Ground-state Magnetic Structure Research Articles

Variational cluster approximation study of Mott transition with strong spin-orbit coupling

Motivated by recent experiments on Sr2IrO4, the ground state magnetic and electronic structures are studied theoretically for a two-dimensional three-band Hubbard model with strong spin-orbit coupling. To treat spin-orbit coupling, local Coulomb interactions, and band structure effects on the same footing, the variational cluster approximation based on the self-energy functional theory is employed. It is found that for a relatively large coupling region, the ground state is an anisotropic antiferromagnetic Mott insulator of an effective local angular momentum Jeff = 1/2 with xy plane as an easy plane. This anisotropy is caused by the strong spin-orbit coupling along with the inter-orbital Hund's coupling. The momentum resolved one-particle excitations are also studied for the Mott insulating phase. It is found that the low-energy one-particle excitations consist mostly of the Jeff = 1/2 state, a direct evidence of a novel Jeff = 1/2 Mott insulator.

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We have determined the ground state of the layered compound CaMn 2Sb 2 which crystalizes in the CaAl 2Si 2 structure. We have performed specific heat and neutron powder diffraction measurements at different temperatures. The neutron powder diffraction results reveal that the system orders antiferromagnetically at 88 K which agrees with the specific heat measurements. The ground state magnetic structure is consistent with previous theoretical calculations.

Magnetic properties of transition-metal-dopedZn1−xTxO(T=Cr, Mn, Fe, Co, and Ni) thin films with and without intrinsic defects: A density functional study

Theoretical calculations based on density-functional theory and generalized gradient approximation have been carried out in studying the electronic structure and magnetic properties of transition-metal-doped ${\text{Zn}}_{1\ensuremath{-}x}{T}_{x}\text{O}$ ($T=\text{Cr}$, Mn, Fe, Co, and Ni) $(11\overline{2}0)$ thin films systematically with and without intrinsic point defects (e.g., vacancies and interstitials), and as function of concentration and distribution of dopants and vacancies. Using large supercells and geometry optimization without symmetry constraint, we are able to determine the sites that metal atoms prefer to occupy, their tendency to cluster, the preferred magnetic coupling between magnetic moments at transition-metal sites, and the effect of intrinsic point defects on the nature of their coupling. Except for Mn atom, which distributes uniformly in ZnO thin films in dilute condition, transition-metal atoms occupying Zn sites prefer to reside on the surface and couple antiferromagnetically. The presence of native point defects has a large effect on the ground-state magnetic structure. In particular, $p$-type defects such as Zn vacancies play a crucial role in tuning and stabilizing ferromagnetism in ${\text{Zn}}_{1\ensuremath{-}x}{T}_{x}\text{O}$ thin films ($T=\text{Cr}$, Mn, Fe, and Ni), while $n$-type defects such as O vacancies or Zn interstitials greatly enhance the ferromagnetic coupling in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ thin films. The present study provides a clear insight into the numerous conflicting experimental results on the magnetic properties of $T$-doped ZnO systems.

Multi-step magnetic transition in non-centrosymmetric compound CeCoGe3

Single crystal neutron diffraction experiments on non-centrosymmetric pressure-induced superconductor CeCoGe3 were carried out in order to reveal a magnetic structure in the ground state. This compound shows multi-step magnetic transitions at TN1=20 K, TN2=11.5 K and TN3=7.5 K. We have observed the appearance of the magnetic reflection at (1 01/2) together with the weaker one at (10 1/4) and its higher-order harmonics at 2.9K. These results indicate that the ground state magnetic structure consists of two components with dominant q1=(0 01/2) and q2=(0 03/4). No magnetic peak was observed along (0 0 l), indicating that the directions of corresponding antiferromagnetic moments are parallel to the c-axis. Consequently, CeCoGe3 has at least two Ce sites in the ground state with respect to the magnitude of the magnetic moment.

