Perpendicular magnetization and exchange bias in epitaxial NiO/[Ni/Pt]_(2)multilayers

The realization of perpendicular magnetization and perpendicular exchange bias(PEB)in magnetic multilayers is important for the spintronic applications.NiO(t)/[Ni(4 nm)/Pt(1 nm)]_(2)multilayers with varying the NiO layer thickness t have been epitaxially deposited on SrTiO;(001)substrates.Perpendicular magnetization can be achieved when t<25 nm.Perpendicular magnetization originates from strong perpendicular magnetic anisotropy(PMA),mainly resulting from interfacial strain induced by the lattice mismatch between the Ni and Pt layers.The PMA energy constant decreases monotonically with increasing t,due to the weakening of Ni(001)orientation and a little degradation of the Ni–Pt interface.Furthermore,significant PEB can be observed though NiO layer has spin compensated(001)crystalline plane.The PEB field increases monotonically with increasing t,which is considered to result from the thickness dependent anisotropy of the NiO layer.

Numerical Analysis of the Magnetization Behavior in Magnetic Resonance Imaging in the Presence of Multiple Chemical Exchange Pools

The purpose of this study was to demonstrate a simple and fast method for solving the time-dependent Bloch-McConnell equations describing the behavior of magnetization in magnetic resonance imaging (MRI) in the presence of multiple chemical exchange pools. First, the time-dependent Bloch- McConnell equations were reduced to a homogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation and Kronecker tensor product. From these solutions, the longitudinal relaxation rate (R1ρ) and transverse relaxation rate in the rotating frame (R2ρ) and Z-spectra were obtained. As illustrative examples, the numerical solutions for linear and star-type three-pool chemical exchange models and linear, star- type, and kite-type four-pool chemical exchange models were presented. The effects of saturation time (ST) and radiofrequency irradiation power (ω1) on the chemical exchange saturation transfer (CEST) effect in these models were also investigated. Although R1ρ and R2ρ were not affected by the ST, the CEST effect observed in the Z-spectra increased and saturated with increasing ST. When ω1 was varied, the CEST effect increased with increasing ω1 in R1ρ, R2ρ, and Z-spectra. When ω1 was large, however, the spillover effect due to the direct saturation of bulk water protons also increased, suggesting that these parameters must be determined in consideration of both the CEST and spillover effects. Our method will be useful for analyzing the complex CEST contrast mechanism and for investigating the optimal conditions for CEST MRI in the presence of multiple chemical exchange pools.

Topology-like dynamical behavior of magnetization reversal in exchange-bias systems

In an exchange-bias system, the barriers with intrinsic potential energy may be asymmetric due to unidirectional anisotropy. Based on the Stoner–Wohlfarth model, we show that the asymmetric barriers may lead to four kinds of dynamical process underlying the hysteresis-loop measurement. These kinds of dynamical processes are different in a topology-like property, which can be controlled by the orientation of the external field. In our study, a new analysis approach has been proposed to reveal the dynamical behaviors of magnetization reversal. With this approach, coercivity, exchange-bias field, and asymmetry of hysteresis loops can be quantitatively obtained.

Investigation of magnetization reversal process in pinned CoFeB thin film by in-situ Lorentz TEM

Exchange bias effect has been widely employed for various magnetic devices.The experimentally reported magnitude of exchange bias field is often smaller than that predicted theoretically,which is considered to be due to the partly pinned spins of ferromagnetic layer by antiferromagnetic layer.However,mapping the distribution of pinned spins is challenging.In this work,we directly image the reverse domain nucleation and domain wall movement process in the exchange biased Co Fe B/Ir Mn bilayers by Lorentz transmission electron microscopy.From the in-situ experiments,we obtain the distribution mapping of the pinning strength,showing that only 1/6 of the ferromagnetic layer at the interface is strongly pinned by the antiferromagnetic layer.Our results prove the existence of an inhomogeneous pinning effect in exchange bias systems.

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlOx/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.

The magnetization reversal behaviour for SmCo_(6.8)Zr_(0.2) and SmCo_(6.8)Zr_(0.2)/α-(Fe,Co) nanocrystalline magnets at low temperature

This paper reports that the SmCo6.8Zr0.2 nanocrystalline permanent magnets and SmCo6.8Zr0.2/a-（Fe,Co） nanocomposite permanent magnets are successfully produced by mechanical alloying and subsequently annealing at 700℃ for 10 minutes. The x-ray diffraction results show that the phase structure of SmCo6.8Zr0.2 nanocrystalline permanent magnets is composed of SmCo7 phase and SmCo6.8Zr0.2/a-（Fe,Co） nanocomposite permanent magnets is composed of SmCo7 and a-（Fe,Co） phases. The mechanism of magnetization reversal is mainly controlled by inhomogeneous domain wall pinning in SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets. The inter-grain exchange interaction at low temperature is investigated, which shows that the inter-grain exchange interaction of SmCo6.8Zr0.2/a-（Fe,Co） magnets increases greatly by the decrease of the measured temperature. According to Amirr-H/Hcj, Amrev-H/Hcj and Xirr-H/Hcj curves at room temperature and 100 K, the changes of irreversible and reversible magnetization behaviours of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets with the decreasing temperature are analysed in detail. The magnetic viscosity and the activation volume of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/a-（Fe,Co） magnets at different temperatures are also studied.

Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations,we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.

Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Effect of zirconium content on exchange coupling and magnetization reversal of nanocrystalline Nd_(12.3)Fe_(81.7-x)Zr_xB_6 alloy

The effect of Zr content on exchange coupling and magnetization reversal of the Ndl2.3Fe81.7_xZrxB6 （x=0-3.0） ribbons was systematically investigated. Interaction domains were imaged by magnetic force microscopy （MFM）. The strength of interactions determined by Wohlfarth＇s analysis increased first with Zr content x increasing, reached the maximum value at x=l.5, and then decreased with x further increasing. Initial magnetization curves and dependence of coercivity and remanence on applied magnetic field showed that the mechanism of coercivity in all samples was mainly of exchange coupling pinning type, which was enhanced with x increasing. It was found by three-dimensional atom probe （3DAP） that Zr atoms did not partition into the Nd2Fe14B hard magnetic phase, but significantly enriched at the interfacial region.

Superexchange Interactions and Magnetic Anisotropy in MnPSe_(3)Monolayer

Two-dimensional van der Waals magnetic materials are of great current interest for their promising applications in spintronics.Using density functional theory calculations in combination with the maximally localized Wannier functions method and the magnetic anisotropy analyses,we study the electronic and magnetic properties of MnPSe_(3)monolayer.Our results show that it is a charge transfer antiferromagnetic(AF)insulator.For this Mn^(2)+3d^(5)system,although it seems straightforward to explain the AF ground state using the direct exchange,we find that the nearly 90oMn-Se-Mn charge transfer type superexchange plays a dominant role in stabilizing the AF ground state.Moreover,our results indicate that,although the shape anisotropy favors an out-of-plane spin orientation,the spin-orbit coupling(SOC)leads to the experimentally observed in-plane spin orientation.We prove that the actual dominant contribution to the magnetic anisotropy comes from the second-order perturbation of the SOC,by analyzing its distribution over the reciprocal space.Using the AF exchange and anisotropy parameters obtained from our calculations,our Monte Carlo simulations give the Néel temperature T_(N)=47 K for MnPSe_(3)monolayer,which agrees with the experimental 40 K.Furthermore,our calculations show that under a uniaxial tensile(compressive)strain,Néel vector would be parallel(perpendicular)to the strain direction,which well reproduces the recent experiments.We also predict that T_(N)would be increased by a compressive strain.

陕西渭北晚更新世黄土-古土壤结合水膜厚度的试验研究

为深入剖析黄土与古土壤工程性质的影响因素,以陕西渭北晚更新世黄土与古土壤为研究对象,基于核磁共振、比表面积测试并结合理论分析对结合水膜厚度进行研究。试验结果显示:黄土与古土壤的结合水膜厚度存在明显差异,黄土的结合水膜厚度明显大于古土壤,原因在于古土壤孔隙体积及孔径小于黄土,而古土壤中高价阳离子含量大于黄土,同时古土壤颗粒表面分布铁锰质薄膜,其亲水性较弱。分析结合水膜与黄土湿陷性、回弹变形等工程特性间的关系,发现黄土-古土壤的湿陷性随结合水膜厚度的增大而增强,随结合水膜厚度的减小而减弱,结合水膜厚度与卸荷变形系数呈正相关关系。综合分析说明,高价阳离子含量、颗粒表面特性,及孔隙性的不同造成了黄土与古土壤结合水膜厚度的差异,从而导致黄土与古土壤的湿陷性等工程性质有显著区别。

Simulation of the SMILE Soft X-ray Imager response to a southward interplanetary magnetic field turning

The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)Soft X-ray Imager(SXI)will shine a spotlight on magnetopause dynamics during magnetic reconnection.We simulate an event with a southward interplanetary magnetic field turning and produce SXI count maps with a 5-minute integration time.By making assumptions about the magnetopause shape,we find the magnetopause standoff distance from the count maps and compare it with the one obtained directly from the magnetohydrodynamic(MHD)simulation.The root mean square deviations between the reconstructed and MHD standoff distances do not exceed 0.2 RE(Earth radius)and the maximal difference equals 0.24 RE during the 25-minute interval around the southward turning.

