Crystal structure and high-field magnetism of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group $Bmab$ was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined to drive a continuous reorientation of the copper spins from the orthorhombic $b$ axis to the $c$ axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a $c$-axis oriented field previously reported for nonstoichiometric ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ is also observed, but the transition field $\left(11.5\mathrm{T}\right)$ is significantly larger than that in the previous work.

Figure 1

Crystal structure of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ at $325\mathrm{K}$. Shown are the distorted corner-shared $\mathrm{Cu}{\mathrm{O}}_6$ octahedra in the $ab$ plane. The thermal ellipsoids of the atoms are plotted at the 90% probability level.

Figure 2

Temperature dependence of the intensity of the 100 magnetic Bragg reflections of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ and ${\mathrm{La}}_{1.99}{\mathrm{Sr}}_{0.01}\mathrm{Cu}{\mathrm{O}}_4$.

Figure 3

Magnetic structure types of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$. Shown are the magnetic structure in zero field (middle) and applied magnetic field parallel to the $b$ and $c$ axes. The components with the modes $F_z(H∕∕c)$, $A_z(H=0)$, and $F_y(H∕∕b)$ are rather weak. For clarity, they are exaggerated in the figures.

Figure 4

Magnetic field dependence of the magnetic Bragg reflections 100 and 101 of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ for a magnetic field applied within the $ab$ plane.

Figure 5

Magnetic field dependence of the magnetic reflections 100 and 210 of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ for a magnetic field applied parallel to the $c$ axis, measured at $T=1.8\mathrm{K}$. The data were taken with increasing field. Lines are guides to the eye.