Annealing effect of NiO/Co90Fe10 thin films: From bilayer to nanocomposite

Exchange-biased bilayers are widely used in the pinned layers of spintronic devices. While magnetic field annealing (MFA) was routinely engaged during the fabrication of these devices, the annealing effect of NiO/CoFe bilayers is not yet reported. In this paper, the transition from NiO/Co₉₀Fe₁₀ bilayer to nanocomposite single layer was observed through rapid thermal annealing at different temperatures under magnetic field. The as-deposited and low-temperature (<623 K) annealed samples had rock salt (NiO) and face center cubic (Co₉₀Fe₁₀) structures. On the other hand, annealing at 623 K and 673 K resulted in nanocomposite single layers composed of oxides (matrix) and alloys (precipitate), due to grain boundary oxidization and strong interdiffusion in the NiO/CoFe and CoFe/SiO₂ interfaces. The structural transition was accompanied by the reduction of grain sizes, re-ordering of crystallites, incensement of roughness, and reduction of Ni²⁺. When measured at room temperature, the bilayers exhibited soft magnetism with small room-temperature coercivity. The nanocomposite layers exhibited an enhanced coercivity due to the changes in the magnetization reversal mechanism by pinning from the oxides. At 10 K, the increased antiferromagnetic anisotropy in the NiO resulted in enhanced coercivity and exchange bias in the bilayers. The nanocomposites exhibited weaker exchange bias compared with the bilayers due to frustrated interfacial spins. This investigation on how the magnetic properties of exchange-biased bilayers are influenced by magnetic RTA provides insights into controlling the magnetization reversal properties of thin films.     

Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment

Changes in morphological and magnetic properties of Fe₃O₄ nanoparticles before and after annealing are investigated in the present work. The nanoparticles are synthesized in a standard capacitively coupled plasma enhanced chemical vapour deposition system with two electrodes using ferrocene as the source compound. Post annealing, due to the sintering process, the particles fuse along with recrystallization. This results in increased size of the nanoparticles and the interparticle interaction, which play a major role in deciding the magnetic properties. X-ray diffraction patterns of the samples before and after annealing indicate a phase change from Fe₃O₄ to Fe₂O₃. Annealing at 200 ^°C causes the apparent saturation magnetization to increase from 6 emu?g^?1 to 15 emu?g^?1. When annealed at 500 ^°C, the magnetic properties of the nanoparticles resemble those of the bulk material. The evidence for the transition from a superparamagnetic state to a collective state is also observed when annealed at 500 ^°C. Variation of the magnetic relaxation data with annealing also reflects the change in the magnetic state brought about by the annealing. The correlation between annealing temperature and the magnetic properties can be used to obtain nanocrystallites of iron oxide with different sizes and magnetic properties.     

Mechanical alloyed Ho doping in CoFe2O4 spinel ferrite and understanding of magnetic nanodomains

We doped Ho³⁺ in CoFe_1.95Ho_0.05O₄ spinel ferrite by mechanical alloying and subsequent annealing at different temperatures (600-1200 °C). We understood the structural and magnetic properties of the samples using X-ray diffraction, SEM, Thermal analysis (TGA and DTA), and VSM measurement. The samples have shown structural stabilization within cubic spinel phase for the annealing temperature (T_AN)≥800 °C. Thermal activated grain growth kinetics has been accompanied with the substantial decrease in lattice strain. The gain size dependent magnetism is evident from the variation of magnetic moment, remanent magnetization and coercivity of the material. The paramagnetic to ferrimagnetic transition temperature T_C (∼805 K) seems to be grain size independent in the present material. The magnetic nanograins, either single domain/pseudo-single domain (50-64 nm) or multi-domain (above 64 nm) regime, showed superparamagnetic blocking below T_m, which is below T_C (805 K) and also well above the room temperature.     

