Microstructure, magnetic and optical properties of sputtered polycrystalline ZnO films with Fe addition

Microstructure, magnetic and optical properties of polycrystalline Fe-doped ZnO films fabricated by cosputtering with different Fe atomic fractions (x_Fe) have been examined systematically. Fe addition could affect the growth of ZnO grains and surface morphology of the films. As x_Fe is larger than 7.0%, ZnFe₂O₄ grains appear in the films. All the films are ferromagnetic. The ferromagnetism comes from the ferromagnetic interaction activated by defects between the Fe ions that replace Zn ions. The average moment per Fe ion reaches a maximum value of 1.61 μ_B at x_Fe = 4.8%. With further increase in x_Fe, the average moment per Fe ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Fe ions. The optical band gap value decreases from 3.245 to 3.010 eV as x_Fe increases from 0% to 10%. Photoluminescence spectra analyses indicate that many defects that affect the optical and magnetic properties exist in the films.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.     

The influence of hydrogen annealing on magnetism of Co-doped TiO2 films prepared by sol–gel method

Co-doped TiO₂ (Co_xTi_1−xO₂, 0.05?x?0.2) films have been prepared on Si (0 0 1) substrates by sol–gel method. When heat treated in air, Co_xTi_1−xO₂ films are non-ferromagnetic at room temperature. However, after further annealed in a flowing hydrogen atmosphere, Co_xTi_1−xO₂ films show room-temperature ferromagnetism (RTFM). Measurements of magnetization (M) vs. temperature (T), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) fail to detect Co clusters in the hydrogenated Co_0.1Ti_0.9O₂ films, suggesting that RTFM in the hydrogenated Co_0.1Ti_0.9O₂ films may be intrinsic. But, metal Co appears in the hydrogenated Co_0.2Ti_0.8O₂ films, showing that RTFM in the hydrogenated Co_0.2Ti_0.8O₂ films is as least partly due to metal Co. These results indicate that hydrogen annealing can produce room-temperature ferromagnetism in Co_xTi_1−xO₂ films, but it should be carefully designed to avoid the formation of metal Co in the hydrogenated Co_xTi_1−xO₂ films.     

Magnetic properties of the compounds ThCO2Ge2 and ThCo2Si2

Rather old preparation of the compounds ThCo₂Ge₂ and ThCo₂Si₂ and their magnetic study in the temperature range 100–570 K, published by Omejec and Ban [Z. Anorg. Allg. Chem. 380 (1971) 111], indicated that both compounds ordered ferrromagnetically below 100 K. In order to verify the old data, polycrystalline samples of ThCo₂Ge₂ and ThCo₂Si₂ have been prepared by arc melting and subsequent annealing, and studied by X-ray diffraction at room temperature (RT), by superconducting quantum interference device (SQUID)-magnetization and AC-susceptibility measurements at 2–320 K, and by dc-magnetization measurements in variable magnetic fields up to 120 kOe at 5, 80, and 283 K. The magnetic measurements confirm the ferromagnetic ordering in both compounds, but with totally different Curie temperatures: ≈120(20) K for ThCo₂Ge₂ and above 320 K for ThCo₂Si₂. The paramagnetic values of ThCo₂Ge₂ and the ordering of both compounds are discussed and compared with the old results of Omejec and Ban.     

Interplay of local structure and magnetism in Co-doped TiO2 anatase

We report ab initio density-functional theory investigations on the local structure and magnetization of Co ions doped in TiO₂ anatase. The calculated formation energy of the pair of substitutional Co ions indicates that they have a tendency to cluster; but clustering has no noticeable effect on the low-spin state of Co. The interstitial Co, which is energetically unstable in reference to bulk cobalt, is found to be strongly attracted to a substitutional Co, and even more strongly to a substitutional Co pair. Interestingly, in a one-to-one binding, the interstitial Co enhances the magnetization of the two; whereas in a one-to-two binding, it destroys the magnetic moment of the substitutional Co pair and therefore reduces the average magnetic moment of Co ions. Our results could explain the strong sample-to-sample variability of the magnetic moment of Co measured in experiments. The magnetic interaction between substitutional and interstitial Co is discussed with bonding analysis.     

