Magnetic Properties of ZnO Nanoparticles

The Journal of Physical Chemistry C, 2012

The combined element-specific X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) study of ZnO nanoparticles (NPs) capped with organic molecules confirms the occurrence of intrinsic ferromagnetic-like (FML) behavior up to room temperature (HTFM). Zn K-edge XMCD measurements reveal the coexistence of two different magnetic contributions: a paramagnetic response from the core of the NP, and a ferromagnetic-like contribution stemming from the interface formed between the ZnO core of the NP and the organic molecule. The extent and conformation of this interface depends on the capping molecule, and as demonstrated in ZnO/ZnS heterostructures, the FML behavior is reinforced when neat interfaces are formed.

Physica B: Condensed Matter, 2014

We report systematic investigations on the evolution of nanostructured ZnO, room temperature ferromagnetism, and tunable optical properties of ZnO nanoparticles prepared by a mechanical alloying process. It was observed that both un-milled and as-milled powders exhibited a wurtzite structure, but average crystallite size decreased and the effective strain increased for the initial periods of milling. Paramagnetic nature observed in un-milled ZnO gradually unveils room temperature ferromagnetic ordering with modest moment and coercivity. A maximum moment of 0.013m B /f.u. at 12 kOe applied field and a coercivity of 172 Oe were obtained for 40 h milled ZnO powder. Thermo-magnetization data reveal a clear magnetic phase transition from ferromagnetic to paramagnetic state around 500 1C, which shifts slightly towards higher temperature with an increasing milling period up to 20 h. Annealing of asmilled ZnO powder and UV-vis studies display a drastic reduction in room temperature magnetic moment and blue-shifting of excitonic absorption peak. The observed ferromagnetic properties are intrinsic and discussed on the basis of finite size effect, defect density due to oxygen vacancy. These nanoparticles with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic devices.

Journal of Magnetism and Magnetic Materials, 2012

Zn 1-x Fe x O (x ¼0-0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid-liquid system at 80 1C, and the Zn 1 À x Fe x O nanoparticles were obtained by annealing at 300 1C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn 1 À x Fe x O. The optical properties were determined by Infrared and Ultraviolet-visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe 0 þ and the Fe 3 þ in the representative Zn 0.95 Fe 0.05 O sample. The optical band gap of Zn 1 À x Fe x O is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples.

Solid State Communications, 2011

ZnO nanoparticles with Wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. Nanoparticles are in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization vs temperature, M(T), and isothermal measurements M(H) were determined. The coercive field clearly shows ferromagnetism above room temperature. An exchange bias was observed, and we related this behavior at a core shell structure presented in the samples. The chemical synthesis, structure, defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior.

Journal of Magnetism and Magnetic Materials, 2009

The ZnO:Fe nanoparticles of mean size 3–10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe–Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe3+ nature of the iron atom in ZnO matrix.

Physical Review B, 2010

We present here direct experimental evidence of the magnetic polarization of Zn atoms in ZnO nanoparticles capped with different materials by means of x-ray magnetic circular dichroism ͑XMCD͒. Our results demonstrate that the magnetism in this material is intrinsic and relays in the ZnO conduction band. The analysis of both x-ray absorption spectroscopy and XMCD signals points out the formation of a well-defined interface between ZnO and the capping molecule in which the exotic magnetism arises at the hybridized band formed among Zn and the bonding atom of the molecule. The magnetic properties of these systems should critically depend on the details of this interface which may offer a new insight into the different observations for seemingly identical materials.

Nanoscale advances, 2020

In this paper, we report the existence of defect induced intrinsic room-temperature ferromagnetism (RTFM) in Cu doped ZnO synthesized via a facile sol-gel route. The wurtzite crystal structure of ZnO remained intact up to certain Cu doping concentrations under the present synthesis environment as confirmed by the Rietveld refined X-ray diffraction pattern with the average crystallite size between 35 and 50 nm. Field emission scanning electron microscopy reveals the formation of bullet-like morphologies for pure and Cu doped ZnO. Diffuse reflectance UV-vis shows a decrease in the energy band gap of ZnO on Cu doping. Further, these ZnO samples exhibit strong visible photoluminescence in the region of 500-700 nm associated with defects/vacancies. Near-edge X-ray absorption fine-structure measurements at Zn, Cu L 3,2-and O K-edges ruled out the existence of metallic Cu clusters in the synthesized samples (up to 2% doping concentration) supporting the XRD results and providing the evidence of oxygen vacancy mediated ferromagnetism in Cu : ZnO systems. The observed RTFM in Cu doped ZnO nanostructures can be explained by polaronic percolation of bound magnetic polarons formed by oxygen vacancies. Further, extended X-ray absorption fine-structure data at Zn and Cu K-edges provide the local electronic structure information around the absorbing (Zn) atom. The above findings for ZnO nanostructures unwind the cause of magnetism and constitute a significant lift towards realizing spinrelated devices and optoelectronic applications.

The Journal of Physical Chemistry C, 2011

2011

Journal of Applied Physics, 2010

We have studied the electronic structure of Zn0.9Fe0.1O nano-particles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy (XPS), resonant photoemission spectroscopy (RPES), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe 3+ ionic state with mixture of a small amount of Fe 2+ and that Fe 3+ ions are dominant in the surface region of the nano-particles. It is shown that the room temperature ferromagnetism in the Zn0.9Fe0.1O nano-particles is primarily originated from the antiferromagnetic coupling between unequal amounts of Fe 3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼ 2-3 nm.