交换耦合双层膜PtMn/Co的微观结构和磁性

采用磁控溅射法在Si（100）基片上沉积了PtMn／Co双层膜，研究退火温度与样品微结构以及磁性的关系，发现退火温度越高，反铁磁性PtMn层由fcc非磁性相向fct反铁磁性相转变越充分，交换偏置场值Hex越高。同时，随着退火温度的升高，反铁磁层的晶粒尺寸逐渐增大，交换偏置场Hex值随着反铁磁层晶粒尺寸的增大几乎呈线性增加。研究了交换偏置场与铁磁层、反铁磁层厚度的关系，结果表明，在250℃退火时Hex与PtMn厚度关系为有峰值的曲线，此退火温度下反铁磁层临界厚度为20nm，最佳反铁磁层厚度为60nm。还发现了交换偏置场Hex随着铁磁性Co层厚度的增大几乎呈线性降低，充分说明铁磁、反铁磁交换耦合作用是一种界面耦合效应。     

Mn成分对CoFe/Pt_(50)(Cr_(100-x)Mn_x)_(50)体系交换偏置的影响

采用直流磁控溅射的方法制备了CoFe/Pt-CrMn交换偏置体系,反铁磁的Pt-CrMn是利用[Pt/CrMn]n多层膜并经过300℃、2h的退火获得。通过调整Mn的成分,系统地研究了体系交换偏置场的变化。获得了钉扎性能良好的L10相反铁磁钉扎材料Pt-CrMn,即CoFe/Pt50(Cr88Mn12)50钉扎体系,其界面交换耦合能为0.22×10-7J/cm2,截止温度(blocking temperature)为480℃。     

硬磁/反铁磁交换耦合对反铁磁FeMn稳定性影响研究

采用磁控溅射方法在SiO2基体上制备了FePt/FeMn/NiFe/Ta多层膜样品,通过FeMn/NiFe双层膜交换偏置的变化研究了硬磁FePt不同磁化状态对反铁磁层FeMn的影响。实验表明,磁化了的L10相FePt能使FeMn在较薄的情况下(4.5nm)对NiFe产生比较强的交换偏置;而未被磁化的FePt对FeMn/NiFe交换偏置影响并不明显。认为更薄的反铁磁层对另外的铁磁层产生交换偏置是由于硬磁与反铁磁的界面交换耦合作用能增强反铁磁的稳定性。     

磁性材料交换偏置效应研究进展

磁性材料中交换偏置效应对自旋电子学的基础研究和应用发展起到了至关重要的作用,因此也成为无机非金属材料的研究中最为活跃的领域之一.由于交换偏置效应来源于铁磁/反铁磁界面处的交换耦合作用,所以相关的研究工作主要集中在铁磁/反铁磁双层膜体系;在一些基态为反铁磁的类钙钛矿锰氧化物中,由于存在相分离形成的铁磁团簇也观察到了交换偏置现象;此外,很多磁性材料纳米化后也出现了交换偏置效应.主要从这3方面介绍了在新材料体系中交换偏置效应的研究进展,以及交换偏置效应在自旋相关器件中的应用,提出了一些研究中面临的挑战并对发展方向作出展望.     

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。     

界面掺杂FeMn对CoFe/CrPt交换偏置体系的影响

采用磁控溅射的方法制备了CoFe/CrPt钉扎的交换偏置体系,用外加磁场真空退火以获得钉扎场.通过把反铁磁的FeMn掺入到该钉扎体系中发现,约0.7nm厚度的FeMn掺入在CoFe/CrPt的界面时,可以使体系的钉扎场从原来的5.6×103A/m增加到1.55×104A/m,而体系的Blocking温度仍然可以达到600℃.     

NiO/CoFe双层膜的交换机制研究

通过改变制备NiO薄膜的氩气压和衬底材料,研究了NiO的结构、表面粗糙度对NiO/CoFe双层膜交换耦合场Hex的影响.实验表明完全自旋未补偿面与交换耦合场的产生没有直接联系,但交换耦合场Hex与界面状况密切相关.增大NiO的表面粗糙度会使交换耦合场Hex减小.应用随机场理论在考虑了实际界面存在的粗造度、杂质和缺陷等实际情况下,正确地预测了交换耦合场的数量级,而且对交换耦合场与铁磁层厚度tFM、反铁磁层厚度tAFM以及交换耦合场的温度特性等实验结果做出了合理解释.并应用随机场模型对反铁磁/铁磁双层膜中铁磁层矫顽力Hc与铁磁层厚度tFM的关系进行了定量计算,发现矫顽力Hc与铁磁层厚度1/tFM成正比,这一结果表明理论计算与我们的实验数据符合得很好.     

