Role of Mn doping for obtaining of hexagonal phase in Ba0.98Zn0.02TiO3 ceramics

This paper reports the observation of hexagonal phase of barium titanate by Mn doping and its effect on dielectric and magnetic properties. Ceramic samples of Ba_0.98Zn_0.02Ti_1−xMn_xO₃ (where, x= 0.04, 0.06 and 0.08) were prepared by traditional solid-state reaction route. The hexagonal phase is stabilized in the composition Ba_0.98Zn_0.02Ti_0.92Mn_0.08O₃ and a very feeble M–H loop is also observed in that composition. This induced magnetism is expected due to the exchange interactions between magnetic polarons formed by oxygen vacancies with Mn ions. The dielectric constant as well as the ferroelectric to paraelectric transition temperature is systematically decreased with increasing of Mn doping concentration. Further to that, the temperature dependent dielectric constant curve is also broadened at transition temperature with increasing of Mn concentration. However, the ferroelectric to paraelectric transition temperature is well above room temperature.     

No room temperature ferromagnetism in Mn over-doped Zn1−xMnxO (x>0.02)

Mn-doped ZnO samples having composition Zn_1−xMn_xO (x=0.02, 0.04 and 0.05) were synthesized by solid state reaction technique with varying concentration of Mn from 0.02 to 0.05. Evidence of room temperature ferromagnetism was observed only in the composition Zn_0.98Mn_0.02O sintered at 500 °C. Our XRD pattern confirms the presence of Mn₃O₄ impurity phase in all the Zn_1−xMn_xO samples with the exception of Zn_0.98Mn_0.02O. We emphasize that the appearance of Mn₃O₄ phase in the system forbids the exchange type of interaction between the Mn ions and suppresses the ferromagnetism in all the Mn over-doped Zn_1−xMn_xO (x>0.02) system. SEM microstructure study also supports the interruption of exchange type of interaction inside the system with the increase in Mn concentration in the sample. Interestingly, for this particular composition, Zn_0.98Mn_0.02O sintered at 500 °C, glassy ferromagnetism type of transition is observed at low temperature. This type of transition is attributed to the formation of the oxides of Mn clusters at low temperature.     

On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O

Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films. It is found that ferromagnetic Zn_0.8Mn_0.2O has a higher Curie temperature than Zn_0.8Co_0.2O. In addition, the higher ratio of grain-boundary area to grain volume of Zn_0.8Mn_0.2O than Zn_0.8Co_0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism.     

Effect of carbon co-doping on the magnetic properties of bulk Mn doped ZnO

We report the systematic study on the effect of C-doping and annealing atmosphere (air, O₂, N₂ and Ar) on the magnetic properties of pure ZnO as well as Zn_0.98Mn_0.02O (ZMO). We found diamagnetic behavior for pure ZnO samples in all the annealing atmospheres whereas all ZMO samples are paramagnetic except the one, which is annealed in argon. While maintaining the same annealing atmospheres and varying the C content we found that on increasing the C-doping concentration the ferromagnetic fraction in the sample increases. The observed RTFM in C-doped ZMO is possibly due to carriers introduced into the samples due to oxygen vacancies and the substitution of C at O sites.     

Structural transitions in La 0.95 Ba 0.05 Mn 0.98 57 Fe 0.02 O 3 under heat treatment

Structural transitions in polycrystalline Ba-doped lanthanum manganite La_0.95Ba_0.05Mn_0.98Fe_0.02O_3?+?δ have been investigated under different cooling conditions after vacuum annealing (fast and slow cooling) by Mössbauer spectroscopy and X-ray diffraction (XRD) analysis. A rhombohedral structure of the synthesized La_0.95Ba_0.05Mn_0.98Fe_0.02O_3?+?δ sample transfers into a mixture of the orthorhombic PnmaI, PnmaII* and PnmaII phases (common space group Pnma) with a stoichiometric oxygen composition under vacuum annealing. The further vacuum annealing leads to fluctuations in a partial relation of the orthorhombic phases on fast cooling. This unusual behavior of the structural transitions are discussed.     

Effect of temperature conditions of ion implantation on percolation ferromagnetism in Ge0.98Mn0.02 thin films

The effect of temperature conditions of ion implantation on the magnetic properties of Ge_0.98Mn_0.02 thin films has been studied. It has been shown that a decrease in the implantation temperature significantly increases the temperature of percolation magnetic ordering in the subsystem of dispersed Mn²⁺ ions. It has been demonstrated that the observed effect can be due to suppression of the thermally activated aggregation of Mn²⁺ ions into Ge₃Mn₅ clusters and increase in their concentration in the dispersed state.     

