The magnetic structure of cubic ErAl3

The magnetic structure of cubic or β-ErAl₃ has been investigated by neutron diffraction from powder samples. ErAl₃ undergoes a transition at 5·1°K, to an antiferromagnetic state with an enlarged tetragonal unit cell. The moments are directed perpendicular to the tetragonal c-axis. Crystal field effects are large and dominant in this compound. The extrapolated saturation moment is 5·1/μ_B, which corresponds to the Γ₈ ground state.     

Linear magnetic birefringence in the quadratic-layer antiferromagnet K2MnF4

The linear optical birefringence of the quasi-Heisenberg antiferromagnet K₂MnF₄ has been determined between 5 and 300 K. A large magnetic contribution (LMB) is found at temperatures below 200 K. According to the earlier results with the iron-group difluorides the LMB can be related to the magnetic internal energy. Below 30 K the LMB of K₂MnF₄ obeys a T³-law. The temperature derivative of the LMB follows a course expected for the magnetic heat capacity of pronounced two-dimensional systems: a broad, rounded maximum above the three -dimensional transition point. At T_N a kink occurs in the LMB.     

Irreversibility line and anisotropy of magnetic melt textured Y1Ba2Cu3O7−δ studied by AC susceptibility

Magnetically melt textured YBa₂Cu₃O_7−δ samples have been studied by AC initial susceptibility measurements in the low field limit. The critical dynamic fields h^*_c and the relative position of the irreversibility line and of the critical line show that magnetic melt texturing leads to the creation of strong pinning sites. The anisotropy factor A is in agreement with the value obtained from magnetization under high DC fields. The A values are lower than in single cystals, but in agreement with the important orientation effects introduced by magnetic field texturing. These values also are typical of fabrication techniques involving a melting at some stage.     

Thermal vibration in the cubic phase of K2SnCl6

The rectilinear and angular mean-square displacements of the atoms in K₂SnCl₆ have been measured from 293 to 450 K using single-crystal X-ray diffraction. The motion of the tin and chlorine atoms has been found to consist, essentially, of rigid-body translation and libration of the SnCl₆ octahedra. The root-mean-square angle of libration of the SnCl₆ octahedra, $\text{φ}{}^2$^{$\text{1}\text{2}$}, ranged from 5.7 to 6.1°. The slow variation of $\text{φ}{}^2$ with temperature, together with the measured non-zero value of one of the fourth cumulants of the chlorine atom displacement, are indicative of highly non-linear coupled motion associated with the soft-mode phase transition at 261.5 K.     

Sound propagation in the two dimensional magnetic system K2NiF4

We have measured the classic constants C₁₁, C₃₃, C₅₅ and C₆₆ in K₂NiF₄ as a function of temperature between 4.2 and 200 K. A small velocity and attenuation anomaly was observed in the C₁₁ mode at T_N = 97.2 K.     

Two-exciton transition in the antiferromagnetic KNiF3 and K2NiF4

For the sharp band located at 31,200 cm^?1 in the three dimensional antiferromagnetic KNiF₃, the dependence of the oscillator strength on the Ni ion concentration and the stress-induced linear dichroic spectrum are studied. The polarization dependence of the corresponding band in the two dimensional antiferromagnetic K₂NiF₄ is also measured. The weak structure located at 30,780 cm^?1 is assigned as two-exciton transition, and the band at 31,200 cm^t?1 as a two-exciton transition accompanied with a T_1u phonon.     

High-Resolution Infrared Spectra of the ν2, ν3, ν4, and 2ν3 Bands of SO3

New measurements are reported for the infrared spectrum of sulfur trioxide, ³²S¹⁶O₃, with resolutions ranging from 0.0015 cm⁻¹ to 0.0025 cm⁻¹. Rovibrational constants have been measured for the fundamentals ν₂, ν₃, and ν₄ and the overtone band 2ν₃. Comparisons are made with the earlier high-resolution measurements on SO₃, and the high correlation among some of the constants related to the Coriolis coupling of the ν₂ and ν₄ levels is discussed in order to understand the areas of disagreement with the earlier work. Splittings of some of the levels are observed and the splitting constant for K=3 of the ground state is determined for the first time. Other observed splittings include the K=1 levels of 2ν₃ (l=2), the K=2 levels of ν₃ and ν₄, and the K=3 levels of ν₂. The analysis shows that there are level crossings between the l=0 and l=2 states of 2ν₃ that allow one to determine the separation of the subband centers for these two states even though access to the l=0 state from the ground state is electric-dipole forbidden. This is a generalized phenomenon that should be found for many other molecules with the same symmetry. The l-type resonance constant, q₃, that causes the splitting of the l₃=±1, k=±1 levels of ν₃ also couples the l₃=0 and 2 states of 2ν₃.     

