CoFe/Cu/CoFe/IrMn自旋阀结构多层膜磁化反转过程的研究

通过高真空直流磁控溅射的方法制备了结构为//Ta(5nm)/Co75 Fe25(5nm)/Cu(2.5nm)/Co75Fe25(5nm)/Ir20 Mn80(12nm)/Ta(8nm)的顶钉扎自旋阀结构多层膜,研究了磁场循环次数、反向饱和场等待时间和磁场变化率对自旋阀结构多层膜磁化反转过程的影响.结果表明,磁场循环次数和反向饱和场等待时间对自由层的磁化反转过程没有影响,而在被钉扎层中出现了练习效应和时间效应;磁场变化率对被钉扎层和自由层的前、后支反转场的影响变化趋势相似,但反铁磁层对被钉扎层的反转有一定的影响.     

Exchange-Coupled IrMn/CoFe Mulitlayers for RF-Integrated Inductors

The high-frequency characteristics of the RF integrated inductors with antiferromagnetic/ferromagnetic (AF/F) multilayers (MLs) are studied. Each AF layer is 8-nm IrMn and F is Co₅₀Fe₅₀ with a 200-nm splitting into N = 1 to 5 repeats of MLs. This exchange coupled {IrMn/CoFe(d nm)}_N MLs are deposited on top of the two-port inductors with five-turn square spiral coils, a dimension of 100 mum times 100 mum and line/space of 5 mum/2 mum. The inductor surface and magnetic layer is separated by 1-mum-thick SiO₂. The enhancement of inductance (DeltaL) is 20% compared to an air-core of the same coil size. The resonance peak gradually shifted to a higher frequency with increasing N, and reached at a maximum of 4.3 GHz when N = 5. This is in good agreement with our magnetic data which revealed that the anisotropy field (H_k) and ferromagnetic resonance frequency (f_FMR) of {IrMn/CoFe(d nm)}_N MLs are increased with increasing N. The quality factor, Q is improved by 6.8% at 1.5 GHz for the {IrMn/CoFe(40 nm)}_N=5 integrated inductors compared to air-core inductors.     

Evidence of high dissipation in magnetization reversal processes of five Co/Cu bilayers

Heat and charge transport perpendicular to the plane in five Co/Cu stacks are studied using AC temperature gradients in the presence of a DC current. Large peaks in AC voltage response versus applied field are observed. This effect, measured only in reversible magnetization reversal modes and at sharp values of magnetic fields, suggests that an extra dissipation process is produced at well-defined magnetic configurations.     

Positive and negative exchange bias in IrMn/NiFe bilayers

We present the observation of double shifted hysteresis loops in IrMn/NiFe bilayer structures. The bilayer structures were fabricated using high vacuum DC magnetron sputtering system. The hysteresis loops of the as deposited samples show the double shifted loops at NiFe layer thicknesses 5 nm and 6 nm, whereas the IrMn layer thickness was kept constant at 15 nm. The results were interpreted as the contribution of both positive and negative exchange bias fields. We suppose that this phenomenon is occurring due to the ferromagnetic (FM) layer exchange coupled with the antiferromagnetic (AFM) layer in two different magnetization directions. The ferromagnetic coupling of the interface spins in some regions of the film generates the hysteresis loop shift toward negative fields and antiferromagnetic coupling toward positive fields in the other regions. The double shifted hysteresis loops disappeared after magnetic field annealing of the samples above Neel temperature of the AFM layer. The X-ray diffraction patterns of the sample show the IrMn (111) crystalline growth necessary for the development of exchange bias field in this system. The correlation between the Magnetic Force Microscopy (MFM) domain structures of the as deposited sample and the magnetization reversal process of the double shifted hysteresis loops were discussed. The results suggest that the larger multidomain formation in the AFM layer with different magnetization directions was responsible for the positive and negative exchange bias fields in IrMn/NiFe bilayer samples.     

Real-time temperature dynamics in exchange-biased bilayers upon laser excitation

A hot-spin and phonon gas in an exchange-biased metallic bilayer is induced by an 8.5-ps laser excitation. The spin-lattice temperature dynamics is sensed in real time by the time evolution of the exchange bias field on the picosecond time scale. A calibration with temperature-dependent quasi-static Kerr measurements yields a pump-pulse induced temperature increase of about 100/spl deg/C at the interface. Upon photoexcitation, the exchange coupling across the interface between the ferromagnetic and antiferromagnetic layer is reduced within the first 10 ps, leading to a reduction of the bias field to about 50% of its initial value. The fast thermal unpinning of the exchange coupling is followed by a heat-diffusion dominated recovery with a relaxation time on the order of 160 ps. A heat transport analysis reveals the diffusivity of the bilayer system.     