Vacancy defects in (Zn, Mn)O

We have used positron annihilation spectroscopy to study the vacancy defects in (Zn, Mn)O crystals grown by chemical vapor transport (CVT). Our results show that Zn vacancies are present in both as-grown and high temperature annealed ZnO and Zn_0.985Mn_0.015O. In addition, we observe O vacancies in ZnO with no Mn. After annealing in O₂ at 1000 C, there is no change in the vacancy distribution in ZnO, while the Zn vacancy concentration increases by an order of magnitude in Zn_0.985Mn_0.015O.     

Origin of high-temperature ferromagnetism in Zn0.98Fe0.02O alloys prepared by hydrothermal method

Zn_0.98Fe_0.02O alloys were prepared by the hydrothermal method. X-ray diffraction had not found any peak from secondary phase. The alloy showed ferromagnetism with a high Curie temperature. Detailed microscope analysis showed that there existed secondary phase with spinel structure. Further analysis proved that the inhomogeneity, ZnFe₂O₄, is the origin of ferromagnetism in the Zn_0.98Fe_0.02O alloys.     

Effect of annealing atmosphere on ferromagnetism in Mn doped ZnO films

This paper reports that Zn_0.97Mn_0.03O thin films have been prepared by radio-frequency sputtering technology followed by rapid thermal processing in nitrogen and oxygen ambient respectively. Magnetic property investigation indicates that the films are ferromagnetic and that the Curie temperature (T_c) is over room temperature. It is observed that the saturation magnetization of the films increases after annealing in nitrogen ambience but decreases after annealing in oxygen. Room temperature photoluminescence spectra indicate that the amount of defects in the films differs after annealing in the different ambiences. This suggests that the ferromagnetism in Zn_0.97Mn_0.03O films is strongly related to the defects in the films.     

Temperature-dependent magnetization in (Mn, N)-codoped ZnO-based diluted magnetic semiconductors

The influences of Mn doping on the structural quality of the Zn_xMn_1−xO:N alloy films have been investigated by XRD. Chemical compositions of the samples (Zn and Mn content) and their valence states were determined by X-ray photoelectron spectrometry (XPS). Hall effect measurements versus temperature for Zn_xMn_1−xO:N samples have been designed and studied in detail. The ferromagnetic transitions happened at different T_C should explain that the magnetic transition in field-cooled magnetization of Zn_1−xMn_xO:N films at low temperature is caused by the strong p-d exchange interactions besides magnetic transition at 46 K resulting from Mn oxide, and that the room temperature ferromagnetic signatures are attributed to the uncompensated spins at the surface of anti-ferromagnetic nano-crystal of Mn-related Zn(Mn)O.     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.     

Microstructure,optical and structural characterization of Cd0.98Fe0.02S thin films co-doped with Zn by chemical bath deposition method

Fe-doped CdS (Cd_0.98Fe_0.02S) and Fe, Zn co-doped CdS (Cd_0.98−xZn_xFe_0.02S (x=0.02, 0.04, and 0.06)) thin films have been successfully deposited on glass substrate by chemical bath deposition technique using aqueous ammonia solution at pH = 9.5. Phase purity of the samples having cubic structure with (111) as the preferential orientation was confirmed by X-ray diffraction technique. Shift of X-ray diffraction peak position towards higher angle side and decrease of lattice parameters, volume and crystallite size confirmed the proper incorporation of Zn into Cd–Fe–S except Zn=6%. The compositional analysis (EDX) showed that Cd, Fe, Zn and S are present in the films. The enhanced band gap and higher transmittance observed in Cd_0.94Zn_0.04Fe_0.02S films are the effective way to use solar energy and enhance its photocatalytic activity under visible light. The enhanced green band emission than blue band by Zn-doping evidenced the existence of higher defect states.     

The study on the ZnO and Zn0.95Mn0.05O added YBCO system: Investigation of microstructure and transport properties

The effect of nano-size Zn_0.95Mn_0.05O and ZnO (30 nm) addition on the microstructure and the normal state transport properties of polycrystalline YBa₂Cu₃O_y (YBCO) was systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nano-sized particles up to 10 wt.%. When Zn_0.95Mn_0.05O and ZnO are added to the YBCO the orthorhombic structure maintained even at the highest concentration. TEM and EDS analyses show the presence of inhomeginities embedded in the superconducting matrix. To interpret the normal state properties of the samples, the percolation theory based on localized states is applied. A cross-over between variable-range hopping and Coulomb gap mechanisms is observed as a result of increasing the nano-particles concentration. The ZnO addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to Zn_0.95Mn_0.05O addition. The calculated values of the localization length, d, are greater in the case of Zn_0.95Mn_0.05O addition. This result can be interpreted by the internal structure defects.     

