Study of the interfacial magnetism in NiO/NiFe system

The interfacial magnetism of NiO/NiFe bilayers with different NiFe layer thicknesses, produced by DC and RF magnetron sputtering, has been studied by magnetometry and X-ray magnetic circular dichroism (XMCD). In magnetic hysteresis loops, the exchange bias field was found to be inversely proportional to the NiFe layer thickness. The fit using the Meiklejohn and Bean model gives a coupling energy at the NiO/NiFe interface of approximately 0.027 mJ/m². The analysis of the XMCD spectra of Fe and Ni, using the sum rules, shows a reduction of the effective spin magnetic moments in bilayers with NiFe thickness less than 4 nm. This reduction is attributed to hybridization of ferromagnetic and antiferromagnetic atoms d orbitals near the interface and/or formation of antiferromagnetic alloys due to atomic diffusion at the interfaces.     

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.     

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.     

A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction

A spin valve is a microelectronic device in which high- and low-resistance states are realized by using both the charge and spin of carriers. Spin-valve structures used in modern hard-drive read heads and magnetic random access memoriescomprise two ferromagnetic electrodes whose relative magnetization orientations can be switched between parallel and antiparallel configurations, yielding the desired giant or tunnelling magnetoresistance effect. Here we demonstrate more than 100% spin-valve-like signal in a NiFe/IrMn/MgO/Pt stack with an antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. Ferromagneticmoments in NiFe are reversed by external fields of approximately 50 mT or less, and the exchange-spring effect of NiFe on IrMn induces rotation of antiferromagnetic moments in IrMn, which is detected by the measured tunnelling anisotropic magnetoresistance. Our work demonstrates a spintronic element whose transport characteristics are governed by an antiferromagnet. It demonstrates that sensitivity to low magnetic fields can be combined with large, spin-orbit-coupling-induced magnetotransport anisotropy using a single magnetic electrode. The antiferromagnetic tunnelling anisotropic magnetoresistance provides a means to study magnetic characteristics of antiferromagnetic films by an electronic-transport measurement.     

Study of magnetization reversal by minor loops in IrMn/CoFe exchange-biased bilayers

We report a detailed investigation of the magnetization reversal by minor loops in Co75Fe25 (t) single layer and Ir22Mn78(10 nm)/CoFe(t) exchange-biased bilayers with different CoFe thicknesses. With increasing CoFe layer thickness in IrMn/CoFe bilayers, the magnetization reversal process shows a transition from the coherent rotation to the domain-wall motion, which is attributed to the competition among the antiferromagnetic domain wall energy, ferromagnetic domain wall energy, and the interface coupling between antiferromagnetic and ferromagnetic layers.     

NiFe/IrMn多层膜制备及MgO掺杂研究

采用磁控溅射制备了Ta/NiFe/IrMn/Ta薄膜,研究了反铁磁IrMn的溅射功率和铁磁层NiFe厚度对多层膜交换偏置场的影响。在反铁磁IrMn中插入MgO,发现MgO含量对交换偏置场有一定影响。随着MgO含量的增加,多层膜的交换偏置场逐渐增大,当MgO的含量约为2.5%交换偏置场达到最大值。随着MgO含量进一步增加交换偏置场下降。在IrMn中插入适量的MgO可以有效地增加交换偏置场。     

Asymmetry of Magnetization Reversal of Pinned Layer in NiFe/Cu/NiFe/IrMn Spin-Valve Structure

Two types of asymmetry in giant magnetoresistance (GMR) are observed which are not related to a training effect, but indicate different mechanisms of magnetization reversal of the pinned layer in spin-valve (SV) structures for ascending and descending field scans. GMR, exchange bias and coercivity in Si/Ta/NiFe/Cu/NiFe/IrMn/Ta SV-structures were investigated as functions of the thickness of the nonmagnetic spacer. The spacer thickness effects are discussed in correlation with layers microstructure and interfaces morphology variations.     

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.     

Specific features of unidirectional anisotropy in exchange-coupled DyCo/NiFe film structures

The dependence of the unidirectional anisotropy in a DyCo/NiFe bilayer system on the thickness of the magnetically soft layer has been studied using dynamic and static techniques. The magnitude of the exchange interaction between layers evaluated using the ferromagnetic resonance method is two times that determined from an analysis of the hysteresis loops. It is established that this difference is related to features of the magnetic microstructure of the DyCo layer.     