磁性薄膜材料中的交换耦合

介绍了在磁性薄膜材料中的交换耦合的研究进展。制备了铁磁/反铁磁/铁磁3层结构不同成分的薄膜。利用X射线粉末衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)等测试分析技术,系统研究了磁性多层薄膜的相组成、界面及微观结构等。利用超导量子干涉仪(SQUID)研究薄膜的磁、电性能和交换耦合。在Co/反铁磁/Fe结构中发现了非常明显的与温度相关的铁磁/反铁磁界面耦合与铁磁/铁磁层间耦合之间的竞争效应。含有不同铁磁层Fe、Co、Fe20Ni80的3层膜FM1/Cr2O3/FM2对交换耦合随温度的变化存在较强的影响,发现铁磁层的磁晶各向异性和跟Cr2O3接触的自旋非对称性反射系数体系的界面和层间耦合有很大的影响。铁磁层对FM/AFM的交换耦合强度的影响甚大,这种影响和铁磁层的各向异性的相关性要强于和铁磁层饱和磁化强度的相关性。通过面内预加场和场冷的方式,在易轴互相垂直的[Pt/Co]n/NiFe/NiO异质结中实现了交换偏置的4种状态,可等温调控偏置。研究了Co/NiO反点阵列和连续膜的交换偏置,与连续膜相比,纳米反点阵列既能增大偏置,也能减小偏置,具有更高的热稳定性。     

NiO/FeCo磁性多层膜的高频磁性

制备了一系列具有铁磁/反铁磁交换偏置作用的[NiO/Fe65Co35]10多层膜,使用振动样品磁强计(VSM)测量了样品的静磁参数,利用微带线法测量了样品4GHz-10GHz的磁谱,首次制备并测得了自然共振频率(fr)在6GHz以上,最高到fr=9.6GHz的薄膜样品。结果表明交换偏置场(Hex)、各向异性场(Hua)、以及矫顽力(Hc)随铁磁层厚度(tFM)增大而减小;基于Landau-Lifishitz(L-L)方程对静磁参量和磁谱进行了比较,发现样品自然共振频率较L-L方程计算值偏大30%以上。     

柔性衬底上Ni_(81)Fe_(19)/Ni_(50)Mn_(50)双层膜交换偏置效应研究

目的:探究制备在聚酰亚胺PI柔性衬底上的Ni_(81)Fe_(19/)Ni_(50)Mn_(50)双层膜的交换偏置效应。方法:采用直流磁控溅射技术分别在硅片和PI衬底上制备Ni_(81)Fe_(19)/Ni_(50)Mn_(50)双层膜,经相同温度退火处理后使用振动样品磁强计测试样品的磁滞回线。结果:柔性衬底上生长的双层膜的交换偏置场大小基本等于硅衬底上生长的,但其矫顽力始终比硅衬底上的大;薄膜的交换偏置场和矫顽力随着铁磁层厚度增大呈线性减小,随着反铁磁层厚度增大呈先增大后减小最后稳定的趋势,柔性衬底上薄膜的交换偏置场和矫顽力还会随着测量温度的升高而下降直至消失。结论:Ni_(81)Fe_(19/)Ni_(50)Mn_(50)双层膜的交换偏置场和矫顽力会受到铁磁层厚度、反铁磁层厚度和测量温度的影响,其中矫顽力还会受到衬底类型的影响。     

Hysteresis studies in Clarke solder-blob junctions

A study of hysteresis in Clarke solder-blob junctions has been carried out in the temperature range 4.2–1.2 K. The temperature dependence of the critical current, the variation of the constant-temperature critical current with sequential sweep cycles, and the effects of an external field are studied. All the experimental results are explained as due to a net trapping of magnetic flux in the junction loop whenever a hysteretic curve is traced. The origin of the flux can be traced to the inductance of the superconducting loop and the electrical or geometrical asymmetry.     

Analysis of minor loop behavior during alternating flux reversal in rectangular-loop toroidal cores

The analysis of minor hysteresis loop behavior during partial alternating flux reversal in toroidal cores with a rectangular hysteresis loop is considered. A simple flux-reversal model is proposed on the basis of which, with the aid of dimensionless units, the asymmetry of minor hysteresis loops is derived. The drift of minor hysteresis loops during alternating flux reversal is investigated on the basis of loop asymmetry and by means of a proposed indirect method. The method involves compensation of the minor-loop drift by means of a dc component of the exciting field, the magnitude and sense of which are a measure of the drift. The experimental section deals partly with a qualitative comparison of the character of the actual asymmetry of a minor loop with that derived from the flux reversal model, and partly with comparing the actual minor-loop drift to the drift derived theoretically on the basis of the indirect method. Experimental results indicate that in the case of the 50 percent nickel-iron alloy, cold-rolled at a great reduction, there appear additional factors which affect the instability of the minor loop. This effect is further investigated, and a simplified mechanism is proposed for its stimulation. Finally, the proposed mechanism is verified experimentally.     