Synthesis and magnetic properties of Zn1−xMnxO films prepared by the sol-gel method

We report on the ferromagnetic characteristics of Zn_1−xMn_xO films (x=0.1-0.3) prepared by the sol-gel method on silicon substrates using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Magnetic measurement show that the Curie temperature (T_C) and the coercive field (H_C) were ∼39 K and ∼2100 Oe for the film of x=0.2, respectively. EDS and TEM measurements indicate that Mn content at the interface is significantly higher than that at the center of the Zn_0.8Mn_0.2O film showing the ratio, Zn:Mn:O≅1:12:15. This experimental evidence suggests that ferromagnetic precipitates containing manganese oxide may be responsible for the observed ferromagnetic behavior of the film.     

A study of room-temperature ferromagnetism in transition metal and fluorine-doped spray-pyrolyzed SnO2 thin films

Room-temperature ferromagnetism has been observed in Co- or Mn-doped SnO₂ and Co- and F-co-doped SnO₂ thin films. A maximum magnetic moment of 0.80μ_B/Co ion has been observed for Sn_0.90Co_0.10O_1.925−δF_0.075 thin films, whereas in the case of Sn_1−xMn_xO_2−δ it was 0.18μ_B/Mn ion for x=0.10. The magnetization of both Sn_1−xCo_xO_2−δ and Sn_1−xCo_xO_2−y−δF_y thin films depends on the free carrier concentration. An anomalous Hall effect has been observed in the case of Co-doped SnO₂ films. However, the same was not observed in the case of Mn-doped SnO₂ thin films. Carrier-mediated interaction is convincingly proved to be the cause of ferromagnetism in the case of Co:SnO₂. It is, however, proposed that no carrier-mediated interaction exists in the case of Mn:SnO₂. Present studies indicate that dopants and hence electronic cloud-lattice interaction plays an important role in inducing ferromagnetism.     

Effect of SnO2 coating on the magnetic properties of nanocrystalline CuFe2O4

Nanocrystalline CuFe₂O₄ and CuFe₂O₄/xSnO₂ nanocomposites (x=0, 1, 5 wt%) have been successfully synthesized by one-pot reaction of urea-nitrate combustion method. The transmission electron microscope study reveals that the particle size of the as synthesized CuFe₂O₄ and CuFe₂O₄/5 wt%SnO₂ are 10 and 20 nm, respectively. The SnO₂ coating on the nanocrystalline CuFe₂O₄ was confirmed from HRTEM studies. The resultant products were sintered at 1100 °C and characterized by XRD and SQUID for compound formation and magnetic studies, respectively. The X-ray diffraction pattern shows the well-defined sharp peak that confirms the phase pure compound formation of tetragonal CuFe₂O₄. The zero field cooled (ZFC) and field cooled (FC) magnetization was performed using SQUID magnetometer from 2 to 350 K and the magnetic hysteresis measurement was carried out to study the magnetic properties of nanocomposites.     

Magnetic and structural study of Cu-doped TiO2 thin films

Transparent pure and Cu-doped (2.5, 5 and 10 at.%) anatase TiO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. The samples were structurally characterized by X-ray absorption spectroscopy and X-ray diffraction. The magnetic properties were measured using a SQUID. All films have a FM-like behaviour. In the case of the Cu-doped samples, the magnetic cycles are almost independent of the Cu concentration. Cu atoms are forming CuO and/or substituting Ti in TiO₂. The thermal treatment in air promotes the CuO segregation. Since CuO is antiferromagnetic, the magnetic signals present in the films could be assigned to Cu substitutionally replacing cations in TiO₂.     

