Comparative evaluation of performance of percolated perpendicular media using FePt–MgO/Pt/Cr trilayer and FePt–MgO/MgO bilayer

(Fe₄₈Pt₅₂)_100−x–(MgO)_x films were used to examine the performance of a perpendicular percolated medium. Two underlayers, Pt(0 0 1)/Cr(0 0 2) and MgO(0 0 2), were used for comparison. The (Fe₄₈Pt₅₂)_100−x–(MgO)_x film with the MgO underlayer exhibits a strong preference to segregate at FePt grain boundaries. The microstructure with small closely packed MgO particles (2–4 nm) dispersed uniformly in the L1₀ FePt matrix was achieved in the Pt/Cr underlayered sample. Structural data reveal that the precipitate is crystallographically coherent with the surrounding L1₀ FePt phase and preserves good lattice alignment. Magnetic results indicate significant pinning behavior for those introduced non-magnetic columns with an enhanced coercivity of about 70%—much greater than that of the MgO underlayered samples. Percolated perpendicular medium can be realized in the FePt system and a Pt(0 0 1)/Cr(0 0 2) underlayer promotes the formation of pinning sites within the FePt grains.     

Microstructural and magnetic studies of L10 FePt–SiO2 nano-composite thin film with columnar structure for perpendicular magnetic recording

(Fe₅₀Pt₅₀)_100−x-(SiO₂)_x films (x=0–30 vol%) were grown on a textured Pt(0 0 1)/CrRu(0 0 2) bilayer at 420 °C using glass substrates. FePt(0 0 1) preferred orientation was obtained in the films. Interconnected microstructure with an average grain size of about 30 nm is observed in the binary FePt film. As SiO₂ is incorporated, it precipitates as particles are dispersed at FePt grain boundaries. When the content of SiO₂ is increased to 13 vol%, columnar FePt with (0 0 1) texture separated by SiO₂ is attained. The FePt columns have a length/radius ratio of 2:1. Additionally, the mean grain size is reduced to about 13 nm. The development of this well-isolated columnar structure leads to an enhancement in coercivity by about 44% from 210 to 315 kA/m. As the SiO₂ content exceeds 20 vol%, a significant ordering reduction is found accompanied by a transformation of preferred orientation from (0 0 1) to (2 0 0) and the columnar structure disappears, resulting in a drastic degradation in magnetism. The results of our study suggest that isolated columnar, grain refined, (0 0 1)-textured FePt film can be achieved via the fine control of SiO₂ content. This may provide useful information for the design of FePt perpendicular recording media.     

Texture development and magnetic properties of [ZrO2/CoPt]n/Ag nanocomposite films

The L1₀ CoPt films with (0 0 1) preferred orientation are achieved by fabricating on the glass substrates and post annealing at 600° C for 30 min. The preferred orientation of [ZrO₂/CoPt]_n/Ag films dependence of the Ag underlayer thickness, ZrO₂ and CoPt interlayer thickness is investigated. A large perpendicular magnetic anisotropy and a nearly perfect L1₀ CoPt (0 0 1) texture are obtained in the [ZrO₂ (3 nm)/CoPt (5 nm)]₃/Ag (10 nm) film. The existence of ZrO₂ plays an important role in reducing the intergranular interactions and in determining the size of CoPt grains. Magnetic reversal in textured CoPt films are close to a Stoner-Wolfarth rotation.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Structural transformations and magnetic properties of Fe60Pt15B25 and Fe60Pt25B15 nanocomposite alloys

Structural and magnetic properties of two rapidly solidified and post-annealed Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅ alloys are compared. The as-quenched Fe₆₀Pt₁₅B₂₅ ribbon was fully amorphous whereas in the Fe₆₀Pt₂₅B₁₅ alloy the amorphous phase coexists with an fcc FePt disordered solid solution. Differential scanning calorimetry curves of both alloys reveal a single exothermal peak with onset temperatures of 873 and 847 K for Fe₆₀Pt₁₅B₂₅ and Fe₆₀Pt₂₅B₁₅, respectively. Magnetically hard, tetragonal ordered L1₀ FePt and magnetically soft Fe₂B nanocrystalline phases were formed due to the annealing of the alloys, as indicated by X-ray diffraction and Mössbauer spectroscopy measurements. Two-phase behavior was detected in the temperature dependence of magnetization of the annealed samples. A magnetic hardening was observed for all annealed ribbons. Magnetic properties of the annealed alloys, studied by hysteresis loop measurements, were related to the differences in the relative fractions of the hard and soft magnetic phases calculated from Mössbauer spectra. The alloy with 25 at% Pt exhibits better hard magnetic properties (H_c=437 kA/m, M_r/M_s=0.74) than the alloy with smaller Pt content (H_c=270 kA/m, M_r/M_s=0.73) mainly due to the larger abundance of the ordered tetragonal FePt phase.     

