Nonzero Magnetization Research Articles

Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: From spin-spiral states toward ferromagnetic order

We present a detailed study of the ground-state magnetic structure of ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster expansion technique in combination with the relativistic disordered local moment scheme to obtain parameters of spin models and then determine the favored magnetic structure of the system by means of a mean-field approach and atomistic spin dynamics simulations. For the case of a single monolayer of Fe, we find that layer relaxations very strongly influence the ground-state spin configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic couplings also have remarkable effects. To characterize the latter effect, we introduce and analyze spin collinearity maps of the system. While for two monolayers of Fe we find a single-$q$ spin spiral as ground state due to DM interactions, for the case of four monolayers, the system shows a noncollinear spin structure with nonzero net magnetization. These findings are consistent with experimental measurements indicating ferromagnetic order in films of four monolayers and thicker.

Singlet–Triplet Excitations and High-Field Magnetization in CuTe2O5

By measuring the THz electron spin resonance (ESR) transmission spectra and high-field magnetization on the spin-gapped system CuTe$_2$O$_5$, we identified the singlet-triplet excitations in the dimerized non-magnetic ground state. The determined spin-gap value of $h\nu_0=4.94$ meV at the $\Gamma$ point ($\mathbf{Q}\simeq\mathbf{0}$) is significantly smaller than the strongest antiferromagnetic exchange interaction between the Cu ions predicted by theoretical investigations. We also observed the critical field $H_{c1}^{a^*}=37.6$ T for \textbf{H} $\bot$ \emph{bc}-plane and $H_{c1}^{bc}=40.6$ T for \textbf{H} $\|$ \emph{bc}-plane at the onset of non-zero magnetization, consistent with the gap value and corresponding anisotropic \emph{g}-factors determined previously. The observed singlet-triplet excitations in Faraday and Voigt configurations suggest a mixing of the singlet state with the $S_z=0$ triplet state and the $S_z=\pm 1$ triplet states, respectively, due to the Dzyaloshinskii-Moriya (DM) interaction with a DM vector perpendicular to the crystalline \emph{bc}-plane.

Origin of ferromagnetism in BaTiO3 nanoparticles

AbstractBased on a microscopic approach we demonstrate that the unexpected ferromagnetic properties of BaTiO3 (BTO) or PTO observed recently at room temperature are due to oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ti3+ or Ti2+ ions with nonzero net spin. The resulting different valence states composed of Ti3+ or Ti2+ offer a nonzero magnetization which decreases with increasing particle size. The system shows a multiferroic behavior below a critical size of the nanoparticles and the related polarization tends to a saturation value when the particle size is enhanced. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles investigation of higher oxides of uranium and neptunium:U3O8and Np2O5

A computational study is presented of the structural, electronic, and magnetic properties of ${\mathrm{U}}_{3}$O${}_{8}$ and Np${}_{2}$O${}_{5}$, which are actinide oxides in a higher oxidation state than the tetravalent state of the common dioxide phases, UO${}_{2}$ and NpO${}_{2}$. The calculations are based on the density functional theory+$U$ approach, in which additional Coulomb correlations on the actinide atom are taken into account. The calculated properties of these two higher oxidized actinide oxides are analyzed and compared to those of their tetravalent analogs. The optimized structural parameters of these noncubic oxides are found to be in reasonable agreement with available experimental data. ${\mathrm{U}}_{3}$O${}_{8}$ is predicted to be a magnetic insulator, having one U atom in a hexavalent oxidation state and two U atoms in a pentavalent oxidation state. For Np${}_{2}$O${}_{5}$, which is also predicted to be an insulator, a complicated noncollinear magnetic structure is computed, leading to a nonzero overall magnetization with a slight antiferromagnetic canting. The calculated electronic structures are presented and the variation of the U $5f$ or Np $5f$--O $2p$ hybridization with the oxidation state is analyzed. With increasing oxygen content, the nearly localized 5$f$ electrons of the actinide elements are more positioned near the Fermi level and the hybridization between 5$f$ and 2$p$ states is markedly increased.