Electronic transport properties and Microstructures of TiO2:Co magnetic semiconductors

TiO₂:Co thin films of high Co concentration were investigated by the high resolution transmission electron microscopy, physical property measurement system and energy dispersive X-ray. The as-deposited films are amorphous magnetic semiconductors and we did not find any Co metal particles in them. The electronic transport process in the low-temperature range below 80 K could be described by the spin-dependent variable-range-hopping process. However, after the samples were annealed at 300 °C, large amounts of Co metal particles were observed to connect each other and the films show a metallic behavior. The origin of ferromagnetism of the thin films is also discussed.     

Intrinsic magnetic properties of In2O3 and transition metal-doped-In2O3

This paper reports on the influence of the sintering temperature and atmosphere and transition-metal doping on the magnetic properties of nanocrystalline and bulk In₂O₃. Undoped nanocrystalline In₂O₃ is diamagnetic whatever the sintering temperature and atmosphere. All single-phase transition-metal-doped In₂O₃ samples are paramagnetic, with a paramagnetic effective moment originating from weakly interacting transition metal ions. No trace of ferromagnetism has been detected even with samples sintered under argon, except extrinsic ferromagnetism for samples with magnetic dopant concentrations exceeding the solubility limit.     

Metastable ferromagnetic phases with predictable Tcs in La2MnCo1−xNixO6

Metastable ferromagnetic phases, for different compositions in La₂MnCo_1−xNi_xO₆, are obtained for samples synthesized by a low-temperature method and annealed in air at different temperatures in the range 200-1350 °C. The T_cs of the ferromagnetic phases vary linearly between those of the phases of the end members. T_cs of the different phases of La₂MnCo_1−xNi_xO₆ can be predicted based on the T_cs and spin states of Mn, Co and Ni in the different phases of the end members, La₂MnCoO₆ and La₂MnNiO₆.     

Magnetic properties of Co-doped SnO2 at different carrier concentrations

Magnetic properties of Co-doped wide-gap semiconductor SnO₂ were studied theoretically by using the PPLCAO first-principles computational scheme. Since the carrier plays an important role on magnetic properties about diluted magnetic semiconductors (DMS) materials, we discuss the origin of magnetic moments and the magnetic ordering mechanism with different carrier concentration in Co-doped SnO₂ based on calculated spin density distribution. It is found that, the RKKY interaction is dominated in the magnetic coupling in Co-doped SnO₂.     

Enhancement of giant magneto-impedance effect in Ni80Fe20/SiO2/Cu composite wires

Ni₈₀Fe₂₀/SiO₂/Cu composite wires of Cu core 60 μm in diameter and coated with layers of SiO₂ and Ni₈₀Fe₂₀ were prepared by RF magnetron sputtering. The influences of the insulator layer thickness, the measurement mode and the magnitude of the driving current on the giant magneto-impedance (GMI) effect were investigated. The results showed that there was an optimum thickness of the insulator layer and the driving current can influence the shape of the MI curve. Resonance enhancement of the GMI was found in the new measurement mode. The results are discussed by taking account of the electromagnetic interactions.     

Antiferromagnetic transition in botallackite Cu2Cl(OH)3

The natural cuprate botallackite, Cu₂Cl(OH)₃, is found to be a new antiferromagnet with Magnetic susceptibility properties under strong field show non-linear M-H properties indicating metamagnetism. The T_N and the super-exchange coupling are discussed and compared with its polymorph atacamite and other copper oxides on the basis of their structural parameters.     

First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form

Structural, optical and magnetic studies of Co-doped ZnO have been carried out for bulk as well as thin films. The magnetic studies revealed the superparamagnetic nature for low-temperature synthesized samples, indicating the presence of cobalt metallic clusters, and this is supported by the optical studies. For the high-temperature sintered samples one obtains paramagnetism. The optical studies reveal the presence of Co²⁺ ions in the tetrahedral sites indicating proper doping. Interestingly, the films deposited by laser ablation from the paramagnetic target showed room temperature ferromagnetism. It appears that the magnetic nature of this system is process dependent.