Anisotropy of the hopping magnetoresistance of antiferromagnetic La2CuO4+δ single crystals

Abstract

The anisotropy of the hopping conductivity of antiferromagnetic La2CuO4+δ single crystals with TN≈188 K is investigated in the temperature range 5–295 K and the anisotropy of their magnetoresistance, in the temperature range 5–55 K. The resistance is measured by the Montgomery method for different combinations of directions of the transport current and magnetic field relative to the crystallographic axes. For the case when the field and transport current are directed parallel to the CuO2 layers, a transition from negative to positive magnetoresistance is observed when the temperature is raised to T≈20 K. For fields perpendicular to the CuO2 layers, only negative magnetoresistance is observed. The nature of the positive magnetoresistance is discussed. It is shown that the effect is most likely due not to the interaction of the spin of the charge carriers with the surrounding magnetic medium but to the orbital motion of these carriers. The corresponding values of the positive magnetoresistance and its behavior as a function of magnetic field and temperature are found using the well-known model of Shklovskii and Efros, which is based on taking into account the compression of the impurity wave functions of the charge carriers in the magnetic field.

Magnetoresistive study of the antiferromagnetic–weak ferromagnetic transition in single-crystal La2CuO4+δ

“…The ratio is close to unity at T ≈ 11-12 K, but it increases with temperature and approaches value of about 3 at room temperature. A similar behavior was found in the previously studied sample with somewhat higher T N ≈ 188 K [18]. The a-b anisotropic conductivity behavior in a twinned sample (in the case that the conductivities σ a and σ b are inherently different) can be observed only when, first, available twins are few in number (so that measured resistivity is not properly averaged between the two possible crystal orientations), and, second, a given current direction is really parallel to axis a (or b) in the most part of the crystal.…”

“…3). In previous studies, negative MR in the AF La 2 CuO 4+δ for the case of both the current and field parallel to CuO 2 was found and discussed to a certain degree [14,18]. The nature of the negative MR in rather low field H c and J a,b revealed in this work is not clear and is worthy of additional study.…”

“…A ratio σ ab /σ c is strongly temperature dependent. It is minimal (about 10) in liquid-helium temperature range, but increases dramatically with temperature and saturates above 200 K to a maximal values order of 100 [16,17,18]. The 3D character of VRH in La 2 CuO 4+δ testifies that a hole transfer between CuO 2 is likely not only at J c, but at J a,b as well.…”

“…A single-crystal La 2 CuO 4+δ sample with dimensions 1.3×0.3×0.39 mm is investigated. This sample was studied previously in work [18], where it was indicated as a sample No. 1 with T N = 188 K. After that study, the sample was annealed additionally in oxygen atmosphere (700 • C, 5.5 days) in the hope that oxygen content (that is, δ) will be increased.…”

“…This sample was studied previously in work [18], where it was indicated as a sample No. 1 with T N = 188 K. After that study, the sample was annealed additionally in oxygen atmosphere (700…”