Magnetoelectric coupling in layered LSMO/PZT nanostructures

Multiferroic thin film composites with electric field-effect driven magnetoelectric (ME) coupling offer the possibility to reversibly tune magnetic properties in materials intended for device applications. The structural and functional versatility of such artificial heterostructures makes them attractive not only for various data processing, storage and sensor applications but also for studying the fundamental ME coupling mechanisms. La_(1−x)Sr_(x)MnO_(3) (LSMO)/PbZr_(y)Ti_(1−y)O_(3) (PZT) is an ideal choice for such a composite, combining the unrivaled ferroelectric (FE) properties of PZT with the multiple electronic and magnetic phenomena exhibited by the mixed valency manganite LSMO. The main physical feature used in realization of the LSMO/PZT ME composites is a striking sensitivity of LSMO magnetism to the charge carrier density. Here, the low-doping region is of particular interest, where the competition between the fundamental magnetic coupling mechanisms, Double-Exchange (DE) versus Superexchange (SE), is most distinctive. In the present work an unconventional sputtering technique – the Large-Distance Magnetron Sputtering (LDMS) method – has been established, which allowed for epitaxial deposition of these heterostructures with highest crystallinity and markedly smooth interfaces, necessary for effective field-effect control of magnetism. The large target-substrate distance effectively suppressed the destructive oxygen ion bombardment, inherently connected with oxide sputtering, and yielded an outstanding lateral uniformity of the film stack. The latter was vital for the fabrication of large capacitor structures of several square millimeter area that were required for detecting the ME coupling in a Superconductive Quantum Interference Device (SQUID) magnetometer. The growth of LSMO on various single crystalline substrates was mastered by exploring a vast deposition parameter space, encompassing Radio Frequency (RF) and Direct Current (DC) sputtering. Commensurately grown on SrTiO_(3) (STO) substrates, the x = 13 %-doped LSMO thin films were found to be stabilized in a metallic low-temperature phase, exhibiting an elevated Curie temperature (T_(C)), as compared to their bulk counterparts. Regarding the PZT deposition, the LDMS technique naturally compensated for high volatility of the PbO vapor yielding a stoichiometric and phase pure film. Thus, with an optimal choice of deposition conditions, LSMO/PZT/Au thin film capacitors with excellent FE properties, i.e., high polarizability and long retention time, were obtained. The high yield of 75 % of 1 mm² large capacitor structures was an excellent starting point for the ME tuning studies which were then carried out in a SQUID magnetometer in order to enable a quantitative analysis of the ME coupling. For this purpose, the measurement device needed to be modified, to allow for in situ application of electric fields. Furthermore, a SQUID measurement cell was designed to be used at low-temperatures and to keep the spurious magnetic signals as low as possible. All key in situ measurements were performed in FE remanence mode in order to avoid artifacts of leakage current and to protect the samples from Time-Dependent Dielectric Breakdown (TDDB). The ME tuning measurements revealed a direct correlation of the FE remanent hysteresis and the magnetic response of the LSMO layer, evidencing a purely field-effect driven coupling and a virtual absence of any magnetostrictive coupling to possible piezo-strain of the PZT layer. In temperature-dependent measurements, a reversal of the sign of the ME effect was observed, with a positive extremum for the electron hole (h^(+)) accumulation mode around the Curie temperature of the magnetic transition. The effect gradually decreased with the lowered temperature to become negative eventually. On the basis of the phase diagram and the dependence of the Mn magnetic moment on the Sr doping level, the shape of the curve was phenomenologically modeled by a transformation of the original Field Cooling (FC) curve. The resultant transformation was a superposition of a FC curve shift along the temperature axis and a rescaling of the magnetic moment. The dependence of the magnetic response to the modulation of the surface charge showed a mostly linear behavior with a coupling coefficient of α_(MQ) ≈ −3.6 μ_(B)/h^(+) for low charge concentration. As this value stunningly matches the magnetic moment of a Mn atom per one formula unit, this result suggests an appearance of antiferromagnetic (AF) coupling upon surface charging. At higher charge modulation, the absolute value of the tuning coefficient decreased, indicating the onset of another magnetic exchange mechanism. Eventually, the quantitative analysis of the ME coupling at the LSMO/PZT interface has allowed for developing a physical picture based on the electronic phase separation of competing AF and ferromagnetic (FM) phases immanent to LSMO