Ferromagnetic signature in nanoparticles of diluted magnetic semiconductors of ZnO:V

Nanoparticles of ZnO:V prepared using sol–gel method are investigated for the magnetic properties. The particles of them possess polycrystalline and hexagonal wurtzite structure. The average size of particles is found to be around 60 nm. In the optical absorption study of the samples, a red shift of optical band edge and a narrowing of the optical band gap are observed when vanadium concentration is increased. The observed non-zero magnetization and coercive field determined from magnetic hysteresis loops indicate the exchange and anisotropy effects developed in nanoparticles of ZnO:V. EPR measurements provide substantial evidence for the presence of defect sites.

Unconventional magnetism in imbalanced Fermi systems with magnetic dipolar interactions

We study the magnetic structure of the ground state of an itinerant Fermi system of spin-\nicefrac{1}{2} particles with magnetic dipole-dipole interactions. We show that, quite generally, the spin state of particles depend on its momentum, i.e., spin and orbital degrees of freedom are entangled and taken separately are not ``good'' quantum numbers. Specifically, we consider a uniform system with non-zero magnetization at zero temperature. Assuming the magnetization is along $z$-axis, the quantum spin states are $\v{k}$-dependent linear combinations of eigenstates of the $\sigma_z$ Pauli matrix. This leads to novel spin structures in \textit{momentum space} and to the fact that the Fermi surfaces for ``up'' and ``down'' spins are not well defined. The system still has a cylindrical axis of symmetry along the magnetization axis. We also show that the self energy has a universal structure which we determine based on the symmetries of the dipolar interaction and we explicitly calculated it in the Hartree-Fock approximation. We show that the bare magnetic moment of particles is renormalized due to particle-particle interactions and we give order of magnitude estimates of this renormalization effect. We estimate that the above mentioned dipolar effects are small but we discuss possible scenarios where this physics may be realized in future experiments.

Iron oxides of Fazendão Deposit, East Border of Quadrilátero Ferrífero, Minas Gerais, Brazil

The iron oxides highly influence the soil structure and aggregation of mineral particles in soil. They also play an important role in some economical variables, as those related to the use of land for agriculture practices or mineral exploitation, in ore mining activities. About 60 % of all industrial activities on iron processing in Brazil is based on ores mined in the geodomain of Quadrilátero Ferrífero, state of Minas Gerais, Brazil. Samples of a mining area for iron, the Fazendão Deposit in the east border of Quadrilátero Ferrífero have been studied in an attempt to contribute to a better understanding of the key chemical and mineralogical pathways related to the formation and transformation of iron oxides, involving hematite. From powder diffraction patterns, it is confirmed that hematite is the main mineralogical phase in all samples. The total iron contents were found to range between 65.15 and 70.00 mass%. The saturation magnetization values, σ = 6.9 and 2.1 J T−1 kg−1 are significantly measurable only for the two samples showing some evidences of magnetite. 298 K-Mössbauer data confirmed the dominant occurrence of hematite in all samples, along with magnetite for the two samples showing non-zero magnetization. 110 K-Mössbauer data indicate that the hematite in all samples undergoes the Morin transition (TM ≈ 260 K), as expected for the relatively pure oxide. Maghemite could not be detected in any sample. New numerical analysis are now being performed, using Rietveld refinement of XRD data, in an attempt to obtain crystallographic results that could indicate more reliable evidences about the mechanisms of formation of hematite, particularly in the magnetic samples, for which the precursor is presumably magnetite.

Ferronematic Ground State of the Dilute Dipolar Fermi Gas

It is shown that a homogeneous two-component Fermi gas with (long-range) dipolar and short-range isotropic interactions has a ferronematic phase for suitable values of the dipolar and short-range coupling constants. The ferronematic phase is characterized by having a nonzero magnetization and long-range orientational uniaxial order. The Fermi surface of the spin-up (-down) component is elongated (compressed) along the direction of the magnetization.

