Effect of thermal deformation on giant magnetoresistance of flexible spin valves grown on polyvinylidene fluoride membranes

We fabricated flexible spin valves on polyvinylidene fluoride(PVDF) membranes and investigated the influence of thermal deformation of substrates on the giant magnetoresistance(GMR) behaviors. The large magnetostrictive Fe_(81)Ga_(19)(Fe Ga) alloy and the low magnetostrictive Fe_(19)Ni_(81)(Fe Ni) alloy were selected as the free and pinned ferromagnetic layers.In addition, the exchange bias(EB) of the pinned layer was set along the different thermal deformation axes α_(31) or α_(32) of PVDF. The GMR ratio of the reference spin valves grown on Si intrinsically increases with lowering temperature due to an enhancement of spontaneous magnetization. For flexible spin valves, when decreasing temperature, the anisotropic thermal deformation of PVDF produces a uniaxial anisotropy along the α_(32) direction, which changes the distribution of magnetic domains. As a result, the GMR ratio at low temperature for spin valves with EB α_(32)becomes close to that on Si, but for spin valves with EB α_(31)is far away from that on Si. This thermal effect on GMR behaviors is more significant when using magnetostrictive Fe Ga as the free layer.     

Large-scale synthesis and magnetic properties of cubic CoO nanoparticles

We present the synthesis, microstructural and magnetic characterization of cubic CoO nanoparticles with well-controlled size and shape. The as-synthesized CoO nanoparticles are stable because of the organic coating that occurred in situ. The Néel temperature is 225 and 280 K for the 42 and 74 nm CoO particles, respectively. The CoO nanoparticles exhibit anomalous magnetic properties, such as large moments, coercivities and loop shifts. These results provide evidence for the formation of spin compensated random system in CoO. The structurally distorted and magnetically disordered surface layer ferromagnetic phase played an important role in the magnetic behavior of CoO nanoparticles. The smaller is the particle size, the stronger is the contribution of the ferromagnetic phase and the more is the surface layer helpful to enhance the observed coercivity and the exchange bias.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

Spin-glass-like behavior in CoO nanoparticles and the origin of exchange bias in layered CoO/ferromagnet structures

The magnetic behavior of CoO nanoparticles and layered CoO/ferromagnetic (FM) structures has been investigated by magnetization and hysteresis loop measurements. In the amorphous CoO, a large uncompensation of spins is found that is closely related to spin-glass-like behavior below a freezing temperature T(F) approximately 215-220 K. The spin-glass-like phase may be described by the de Almeida-Thouless line for Ising spin systems. The exchange bias in the layered CoO/FM structures is explained by the spin-glass-like state in the nanoparticles constituting the CoO film.     

Diluted antiferromagnets in exchange bias: proof of the domain state model

The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias.     

Improving exchange-coupling field in the same thickness of pinned magnetic layer

A conventional Ta/NiFe/Cu/NiFe/FeMn spin valve was prepared to investigate the exchange bias properties with the variations of deposition field. By enhancing the deposition magnetic fields from 50 to 650 Oe, increase of exchange bias fields at a given thickness of the pinned NiFe layer has been found in the spin valves. In this paper, we show that this increase is due to the change of magnetic moment distribution at the ferromagnetic and antiferromagnetic interface by comparison of measured results with the interfacial uncompensated model. Therefore, by enhancing deposition magnetic fields, a large exchange-coupling field can be achieved in relatively thicker magnetic films for application.     

Effect of the nonmagnetic layer in a Co/Cu/CoO trilayer structure on the exchange coupling in it

The dependence of the exchange bias of epitaxial single-crystal Co/Cu/CoO trilayer films on the copper layer thickness and temperature is studied. The exchange bias of the hysteresis loops of the ferromagnetic cobalt layer as a function of the copper layer thickness is found to have a well-pronounced oscillating character. The oscillations manifest themselves over the entire temperature range in which an exchange bias takes place (77–220 K). The complex variation of the oscillation amplitude with the nonmagnetic layer thickness can be explained by the superposition of two interlayer exchange coupling oscillation periods (λ₁ ≈ 10–11 Å, λ₂ ≈ 20 Å) having differentamplitudes and temperature dependences.     

Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

Micromagnetic Simulation of Transfer Curve in Giant-Magnetoresistive Head

The transfer curve of the giant-magnetoresistive （GMR） magnetic head represents its most important property in applications, and it is calculated by the micromagnetic modeling of the free layer and the pinned layer in the heart of the GMR head. Affections of the bias hard magnetic layer and the anti-ferromagnetic pinning layer are modeled by effective magnetic fields. The simulated transfer curve agrees with experiment quite well, therefore the values of these effective magnetic fields can be determined by the model. A synthetic antiferromagnetic spin valve structure GMR head is also analyzed for comparison.     

O2 plasma oxidation effect on magnetic properties of spin valves

The use of nano-oxide to improve the performance of spin valves has been extensively studied. But most of the investigations so far have been carried out on different samples. This may make some of the conclusions drawn from the experiments inconsistent because of the fluctuation in deposition conditions and device structures. In this work, the effect of nano-oxide on the properties of spin valves has been investigated through post-growth oxidation of the same sample in oxygen plasma for different rf powers and durations. The sample investigated was a bottom spin valve with the structure Si/SiO₂/Ta/NiFe/IrMn/CoFe/Cu/CoFe/Ta. A relative increase of 20% and 12% was obtained in the giant magnetoresistance (GMR) ratio of as-deposited and annealed samples, respectively. It was found that, at a fixed rf power, there is a peak of the GMR ratio as the oxidation time increases. A higher peak value of the GMR ratio was obtained for lower rf power, although the required oxidation time is longer. This result can be well understood by considering both the enhanced specularity at the insulator/metal interface and the loss of magnetic effective thickness of the free layer by the oxidation. Magnetic parameters such as the interlayer coupling field (H₀) and the coercivity of the free layer (H_cf) were also greatly influenced by the oxidation process. When only the Ta layer was oxidized, H₀ increases very slightly, and H_cf increases with the oxidation time. However, when the CoFe free layer was oxidized, a significant increase was found for H₀, and H_cf changes to decreasing. These results can be explained based on the Néel and RKKY coupling models. Received: 25 October 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002     

Microstructural and magnetic properties of passivated Co nanoparticle films

Co nanoparticle films were prepared by plasma–gas-condensation-type particle beam deposition system. High-resolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of 20 nm, and each of the Co nanoparticles is covered with an 3 nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5 T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process.     

Dual spin valves with nano-oxide layers

A novel method is proposed for increasing the giant magnetoresistance (GMR) of dual spin valves of the CoFe/Cu/CoFe/Cu/CoFe type by inserting nano-oxide layers (NOLs) into the pinned layers. Using this method, MR ratio of 23.5% was obtained, a value equal to those of specular spin valves with antiferromagnetic oxide, e.g., NiO. This method allows the selection of metallic materials for the antiferromagnetic layer. In addition, we obtained the specularity factor of upper NOLs, 0.8, and that of lower NOLs, 0.7, by calculating Boltzmann equations taking into account the roughness of each interlayer. This model shows that the MR ratio would be 27.5% for dual spin valves with ideal NOLs.     

A study of the induced magnetism in the Au spacer layer of Co/Au/CoO exchange-bias trilayers and related systems

We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at a magnetic/non-magnetic interface, applying low temperature nuclear orientation (LTNO) to Co/Au(x)/CoO trilayer systems. This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au γ-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurements on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.     

Enhancing exchange bias with diluted antiferromagnets

The exchange bias H(E) of coupled polycrystalline films of antiferromagnetic CoO and ferromagnetic Co was significantly enhanced by the systematic substitution of nonmagnetic Mg for Co in CoO. Samples in which either Co or Co(1-x)Mg(x)O were deposited first were investigated at temperatures from 10 to 300 K. With Co(1-x)Mg(x)O on the bottom, the increased interfacial uncompensated spin density of the single antiferromagnetic domain Co(1x)Mg(x)O crystallites produced the enhanced H(E). With Co on the bottom, a thin interfacial oxide layer was primarily responsible for the strongly increased H(E).     