Structure and magnetic properties of (Nd,Dy)16(Fe,Co)76−xTixB8 powders prepared by mechanical alloying

Nanocrystalline (Nd,Dy)₁₆(Fe,Co)_76−xTi_xB₈ magnets were prepared by mechanical alloying and respective heat treatment at 973–1073 K/30–60 min. An addition of 0.5 at % of Ti results in an increase of coercivity from 796 to 1115 kA m⁻¹. Partial substitution of Nd by Dy results in an additional increase of coercivity up to 1234 kA m⁻¹. Mössbauer investigations shows that for x?1 the (Nd,Dy)₁₆(Fe,Co)_76−xTi_xB₈ powders are single phase. For higher Ti contents (x>1) the mechanically alloyed powders heat treated at 973 K are no more single phase, and the coercivity decreases due to the presence of an amorphous phase. A heat treatment at a higher temperature (1073 K) for longer time (1 h) results in the full recrystallisation of powders. The mean hyperfine field of the Nd₂Fe₁₄B phase decreases for titanium contents of 0?x?1, and remains constant for x>1. This indicates that the Ti content in the Nd₂Fe₁₄B phase reaches its maximum value.     

Effect of Cr100−xTix (x = 0-19 at.%) alloy underlayers on the high coercivity of FePt thin film below the substrate temperature of 250 °C

The effect of Cr_100−xTi_x underlayer on orderd-L1₀ FePt films was investigated. A low-temperature ordering of FePt films could be attained through changing the Ti content of Cr_100−xTi_x underlayer. The ordering temperature of the 30 nm FePt film grown on 20 nm Cr₉₀Ti₁₀ underlayer was reduced to 250 °C which is practical manufacture process temperature. An in-plane coercivity was very high to 6000 Oe and a ratio of remnant magnetization (M_r) to saturation magnetization (M_s) was as large as 0.85. This result indicates that the coercivity obtained at 250 °C by the effect of CrTi underlayer is significantly higher than those obtained at 250-275 °C by the effect of underlayers in other conventional studies. The prominent improvement of the magnetic properties of ordered FePt thin films at low temperature of 250 °C could be understood with considering the strain-induced ordering phase transformation associated with lattice mismatch between Cr underlayer and FePt magnetic layer due to an addition of Ti content.     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Structural and magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

Positive exchange bias in a Ni80Fe20/NixFe1−xO thin-film bilayer

We have measured positive exchange bias in a Ni₈₀Fe₂₀/Ni_xFe_1−xO thin-film nanocrystallite system. A series of solid solution Ni_xFe_1−xO 40 nm thick films capped with 25 nm thick Ni₈₀Fe₂₀ were deposited using a range of %O₂/Ar bombardment energies (i.e. End-Hall voltages). Proper tuning of the deposition conditions results in a Ni₈₀Fe₂₀/Ni_xFe_1−xO (30%O₂/Ar) based bilayer that exhibits a positive exchange bias loop shift of H_ex∼60 Oe at 150 K.     

Influence of doping concentration on room-temperature ferromagnetism for Fe-doped BaTiO3 ceramics

Ba(Ti_1−xFe_x)O₃ ceramics (x=7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO₃-type hexagonal perovskite structure. Mössbauer spectra show all Fe atoms to be present as Fe³⁺ in BaTiO₃ lattice, occupying M1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti_1−xFe_x)O₃, originating from super-exchange interactions between Fe³⁺ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.     

EPR and magnetic properties of MgO-CaO-SiO2-P2O5-CaF2-Fe2O3 glass-ceramics

Glass-ceramics have been derived from 4.5MgO(45−x)CaO34SiO₂16P₂O₅0.5CaF₂xFe₂O₃ (x=5, 10, 15, 20 wt%) glasses by heat treatment. Room temperature electron paramagnetic resonance (EPR) spectra and temperature-dependent magnetic susceptibility (χ) of the glass-ceramics have been obtained. The EPR absorption line centered at g≈4.3 disappeared at higher concentrations of iron oxide. The intensity and line width of the EPR absorption line centered at g≈2.1 increased as the iron oxide concentration was increased. Temperature-dependent magnetization of samples with low iron oxide content revealed ferrimagnetic as well as paramagnetic contributions. Information about the structural changes involving iron ions, their valence state and the type of magnetic interactions between the Fe ions as a function of composition was obtained using EPR and χ studies.     

