Interfacial roughness and angle dependence of giant magnetoresistance in magnetic superlattices

Using the two-point conductivity formula, we numerically evaluate the giant magnetoresistance (GMR) in magnetic superlattices with currents in the plane of the layers (CIP), from which the effect of the interfacial roughness and magnetization configuration on the GMR is studied. With increasing interfacial roughness, the maximal GMR ratio is found to first increase and then decrease, exhibiting a peak at an optimum strength of interfacial roughness. For systems composed of relatively thick layers, the GMR is approximately proportional to ,where is the angle between the magnetizations in two successive ferromagnetic layers, but noticeable departures from this dependence are found when the layers become sufficiently thin. Received 21 September 1998 and Received in final form 22 December 1998     

Degradation of the Giant Magnetoresistance in Fe/Cr Multilayers Due to Ar-Ion Beam Mixing

The influence of 200 keV Ar-ion irradiation on the interlayer coupling in the Fe/Cr multilayer system exhibiting the giant magnetoresistance effect (GMR) is studied by conversion electron Mössbauer spectroscopy (CEMS), VSM hysteresis loops, magnetoresistivity and electric resistivity measurements and supplemented by the small-angle X-ray diffraction (SAXRD). The increase of Ar ion dose causes an increase of interface roughness, as evidenced by the increase of the Fe step-sites detected by CEMS as a result of which the GMR gradually decreases and vanishes at doses exceeding 1×10¹⁴ Ar/cm². A degradation of GMR with increasing Ar-ion dose is related to the formation of pinholes between Fe layers and the decrease of the antiferromagnetically coupled fraction.

     

Current-in-plane giant magnetoresistance: The effect of interface roughness and spin-depolarization due to the proximity of a buffer layer

We report on proximity effects of a Au buffer layer on the current-in-plane giant magnetoresistance effect (CIP-GMR) in high-quality, epitaxial Fe/Cr/Fe(001) trilayers. The lower Fe layer is grown in the shape of a wedge and allows simultaneous preparation of 24 GMR stripe-elements with different lower Fe thicknesses in the range from 13 to 14.5 ML. The layer-by-layer growth mode in combination with the small thickness variation gives rise to: (i) well-controlled roughness changes from stripe to stripe as confirmed by reflection high-energy electron diffraction (RHEED), and (ii) to a varying influence of the underlying Au buffer. The oscillatory roughness variation along the wedge yields an oscillatory GMR behavior as a function of Fe thickness and confirms the previous result that slightly increased interface roughness causes a higher GMR ratio. The proximity of the Au buffer to the GMR trilayer results in an increase of the GMR ratio with increasing Fe thickness. The latter effect is explained by spin-depolarization at the Fe/Au interface and in the bulk of the Au buffer.     

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.     

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.     

High efficiency giant magnetoresistive device based on two-dimensional MXene (Mn2NO2)

Due to the unique electronic structure of half-metals, characterized by the conductivity of majority-spin and the band gap of minority-spin, these materials have emerged as suitable alternatives for the design of efficient giant magnetoresistive (GMR) devices. Based on the first-principles calculations, an excellent GMR device has been designed by using two-dimensional (2D) half-metal Mn₂NO₂. The results show that Mn₂NO₂ has sandwiched between the Au/nMn₂NO₂ (n = 1, 2, 3)/Au heterojunction and maintains its half-metallic properties. Due to the half-metallic characteristics of Mn₂NO₂, the total current of the monolayer device can reach up to 1500 nA in the ferromagnetic state. At low voltage, the maximum GMR is observed to be 1.15 × 10³¹ %. Further, by increasing the number of layers, the ultra-high GMR at low voltage is still maintained. The developed device is a spintronic device exhibiting the highest magnetoresistive ratio reported theoretically so far. Simultaneously, a significant negative differential resistance (NDR) effect is also observed in the heterojunction. Owing to its excellent half-metallic properties and 2D structure, Mn₂NO₂ is an ideal energy-saving GMR material.     

Perpendicular giant magnetoresistance in magnetic multilayered nanowires

We present giant magnetoresistance (GMR) measurements performed on electrodeposited Co/Cu multilayered nanowires. The variation of the GMR with the thicknesses of the Cu and Co layers over wide ranges is discussed in the framework of the Valet-Fert model for perpendicular GMR. The interface and bulk spin-dependent scattering parameters as well as the spin diffusion lengths in the nonmagnetic and ferromagnetic layers are extracted from this analysis.     

Design of internal Brewster guided-mode resonance filter

An internal Brewster guided-mode resonance （GMR） filter is designed. For this kind of GMR filter, the Brewster reflection occurs at the interface of grating/waveguide layers rather than at the interface of air/grating layers. At Brewster angle of 60~, the GMR filter owns almost 100% reflection at the resonance wavelength of 800 nm with the full-width at half-maximum （FWHM） of 0.2 nm. Its angle response changing with the fabrication deviation is also discussed.     