Influence of manganese substitution and annealing temperature on the formation,microstructure and magnetic properties of Mn–Zn ferrites

The effects of annealing temperature and manganese substitution on the formation, microstructure and magnetic properties of Mn_xZn_1−xFe₂O₄ (with x varying from 0.3 to 0.9) through a solid-state method have been investigated. The correlation of the microstructure and the grain size with the magnetic properties of Mn–Zn ferrite powders was also reported. X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of variation of manganese substitution and its impact on crystal structure, crystalline size, microstructure and magnetic properties of the ferrite powders formed. The XRD analysis showed that pure single phases of Mn–Zn ferrites were obtained by increasing the annealing temperature to 1200–1300 °C. Increasing the annealing temperature to ?1300 °C led to abnormal grain growth with inter-granular pores and this led to a decrease in the saturation magnetization. Moreover, an increase in the Mn²⁺ ion substitution up to x=0.8 increased the lattice parameter of the formed powders due to the high ionic radii of the Mn²⁺ ion. Mn–Zn ferrites phases were formed and the positions of peaks were shifted by substituting manganese. The average crystalline size was increased by increasing the annealing temperature and decreased by increasing the substitution by manganese up to 0.8. The average crystalline size was in the range 95–137.3 nm. The saturation magnetization of the Mn–Zn-substituted ferrite powders increased continuously with an increase in the Mn concentration up to 0.8 at annealing temperatures of 1200–1300 °C. Further increase of Mn substitution up to 0.9 led to a decrease of saturation magnetization. The saturation magnetization increased from 17.3 emu/g for the Mn_0.3Zn_0.7Fe₂O₄ phase particles produced to 59.08 emu/g for Mn_0.8Mn_0.2Fe₂O₄ particles.     

Structure,magnetic properties and magnetostriction of Fe81Ga19 thin films

The structure, magnetic properties and magnetostriction of Fe₈₁Ga₁₉ thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe₈₁Ga₁₉ thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (M_r/M_s) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe₈₁Ga₁₉ thin film fabricated under an applied magnetic field exists along 〈1 0 0〉 direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 90° rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film.     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Synthesis of cobalt ferrite nano-particles and their magnetic characterization

Cobalt ferrite nano-particles (CoFe₂O₄) were synthesized by the co-precipitation method with ammonium hydroxide as an alkaline solution. The reactions were carried out at different temperatures between 20 and 80 °C. The nano-particles have been investigated by magnetic measurements, X-ray powder diffraction and transmission electron microscopy. The average crystallite size of the synthesized samples was between 11 and 45 nm, which was found to be dependent on both pH value of the reaction and annealing temperatures. However, lattice parameters, interplane spacing and grain size were controlled by varying the annealing temperature. Magnetic characterization of the nano-samples were carried out using a vibrating sample magnetometer at room temperature. The saturation magnetization was computed and found to lie between 5 and 67 emu/g depending on the particle size of the studied sample. The coercivity was found to exhibit non-monotonic behavior with the particle size. Such behavior can be accounted for by the combination between surface anisotropy and thermal energies. The ratio of remanence magnetization to saturation magnetization was found to exhibit almost linear dependence on the particle size.     

Investigation of the magnetic properties and magnetic structure parameters of nanocrystalline Fe<Subscript>79</Subscript>Zr<Subscript>10</Subscript>N<Subscript>11</Subscript> films

The magnetic properties (magnetization curve, ferromagnetic resonance spectrum) of nanocrystalline Fe₇₉Zr₁₀N₁₁ films obtained by RF magnetron sputtering with subsequent annealing were studied experimentally, along with the fundamental magnetic constants of these films (saturation magnetization M _S, local magnetic anisotropy energy K, and the exchange coupling constant A). The magnetic properties are discussed within the random magnetic model, which determines the correlation of the magnetic properties with the fundamental magnetic constants and nanostructure parameters (grain size, magnetic anisotropy, and correlation radius R _C). The exchange correlation length 2R _L for the film magnetic microstructure was determined by correlation magnetometry.     

Effect of thermal treatment on the magnetization processes of nanocrystalline Fe80Ge3Nb10B7 alloys

The dynamic magnetization processes of nanocrystalline Fe₈₀Ge₃Nb₁₀B₇ alloys after annealing at different temperatures are studied through the permeability spectroscopy. Three steps of crystallization are found when amorphous Fe₈₀Ge₃Nb₁₀B₇ alloys are heated from 300to 1200 K. The dominant magnetization process varies with different annealing temperatures. Domain wall bulging is the main magnetization mechanism under weak applied field. When the applied field exceeds pinning field H_p, the depinning-involved domain wall displacement occurs. Different annealing temperature results in different H_p. The lower value of μ′ and high relaxation frequency after heating at 923 and 973 K are due to the strengthened domain wall pinning and the increase of magnetocrystalline anisotropy.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Evolution of magnetic order in mechanically alloyed Al–1 at%Fe

