Enhancement of giant magneto-impedance effect in Ni80Fe20/SiO2/Cu composite wires

Ni₈₀Fe₂₀/SiO₂/Cu composite wires of Cu core 60 μm in diameter and coated with layers of SiO₂ and Ni₈₀Fe₂₀ were prepared by RF magnetron sputtering. The influences of the insulator layer thickness, the measurement mode and the magnitude of the driving current on the giant magneto-impedance (GMI) effect were investigated. The results showed that there was an optimum thickness of the insulator layer and the driving current can influence the shape of the MI curve. Resonance enhancement of the GMI was found in the new measurement mode. The results are discussed by taking account of the electromagnetic interactions.     

Ni80Fe20 permalloy nanoparticles: Wet chemical preparation, size control and their dynamic permeability characteristics when composited with Fe micron particles

Ni₈₀Fe₂₀ permalloy nanoparticles (NPs) have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be precisely controlled in the size range of 20-440 nm by proper addition of K₂PtCl₄ agent. X-ray diffraction results show that the Ni-Fe NPs are of FCC structure, and a homogeneous composition and a narrow size distribution of these NPs have been confirmed by scanning electron microscopy assisted with energy dispersion spectroscopy of X-ray (SEM-EDX). The saturation magnetization of ~440nm NPs is 80.8 emu/g that is comparable to that of bulk Ni₈₀Fe₂₀ alloys, but it decreases to 28.7 emu/g for ~20 nm NPs. The coercive force decreases from 90 to 3 Oe with decreasing NP size. The wide range of particle size is exploited to seek for high permeability composite particles. The planar type samples composed of the NiFe NPs exhibit low initial permeability due to the deteriorated magnetic softness and low packing density. However, when they are mixed with Fe micron particles, the initial permeability significantly increases depending on the mixing ratio and the NiFe NP size. A maximum initial permeability is achieved to be ~9.1 at 1 GHz for the Fe-10 vol%NiFe (~20 nmΦ), which is about three times that of pure Fe micron particles. The effects of Ni-Fe particle size, volume percentage and solvent on the static and dynamic permeability are discussed.     

Domain observation and transition from giant magnetoresistance to anisotropy magnetoresistance in Ag1−x(Ni0.8Co0.2)x granular films

A series of Ag_1−x(Ni_0.8Co_0.2)_x granular film samples were prepared using an ion-beam cosputtering technique. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed to investigate the microstructure of these samples. The results measured using a vibrating sample magnetometer (VSM) show a gradual change from superparamagnetism to ferromagnetism as x increases in these samples. Magnetoresistance was measured using a conventional four terminal method at room temperature. As x increases, a transition from giant magnetoresistance (GMR) to anisotropic magnetoresistance (AMR) has been observed. The stripe-type domains have been observed using magnetic force microscopy (MFM) in the high x samples, and the domains gradually disappear as x decreases. It suggests that the transition from GMR to AMR may result from intergranular interaction (not only dipolar) in the samples as x increases.     

Magnetic structures in Ni80Fe20/Ru multilayers

Using polarized neutron reflectivity (PNR) measurements together with associated simulation, magnetic structures of two Ni₈₀Fe₂₀ (1 1 1)/Ru (0 0 0 1) multilayer samples with Ru thickness of 9 and 21 Å were investigated under various external magnetic fields at room temperature. The results reveal the existence of layer thickness, interface roughness, magnetic moment, interlayer coupling angle and non-collinear coupling. The former three are independent of Ru thickness; while the latter two are strongly dependent of Ru thickness.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

Giant magneto-impedance effect of magnetron sputtered Ni80Fe20/SiO2/Cu composite wires

In this work, Ni₈₀Fe₂₀/Cu and Ni₈₀Fe₂₀/SiO₂/Cu composite wires of Cu core 100 μm in diameter and coated with a layer of Ni₈₀Fe₂₀ were produced by RF magnetron sputtering. In order to obtain a uniform coating, the wires were spun during sputtering. The influences of the magnetic coating and insulator thickness on the GMI effect of the composite wires were investigated. The results showed that the film thickness has a significant effect on the magnitude and the optimum frequency of the GMI effect. After the addition of an insulator layer, the MI ratio of the composite wires was observed to change with varying thickness of the insulator layer. This observed trend was attributed to the interaction between the conductive layer and the high-permeability magnetic coating.     