IrMn基反铁磁自旋阀的巨磁电阻效应

用第一性原理方法研究了在微观尺度具有三重对称磁结构的IrMn合金的反铁磁自旋阀(AFSV)的电子输运.研究表明:基于有序L1₂相IrMn合金的Co/Cu/IrMn自旋阀的巨磁电阻(GMR)效应具有三重对称性,可以利用这一特性区分反铁磁材料的GMR与传统铁磁材料的GMR.基于无序γ相IrMn合金的IrMn(0.84 nm)/Cu(0.42 nm)/IrMn(0.42 nm)/Cu(0.42 nm)(111) AFSV的电流平行平面构型的GMR约为7.7％,大约是电流垂直平面构型的GMR(3.4％)的两倍,明显大于实验中观测到的基于共线磁结构的FeMn基AFSV的GMR. 关键词： 反铁磁自旋阀 巨磁电阻效应 非共线磁结构 电流平行平面结构     

Shape-induced oscillation of exchange bias in the Co nanopillar with lateral oxidation

A modified Monte Carlo Metropolis method is performed to simulate the hysteresis loops of a laterally oxidized Co nanopillar with an excised arc or an inclined circular plane. At low temperature after field cooling, an oscillatory exchange bias is observed with excised length or angle of inclination. However, the behaviors of coercivity exhibit a monotonous decrease with excision but an oscillation with tilt. The phenomena are intriguing and corroborated by means of the microscopic spin configurations and the magnetization reversal mechanisms. The occurrence of oscillatory exchange bias in such a Co/CoO composite nanopillar with direct exchange couplings is mainly because of the shape inducement in the nanopillar, which results in the energy fluctuations of interfacial coupling and the nonuniformity of domain distribution.     

Ferromagnetic/antiferromagnetic exchange coupling in SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/CoO composites

The surface of SrFe₁₂O₁₉ coated with a CoO layer reveals a strong exchange bias characterized by magnetic hysteresis loops. The low-temperature coercivity, H_C, and the squareness, M_R/M_S, of a permanent magnet of SrFe₁₂O₁₉/CoO powder prepared by the sol–gel method are enhanced after field cooling through the Néel temperature (T_N=290 K) when compared to those after zero-field cooling. The existence of loop shifts and the enhancement of H_C indicate that exchange-bias effects, which are induced by the ferromagnetic/antiferromagnetic (FM/AFM) exchange-coupling interactions, are responsible for these behaviors. According to our experimental results, some of the factors controlling the exchange bias, such as FM/AFM interfaces and the CoO amount of the antiferromagnetic layer, are discussed. PACS 75.30.Et; 75.50.Ee; 75.60.Ej; 75.60.Gm; 75.70.Cn     

Soft X-ray resonant magnetic scattering studies on Fe/CoO exchange bias system

We have used soft X-ray resonant magnetic scattering (XRMS) to search for the presence of an effective ferromagnetic moment belonging to the antiferromagnetic (AF) layer which is in close contact with a ferromagnetic (F) layer. Taking advantage of the element specificity of the XRMS technique, we have measured hysteresis loops of both Fe and CoO layers of a CoO(40 Å)/Fe (150 Å) exchange bias bilayer. From these measurements we have concluded that the proximity of the F layer induces a magnetic moment in the AF layer. The F moment of the AF layer has two components: one is frozen and does not follow the applied magnetic field and the other one follows in phase the ferromagnetic magnetization of the F layer. The temperature dependence of the F components belonging to the AF layer is shown and discussed.     

Progress in spintronics

The discovery of the giant magnetoresistance (GMR) by Peter Grünberg and Albert Fert in 1988, which was awarded with the Nobel Prize for Physics in 2007, initiated an upsurge of experimental and theoretical investigations on spin dependent transport phenomena. Since then, spin valves have been introduced, switching via spin torque was proposed and confirmed, the tunneling magneto-resistance effect has matured to marketability, and magnetic domain walls and their propagation are being developed for memory storage devices with enhanced density. This field, which embraces spin-structures and spin-transport on the nanoscale, was coined spin electronics or short spintronics. A brief overview on the development of spintronics from the early discovery of the GMR effect to the present activities is provided.     

Perpendicular exchange bias of Co/Pt multilayers

Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.