The role of oxygen vacancies in magnetism of CoxTi1−xO2−δ films on Si(001) prepared by sol-gel method

Co_xTi_1−xO_2−δ films have been prepared on Si(001) substrates by sol-gel method. When heat treated in air, Co_xTi_1−xO_2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co_xTi_1−xO_2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co_xTi_1−xO_2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co_xTi_1−xO_2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization (M) vs temperature (T) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co_xTi_1−xO_2−δ films. Oxygen vacancies are formed in Co_xTi_1−xO_2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co_xTi_1−xO_2−δ films.     

Dopant Spin States and Magnetic Interactions in Transition Metal (Fe3+)-Doped Semiconductor Nanoparticles: An EMR and Magnetometric Study

In this work, electron magnetic resonance (EMR) spectroscopy and magnetometry studies were employed to investigate the origin of the observed room-temperature ferromagnetism in chemically synthesized Sn_1?x Fe _x O₂ powders. EMR data clearly established the presence of two different types of signals due to the incorporated Fe ions: paramagnetic spectra due to isolated Fe³⁺ ions and broad ferromagnetic resonance (FMR) spectra due to magnetically coupled Fe³⁺ dopant ions. EMR data analysis and simulation suggested the presence of high-spin (S = 5/2) Fe³⁺ ions incorporated into the SnO₂ host lattice both at substitutional and at interstitial sites. The FMR signal intensity and the saturation magnetization M _s of the ferromagnetic component increased with increasing Fe concentration. For Sn_0.953Fe_0.047O₂ samples, well-defined EMR spectra revealing FMRs were observed only for samples prepared in the 350–600°C range, whereas for samples prepared at higher annealing temperatures up to 900°C, the FMRs and saturation magnetization were vanished due to diffusion and eventual expulsion of the Fe ions from the nanoparticles, in agreement with data obtained from Raman and X-ray photoelectron spectroscopy.     

Observation of higher magnetization on the substitution of Al for Co in La2MnCoO6

Upon substitution of non-magnetic Al³⁺ for diamagnetic, low-spin, Co³⁺ in ferromagnetic La₂MnCoO₆, the ferromagnetic moment, measured at 82 K and 15 kOe, is found to increase initially with Al content and then decreases, though the magnetic transition temperature decreases continuously on increasing x in La₂MnCo_1−xAl_xO₆.     

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.     

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.     

The large low-field magnetic entropy changes in Ni43Mn46Sn11−xSbx alloys

A series of Ni₄₃Mn₄₆Sn_11−xSb_x (x=0, 1, and 3) alloys were prepared by an arc melting method. The martensitic transition shifts to higher temperature with the increasing Sb content. The isothermal magnetization curves and Arrott plots around martensitic transition temperatures show a typical metamagnetic behavior. Under a low applied magnetic field of 10 kOe, large magnetic entropy changes around the martensitic transition temperature are 10.4, 8.9, and 7.3 J/kg K, for x=0, 1, and 3, respectively. The origin of the large magnetic entropy changes and potential application for Ni₄₃Mn₄₆Sn_11−xSb_x alloys as working substances in magnetic refrigeration are discussed.     

Room-temperature ferromagnetism in Sn1−xMnxO2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis

Sn_1−xMn_xO₂ (x=0.01-0.05) thin films were synthesized on quartz substrate using an inexpensive ultrasonic spray pyrolysis technique. The influence of doping concentration and substrate temperature on structural and magnetic properties of Sn_1−xMn_xO₂ thin films was systematically investigated. X-ray diffraction (XRD) studies of these films reflect that the Mn³⁺ ions have substituted Sn⁴⁺ ions without changing the tetragonal rutile structure of pure SnO₂. A linear increase in c-axis lattice constant has been observed with corresponding increase in Mn concentration. No impurity phase was detected in XRD patterns even after doping 5 at% of Mn. A systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700 °C for Sn_1−xMn_xO₂ (x=0.01) films. Magnetic studies reveal room-temperature ferromagnetism (RTFM) with 3.61×10⁻⁴ emu saturation magnetization and 92 Oe coercivity in case of Sn_1−xMn_xO₂ (x=0.01) films deposited at 500 °C. However, paramagnetic behavior was observed for the films deposited at a higher substrate temperature of 700 °C. The presence of room-temperature ferromagnetism in these films was observed to have an intrinsic origin and could be obtained by controlling the substrate temperature and Mn doping concentration.     

