Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

Fabrication and magnetic properties of Fe3O4 nanowire arrays in different diameters

Fe₃O₄ nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature T_B increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields (H) were measured and a power relationship between T_B and H were found. The temperature dependence of coercivity below T_B was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.     

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.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Morphology and magnetic properties of Co0.8Fe2.2O4 films prepared by the chemical solution deposition

Co_0.8Fe_2.2O₄ ferrite thin films have been prepared on Si(0 0 1) substrates by the chemical solution deposition. Structural characteristics indicate all films are single phase with spinel structure and the space group and the mean grain size increases from 8 to 30 nm with the increase of annealing temperature. The magnetic properties of Co_0.8Fe_2.2O₄ thin films are highly dependent on annealing temperature. The sample annealed at 800 °C possesses high saturation magnetization, moderate coercivity and squareness ratio, making it a promising application candidate in high-density record and magneto-optical materials.     

Effect of composition on phase formation,microstructure and magnetic properties of Nd–Fe–B thin films

Nd–Fe–B hard magnetic thin films were deposited on a combined Cr/Ta buffer layer on heated MgO(1 0 0) substrates by pulsed laser deposition. The effect of composition on phase formation, morphology and magnetic properties was investigated. For a quantification of phase formation, hysteresis measurements along the hard axis are used. Optimum phase formation is obtained at a Nd/Fe ratio around 0.3. For this Nd content an excellent magnetic texture and a coercivity up to 1 T can be obtained; however, remanence is reduced and the films have a granular microstructure with high roughness. Also a certain B surplus is of benefit. However, with a too-high B or Nd surplus phase formation is more difficult.     

TbMnO3 epitaxial thin films by pulsed-laser deposition

Epitaxial TbMnO₃ films have been fabricated on SrTiO₃(001) and LaAlO₃(001) substrates by pulsed laser deposition (PLD), the structure and surface morphology of the films were characterized by X-ray diffraction with Cu K_α radiation and atomic force microscopy. The electrical transport and magnetic properties of the TbMnO₃ films and bulk were examined, the resistivity and the forbidden band width E_g change with epitaxial orientation, semiconductor transport properties are found in the films and bulk, the average of the E_g of the films on SrTiO₃ and on LaAlO₃ is equal to the E_g of the bulk. The two TMO films have different magnetization mode, the magnetization of the film on SrTiO₃ have an analogy to that of TbMnO₃ single crystal.     

Spin glass ordering of Zn0.75Co0.25Fe0.5Cr1.5O4 cubic spinel

The linear and nonlinear low field AC susceptibilities of Zn_0.75Co_0.25Fe_0.5Cr_1.5O₄ show peaks due to non-critical contributions, which mask the peak due to spin glass ordering. They extend into the region of temperatures in which Mössbauer spectra do not show any magnetic component. When a DC field of 200 Oe suppresses the non-critical contributions, peak due to spin glass ordering is clearly visible. The spin glass ordering is thus shown to be a thermodynamic transition. The critical exponent is found to fall within the range found using other spin glasses. Mössbauer spectra in zero fields provide T_SG, which agrees with the peak temperature of AC susceptibilities in the absence of non-critical contributions. 〈S_Z〉 determined using Mössbauer spectra does not show any anomaly. In the presence of a field of 5 T, the spectra show SG ordering at 4.2 K, which converts into ferrimagnetic ordering at higher temperatures.     

Photocontrol of magnetization in Al-substituted Fe3O4 thin films

Thin films of (111)-oriented spinel ferrite Al_0.5Fe_2.5O₄ have been prepared by a pulsed-laser deposition (PLD) technique on α-Al₂O₃ (0001) substrates. The films exhibit cluster-glass behaviors with a spin-freezing temperature, T_g, near or above room temperature. The magnetization was found to increase following light irradiation below T_g, which indicates the photoinduced melting of cluster-glass states. An analysis comparing the dynamic behavior of magnetic response to light irradiation between zero-field-cooled (ZFC) states and field-cooled (FC) states at 10 K under various light intensities, I, revealed that the direct photoexcitation of spins occurs when I≤0.78 mW/mm², while the thermal heating effect following the light absorption of the samples also contributes to the enhancement of magnetization when I≥1.22 mW/mm². The magnetization of the films could be controlled by light irradiation even at room temperature. This suggests the possibility of utilizing these films in the development of novel magneto-optical memory devices.     

