Room temperature ferromagnetism in Mn-doped ZnO films mediated by acceptor defects (original) (raw)
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Room temperature ferromagnetism promoted by defects at zinc sites in Mn-doped ZnO
Journal of Alloys and Compounds, 2016
The nature of the room temperature ferromagnetism in dilute magnetic oxides is still a matter of debate. However, there is a consensus that structural point defects play an important role to achieve a desired long range ferromagnetic order. In this report we have clearly established a correlation between an observed room temperature ferromagnetism and defects at zinc sites for Mn-doped ZnO (Zn 0.93 Mn 0.03 O) samples prepared by the solid state reaction method and subject to post-annealing under reducing atmosphere. Detailed microstructural analysis was carried out to exclude the presence of extrinsic sources of ferromagnetism. Photoluminescence and Hall measurements reveal that the main present defects in the samples are associated to defects at the zinc sites. Magnetic characterization demonstrates a room temperature ferromagnetic behavior associated to a paramagnetic CurieeWeiss component. The magnetization and density of defects expressively reduces after the post-annealing. In this context, the defect mediated magnetic coupling between Mn atoms under the scope of the bound magnetic polaron model and the d 0 ferromagnetism are used to interpret the magnetic properties.
Journal of Alloys and Compounds, 2009
There are several confronting reports of room temperature ferromagnetic (FM) ordering in bulk as well as thin films of dilutely doped or even some undoped semiconductors. We have synthesized and characterized dilute Mn-doped (2 and 4%) ZnO pellets. SQUID measurements confirm that the 2% Mn sample shows the FM ordering above the room temperature and the FM contribution coming mainly from the bulk. However, the ordering gets completely quenched for 4% Mn doping. Upon cooling down, the 2% Mn-doped sample shows further enhancement in the magnetic properties while the 4% sample did not show any FM ordering down to 5 K. The powerful X-ray photoemission spectroscopy (XPS) was employed to compare the electronic structure of these two samples. The XPS results show that the manganese shifts toward the higher valence state upon Mn doping while there is no change in the zinc and oxygen valence. The atomic concentration of divalent Mn state is found to be dominant in the ferromagnetic sample. For the non-ferromagnetic sample, a larger contribution of higher oxidation Mn states is present that is correlated to the suppressed ferromagnetism. Interestingly, the oxygen content is also found to be higher in the 4% Mn sample than that in the 2% Mn sample that has been attributed to the charge neutrality of the samples. The present study provides evidence that the magnetization originates neither from any precipitating secondary phase nor from the oxygen content but the Mn 2+ state plays a significant role for the FM properties in the Mn-doped ZnO system.
Physica B: Condensed Matter, 2018
Understanding of origin of ferromagnetism in dilute magnetic oxides (DMO's) has become one of the most challenging research problems in condensed matter physics. Here we are reporting a detailed study of magnetic properties and electronic structure of two 5% Co-doped ZnO samples (the as-prepared sample Zn 0.95 Co 0.05 O and the hydrogenated sample Zn 0.95 Co 0.05 O:H). The as-prepared sample is found to be paramagnetic while through hydrogenation, we observed inducement of remarkable ferromagnetism in it. The H-mediated magnetic transition is accompanied by electronic structure modifications with no structural deviations. To get in-depth information into electronic structure correlations of the observed ferromagnetism, we have investigated their electronic properties in detail. For this purpose, we have employed the site-selective and element-sensitive X-ray-absorption spectroscopy (XAS) in the vicinity of the Cobalt L 2,3 edge, the oxygen K edge, and the Zinc L 3 edge using synchrotron radiation. The Co L 2,3 edge spectra clearly show that Co dopants reside at the Zn sites for both these samples and that they are tetrahedrally coordinated with the ligand O atoms. Very minor changes are observed in the Zn L 3 edge spectra. However, the O 1s edge spectra display dominant additional components in the ferromagnetic hydrogenated sample Zn 0.95 Co 0.05 O:H, not observed in the asprepared non-magnetic sample Zn 0.95 Co 0.05 O. We conclude that the observed spectral features can be attributed to the presence of O vacancies and the hybridization of Co 3d states with O 2p vacancy states. These two factors together are likely to play important role in inducement of ferromagnetic ordering in this Co-doped ZnO system. However, which of these two weighs more in this mechanism, cannot be pinpointed and more studies are required in this regard.
Defects induced ferromagnetism in Mn doped ZnO
Journal of Magnetism and Magnetic Materials, 2011
Single phase Mn doped (2 at %) ZnO samples have been synthesized by solid-state reaction technique. Before the final sintering at 500 C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 hours). The grain sizes of the samples are very close to each other (~ 32 \pm 4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 {\mu}_B / Mn atom) was achieved for 96 hours milled sample. Electrical resistivity has been found to increase with increasing milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.
