Neutron scattering study on ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$

We present neutron scattering data on two single crystals of the high temperature superconductor ${\mathrm{La}}_{2-x}{(\mathrm{Ca},\mathrm{Sr})}_x\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$. The ${\mathrm{Ca}}_{0.1}$-doped crystal exhibits a long-range antiferromagnetically ordered ground state. In contrast, the ${\mathrm{Sr}}_{0.15}$-doped crystal exhibits short-range antiferromagnetic order as well as weak superconductivity. In both crystals antiferromagnetic correlations are commensurate; however, some results on the ${\mathrm{Ca}}_{0.1}$-doped crystal resemble those on the spin-glass phase of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$, where magnetic correlations became incommensurate. In addition, both crystals show a structural transition from tetragonal to orthorhombic symmetry. Quite remarkably, the temperature dependence and correlation length of the magnetic order is very similar to that of the orthorhombic distortion. We attribute this behavior to an orthorhombic strain-induced interbilayer magnetic coupling, which triggers the antiferromagnetic order. The large size of the crystals made it also possible to study the magnetic diffuse scattering along rods perpendicular to the $\mathrm{Cu}{\mathrm{O}}_2$ planes in more detail. For comparison we show x-ray diffraction and magnetization data. In particular, for the ${\mathrm{Ca}}_{0.1}$-doped crystal these measurements reveal valuable information on the spin-glass transition as well as a second anomaly associated with the Néel transition.

Figure 1

Unit cell of ${\mathrm{La}}_{2-x}{(\mathrm{Sr},\mathrm{Ca})}_x\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ in the tetragonal high temperature phase.

Figure 2

Reciprocal space for the $(h,h,l)$ and $(h,k,2k)$ zones with typical scans indicated by arrows. X-ray diffraction experiments (XRD) were mainly performed at $(\frac{3}{2},\frac{3}{2},2)$ and $(\frac{5}{2},\frac{5}{2},2)$.

Figure 3

Elastic scans along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},l)$ and $(\frac{3}{2},\frac{3}{2},l)$ for ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (top) and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (bottom). M indicates magnetic Bragg reflections and S predominantly nuclear superlattice reflections.

Figure 4

Intensity of the magnetic Bragg peaks $(\frac{1}{2},\frac{1}{2},3)$ and $(\frac{1}{2},\frac{3}{2},3)$, and the orthorhombic superlattice peaks $(\frac{1}{2},\frac{1}{2},4)$, $(\frac{1}{2},\frac{1}{2},2)$, and $(\frac{3}{2},\frac{3}{2},4)$, of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (a), (b) and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (c), (d) as a function of temperature. Data sets Nos. 1 and 2 were collected during different experiments (see text). In (a) and (b) errors are within point size. Inset: full width at half-maximum of magnetic Bragg and orthorhombic superlattice peaks in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ as a function of temperature. Spectrometer resolution indicated by dotted line.

Figure 5

Elastic $l$-scans for ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$. (a) along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},l)$ at $9\mathrm{K}$ (logarithmic intensity scale). The peak at $l=7.46$ as well as the left shoulder of the peak at $l=9$ originate from Al powder ring reflections. (b) along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},l)$ at $9\mathrm{K}$ and $40\mathrm{K}$. (c) along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},l)$ and $(\frac{3}{2},\frac{3}{2},l)$ at $9\mathrm{K}$. The origin of the peaks at $l=6.5$ in (a) and at $l=0.35$ in (c) is not clear, but there seems to be no systematic appearance of further unidentified peaks.

Figure 6

Elastic scans through the magnetic Bragg peak $(\frac{1}{2},\frac{1}{2},3)$ of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ at different temperatures. (a) $h$-scans. (b) $l$-scans. (c) Background signal as well as diffuse scattering intensity extracted from $h$- and $l$-scans.

Figure 7

Elastic diffuse scattering intensity at $(\frac{1}{2},\frac{1}{2},\frac{5}{2})$ and $(\frac{5}{2},\frac{5}{2},\frac{5}{2})$ in ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ as a function of temperature.

Figure 8

Inelastic scans with $\hslash \omega =6\mathrm{meV}$ along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},l)$ for ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$.

Figure 9

Results from x-ray diffraction: Normalized integrated intensity and full width at half maximum of the orthorhombic superlattice peaks $(\frac{3}{2},\frac{3}{2},2)$ in ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (left) and $(\frac{5}{2},\frac{5}{2},2)$ in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ (right) as a function of temperature. Solid line in top left graph is a guide to the eye.

Figure 10

Static magnetic susceptibility of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ as a function of temperature and different directions of the applied field. (a) Zero field cooled data. (b), (c) Zero field cooled and field cooled data at low temperatures.

Figure 11

Static magnetic susceptibility of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ for different magnetic fields as a function of increasing and decreasing temperature. (a) for $H\Vert ab1$ and (b) for $H\Vert ab2$. Curves shifted for clarity.

Figure 12

Static magnetic susceptibility of ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{6+\delta }$ as a function of temperature and different directions of the applied field.