Polarized neutron reflectometry study of the magnetization reversal process in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7/{\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3$ superlattices grown on ${\text{SrTiO}}_3$ substrates

Using polarized neutron reflectometry we investigated the reversal of the magnetization of a high-$T_c$ superconductor/ferromagnet superlattice that consists of eight bilayers of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7\left(25.6\text{nm}\right)/{\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3\left(25.6\text{nm}\right)$ grown on a ${\text{SrTiO}}_3$ substrate. The measurements were performed during a magnetization hysteresis loop at 5 K. We obtained evidence that the reversal in the vicinity of the coercive field proceeds via the switching of micrometer-sized magnetic domains that are considerably larger than the typical domains of ${\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3$. Furthermore, these large magnetic domains appear to be more strongly correlated along the vertical direction of the superlattice than along the lateral one. We provide evidence that this unusual behavior may be induced by the ${\text{SrTiO}}_3$ substrate which undergoes a series of structural phase transitions, some of which give rise to the formation of micrometer-sized surface facets that are tilted with respect to each other. These facets and the resulting strain fields are transmitted throughout the superlattice and thus may act as templates for the large magnetic domains in the ${\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3$ layers whose magnetic properties are very susceptible to the lattice strain.

Figure 1

Sketch of the geometry of the neutron beams and the applied magnetic field, $\mathbf{H}$, for the neutron reflectometry measurements. The scattering plane is spanned by the incident beam ${\mathbf{k}}_i$ and the final beam ${\mathbf{k}}_f$ and contains the surface normal $\mathbf{z}$. The angle of incidence relative to the (averaged) surface is denoted as $\omega$ while the scattering angle (detector angle) is $2\theta$. For the experiments presented here, $\mathbf{H}$ is applied normal to the scattering plane.

Figure 2

Illustration of the wave fields formed in a multilayer by refraction and multiple reflection at the interfaces (top left); and the corresponding depth profile of the potential $V$ (top right). The solid line in the depth profile stands for the nuclear contribution, the shaded areas mark the potential including magnetic contributions for $|-⟩$ and $|+⟩$ neutrons. The graph shows a calculated $R\left(q_z\right)$ curve for a nonmagnetic sample (solid line) and for the case where one material is magnetic. The dotted line corresponds to spin-down, the dashed one to spin-up neutrons.

Figure 3

Magnetization loop of the $\left[{\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7\left(25.6\text{nm}\right)/{\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_3\left(25.6\text{nm}\right)\right]\times 8$ superlattice performed with a vibrating sample magnetometer at $T=5\text{K}$. The main part shows the low field regime which yields a coercive field of ${\mu }_0{H}_{\text{coerc}}=0.028\text{T}$ and the absence of an exchange bias effect. The inset displays the positive branch up to 7 T from which a saturation moment of about $2.9{\mu }_{\text{B}}$ per Mn is derived. The diamagnetic signal of the ${\text{SrTiO}}_3$ substrate has been subtracted.

Figure 4

PNR curves taken at 5 K right before (a), during [(b) and (c)] and after the magnetization reversal process (d) and finally in saturation (e). The up (down) triangles show the curves for the spin-up (spin-down) polarization of the neutron spins, i.e., for $|+⟩$ $\left(|-⟩\right)$. The dashed line in (a) shows the unpolarized reflectivity curve at room temperature in the nonmagnetic state, the solid lines in (e) show reflectivity curves that were calculated with a model as described in the text.

Figure 5

(a) and (b) Comparison of the measured polarized reflectivity curves at ${\mu }_0{H}_{\text{appl}}=-0.023\text{T}$ close to the coercive field (points with error bars) with representative fits using a model that assumes a coherent superposition of the signal from small magnetic domains (as compared to the neutron coherence volume). It shows that a good fit to the spin-down channel results in a very poor fit for the spin-up channel (a) and vice versa (a). (c) and (d) Magnetic depth profiles that have been used for the fits in (a) and (b), respectively.

Figure 6

(a)–(h) Measured reflectivity curves during the reversal process (points with error bars). The data during magnetization reversal are shown to be reasonably described by a linear combination of the reflectivity curves measured at ${\mu }_0{H}_{\text{appl}}=0.008\text{T}$ and ${\mu }_0{H}_{\text{appl}}=-0.048\text{T}$. The lines in the figures are calculated with the formula $R^{\pm }=x{R}_{-0.048\text{T}}^{\pm }+\left(1-x\right)R_{0.008\text{T}}^{\pm }$ with $x$ given in the respective figure. (i) Measured $M\text{−}H$-hysteresis loop with the points indicated where the reflectivity curves (a)–(h) have been measured.

Figure 7

Off-specular maps at temperature of 300 K (a) and 15 K (b) plotted as a function of the angle of the incident beam, ${\alpha }_i$, versus the one of the reflected beam, ${\alpha }_f$. The significant broadning of the specular signal in (b) arises from the buckling of the sample surface which gives rise to an angular spread of about $0.3°–0.4°$.

Figure 8

Magneto-optical images showing the occurrence of a domain state on the micrometer scale which appears as a stripelike variation from bright to dark contrast along the diagonal direction of the sample. This domain state is most pronounced at 5 K and disappears above 65 K.