The evolution of ferromagnetic order in high-energy ball-milled Al–1 at% Fe before the onset of a considerable Fe–Al solid solution phase has been investigated using ⁵⁷Fe Mössbauer and bulk magnetization studies. The unmilled sample does not exhibit bulk magnetic properties and an onset of bulk magnetization is observed only after 30 min of milling, when the grain size becomes comparable to the ferromagnetic exchange length. The Curie temperatures of all the samples are less than that of pure iron. The reduction in grain size is accompanied by an increase in coercivity and reduced remanence and a decrease in T_C. The effective magnetic moment per iron atom decreases with the development of a non-magnetic, Al-rich Fe–Al solution on longer milling. The clustering of Fe at grain boundaries is responsible for the observed bulk magnetic ordering. The systematic variation of the magnetic properties has been qualitatively correlated with the evolution of microstructure, reduction in grain size and enhanced inter-granular exchange coupling.     

Improvement of magnetic properties and read/write characteristics in SmCo5 perpendicular thin films

An SmCo₅ alloy is a promising candidate for ultra-high density magnetic recording media because of its strong uniaxial magnetocrystalline anisotropy, whose constant, K_u, is more than 1.1×10⁸ erg/cm³. Recently, we successfully obtained high perpendicular magnetic anisotropy for a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, it is necessary to improve magnetic properties and read/write (R/W) characteristics for applying SmCo₅ thin films to perpendicular magnetic recording media. In this study, we focused on reduction of magnetic domain size and change of a magnetization reversal process of SmCo₅ perpendicular magnetic thin films by introducing carbon (C) atoms into the constituent Cu underlayer. The magnetic domain size became small and the ratio of coercivity (H_c) against magnetic anisotropy (H_k) which is an index of the magnetization reversal process was increased by adding C atoms. We also evaluated the R/W characteristics of SmCo₅ double-layered media including C atoms. The medium noise was decreased and signal-to-noise ratio increased by introducing the C. The addition of C into the Cu underlayer is effective for changing the magnetization reversal process, reducing medium noise and increasing SNR.     

Structure and magnetic properties of annealed metastable FeAg/Pt films

FeAg and FeAg/Pt films were prepared by dc magnetron sputtering at room temperature. The effects of Ag volume fraction in FeAg films and postannealing temperature and time on structure and magnetic properties of FeAg and FeAg/Pt films have been investigated. The results show that the as-deposited FeAg films are metastable. After annealing at 300°C, the phase separation of metastable FeAg films happened and the highest coercivity is obtained in Fe₅₀Ag₅₀/Pt film. With increasing annealing temperature, the ordering and the magnetic properties of the Fe₅₀Ag₅₀/Pt films were improved. When the Fe₅₀Ag₅₀/Pt films are annealed at 600°C for different annealing times, a long annealing time enhances the ordering of the metastable Fe₅₀Ag₅₀/Pt films and affects the orientation development. When the films are annealed for a long time, the grain size and the magnetic domain size also increase, which lead to an increase of correlation length due to the growth of FePt grains.     

Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels

The effects of annealing prior to cold rolling on the microstructure, magnetic and mechanical properties of low-C grain non-oriented (GNO) electrical steels have been investigated. The grain structure of hot-rolled electrical steel strips is modified by annealing at temperatures between 700 and 1050 °C. Annealing at temperatures less than the ferrite to austenite+ferrite transformation temperature on heating (Ac₁) causes a marginal effect on the grain size. However, annealing in the intercritical region at temperatures between Ac₁ and Ac₃ (the ferrite+austenite to austenite transformation temperature on heating) causes rapid decarburization and development of large columnar ferrite grains free of carbide particles. This microstructure leads, after cold rolling and a fast annealing treatment, to carbide free, large ferrite grain microstructures with magnetic and mechanical properties superior to those observed typically in the same steel in the industrially fully processed condition. These results are attributed to the increment in grain size and to the {1 0 0} fiber texture developed during the final annealing at temperatures up to 850 °C. Annealing at higher temperatures, T>Ac₃, results in a strong {1 1 1} fiber texture and an increase of the quantity of second phase particles present in the microstructure, which lead to a negative effect on the final properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low C GNO electrical steels strips.     

