Insight into magnetic properties in zinc ferrite thin films by tuning oxygen content

In this investigation ZnFe₂O₄ thin films were fabricated by RF magnetron sputtering and the magnetic behavior was examined under the influence of the growth conditions in three types of atmospheres, using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and vibrating sample magnetometry (VSM). XPS revealed that films under an Ar/O₂ ratio 1:2 and 2:1 have Fe/Zn atomic ratios similar to the Fe/Zn stoichiometric composition; while the films deposited in pure Ar atmosphere, showed a formation of Fe ions (Fe⁰) reduced by Ar ion sputtering. VSM revealed an enhancing of the saturation magnetization for the ZFO in thin films unlike the samples in bulk, suggesting that Fe ions migrate to A sites and the interaction J_AB dominates the magnetic properties instead of J_BB interaction.     

Room temperature single-step electrosynthesized copper ferrite thin films and study of their magnetic properties

A single-step electrosynthesis of copper ferrite thin films from aqueous bath (which avoids anodization step for an incorporation of oxygen species into deposit) has been carried out at room temperature. Observed tetrahedral structured nanocrystalline copper ferrite thin films showed smooth, uniform and compact surface morphology. After annealing, increase in dielectric constant and reduced dielectric loss were observed. The saturation magnetization for annealed films was 292 emu/cm³ comparable to that of other reported ferrites.     

Influence of citric acid on the morphology and magnetic properties of barium ferrite thin films

Hexagonal barium ferrite (BaFe₁₂O₁₉) thin films were fabricated by spin coating of precursors from sol-gel methods. Different amount of citric acid was added. Weight loss and changes of chemical bonds during the heat treatment below 500 °C were recorded by thermogravity analyzer and Fourier transform infrared spectroscopy, respectively. The morphology of the films, i.e. the amount of acicular crystallites in the film, was controlled by adjusting the quantity of citric acid. The X-ray diffraction shows that the films are c-axis oriented, but the crystallization is less perfect with the increase of the citric acid. With the increase of the citric acid, the percentage of the acicular crystallites increased. At the same time, the coercivity force increased. The acicular crystallites were not exchange-coupled to the platelet crystallites. The citric acid was understood to form complex with iron ions and accelerate the exothermal auto-combustion. The c-axis oriented growth of the film was thus deteriorated.     

Cationic behavior and the related magnetic and magnetotransport properties of manganese ferrite thin films

Manganese ferrite (Mn_xFe_3−xO₄) thin films have been prepared by a sol–gel method. The samples (x≤1.25) are polycrystalline containing well-defined grains and maintain cubic spinel structure with increasing lattice constant with x. The substituting Mn ions were found to have multi-valence, +2 and +3, by X-ray photoelectron spectroscopy. The saturation magnetization of the Mn-substituted films measured by vibrating sample magnetometry was found to increase from that of Fe₃O₄ at low x and then gradually decreases as x increases further. Such magnetic behavior can be explained in terms of the dominance of Mn²⁺ species over Mn³⁺ at low x and the gradual decrease in the population ratio Mn²⁺/Mn³⁺ as x increases. The observed decrease in the coercivity with increasing x implies the increase in octahedral Mn²⁺ population. The Mn-substituted samples exhibit magnetoresistance (MR) effect the maximum intensity of which is gradually reduced with x from that of Fe₃O₄. All samples show increasing MR with increasing external field (H), while their magnetization curves start to saturate near H=2 kOe. Such increasing MR with H can be explained in terms of the tunneling of spin-polarized carriers across grain boundaries. The reduction in the MR intensity with x can be partly explained in terms of the decrease in spin-polarized carrier density associated with octahedral occupation of Mn²⁺ ions.     

Spin-deposited nanocrystalline lithium ferrite thin films: Fabrication and characterization

Thin films of lithium ferrite (with general composition Li_0.5Fe_2.5O₄) were fabricated at low temperatures (up to 650 °C) by citrate-route using spin-deposition technique. Deposited films consisted of nanometer-sized grains as evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD patterns for annealed films showed broad peaks exhibiting a spinel phase. Size of nanocrystallites is estimated to be 3-7 nm using Scherrer's equation. Average grain size ∼8.5 nm is observed from TEM images of films annealed at 650 °C. Scanning electron micrographs show the formation of spherical aggregates of around 130 nm in diameter. The AFM analysis clearly evidenced the development of nanograins even at low (∼500 °C) annealing temperatures. Significant decrease in complex dielectric permittivity (∈′ − j∈″) with frequency is observed in the low frequency (100 Hz-1 MHz) as well as in X-band microwave frequency (8-12 GHz) region. ∈′ is found to be in the range of 15.7-33.9 in low frequency region, whereas in X-band microwave frequency region, it is found to lie between 3.9 and 4.9. Similarly, ∈″ is found to be 0.16-5.9 in the low frequency region, and 0.002-0.024 in the X-band microwave frequency region. Room temperature dc resistivity of these films is estimated to lie in the range of 10⁶-10⁸ Ω cm. These results strongly suggest that citrate-route processed nanocrystalline lithium ferrite thin films are promising candidates for monolithic microwave integrated circuits (MMICs).     