Thermal entanglement of spin chains with quantum critical behavior

Results for the characteristics of the macroscopic thermal entanglement for integrable spin-1/2 antiferromagnetic chain models with multispin interactions in the thermodynamic limit are presented. Such interactions cause quantum-phase transitions to incommensurate phases with partial spontaneous magnetization. We have derived exact equations, which determine the thermal and magnetic field behavior of the concurrence of spin chains. The analysis of these equations shows that there exists a critical temperature, below which the thermal entanglement of considered spin chains is nonzero. We have shown that at those quantum-phase transitions, caused by multispin interactions, the thermal entanglement is reduced. Also, we have shown that the thermal entanglement is reduced due to nonzero magnetization of spin chains in the ground state, caused by multispin interactions.

Out-of-equilibrium phase re-entrance(s) in long-range interacting systems

Systems with long-range interactions display a short-time relaxation toward quasistationary states (QSSs) whose lifetime increases with system size. The application of Lynden-Bell's theory of "violent relaxation" to the Hamiltonian Mean Field model leads to the prediction of out-of-equilibrium first- and second-order phase transitions between homogeneous (zero magnetization) and inhomogeneous (nonzero magnetization) QSSs, as well as an interesting phenomenon of phase re-entrances. We compare these theoretical predictions with direct N -body numerical simulations. We confirm the existence of phase re-entrance in the typical parameter range predicted from Lynden-Bell's theory, but also show that the picture is more complicated than initially thought. In particular, we exhibit the existence of secondary re-entrant phases: we find unmagnetized states in the theoretically magnetized region as well as persisting magnetized states in the theoretically unmagnetized region. We also report the existence of a region with negative specific heats for QSSs both in the numerical and analytical caloric curves.

Ferromagnetic properties ofp-(Cd,Mn)Te quantum wells: Interpretation of magneto-optical measurements by Monte Carlo simulations

In order to single out dominant phenomena that account for carrier-controlled magnetism in $p{\text{-Cd}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{Te}$ quantum wells we have carried out magneto-optical measurements and Monte Carlo simulations of time-dependent magnetization. The experimental results show that magnetization relaxation is faster than 20 ns in the paramagnetic state. Decreasing temperature below the Curie temperature ${T}_{C}$ results in an increase of the relaxation time but to less than $10\text{ }\ensuremath{\mu}\text{s}$. This fast relaxation may explain why the spontaneous spin splitting of electronic states is not accompanied by the presence of nonzero macroscopic magnetization below ${T}_{C}$. Our Monte Carlo results reproduce the relative change of the relaxation time on decreasing temperature. At the same time, the numerical calculations demonstrate that antiferromagnetic spin-spin interactions, which compete with the hole-mediated long-range ferromagnetic coupling, play an important role in magnetization relaxation of the system. We find, in particular, that magnetization dynamics is largely accelerated by the presence of antiferromagnetic couplings to the Mn spins located outside the region, where the holes reside. This suggests that macroscopic spontaneous magnetization should be observable if the thickness of the layer containing localized spins will be smaller than the extension of the hole wave function. Furthermore, we study how a spin-independent part of the Mn potential affects ${T}_{C}$. Our findings show that the alloy disorder potential tends to reduce ${T}_{C}$, the effect being particularly strong for the attractive potential that leads to hole localization.

Effect of microstructural evolution on magnetic properties of Ni thin films

The magnetic properties of Ni thin films, in the range 20–500 nm, at the crystalline-nanocrystalline interface are reported. The effect of thickness, substrate and substrate temperature has been studied. For the films deposited at ambient temperatures on borosilicate glass substrates, the crystallite size, coercive field and magnetization energy density first increase and achieve a maximum at a critical value of thickness and decrease thereafter. At a thickness of 50 nm, the films deposited at ambient temperature onto borosilicate glass, MgO and silicon do not exhibit long-range order but are magnetic as is evident from the non-zero coercive field and magnetization energy. Phase contrast microscopy revealed that the grain sizes increase from a value of 30–50 nm at ambient temperature to 120–150 nm at 503 K and remain approximately constant in this range up to 593 K. The existence of grain boundary walls of width 30–50 nm is demonstrated using phase contrast images. The grain boundary area also stagnates at higher substrate temperature. There is pronounced shape anisotropy as evidenced by the increased aspect ratio of the grains as a function of substrate temperature. Nickel thin films of 50 nm show the absence of long-range crystalline order at ambient temperature growth conditions and a preferred [111] orientation at higher substrate temperatures. Thin films are found to be thermally relaxed at elevated deposition temperature and having large compressive strain at ambient temperature. This transition from nanocrystalline to crystalline order causes a peak in the coercive field in the region of transition as a function of thickness and substrate temperature. The saturation magnetization on the other hand increases with increase in substrate temperature.