Exchange模型的DCE-MRI定量研究评价肝硬化的价值

目的探讨Exchange模型的动态对比增强磁共振(DCE-MRI)定量研究评价肝硬化的价值。方法符合纳入标准和排除标准的肝硬化代偿期和失代偿期患者各15例,正常对照组15名,均行肝脏DCE-MRI扫描,采用Exchange模型计算出各组定量参数,包括对比剂容积转运常数(Ktrans)、速率常数(Kep)、血管外细胞外间隙容积分数(Ve)、血浆容积分数(Vp)、肝动脉灌注指数(HPI)、血容量(BV)、血流量(BF)、对比剂平均通过时间(MTT)。3组定量参数的比较采用单因素方差分析,组间两两比较采用LSD-t检验。绘制ROC曲线判断Ktrans、Ve、HPI、MTT对肝硬化的诊断效能。结果正常对照组、代偿期肝硬化组和失代偿期肝硬化组的Ktrans分别为(0.85±0.34)min^(-1)、(0.67±0.36)min^(-1)、(0.39±0.18)min^(-1),Ve分别为0.47±0.42、0.34±0.13、0.16±0.06,HPI分别为0.35±0.09、0.57±0.17、0.76±0.20,MTT分别为(17.38±15.38)s、(34.26±11.37)s、(57.33±16.73)s,BF值分别为(97.22±36.29)ml/(min·100g)、(69.30±35.22)ml/(min·100g)、(24.64±28.24)ml/(min·100g),差异均有统计学意义(P<0.05),且HPI和MTT的组间两两比较差异有统计学意义(P<0.05)。Kep、Vp、BV在3组间比较差异无统计学意义(P>0.05)。Ktrans和Ve判断失代偿期肝硬化的曲线下面积(AUC)为0.86、0.92,敏感度为90%、90%,特异度为80%、80%;HPI和MTT判断代偿期肝硬化的AUC为0.79、0.76,敏感度为85%、85%,特异度为75%、75%;HPI和MTT判断失代偿期肝硬化的AUC为0.91、0.88,敏感度为95%、90%,特异度为80%、80%。结论 Exchange模型的DCE-MRI定量研究获得多个定量参数,可反映肝硬化的血流动力学改变及血管微环境变化,评价肝硬化的严重程度。

酰胺质子转移MRI技术在预测急性缺血性脑卒中预后情况的可行性研究

目的:研究酰胺质子转移(APT)磁共振成像(MRI)技术在预测脑卒中预后情况的可行性。方法:选取2022年9月至2023年5月南京医科大学附属南京医院收治的71例急性脑卒中患者,入院前均进行美国国立卫生研究院卒中量表(NIHSS)评分,并进行化学交换饱和转移(CEST)MRI检查,根据预后1个月随访的调制等级分数(mRS)分为两组,将mRS<2分的患者纳入恢复良好组(44例);mRS≥2分的患者纳入恢复不良组(27例)。分析数据获得不对称磁化转移比(MTRasym)图像的APT,并计算出缺血区和对侧正常组织APT成像信号的差值(△APTw)。对比分析两组△APTw的差异性,采用Pearson相关性分析△APTw与NIHSS及mRS评分之间的相关性。绘制受试者工作特征(ROC)曲线,计算ROC曲线下面积(AUC)。结果:患者图像的△APTw与NIHSS以及mRS评分之间存在显著的正相关(R^(2)=0.659、0.522,P<0.001),恢复良好组与恢复不良组之间的△APTw、NIHSS和mRS评分比较,差异有统计学意义(t=5.73、6.36、13.92,P<0.05)。APT预测脑卒中预后的AUC为0.886,灵敏度和特异度分别为77.8%和95.5%,阳性预测值为91.3%,阴性预测值为87.5%。结论:APT成像技术在预测急性脑缺血性脑卒中的预后情况具有可行性。

Types of the jump phenomenon in the angular dependence of the noncollinear exchange bias

Based on the principle of minimal energy and the coherent rotation model, two types of the jump phenomena, complete and incomplete jump phenomenon, are proved to exist in the angular dependence of the exchange bias with noncollinear unidirectional and uniaxial anisotropies. It is found that the transition between complete and incomplete jump phenomena occurs on condition that the exchange-coupling constant exceeds a critical value. Additionally, two different modes of the magnetization rotation, the whole-plane rotation, and the half-plane rotation are present in the magnetization reversal process, and they are dependent on the direction of the external field. Furthermore, the equations of the critical angle, at which orientation the exchange bias field reaches a maximum value and the coercivity disappears, are also derived in this paper. The numerical calculations in this paper are consistent with the relevant experimental observations, indicating that our method to study the angular dependence of the exchange bias as well as the magnetization reversal behaviors is valid. Our discussion about the jump phenomenon, the critical angle, and the modes of the magnetization reversal can explain the observed differences in results between different experiments.

