Structural and magnetic investigations of a La2CuO4 single crystal with polarization analysis

Neutron scattering results on magnetic and structural ordering phenomena in a La₂CuO₄ single crystal are presented. Polarized neutrons and polarization analysis were used. A general discussion of the possibilities of this method in the study of single crystals is given. Precise information on magnetic structure, ordered moment as a function of temperature, and a lowering of crystalline symmetry at low temperature are obtained.     

Spin‐freezing in superconducting La2CuO4.03 single crystal

Electron spin‐freezing at T_f=8 K has been detected in superconducting (T_c=12\ K) single crystal La₂CuO_4+y (y\simeq0.03) by ZF‐μSR. According to diffraction data, the crystal is in Bmab phase without any traces of structural phase separation. TF‐μSR experiment has shown that no Abrikosov flux line lattice is formed below T_c. The data allow us to assume that the magnetic and superconducting regions in the crystal are space separated on the microscopic scales ~ 10² Å. The presence of large field induced broadening of the Knight shift distribution k_\sigma > 1000 ppm indicates that the crystal contains micro‐regions possessing enhanced magnetic susceptibility.     

Magnetic properties of La2CuO4 single crystals

Magnetic susceptibility x of La₂CuO₄ crystals have been measured with external magnetic field perpendicular to and within the c plane. A kind of ordering or condensation of the spin system with the spin direction parallel to the c axis has been found although it is difficult to consistently explain all of the observed behaviors. Even above the ordering temperature (⋍ 250K), uniaxial anisotropy of x has also been observed.     

Local hyperfine field vector of positive muon in mono-domain single crystal of antiferromagnetic La2CuO4

By using a monodomain single crystal La₂CuO₄ grown by the travelling solvent floating zone method, the strength and direction of the local hyperfine field vector at positive muon sites were determined. Combining with the ⁺ location determined separately, we found the importance of the local hyperfine field contributions with dipolar symmetry as well as the distant dipolar field from surrounding point dipoles of Cu atomic moments.     

Crystal-field excitations in Pr2CuO4

Neutron spectroscopy has been employed to study the crystal-field excitations in Pr₂CuO₄. The energetic ordering of the crystal-field levels and the crystal-field parameters are determined from the observed transitions. Our results are consistent with the large magnetic anisotropy observed in single-crystal susceptibility data.     

Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in La1.88Sr0.12CuO4

We report electronic Raman scattering experiments on a superconducting La(1.88)Sr(0.12)CuO(4) single crystal in a magnetic field. At low temperatures, the spectral weight of the high-energy two-magnon peak increases linearly with field and is amplified by a factor of more than two at 14 T. The effect disappears at elevated temperatures and is not present in undoped La(2)CuO(4). This observation is discussed in terms of an electronically inhomogeneous state in which the field enhances the volume fraction of a phase with local antiferromagnetic order at the expense of the superconducting phase.     

Anomalous magnetism of La2CuO4+x single crystals

Measurements of the differential magnetic susceptibility are performed to study the changes in the magnetic properties of La₂CuO_4+x due to oxygen doping in the concentration interval 0<x<0.011. For crystals with 0.005<x<0.011, a ferromagnetic-type phase transition is observed at T _c >T _N , and it occurs even in crystals with T _N ～0. The concentration dependence of the transition temperature T _c (x) is obtained.     

Effect of magnetic (phase) inhomogeneities on the conductivity of an antiferromagnetic La2CuO4 + δ single crystal at low temperatures

It has been established that, at a strongly nonuniform distribution of oxygen in a La₂CuO_{4 + δ} single crystal, isolated superconducting inclusions can be formed at an average hole concentration not higher than 0.0024 per copper atom. The behavior of the conductivity in a magnetic field below 20 K is consistent with possible existence of a insulating low-temperature magnetic phase (spin density waves), which is typical of cuprates with excess oxygen.     

Manifestation of ferromagnetism in lightly oxygen-doped La2CuO4 single crystals in magnetic fields H<50 Oe

The temperature and field dependences χ(T,H) in La₂CuO_4+δ single crystals with δ<0.015 have been investigated in magnetic fields 0.1<H<450 Oe by the differential magnetic susceptibility method. It was found that under oxygen doping conditions ferromagnetic regions are formed. These regions produce a characteristic curve of the magnetic susceptibility χ(T,H), which is observed only in magnetic fields of less than 50 Oe. This can be explained by the formation of ferrons [A. Aharony et al. Phys. Rev. Lett. 60, 1330 (1988); L. I. Glazman and A. S. Ioselevich, Z. Phys. B 80, 268 (1990)] in an antiferromagnetic matrix. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 152–155 (10 August 1996)     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Magnetic studies of phase separation in La2CuO4+° single crystals

The phase seperation effect in La₂CuO_4.015 single crystal has been studied by magnetic susceptibility and EPR experiments. The experiments confirm the percolative phase separation model based on the concept of magnetic polarons formed by hole doping. It has been shown that the superconducting subphase has specific spatial structure which is responsible for unconventional low temperature behavior of susceptibility. The EPR signal as well as the paramagnetic contribution to the lowtemperature susceptibility have been shown to arise from isolated Cu²⁺ ions located inside the phase domain walls.     

Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La_1.85Sr_0.15CuO₄ superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field strength. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 683–687 (10 December 1997)