Enhancement of magnetoresistances at room temperature in YSZ doping La0.67Sr0.33MnO3 system

The temperature dependence of the resistance of composite samples (1−x)La_0.67Sr_0.33MnO₃+xYSZ with different YSZ doping level x was investigated at magnetic fields 0-3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal-insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal-insulator transition temperature values T_P of the composites decrease, but T_P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal-insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10-350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.     

Magnetic phase transitions and structures of Co3V 2O8

Co₃V ₂O₈ is a spin- 3/2 system on a Kagomé staircase and is known to undergo two magnetic phase transitions between 6 and 11 K. The H-T phase diagram of Co₃V ₂O₈ derived by magnetization measurements on a single crystal is presented. Additionally both ordered magnetic structures were investigated by neutron powder diffraction experiments and solved using Bertaut’s macroscopic theory. For the ferromagnetic phase the magnetic moments of the Co²⁺ ions were found to be 1.5(3)μ_B and 2.7(1)μ_B at 3.5 K along the crystallographic a axis for the (4a) and (8e) sites, respectively. The antiferromagnetic phase exhibits a magnetic cell with a doubled b axis with respect to the nuclear one. The magnetic moments point along the a axis being 1.8(2)μ_B (4a) and 1.8(1)μ_B (8e) at 8 K.     

Magnetic anisotropy of Tb1−xGdxMn6Sn6 compounds

Magnetization curves of Tb_1−xGd_xMn₆Sn₆ compounds (0?x?1) have been measured for aligned powder samples in the temperature range 4.2–300 K in pulsed magnetic fields up to 30 T. Temperature and concentration dependences of the magnetocrystalline anisotropy constants K₁ and K₂ and concentration dependence of the temperature of spontaneous spin-reorientation transition have been determined. Using these data, we estimated the contribution of the manganese and terbium atoms to the magnetic anisotropy of Tb_1−xGd_xMn₆Sn₆ and analyzed the origin of the appearance of field-induced first-order magnetic phase transition in these compounds.     

Magnetic disorder induced enhanced magneto-resistance in La0.5Sr0.5Co1−xRuxO3

We observe a sharp increase in negative magneto-resistance ratio up to 40% for x=0.1, in La_0.5Sr_0.5Co_1−xRu_xO₃ which is due to the magnetic disorder induced by an anti-ferromagnetic interaction between Co and Ru ions. We also observe a metal to insulator and a ferromagnetic to anti-ferromagnetic transition for 0≤x≤0.3. Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co³⁺:t_2g⁵e_g¹), forcing a double exchange mediated ferromagnetic state to an anti-ferromagnetic spin state for x≥0.2.     

Preference orientation and magnetic properties of CoxFe3−xO4 nanocrystalline thin films on quartz substrate

Nanocrystalline spinel ferrite thin films of Co_xFe_3−xO₄ (x=0.3$x=0.3$, 0.5, 0.8, and 1.0) have been prepared by RF sputtering on quartz substrate without a buffer layer at room temperature and annealed at the temperature range from 200 to 600 °C in air. The as-sputtered films exhibit the preferred orientation and the high magnetization and coercivity. After annealing, the preferred orientations become poor, but the magnetization and coercivity increase. The sample with a magnetization of 455 emu/cm³, a coercivity of 2.8 kOe, a remanence ratio of 0.72, and a maximum energy product of 2.4 MGOe has been obtained. The influence of Co ions and annealing temperature on the magnetic properties has been discussed.     

