GdFeCo磁光薄膜中RE-TM反铁磁耦合与激光感应超快磁化翻转动力学研究

使用飞秒时间分辨抽运-探测磁光克尔光谱技术,研究了激光加热GdFeCo磁光薄膜跨越铁磁补偿温度时稀土-过渡金属(RE-TM)反铁磁交换耦合行为和超快磁化翻转动力学. 实验观察到由于跨越铁磁补偿温度、净磁矩携带者交换而引起的磁化翻转反常克尔磁滞回线以及在同向外磁场下,反常回线上大于和小于矫顽力部分的饱和磁化强度不同,显示出GdFeCo中RE与TM之间的非完全刚性反铁磁耦合. 在含有Al导热底层的GdFeCo薄膜上观测到饱和磁场下激光感应磁化态翻转及再恢复的完整超快动力学过程. 与剩磁态的激光感应超快退磁化过 关键词： 补偿温度 磁化翻转 反铁磁耦合 GdFeCo     

铁磁和反铁磁双层膜中铁磁共振的研究

采用微磁学理论研究了铁磁/反铁磁双层膜中的铁磁共振现象.本模型将铁磁薄层抽象为一个单晶,具有立方磁晶各向异性和单轴磁晶各向异性,而反铁磁层视为厚度趋近于半无穷,且只有单轴磁晶各向异性.推导出了该系统的铁磁共振频率和频率谱宽度的解析式.数值计算表明,铁磁共振模式分两支,取决于立方磁晶各向异性.而界面的交换耦合,是磁易轴具有单向性的起因.     

铁磁/反铁磁/铁磁三层膜系统中反铁磁自旋结构及其交换各向异性

研究铁磁/反铁磁/铁磁三层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明,其反铁磁膜中的磁矩转动存在可逆"恢复行为"、不可逆"连续倒转行为"以及不可逆"中断倒转行为"三种情形,三种情形的出现强烈地依赖于两界面处的线性耦合和双二次耦合.钉扎界面的交换耦合与旋转界面的交换耦合相互竞争,当钉扎界面耦合占主导时,反铁磁磁矩发生可逆"恢复行为",系统出现交换偏置.在旋转界面耦合占主导情形下,其线性耦合与双二次耦合也相互竞争,分别导致反铁磁磁矩发生不可逆"连续倒转行为"和不可逆"中断倒转行为",系统都不出现交换偏置,但矫顽场都得以增强.相关结论为实验上观测的磁滞能耗以及界面垂直耦合提供了可能的解释.     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

热效应在电流驱动反铁磁/铁磁交换偏置场翻转中的显著作用

电流驱动的面内交换偏置场翻转具有无需外磁场辅助、抗磁场干扰以及强磁各向异性等优势,受到广泛关注.然而,在纳米级厚度薄膜系统中,反铁磁/铁磁异质结的阻塞温度较低,同时电流脉冲会产生大量的焦耳热,理论上电流热效应对于交换偏置场翻转有着显著作用,但是其作用机制缺乏相关研究和验证.我们制备了一系列反铁磁IrMn厚度不同的Pt/IrMn/Py异质结,系统性地研究了热效应在电流翻转交换偏置场中的作用机制.结果表明,在毫秒级电流脉冲下,焦耳热能够使得器件升温至阻塞温度以上,解除反铁磁/铁磁界面的交换耦合,同时电流产生的奥斯特场和自旋轨道矩能够翻转铁磁磁矩,在降温过程中完成交换偏置场的翻转.并且,在翻转过程中,反铁磁/铁磁异质结的各向异性磁阻曲线呈现与温度相关的两步磁化翻转现象,分析表明该现象起源于交换偏置耦合与铁磁直接交换作用之间的竞争关系.本文的研究结果厘清了热效应在电流驱动交换偏置场翻转过程中的重要作用,有助于推动基于电控交换偏置场的自旋电子器件发展.     