Ferromagnetic Domain Behaviors in Mn doped ZnO Film
Journal of Magnetics, 2011
Mn doped ZnO films were prepared on Si (100) substrates using sol-gel method. The prepared films were annealed at 550ºC for decomposition and oxidation of the precursors. XRD analysis revealed the presence of ZnMnO hexagonal wurtzite phase along with the presence of small quantity of ZnMn 2 O 3 secondary phase and poor crystalline nature. The 2D, 3D views of magnetic domains and domain profiles were obtained using magnetic force microscopy at room temperature. Rectangular shaped domains with an average size of 4.16 nm were observed. Magnetic moment measurement as a function of magnetic field was measured using superconducting quantum interference device (SQUID) magnetometry at room temperature. The result showed the ferromagnetic hysteresis loop with a curie temperature higher than 300 K.
Defect Mediated Magnetic Interaction and High Tc Ferromagnetism in Co Doped ZnO Nanoparticles
2011
Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles (NPs). ZnO NPs are doped with 3% and 5% Co using ball milling and ferromagnetism (FM) is studied at room temperature and above. A high Curie temperature (T c has been observed from the Co doped ZnO NPs. X-ray diffraction and high resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption and photoluminescence studies on the doped samples show change in band structure and oxygen vacancy defects, respectively. Micro-Raman studies of doped samples shows defect related additional strong bands at 547 and 574 cm −1 confirming the presence of oxygen vacancy defects in ZnO lattice. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear M-H loop with saturation magnetization and coercive field of the order of 4-6 emu/g and 260 G, respectively. Temperature dependence of magnetization measurement shows sharp ferromagnetic to paramagnetic transition with a high T C = 791 K for 3% Co doped ZnO NPs. Ferromagnetic ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. We show that the observed FM data fits well with the BMP model involving localised carriers and magnetic cations.
MRS Proceedings, 2009
Evidence for long range ferromagnetic order above room-temperature, RTFM, in pristine ZnO, In2O3, TiO2 nanoparticles and thin films, containing no nominal magnetic elements have been reported recently. This could question the origin of RTFM in doped dilute alloys if for example the ZnO matrix itself develops a defect induced magnetic order with a significant moment per unit cell. In this presentation we report a systematic study of the film thickness dependence of RTFM in pure ZnO deposited by DC Magnetron Sputtering. We observe a maximum in the saturation magnetization, MS, value of 0.62 emu/g (0,018 μB/unit cell), for a ˜480 nm film deposited in an oxygen ambience of appropriate pressure. Above a thickness of around 1 μm the films are diamagnetic as expected. We thus see a sequential transition from ferromagnetism to para- and eventual diamagnetism as a function of film thickness in ZnO. We also find that in such a ZnO matrix with a maximum intrinsic defect induced moment, on dopi...
With a motivation to compare the magnetic property, we synthesised undoped, transition metal (TM) Mn doped and (Mn:Fe) co-doped ZnO ceramics in the compositions ZnO, Zn 0.98 Mn 0.02 O and Zn 0.96 (Mn 0.02 Fe 0.02 )O. Systematic investigations on the structural, microstructural, defect structure and magnetic properties of the samples were performed. Low temperature as well as room temperature ferromagnetism has been observed for all our samples, however, enhanced magnetisation at room temperature has been noticed when ZnO is co-doped with Fe along with Mn. Particularly the sample with the composition Zn 0.96 Mn 0.02 Fe 0.02 O showed a magnetisation value more than double of the sample with composition Zn 0.98 Mn 0.02 O, indicating long range strong interaction between the magnetic impurities leading to higher ferromagnetic ordering. Raman and PL studies reveal presence of higher defects in form of oxygen vacancy clusters created in the sample due to Fe co doping. PL study also reveals enhanced luminescence efficiency in the co doped sample. Temperature dependent magnetisation study of this sample shows the spin freezing temperature around 39 K indicating the presence of small impurity phase of Mn 2 À x Zn x O 3 type. Electron Spin Resonance signal obtained supports ferromagnetic state in the co doped sample. Enhancement of magnetisation is attributed to interactions mediated by magnetic impurities through large number of oxygen vacancies created by Fe 3 þ ions forming bound magnetic polarons (BMP) and facilitating long range ferromagnetic ordering in the co-doped system.
Single phase Mn doped (2 at %) ZnO samples have been synthesized by solid-state reaction technique. Before the final sintering at 500 o C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 hours). The grain sizes of the samples are very close to each other (~ 32 ± 4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 B / Mn atom) was achieved for 96 hours milled sample. Electrical resistivity has been found to increase with increasing milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.