Enhanced room temperature ferromagnetism in electrodeposited Co-doped ZnO nanostructured thin films by controlling the oxygen vacancy defects

Journal Article · Sun Jun 07 04:00:00 EDT 2015 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.4922050· OSTI ID:22412876

Simimol, A. ^[1]; Anappara, Aji A. ^[2]; Greulich-Weber, S. ^[3]; Chowdhury, Prasanta ^[1]

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1. Nanomaterials Research Lab, Surface Engineering Division, CSIR-National Aerospace Laboratories, Post Bag No. 1779, Bangalore 560017 (India)
2. Department of Physics, National Institute of Technology Calicut, Calicut 673601 (India)
3. Department of Physics, Nanophotonic Materials, Faculty of Science, University of Paderborn, 33095 Paderborn (Germany)

We report the growth of un-doped and cobalt doped ZnO nanostructures fabricated on FTO coated glass substrates using electrodeposition method. A detailed study on the effects of dopant concentration on morphology, structural, optical, and magnetic properties of the ZnO nanostructures has been carried out systematically by varying the Co concentration (c.{sub Co}) from 0.01 to 1 mM. For c.{sub Co }≤ 0.2 mM, h-wurtzite phase with no secondary phases of Co were present in the ZnO nanostructures. For c.{sub Co} ≤ 0.2 mM, the photoluminescence spectra exhibited a decrease in the intensity of ultraviolet emission as well as band-gap narrowing with an increase in dopant concentration. All the doped samples displayed a broad emission in the visible range and its intensity increased with an increase in Co concentration. It was found that the defect centers such as oxygen vacancies and zinc interstitials were the source of the visible emission. The X-ray photoelectron spectroscopy studies revealed, Co was primarily in the divalent state, replacing the Zn ion inside the tetrahedral crystal site of ZnO without forming any cluster or secondary phases of Co. The un-doped ZnO nanorods exhibited diamagnetic behavior and it remained up to a c.{sub Co} of 0.05 mM, while for c.{sub Co }> 0.05 mM, the ZnO nanostructures exhibited ferromagnetic behavior at room temperature. The coercivity increased to 695 G for 0.2 mM Co-doped sample and then it decreased for c.{sub Co }> 0.2 mM. Our results illustrate that up to a threshold concentration of 0.2 mM, the strong ferromagnetism is due to the oxygen vacancy defects centers, which exist in the Co-doped ZnO nanostructures. The origin of strong ferromagnetism at room temperature in Co-doped ZnO nanostructures is attributed to the s-d exchange interaction between the localized spin moments resulting from the oxygen vacancies and d electrons of Co{sup 2+} ions. Our findings provide a new insight for tuning the defect density by precisely controlling the dopant concentration in order to get the desired magnetic behavior at room temperature.

• OSTI ID: 22412876
• Journal Information: Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 21 Vol. 117; ISSN JAPIAU; ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
COBALT
COERCIVE FORCE
CONCENTRATION RATIO
CRYSTALS
DOPED MATERIALS
ELECTRODEPOSITION
FERROMAGNETISM
INTERSTITIALS
MAGNETIC PROPERTIES
NANOSTRUCTURES
PHOTOLUMINESCENCE
SUBSTRATES
TEMPERATURE RANGE 0273-0400 K
THIN FILMS
ULTRAVIOLET RADIATION
VACANCIES
X-RAY PHOTOELECTRON SPECTROSCOPY
ZINC OXIDES