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.     

Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.     

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.     

Electrodeposition and characteristics of Ni80Fe20/Cu composite wires

Electrodeposited Ni₈₀Fe₂₀/Cu composite wire has significant advantages over amorphous wire as sensing element for weak magnetic field sensors, such as orthogonal fluxgate sensors, due to its non-ferromagnetic and conductive core structure. In this study, the key processing parameters are investigated, including electrodeposition current density, duty cycle, electrolyte solution, pH value, applied magnetic field, effect of seeded layers, and post annealing.     

真空退火对Ni80Fe20/Cu多层膜微结构的影响

用直流磁控溅射法在Si(001)衬底上制备了以Ta为缓冲层、含有15周期的Ni₈₀Fe₂₀（4nm)/Cu(6nm)多层膜.样品分别在150,250,350℃进行了真空退火处理.用低角和高角X射线衍射法研究了多层膜的微结构.结果表明,所有样品均有较好的[111] 取向,而且随退火温度或时间的增加,[111]取向程度变得更高.超晶格周期、平均面间距在退火后略有减小,表明多层膜结构在退火后变得更为致密.多层膜界面粗糙度随退火温度或时间的增加而增大,平均相关长度随退火温度或时间的增加而减小,分析认为这是由于Ni₈₀Fe₂₀/Cu界面存在严重的互扩散所导致的.模拟Ni₈₀Fe₂₀/Cu多层膜高角X射线衍射谱,发现在Ni₈₀Fe₂₀/Cu蜀面有非常厚的混合层存在,而且混合层厚度随退火温度或时间的增加而增大.模拟结果还表明,随退火温度或时间的增加,Ni₈₀Fe₂₀层面间距几乎保持不变,Cu层面间距则随退火温度的增加而略有减小. 关键词：     

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.     

磁性隧道结Ni80Fe20/Al2O3/Co的研究

用等离子体氧化形成中间绝缘层的方法可重复制备出具有隧道磁电阻（TMR)效应的Ni₈₀Fe₂₀/Al₂O₃/Co磁性隧道结.光透射谱等实验结果表明等离子体氧化能可控制地制备较致密的Al₂O₃绝缘层.样品的TMR比值在室温下最高可达6.0％,反转场可低于800A/m,相应的平台宽度约为2400A/m.结电阻R_j的变化范围从百欧到几百千欧,并且TMR比值随零磁场结偏压增大单调减小. 关键词：     

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.     

Synthesis and characterization of electrodeposited permalloy (Ni80Fe20)/Cu multilayered nanowires

Permalloy (Ni₈₀Fe₂₀)/Cu multilayered nanowires (NWs) were electrodeposited using a template directed method from sulfate baths via pulse potential technique. Microstructures and compositions of the nanowires were characterized using various microscopy and spectroscopy techniques. To synthesize compositionally uniform nanowires with high efficiency, new sulfate baths with a high content of Ni²⁺ were developed. The effects of deposition potential and concentration of metal ions were optimized to reduce composition inhomogeneity and incorporation of copper in the permalloy layers. Composition of the NiFe layers was found to be close to 20 at% Fe with a maximum of 5 at% Cu. TEM analysis indicated that individual nanowires exhibit distinct and coherent layering structure with rough and wavy interfaces. A synthesized single nanowire was also AC dielectrophoretically assembled across the microfabricated gold electrodes for subsequent magnetoresistance measurements.     

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.     

合成反铁磁Ni81Fe19/Co90Fe10/Ru/Co90Fe10/Ni81Fe19的研究

采用直流磁控溅射方法制备了一系列的合成反铁磁及以其为自由层的自旋阀.研究发现,在Ni₈₁Fe₁₉与Ru层之间插入适当厚度的Co₉₀Fe₁₀层后,可有效地提高合成反铁磁两磁性层间的反铁磁耦合强度,得到具有饱和场H_s更高、饱和磁化强度M_s更低、热稳定性更好的合成反铁磁.另外,以这种合成反铁磁作自旋阀的自由层时,可有效提高自旋阀的稳定性. 关键词： 合成反铁磁 退火 自旋阀     

Spin wave resonance study of exchange-coupled Ni80Fe20 double layers separated by thin interlayers of Cu, Cr and Pd

We have performed ferromagnetic resonance measurements on thin polycrystalline Ni₈₀Fe₂₀ (permalloy) double layers with interlayers of Cu, Cr and Pd. Furthermore, a mathematical model is presented, which allows a calculation of resonance fields in the parallel configuration. Comparison of the calculated and measured resonance field values yields an interlayer exchange coupling constant A₁₂ as a function of interlayer thickness d. We observe exponential dependencies of A₁₂ on d for Cr, Cu and Pd interlayers with the strongest decay for Cr. Additionally, we present FMR measurements and calculations for a Ni₈₀Fe₂₀/Cr/Co heterostructure layer system.     