Enhanced magneto-optical Kerr effects in nanocrystalline Sc-doped CoFe2O4 thin films

Nanocrystalline CoFe_2−xSc_xO₄ (x=0-0.4) thin films were prepared on silicon substrates at reduced temperature by a sol-gel process, and the doping effects of scandium on the microstructure, magnetism and polar magneto-optical Kerr effects of the as-deposited films were examined. It was shown that the intensities of both of the Kerr rotation peaks increase with the doping content x of Sc³⁺. The increase for the peak at 540 nm is due to the decrease of the electrostatic polarization of O²⁻ resulting from the relatively large radius of Sc³⁺, and that for the peak at 620 nm was a result of the migration of Co²⁺ from octahedral to tetrahedral sites in the presence of the dopant of Sc³⁺.     

Epitaxial growth and characterization of CaVO3 thin films

Epitaxial thin films of CaVO₃ were synthesized on SrTiO₃, LaAlO₃ and (La_0.27Sr_0.73)(Al_0.65Ta_0.35)O₃ substrates by pulsed laser deposition. All CaVO₃ films, independent of epitaxial strain, exhibit metallic and Pauli paramagnetic behavior as CaVO₃ single crystals. X-ray absorption measurements confirmed the 4+ valence state for Vanadium ions. With prolonged air exposure, an increasing amount of V³⁺ is detected and is attributed to oxygen loss in the near surface region of the films.     

Room-temperature magnetoresistance effects of Ag-added Fe3O4 films with single-domain grains

Polycrystalline Ag_x(Fe₃O₄)_1−x films (x=0, 0.1, 0.2 and 0.3) have been prepared by the sol-gel method in combination of the spin-coating technique with a precursor solution containing polyvinyl alcohol (PVA) on fused quartz substrates. XRD analysis and SEM images indicate that the Fe₃O₄ grains are nearly spherical single-domain particles. The coercivities of the films are about 290 Oe for x=0.1 and 360 Oe for x=0.3, respectively, which are nearly the same as the magnetocrystalline anisotropic effective field H_K of Fe₃O₄. At 300 K, the x=0.1 film has a maximal magnetoresistance of −8.7% at a magnetic field of 50 kOe and −3.5% at 8.8 kOe, while the pure Fe₃O₄ film is only −2.2% at 8.8 kOe. This enhancement of the MR can be attributed to the contribution from the spin-dependent scattering at the Ag-Fe₃O₄ interfaces as well as the spin-polarized tunneling at boundaries of Fe₃O₄ grains of the spin-polarized electrons. In addition, different MR behaviors for Ag-added Fe₃O₄ bulk polycrystalline samples and polycrystalline films are discussed.     

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Magnetic nanocomposite thin films of BaFe12O19 and TiO2 prepared by sol-gel method

The composite films with different weight ratio of barium ferrite to titanium dioxide are successfully prepared using sol-gel method for the first time. The morphology, crystal structure and magnetic properties of composite films are investigated with atomic force microscopy, X-ray diffraction and vibrating sample magnetometry. The results show that the composite films are uniform with no microcracks. The grain diameters are less than 100 nm. With the increase of barium ferrite, the grain diameter decreases. The composite films are composed of M-type hexagonal barium ferrite and rutile titanium dioxide. The composite films possess the excellent magnetic properties. The specific saturation magnetization and coercivity reach 18.3 emu/g and 3350 Oe, respectively. The application of composite films in magnetic recording and electromagnetic absorption fields is promising.     

Magnetic properties of ZnFe2O4 nanoparticles produced by a low-temperature solid-state reaction method

ZnFe₂O₄ nanoparticles with average grain size ranging from 40 to 60 nm behaving superparamagnetic at room temperature have been produced using a low-temperature solid-state reaction (LTSSR) method without ball-milling process. Abnormal magnetic properties such as S-shape hysteresis loops and non-zero magnetic moments were observed. ZnFe₂O₄ nanoparticles were also synthesized using a NaOH coprecipitation method and a PVA sol-gel method to study the relationship between the preparation processes and the magnetic properties. Spin-glass behavior was observed in the low temperature solid-state reaction produced Zn ferrite in the zero-field cooled (ZFC) measurement. Our work proves that the various preparation methods will to some extent determine the properties of magnetic nanoparticles.     

