【作者】 刘益；

【导师】 钱正洪； 朱建国；

【作者基本信息】 四川大学 ， 凝聚态物理， 2021， 博士

【摘要】 磁性斯格明子（以下简称斯格明子）是一种在磁性材料中广泛存在的有准粒子特性的自旋拓扑结构。斯格明子可在极低极化电流密度作用下移动或振荡,同时具有较小的尺寸及拓扑稳定性,因而受到广泛关注。由于近年发现了多种室温斯格明子材料,使得斯格明子有望应用于赛道存储器、斯格明子晶格、自旋转移纳米振荡器等新型自旋电子器件中。人工斯格明子是一种无需Dzyaloshinskii–Moriya（DM）作用而可在室温下稳定存在的斯格明子,对深入研究室温斯格明子有非常重要的参考价值。因此本论文通过微磁学模拟,构造了多种室温斯格明子材料体系,并通过施加磁场方式,对斯格明子的稳定性和动力学特性进行了模拟和分析。主要研究结果如下:（1）构造了硬磁/软磁双层膜结构Ni Fe（8 nm）/Co Cr Pt（10 nm）,并通过模拟获得了人工斯格明子。通过施加磁场,发现人工斯格明子的磁场稳定性高于涡旋结构。垂直于薄膜方向的磁场对斯格明子的涡旋核的直径有重要影响。通过构造四层膜结构Co Cr Pt/Cu/Co Cr Pt/Ni Fe可获得涡旋核尺寸最小为13 nm的人工斯格明子结构。对斯格明子施加频率f为0.79 GHz的交变磁场,发现斯格明子的涡旋核直径会变小且涡旋核会围绕硬磁层中心转动。斯格明子的极性对斯格明子涡旋核的转动方向有影响,而其螺旋度对斯格明子的起振方向有影响。人工斯格明子和涡旋结构之间互相转化时,自旋结构的螺旋方向不受影响,会保持其原来的螺旋方向。（2）通过对硬磁/软磁材料体系设定不同的材料参数,发现布洛赫型斯格明子可转化为奈尔型斯格明子等其它三种类型的自旋结构。当硬软磁层之间的交换常数为0 J/m,垂直磁各向异性常数为1.0～1.5×10⁵J/m³时;或者硬软磁层之间交换常数为0 J/m,饱和磁化强度为5.2～6.9×10⁵A/m时,可获得奈尔型人工斯格明子。当尺寸大于一定值时（>800×800nm²）,硬磁层尺寸对人工斯格明子的稳定性影响有限。（3）在Fe₆₅Co₃₅（0.6 nm）/Fe Pt（0.6 nm）/Pt（1.2 nm）三层膜中通过模拟实现了奈尔型斯格明子。发现DM作用对Fe₆₅Co₃₅/Fe Pt/Pt三层膜中奈尔型斯格明子的形成起着重要作用,即使初始的自旋结构为布洛赫型斯格明子,只要DM作用常数大于1.2 J/m²,布洛赫型斯格明子会转化为奈尔型斯格明子。缺陷强度（缺陷中心位置垂直磁各向异性常数相对无缺陷位置的变化量）越大,斯格明子在+z方向的临界磁场越大,最大达到885m T。当缺陷位于软磁层底部时,只需要非常小的缺陷强度（λ=0.1,即缺陷中心垂直磁各向异性常数的相对变化量为10%）,在磁场的作用下,可使得斯格明子涡旋核迁移。而当缺陷位于软磁层外面时,只有当缺陷的长度为25 nm时,在外加磁场的作用下斯格明子涡旋核才会移动。在Fe₆₅Co₃₅/Fe Pt/Pt三层膜的基础上构造了具有不同形状（三角形、正六边形、正方形）的Fe₆₅Co₃₅软磁层结构。发现这些结构也可形成涡旋,并且人工斯格明子可在硬磁层中实现。通过软磁层的尺寸（6～100 nm）的调节可使得斯格明子涡旋核尺寸在（6～60 nm）之间变化。当软磁层尺寸较小时（<20 nm）,所形成的斯格明子不够稳定,在很小的-z方向的磁场（<60m T）作用下涡旋结构会消失。构造了人工斯格明子晶格,发现软磁层磁盘直径小于23 nm时,即使形成了人工斯格明子晶格,但也是不稳定的,这一晶格结构会自动消失。（4）在Co层上构造了Ni Fe纳米触点阵列,并通过模拟在纳米触点底部构造了人工斯格明子。研究了自由斯格明子在自旋极化电流作用下迁移时,人工斯格明子产生势垒,人工斯格明子和自由斯格明子之间的排斥作用对自由斯格明子移动的影响。发现该势垒主要来源于垂直磁各向异性和DM作用,会使得自由斯格明子向-y方向迁移。双排人工斯格明子结构对自由斯格明子的斯格明子霍尔效应有所抵消作用。由于人工斯格明子的周期性排列而呈现周期性变化,人工斯格明子结构和自由斯格明子之间的作用也呈现周期性变化,使得这一抵消作用不是连续存在的,它会随着自由斯格明子位置的变化而周期性变化。更多还原