Ferromagnetism at 300 K in spin-coated films of Co doped anatase and rutile-TiO2

Thin films of Ti_1−xCo_xO₂ (x=0 and 0.03) have been prepared on sapphire substrates by spin-on technique starting from metalorganic precursors. When heat treated in air at 550 and 700 °C, respectively, these films present pure anatase and rutile structures as shown both by X-ray diffraction and Raman spectroscopy. Optical absorption indicate a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1×10⁻⁵ Torr at 500 and 600 °C, respectively, the magnetic moment, at 300 K, is strongly enhanced reaching 0.36μ_B/Co for the anatase sample and 0.68μ_B/Co for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Magnetic and magnetocaloric properties of Y bMn6Sn6−xInx

We have synthesized three Y bMn₆Sn_6−xIn_x representatives (x=0.45, 0.80, 1.20), the first pseudo-ternary RMn₆Sn_6−xX′_x compounds involving a divalent R metal. The crystal structure is found to evolve with the In concentration without modification of the Yb valency: Y bMn₆Sn_5.55In_0.45 is isotypic with HoFe₆Sn₆ (Immm) while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 crystallize in the TbFe₆Sn₆-type (Cmcm). The In content is also determining as regards the magnetic and magnetocaloric properties: Y bMn₆Sn_5.55In_0.45 () almost behaves like a simple ferromagnet while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 also order ferromagnetically but at significantly lower temperatures ( and 129 K, respectively) and are further characterized by the interference of low temperature antiferromagnetic interactions. The results are discussed and compared to previously published data.     

Polymorphism in Bi2Sn2O7

Single crystals of Bi₂Sn₂O₇ were grown in a Bi₂O₃ flux. Phase transitions were identified at about 90 and 680° using X-ray, SHG, DSC, dielectric, and optical data. γ-Bi₂Sn₂O₇, which exists above 680°C is centric and cubic with a = 10.73 Å at 700°, and it probably has the ideal pyrochlore structure. β-Bi₂Sn₂O₇, which exists between 680° and about 90°C, is acentric but remains cubic with a = 21.40 Å. α-Bi₂Sn₂O₇, which exists from about 90°C to below room temperature, is acentric and noncubic, probably tetragonal with a = 21.328 and c = 21.545 Å. The α-β transition is first order, and the β-γ transition appears to be second order. Substitutions of Pb²⁺ or Cd²⁺ for Bi³⁺ and of Ga³⁺, Rh³⁺ Sc³⁺, In³⁺, Sb⁵⁺ Nb⁵⁺ or Ta⁵⁺ for Sn⁴⁺ lower the α-β transition temperature.     

The exchange bias in MnPd/Co1−xFex bilayers

A systematic study of exchange bias in MnPd/Co and MnPd/Co_1−xFe_x bilayers has been carried out. Very large unidirectional anisotropy constant of 2.2 erg/cm² and the appearance of double-shifted loops, ascribed to the coexistence of positive and negative exchange bias, have been observed. The dependence of exchange bias, unidirectional anisotropy constant and coercivity on thickness, temperature, annealing regime and Fe content has been investigated and discussed.     

Effects of sintering temperature and atmosphere on magnetism in Mn-doped TiO2 bulk samples

Magnetic properties have been investigated on Mn doped TiO₂(Ti_0.98Mn_0.02O₂) bulk samples prepared by solid state reaction, which were sintered at different temperature ranging from 450 ^°C to 900 ^°C in air and argon atmosphere, respectively. The results show that the magnetic properties were strongly dependent on the sintering temperature and atmosphere. For samples sintered in air, the magnetization initially increase with the increase of sintering temperature up to 600 ^°C and thereafter it decrease. While the magnetization of samples sintered in argon atmosphere decreases monotonically with the increase of sintering temperature. Furthermore, for samples sintered at 600 ^°C in air, the magnetic susceptibility exhibits a dominant Curie-Weiss behaviour and no magnetic transition is observed over the temperature range from 10 to 300 K. In contrast, for samples sintered in argon atmosphere, besides the magnetic transition near 45 K perhaps caused by Mn₃O₄, another magnetic transition appears near room temperature.     

Calcium and oxygen doping in Y Ba2Cu3Oy

Both oxygen and calcium play important roles in inducing superconductivity in Y Ba₂Cu₃O_y (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 ^°C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y _1−xCa_xBa₂Cu₃O_y with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 ^°C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.