A comment on the structure of K2CuF4

The structure of K₂CuF₄ has been investigated by X-ray diffraction. Some samples displayed a multi-domain structure which arises from the possibility of the co-existence of the x and y axes in the [110]_p direction of a pseudo K₂NiF₄-type structure. The structure was determined using a single-domain crystal to be of orthorhombic symmetry, D¹⁸_2h-Bbcm, with four molecules per unit cell. The displacement of F^- ions can be explained by the orbital ordering model assumed by Khomskii and Kugel.     

Thermomagnetic determination of Fe3O4 magnetic nanoparticle diameters for biomedical applications

The utility and promise of magnetic nanoparticles (MagNPs) for biomedicine rely heavily on accurate determination of the particle diameter attributes. While the average functional size and size distribution of the magnetic nanoparticles directly impact the implementation and optimization of nanobiotechnology applications in which they are employed, the determination of these attributes using electron microscopy techniques can be time-consuming and misrepresentative of the full nanoparticle population. In this work the average particle diameter and distribution of an ensemble of Fe₃O₄ ferrimagnetic nanoparticles are determined solely from temperature-dependent magnetization measurements; the results compare favorably to those obtained from extensive electron microscopy observations. The attributes of a population of biocompatible Fe₃O₄ nanoparticles synthesized by a thermal decomposition method are obtained from quantitative evaluation of a model that incorporates the distribution of superparamagnetic blocking temperatures represented through thermomagnetization data. The average size and size distributions are determined from magnetization data via temperature-dependent zero-field-cooled magnetization. The current work is unique from existing approaches based on magnetic measurement for the characterization of a nanoparticle ensemble as it provides both the average particle size as well as the particle size distribution.     

Mössbauer study of magnetic ordering in K2FeF4

The sublattice magnetization of the quadratic-layer basal-plane antiferromagnet K₂FeF₄ has been studied by use of Mössbauer spectroscopy. A distribution of Néel temperatures with a width ～3 K is found at a mean T_N = 67.2±0.3 K. For 0.3 < T/T_N < 0.99 the sublattice magnetization is described by a power law with critical exponent β = 0.17±0.01.     

Experimental determination of the spin-wave spectrum of the two-dimensional ferromagnet K2CuF4

K₂CuF₄ is a quasi-bidimensional ferromagnet with an easy magnetization plane. The Curie temperature is 6.25 K. We have measured the spin-wave spectrum from 2 to 14 K, and found an exchange integral of 11.4 K between the first neighbours in the basal plane. No measurable dispersion was found along the axis perpendicular to that plane. We have observed a shift in the energy of the magnons, and a broadening of the magnon peak both of which behave differently below and above the Curie temperature.     

Cubic magnetic anisotropy of the antiferromagnetically ordered Cu3TeO6

A combination of highly sensitive torque magnetometry in low magnetic fields and a phenomenological approach to magnetic anisotropy is used to probe the symmetry of the antiferromagnetically ordered state of spin S=1/2 system Cu₃TeO₆. The results show that the ordered state has four antiferromagnetic domains with spin axis in the 〈1±1±1〉 directions, in agreement with the previously reported neutron measurements. These results show that this approach, previously applied to ferromagnets and highly anisotropic antiferromagnets, is also successful in determining the symmetry of weakly anisotropic Heisenberg antiferromagnets with multiple spin domains. Possible microscopic origin of magnetic anisotropy is briefly discussed.     

Two models for the transport properties of YBa2Cu3O7−δ in its normal state

The high-temperature superconductor YBa₂Cu₃O_7−δ in its normal state, shows unusual dependence of its transport properties on the oxygen deficiency parameter δ and on temperature: for δ ≈ 0 both the resistivity and the Hall number rise linearly with temperature, while the thermoelectric power is very small. In order to interpret this unusual combination of properties we propose two alternative models, a two-dimensional tight-binding wide-band model, and a narrow-band model. In the first case we assume scattering by a fully excited boson field, with a mean free path Λ ∝ 1/T. In the second model we assume a band composed of two parts, where the upper smaller part does not contribute to transport (as would result from the existence of a mobility edge), and Λ is independent of temperature. The calculated results are compared with experiments.     

Electronic energy levels of CsCdCl3

The excitation and emission spectra of CsCdCl₃ crystal grown by the Bridgman growth technique, are observed. Its dominant excitation spectra correspond to the transitions between the electronic energy levels of the face-shared [CdCl₆]⁴⁻ octahedron complexes with C_3v symmetry. With the help of the molecular orbital diagram of the [CdCl₆]⁴⁻ molecule complex, an approximate electronic energy level diagram of this crystal is constructed using one-electron approximation.     