反铁磁耦合硬磁-软磁-硬磁三层膜体系的不可逆交换弹性反磁化过程

采用一维原子链模型研究了反铁磁耦合的硬磁/软磁/硬磁三层膜体系的反磁化过程. 研究结果表明,当考虑了软磁层的磁晶各向异性能后,软磁层厚度和界面交换耦合强度的改变都有可能导致软磁层的交换弹性反磁化过程由可逆过程转变为不可逆过程. 对软磁层很薄的体系,其反磁化过程是典型的可逆交换弹性反磁化过程. 然而,当软磁层厚度超过某一临界厚度t_c时,反磁化过程转变为不可逆的交换弹性反磁化过程. 软磁-硬磁界面交换耦合强度A_sh对反磁化行为也有很大的影响. 对于软磁层厚度小于临界厚度t_c的体系,也存在一个临界界面交换耦合强度A_sh^c. 当A_sh大于A_sh^c时,软磁层的反磁化过程是可逆的交换弹性反磁化过程;而当A_sh小于A_sh^c时,这一过程变为不可逆. 给出了体系的可逆与不可逆交换弹性反磁化过程随软磁层厚度和界面交换耦合强度变化的磁相图. 同时还研究了偏转场随软磁层厚度的变化关系. 关键词： 反铁磁耦合三层膜 交换弹性反磁化过程 反磁化机理 磁相图     

铁磁结构中涡旋的动力学特性模拟研究

基于微磁学基本方程Landau-Lifshitz-Gilbert(LLG)方程,我们建立了软磁薄膜体系顺磁-铁磁转变过程中涡旋数目随时间的变化关系模型.磁化强度运动方程采用了传统的Runge-Kutta数值方法求解.计算结果发现:不同的交换场下,涡旋数变化可以分为两个阶段:第一阶段涡旋数目随时间急剧减少;第二阶段涡旋数目缓慢减少,直至不再变化,交换系数越小剩余的涡旋数会越多.退磁能对相变过程影响甚微,只有在交换系数(1.3E-12)较小时有可观察到的效应:有退磁场涡旋数目稍小.     

脱氧La0.5Ca0.5MnO3样品的铁磁-反铁磁转变和电阻率变化

研究了氧空位对La0.5Ca0.5MnO3 (LCMO)多晶块材的电输运和磁性质的影响. 随着氧空位的增加, 样品在高温段的电阻率一直增加, 并满足绝热小极化子模型, 而低温段的电阻率先下降后上升, 并出现明显的dR/dT>0的行为, 直至最后变为绝缘的. 氧空位的增加抑止了反铁磁相的出现, 使得脱氧的LCMO样品不发生反铁磁转变, 进一步增加氧空位则会抑制铁磁相.     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

单畴Ni球观测及NicoreNiOshell的铁磁/反铁磁耦合研究

 使用微波辅助聚合方法制备了单分散单畴Ni纳米球，由MFM发现，尺度分布在100～180 nm的Ni球的一个相关特征是条型磁畴结构。用XRD、TEM、XPS以及EDAX测量了由Ni球进一步制备的Ni_coreNiO_shell高度球型纳米结构。用VSM 和SQUID进一步讨论了其铁磁/反铁磁界面耦合效应，估算了交换耦合场与粒子尺寸的关系。     

First-order reversal curves of magnetic recording tapes

The interaction and its variation between magnetic grains in two kinds of magnetic recording tapes are investigated by first-order reversal curves (FORC) and the $\delta M$ method. The composition and microstructure of the samples are characterised by x-ray diffraction and scanning electron microscope. The FORC diagram can provide more accurate information of the interaction and its variation, but the $\delta M$ curves cannot. The positive interaction field and the large variation of the interaction field have opposite effects on the $\delta M$ curve.     

矩形磁性纳米点动力学反磁化过程的微磁学研究

采用微磁学模拟方法研究了初始态为C形磁结构的矩形CoFe纳米点在方波脉冲场作用下的动力学反磁化过程.研究发现,随着脉冲场强的增强,磁体的反磁化模式发生了改变.当场强较弱时反磁化过程通过畴壁移动-单涡旋的形成和移动来完成;当场强较大时反磁化过程模式转变为畴壁移动-双涡旋的形成与移动;在更强的场强下反磁化过程通过畴壁的移动-多涡旋的形成与湮没来实现.由于反磁化模式随场强的变化而改变,反磁化时间随场强的增大出现振荡变化现象. 关键词： 动力学反磁化过程 反磁化时间 微磁学模拟     

Effect of the soft layer thickness on magnetization reversal process of exchange-spring nanomagnet patterns

A systematic study of the magnetization reversal behavior in the regular arrangement of L1₀-FePt based exchange-spring nanomagnets with different thicknesses of the Co soft magnetic layer is presented. The magnetic property of the hard magnet is compared to two tuned exchange-spring magnets: its systems of 20 nm L1₀-FePt/3 nm, and 7 nm Co. In particular, we focus on the switching field distribution. The exchange coupling showed narrower SFD, in spite of the decoupled part switches earlier. The magnetization switching mechanism of exchange-spring nanomagnets patterns has been revealed with a first-order reversal curves technique and the switching field distribution. Further, the microscopic results using magnetic force microscopy show that the spin rotation of the non-interacting part in the thicker soft layered exchange-spring magnet. The part influences the magnetization reversal process. According to the experimental results, exchange coupling strength can be tuned by the thickness of the soft magnetic layer.     

