Twisted magnetization phases in orbital-dominant rare-earth nitrides

<p>In this thesis we investigate the magnetic properties of NdN and SmN, members of the rare-earth nitrides, a series of intrinsic ferromagnetic semiconductors. In rare-earth systems, the strong spin-orbit coupling of the partially filled 4ƒ shell ensures that there is a substantial orbital contribution to the ferromagnetic moment, in contrast to many transition metal systems where the orbital moment is usually quenched. In SmN and NdN the orbital moment actually exceeds the spin moment, and the resulting orbital dominant magnetization allows for the fabrication of a magnetic heterostructures showing novel behavior. We report a new theoretical study of the magnetic properties on both SmN and NdN by considering the atomic-like 4ƒ electrons. These calculations incorporate spin-orbit coupling, the exchange interaction in a self-consistent mean-field approach, and crystal field interactions in an arbitrary-multiplet point-charge model. Our findings show excellent agreement with the experimentally measured ferromagnetic moments of SmN and NdN, representing an advance from previous theoretical studies. We also report an experimental study on SmN/GdN heterostructures using the element-resolved method of x-ray magnetic circular dichroism (XMCD) to probe the magnetism. The competition between the orbital-dominant Zeeman coupling in SmN and the ferromagnetic spin-based interface exchange with GdN, which has purely a spin moment, results in a twisted magnetization profile. The depth profile of the magnetization derived from XMCD measurements showed good agreement with an analytical model developed to describe the competing interactions. In a second study, a superlattice of NdN/GdN was investigated via XMCD and standard magnetometry techniques. A twisted magnetization was shown to be present due to the same mechanism as in the SmN/GdN system. By varying the maximum applied field and temperature, twisted phases were shown to develop in both GdN and NdN layers. These twisted phases in orbital-dominant ferromagnetic semiconductors represent a departure from previously explored spin-dominant metallic systems displaying similar twisted phases.</p>

Twisted magnetization phases in orbital-dominant rare-earth nitrides

<p>In this thesis we investigate the magnetic properties of NdN and SmN, members of the rare-earth nitrides, a series of intrinsic ferromagnetic semiconductors. In rare-earth systems, the strong spin-orbit coupling of the partially filled 4ƒ shell ensures that there is a substantial orbital contribution to the ferromagnetic moment, in contrast to many transition metal systems where the orbital moment is usually quenched. In SmN and NdN the orbital moment actually exceeds the spin moment, and the resulting orbital dominant magnetization allows for the fabrication of a magnetic heterostructures showing novel behavior. We report a new theoretical study of the magnetic properties on both SmN and NdN by considering the atomic-like 4ƒ electrons. These calculations incorporate spin-orbit coupling, the exchange interaction in a self-consistent mean-field approach, and crystal field interactions in an arbitrary-multiplet point-charge model. Our findings show excellent agreement with the experimentally measured ferromagnetic moments of SmN and NdN, representing an advance from previous theoretical studies. We also report an experimental study on SmN/GdN heterostructures using the element-resolved method of x-ray magnetic circular dichroism (XMCD) to probe the magnetism. The competition between the orbital-dominant Zeeman coupling in SmN and the ferromagnetic spin-based interface exchange with GdN, which has purely a spin moment, results in a twisted magnetization profile. The depth profile of the magnetization derived from XMCD measurements showed good agreement with an analytical model developed to describe the competing interactions. In a second study, a superlattice of NdN/GdN was investigated via XMCD and standard magnetometry techniques. A twisted magnetization was shown to be present due to the same mechanism as in the SmN/GdN system. By varying the maximum applied field and temperature, twisted phases were shown to develop in both GdN and NdN layers. These twisted phases in orbital-dominant ferromagnetic semiconductors represent a departure from previously explored spin-dominant metallic systems displaying similar twisted phases.</p>

Ferromagnetism out of charge fluctuation of strongly correlated electrons in κ-(BEDT-TTF)2Hg(SCN)2Br

We perform magnetic susceptibility and magnetic torque measurements on the organic κ-(BEDT-TTF)2Hg(SCN)2Br, which is recently suggested to host an exotic quantum dipole-liquid in its low-temperature insulating phase. Below the metal-insulator (MI) transition temperature, the magnetic susceptibility follows a Curie–Weiss law with a positive Curie–Weiss temperature, and a particular $$M\propto \sqrt{H}$$ M ∝ H curve is observed. The emergent ferromagnetically interacting spins amount to about 1/6 of the full spin moment of localized charges. Taking account of the possible inhomogeneous quasi-charge-order that forms a dipole-liquid, we construct a model of antiferromagnetically interacting spin chains in two adjacent charge-ordered domains, which are coupled via fluctuating charges on a Mott-dimer at the boundary. We find that the charge fluctuations can draw a weak ferromagnetic moment out of the spin singlet domains.

Magnetic Field Perturbations to a Soft X-ray-Activated Fe (II) Molecular Spin State Transition

The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.