Novel magnetic behavior of Mn-doped ZnO hierarchical hollow spheres

In this study, unique three-dimensional Zn_0.98Mn_0.02O hierarchical hollow microspheres (HHMs) with diameters of 5–8 μm have been synthesized by a simple hydrothermal approach. In particular, room-temperature magnetization measurements indicate that novel co-existence of ferromagnetism (FM)/paramagnetism (PM) and only PM behaviors for the as-annealed Zn_0.98Mn_0.02O HHMs at 673 and 1,073 K, respectively, in Ar gas atmosphere appear, whereas the as-synthesized ones show merely pure FM. Based on the Photoluminescence and Raman spectra, it is confirmed that the concentrations of oxygen vacancies in Zn_0.98Mn_0.02O HHMs were becoming larger and larger with increasing annealing temperature. Corresponding magnetic evolution mechanism is proposed to relate to oxygen vacancies based on annealing processes. This novel magnetic property will enrich our understanding of diluted magnetic semiconductors.     

Paramagnetism in Cu-doped ZnO

Zn0.98 Cu0.02 O powders are prepared by the sol-gel method.A small number of CuO impurity phases are observed by the x-ray diffraction,indicating the solubility of Cu in ZnO is less than 2 at.%.The Zn0.98 Cu0.02 O powders exhibit diamagnetism at 300 K and paramagnetism at 5 K.After subtracting the diamagnetic contribution of ZnO bulk and the paramagnetic contribution of defects,Cu ions exhibit weak paramagnetism.By codoping Cu with Co or Mn in ZnO,only paramagnetism is observed at room temperature.     

Raman spectroscopy of Cu doping in Zn1−xCoxO diluted magnetic semiconductor

The room‐temperature ferromagnetism and the Raman spectroscopy of the Cu‐doped Zn_1−xCo_xO powders prepared by the sol–gel method are reported. The x‐ray diffraction (XRD) data confirmed that the wurtzite structure of ZnO is maintained for ZnO doped with Co below 10 at%. The magnetization–field curves measured at room temperature demonstrated that all Co‐doped ZnO powders were paramagnetic. Ferromagnetic ordering is observed for the samples doped with Cu in Zn_0.98Co_0.02O and strongly depends on the concentration of Cu. The relative strength of the second‐order LO peak to the first‐order one in the Raman spectra, which is related to the carrier concentration, of the Cu‐doped Zn_0.98Co_0.02O powder is strongly correlated with the saturation magnetic moment of the system. This seems to be in favor of the Ruderman‐Kittel‐Kasuya‐Yosida (RKKY) or double exchange mechanism of the ferromagnetism in this system. Copyright © 2009 John Wiley & Sons, Ltd.     

On the room-temperature ferromagnetism in (ZnO)0.98(MnO2)0.02

Room-temperature ferromagnetism (RTFM) is investigated in the polycrystalline bulk (ZnO)_0.98(MnO₂)_0.02 samples prepared by a modified solid-state sintering route. Successive sintering of a sample was carried out in air at different temperatures in the range of 400-1000 °C. The study of magnetization and phase-investigation in the sample was carried out after each sintering step. The progressive suppression of impurities and the consequent reduction in RTFM is clearly observed in the samples with increase in the sintering temperature up to 800 °C. The subsequent successive sintering of the (ZnO)_0.98(MnO₂)_0.02 sample up to 1000 °C yields fully paramagnetic sample exhibiting wurtzite structure. The studies support the conjecture (Kundaliya et al., Nat. Mater. 3 (2004) 709 [18]) that RTFM in this system has an origin related to a randomly distributed impurity phase produced by local dissolution of ZnO and MnO₂.     

Nonlinear excitations (solitons) in the antiferromagnetic chain of (NH4)2Mn0.98Fe0.02F5

Measurements of the⁵⁷Fe Mössbauer effect and of the magnetic susceptibilities on a single crystal have been performed on the quasi-1-d antiferromagnetic chain of (NH₄)₂Mn_0.98Fe_0.02F₅ as a function of temperature. Particular attention was paid to the region very near the Néel point. The Mössbauer spectra fitted by the Blume-Tjon model show definite relaxation effects, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of nonlinear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy a local anisotropy energyD/k of –3.9 K was derived.     

Synthesis and structural,magnetic characterization of Zn(Mn)O diluted magnetic semiconductor

The bulk samples with nominal composition Zn_1−x Mn_x O [x = 5% and 7%] were synthesized at 930 °C by Standard Solid State Reaction method. The structural analysis reveals the single phase nature. The Topography study indicates the distribution of the particles. Magnetic property was affirmed by Vibrating Sample Magnetometer, Zn_1−x Mn_x O (with x = 5%), low concentration of dopant shows good ferromagnetism compared to high concentration in Zn_1−x Mn_x O (with x = 7%).     

Excitons and photoluminescence in ZnO and Zn0.99Mn0.01O nanocrystals

The photoluminescence and photoluminescence excitation spectra for Zn_1?x Mn _x O nanocrystals are presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreases noticeably. This suggests that the oxygen vacancies affect the Zhang-Rice-like states appearing due to strong d-p-hybridization, which is confirmed by an increase in the band gap of Zn_1?x Mn _x O for low x. The origin of the 2.9-eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn_1?x Mn _x O nanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2–3.0) eV.