Effect of grain size on the magnetic properties of superparamagnetic Ni0.5Zn0.5Fe2O4 nanoparticles by co-precipitation process

Ni_0.5Zn_0.5Fe₂O₄ (NZFO) spinel-type nanoparticles were directly fabricated by the chemical co-precipitation process using metal nitrate and acetate as precursors since nitrogen and carbon would be taken away in the forms of oxynitride and oxycarbide, respectively, after the precursors were annealed and then investigated in detail by employing X-ray diffraction (XRD), magnetic measurement and Raman spectroscopy. XRD analysis indicates that the as-prepared nanocrystals are all of a pure cubic spinel structure with their sizes ranging from 20.8 to 53.3 nm, as well as peaks of some samples shifting to lower angles due to lattice expansion. Calculations from the derived XRD data indicate that the activation energy is 30.83 kJ/mol. The magnetic measurements show that these samples are superparamagnetic. The saturation magnetization increases with annealing temperature, which may be explained by super-exchange interactions of Fe ions occurring at A- and B-sites. The variation of coercivity with particle size is interpreted on the basis of domain structure and crystal anisotropy. Furthermore, these nanoparticles exhibit a redshift phenomenon at lower temperatures seen in the Raman spectra, which could be related to ionic substitution.     

Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography

Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd₂Fe₁₄B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning.     

Magnetoresistance and magnetic properties of Fe3O4 nanoparticle compacts

In this paper,we report on the magnetic properties of Fe3O4 nanoparticles with different grain sizes under different pressures.In all the samples,the saturated magnetization Ms shows a linear decrease with increasing pressure.The thickness of the magnetic dead layer on the nanoparticle surface nuder different pressures was roughly estimated,which also increases with increasing pressure.The transport measurements of the nanoparticle Fe3O4 compacts show that the low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range;however,the high-field MR value is dependent on grain size,especially at low temperatures.These experimental results can be attributed to the different surface states of the nanoparticles.     

The influence of NiO addition on the magnetic properties of polycrystalline yttrium iron garnet

The influence of NiO addition on the magnetic properties of polycrystalline Y₃Fe₅O₁₂ is studied for the saturation magnetization, Curie temperature, initial magnetic permeability and ferromagnetic resonance line width. Dependence of saturation magnetization on NiO addition suggest that Ni²⁺ ions enter octahedral sites of the garnet lattice. Real part of the complex initial permeability versus temperature curves reveal the single phase for samples with NiO content. The absence of any additional peak in these curves and the invariance of Curie temperature suggest that NiO addition cannot alter the magneto-crystalline anisotropy of the material. Variations of initial permeability with NiO content are due to change of saturation magnetization and grain size of the materials. The ferromagnetic resonance line width varies linearly with the porosity of samples with NiO showing no anisotropy contribution in it.     

Grain growth process of two-phase nanocrystalline soft magnetic materials

In order to clarify the origin of the high thermal stability of the microstructure in bcc-Fe/amorphous two-phase nanocrystalline soft magnetic materials, we have investigated the changes in the magnetic and microstructural properties upon isothermal annealing at 898 K for an Fe₈₉Zr₇B₃Cu₁ alloy by means of transmission electron microscopy, Mössbauer spectroscopy and DC magnetometry. The mean grain size was found to remain almost unchanged at the early stage of annealing. However, rapid grain coarsening was evident at an annealing time of 7.2 ks where the intergranular amorphous phase begins to crystallize into Fe₂₃Zr₆. The grain growth process with a kinetic exponent of 1.6 is observed for the growth process beyond this annealing time, reflecting the disappearance of the intergranular amorphous phase. Our results confirm that the thermal stability of the bcc-Fe/amorphous two-phase nanocrystalline soft magnetic alloys is governed by the residual amorphous phase.     

Variation of magnetic properties in heat treated and ball milled Fe3Al alloy

The magnetic properties of three samples of Fe₃Al—as melted and annealed, high energy ball milled and milled sample followed by annealing—have been studied using a combination of X-ray diffraction, Transmission electron microscopy, room temperature ⁵⁷Fe M?ssbauer spectroscopy and DC magnetization. The different magnetic contributions in the M?ssbauer spectra have been explained in terms of the nearest neighbour Al configuration of Fe. These correlate well to the bulk magnetic properties determined by DC magnetization. High temperature DC magnetization studies show the presence of antiferromagnetic contributions from grain boundaries in the ball milled, nano sized sample.