Effect of mechanical milling on the magnetic properties of garnets

Rare earth garnets after milling to nanosizes are found to decompose into rare earth orthoferrite and other rare earth and iron oxide phases. The magnetization for the yttrium iron garnet decreases in the nano state due to the formation of antiferromagnetic phases. But for the gadolinium iron garnet when milled up to 25 h, the room temperature magnetization increases despite the formation of antiferromagnetic and non-magnetic phases. This is attributed to the uncompensated moments of the sublattices because of the weakening of the superexchange interaction due to change in bond angles and the breaking of some superexchange bonds on account of the defects and oxygen vacancies introduced on milling. For the 10 h milled gadolinium iron garnet at 5 K, after correcting for the non-magnetic phases present, there is an increase in the magnetic moment of about 10% as compared to the value for the as-prepared garnet. The magnetic hyperfine fields corresponding to the various phases were measured using ⁵⁷Fe Mössbauer spectroscopy at 16 K. The isomer shift values indicate the loss of oxygen for the samples milled for larger duration.     

Mössbauer studies on the superparamagnetic behavior of CoFe2O4 with a few nanometers

CoFe₂O₄ nanoparticles with a cubic spinel structure are prepared by a high-temperature thermal decomposition method. The average particle sizes are 4.6  and 5.7 nm for CoFe₂O₄ made with two kinds of solvents by TEM. Mössbauer spectra of 4.6 nm particles displayed a superparamagnetic behavior as demonstrated by a single line with zero hyperfine fields, but that of 5.7 nm particles did not at room temperature. It is considered that anisotropy energy was still more superior to thermal energy because of particle size of 5.7 nm CoFe₂O₄. Furthermore, Mössbauer spectra exhibited the typical spectrum shapes of the CoFe₂O₄ at 4.2 K. The spectrum at 4.2 K was fitted using two magnetic components of hyperfine fields _Hhf=540.4,512.6$H_{\mathrm{hf}}=540.4,512.6$ kOe and isomer shifts δ=0.40,0.30$\delta =0.40,0.30$ mm/s for 4.6 nm and _Hhf=542.7,512.8$H_{\mathrm{hf}}=542.7,512.8$ kOe and δ=0.41,0.29$\delta =0.41,0.29$ mm/s for 5.7 nm corresponding to Fe³⁺ ions at site A and site B, respectively.     

Synthesis and magnetic properties of spinel CoFe2O4 nanowire arrays

Spinel CoFe₂O₄ nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO) template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into the nanopores by vacuum impregnation. After heat treatment, it transformed to spinel CoFe₂O₄ nanowires. The structure, morphology and magnetic properties of the sample were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The results indicate that the nanowire arrays are compact. And the individual nanowires have a high aspect ratio, which are about 80 nm in diameter and 10 μm in length. The nanowires are polycrystalline spinel phase. Magnetic measurements indicate that the nanowire arrays are nearly magnetic isotropic. The reason is briefly discussed. Moreover, the temperature dependence of the coercive force of the nanowire arrays was studied.     

Preparation and characterizations of SiO2-coated nanoparticles of Mn0.4Zn0.6Fe2O4

Core/shell nanoparticles consisting of a magnetic core of zinc-substituted manganese ferrite (Mn_0.4Zn_0.6Fe₂O₄) and a shell of silica (SiO₂) are prepared by a sol-gel method using tetraethyl orthosilicate (TEOS) as a precursor material for silica and salts of iron, manganese and zinc as the precursor of the ferrite. Three weight percentages of the shell materials of SiO₂ are used to prepare the coated nanoparticles. The X-ray diffractograms (XRD) of the coated and uncoated magnetic nanoparticles confirmed that the magnetic nanoparticles are in their mixed spinel phase in an amorphous matrix of silica. Particles sizes of the samples annealed at different temperatures are estimated from the width of the (3 1 1) line of the XRD pattern using the Debye-Sherrer equation. The information regarding the crystallographic structure together with the particles sizes extracted from the high-resolution transmission electron microscopy (HRTEM) of a few selected samples are in agreement with those obtained from the XRD. HRTEM observations revealed that particles are coated with silica. The calculated thickness is in agreement with that obtained from the HRTEM pictures. Hysteresis loops observed in the temperature range 300 down to 5 K and Mössbauer spectra at room temperature indicate superparamagnetic relaxation of the nanoparticles.