Nicola A. Hill

FIRST-PRINCIPLES INVESTIGATION OF FERROMAGNETISM AND FERROELECTRICITY IN BISMUTH MANGANITE

We present results of local spin density approximation (LSDA) pseudopotential calculations for the perovskite structure oxide, bismuth manganite (BiMnO3). The origin of the differences between bismuth manganite and other perovskite manganites is determined by first calculating total energies and band structures of the high symmetry cubic phase, then sequentially lowering the magnetic and structural symmetry. Our results indicate that covalent bonding between bismuth cations and oxygen anions stabilizes different magnetic and structural phases compared with the rare earth manganites. This is consistent with recent experimental results showing enhancement of charge ordering in doped bismuth manganite.

Why are there any magnetic ferroelectrics

Abstract Multiferroic magnetoelectrics are rare materials which are both ferromagnetic and ferroelectric in the same phase. In this paper we show that their scarcity results from transition metal d electrons, which are essential for magnetism, de-stabilizing the off-center ferroelectric distortion. By studying some known multiferroics, we identify the unconventional driving forces which can cause ferromagnetism and ferroelectricity to occur simultaneously.

Influence of Quantum Confinement on the Electronic and Magnetic Properties of (Ga,Mn)As Diluted Magnetic Semiconductor

We investigate the effect of quantum confinement on the electronic structure of diluted magnetic semiconductor Ga1-xMnxAs using a combination of tight-binding and density functional methods. We observe half metallic behavior in the clusters as well as a strong majority-spin Mn d−As p hybridization down to sizes as small as 20 A in diameter. Below this size, the doped holes are significantly self-trapped by the Mn sites, signaling both valence and electronic transitions. Our results imply that magnetically doped III−V nanoparticles will provide a medium for manipulating the electronic structure of dilute magnetic semiconductors while conserving the magnetic properties and even enhancing them in certain size regimes.

First-principles study of strain-electronic interplay in ZnO: stress and temperature dependence of the piezoelectric constants

We present a first-principles study of the relationship between stress, temperature, and electronic properties in piezoelectric ZnO. Our method is a plane wave pseudopotential implementation of density-functional theory and density-functional linear response within the local-density approximation. We observe marked changes in the piezoelectric and dielectric constants when the material is distorted. This stress dependence is the result of strong, bond-length dependent hybridization between the O 2p and Zn 3d electrons. Our results indicate that fine tuning of the piezoelectric properties for specific device applications can be achieved by control of the ZnO lattice constant, for example by epitaxial growth on an appropriate substrate.

Density functional calculations for III-V diluted ferromagnetic semiconductors: A review

In this paper we review the latest achievements of density functional theory in understanding the physics of diluted magnetic semiconductors. We focus on transition-metal-doped III–V semiconductors, which show spontaneous ferromagnetic order at relatively high temperature and good structural compatibility with existing III–V devices. We show that density functional theory is a very powerful tool for (i) studying the effects of local doping defects and disorder on the magnetic properties of these materials, (ii) predicting properties of new materials, and (iii) providing parameters, often not accessible from experiments, for use in model Hamiltonian calculations. Such studies are facilitated by recent advances in numerical implementations of density functional theory, which make the study of systems with a very large number of atoms possible.

Magnetic stress as a driving force of structural distortions: the case of CrN

We show that the observed transition from rock salt to orthorhombic P(nma) symmetry in CrN can be understood in terms of stress anisotropy. Using local spin density functional theory, we find that the imbalance between stress stored in spin-paired and spin-unpaired Cr nearest neighbors causes the rock-salt structure to be unstable against distortions and justifies the observed antiferromagnetic ordering. This stress has a purely magnetic origin and may be important in any system where the coupling between spin ordering and structure is strong.

Influence of the local As antisite distribution on ferromagnetism in (Ga, Mn)As

The effect of the inclusion of As antisites in the diluted magnetic semiconductor (Ga, Mn)As is studied within the density functional theory in the local spin density approximation. In the case of the homogeneous distribution of Mn ions, we find that the ferromagnetism is weakened by the presence of the antisites. This is due to the compensation of the free holes which mediate the long-range ferromagnetic order. In contrast, when two Mn ions are coupled through only one As ion, the ferromagnetic and antiferromagnetic states are comparable in energy. In this case, the magnetic ground state depends on: (i) the position of the As antisites relative to the Mn, and (ii) the As antisite concentration. We explain our results using a model of competing antiferromagnetic super exchange and ferromagnetic double exchange via localized Zener carriers.

First principles study of structural, electronic and magnetic interplay in ferroelectromagnetic yttrium manganite

Abstract We present results of local spin density approximation pseudopotential calculations for the ferroelectromagnet, yttrium manganite (YMnO 3 ). The origin of the differences between ferroelectric and non-ferroelectric perovskite manganites is determined by comparing the calculated properties of yttrium manganite in its ferroelectric hexagonal and non-ferroelectric orthorhombic phases. In addition, orthorhombic YMnO 3 is compared with the prototypical non-ferroelectric manganite, lanthanum manganite. We show that, while the octahedral crystal field splitting of the cubic perovskite structure causes a centro-symmetric Jahn–Teller distortion around the Mn 3+ ion, the markedly different splitting in hexagonal perovskites creates an electronic configuration consistent with ferroelectric distortion. We explain the nature of the distortion, and show that a local magnetic moment on the Mn 3+ ion is a requirement for it to occur.

Ab initio transport theory for digital ferromagnetic heterostructures.

We present a theoretical density functional study of the electronic, magnetic, and transport properties of digital ferromagnetic heterostructures, obtained by delta doping GaAs with Mn. In the absence of intrinsic donors these systems have a half metallic density of states, with an exchange interaction much stronger than that of a random alloy with the same Mn concentration. Our ab initio ballistic transport calculations show that the carriers at the Fermi energy are strongly confined within a few monolayers around the MnAs plane. This strong confinement is responsible for the large exchange coupling and the two-dimensional half metallic behavior.

First principles study of intrinsic defects in (Ga,Mn)As

We present density functional calculations of the structural, electronic and magnetic properties of various intrinsic defects in (Ga,Mn)As. Our main finding is that As excess generally weakens the ferromagnetic coupling between Mn ions. Moreover, we find that the interaction is strongly dependent on the microscopic configuration of the Mn ions and the defects, suggesting that a mean field description is not always appropriate. r 2002 Elsevier Science B.V. All rights reserved.