It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.
Neutron diffraction and inelastic spectroscopy is used to characterize the magnetic Hamiltonian of SrHo 2 O 4 and SrDy 2 O 4 . Through a detailed computation of the crystal-field levels we find site-dependent anisotropic single-ion magnetism in both materials, and diffraction measurements show the presence of strong one-dimensional spin correlations. Our measurements indicate that competing interactions of the zigzag chain, combined with frustrated interchain interactions, play a crucial role in stabilizing spin-liquid type correlations in this series.
We have used a spin-wave model to calculate the temperature dependence of the ͑sublattice͒ magnetization of magnetic nanoparticles. The uniform precession mode, corresponding to a spin wave with wave vector q = 0, is predominant in nanoparticles and gives rise to an approximately linear temperature dependence of the ͑sublattice͒ magnetization well below the superparamagnetic blocking temperature for both ferro-, ferri-, and antiferromagnetic particles. This is in accordance with the results of a classical model for collective magnetic excitations in nanoparticles. In nanoparticles of antiferromagnetic materials, quantum effects give rise to a small deviation from the linear temperature dependence of the ͑sublattice͒ magnetization at very low temperatures. The complex nature of the excited precession states of nanoparticles of antiferromagnetic materials, with deviations from antiparallel orientation of the sublattice magnetization vectors, results in a contribution to the susceptibility, which increases with increasing temperature.
The aim of this study was to compare the accuracy of approximal caries lesion depth measurements in radiographs from four digital systems. Two CCD-based sensors, Dixi (Planmeca) and Sidexis (Sirona), and two phosphor plate systems, Digora (Soredex) and DenOptix (Gendex), were used to record radiographs of 177 extracted human teeth. Sixty-four radiographically visible lesions were selected and measured by 4 observers in the digital radiographs and in digital photographs of histological sections of the teeth. The gold standard (true measure) was defined as the mean of the 4 observers’ measurements on the histological sections. Two untrained observers underestimated general lesion depth in all the radiographic systems while each of 2 trained observers underestimated the depth in one of the systems. Analysis of variance revealed that Digora images resulted overall in the smallest underestimation, followed by Dixi, DenOptix and Sidexis. The differences between Digora and Sidexis and between Dixi and Sidexis were statistically significant (p < 0.05). Analysis of variance using the regression coefficient as the test variable demonstrated that the coefficients differed between DenOptix and the remaining systems (p < 0.05) while there were no significant differences between these (p > 0.05). It can be concluded that radiographs obtained with the Dixi and Digora systems were more accurate than Sidexis and DenOptix images for measurement of caries lesion depth. Still, a high probability exists that the individual lesion measurement is either overestimated or underestimated compared to the true lesion depth.
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