On the ground-state magnetic structure in Er2Ni2Pb

We have studied the unusual low-temperature magnetic phase of Er2Ni2Pb using powder neutron diffraction measurements in zero field down to 460 mK. Our previous neutron diffraction experiments down to 1.5 K showed that magnetic Bragg reflections seen in Er2Ni2Pb can be indexed by several propagation vectors that partially coexist. All the incommensurate propagation vectors seemed to disappear in the low temperature limit. The present study, however, shows that reflections belonging to the propagation vector q’ = (0.47 0 1/2) do not disappear but remain present down to 460 mK. This highly unexpected result suggests that the magnetic structure described by this propagation vector might not be a simple sine-wave modulation. One interesting possibility here is a spin-slip structure as the ground state.

Competing magnetic structures and magnetic transitions inEr2Ni2Pb: Powder neutron diffraction measurements

We have studied the magnetic structures of ${\text{Er}}_{2}{\text{Ni}}_{2}\text{Pb}$ using a powder neutron diffraction technique in zero field. Previous bulk measurements suggested three distinct magnetic phase transitions. Our neutron diffraction experiments, which were made in the range 1.5\ensuremath{-}5 K, showed that magnetic Bragg reflections in ${\text{Er}}_{2}{\text{Ni}}_{2}\text{Pb}$ can be indexed by several propagation vectors that coexist over an extensive temperature range. Rather than a homogeneous magnetic structure that is simultaneously described by all the existing propagation vectors, several spatially separated structures appear to exist in ${\text{Er}}_{2}{\text{Ni}}_{2}\text{Pb}$. The appearance/disappearance of representative reflections at ${T}_{N}=3.5\text{ }\text{K}$, ${T}_{m1}=3.0\text{ }\text{K}$, ${T}_{m2}=2.3\text{ }\text{K}$, and ${T}_{m3}=1.8\text{ }\text{K}$ denote magnetic phase transitions. The only magnetic state that is determined by a single propagation vector exists just below ${T}_{N}$. In all other magnetic states, more than one propagation vectors are stable. Except for the lowest temperature state, which is commensurate, all other propagation vectors are incommensurate with respect to the crystal structure. Although the coexistence of several spatially separated magnetic structures can be explained by the competition of magnetic interactions along particular crystallographic directions, some of the details, e.g., the exact ground-state magnetic structure, are still unclear and need further study.

Magnetic Ground State of Coupled Edge-SharingCuO2Spin Chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO2 spin chains, as found in the (La,Ca,Sr)14Cu24O41 system, for instance. Our data rely on spin-polarized electronic structure calculations including on-site interaction [local density approximation plus the multiorbital mean-field Hubbard model (LDA+U)] and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO2 spin chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.

Ground state magnetic structure of Ce2Ni3Ge5

Abstract We have carried out neutron diffraction experiments on Ce2Ni3Ge5 single crystal. Ce2Ni3Ge5 orders antiferromagnetically below TN = 5 K and exhibits a successive transition at T* = 4.3 K. We have confirmed that the ground state antiferromagnetic structure is characterized with the propagation vector q = [0 1 0]. We have revealed that this magnetic structure stabilizes only below T* = 4.3 K from the simultaneous neutron diffraction and electrical resistivity measurements, contrary to the previously reported neutron powder diffraction result. From the analysis of magnetic reflection intensities, we suggest that the ground state magnetic structure is a canted AF structure, where the moments orient about 20° tilted from the a-axis in the ab-plane. The obtained AF components are 0.45 μB/Ce along the a-axis and 0.15 μB/Ce along the b-axis. It is suggested that the high temperature phase is characterized with another magnetic phase having a different propagation vector, which is not determined in the present study.

Magnetic phase transitions of NdRhSn

A NdRhSn single crystal has been grown and investigated with respect to crystal structure and magnetic behavior. The magnetization and specific heat were measured as functions of temperature and the magnetic field applied along the crystallographic a- and c-axis, respectively. A strong uniaxial magnetocrystalline anisotropy with the c-axis as the easy magnetization direction has been observed. The obtained experimental data can be conceived within the following scenario of magnetism in NdRhSn: the paramagnetic–antiferromagnetic phase transition at 9.8 K and at 7.6 K, an order-to-order magnetic phase transition takes place. The magnetic state below 7.6 K is characterized by a spontaneous magnetization corresponding to ∼2/3 of the value of the Nd 3+ free-ion moment. This may be either due to crystal-field effects or due to a complex ground-state magnetic structure.

Finite-temperature magnetism of tetragonal iron

Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Néel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration.