High temperature ferromagnetism in cubic Mn-doped ZrO2 thin films

Theory has predicted that high temperature ferromagnetism (FM) should be found in cubic fake-diamonds, Mn-doped ZrO₂. Experimentally, it is shown that Mn-doped ZrO₂ ceramics are not ferromagnetic, but the nanosized Mn-doped ZrO₂ thin films grown on LaAlO₃ substrates can be ferromagnets with T_C above 400 K. The largest saturated magnetic moment (M_s) is huge as of about 230 emu/cm³ for the Mn_0.05Zr_0.95O₂ films, and it decreases as the Mn content increases. The intrinsic FM is strongly associated with the cubic structure of Mn-doped ZrO₂, and the Mn–Mn interactions via oxygen intermediates are important. No electrical conductivity is observed. Mn-doped ZrO₂ thin films can be truly considered as excellent candidates for spintronic applications.     

Effect of Bi substitution on the magnetic and dielectric properties of epitaxially grown BaFe0.3Zr0.7O3−δ thin films on SrTiO3 substrates

The structural, dielectric and magnetic properties of single crystalline Ba_1−xBi_xFe_0.3Zr_0.7O_3−δ (x=0.0-0.29) thin films have been studied. The pseudotetragonal epitaxial thin films were obtained by pulsed laser-beam deposition (PLD) on (0 0 1) SrTiO₃ (STO) single-crystal substrates. The Bi substitution for the Ba ions up to an extent of x=0.18 caused a slight improvement in the leakage current properties, as well as an enhancement of the apparent dielectric constant. The saturation magnetization of the films was significantly decreased following Bi substitution. These changes were thought to be related to the increase in oxygen deficiencies in the films. The effect of the Bi substitution on the dielectric and magnetic properties was analyzed in conjunction with the change in valence value of the Fe ions.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Influence of Fe cap layer on the structural and magnetic properties of Fe49Pt51/Fe bi-layers

The influences of an Fe cap layer on the structural and magnetic properties of FePt/Fe bi-layers are investigated. Compared with single FePt alloy films, a thin Fe layer can affect the crystalline orientation and improve the chemical ordering of L10 FePt films. Moreover, the coercivity increases when a thin Fe layer covers the FePt layer.Beyond a critical thickness, however, the Fe cover layer quickens the magnetization reversal of Fe49Pt51/Fe bi-layers by their exchange coupling.     

Stabilization of a very high-k crystalline ZrO2 phase by post deposition annealing of atomic layer deposited ZrO2/La2O3 dielectrics on germanium

The impact of the ZrO₂/La₂O₃ film thickness ratio and the post deposition annealing in the temperature range between 400 °C and 600 °C on the electrical properties of ultrathin ZrO₂/La₂O₃ high-k dielectrics grown by atomic layer deposition on (1 0 0) germanium is investigated. As-deposited stacks have a relative dielectric constant of 24 which is increased to a value of 35 after annealing at 500 °C due to the stabilization of tetragonal/cubic ZrO₂ phases. This effect depends on the absolute thickness of ZrO₂ within the dielectric stack and is limited due to possible interfacial reactions at the oxide/Ge interface. We show that adequate processing leads to very high-k dielectrics with EOT values below 1 nm, leakage current densities in the range of 0.01 A/cm², and interface trap densities in the range of 2-5 × 10¹² eV⁻¹ cm⁻².     

In vitro evaluation of bioactivity of CaO-SiO2-P2O5-Na2O-Fe2O3 glasses

Glasses with compositions 41CaO(52 − x)SiO₂4P₂O₅·xFe₂O₃3Na₂O (2 ≤ x ≤ 10 mol.%) were prepared by melt quenching method. Bioactivity of the different glass compositions was studied in vitro by treating them with simulated body fluid (SBF). The glasses treated for various time periods in SBF were evaluated by examining apatite formation on their surface using grazing incidence X-ray diffraction, Fourier transform infrared reflection spectroscopy, scanning electron microscopy and energy dispersive spectroscopy techniques. Increase in bioactivity with increasing iron oxide content was observed. The results have been used to understand the evolution of the apatite surface layer as a function of immersion time in SBF and glass composition.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

Synthesis and characterization of room-temperature ferromagnetism in Fe- and Ni-co-doped In2O3

Observation of room-temperature ferromagnetism in Fe- and Ni-co-doped In₂O₃ samples (In_0.9Fe_0.1−xNi_x)₂O₃ (0?x?0.1) prepared by citric acid sol-gel auto-igniting method is reported. All of the samples with intermediate x values are ferromagnetic at room-temperature. The highest saturation magnetization (0.453 μ_B/Fe+Ni ions) moment is reached in the sample with x=0.04. The highest solubility of Fe and Ni ions in the In₂O₃ lattice is around 10 and 4 at%, respectively. The 10 at% Fe-doped sample is found to be weakly ferromagnetic, while the 10 at% Ni-doped sample is paramagnetic. Extensive structure including Extended X-ray absorption fine structure (EXAFS), magnetic and magneto-transport including Hall effects studies on the samples indicate the observed ferromagnetism is intrinsic rather than from the secondary impurity phases.