Bridging magnetic and electronic structure techniques via atomistic approaches

构建具有磁性材料的结构-性质相图的新能力。在上个世纪这类材料的研究中，难题之一是总能量计算(目前基于密度泛函理论(DFT))和使用模型哈密顿方法的磁结构计算之间存在明显的障碍。在前者中，人们往往不清楚磁性成分在结构性质随温度、压力或化学势的变化中所起的作用。从DFT的观点来看，磁性作为自旋或磁矩的表现立即显现出来，但特定的相变可能是由系统的其他属性(如热膨胀)“驱动”的。从磁学的角度来看，从电子结构计算中获得的信息反过来可能是令人沮丧的不透明的，因为它没有澄清在特定条件下哪种类型的物理相互作用是主导的。为了将这两种方法结合在一起，我们建议从电子结构的许多细节中抽象出来，只集中在那些将直接影响磁性的细节上。系统的剩余相互作用可以用传统的原子间势来解释。将原子视为(可能可极化的)点电荷的原子论方法的威力将通过用点状磁矩来增强这一描述。因此，组合的总能量描述将包括半经典原子贡献和磁(自旋)模型项，该项也可以半经典或用适当的量子力学方法获得。后者通常需要大量的格子蒙特卡罗模拟，这将需要利用高性能计算机。学生的应用项目将涉及三元锰基氧化物，最初专注于模拟CaMnO_3，LaMnO_3及其固溶体。这项工作将涉及三个方面：(I)研究小单胞中磁有序的DFT计算，目的是获得磁相互作用的参数，将用于(Ii)磁蒙特卡罗模拟，目的是从模型哈密顿量和实验数据中获得序参数信息；以及(Iii)包括这两种类型的相互作用的两个纯体系的原子GUP模拟。当主要模型参数确定后，工作将转移到固溶体和构建相应的相图。

New capability of constructing structure-property phase diagrams of materials with magnetic properties. One of conundrums in the last century of research in this class of materials was a distinct barrier between total energy calculations, nowadays based on the Density Functional Theory (DFT), and magnetic structure calculations using model Hamiltonian approaches. In the former field it is often unclear what roles the magnetic components play in the structure property evolution with temperature, pressure or chemical potentials. The magnetism from the DFT point of view is immediately apparent as a manifestation of spin or the magnetic moment, but a particular phase transition may be "driven" by other properties of the system such as thermal expansion. From the magnetic perspective the information available from the electronic structure calculations in turn can be frustratingly opaque in that it does not clarify which types of the physical interactions are dominant for a particular set of conditions. To bring these two methods together, we propose to abstract from many details of the electronic structure and concentrate only on those that will directly affect the magnetic properties. The remaining interactions of the system can be accounted for using traditional interatomic potentials. The power of the atomistic approaches that treat atoms as (potentially polarisable) point charges will be enhanced by augmenting this description with point-like magnetic moments. A combined total energy description therefore will include semi-classical atomistic contribution and a magnetic (spin) model term that can be obtained also semi-classically or with an appropriate quantum-mechanical approach. The latter will typically require extensive lattice Monte Carlo simulations that will need to exploit High Performance Computers. The student's application project will concern ternary manganese based oxides initially concentrating on modelling CaMnO3, LaMnO3 and their solid solution. The work will involve three aspects: (i) DFT calculations studying magnetic ordering in a small unit cell aimed at obtaining parameters of magnetic interactions that will be used in (ii) magnetic Monte Carlo simulations that will aim to derive order parameter information from model Hamiltonians and the experimental data; and (iii) atomistic GULP simulations of the two pure systems including both types of interactions. When major model parameters will be established - the work will be moved to the solid solution and construction of the corresponding phase diagram.