Dzyaloshinsky-Moriya spin canting in the low-temperature tetragonal phase of La_2−x−yEu_ySr_xCuO₄

Magnetization measurements in magnetic fields up to 14 Tesla have been used to study the magnetism of the Cu spins in La_2−x−yEu_ySr_xCuO₄ (x⩽0.17; y⩽0.2). This system exhibits weak ferromagnetism due to a combination of the Dzyaloshinsky-Moriya interaction and tilting of the CuO₆ octahedra. In the low-temperature orthorhombic (LTO) phase, the magnetic structure is the same as in the LTO phase of pure La_2−xSr_xCuO₄; however, the Eu-doped system also exhibits low-temperature transitions to structural phases with different octahedral tilt patterns. There has been a long-standing debate about whether the spin-canting of the LTO phase continues to exist in the low-temperature tetragonal (LTT) phase. In contrast to theoretical predictions, our results clearly show that Cu spin canting is present in the LTT phase (within the antiferromagnetic regime) as well as in an intermediate low-temperature less-orthorhombic (LTLO) phase. Moreover, in La_1.8Eu_0.2CuO₄ the canted moment is about 50% larger than in pure La₂CuO₄, which we attribute to the larger tilt angle of the octahedra in the Eu-doped compound. We also find clear evidence that the size of the canted moment does not change significantly at the structural transition itself. The most important change induced by the transition is a significant reduction of the magnetic coupling between the CuO₂ planes. As a consequence, the spin-flip for magnetic field perpendicular to the CuO₂ planes, which is the most characteristic fingerprint of the canted Cu spin structure in the LTO phase, disappears in the LTT phase. The shape of the magnetization curves changes from the well known spin-flip type to a weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed even at very low temperatures, which seems to indicate that the interlayer decoupling in our samples is not perfect. Nonetheless, a small fraction (≲15%) of the canted Cu spin moments can be remanently magnetized throughout the entire antiferromagnetically ordered LTT/LTLO phase, i.e., for T≲135 K and x<0.02. It appears that the remanent canted moment is perpendicular to the CuO₂ planes. A small number of experiments were performed with the magnetic field applied parallel to the CuO₂ planes, where in pure La₂CuO₄ a spin-flop transition was observed. We find that in La_1.8Eu_0.2CuO₄ the critical field of the spin-flop seems to decrease in the LTLO phase, which might indicate a competition between different in-plane anisotropies. To study the Cu spin magnetism in La_2−x−yEu_ySr_xCuO₄, a careful analysis of the Van Vleck paramagnetism of the Eu³⁺ ions was performed.