Line Positions and Intensities of the ν1+ ν2+ 3ν3, ν2+ 4ν3, and 3ν1+ 2ν2Bands of Ozone

Using a Fourier transform spectrometer, we have recorded the spectra of ozone in the region of 4600 cm⁻¹, with a resolution of 0.008 cm⁻¹. The strongest absorption in this region is due to the ν₁+ ν₂+ 3ν₃band which is in Coriolis interaction with the ν₂+ 4ν₃band. We have been able to assign more than 1700 transitions for these two bands. To correctly reproduce the calculation of energy levels, it has been necessary to introduce the (320) state which strongly perturbs the (113) and (014) states through Coriolis- and Fermi-type resonances. Seventy transitions of the 3ν₁+ 2ν₂band have also been observed. The final fit on 926 energy levels withJ_max= 50 andK_max= 16 gives RMS = 3.1 × 10⁻³cm⁻¹and provides a satisfactory agreement of calculated and observed upper levels for most of the transitions. The following values for band centers are derived: ν₀(ν₁+ ν₂+ 3ν₃) = 4658.950 cm⁻¹, ν₀(3ν₁+ 2ν₂) = 4643.821 cm⁻¹, and ν₀(ν₂+ 4ν₃) = 4632.888 cm⁻¹. Line intensities have been measured and fitted, leading to the determination of transition moment parameters for the two bands ν₁+ ν₂+ 3ν₃and ν₂+ 4ν₃. Using these parameters we have obtained the following estimations for the integrated band intensities,S_V(ν₁+ ν₂+ 3ν₃) = 8.84 × 10⁻²²,S_V(ν₂+ 4ν₃) = 1.70 × 10⁻²², andS_V(3ν₁+ 2ν₂) = 0.49 × 10⁻²²cm⁻¹/molecule cm⁻²at 296 K, which correspond to a cutoff of 10⁻²⁶cm⁻¹/molecule cm⁻².     

The magnetic specific heat anomaly of a two-dimensional ferromagnet K2CuF4

The magnetic specific heat anomaly of a two-dimensional Heisenberg ferromagnet K₂CuF₄ in the immediate neighborhood of its critical temperature was measured and found to diverge logarithmically both above and below T_c. This result is consistent with other critical exponents so far reported on this compound and supports the idea of universality.     

Spectroscopic and magnetic susceptibility analyses of the FJ and D4 energy levels of Tb(4f) in TbAlO3

Detailed analyses of spectroscopic and temperature-dependent magnetic susceptibility data are reported for the crystal-field split energy levels of the ⁷F_J and ⁵D₄ of Tb³⁺ in stoichiometric single crystals of ortho-aluminate TbAlO₃. The spectroscopic data include absorption spectra obtained between 2940 and 480 nm from 8 to 300 K. High resolution fluorescence spectra are reported, representing transitions from ⁵D₄ to ⁷F_6,5,4, at a sample temperature of 85 K. Using crystal-field modeling techniques recently adapted for low symmetry systems, we have assigned all 58 experimental Stark levels within the ⁷F_J and ⁵D₄ manifolds, with a fitting standard deviation of 4.5 cm⁻¹ (3.8 cm⁻¹ rms error). As a further test, the theoretical Stark levels and calculated wavefunctions were used to determine the temperature dependence of the magnetic susceptibility along the c-axis of the TbAlO₃ crystal. Good agreement is obtained between the calculated susceptibility and temperature-dependent magnetic data reported earlier, including a prediction of a 0.2 cm⁻¹ splitting of the ground-state quasi-doublet. The susceptibility calculation also confirms the predicted ordering of states within the ⁷F₆ multiplet manifold.     

“Quantization” of the q-vector in microcrystals of K0.3MoO3 and NbSe3

We report steps of conductivity between discrete conducting states in microsamples of quasi-one-dimensional conductors K_0.3MoO₃ and NbSe₃. The steps reveal single phase-slip events, and the discrete states reveal “quantization” of the wave vector q of the charge-density wave. The “quantization” is observed due to the coherence of the CDW within the sample volume and tight boundary conditions at the contacts. The distribution of steps in temperature gives the temperature dependences of the q-vectors, while the steps' height provides the carriers' mobilities. For the case of NbSe₃ we give the 1st direct confirmation of the extremely high mobility of the “pocket” holes at low temperatures.     

Analysis of the ν1 + ν2 + ν3 band of O3

The high resolution spectrum of the ν₁ + ν₂ + ν₃ band of O₃ in the 2800-cm⁻¹ region has been analyzed using Watson's Hamiltonian. The resulting Hamiltonian constants and previously published line intensities have been used to generate a listing of line assignments, positions, absolute intensities, and ground state energies. These should be useful for atmospheric studies.     

Theoretical studies of structural and magnetic properties of cubic perovskites PrCoO3 and NdCoO3

The structural and magnetic properties of cubic perovskites, PrCoO₃ and NdCoO₃, are studied using the full potential linearized augmented plane wave (FP-LAPW) method within the frame work of density functional theory (DFT). The structural parameters are also investigated by analytical techniques. The calculated structural parameters are consistent with the experimental results. The strong hybridization of the O-2p, Co-3d and Pr/Nd-4 f states around the Fermi level reveals that these compounds are metallic. It is also found that the origin of ferromagnetism in these compounds is double-exchange interaction between Co-3d states via O-2p states (Co-O-Co).