Spectroscopic study of Zn1−xCoxO thin films showing intrinsic ferromagnetism

Dilute magnetic semiconductors are widely studied due to their potential applications in spin-resolved electronics. We report the direct evidences of intrinsic ferromagnetism in the primarily ferromagnetic ZnO:Co thin films using near-edge X-ray absorption fine structure (NEXAFS) and soft X-ray magnetic circular dichroism (XMCD). The single phase Zn_1−xCo_xO thin films with nominal compositions (0.00 ≤ x ≤ 0.15) were synthesized by a spray pyrolysis technique, which exhibit room temperature ferromagnetism as revealed by alternating gradient force magnetometer (AGFM) measurements. The spectroscopic measurements indicate that most of Co dopants have substituted for Zn sites in ZnO matrix and they are present in divalent Co²⁺ (d⁷) state with tetrahedral symmetry according to the atomic multiplet calculations. The O 1s NEXAFS spectra suggest strong hybridization between O 2p and Co 3d electrons within ZnO matrix. The Co 2p XMCD measurements rule out the magnetism due to the presence of Co clusters, and show that Co–O–Co bonding provides localized magnetic moments leading to ferromagnetism.     

XAFS and CEMS study of dilute magneto-optical semiconductor, Fe doped TiO2 films

TiO₂ films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-ray absorption fine spectra (XAFS) and conversion electron Mössbauer spectra (CEMS). The edge energy and spectrum profiles of Fe- and Ti K X-ray absorption showed only Fe³⁺ and Ti⁴⁺ states for rutile TiO₂ films prepared under 10^− 1 Torr, the metallic Fe and Ti⁴⁺ for rutile TiO₂ films prepared in 10^− 6 Torr, and the metallic Fe and the average valance of less than “4+” for Ti in Ti_nO_2n−x films prepared by the PLD under 10^− 8 Torr. The metallic Fe clusters are also found in the TEM images of Ti_nO_2n−x film. Magnetic property of Fe doped TiO₂ films prepared by PLD at high vacuum (10^− 6 and 10^− 8 Torr) is considered to originate mainly from the magnetic metal iron clusters.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Structure, magnetic and magnetostrictive properties of Sm0.7Pr0.3Fex alloys

The structure, magnetic and magnetostrictive properties of Sm_0.7Pr_0.3Fe_x (1.45 ≤ x ≤ 1.95) alloys is investigated. X-ray diffraction analysis confirms the presence of single cubic Laves phase in Sm_0.7Pr_0.3Fe_x alloys with 1.45 ≤ x ≤ 1.85. The saturation magnetization of the alloys tends to increase with increasing Fe content. Sm_0.7Pr_0.3Fe_1.55 has the highest magnetostriction among all the Sm_0.7Pr_0.3Fe_x alloys at low fields and shows a large magnetostriction λ_// − λ_⊥ = −1008 ppm at a magnetic field of 12 kOe.     

Structural, magnetic and dielectric properties of Bi5−xLaxTi3Co0.5Fe0.5O15 ceramics

The Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4) ceramics were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The cell volume of the composites increases monotonously with the increase of La content, which indicates that La ions have been incorporated into the lattice of Bi₅Ti₃Co_0.5Fe_0.5O₁₅. The magnetic measurements show that La doping on Bi sites has enhanced the magnetization of Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4). Both the dielectric constants and loss tangent of all the samples decrease on increasing frequency and then become almost constant at room temperature. The La doped Bi₅Ti₃Co_0.5Fe_0.5O₁₅ samples exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization and lower losses at room temperature.     

Structure and magnetic properties of sputtered (Fe1 − xCrx)0.09Cu0.91 solid solution films

The surface morphology of the (Fe_1 − xCr_x)_0.09Cu_0.91 films does not change significantly as x increases. No Fe or Cr granules form in the films because of the low deposition temperature and the non-equilibrium deposition procedure, suggesting that Fe and Cr atoms disperse in the Cu matrix. With increasing x, the lattice constant c and single cell volume decrease, but the lattice constants (a, b) first increase and latterly decrease. The films show spin-glass-like manner that looks like superparamagnetism at high temperatures and are ferromagnetic at low temperatures. The peak temperature of the zero-field-cooling curves decreases from 37 to 23 K as x increases from 0 to 0.25. Below the peak temperatures, the field-cooled magnetization decreases with decreasing temperature due to the antiferromagnetically coupling of the disordered spin-glass-like moments. The coercivity increases greatly below 50 K because of the pinning effect of the frozen spin-glass-like moments at low temperatures.     