Coverage-dependent magnetic and electronic properties of graphene with Co adatoms

Decorating two-dimensional (2D) materials with transition-metal adatoms is an effective way to bring about new physical properties that are intriguing for applications in electronics and spintronics devices. Here, we systematically studied the coverage-dependent magnetic and electronic properties of graphene decorated by Co adatoms, based on first-principles calculations. We found that if the Co coverage is larger than 1/3[Formula: see text]ML, the Co atoms will aggregate to form a Co monolayer and then a van der Waals bilayer system between the Co monolayer and graphene forms. When the Co coverage is [Formula: see text][Formula: see text]ML, the Co adatom is spin-polarized with spin moment varying from 1.1 to 1.4[Formula: see text][Formula: see text]. The [Formula: see text] and [Formula: see text] orbitals of Co hybridize significantly with the [Formula: see text] bands of graphene, which generates a series of new bands in the energy range from [Formula: see text][Formula: see text]eV to 1[Formula: see text]eV with respect to the Dirac point of graphene. In most cases, the new bands near the Fermi level lead to topological states characterized by the quantum anomalous Hall effect.

Magnetic Field Perturbations to a Soft X-Ray Activated Fe (II) Molecular Spin State Transition

The X-ray induced spin crossover transition of an Fe(II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior that is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.

Dumbbell configuration of silicon adatom defects on silicene nanoribbons

Using density functional theory (DFT), we performed theoretical investigation on structural, energetic, electronic, and magnetic properties of pure armchair silicene nanoribbons with edges terminated with hydrogen atoms (ASiNRs:H), and the absorptions of silicon (Si) atom(s) on the top of ASiNRs:H. The calculated results show that Si atoms prefer to adsorb on the top site of ASiNRs:H and form the single- and/or di-adatom defects depending on the numbers. Si absorption defect(s) change electronic and magnetic properties of ASiNRs:H. Depending on the adsorption site the band gap of ASiNRs:H can be larger or smaller. The largest band gap of 1 Si atom adsorption is 0.64 eV at site 3, the adsorption of 2 Si atoms has the largest band gap of 0.44 eV at site 1-D, while the adsorption at sites5 and 1-E turn into metallic. The formation energies of Si adsorption show that adatom defects in ASiNRs:H are more preferable than pure ASiNRs:H with silicon atom(s). 1 Si adsorption prefers to be added on the top site of a Si atom and form a single-adatom defect, while Si di-adatom defect has lower formation energy than the single-adatom and the most energetically favorable adsorption is at site 1-F. Si adsorption atoms break spin-degeneracy of ASiNRs:H lead to di-adatom defect at site 1-G has the highest spin moment. Our results suggest new ways to engineer the band gap and magnetic properties silicene materials.

A semi exact solution for a metallic phase in a Holstein-Hubbard chain at half filling with Gaussian anharmonic phonons

The nature of phase transition from an antiferromagnetic SDW polaronic Mott insulator to the paramagnetic bipolaronic CDW Peierls insulator is studied for the half-filled Holstein-Hubbard model in one dimension in the presence of Gaussian phonon anharmonicity. A number of unitary transformations performed in succession on the Hamiltonian followed by a general many-phonon averaging leads to an effective electronic Hamiltonian which is then treated exactly by using the Bethe-Ansatz technique of Lieb and Wu to determine the energy of the ground state of the system. Next using the Mott–Hubbard metallicity condition, local spin-moment calculation, and the concept of quantum entanglement entropy and double occupancy, it is shown that in a plane spanned by the electron–phonon coupling coefficient and onsite Coulomb correlation energy, there exists a window in which the SDW and CDW phases are separated by an intermediate phase that is metallic.

Magnetization control by angular momentum transfer from surface acoustic wave to ferromagnetic spin moments

Interconversion between electron spin and other forms of angular momentum is useful for spin-based information processing. Well-studied examples of this are the conversion of photon angular momentum and rotation into ferromagnetic moment. Recently, several theoretical studies have suggested that the circular vibration of atoms work as phonon angular momentum; however, conversion between phonon angular momentum and spin-moment has yet to be demonstrated. Here, we demonstrate that the phonon angular momentum of surface acoustic wave can control the magnetization of a ferromagnetic Ni film by means of the phononic-to-electronic conversion of angular momentum in a Ni/LiNbO3 hybrid device. The result clearly shows that the phonon angular momentum is useful for increasing the functionality of spintronic devices.

Spin Berry points as crucial for ultrafast demagnetization

Laser-induced ultrafast demagnetization has puzzled researchers around the world for over two decades. Intrinsic complexity in electronic, magnetic and phononic subsystems is difficult to understand microscopically. So far, it is not possible to explain demagnetization using a single mechanism, which suggests a crucial piece of information still missing. In this paper, we return to a fundamental aspect of physics: spin and its change within each band in the entire Brillouin zone. We employ face-centered cubic (fcc) Ni as an example and use an extremely dense k mesh to map out spin changes for every band close to the Fermi level along all the high symmetry lines. To our surprise, spin angular momentum at some special k points abruptly changes from [Formula: see text] to [Formula: see text] simply by moving from one crystal momentum point to the next. This explains why intraband transitions, which the spin superdiffusion model is based upon, can induce a sharp spin moment reduction, and why electric current can change spin orientation in spintronics. These special k points, which are called spin Berry points [M. V. Berry, Proc. R. Soc. Lond. A 393 (1984) 45], are not random and appear when several bands are close to each other, so the Berry potential of spin majority states is different from that of spin minority states. Although within a single band, spin Berry points jump, when we group several neighboring bands together, they form distinctive smooth spin Berry lines. It is the band structure that disrupts those lines. Spin Berry points are crucial to laser-induced ultrafast demagnetization and spintronics.