Domain dynamics in strained metal films with perpendicular magnetic anisotropy

目前，人们越来越感兴趣的是试图操纵磁性，包括磁性薄膜和纳米结构中的磁化方向，不仅使用磁场或电流，而且使用电场。其实际原因是，与磁场或电流相比，电场在切换磁化时消耗的能量更少，这可能对新一代包含纳米磁体的电子设备（如磁性随机存取存储器）产生重要影响。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器件所需的功率密度，并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磁化指向特定方向的趋势，称为磁各向异性。电场使外电子轨道扭曲并耦合到原子磁矩，从而能够操纵磁各向异性，从而操纵磁化方向。多铁性材料，其中磁性可以由电场控制（反之亦然），本身不是很有用，因为只有其中之一，铁酸铋，在室温下是多铁性的，磁电效应通常相当小，但与铁磁薄膜结合，可以通过两者之间的“磁交换”耦合来实现改进的电控制。然而，这是电场控制的第三种方法，将在本研究中使用，即，结合联合收割机压电和铁磁材料，并利用它们之间的应变耦合。施加到压电体上的电压使其膨胀和压缩，从而使顶部的磁性薄膜应变，改变其磁性，包括其各向异性。从基础研究的角度来看，到目前为止，很少有工作来研究应变对具有垂直于平面的磁化方向的薄膜的磁性的影响，特别是对磁化的切换。这项研究将集中在这种垂直的薄膜，其中利兹集团擅长制作，制备它们在单晶和合金的形式，并使用磁成像研究的磁化dynamics.Magnetic成像的应变的影响，使磁化方向的薄膜，这通常是不均匀的，但分裂成区域，或“域”的磁化方向是不同的直接可视化。在垂直膜中，畴中的磁化相对于膜平面可以指向上或指向下。从薄膜反射的光的偏振旋转的量取决于磁化方向（磁光克尔效应），这使得域被描绘在适当设计的显微镜。这种显微镜提供了优于其他形式的磁成像，因为它产生大面积的图像非常快，不需要特殊的样品制备。原则上，它也可以用于研究具有非常高（亚纳秒）时间分辨率的畴图案的变化。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，纳秒级的磁场脉冲将改变磁畴模式，在每个脉冲之前和之后拍摄图像将深入了解磁畴动态。这将作为来自压电的电压诱导应变的函数来完成，从而提供了理解电场如何提高电子器件功能效率的途径。

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.