Tuning the exchange bias in NiFe/Fe-oxide bilayers by way of different Fe-oxide based mixtures made with an ion-beam deposition technique

We have investigated the structural and magnetic properties of ion-beam deposited polycrystalline NiFe (25 nm)/Fe-oxide (35 nm) bilayers. A film prepared with an assist beam O2 to Ar gas ratio of 0% during deposition had a bottom layer that consisted of pure b.c.c. Fe (a = 2.87 A) whereas films prepared with 19%O2/Ar and 35%O2/Ar had either Fe3O4 (a = 8.47 angstroms) or alpha-Fe2O3 (a = 5.04 angstroms, c = 13.86 angstroms) bottom layers, respectively. Cross-sectional transmission electron microscopy revealed a smooth interface between the top nano-columnar NiFe and bottom nano-columnar Fe-oxide layer for all films. At room temperature, the observed coercivity (Hc approximately 25 Oe) for a film prepared with 19% O2/Ar indicates the existence of a magnetically hard ferrimagnetic Fe3O4 phase that is enhancing the plain NiFe (Hc approximately 2 Oe) by way of exchange coupling. A significant amount of exchange bias is observed below 50 K, and at 10 K the size of exchange bias hysteresis loops shift increases with increasing oxygen in the films. Furthermore, the strongest exchange coupling (H(ex) approximately 135 Oe at 10 K) is with alpha-Fe2O3 (35% O2/Ar) as the bottom film layer. This indicates that the pure antiferromagnetic phases work better than ferrimagnetic phases when in contact with ferromagnetic NiFe. H(ex) (T) is well described by an effective AF domain wall energy that creates an exchange field with a (1 - T/T(crit)) temperature dependence. Hc (T) exhibits three distinct regimes of constant temperature that may indicate the existence of different AF spin populations that couple to the FM layer at different temperatures.     

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.     

Fe掺杂对YCrO3微结构及磁特性的影响

利用溶胶-凝胶法制备YCr0.6Fe0.4O3样品,研究了Fe^3+掺杂对YCrO3的微结构与磁特性的影响。结果表明,样品具有良好的单相正交结构,属于Pnma空间群。拉曼及傅里叶红外光谱结果显示,Fe^3+掺杂使得Y/Cr-O键长被拉伸,增加了晶格畸变,导致振动模式向低波数偏移。热磁M-T曲线表明,体系在TN^183K发生反铁磁相变,随温度降低磁化强度增加,FC曲线在128K出现较宽的驼峰,温度降至85K,磁化强度达到最小(0.08 A·m^2/kg),随后逐渐增加。这主要是Fe^3+掺杂使YCrO3样品内Cr^3+O^2-Cr^3+、Fe^3+O^2-Fe^3+反铁磁有序与Cr^3+O^2-Fe^3+铁磁相互作用共存,Fe/Cr团簇导致磁化反转现象。当T<80K时,M-H曲线出现磁台阶效应,T=50K时出现交换偏置现象,场冷磁场增加,磁滞回线基本重合,表明样品内反铁磁和铁磁界面处产生钉扎效应。     

Magnetism of Nano-Graphene with Defects: A Monte Carlo Study

The effect of the defects on magnetic properties of a bilayer Ising ferromagnetic antiferromagnetic model is studied by Monte Carlo simulations, for a nano-graphene lattice with spins that can take the values σ=3/2 and S=5/2. We consider two ferromagnetic and antiferromagnetic bilayers with N=42 spins, with a random number of defects. We only consider the nearest-neighbor interactions between the site i and j on each layer. The effects of the defects on magnetization are investigated for fixed temperature, crystal field, and magnetic field values. The thermal dependency of each layer magnetization is calculated for fixed defect rate values K ₃ and K ₅, the crystal field, and external magnetic field. The magnetization hysteresis loops for several rate defects are also investigated as a function of the external magnetic field.     

磁控溅射法制备IrMn底钉扎自旋阀研究

采用高真空直流磁控溅射的方法,在玻璃衬底上制备了结构为Ta／buffer layer／IrMn／CoFe／Cu／CoFe／NiFe／Ta的IrMn底钉扎自旋阀。研究了NiFe和Cu作为缓冲层对自旋阀磁性能的影响,并对缓冲层厚度进行了参数优化,当缓冲层厚度为2nm时自旋阀各项性能达到最佳。研究了退火制度对底钉扎自旋阀性能的影响,得到了30000e强磁场下200℃保温1h为最佳处理条件。通过结构的改善和工艺的优化,得到的底钉扎自旋阀的磁电阻率8．51％,矫顽场为0．50e,交换偏置场超过8000e。最后对自旋阀的底钉扎和顶钉扎结构进行了比较。     

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.     