磁性多层膜的巨磁电阻随铁磁和非磁层厚度变化的唯象理论计算

根据唯象理论，并采用以铁磁─非磁混合层代替铁磁／非磁层界面的理论方法，计算了Ｆｅ／Ｃｒ多层膜的巨磁电阻随铁磁和非磁层厚度的变化关系与实验结果做了比较，发现它们符合得较好．还绘出了巨磁电阻随铁磁和非磁层厚度变化的二元函数图 关键词：     

SPIN-DEPENDENT INTERFACIAL TUNNELING AND MAGNETO RESISTANCE IN THE FAMILY OF GRANULAR PEROVSKITE La1-xSrxMnO*3

The magnetic transport properties in granular perovskite system La_1-xSr_xMnO^*₃ have been investigated. The spin-dependent inter facial tunneling and the corresponding giant magnetoresistance (GMR) effect have been observed in the whole temperature range below the Curie point T_c for the samp les with concentration x from 0.05 to 0.45. Theoretical analysis shows that the interfacial tunneling originates from the difference in magnetism between surfaces and cores, and the tunnel-type GMR stems from the field-induced change of interfacial magnetic order.     

不同过渡层上Co/Cu/Co三明治结构的巨磁电阻效应研究

通过对不同过渡层上Co(5.5nm)/Cu(3.5nm)/Co(5.5nm)三明治结构的研究,发现过渡层的磁性及过渡层诱导的三明治晶格结构对材料的巨磁电阻效应有重要影响.反铁磁Cr过渡层由于和相邻铁磁Co层之间存在着反铁磁耦合,可以获得6％以上的巨磁电阻值,但它同时使材料的矫顽力较大,因此磁灵敏度不高.Ni和Ti过渡层上Co/Cu/Co三明治结构,由于形成了强的（111）织构,其巨磁电阻值也达到5％以上.磁性材料Ni过度层还使三明治结构材料的矫顽力大为下降,从而显著提高了材料的磁灵敏度. 关键词：     

[Fe/Cr]多层膜及掺入Si中介层后的层间耦合和磁电阻效应

用真空蒸镀方法制备了［Ｆｅ／Ｃｒ］，［Ｆｅ／Ｃｒ／Ｓｉ］和［Ｆｅ／Ｓｉ］多层膜．研究了Ｃｒ层、Ｓｉ层和Ｃｒ＋Ｓｉ层厚度变化对层间耦合和磁电阻的影响．Ｆｅ层厚为２ｎｍ，Ｃｒ层厚度变化存在耦合振荡和巨磁电阻及其振荡．磁电阻值为１４.６％（４.２Ｋ）．在Ｃｒ层中加入一半Ｓｉ层或全部由Ｓｉ层替代，振荡消失，磁电阻减小到千分之几．根据掺Ｓｉ层后多层膜的电阻率变化，认为Ｓｉ加入使非磁层中自由电子数减少，随之极化效应也变弱，导致振荡消失，磁电阻大为降低 关键词：     

Interactive effect of impurities on giant magnetoresistance of Co–Fe/Cu multilayers

Giant magnetoresistance of Co–Fe–B/Cu multilayers fabricated in the sputtering atmosphere, where the amount of oxygen impurity is varied, is discussed in connection with their interfacial roughness. The magnetoresistance (MR) ratio of Co–Fe/Cu multilayers is enhanced by up to 33% when the oxygen content is varied between 10 and 100 ppm of processing Ar gas. The enhancement of the MR ratio was due to the flattening effect of impurity oxygen on the multilayer interfaces: the root mean square roughness of the multilayer was decreased from 7.5 to 5 Å. With increasing boron content in Co–Fe layers, however, the enhancing effect of MR ratio by oxygen diminished and nearly vanished for 12 at%-B–(Co–Fe) case. The strong affinity of boron for oxygen is suggested as a probable mechanism.     

Coupling-induced spectral splitting for plasmonic sensing with ultra-high figure of merit

We investigate a kind of spectral splitting effect in a plasmonic multilayer system, which consists of stacked Al_2 O_3 and SiO_2 layers, a thin metal film, and a dielectric prism substrate. The results illustrate that an obvious peak appears in the center of the surface plasmon resonance(SPR)-induced reflection spectral dip in the structure with the SiO_2/Al_2 O_3/SiO_2 layers. This spectral splitting response can be regarded as an electromagnetically induced transparency(EIT) like effect,which is attributed to the coupling and interference between the SPR on the metal film and guided-mode resonance(GMR)in the Al_2 O_3 layer. The theoretical calculations agree well with the numerical simulations. It is also found that the reflection spectrum will be further split by the introduction of another Al_2 O_3 layer into the multilayer structure. The reintroduced GMR in the Al_2 O_3 layer changes the coupling and interference process between the SPR and GMR field, giving rise to the generation of ultra-narrow reflection dip. Especially, the spectral splitting can facilitate the realization of plasmonic sensors with ultra-high figure of merit(583), which is about 5 times larger than that of traditional SPR sensors. These results will provide a new avenue to the light field manipulation and optical functionalities, especially biochemical and environmental sensing.     