Current density effect on magnetic properties of nanocrystalline electroplated Ni80Fe20/Cu composite wires

In developing high-sensitivity micro sensors for very weak magnetic field, extremely high permeability magnetic material is essential for the sensing element. In this study, the effect of plating current density in nanocrystalline electrodeposition of permalloy on the crystal grain size and consequently on the soft magnetic properties of the deposited layer of Ni₈₀Fe₂₀/Cu composite wire is investigated. It is found that the coercivity of the deposited Ni₈₀Fe₂₀ increases and the MI effect ratio of the Ni₈₀Fe₂₀/Cu wire decreases with increasing current density in the lower range of current density (0.6–2 A/dm²) while the opposite trend is observed as the current density range increases in a higher range (2–8 A/dm²). It seems that increasing plating current density has the effect of decreasing the crystal grain size of the plated material, resulting in lower coercivity of the plated material. This effect, due to decreased grain size, is dominating in the higher range of plating current density. However, it also has the effect of increasing residual stresses in the plated material, which is dominating in the lower range of plating current density, resulting in higher coercivity of the plated materials.     

PLD of Fe3O4 thin films: Influence of background gas on surface morphology and magnetic properties

Ablation of Fe₃O₄ targets has been performed using a pulsed UV laser (KrF, λ = 248 nm, 30 ns pulse duration) onto Si(100) substrates, in reactive atmospheres of O₂ and/or Ar, with different oxygen partial pressures. The as-deposited films were characterised by atomic force microscopy (AFM), X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS) and extraction magnetometry, in order to optimise the deposition conditions in the low temperature range. The results show that a background mixture of oxygen and argon improves the Fe:O ratio in the films as long as the oxygen partial pressure is maintained in the 10⁻² Pa range. Thin films of almost stoichiometric single phase polycrystalline magnetite, Fe_2.99O₄, have been obtained at 483 K and working pressure of 7.8 × 10⁻² Pa, with a high-field magnetization of ∼490 emu/cm³ and Verwey transition temperature of 112 K, close to the values reported in the literature for bulk magnetite.     

Controlled domain wall motion by current into patterned-U Ni80Fe20 wires

The current-induced domain wall motion was observed experimentally in the case of the domain wall trapped at the semicircular arc within the U shape Ni₈₀Fe₂₀ wire. The measurement of the current-induced domain wall motion was achieved by adding a biased field before switching field and a critical current density was measured. We found two magnetic domain structures in the U pattern. At zero fields, the vortex domain wall nucleated at the semicircular arc of the U pattern. Continuous magnetic state without wall was investigated in near-switching field.     

Perpendicular magnetic anisotropy of FePt film on Si substrate with SiO2 underlayer and B4C interlayers

FePt multilayer composite films with and without B₄C interlayer have been prepared by magnetron sputtering, respectively, and subsequent annealing in vacuum. It was found that the B₄C layers effectively serve as spacers to separate the FePt layers, enhancing (0 0 1) orientation of FePt alloy. Our results show that highly (0 0 1) oriented FePt/B₄C films have significant potential as perpendicular recording media.     

Nanostructure and excellent magnetic properties of Co19.35Fe53.28Hf7.92O19.35 films

This letter reports the novel nanostructure and excellent magnetic properties of Co_19.35Fe_53.28Hf_7.92O_19.35 films with varying thicknesses. Among the samples investigated, the film with a thickness of 432 nm exhibits the most excellent magnetic properties: high saturation magnetization, , low coercivity, , and high hard-axis anisotropy field of . The magnetic permeability remains almost stable up to 3 GHz and reaches a maximum at the ferromagnetic resonant frequency of 4.024 GHz. The excellent magnetic characteristics of this film in addition to a very high electrical resistivity of 3569 μΩ cm make it ideal for uses in high-frequency applications of micromagnetic devices. It reveals that these superior properties are ascribed to the formed peculiar nanostructure. A magnetic phase separation appears to occur strongly as the film thickness increases over 437 nm, which, in turn, modifies the high-frequency behavior.     

Preparation and magnetoresistance of thin CrO2 films

Highly textured chromium dioxide (CrO₂) films have been deposited on Al₂O₃ single-crystal substrates by atmospheric pressure chemical vapor deposition method (CVD). X-ray diffraction patterns show that the CrO₂ films are (1 0 0)-oriented on Al₂O₃ (0 0 1) substrates, and are (1 0 1)-oriented on Al₂O₃ (0 1 2) substrates. Scanning electron microscopy images indicate that the (1 0 0)-oriented CrO₂ films grown on Al₂O₃ (0 0 1) substrates have smoother surface and better qualities than that grown on Al₂O₃ (0 1 2) substrate. At room temperature, the magnetoresistance of the (1 0 0)- and (1 0 1)-oriented CrO₂ films are nearly same, and both show a linear dependence on applied magnetic field. While at 80 K, the (1 0 1)-oriented CrO₂ films show a much larger magnetoresistance compared with the (1 0 0)-oriented CrO₂ films. The reasons are briefly discussed.     

