Picosecond Dynamics of Magnetic Exchange Springs

铁磁材料在现代生活所依赖的电磁技术中随处可见。它们的范围从电机和发电机中的大宗材料到用于在硬盘驱动器中存储数据的薄膜。在铁磁体中，每个原子都有一个磁矩，就像地球一样，有北极和南极。交换相互作用(EI)迫使相邻原子的磁矩指向同一方向，这是一种纯粹的量子力学效应，是磁性中最强大的力量，产生的有效磁场最强可达地球磁场的1亿倍。我们日常的经验是，一些铁磁体保持永久磁化，而另一些则不是。在后一种情况下，磁矩在被称为磁区的微观区域内平行排列，但不同的磁区具有指向不同方向的磁矩，因此总体上没有净磁矩。相邻的磁区由磁区壁隔开，磁区壁宽约10 nm(100个原子直径)，磁矩的取向逐渐以螺旋结构旋转。磁区壁的有限宽度是电致发光的结果，磁区壁像弹簧一样存储交换能量。建议的项目涉及通过多层薄膜厚度形成的交换弹簧(ES)结构。交替层被称为硬层和软层，因为在后者中更容易形成螺旋结构。通过施加磁场或通过改变不同硬层中的磁矩的相对排列来诱导螺旋结构，从而扭转中间的软层中的磁矩。通过研究ES结构的形式及其对外界刺激的响应，我们可以获得关于EI强度如何随着结构的变化而变化的信息。然而，用于最强永磁体或硬盘驱动器中的记录介质的磁性材料远非完美，由纳米级的微晶组成，这些微晶通过晶界的Ei相互作用。此外，下一代磁记录技术将利用磁场和短激光脉冲的联合影响来切换磁矩的方向，从而表示二进制信息。关于晶界区域内的EI，或者在施加激光脉冲后EI是如何立即被修改的，我们知之甚少。本项目的目的是在这种情况下利用ES弹簧结构来获得有关EI的新信息。目前最先进的薄膜沉积将被用于制备Es结构，其中原子尺度结构可以被仔细地控制，以便更好地理解磁性和结构性质之间的关系。微波辐射将被用来激励ES，从而使磁矩以特征频率振荡，从而可以推断ES不同区域内的EI强度。特别是，X射线将用于探测运动，因为通过调节从同步加速器获得的X射线光子的能量，可以分别确定不同原子物种的响应，从而提供关于振荡模式的更详细信息。最后，ES将被超快激光脉冲激发，以软化一个或多个硬层内的磁矩，从而使ES可以解开。这种解卷运动将提供有关材料的磁性参数(包括EI)如何被激光脉冲改变的信息，并将探索硬层的磁矩改变其取向所需的条件。因此，ESS作为激光辅助记录介质的潜力将被确定。

Ferromagnetic materials are found throughout the electromagnetic technology upon which modern life depends. They range from the bulk materials found in motors and dynamos to thin films used to store data in hard disk drives. Within a ferromagnet each atom has a magnetic moment, like planet earth, with north and south poles. The magnetic moments of adjacent atoms are forced to point in the same direction by the exchange interaction (EI), a purely quantum-mechanical effect, which is the most powerful force in magnetism, generating effective magnetic fields up to one hundred million times as strong as the earth's magnetic field.Our everyday experience is that some ferromagnets remain permanently magnetized while others do not. In the latter case, the magnetic moments have parallel alignment within microscopic regions known as domains, but different domains have magnetic moments pointing in different directions, so that there is no net magnetic moment overall. Neighbouring domains are separated by domain walls, about 10 nm (100 atomic diameters) wide, through which the orientation of the magnetic moments gradually rotates in a helical structure. The finite width of the domain wall is a consequence of the EI and the wall stores exchange energy like a spring. The proposed project is concerned with exchange spring (ES) structures that form through the thickness of multilayered thin films. Alternate layers are termed hard and soft because it is easier to form the helical structure in the latter. The helical structure is induced either by applying a magnetic field or by changing the relative alignment of the magnetic moments in different hard layers so as to twist the magnetic moments in the soft layers in between. By studying the form of the ES structure, and its response to external stimuli, we can obtain information about how the strength of the EI varies through the structure.The EI present in perfect crystals can already be calculated accurately. However, the magnetic materials used in the strongest permanent magnets, or as recording media in hard disk drives, are far from perfect and consist of nanoscale crystallites that interact with each other through the EI at their grain boundaries. Furthermore, the next generation of magnetic recording technology will use the combined influence of a magnetic field and a short laser pulse to switch the orientation of the magnetic moments so as to represent binary information. Rather little is known about the EI within the grain boundary regions, or how the EI is modified immediately after application of a laser pulse. The aim of this project is to use ES spring structures to obtain new information about the EI in such circumstances.State of the art thin film deposition will be used to fabricate ES structures in which the atomic scale structure can be carefully controlled so that the relationship between magnetic and structural properties can be better understood. Microwave radiation will be used to excite the ES so that magnetic moments oscillate with characteristic frequencies that allow the strength of the EI within different regions of the ES to be deduced. In particular, x-rays will be used to detect the motion, since by tuning the energy of the x-ray photons obtained from a synchrotron, the response of different atomic species can be separately determined, providing more detailed information of the mode of oscillation. Finally, the ES will be excited with an ultrafast laser pulse to soften the magnetic moments within one or more hard layer so that the ES can unwind. This unwinding motion will provide information about how the magnetic parameters of the material, including the EI, are modified by the laser pulse, and the conditions required for the magnetic moments of the hard layer to switch their orientation will be explored. The potential of ESs as laser assisted recording media will hence be determined.