Synthesis and weak ferromagnetism of Dy-doped BiFeO3 powders

Dy-doped BiFeO₃ powders were synthesized by sol-gel auto-combustion method. X-ray diffraction of the powders were recorded and analyzed for the confirmation of crystal structure and the calculation of the lattice parameters. X-ray photoelectron spectroscopy was used to identify the oxidation state of Fe. Detailed studies of vibrating sample magnetometer indicated a significant effect of Dy doping on the magnetic properties of Bi_1 − xDy_xFeO₃ powders at room temperature. When x = 0.1, BDFO shows the best magnetic property. The origin of weak ferromagnetism was discussed in this paper.     

Enhanced ferromagnetic properties of multiferroic BiCoxFe1 − xO3 synthesized by hydrothermal method

The BiCo_xFe_1 − xO₃ samples have been successfully synthesized by hydrothermal process. The resulting products were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray (EDS), differential thermal analysis (DTA), and physical property measurement system (PPMS).It was found that the magnetization of the obtained products was greatly enhanced by Co substituting for Fe ions. Furthermore, the value of magnetism of BiCo_xFe_1 − xO₃ samples can be adjusted by Fe doping concentration. DTA curve indicates the ferroelectric properties of the obtained BCFO samples are not affected by Co substitution. Therefore, it would be interesting to realize thin films with similar compositions and study their properties in the interest of device applications.     

Finite size effect on Ni doped nanocrystalline NixZn1 − xFe2O4 (0.1 ≤ x ≤ 0.5)

Nanocrystalline nickel ferrite with different concentration of Ni and Zn (Ni_xZn_1 − xFe₂O₄ where x = 0.1, 0.3, 0.5) were synthesized using chemical co-precipitation method. The effect of doping ion concentration on physical properties like crystalline phase, crystallite size, particle size, and saturation magnetization are investigated. The X-ray diffraction pattern confirms the synthesis of single crystalline Ni_xZn_1 − xFe₂O₄ nanoparticles. The lattice parameter decreases with increase Ni content resulting in reduction of lattice strain. HRTEM images revealed that the as-prepared nanoparticles were crystalline with particle size distribution in 10-30 nm range. The saturation magnetization show the superparamagnetic nature of sample for x = 0.1 and x = 0.3 whereas for x = 0.5, the material is ferromagnetic. The saturation magnetization value is 23.95 emu/gm for Ni_0.1Zn_0.9Fe₂O₄ sample and it increases with increase in Ni content.     

Enhanced multiferroic properties and tunable magnetic behavior in multiferroic BiFeO3-Bi0.5Na0.5TiO3 solid solutions

A series of multiferroic (1−x)BiFeO₃−x(Bi_0.5Na_0.5)TiO₃ (BF-BNT) (x = 0 − 0.6) solid solution ceramics were prepared by a sol-gel method. The XRD results show that increasing BNT content induce a gradual phase transformation from rhombohedral to pseudocubic structure near x = 0.4. Compared with pure BiFeO₃, superior multiferroic properties are obtained for x = 0.3 with remnant polarization P_r = 1.49 μC/cm² and saturated magnetization M_s = 0.51 emu/g. Importantly, the paramagnetic (PM) to ferromagnetic (FM) transition is observed for the solutions, and the Curie temperature (T_C) can be tuned by varying the content of BNT. This observed FM ordering is discussed in terms of the possible existence of the long-range superexchange interaction of Fe³⁺-O-Ti-O-Fe³⁺ in the chemically ordered regions.     

Effect of chromium substitution on structure and magnetic properties of Bi2Fe4O9

Orthorhombic Bi₂Fe_4 − xCr_xO₉ (x = 0.0, 0.25, and 0.75) nanoplatelets were synthesized by a simple hydrothermal method. The structure, morphology, and magnetic properties of the obtained powders have been characterized. Calculation of the lattice parameters of Bi₂Fe_4 − xCr_xO₉, as well as bond lengths and angles, was carried out by X-ray diffraction Rietveld refinement. The volumes of the metal-oxygen tetrahedra and octahedra were calculated to be sequentially increasing as the Cr doping level increases. The samples undergo an antiferromagnetic transition at 250 ± 5 K. The magnetic moments of the samples increase with higher Cr doping level. The 3d electron spin state for Fe³⁺ in the as-prepared samples is different, which is possibly due to the distortion of Fe-O tetrahedra and octahedra in the crystal structure after chromium substitution.     