Effect of thickness on the microstructure and soft magnetic properties of CoFeHfO thin films

The thickness effects on the microstructure and soft magnetic properties of CoFeHfO thin films have been investigated in the range of 100–600 nm. There was a significant change in the coercivity (H_c) and the anisotropy (H_k) value with increasing film thickness, but the saturation induction and the resistivity almost remain unchanged. H_c and H_k reached a minimum value of 0.19 Oe and a maximum value of 50 Oe, respectively at 200 nm film thickness. The high saturation magnetic induction is 21 kG and resistivity is 500 μΩ cm. The origin of the changing H_c and H_k values is discussed in detail based on change of microstructure along with film thickness.     

溅射功率对Fe-Si-B-Nb-Cu薄膜微结构与磁性的影响

通过射频磁控溅射法制备了Fe_Si_B_Nb_Cu薄膜，采用x射线衍射与Mssb auer谱相结合分析了薄膜的微结构形态，研究了不同溅射功率对薄膜微结构的影响.其结果 表明：在较低溅射功率密度下，薄膜为无定型结构；随着溅射功率密度升高，沉积薄膜无需 热处理，便呈现出晶态和非晶态的混合相结构，晶态为纳米级的α_Fe(Si)和α_Fe(B)固溶 体；α_Fe(Si)相和α_Fe(B)相的体积分数、微结构组态、磁矩取向及宏观磁性能均随着溅 射功率的变化而变化. 关键词： 溅射功率 Fe_Si_B_Nb_Cu合金 薄膜微结构 磁矩取向     

Correlation between site preference and magnetic properties of substituted strontium ferrite thin films

SrFe_12−x(Sn_0.5Zn_0.5)_xO₁₉ thin films with x=0−5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). The site preference and magnetic properties of Zn-Sn substituted strontium ferrite thin films were studied using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra displayed that the Zn-Sn ions preferentially occupy the 2b and 4f₂ sites. The preference for these sites is responsible for the anomalous increase in the magnetization at high Zn-Sn substitutions. X-ray diffraction (XRD) patterns and field emission scanning electron microscope (FE-SEM) micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. Vibrating sample magnetometer (VSM) was employed to probe magnetic properties of samples. The maximum saturation of magnetization and coercivity at perpendicular direction were 265 emu/g and 6.3 kOe, respectively. It was found that the complex susceptibility has linear variation with static magnetic field.     

Structural and magnetic properties of single-step electrochemically deposited nanocrystalline cobalt ferrite thin films

In this paper, we study the structural, surface morphological and magnetic properties of single-step electrochemically deposited cobalt ferrite thin films. The prepared films were nanocrystalline with cubic crystal structure. Scanning electron micrograph image showed that the cobalt ferrite thin film was uniformly distributed over the substrate in addition to some random overgrowth of porous particles. The saturation magnetization of 298 emu/c was confirmed when films were used in magnetic studies.     

Effects of the Hf content on the microstructure and magnetic properties of Co-Hf-Ta thin films

Effects of the Hf content in Co-Hf-Ta thin films on the microstructure and magnetic properties were investigated in this study. It was found that appropriate Hf addition can effectively refine the Co grain size. Co grain sizes sharply decreased from 50 nm down to 2.3 nm with increasing the Hf content from 1.02 at.% to 2.81 at.%, leading to the reduced magneto-crystalline anisotropy. The Co-Hf-Ta thin films with small Co grains reveal low anisotropy field, low coercivity, and high resistivity. By optimizing the Hf content, the film with Hf concentration of 2.81 at.% exhibits excellent soft magnetic properties: high saturation magnetization (4πM_S ∼ 13.6 kG), and low coercivity (H_C ∼ 0.6 Oe). The effective permeability of the film reaches 800 and remains constant up to 1 GHz.     

Structure and magnetic properties of Co-substituted NiZn ferrite thin films synthesized by the sol-gel process

Co-substituted NiZn ferrite thin films, Ni_0.5Zn_0.5Co_xFe_2−xO₄ (0≤x≤0.2), were synthesized by the sol-gel process. The structure and magnetic properties of Ni_0.5Zn_0.5Co_xFe_2−xO₄ ferrite thin films have been investigated. The diffraction peak shifted towards the lower angle and the lattice parameter increased with Co substitution. There is little influence of Co substitution on the microstructure of NiZn ferrite thin films. The saturation magnetization gradually increases with the increase in Co substitution when x≤0.10, and decreases when x>0.10. Meanwhile, the coercivity initially decreases with the increase in Co substitution when x≤0.10, and increases when x>0.10.     