【Abstract】 Magnetic skyrmions（Later referred to as skyrmion）are a topological structure with quasi-particle characteristics.Magnetic skyrmions widely exist in magnetic materials.Because magnetic skyrmion can move or oscillate under the action of extremely low polarization current density,and at the same time its small size,and its topological stability,magnetic skyrmions have received widespread attention.Due to the discovery of a variety of room temperature skyrmions materials in recent years,skyrmions are expected to be widely used in new spintronic devices such as racetrack memory,skyrmion lattices,and spin transfer nano-oscillators in the future.The room temperature skyrmion materials have very important value for the practical application of skyrmion.Artificial skyrmions is another kind of skyrmion materials that have been experimentally realized at room temperature with Dzyaloshinskii–Moriya（DM）interaction,and it has very important reference value for in-depth study of room temperature skyrmions.Therefore,in this thesis,a variety of room temperature skyrmion materials systems were constructed through micromagnetic simulations,and the stability and dynamic characteristics of skyrmions were simulated by applying a magnetic field,and the simulation results were analyzed.The main results are as follows:（1）Hard/soft magnetic（Ni Fe（8 nm）/Co Cr Pt（10 nm））bilayer films were constructed,and the artificial skyrmion was realized in the hard magnetic layer by micromagnetic simulation.By applying a magnetic field,it was found that the stability of artificial skyrmions is higher than that of the vortex structure.Magnetic field perpendicular to the film direction has an important influence on the core diameter of the skyrmion.At the same time,artificial skyrmion with a minimum skyrmion core diamater of 13 nm was obtained by constructing four-layer films Co Cr Pt/Cu/Co Cr Pt/Ni Fe.Applying an alternating magnetic field with a frequency f of 0.79 GHz to skyrmions,it was found that the core diameter of skyrmion decreases,and the skyrmion core revolves around the center of the hard mangetic layer.The polarity of skyrmions has an effect on the rotation direction of the skyrmions core,and its helicity has an effect on the direction of skyrmions’starting vibration.When the artificial skyrmion and the vortex were transformed into each other,the spiral direction of the spin structure will not be affected,and the original spiral direction will be maintained.（2）By setting different magnetic materials parameters for the hard/soft magnetic bilayer films,it was found that Bloch-type skyrmions can be transformed into three types spin structures,such as:Néel-type skyrmions.When the exchange constant between the hard and soft magnetic layers is 0J/m,the perpendicular magnetic anisotropy（PMA）constant is 1.0～1.5×10⁵J/m³;or when the exchange constant is 0 J/m,the saturation magnetization is5.2～6.9×10⁵A/m,an artificial Néel-type skyrmion can realize.When the size of the hard magnetic layer is larger than a certain value（>800×800 nm²）,the size of the hard magnetic layer has a limited effect on the stability of artificial skyrmion.（3）Néel-type skyrmions were realized in Fe₆₅Co₃₅（0.6 nm）/Fe Pt（0.6nm）/Pt（1.2 nm）three-layer films by micromagnetic simulation.It was found that DM interaction plays a key role in the formation of Néel-type skyrmions in Fe₆₅Co₃₅/Fe Pt/Pt three-layer films.Even if the initial spin structure is Bloch-type skyrmions,as long as the DM interaction constant is greater than 1.2 J/m²,Bloch-type skyrmions will be transformed into Néel-type skyrmions.The stronger the intensity of the defect is,the larger the+z critical magnetic field of skyrmions is（The strength of the defect represents the relative change of the PMA constant in the defect center position relative to the defect-free position.）.Finally,the maximum value of the+z critical magnetic field could be reach 885 m T.When the defect is located at the bottom of the soft magnetic layer,only a very small defect intensity（λ=0.1,that is,the relative variation of the perpendicular anisotropy constant of the defect center is 10%）is required.Under the action of magnetic field,the skyrmion core can migrate.On the basis of the Fe₆₅Co₃₅/Fe Pt/Pt three-layer film,Fe₆₅Co₃₅soft magnetic layer with different shapes（triangular,regular hexagon,square,etc.）was constructed.It was found that the soft magnetic layer of these structures can form a vortex structure,and artificial skyrmion was realized in the hard magnetic layer.By adjusting the size of the soft magnetic layer（6～100 nm）,the size of the skyrmion core can be changed（6～60 nm）.When the size of the soft magnetic layer is small（<20 nm）,the skyrmions formed are not stable enough,and the artificial skyrmion will disappear under the action of a small-z-direction magnetic field（<60 m T）.An artificial skyrmion lattice was constructed.It was found that when the diameter of the soft magnetic layer disk is less than 23 nm,even if an artificial skyrmion lattice was formed,it is unstable,and the lattice will automatically disappear.（4）Ni Fe nanocontacts array was constructed on the Co magnetic layer,and artificial skyrmions were realized on the bottom of the nanocontacts by micromagnetic simulation.When free skyrmions migrate driven by spin-polarized current,the artificial skyrmions produce a potential barrier.The repulsion between the artificial skyrmions and the free skyrmions,and the potential barrier affect the movement of the free skyrmions.It was found that the potential barrier is mainly derived from the PMA and DM interaction.The potential barrier causes the free skyrmions to migrate in the-y direction.Double artificial skyrmion arrays can offset the skyrmion Hall effect of free skyrmions.Due to the periodic arrangement of artificial skyrmions,the interaction between artifical skyrmion and free skyrmion also presents a periodical change,making this offsetting effect not continuous,and the offsetting effect will be cyclical as the position of the free skyrmions change.更多还原