We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V(O)) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic "indirect double-exchange" model, in which alignments of localized large moments of Cu in the vicinity of the V(O) are mediated by the large-sized vacancy orbitals.

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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 + ...

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe 16 N 2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe 16 N 2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe 16 N 2 + Fe 8 N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed "cluster+atom" theory provides promising directions on designing novel magnetic materials with unique performances.

Two- and three-dimensional arrays were prepared by self-assembly of iron nanoparticles with similar magnetic moments and interparticle separations, and characterized both magnetically and structurally. The rapid magnetization decay in the three-dimensional (3D) arrays suggests a relaxation mechanism than has been previously reported, perhaps associated with the existence of domain walls within large structurally ordered regions. Small angle x-ray scattering indicates the presence of such regions in the 3D arrays.