Domain boundary in multi-FERROIC materials

如今，纳米线中磁畴壁的传播已经被用作一种存储设备：每当磁畴壁通过拾取线圈时，就会发射一个信号并进行编码。同样的技术显然不适用于铁电或铁弹性畴壁，因为畴壁的运动不那么平滑：它可以“抽搐”并自发发出声波或电信号。这些信号是不需要的，在任何设备应用中都会导致“噪声”。不过，这种限制并不是物理上的需要。在铁弹性SrTiO3中可以看到平滑的运动，而大多数多孔材料几乎完全是“不稳定的”，并形成在任何设备应用中都无法控制的畴壁雪崩。该方案的第一个目的是研究噪声和无声墙传播之间的交叉。铁弹性畴壁和铁电畴壁（通常两者都有）还有另一个奇妙的特性：它们可以通过掺杂很容易地改变，它们可以弯曲，它们可以形成复杂的畴壁，这些畴壁包含比单一磁畴壁多得多的信息。例如WO3中的超导畴壁，BiFeO3中的高导电和光伏畴壁以及CaTiO3中的极性畴壁。所有这些材料都是众所周知的，并且可以在适当的衬底上常规沉积为薄膜。我们所缺少的是关于这种壁改变其局部结构的机制以及这种影响如何改变其流动性的知识。壁非常薄（1-10nm），因此通过实验手段研究其结构特性非常困难和昂贵。我们已经在这一领域做了一些最先进的实验工作，但现在通过计算机模拟相关的领域模式来推进我们的理论研究是及时的。适当的理论是基于具有相互作用的序参数的复杂朗道-金兹堡理论，领域模式的计算机模拟是基于与大量相互作用的“原子”相互作用的局部状态参数的力学非局部模型。在这里，至少需要100万个粒子才能看到边界效应、传播扭结激发和畴壁的相互干扰。我们将把这个规模扩大到1000万个粒子。因此，我们的第二个目标是推导出相互作用畴壁的实际热力学势，并在足够大的尺度上模拟图案的形成，以实现可能的器件应用。

The propagation of magnetic domain walls in a nanowire is already used as a memory device today: each time a domain wall passes a pick-up coil, a signal is emitted and encoded. The same technology does not apparently work in ferroelectric or ferroelastic domain walls because the movement of the wall is much less smooth: it can 'jerk' and emit spontaneously acoustic or electric signals. These signals are unwanted and lead to 'noise' in any device application. This limitation is not a physical necessity, though. Smooth movements are seen in ferroelastic SrTiO3 while most porous materials are almost totally 'jerky' and form avalanches of domain walls which can not be controlled in any device application. The first aim of the proposal is to investigate the crossover between noisy and silent wall propagation.Ferroelastic and ferroelectric domain walls (they are often both) have another fantastic property: they can be changed easily by doping, they can be bent, and they can form complex domain patterns which contain much more information than can be encoded in single magnetic domain walls. Examples are superconducting domain walls in WO3, highly conducting and photovoltaic walls in BiFeO3 and polar walls in CaTiO3. All these materials are well known and can be deposited routinely as thin films on appropriate substrates. What is missing is the knowledge of the mechanism by which such walls change their local structure and how this effect changes their mobilities. Walls are very thin (1-10nm) and it is therefore extremely difficult and costly to investigate their structural properties by experimental means. We have done some of the most advanced experimental work in this field but now it has become timely to advance our research theoretically by computer simulation of the relevant domain patterns. The appropriate theory is based on complex Landau-Ginzburg theory with interacting order parameters, the computer simulation of the domain patterns is based on mechanical, non-local models of interacting local state parameters with a large number of interacting 'atoms'. Here a minimum of 1 million particles are required to see boundary effects, propagating kink excitations and mutual jamming of domain walls. We will extend this size to >10 million particles.Our second aim is hence to derive realistic thermodynamic potentials for interacting domain walls and simulate the pattern formation on a large enough scale to be realistic for possible device applications.