,Dy,Ho) via thermal expansion anomalies

Thermal expansion measurements were done on single-crystal RMn2O5 (R=Tb,Dy,Ho) along the principal crystallographic axes. Distinctive anomalies were observed in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order (TN1), ferroelectricity (TC1), as well as at temperatures when anomalous changes in the polarization, dielectric constant, and spin wave incommensurability have been previously reported. These observations suggest that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion. In particular, a possible scenario as to how ferroelectricity arises and subsequently is altered as a result of the subtle changes in the highly frustrated magnetic order, is discussed in the light of the strong spin-lattice coupling observed in these materials. Neutron diffraction measurements on HoMn2O5 show a spin reorientation at 23K corresponding to a step-like anomaly in the dielectric constant.

Dilatometric and ultrasound study of [NH2(CH3)2]MnCl3 · 2H2O crystals

The unit-cell parameters of [NH2(CH3)2]MnCl3 · 2H2O crystals are determined by X-ray diffraction analysis and the velocities of longitudinal ultrasonic waves in these crystals are measured by the echo-pulse method in the temperature range 100–315 K. The coefficients of thermal expansion along the principal crystallographic axes are derived from the temperature dependences of the unit-cell parameters. The temperature dependences of the characteristics studied reveal kink anomalies at temperatures of ∼125, 179, 203, 260, and 303 K. These anomalies are indicative of structural transformations in the [NH2(CH3)2]MnCl3 · 2H2O crystals, which may be related to the dynamics of dimethylammonium cations.

Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C

Experiments have been carried out to determine the temperature, oxygen fugacity ( fO 2) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO 2 (1/3). This makes a knowledge of the T- fO 2 path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO 2 for Ca diffusion in olivine at a given oxygen fugacity ( fO 2*) are given by: diffusion along [100]: log [Do (m 2/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/mol diffusion along [010]: log [Do (m 2/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/mol diffusion along [001]: log [Do (m 2/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/mol where Δ log fO 2 = log[ fO 2*] − log[10 −12] with fO 2* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo 83 and Fo 92. Diffusion in Fo 100 has a much higher activation energy than in Fe-bearing olivine and has a weaker fO 2 dependence.

Natural behavior of the magnetization under spontaneous reorientation: TmFeO3, ErFeO3

A SQUID magnetometer is used to study the behavior of the magnetization of TmFeO3 single crystals along the a and c principal crystallographic axes in the Γ4–Γ24–Γ2 spin reorientation range. The temperature dependences are obtained as the moduli of the magnetization vector M and its turn angle θ in the reorientation range. The results are compared with the same results for ErFeO3. It is shown that even though the experimental dependences |M|(T) and θ(T) are qualitatively different in TmFeO3 and ErFeO3 they can all be convincingly described on the basis of a modified mean-field theory previously proposed by the authors. Since the theoretical analysis does not include any parameters which are not known from experiment, the agreement between theory and experiment confirms that the model proposed for describing Γ4–Γ24–Γ2 phase transitions in orthoferrites is a general model. Dedicated to E. S. Borovik—Scientist and Humanitarian “Who can say what influence the silent presence of one person has on another?” Walter De la Mar

Magnetic and transport properties of [formula omitted] single-crystal

Magnetization, susceptibility and resistivity were measured on the GdFe2Si2 (antiferromagnetic with TN=8K) single crystal along both principal crystallographic axes in the temperature range 2–300K and in magnetic fields up to 14T. Magnetic properties were found to be isotropic. The susceptibility obeys the Curie–Weiss law with the effective magnetic moment μeff=8.26μB/f.u. A saturated moment of 7.2μB/f.u. was determined at 2K. On the other hand, resistivity and magnetoresistivity are anisotropic and show an anomaly at TN. The anomalies in the susceptibility and resistivity data at T=8K are associated with the magnetic phase transition.