Magnetic structure of the local-moment antiferromagnet CeCuSn

We report on single-crystal neutron diffraction studies of the local-moment antiferromagnet CeCuSn. The ground-state magnetic structure is characterized by a magnetic wave vector k=(0.115,0,0). The onset of antiferromagnetic order occurs around 12K with an inflection in the temperature dependence of the magnetic intensities at about 8K. This is in contrast to bulk measurements, which only show broad features at 8–10K. The ordered moments are likely reduced from the free-ion moment for Ce.

Ab Initio Investigations in Magnetic Oxides

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structure of the FeFe-cofactor of the iron-only nitrogenase and possible mechanism for dinitrogen reduction

We use density functional calculations to model the FeFe-cofactor of the iron-only nitrogenase. We determine the ground state geometical and magnetic structure and show that our results are in agreement with experimental observations. We model the reactivity of the FeFeco towards hydrogen and nitrogen and compare to the FeMoco. Our results for the difference in reactivity can be linked to experimental observations.

Mössbauer-effect study of competitive exchange interactions in UNi 1− xCo xGa solid solutions

Results of Mössbauer spectroscopy of 119Sn atoms substituted on Ga sites in the UNi 1− x Co x Ga solid solutions are presented and discussed together with previously reported magnetization data in terms of evolution of magnetism between the antiferromagnet UNiGa and ferromagnet UCoGa. The Mössbauer spectra were analyzed by considering the hyperfine field transferred to the 119Sn nucleus from the six nearest U magnetic moments and a weak signal due to anisotropic polarization of Sn 5p states. The low-temperature spectra for UNiGa and UCoGa are consistent with their ground-state magnetic structures. The nonmagnetic component observed for UNiGa persists in the spectra for x up to 0.4 which indicates the presence of antiferromagnetically coupled U moments. The nonmagnetic component emerging in the spectra for x around 0.77 is tentatively attributed to clusters of frustrated U magnetic moments surrounding the 119Sn nuclei.

The ab initio ground state properties and magnetic structure of plutonium

Using the fully relativistic, full potential, spin-polarized, linearizedaugmented-plane-wave (RSPFLAPW) method, the energy differences, theelectronic structure and magnetic moments of alpha- and delta-plutonium withdifferent alignments of the atomic magnetic moments have been calculated. Wehave found in our calculations that both α-Pu and δ-Pustructures have an antiferromagnetic order at their experimental equilibrium volumes.Similar results have also been obtained by other researchers for δ-Pu. Animportant conclusion from the present work is that taking the spin polarization intoaccount improves the calculated density of states for α-Pu.This fact may be considered as an argument for the importance of magneticinteractions in the low temperature phase of plutonium.

First-principles calculations of spin spirals inNi2MnGaandNi2MnAl

We report here non-collinear magnetic configurations in the Heusler alloys Ni2MnGa and Ni2MnAl which are interesting in the context of the magnetic shape memory effect. The total energies for different spin spirals are calculated and the ground state magnetic structures are identified. The calculated dispersion curves are used to estimate the Curie temperature which is found to be in good agreement with experiments. In addition, the variation of the magnetic moment as a function of the spiral structure is studied. Most of the variation is associated with Ni, and symmetry constraints relevant for the magnetization are identified. Based on the calculated results, the effect of the constituent atoms in determining the Curie temperature is discussed.

Electronic Structure And Magnetism of Novel Diluted Magnetic Semiconductors CdGeP 2 : Mn and ZnGeP 2 : Mn

Chalcopyrite II-IV-V 2 semiconductors CdGeP 2 : Mn and ZnGeP 2 : Mn are new types of diluted magnetic semiconductors (DMSs). Since their ferromagnetic Curie temperatures are much higher than room temperature, these DMSs are good candidates for materials to be used in spintronics devices. Their electronic and magnetic structures have been investigated using the first-principles calculations based on the Korringa-Kohn-Rostoker coherent-potential-approximation and local-density-approximation (KKR-CPA-LDA) methods. When Cd or Zn atoms are substituted by Mn atoms, the ground state magnetic structure is spinglass-like. On the other hand, if Mn atoms substitute Ge atoms, the system becomes ferromagnetic through the double-exchange mechanism. However, the calculation of the formation energies shows that this system is not energetically favorable. Instead, the system with vacancies (Cd, Vc, Mn)GeP 2 or a non-stoichiometric (Cd, Ge, Mn)GeP 2 are also ferromagnetic and, moreover, energetically stable. We conclude that either of these variants possess a ferromagnetic phase of the kind CdGeP 2 : Mn. Similar conclusions are obtained for ZnGeP 2 : Mn.