Structural,transport and spectroscopic properties of Ti substituted magnetite: Fe3−xTixO4

The effect of Ti⁴⁺ ion on the formation of magnetite, which were prepared by solid-state route reaction method, were studied by resistivity, Raman and ⁵⁷Fe Mössbauer spectrometry. Resistivity measured in the range of 10 < T < 300 K for Ti⁴⁺ magnetite Fe_3−xTi_xO₄ exhibit first order phase transformations at the Verwey transition T_v for Fe₃O₄, Fe_2.98Ti_0.02O₄ and Fe_2.97Ti_0.03O₄ at 123 K, 121 K and 118 K, respectively. No first order phase transition was observed for Fe_2.9Ti_0.1O₄ and small polaron model retraces the semiconducting resistivity behavior with activation energy of about 72 meV. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Ti doping. The Raman active mode for Fe_2.9Ti_0.1O₄ at ≅634.4 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅670 cm⁻¹, inferring that Mn²⁺ ions are located mostly on the octahedral sites. ⁵⁷Fe Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Ti⁴⁺ ions are located mostly on the octahedral sites of the Fe_3−xTi_xO₄ spinel structure.     

Low temperature step magnetization and magnetodielectric study in Bi0.95La0.05Fe1−xZrxO3 ceramics

Single-phase, co-doped (La³⁺, Zr⁴⁺) in polycrystalline Bi_0.95La_0.05Fe_1−xZr_xO₃ (with x = 0, 0.02, 0.04 and 0.06) ceramics (particle size ∼650 nm; tolerance factor ∼0.883) were prepared by solid state reaction of oxides, followed by rapid quenching of samples. Enhanced magnetization was observed in co-doped (La³⁺, Zr⁴⁺) BiFeO₃ which may be ascribed to the collapse of the spiral spin structure. Step magnetization was observed in zero field cooled (ZFC) and field cooled (FC) curves. The coexistence of ferromagnetism and ferroelectricity has been confirmed in the co-doped (La³⁺, Zr⁴⁺) in BiFeO₃ ceramics by means of (M–H) and (P–E) loops measurements. Magnetodielectric properties have been observed at room temperature.     

Structural and transport properties of stoichiometric Mn-doped magnetite: Fe3−xMnxO4

The polycrystalline samples of Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50) were prepared by a solid-state route reaction method. X-ray diffraction pattern shows that Mn²⁺ doped magnetites are in single phase and possess cubic inverse spinel structure. The resistivity measurements (10 < T < 300 K) for x = 0.0 and 0.01 confirms the first order phase transition at the Verwey transition T_V = 123 K and 117 K, respectively. No first order phase transition was evidenced for Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50). Small polaron model has been used to fit the semiconducting resistivity behavior and the activation energy ?_a, for samples x = 0.10 and 0.50 is about 72.41 meV and 77.39 meV, respectively. The Raman spectra of Fe_3−xMn_xO₄ at room temperature reveal five phonons modes for Fe_3−xMn_xO₄ (0.01 ≤ x ≤ 0.50) as expected for the magnetite (Fe₃O₄). Increased Mn²⁺ doping at Fe site leads to a gradual changes in phonon modes. The Raman active mode for Fe_3−xMn_xO₄ (x = 0.50) at ≅641.5 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅669.7 cm⁻¹, inferring that Mn⁺² ions are located mostly on the octahedral sites. The laser power is fixed to 5 mW causes the bands to broaden and to undergo a small shift to lower wave numbers as well as increase in the full width half maxima for A_1g phonon mode with the enhancement of Mn²⁺ doping. Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Mn⁺² ions are located mostly on the octahedral sites of the Fe_3−xMn_xO₄ spinel structure.     

The effect of Cu content on the structure of Ni1−xCuxFe2O4 spinels

A series of Ni_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.5) spinels were synthesized employing sol-gel combustion method at 400 °C. The decomposition process was monitored by thermal analysis, and the synthesized nanocrystallites were characterized by X-ray diffraction, transmission electron microscopy, infra-red and X-ray photoelectron spectroscopy. The decomposition process and ferritization occur simultaneously over the temperature range from 280 °C to 350 °C. TEM indicates the increase of lattice parameter and particle size with the increase of copper content in accordance with the XRD analysis. Cu²⁺ can enter the cubic spinel phase and occupy preferentially the B-sites within x = 0.3, and redundant copper forms CuO phase separately. A broadening of the O 1s region increases with the increment of copper content compared to pure NiFe₂O₄, showing different surface oxygen species from the spinel and CuO. Cu²⁺ substitution favors the occupancy of A-sites by Fe³⁺.     