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.     

Measurement and simulation of specific heat for metastable liquid Ni80Fe10Cu10 alloy

The specific heat of undercooled liquid Ni₈₀Fe₁₀Cu₁₀ alloy was experimentally measured by electromagnetic levitation drop calorimeter, and also numerically simulated by the molecular dynamics method. The achieved maximum undercooling is up to 252 K (0.15 T _L) in the experiments, and the measured result is 41.67 J mol⁻¹ K⁻¹. The simulation provides calculated data within 0∼702 K undercooling range, which is much broader than the experimental regime. The simulated value is 37.02 J mol⁻¹ K⁻¹. Although there exists a difference of 4.65 J mol⁻¹ K⁻¹ between them, the result is quite acceptable for simulation. Furthermore, the liquid structure of undercooled Ni₈₀Fe₁₀Cu₁₀ alloy is studied in terms of the total and partial pair distribution functions, which display that the ordered degree of atoms enhances from a normal liquid to metastable state.     

Ni81Fe19/Cr82Al18双层膜中的交换偏置

研究了直流磁控溅射法制备的Ni₈₁Fe₁₉/Cr₈₂Al₁₈双层膜中的交换耦合.样品的室温矫顽力与1/t^{3/2_FM(t_FM为铁磁层厚度）近似成正比例关系,从而表明在Ni₈₁Fe₁₉/Cr₈₂Al₁₈中交换耦合为铁磁/反铁磁界面的随机相互作用.另外还讨论了反铁磁层厚度对交换偏置的影响. 关键词：}     

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.     

Effect of severe plastic deformation by torsion on the properties and structure of the Ni54Mn21Ga25 and Ni54Mn20Fe1Ga25 alloys

The effect of one-percent substitution of iron for manganese on the physical (magnetic, electrical, thermal, and galvanomagnetic) properties and the crystal structure of the Ni₅₄Mn₂₁Ga₂₅ alloy has been investigated. It has been demonstrated that the deviation of the alloy composition from the stoichiometric composition Ni₅₀Mn₂₅Ga₂₅ leads to the formation of a mixed ferromagnetic-antiferromagnetic state. The atomic disordering and nanostructuring of the alloys under investigation due to the severe plastic deformation by torsion in Bridgman anvils to sizes of 10–20 nm result in the suppression of reversible magnetically controlled shape memory effects.     

Medium-range structural order in liquid Ni20Al80 alloy: Experimental and molecular dynamics studies

In this Letter, structural properties of liquid Ni₂₀Al₈₀ have been studied through molecular dynamics (MD) simulations, and results agree well with X-ray experiments. The prepeak in the structure factor manifests the existence of medium-range structural order (MRSO) in liquid Ni₂₀Al₈₀. Analyses in MRSO reveal the dominance of ideal icosahedra and defective icosahedra which, furthermore, form clusters and share common atoms.     

Destruction of pressure-induced superconductivity by long-range antiferromagnetic order in Tm2Fe3Si5

Reentrant superconductivity has been detected in a well characterized sample of Tm₂Fe₃Si₅ by ac susceptibility measurements under applied pressure. The superconducting transition temperature displays a pronounced, non-linear behavior under pressure, starting at low pressures with an increase which is among the largest known for any compound (dT_c1/dp = 0.47 K/kbar). The magnetic transition temperature is relatively pressure independent.     

BiFeO3/Ni81Fe19磁性双层膜中的交换偏置及其热稳定性研究

研究了磁场诱导生长的BiFeO₃/Ni₁₈Fe₁₉磁性双层膜中 的交换偏置及其热稳定性. 结果表明: BiFeO₃/Ni₁₈Fe₁₉双层膜中的交换偏置场H_ex未表现出明显的磁练习效应. 在负饱和磁场等待过程中, BiFeO₃/Ni₁₈Fe₁₉双层膜磁滞回线的前支和后支曲 线都随着在负饱和磁场中等待时间t_sat的增加向正场方向偏移. 交换偏置场H_ex的大小随着等待时间t_sat的增加而减小, 矫顽力H_c基本不变. 交换偏置场H_ex的大小随测量温度T_m的升高变化不明显, 表现出良好的热稳定性; 但矫顽力H_c随T_m的升高而显著减小. 良好的热稳定性应该来源于铁电性和反铁磁性间的共同耦合作用. 关键词： 多铁性 磁性薄膜 交换偏置 热稳定性