Ab initio investigations in magnetic oxides

It is argued that band theory can be used as a valuable tool to investigate and model properties of several classes of oxide systems. This theoretical framework is presented in the first part of the article with details on the density functional theory (DFT) and its approximations leading to the local spin density functional LSDF as well as on the common methods built around it (mainly the augmented spherial wave ASW method used throughout this work). The major part of the article is devoted to case studies meant to illustrate the wealth and limits of the LSDF in addressing the electronic and magnetic structures of a selected variety of transition metal oxides of technological importance. Such systems are iron monoxide “FeO” (mainly the high pressure form), the archetype of half metallic ferro magnets CrO2, used in magnetic recording media, the simple and double perovskite derived systems: SrFeO3 and La2TIrO6 (T=Mn, Fe, Co) as well as manganese oxides (CaMnO3, Ca4Mn4O10 and Tl2Mn2O7) for which a giant magnetoresistive GMR behaviour was identified in recent years. The computed ground state magnetic configurations and electronic structure results are discussed and confrontations with experimental data are carried out when available. The chemical bonding properties are analysed, and a new conceptual approach is provided.

Hydrogen in Laves phaseZrX2(X=V,Cr, Mn, Fe, Co, Ni) compounds: Binding energies and electronic and magnetic structure

The effects of hydrogen on the ground-state electronic and magnetic structures of ${\mathrm{AB}}_{2}$ (A=Zr; $B=\mathrm{V},$ Cr, Mn, Fe, Co, Ni) Laves phase compounds were investigated by first-principles local-density-functional calculations. We calculated the relative stability of atomic hydrogen at various interstitial tetrahedral sites formed by two A atoms and two B atoms $(2A2B$ site), one A atom and three B atoms $(1A3B$ site), and four B atoms $(4B$ site). We find that (i) for ${\mathrm{ZrV}}_{2}$ and ${\mathrm{ZrCr}}_{2},$ hydrogen prefers the site with the largest interstitial hole size (i.e., $2A2B$ site); (ii) for ${\mathrm{ZrMn}}_{2},$ ${\mathrm{ZrFe}}_{2},$ and ${\mathrm{ZrCo}}_{2},$ the hydrogen binding energies at the $1A3B$ site become comparable to or even lower than those at the $2A2B$ site once the antibonding states of B atoms become progressively occupied. In H-free ${\mathrm{ZrFe}}_{2},$ we found a large magnetic moment $(0.5{\ensuremath{\mu}}_{\mathrm{B}})$ at the Zr site, which is coupled antiparallel to the moment at the Fe site $(1.9{\ensuremath{\mu}}_{\mathrm{B}}).$ The hydrogen absorption does not have a strong effect in suppressing the magnetic moments of atoms closest to the absorbed hydrogen but increases the magnetic moments of atoms not having hydrogen as the nearest neighbor. This increase in the moment is partly due to the H-induced lattice expansion.

Temperature-induced magnetization reorientation in arrays of magnetic dots on a magnetic substrate

We studied magnetic properties of regular arrays of magnetic dots on a magnetic layer. We took into account the exchange interaction, the dipolar interaction of amplitude D between spins, and the perpendicular anisotropy A. The spins are of the classical Heisenberg model. Due to the competition between D and A, the ground state (GS) can be perpendicular or parallel to the substrate. We have studied the body-centered-cubic and hexagonal-close-packed dot lattices. We have determined the GS magnetic structure as a function of A and D by a steepest descent method. The results show that below (above) a critical value of D/A, the GS spin configuration is perpendicular (parallel). We have studied finite-temperature properties of our system of dots in the vicinity of the critical D/A by Monte Carlo simulations. The obtained results show spectacular features, in particular on the spin reorientation. A phase diagram in the vinicity of the critical value of D/A is presented.