退火对CoFe/IrMn双层膜结构和磁性能的影响

采用高真空直流磁控溅射的方法制备了结构为//Ta(5nm)/Co75Fe25(5nm)/Ir20Mn80(12nm)/Ta(8nm)的双层膜,通过X射线衍射(XRD)、原子力显微镜(AFM)和振动样品磁强计(VSM)研究了退火对双层膜的结构及磁性能的影响;并通过样品在反向饱和场下停留不同的时间,研究了退火对双层膜的磁稳定性的影响。结果表明,退火使得IrMn(111)织构减弱,表面/界面粗糙度增大,交换偏置场减小,矫顽力增加,退火降低了双层膜的磁稳定性。     

Tuning the permeability spectra with a half-free ferromagnetic underlayer in (NiFe/IrMn)n exchange-biased multilayers

The influence of a half-free ferromagnetic underlayer on the static and microwave magnetic properties in [NiFe/IrMn]_n exchange bias multilayer thin film system has been systematically investigated. By changing the thickness of the half-free ferromagnetic underlayer, the linewidth of the ferromagnetic resonance (FMR) in this system can be tuned from 0.9 to 2.4 GHz. Theoretical fitting of the FMR frequencies based on the Landau-Lifshitz-Gilbert equation is also carried out to quantitatively identify the effective anisotropy fields and Gilbert damping parameters. The results provide an effective and flexible way to tailor the microwave permeability spectra and broaden the frequency linewidth toward the low frequency range in [ferromagnetic/antiferromagnetic]_n exchange bias multilayer films system. This approach has potential application for tunable wideband high frequency noise filters.     

Characterization of Microstructural and Morphological Properties in As-deposited Ta/NiFe/IrMn/CoFe/Ta Multilayer System

Surface morphology and its relationship with microstructure in Ta/NiFe/IrMn/CoFe/Ta multilayer system deposited by pulsed DC magnetron sputtering have been investigated in dependence of Ta buffer and NiFe seed layer thicknesses using atomic force microscopy.The structural parameters such as grain size,dislocation density,texture and strain were calculated.For each surface,a self-affinity behavior with mean fractal dimensions in the range of 2.03-2.18 was found.Additionally,it was also observed that the surface of all samples has locally smooth textured surface structure in the short range.The texture aspect parameter and texture direction index have been obtained for isotropy/anisotropy surface texture.A significant relationship between the surface texture and the strength of the〈111〉texture in IrMn layer has been found.The analysis indicated that the surface roughness is strongly affected by the thicknesses of the NiFe seed and Ta buffer layers.     

Magnetic characterization of MnPt/CoFe bilayers using the MOKE technique

The performance of magnetoresistive devices (spin valves, tunnel junctions), made of two ferromagnetic (FM) layers and separated by a non-magnetic spacer, rely on the existence of two well separated resistance states. For this to occur, one of the FM layer is deposited just adjacent to an antiferromagnetic (AFM) layer. Due to the exchange interaction at the AFM/FM interface, the reversal of the magnetization (M) of such FM-pinned layer occurs at a high applied magnetic field. The magnetization of the other FM layer reverses almost freely when a small magnetic field is applied. Here we study the exchange bias effect in the MnPt (t)/CoFe (50 Å) system, using the Magneto-Optical Kerr Effect (MOKE) and domain imaging techniques. The exchange (H_E) and coercive (H_c) fields increase with increasing AFM thickness, saturating for t > 200 Å (H_E ≈ 670 Oe and H_c ≈ 315 Oe). Furthermore, we observe that the value of the exchange field is almost independent of the applied magnetic field sweeping rate (up to ≈ 300 kOe/s). Domain imaging allowed us to conclude that magnetization reversal in the studied system proceeds essentially by coherent magnetic moment rotation.     

铁磁/反铁磁双层膜中的磁化性质与界面微结构

采用Monte Carlo模拟方法,通过反铁磁层中非磁性掺杂方式调节铁磁层与反铁磁层界面微结构,讨论了其界面微结构对体系的磁滞回线的不对称性、交换偏置场及矫顽场的影响.模拟结果显示:在同一掺杂浓度下,体系的磁滞回线的不对称性及交换偏置场强烈地依赖于掺杂方式,但矫顽场几乎不受影响,其相应的温度特性亦依赖于掺杂方式.它表明铁磁/反铁磁双层膜体系中的磁滞回线的不对称性以及交换偏置场与其界面微结构密切相关,同时实验上人们可通过界面微结构的改变获得交换偏置场大、矫顽场小且热稳定性好的自旋阀结构.     

低剂量Ga~+辐照对CoFe/IrMn双层膜结构和磁性能的影响

采用高真空直流磁控溅射的方法制备了结构为//Ta(5nm)/Co75Fe25(5nm)/Ir20Mn80(12nm)/Ta(8nm)双层膜,通过X射线衍射(XRD)、原子力显微镜(AFM)和振动样品磁强计(VSM)等分析测试手段,研究了低剂量Ga+离子辐照对双层膜结构和磁性能的影响;通过样品在反向饱和场下停留时间研究了低剂量Ga+离子辐照对双层膜的磁稳定性的影响;并利用SRIM2003软件模拟分析了离子辐照后Ga元素在IrMn层中的深度分布。结果表明,低剂量Ga+离子辐照对双层膜中反铁磁层IrMn的〈111〉方向织构影响甚微;而双层膜的交换偏置场以及界面粗糙度随着Ga+离子辐照剂量的增大而减小;低剂量Ga+离子辐照后双层膜磁稳定性降低。     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Effects of isofield uniaxial cyclic stress on the magnetization of2% Mn pipeline steel - behavior on minor hysteresis loops and smallmajor hysteresis loops