Analysis of periodic Mo/Si multilayers: Influence of the Mo thickness

A set of Mo/Si periodic multilayers is studied by non-destructive analysis methods. The thickness of the Si layers is 5 nm while the thickness of the Mo layers changes from one multilayer to another, from 2 to 4 nm. This enables us to probe the effect of the transition between the amorphous and crystalline state of the Mo layers near the interfaces with Si on the optical performances of the multilayers. This transition results in the variation of the refractive index (density variation) of the Mo layers, as observed by X-ray reflectivity (XRR) at a wavelength of 0.154 nm. Combining X-ray emission spectroscopy (XES) and XRR, the parameters (composition, thickness and roughness) of the interfacial layers formed by the interaction between the Mo and Si layers are determined. However, these parameters do not evolve significantly as a function of the Mo thickness. It is observed by diffuse scattering at 1.33 nm that the lateral correlation length of the roughness strongly decreases when the Mo thickness goes from 2 to 3 nm. This is due to the development of Mo crystallites parallel to the multilayer surface.     

Effect of the structural quality of the buffer on the magnetoresistance and the exchange coupling in sputtered Co/Cu sandwiches

The effect of the structural quality of the buffer stack on the structural properties, giant magnetoresistance (GMR) and the quality of the antiferromagnetic coupling has been investigated for Co/Cu/Co sandwiches prepared by DC-magnetron sputtering. Three kinds of buffers were employed: type A: Cr(6 nm)/Co(0.8 nm)/Cu(10 nm), type B: Fe(6 nm)/Co(0.8 nm)/Cu(10 nm) and type C: Cr(4 nm)/Fe(3 nm)/Co(0.8 nm)/Cu(10 nm). For B and C type buffers, the antiferromagnetic alignment is very interesting at zero field with a coupling strength larger than 0.4 erg/cm² and a GMR signal reaching 5% at room temperature. However, for the A type buffer the antiferromagnetic coupling completely disappears, while the GMR drops to about 0.8%. X-ray diffraction, atomic force microscopy and transmission electron microscopy have been performed in order to understand the origin of the observed difference in the magnetic properties. The results show a strong difference in the average surface roughness, 1.15 nm and 0.35 nm, respectively for the A and C types buffers, and demonstrate that the quality of the surface of the buffer is the key to optimize both the GMR and the indirect exchange coupling. Received 11 July 2000     

Effect of annealing temperature for Si0.8Ge0.2 epitaxial thin films

This study investigates the effect of annealing temperature on the Si_0.8Ge_0.2 epitaxial layers. The Si_0.8Ge_0.2 epitaxial layers were deposited by using ultrahigh vacuum chemical vapor deposition (UHVCVD) with different annealing temperatures (400-1000 °C). Various measurement technologies, including high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and interfacial adhesion tester, were used to characterize the materials properties of the SiGe epilayers. The experimental results showed that the SiGe epilayers gradually reduced lattice-mismatch to the underlying substrate as annealing temperature increased (from 400 to 800 °C), which resulted from a high temperature enhancing interdiffusion between the epilayers and the underlying substrate. In addition, the average grain size of the SiGe films increased from 53.3 to 58 nm with increasing annealing temperature. The surface roughness in thin film annealed at 800 °C was 0.46 nm. Moreover, the interfacial adhesion strength increased from 476 ± 9 to 578 ± 12 kg/cm² with increasing the annealing temperature.     

Electrodeposition and characterization of Cu/Co multilayers: Effect of individual Co and Cu layers on GMR magnitude and behavior

The present work discusses the successful electrodeposition of Cu/Co multilayers, exhibiting appreciable GMR of 12-14% at room temperature. The effect of individual Cu and Co layers on the magnitude and behavior of GMR has been studied. By varying the thickness of individual layers the field at which saturation in GMR is observed can be controlled. It was observed that for lower thicknesses of Co layer, the saturation fields are reduced below 1 kOe. The Cu layer thickness seems to control the nature of magnetic coupling and the saturation field, with the two showing a correlation.     

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.     

Thermally assisted writing for perpendicular MRAM

Spin valves composed of TbCo/CoFe/Cu/CoFe/TbFeCo were fabricated with perpendicular magnetization and GMR ratios of 4.5%. The (TbCo/CoFe) layers and (CoFe/TbFeCo) layers are referred to the free and the pinned layers, respectively. The compositions of two layers were chosen to have a lower Curie temperature (130 °C) but higher coercivity (13.2 kOe) of the free layer at room temperature than those of the pinned layer; therefore, the free layer is quite stable at room temperature but its magnetization can be easily switched at a relatively low temperature. Spin valves were patterned into 100-μm-wide cells and their coercivity was reduced with increasing writing current due to the temperature rise by current-heating. When the current density of the writing current was increased to 2.1×10⁶ A/cm², the required switching field for the free layer was only 10 Oe.