Magnetic field dependence of the copper charge density wave order in a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\mathrm{Nd}}_{0.65}{\left({\mathrm{Ca}}_{0.7}{\mathrm{Sr}}_{0.3}\right)}_{0.35}{\mathrm{MnO}}_3$ superlattice

For a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\mathrm{Nd}}_{0.65}{\left({\mathrm{Ca}}_{0.7}{\mathrm{Sr}}_{0.3}\right)}_{0.35}{\mathrm{MnO}}_3$ (YBCO/NCSMO) superlattice, we studied with resonant elastic x-ray scattering (REXS) at the Cu $L_3$ edge how the copper sublattice charge density wave (Cu-CDW) order in YBCO is affected by a large magnetic field up to 6.9 T that weakens the CE-type antiferromagnetic (AF) and the charge/orbital (Mn-CO) orders of the manganite in favor of a ferromagnetic state. While a field of only 2 T induces a strong ferromagnetic moment in the manganite, we find that the Bragg peak of the Cu-CDW hardly changes up to 6 T. Moreover, as the magnetic field is further increased to 6.9 T, the Cu-CDW Bragg peak gets suddenly enhanced and broadened, whereas the ferromagnetic moment of the manganite is already saturated. The observed uncorrelated magnetic field dependence of the charge orders in the cuprate and manganite layers suggests that these orders are not directly coupled across the interface. We rather interpret our data in terms of an indirect coupling via the domain boundaries of the Mn-CO and the related disorder and lattice strain. This interpretation is supported by additional studies of the magnetoelectric response, which provide evidence for a crossover in the dynamics of the Mn-CO in the range between 6 and 7 T, from a low-field state with pinned domains to a high-field state with more mobile and flexible domain boundaries. We attribute the concomitant enhancement and broadening of the Cu-CDW Bragg peak to this crossover.

Figure 1

X-ray diffraction curve ($\theta \text{−}2\theta$ scan) of the $\langle 00L\rangle$ peaks of the NCSMO/YBCO superlattice and the LSAT substrate that confirms its epitaxial growth and high structural quality. Inset: Magnification around the $\langle 002\rangle$ and $\langle 006\rangle$ peaks of NCSMO and YBCO, respectively, showing pronounced satellite peaks that testify for the high structural quality of the superlattice.

Figure 2

(a) Temperature dependence of the DC magnetization in field-cooled mode with the magnetic field applied parallel to the layers of the superlattice. (b) Magnetization versus magnetic field loop at $T=10\mathrm{K}$ after zero-field cooling showing a ferromagnetic hysteresis. Note that the magnetization loop was obtained after zero-field cooling and has a smaller saturation moment than for the field-cooled magnetization in (a). Inset: Magnified view of the hysteresis curve around the origin.

Figure 3

Voltage vs temperature curves obtained in four-contact geometry with an applied current of $I=\pm 10$ $\mu \mathrm{A}$ at (a) $B=2\mathrm{T}$ and (b) $B=8\mathrm{T}$. The Ohmic and non-Ohmic signals have been obtained according to Eqs. (1) and (2). Temperature and magnetic field dependence of (c) the Ohmic signal and (d) the non-Ohmic signal. (Inset: Applied magnetic field dependence of the non-Ohmic signal at 40 and 90 K.)

Figure 4

Schematics of the YBCO/NCSMO superlattice and the configuration of the REXS experiment in grazing incidence geometry.

Figure 5

(a) and (b) Normalized diffraction curves around the Bragg peak of the Cu-CDW of YBCO measured in GI geometry (left panel) and GE geometry (right panel) near the maximum of the Cu $L_3$-edge resonance (931 eV) at $B=0\mathrm{T}$ and temperatures of 40 and 250 K. (c) and (d) Bragg peak of the Cu-CDW after subtracting the curve at 250 K, which represents the background signal. Solid lines show the best fits with a Gaussian function.

Figure 6

(a) and (d), (b) and (e), and (c) and (f) Demonstrate the temperature dependence of the normalized diffraction curves around the Bragg peak of the Cu-CDW of YBCO measured in grazing incidence (GI) geometry and the corresponding background subtracted Bragg peaks of the Cu-CDW obtained by subtracting the curves from those at $250\mathrm{K}$ as background signal at 0 6, and $6.9\mathrm{T}$, respectively. Solid lines show Gaussian fits of the Bragg peak.

Figure 7

Temperature and magnetic field dependence of the Bragg peak due to the Cu-CDW of the YBCO layers. The parameters obtained with Gaussian fits are displayed for (a) the area, (b) the position, (c) the height, and (d) the full-width at half-maximum (FWHM) of the peak at fields of 0 (red), 4 (green), 6 (blue), and 6.9 T (violet). The dashed lines in (a) are guides to the eye.

Figure 8

Magnetic field loops of the spontaneous voltage that develops between two silver contacts on the topmost NCSMO layer without an applied current at temperatures of (a) 30, (b) 90, and (c) 150 K. Gray bars mark the region between 6 and 7 T.