Exchange bias and vertical shift in CoFe2O4 nanoparticles

Magnetic properties of core–shell cobalt ferrite nanoparticles prepared by co-precipitation route in the range 15–48 nm have been studied. It is shown that the coercivity follows non-monotonic size dependence and exhibits a peak at around 26 nm. Field-cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region (with structural and spin disorder) and a core ferrimagnetic region. The dependence of the exchange bias and vertical shift on the particle size and cooling field is found to have significant differences. These differences are explained in the light of recent results that suggest that there is a variation of the pinning strength amongst the interface spins and the vertical shift is affected by the more strongly pinned uncompensated spins.     

Formation and magnetic properties of mechanically alloyed Al65Cu20Fe15 quasicrystal

The formation of icosahedral phase by mechanical alloying of crystalline elemental powder of Al, Cu and Fe has been investigated. The effect of milling time on the formation of icosahedral phase of nominal composition of Al₆₅Cu₂₀Fe₁₅ has been studied using the X-ray diffraction technique. Further studies have been carried out by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), particle size, magnetisation and ferromagnetic resonance studies. All these studies indicate that the icosahedral alloy shows soft ferromagnetic behaviour.     

Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route

Magnetic nanoparticles of cobalt ferrite have been synthesized by wet chemical method using stable ferric and cobalt salts with oleic acid as the surfactant. X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) confirmed the formation of single-phase cobalt ferrite nanoparticles in the range 15–48 nm depending on the annealing temperature and time. The size of the particles increases with annealing temperature and time while the coercivity goes through a maximum, peaking at around 28 nm. A very large coercivity (10.5 kOe) is observed on cooling down to 77 K while typical blocking effects are observed below about 260 K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles.     

Magnetic and structural study of Cu-doped TiO2 thin films

Transparent pure and Cu-doped (2.5, 5 and 10 at.%) anatase TiO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. The samples were structurally characterized by X-ray absorption spectroscopy and X-ray diffraction. The magnetic properties were measured using a SQUID. All films have a FM-like behaviour. In the case of the Cu-doped samples, the magnetic cycles are almost independent of the Cu concentration. Cu atoms are forming CuO and/or substituting Ti in TiO₂. The thermal treatment in air promotes the CuO segregation. Since CuO is antiferromagnetic, the magnetic signals present in the films could be assigned to Cu substitutionally replacing cations in TiO₂.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Investigating the formation and magnetic properties of the Dy0.75Fe1.25O3 orthoferrite prepared by sol-gel method

In this paper, the Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles were synthesized successfully by sol-gel method. Dy_0.75Fe_1.25O₃ orthoferrite nanoparticles are obtained by calcining the flakes at 600 and 700 °C. The magnetic properties of the different samples are investigated using Quantum Design MPMS SQUID magnetometer and MS-500 Mössbauer spectrometer. Magnetic phase γ-Fe₂O₃ coexists in the samples calcined at 600 °C and orthoferrite phase is completely recovered in the samples calcined at 700 °C. Although excessive Fe³⁺ ions were introduced, none of these iron spins couple magnetically with Dy³⁺ ions.     

Fabrication and magnetic properties of Ce1Y2Fe5O12 thin films on GGG and SiO2/Si substrates

Microstructure and magnetic properties of crystalline Ce₁Y₂Fe₅O₁₂ thin films prepared on GGG and on SiO₂/Si substrates by pulsed laser deposition were studied. The results show that highly textured Ce₁Y₂Fe₅O₁₂ film with (4 4 4) preferred orientation prepared on GGG (1 1 1) shows strong paramagnetism superimposed by a weak ferromagnetism. However, polycrystalline Ce₁Y₂Fe₅O₁₂ thin films on SiO₂/Si, which can only be obtained after post-annealing, show strong ferromagnetism with easy axis of magnetization lying in the plane of the film. With post-annealing temperature increasing, CeO₂ segregates from Ce₁Y₂Fe₅O₁₂; then YIG continues to be decomposed, forming Fe₂O₃. Consequently, the saturation magnetization of Ce₁Y₂Fe₅O₁₂ films decreases first and then increases correspondingly, which indicates that the magnetic properties of Ce₁Y₂Fe₅O₁₂ films are mainly related to the microstructure.     

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.