Interface mediated control of microstructure and magnetic properties of FePt-C thin films

The microstructure and magnetic properties of FePt-C film deposited on the MgO/CrRu underlayer are studied in lieu of carbon doping in FePt and RF biasing assisted etching of the MgO underlayer. Increasing amounts of carbon doping in the FePt deteriorates the chemical ordering as a result of reduced kinetic energy of FePt adatoms due to frequent collision with carbon adatoms. This leads to a reduction in the magnetocrystalline anisotropy energy and hence the coercivity. RF etching of the MgO/CrRu underlayer before deposition of FePt helps to reduce the grain size and intergranular exchange interaction without adversely affecting the chemical ordering. Reduced intergranular exchange interaction leads to an increase in the coercivity.     

The study on microstructure and microwave-absorbing properties of lithium zinc ferrites doped with magnesium and copper

Lithium zinc ferrites doped with magnesium and copper were prepared by means of a combination of sol-gel method and subsequent calcination. The crystalline phase and microstructure of different doped lithium zinc ferrites were measured by X-ray powder diffraction and scanning electronic microscopy analysis. The results indicate that there are no remarkable differences in phase composition between pure lithium zinc ferrite and the as-doped lithium zinc ferrites. The effects of magnesium and copper dopants on microwave absorption in low-frequency region were investigated by the transmission/reflection coaxial line method. It was found from the present work that doping with copper improved microwave-absorbing properties, while doping with magnesium had little effect on microwave absorption of pure lithium zinc ferrite.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Effect of annealing conditions on structural and magnetic properties of laser ablated copper ferrite thin films

The effect of annealing conditions on structural and magnetic properties of copper ferrite thin films on (100) Si substrates was examined in detail. After deposition, the ferrite thin films were post-annealed in vacuum and in oxygen atmosphere for several hours. It is found that the crystal structure of CuFe₂O₄ thin films changed drastically depending on different heating process. A maximum magnetization was achieved in the film that was vacuum annealed and it decreased remarkably after oxygen annealing.     

Ferromagnetic resonance and magnetic crystallographic anisotropy of monocrystalline lithium ferrite films

The crystallographic anisotropy constant K₁ of monociystalline lithium ferrite films was measured by the methods of ferromagnetic resonance and rotational moments. The presence of uniaxial anisotropy in the plane of a film with the constant K_u 10₃ erg · cm^–3 is established experimentally. The nature of the uniaxial anisotropy is explained by the anisotropy of the stresses in the plane of the film, a formula is obtained to compute the angle of deflection of the easy magnetization axis from the crystallographic direction. An estimate is made of the difference in the stresses along the axes (_x–_z) 10¹⁰ dyne · cm^–2.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 86–89, September, 1973.     

Magnetic properties of disordered ferrite and ilmenite-hematite thin films

We have synthesized thin films of disordered zinc ferrite (ZnFe₂O₄) and ilmenite-hematite (FeTiO₃-Fe₂O₃) solid solution, the former and the latter of which are interesting from the viewpoints of magnetooptics and spintronics, respectively, by utilizing sputtering and pulsed laser deposition methods, and have explored their magnetic, magnetooptical, and electrical properties. Although ZnFe₂O₄ possesses a normal spinel structure as its stable phase, some of the Fe³⁺ ions occupy the tetrahedral as well as the octahedral sites in ZnFe₂O₄ of which the sputtered thin film is composed. Consequently, the as-deposited thin film manifests large magnetization even at room temperature although the magnetic phase transition temperature of the stable phase of ZnFe₂O₄ is as low as 10 K. Also, the thin film exhibits a cluster spin glass transition at a temperature as high as 325 K. Furthermore, the ZnFe₂O₄ thin films exhibit large Faraday effects at a wavelength of 400 nm or so. The ilmenite-hematite solid solution is one of the ferrimagnetic semiconductors. Most of the compositions possess Curie temperatures higher than room temperature, and the type of carrier can be tuned only by changing the composition. We have succeeded in synthesizing solid-solution thin films of various compositions grown epitaxially on sapphire substrates with a (0 0 0 1) plane, and have shown that the thin films are ferrimagnetic semiconductors.     

多孔硅膜微结构的透射电镜研究

用透射电镜观察到由Ｓｉ（ｌｌｌ）衬底生长的多孔硅膜具有双层结构和明显的不均匀微结构，初步探讨了荧光机理与微结构的关系。