Antiferromagnetically coupled multilayers of (Co90Fe10 tF/Ru tRu)×N and (Ni81Fe19 tF/Ru tRu)×N prepared by ion beam deposition

This work describes multilayers of Co₉₀Fe₁₀ t_F/Ru t_Ru/Co₉₀Fe₁₀ t_F and Ni₈₁Fe₁₉ t_F/Ru t_Ru/Ni₈₁Fe₁₉ t_F (20 Åt_F200 Å) prepared at ambient temperature by ion beam deposition on Si/SiO₂ 3 kÅ substrates. The samples exhibited a maximum antiferromagnetic coupling with t_Ru=3.2 Å and M–H curves characterized by zero remanent magnetic moment and enhanced saturation field. Antiferromagnetic peaks were present with t_Ru17 and 30 Å.     

磁存储器环形带切口结构自由层磁化反转的微磁模拟

针对基于磁性隧道结的赝自旋阀磁随机存储器, 使用带斜面切口环形结构自由层, 抛弃采用厚度改变矫顽力的方式, 降低了磁性隧道结的面积电阻, 改进了垂直电流磁随机存储器. 通过集成工艺中淀积的二次效应生成磁环的切口, 利用微磁学方法计算分析了自由层的磁化反转特性, 结果表明该模型具有低串扰、低面积电阻、高磁阻率以及较强的抗干扰性能. 关键词： 自由层结构 磁化翻转 微磁 VMRAM     

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlO_x/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.     

Effects of notches on the order of flux-closure state formation in bi-rings by micromagnetic simulation

The effects of the number and the location of notches on the formation of flux-closure states in bi-rings with fields applied in the x direction (i.e., along the short axis direction of bi-rings) and y direction (i.e., along the long axis direction of bi-rings) are investigated using micromagnetic simulation. For the bi-rings with one notch and the bi-rings with two notches symmetric about y axis, the order of flux-closure state formation in each ring can be controlled. But the flux-closure state forms simultaneously in each ring for the bi-rings with two notches symmetric about x axis. For the bi-rings with two notches that are symmetric neither about x axis nor about y axis, only one ring can form a flux-closure state in the y-direction field and no flux-closure state can be found in rings in the x-direction field. Therefore, logic states can be defined by controlling the order of flux-closure state formation, which can be utilized in future logic devices.     

磁涡旋极性翻转的局域能量

针对坡莫合金纳米圆盘中的单个磁涡旋结构,采用微磁学模拟研究了磁涡旋极性翻转过程中的局域能量密度.磁涡旋的极性翻转通过与初始涡旋极性相反的涡旋与反涡旋对的生成,以及随后发生的反涡旋与初始涡旋的湮没来实现.模拟结果显示当纳米圆盘样品中局域能量密度的最大值达到一临界值时,磁涡旋将会实现极性翻转,其中交换能起主导作用.基于涡旋极性翻转过程中出现的三涡旋态结构,应用刚性磁涡旋模型对局域交换能量密度进行了理论分析.通过刚性磁涡旋模型得到的磁涡旋极性翻转所需的局域交换能量密度的临界值与模拟结果符合得较好.     

First order transition to oscillation death through an environment

The emergence of dynamical abrupt transitions for the first time in an ensemble of identical limit-cycle and chaotic oscillators coupled via a common environment is reported. The transition from the oscillatory state to the death state and vice versa, in these networks of oscillators are found not only discontinuous as well as irreversible in the parameter space. This first order phase transition in these systems is termed as Explosive Death. The occurrence of such transition is studied in details by using an appropriate order parameter for both limit-cycle and chaotic oscillators, in particular, Stuart–Landau and Rössler oscillators. The backward transition point for this phenomenon is obtained analytically using linear stability analysis and is found to be consistent with the numerical results.