Characteristics of NixFe100−x films deposited on SiO2/Si(1 0 0) by DC magnetron co-sputtering

Ni_xFe_100−x films with a thickness of about 200 nm were deposited on SiO₂/Si(1 0 0) substrates at room temperature by DC magnetron co-sputtering using both Fe and Ni₈₀Fe₂₀ targets. Compositional, structural, electrical and magnetic properties of the films were investigated. Ni₇₆Fe₂₄, Ni₆₅Fe₃₅, Ni₆₀Fe₄₀, Ni₅₅Fe₄₅, Ni₄₉Fe₅₁ films are obtained by increasing the sputtering power of the Fe target. All the films have a fcc structure. Ni₇₆Fe₂₄, Ni₆₅Fe₃₅, Ni₆₀Fe₄₀ and Ni₅₅Fe₄₅ films grow with crystalline orientations of [1 1 1] and [2 2 0] in the direction of the film growth while the Ni₄₉Fe₅₁ film has the [1 1 1] texture structure in the direction of the film growth. The lattice constant of the film increases linearly with increasing Fe content. All of the films grow with thin columnar grains and have void networks in the grain boundaries. The grain size does not change markedly with the composition of the film. The resistivity of the film increases with increasing Fe content and is one order of magnitude larger than that of the bulk. For all the films the magnetic hysteresis loop shows a hard magnetization. The Ni₇₆Fe₂₄ film has the lowest saturation magnetization of 6.75×10⁻² T and the lowest saturation field of 8.36×10⁴ A/m while the Ni₄₉Fe₅₁ film has a largest saturation magnetization of 9.25×10⁻² T and the largest saturation field of 1.43×10⁵ A/m.     

High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4

Sn doping in an n-type transparent conducting oxide MgIn₂O₄ is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg_1+xIn_2−2xSn_xO₄ for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10⁻⁴ K⁻¹ for the parent compound at 600 K.     

Cu-doping effect on structure and magnetic properties of Fe-doped ZnO powders

Fe-doped and Cu, Fe co-doped ZnO diluted magnetic semiconductors powders were synthesized by sol–gel method. The x-ray diffraction (XRD) results showed that Zn_0.97−xFe_0.03Cu_xO (x ≤ 0.02) samples were single phase with the ZnO-like wurtzite structure. X-ray photoelectron spectroscopy (XPS) showed that Fe²⁺ and Fe³⁺ existed in Zn_0.97Fe_0.03O, while Fe²⁺, Fe³⁺and Cu⁺, Cu²⁺ were found in Zn_0.95Fe_0.03Cu_0.02O. Both Zn_0.97Fe_0.03O and Zn_0.95Fe_0.03Cu_0.02O exhibited ferromagnetic performance at room temperature. But the Cu incorporation reduced the saturation magnetization of Fe-doped ZnO diluted magnetic semiconductors.     

Synthesis, structural and magnetic studies of the CuCr1−xRhxO2 delafossite solid solution with 0 ≤ x ≤ 0.2

The CuCr_1−xRh_xO₂ series is investigated by X-ray diffraction, magnetization measurements and Raman spectroscopy on ceramic samples. It is found that a delafossite solid solution is maintained up to x = 0.2 in CuCr_1−xRh_xO₂. The small observed variation in cell parameters is consistent with the small difference between the ionic radii of Cr³⁺ and Rh³⁺. A significant broadening of X-ray reflections is observed and when analyzed using the Williamson-Hall relationship showed that the strain generated by Rh substitution is strongly anisotropic, affecting mainly (Cr,Rh)-O bonds in the ab plane. Room temperature Raman spectra displayed three main Raman active modes. All modes shift to lower frequency and undergo significant changes in intensity with increasing Rh content, showing the effect of Rh atoms on the M³⁺-O bond strength. The magnetic behavior of CuCr_1−xRh_xO₂ samples was investigated as a function of temperature and applied field. At high temperature paramagnetic behavior, and at low temperature, evidence for weak ferromagnetism, reinforced by a hysteresis loop at 4 K is observed. The magnetic behavior of CuCr_1−xRh_xO₂ is attributed to the disorder of Cr and Rh in octahedral sites resulting in short-range Cr-O-Cr and Cr-O-Rh interactions, which give rise to short-range weak ferromagnetism.     

Magnetic properties of Zn1−xCoxO bulks prepared by hot pressing

Zn_1−xCo_xO-diluted magnetic semiconductor bulks have been prepared by hot pressing. Mixed powders of pure ZnO and CoO were compacted under pressure of 10 MPa at the temperature of 1073 K. Then, the samples were annealed in vacuum at the temperature range from 673 K to 873 K for 10 h. The crystal structure and magnetic properties of Zn_1−xCo_xO bulks have been investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). X-ray photoelectron spectroscopy (XPS) and was used to study chemical valence of zinc and cobalt in the samples. The results showed that Zn_1−xCo_xO samples had c-axis-oriented wurtzite symmetry neither cobalt nor cobalt oxide phase was found in the samples if x was less than 0.15. Zn and Co were existed in Zn_0.9Fe_0.1O sample in Zn²⁺ and Co²⁺ states. The results of VSM experiment proved the room temperature ferromagnetic properties (RTFP) of Co-doped ZnO samples. The saturation magnetization and the coercivity of Zn_0.9Co_0.1O sample, observed in the M–H curve, were about 0.22 emu/g and 300 Oe, respectively.