Subloop Magnetic Moment Reversal-Induced Recovery Effect in Exchange-Biased IrMn/Pt/Co/Pt Multilayers

For perpendicular exchange-biased IrMn/Pt/ Co/Pt multilayers with bidomain state, double hysteresis loops (DHLs) are achieved. It is found that exchange bias field and coercivity in full loop and two subloops’ training show different dependencies on the cycle number. Further studies show that the negative-biased (positive-biased) subloop measurement will influence training effect of the positive-biased (negative-biased) subloop, acting as a recovery process. This recovery procedure is different from the conventional ways, i.e., by applying a magnetic field perpendicular to the initial pinning direction or by leaving the sample alone for several days without any applied magnetic fields. This recovery effect can be explained by the re-rotation of antiferromagnet (AFM) spins after corresponding subloop magnetic moment reversal during the full loop measurement, which cannot happen during the subloop measurement.     

自旋阀结构中的力致磁阻效应

采用变分法研究了外应力场下铁磁单层膜、铁磁/反铁磁双层膜系统的磁化性质,进而研究了由铁磁单层膜和铁磁/反铁磁双层膜所构建的自旋阀结构中的磁电阻与外应力场之间的关系.结果表明,铁磁膜中的磁化性质与膜面内所加应力场的大小,方向密切相关,而反铁磁层的嵌入将明显地改变着铁磁层的磁矩向应力场方向磁化的行为.特别地,在应力场方向垂直于铁磁易轴情况下,当应力场日H_λ=2(K_1+K_(up)/3M)时,将发生磁化从易轴方向到应力方向的突变.为此,可采用自旋阀结构,通过其膜面内的应力场所调控的磁电阻效应,构建纳米尺度下的力磁传感器.     

自旋阀多层膜磁化翻转场的调控和磁电阻特性

用磁控溅射方法制备Ta/CoFe/Fe/Au/Fe/IrMn/Ta和Ta/CoFe¹/Au/CoFe²/IrMn/Ta两种多层膜结构的自旋阀,并优化各功能层的溅射参数有效调控了磁化翻转场和磁电阻特性。根据TEM确定了样品多层膜的微观结构和膜厚,使用VSM和加磁场四探针法分别测量了样品的磁滞回线和磁电阻(MR)特性曲线。结果表明,样品中隔离层Au的厚度与MR值之间存在振荡衰减的关系;而钉扎层、自由层和被钉扎层的厚度直接影响各膜层的矫顽力和饱和磁化强度等磁学性能,进而改变MR值。各层厚度为6/6/3.8/6/9/6 nm的Ta/CoFe¹/Au/CoFe²/IrMn/Ta结构自旋阀,具有最佳的MR值。     

Correlation between isotropic ferromagnetic resonance field shift and rotatable anisotropy in polycrystalline NiFe/FeMn bilayers

Polycrystalline NiFe/FeMn bilayers were fabricated by DC magnetron sputtering and studied by ferromagnetic resonance and vibrating sample magnetometer. The isotropic resonance field shift and the rotatable anisotropy are correlated to each other because they both undergo the maximal values around 2.5 nm and then decrease with increasing the antiferromagnetic layer thickness. These results can be explained in terms of thermally activated transition of antiferromagnetic spins.     

Magnetic properties of Ni-NiO (ferromagnetic-antiferromagnetic) nanocomposites obtained from a partial mechanochemical reduction of NiO

The magnetic properties of ferromagnetic (FM)-antiferromagnetic (AFM), Ni-NiO, nanocomposites obtained from a reactive ball milling reduction of NiO in H2 atmosphere have been studied. The formation of ferromagnetic Ni from antiferromagnetic NiO can be accurately followed by the increase of the saturation magnetization. The microstructure of the nanocomposite, consisting of FM Ni nanoparticles embedded in an AFM NiO matrix leads to exchange bias effects, i.e., loop shifts and coercivity enhancement, after field cooling from above the Néel temperature of NiO.