The present results show that the sense of the stress-induced irreversible changes in the unstressed magnetization is not always towards the anhysteretic. Examples where the change is away from the anhysteretic are shown, as well as cases where the cyclic uniaxial stress does not cause an irreversible change in the sample's unstressed magnetization when that magnetization does not coincide with a point along the anhysteretic curve. Examination of the magnetization changes with cyclic stress along the upper or lower branches of a minor hysteresis loop suggests that the distance along that branch from the previous turning point is the most important factor in determining the sense of the stress-induced changes in magnetization. Initially the stress-induced changes in the unstressed magnetization occur in the same sense as they would have before the turning point, but the interaction between the domain walls and the pinning sites in the sample as the applied magnetic field changes after the turning point causes this stress-induced change to decrease in value and then reverse in sign. This suggests that the isofield cyclic stress-induced changes in the magnetization should be considered to be towards a local equilibrium condition rather than a global equilibrium condition (the anhysteretic curve)     

Peculiarities of magnetization reversal in exchange-coupled ferro/ferromagnet NiFe/CoP film structures

The effect of layer thicknesses on the magnetic properties and mechanism of magnetization reversal in exchange-coupled NiFe/CoP film structures has been studied. The process of magnetization reversal was studied by analysis of the magnetic-induction and magneto-optical hysteresis loops. It is established that, as the thicknesses of layers in the NiFe/CoP film structure are increased, the system exhibits a transition from homogeneous magnetization reversal in the structure to exchange spring formation in the soft magnetic layer.     

Perpendicular exchange bias in IrMn/Pt/[Co/Pt]3 multilayers with cone magnetization

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]₃ multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]₃ film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]₃ system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6 nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3 nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]₃ system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.     

Nonlinear magnetization reversal in copper-permalloy composite wires induced by a high-frequency current

The phenomenon of magnetization reversal in copper-permalloy composite wires induced by a high-frequency ac current field was studied. The frequency spectrum of a voltage measured in the probing coil wound on the wire contains only even harmonics in a broad range of amplitudes of the ac excitation current and the bias magnetic field strength. The amplitude of the even harmonics measured is highly sensitive with respect to the bias field. The results can be used to develop weak magnetic field sensors.     

Cyclic dynamic magnetization reversal in system of magnetic dipoles

Dynamics of the magnetic moment of a circular ring system of spherical magnetic bodies has been numerically simulated. Conditions necessary for the cyclic magnetization reversal in this system by means of the sequential excitation of various oscillatory regimes (including a phase with zero total magnetic moment) are determined using bifurcation diagrams.     

CoFe/Cu多层膜巨磁阻效应的研究

采用直流磁控溅射制备了多层膜Ta/缓冲层/[Co95Fe5/Cu]×12/Co95Fe5/Ta。实验发现,多层膜的磁阻性能受到缓冲层材料、各子层厚度以及退火处理的影响。采用优化的多层膜结构:Ni65Fe15Co20缓冲层厚8 nm、CoFe层厚1.55 nm、Cu层厚2.4nm,沉积态薄膜GMR值达到7.6%;而在外加磁场79.6×103A/m下,250℃保温2 h退火处理后,多层膜的GMR值进一步提高到11.9%,磁滞从18.7×102A/m降低到796 A/m。     

Cavity-enhanced magnetooptical observation of magnetization reversal in individual single-domain nanomagnets

Optical studies of nanoscale magnets promise access to ultrafast magnetization dynamics but are challenging because of limited spatial resolution. We demonstrate that cavity enhancement of the magnetooptical Kerr effect increases the sensitivity in nanomagnetooptics significantly. Magnetization switching in individual single-domain magnets in both far-field and near-field Kerr microscopy is observed, and scaling laws are determined. Near-field signals remain nearly constant with reduced magnet diameter, indicating favorable scaling of near-field magnetooptics into the deep nanometer range.     

Peculiarities of the magnetization reversal in heterophase nanocomposite permanent magnets

We have studied the process of magnetization reversal in a thin-film Fe/Sm₂Co₇ exchange coupled bilayer structure under the action of an in-plane external field. An analysis of the local magnetization changes, as measured using the magnetooptical indicator film technique, showed that the magnetization reversal proceeds by inhomogeneous rotation of the magnetic moments in Fe and SmCo layers, both in plane and in the perpendicular direction. It is established that, because of the exchange interaction between layers, the magnetization reversal along the easy axis in the entire structure is determined primarily by the formation of exchange-induced spin helices and domain walls in the magnetically soft layer, whereas the magnetization reversal at an angle of α with respect to the easy axis plays a significant role in the magnetically hard layer and becomes dominating for α=90°.