There is tremendous fl urry of research interest in multiferroic materials that exhibit multiple primary ferroic order parameters simultaneously and that have practical applications. [ 1 ] Much of the recent work on multiferroic materials was directed towards bringing ferroelectricity and ferromagnetism together in a single-phase compound. [ 2 ] The search for these materials is driven by the prospect of controlling charges by applying magnetic fi elds or/and manipulating spins by applying electrical fi eld and using this to construct new paradigms of spintronics devices and data storage applications. [ 1 ] Despite these intriguing characteristics, the simultaneous presence of electric and magnetic dipoles does not guarantee mutual coupling because the microscopic mechanisms of ferroelectricity and ferromagnetism are quite different. [ 2 ] Owing to these fundamental and technological challenges, the long sought after single-phase multiferroic material was hampered and was mainly based on silicon incompatible perovskite materials, [ 1 ] which limits their multifunctional applications.Zinc oxide (ZnO) has been of growing technological importance due to its versatile properties. [ 3 ] The substitution of transition-metal ions into the Zn sites leads to ferromagnetic ordering. [ 4 ] Additionally, doped-ZnO bulk crystals and thin fi lms could exhibit ferroelectric behavior. [ 5 ] Recently, a number of reports have shown the coexistence of ferromagnetism and ferroelectricity in doped ZnO. [ 6 ] However, there are no reports on the demonstration of mutual manipulation between ferromagnetism and ferroelectricity in this class of material. Despite these exciting characteristics, the demonstration of mutual ferromagnetic and ferroelectric coupling (true multiferroic behavior) in doped-ZnO fi lms poses a technology diffi culty because 1) ZnO is not suffi ciently insulating, which undermines the ferroelectric measurement and 2) most of the ferromagnetic and ferroelectric order in doped-ZnO fi lms are mutually exclusive. [ 2 ] Here, we report the mutual manipulation of ferromagneticferroelectric properties in a copper-doped ZnO (ZnO:Cu) fi lm and demonstrate domain structure manipulation in this material. The ZnO:Cu is a particular interesting system for several reasons. First, the fact that neither metallic Cu nor its oxides are ferromagnetic at 300 K is advantageous, removing any possibility of ferromagnetism arising from the presence of precipitates or secondary phases. The ZnO:Cu was reported to possess ferromagnetism, attributing to the presence of oxygen vacancies and Cu ions. [ 7 , 8 , 9 ] Second, Cu atoms in ZnO are well-known as electron traps, which results in a high resistivity fi lm. [ 10 ] A highly resistive fi lm reduces the current leakage, favoring ferroelectric measurement. In this study, we discover a striking multiferroic phenomenon in ZnO:Cu that is attributed to the interplay of Cu ions and oxygen vacancies (V o ). The substitution of Cu 2 + into Zn 2 + sites gives rise to ferroelectricity, while Cu 2 + ...

The surface potential of undoped and copper-doped zinc oxide (ZnO:Cu) films has been studied using the Kelvin probe force microscopy at ambient condition. In contrast to the undoped ZnO with unipolar behavior, the ZnO:Cu film exhibits a bipolar surface potential behavior under a dc bias. The localized hole trapping phenomenon is attributed to the presence of Cu ions in ZnO films. With an appropriate amount of the Cu ions (∼8 at. %), the charge trapping is reasonably stable over a period of 20 h, which can be associated with the presence of oxygen vacancies. This coexistence of Cu ions and oxygen vacancies in ZnO gives rise to stable bipolar behavior, paving way to potential charge storage application.

This paper presents the mechanical polishing effects on the surface domains and crystal structures of relaxor‐based Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) single crystals. In normal sample preparation processes, a “surface deformed layer” composed of distorted crystal structures is produced due to intense compression caused by several polishing steps using a series of lapping films down to 1 μm in particle size. This “surface deformed layer” may contribute to dissimilar properties compared with those of the interior, and also result in pop‐in events in the load–displacement curve (P–h curve) during nanoindentation. An anomaly in the X‐ray diffraction (XRD) profiles is also found, demonstrating a broad minor peak besides the major peak in the intensity. In addition, Piezoresponse Force Microscopy reveals that the domain structures on the crystal surface appear to be distorted and aligned along the polishing direction. Therefore, a controlled fine polishing procedure using Al2O3 slurry of 0.3 μm particle size is adopted to remove this “surface deformed layer.” After polishing to mirror finish, the macroscopic orientations of the domain walls agree well with the permissible domain wall directions. The topography is also altered analogous with the polarization direction. More specifically, the upward domains constitute a depression of ∼10 nm compared with the downward domains, suggesting a different hardness for the head and tail domain sections. Furthermore, the minor peak in the XRD and the pop‐in event in the nanoindentation P–h curve are successfully eliminated after this fine polishing procedure. The removal of the surface layer may also lower the coercive field of the crystals, thus enabling ferroelectric control with a smaller voltage.

The deformation behavior of [001] T -and [011] T -cut single crystal solid solution of Pb(Zn 1/3 Nb 2/3 )O 3 -6% PbTiO 3 (PZN-6%PT) in both unpoled and poled states has been investigated by nanoindentation. Nanoindentation experiments reveal that material pile-up and local damage around the indentation impressions are observed at ultra-low loads. These pile-ups and local damage cause a pop-in event (i.e. a sudden increase in displacement at an approximately constant load) in the nanoindentation load-displacement curve (P-h curve). Detailed studies of the relationships between indentation load (P), displacement (h) and harmonic contact stiffness (S ) suggest that there is a surface layer, possibly due to crystal fabrication processes, which possesses different mechanical properties from the interior. The thickness of this surface layer is estimated to be approximately 300 nm. Furthermore, it is found that [011] T -cut crystal is stiffer than [001] T -cut crystal. On the other hand, both [001] T -and [011] T -cut crystals in unpoled state possess lower contact stiffness than poled crystals. This finding suggests that poling improved the mechanical property of the crystal. In summary, poled [001] T -cut crystals have an elastic modulus of (107 AE 6) GPa and a hardness of (5.1 AE 0.4) GPa. In contrast, the modulus for [011] T -cut crystals is not constant but increases with indentation depth.

This paper presents recent studies on surface and cross-sectional domain structures of Pb(Zn1/3Nb2/3)O3–(6–7)% PbTiO3 (PZN–PT) single crystals using piezoresponse force microscopy and three-point bending technique. The surface domain structures for the rhombohedral-based single crystals in (001) orientation are found to be influenced by polishing process, whereas the surface domains on the (011)-oriented single crystals are aligned along [011¯] direction, unaffected by the polishing process. On the other hand, the domain structures on the cross-sectional fracture surface show preferential alignment which agrees reasonably with the rhombohedral dipoles on the {100} and {110} planes. The differences between the surface and cross-sectional domain structures could be attributed to stress compensation between the surface strain effect and the minimization of elastic energy. In addition, both surface and cross-sectional surface demonstrate nanoscale domains, about 100–200 nm in size. Further fractography observation suggests that the preferred cracking planes for the PZN–PT single crystals are {110} and {100} planes. The {110} planes may be the slip planes along which material pile up is observed upon indentation loading. The pile up results in tensile hoop stress, producing radial cracks along the {100} cleavage planes. To accommodate the localized stress change, new ferroelastic domains by mechanical stress are then formed without interrupting the out-of-plane piezoelectric response. Since the material pile up is thought to cause enhanced toughness along {110} planes, the PZN–PT single crystal in [011]-poled orientation exhibits more superior piezoelectric properties compared to that of the [001]-poled counterpart.