还原磁化焙烧竖炉冷却系统研究

某氧化铁矿石属于难选铁矿石,磁化焙烧工艺是处理此类铁矿石最有效的方法之一。竖炉磁化焙烧工艺加工成本相对较高,其中竖炉冷却系统是竖炉加工成本的重要组成部分,通过对竖炉冷却系统冷却水速、水箱梁水垢、水箱梁冷却水量等方面的研究,探索高效、节能的竖炉冷却系统.

基于Exchange血流动力学模型的MRI扫描参数诊断肝硬化临床价值分析

目的探讨MRI增强扫描Exchange血流动力学模型评估肝硬化患者病情的临床应用价值及其意义。方法在60例慢性乙型肝炎和慢性丙型肝炎患者及30例健康体检者,均接受肝活检和肝脏DCE-MRI检查,通过检测双输入双室Exchange血流动力学模型得到参数指标,包括对比剂容积转运常数(Ktrans)、速度常数(Kep)、血管外细胞外间隙容积分数(Ve)、血浆容积分数(Vp)、肝动脉灌注指数(HPI)以及对比剂平均通过时间(MTT)、达峰时间(TTP)、血容量(BV)、血流量(BF)。结果在60例患者中,经肝组织病理学检查,诊断为S1期3例,S2期3例,S3期1例和S4期53例;肝纤维化患者Ktrans和Ve指标较对照组显著降低(P<0.05),而HPI、MTT、TTP水平较对照组显著升高(P<0.05);ROC曲线分析显示Ktrans、HPI、MTT和TTP诊断肝硬化的效能较高(P<0.05)。结论基于双输入双室Exchange血流动力学模型的DCE-MRI检查可得到多个定量参数,在诊断肝硬化方面具有良好的指导意义。

Effect of annealing time on the exchange coupling interactions and microstruc-ture of nanocomposite Pr_(7.5)Dy_1Fe_(71)Co_(15)Nb_1B_(4.5) ribbons

The influence of annealing time on the magnetic properties and microstructure of nanocomposite Pr7.5Dy1Fe71Co15Nb1B4.5 ribbons was systematically investigated by the methods of vibrating sample magnetometer （VSM）, X-ray diffraction （XRD） and high resolution transmission electron microscopy （HRTEM）. Interaction domains derived from strong exchange coupling interactions between hard and soft magnetic grains were imaged using magnetic force microscopy （MFM）. Maximum remanence, intrinsic coercivity, and maximum energy product values were obtained in the ribbons annealed at 700℃ for 15 min, which were composed of Pr2（Fe, Co）14B, α-（Fe, Co）, and slight Pr2（Fe, CO）17 phases. Although Jr, Hci, and （Bn）max decreased gradually with further increase of annealing time, it is emphasized that comparatively high Jr and Hci and （BH）max were obtained in a wide annealing time period of 15 to 360 min. The shape of initial magnetization curves and hysteresis loops change as a function of annealing time, indicating different magnetization reversal routes, which can be fully explained by the corresponding microstructure.

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic （FM/AF） bilayers has been investigated in detail by extending Slonczewski＇s ＇proximity magnetism＇ idea. Here three important parameters are discussed for FM/AF bilayers, i.e. interracial bilinear exchange coupling J1, interracial biquadratic （spin-flop） exchange coupling J2 and antiferromagnetic layer thickness tAF. The results show that both the occurrence and the variety of the exchange bias strongly depend on the above parameters. More importantly, the small spin-flop exchange coupling may result in an exchange bias without the interracial bilinear exchange coupling. However, in general, the spin-flop exchange coupling cannot result in the exchange bias. The corresponding critical parameters in which the exchange bias will occur or approach saturation are also presented.

Room-temperature electric control of exchange bias effect in CoO1-δ/Co films using Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (110) substrates

Significant electric control of exchange bias effect in a simple CoO1-δ/Co system, grown on piezoelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (110) (PMN-PT) substrates, is achieved at room temperature. Obvious changes in both the coercivity field (HC) and the exchange bias field (HE), of 31% and 5%, respectively, have been observed when the electric field is applied to the substrate. While the change of coercivity is related to the enhanced uniaxial anisotropy in the ferromagnetic layer, the change of the exchange bias field can only originate from the spin reorientation in the antiferromagnetic CoO1-δ layer caused by the strain-induced magnetoelastic effect. A large HE/HC > 2, and HE~ 110 Oe at room temperature, as well as the low-energy fabrication of this system, make it a practical system for spintronic device applications.