Mutual Ferromagnetic–Ferroelectric Coupling in Multiferroic Copper‐Doped ZnO

Abstract

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 + ...

Influence of oxygen partial pressure on the microstructural and magnetic properties of Er-doped ZnO thin films

“…The observed stripe-like magnetic domain indicates a long range magnetic ordering. 17 Similar stripe-like structures are also observed in other Er-doped ZnO films fabricated with different P O2 . Fig.…”

Zinc vacancy and erbium cluster jointly promote ferromagnetism in erbium-doped ZnO thin film

“…The stripe-like structures observed in the MFM amplitude image show a long range magnetic ordering in the film. 25 The similar stripe-like structures are also observed in Zn 1-x Er x O (0.01 ≤ x ≤ 0.04) thin films, which are not present on here. Fig.…”

Piezoelectricity and charge trapping in ZnO and Co-doped ZnO thin films

“…17 The control of ZnO conductivity by means of extrinsic doping can thus lead to a fine tuning of the piezoelectricity, which may also provide it with ferroelectric responses, whereas the ferroelectricity of undoped ZnO has never been reported from experiments on a macroscopic scale. 17 By contrast, local ferroelectricity can be induced by the presence of Li and Cu doping [18][19][20][21] as well as polar defects such as O vacancies at the surface, [22][23][24] and can be probed by performing experiments at the nanoscale.…”