Domain dynamics in strained metal films with perpendicular magnetic anisotropy

目前，人们对试图控制磁性的兴趣越来越大，包括磁性薄膜和纳米结构的磁化方向，不仅使用磁场或电流，还使用电场。实际原因是，电场在转换磁化时比磁场或电流消耗的能量更少，这可能对新一代包含纳米磁铁的电子设备产生重要影响，比如磁性随机存取存储器。即使简单地与现有方法结合使用，电场感应效应也可以降低设备所需的功率密度，减少作为热量损失的能量。足够大的电场可以直接影响薄膜中指向特定方向的磁化趋势，称为磁各向异性。电场使外层电子轨道扭曲并与原子磁矩偶联，从而可以操纵磁各向异性，从而控制磁化方向。多铁性材料，其中磁性可以由电场控制（反之亦然），本身并不是很有用，因为其中只有一种，铋铁氧体，在室温下是多铁性的，磁电效应通常相当小，但与铁磁性薄膜结合，可以通过两者之间的“磁交换”耦合实现改进的电气控制。然而，本研究将使用的是电场控制的第三种方法，即将压电材料和铁磁材料结合起来，利用它们之间的应变耦合。施加在压电上的电压使其膨胀和压缩，从而使顶部的磁膜应变，改变其磁性，包括其各向异性。从基础研究的角度来看，迄今为止很少有研究应变对垂直于平面磁化方向的薄膜磁性能的影响，特别是对磁化开关的影响。这项研究将集中在这种垂直薄膜上，利兹小组擅长制作，制备单晶和合金形式的薄膜，并使用磁成像来研究应变对磁化动力学的影响。磁成像可以直接看到薄膜的磁化方向，薄膜通常不是均匀的，而是分成不同磁化方向的区域或“域”。在垂直的薄膜中，畴内的磁化可以相对于薄膜平面向上或向下。从薄膜反射的光的偏振被旋转了一定的量，这取决于磁化方向（磁光克尔效应），这使得在一个适当设计的显微镜中可以描绘出这些区域。这种显微镜比其他形式的磁成像具有优势，因为它可以非常迅速地产生大面积的图像，并且不需要特殊的样品制备。原则上，它也可以用于以非常高（亚纳秒）的时间分辨率研究域模式的变化。当评估材料对需要高速运行的电子设备的适用性时，这些磁化强度的动态变化是有用的。在这个项目中，纳秒级的磁场脉冲会改变区域的模式，在每次脉冲前后拍摄图像将有助于了解区域的动态。这将作为压电的电压感应应变的函数来完成，为理解电场如何提高电子设备功能的效率提供了一条途径。

There is currently growing interest in trying to manipulate magnetic properties, including the direction of magnetization in magnetic thin films and nanostructures, using not just magnetic fields or electric currents, but electric fields. The practical reason for this is that electric fields are expected to dissipate less energy in switching the magnetization than either magnetic fields or electric currents, and this could have an important effect on new generations of electronic devices that incorporate nanomagnets, such as magnetic random access memories. Even if used simply in conjunction with existing methods, electric field-induced effects could reduce the power density required in a device and lessen the amount of energy that is lost as heat.Sufficiently large electric fields can directly affect the tendency of the magnetization in a thin film to point in a particular direction, known as the magnetic anisotropy. The electric field distorts the outer electronic orbitals and couples to the atomic magnetic moments, enabling manipulation of the magnetic anisotropy and hence the magnetization direction. Multiferroic materials, in which magnetism can be controlled by electric fields (and vice versa), are not very useful in themselves because only one of them, bismuth ferrite, is multiferroic at room temperature and the magnetoelectric effect is generally rather small, but in combination with ferromagnetic thin films improved electric control may be achieved by "magnetic exchange" coupling between the two. However, it is a third method of electric field control that will be used in this research, namely, to combine piezoelectric and ferromagnetic materials and utilise the strain coupling between them. A voltage applied to the piezoelectric makes it expand and compress and thereby strains the magnetic film on top, altering its magnetic properties, including its anisotropy. From the point of view of basic research, there has been very little work so far to investigate the effect of strain on the magnetic properties of thin films with an orientation of the magnetization perpendicular to the plane, and in particular on the switching of the magnetization. This study will focus on such perpendicular films, which the Leeds group excels in making, preparing them in both single crystal and alloy form and using magnetic imaging to study the effect of strain on the magnetization dynamics.Magnetic imaging enables a direct visualisation of the magnetization direction in a thin film, which is generally not uniform but split up into regions, or "domains" where the magnetization direction is different. In perpendicular films the magnetization in the domains may point up or down relative to the film plane. The polarization of light reflected from the film is rotated by an amount that depends on the magnetization direction (the magneto-optic Kerr effect), and this enables the domains to be pictured in an appropriately designed microscope. Such a microscope offers advantages over other forms of magnetic imaging in that it produces large-area images very quickly and requires no special sample preparation. In principle it can also be used to study changes to the domain pattern with a very high (sub-nanosecond) time resolution. These dynamical changes to the magnetization are useful to know when assessing the suitability of the material for electronic devices, which need to operate at high speed. In this project, nanosecond magnetic field pulses will modify the domain pattern, and taking images before and after each pulse will give an insight into the domain dynamics. This will be done as a function of voltage-induced strain from the piezoelectric, providing a route to understanding how electric fields can improve the efficiency of electronic device function.