Asymmetric hysteresis loops of systems of bistable nanoscopic wires

A system of bistable magnetic nanowires of diameter D = 57 nm, length L = 115 nm, magnetization M = 370 emu/cm3 is simulated. The probability distribution of the switching fields of the wires is Gaussian, with mean Hs = 710 Oe and standard deviation u(Hs) = 105 Oe. The wires are randomly distributed on a plane, with directions parallel (OX) or perpendicular (OY) to the axis direction where the magnetic field is applied. The magnetostatic interaction between the wires leads to an asymmetry of the hysteresis loop of the system. Namely, we obtain different curves M(H) for ascending and descending magnetic field. This behaviour is due to the wires which are perpendicular to the applied magnetic field. The directions of their magnetic moments remain often unchanged during the hysteresis experiment, and their contribution to the magnetic moment measured along OX is equal to zero. However, they interact with the wires parallel to the field, and this interaction influences the magnetic state of the parallel wires. The effect can be useful when we are interested in tailoring the shape of the hysteresis loop. Also, a given magnetic configuration of the wires parallel to OY produces a unique asymmetry of the hysteresis loop of the wires parallel to OX. Once an additional field is applied along OY, this unique state is destroyed. The effect can be useful for safety systems.     

Abnormal C–V curve and clockwise hysteresis loop in ferroelectric barium stannate titanate ceramics

Reversible hysteresis loop, defined as integral of small signal dielectric constant with electrical field, represents the contribution of the reversible part of polarization. In barium stannate titanate ceramics, field dependence of small signal dielectric constant was abnormal. The subsequent mathematical integral showed an abnormal clockwise hysteresis loop in the temperature range of 10–40 °C. The ferroelectric hysteresis loop measured by Sawyer–Tower circuit showed slim-waist or pinched shape. This phenomenon may reveal abnormal domain switching mechanism and is believed to be related with the strong interaction between point defects and domain walls.     

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.     

Asymmetric giant magnetoimpedance of amorphous microwires under the action of torsional stresses

The effect of torsional stresses on the direct-current-induced asymmetric giant magnetoimpedance in an amorphous microwire with negative magnetostriction has been theoretically studied. The appearance of asymmetry in the dependence of the impedance on the external magnetic field is related to the inter-action of a circular field of the direct current and a helicoidal anisotropy of the wire that is induced by the applied torsional stresses. It is established that torsional stresses significantly influence a threshold value of the current-generated bias field, above which a hysteresis in the dependence of the impedance on the external field disappears. The obtained results can be used for the creation of weak magnetic-field sensors.     

压电陶瓷的极化反转过程

利用阻抗分析仪和重复迭代法，对Ｐｂ（Ｍｇ１／３Ｎｂ２／３）０．１２５Ｔｉ０．４２Ｚｒ０．４５５Ｏ３压电陶瓷的极化反转过程进行了研究。实验数据显示出了铁电畴随电场变化的整个开关过程，发现极化反转前后压电性有着明显的不对称性，这一异常现象是和场诱应变、电滞回线数所相互对应的。经比较研究后认为，影响极化反转前后压电对称性的机理是空间电荷在晶粒表面积聚形成内偏置电场，由此建立的理论模型可以作出合理的解释。     

Secondary Peak on Asymmetric Magnetization Loop of Type-II Superconductors

Asymmetric magnetization loops with a second peak effect were parameterized by the extended critical-state model. The magnetic field distribution in a sample is considered. An expression is suggested for a peak of the critical current density and corresponding depression on field dependence of the depth of surface layer with equilibrium magnetization. These functions determine the width and the asymmetry of a magnetization loop. The asymmetry of the secondary peak height on magnetization branches for increasing and decreasing field is reproduced on the computed magnetization curves.     

Oxygen implanted Ni/NiO exchange bias system: fabrication, structure and magnetic property

Ion implantation is used to build a nanometer scale anti-ferromagnetic (AFM) cluster embedded exchange bias (EB) system. Ni film with a thickness of 100 nm is deposited on the Si (100) substrate using magnetron sputtering, 140 keV O+ is chosen to implant into the Ni film to form NiO AFM clusters, of which the size is estimated by X-ray diffraction based on synchrotron radiation (SR-XRD). By measuring hysteresis loop after field-cooling, significant shifts of loop along the applied field are observed. By increasing the implantation dose up to 3 x 10(17)/cm2 and annealing samples in N2 atmosphere, we discuss the origin of EB effect observed and the size confinement effect which lowers down the Néel temperature (T(N)) of NiO cluster to below room temperature.     

Temperature-dependent ferroelectric hysteresis properties of modified lead zirconate titanate ceramics

Temperature-dependent ferroelectric hysteresis properties of modified lead zirconate titanate ceramics have been investigated in a wide temperature range from 300 to 433 K. It is observed that remnant polarization, saturation polarization, and coercive field are increasing with an increase of the temperature in a low-field region and decreasing in a high-field region. Such behavior is explained by the competition between switching and backswitching mechanisms. A three-stage dependence of the logarithm of the hysteresis loop area on the logarithm of the electric field is identified. The temperature dependence of backswitching properties has been studied. The obtained results indicate that the temperature dependence of the polarization backswitching can be well described by the Arrhenius law. The activation energy for the domain switching determined from the fitting results exhibits decreasing tendency with the increase of the electric field.