Orbital magnetic moment of a chiral p-wave superconductor

The existence and magnitude of a bulk orbital angular momentum of the chiral condensate in the A phase of superfluid helium-3 is a longstanding matter of controversy. The analogous problem in a chiral p-wave superconducting material is the existence of a finite orbital magnetic moment in the bulk. In Sr2RuO4, the existence of such an orbital moment is strongly suggested by experimental evidence for spontaneous time-reversal symmetry breaking (TRSB) in the superconducting state, but the theories disagree on the expected magnitude of this moment. We show that a nonzero orbital magnetization density arises naturally in a realistic band model for Sr2RuO4, and its temperature dependence is qualitatively similar to those of the muon spin rotation and Kerr effect experimental results. The simplest model that leads to the orbital moment requires at least two degenerate atomic orbitals per Ru, which correspond to the Ru dxz and dyz states. This is in contrast to the theories of orbital angular momentum in the isotropic superfluid 3He, or models of orbital moment in Sr2RuO4 which assume only a single band at the Fermi level. The implications of this surprising result are explored.

Local electron structure and magnetization in [formula omitted]- Fe 86Mn 13C

The aim of the work is to elucidate the origin of magnetization presence in austenitic Fe 86Mn 13C steel after dynamic loading. The observation of microstructures in the region of transition from FCC austenitic Fe 86Mn 13C steel to FK12+FK14 type of Frank–Kasper tetrahedral close packed structure is described. We used the methods of optical microscopy, electron microscopy, electron diffraction and X-ray-diffraction to investigate the phase transition region. Changes of local magnetization were estimated by induction method. To explain the magnetization origin of the sample consisting of austenite grains and intergranular layers, which have Frank–Kasper’s structure (FK12+FK14) typical of β –Fe–Mn, the local electronic structure has been investigated for intergranular layers. The local electron structure of FK12 and FK14 clusters have been simulated by method of self-consistent field to understand the nature of non-zero magnetization of the Fe 87Mn 13 alloy exposed by shock deformation. It was shown, that numbers of states with upward and downward spins are not equal. Therefore the occurrence of magnetization is possible.

Ferrimagnetism of MnV2O4 spinel

The spinel MnV2O4 is a two-sublattice ferrimagnet, with site A occupied by theMn2+ ion and siteB by the V3+ ion. The magnon of the system, the transversal fluctuation of the total magnetization, is acomplicated mixture of the sublattice A and B transversal magnetic fluctuations. As aresult, the magnons’ fluctuations suppress in a different way the manganese andvanadium sublattice magnetization and one obtains two phases. At low temperature(0,T*) the sublattice Mn magnetization and sublattice V magnetization contributeto the magnetization of the system, while at a high temperature(T*,TN), the vanadium sublattice magnetization is suppressed by magnon fluctuations,and only the manganese ions have non-zero spontaneous magnetization. Amodified spin-wave theory is developed to describe the two phases and tocalculate the magnetization as a function of temperature. The anomalousM(T) curve reproduces the experimentally obtained zero-field-cooled (ZFC) magnetization.

Anomalous Hall effect in IV-VI magnetic semiconductors

Topological contribution to the anomalous Hall effect in IV-VI narrow-gap semiconductors with a nonzero spontaneous magnetization due to magnetic impurities is considered theoretically. The off-diagonal conductivity is calculated in the relativistic model of the IV-VI semiconductors. Spin-orbit interaction in these compounds is strong and cannot be treated perturbatively. Therefore, it is included in the Hamiltonian of a clean (defect-free) system. Geometrical interpretation of the topological contribution to the Hall conductivity is also briefly discussed.

Towards the theory of ferrimagnetism

A two-sublattice ferrimagnet, withspin-s1 operatorsS1i at the sublatticeA site and spin-s2 operators S2i at the sublattice B site, is considered. The magnon of the system, the transversal fluctuation ofthe total magnetization, is a complicate mixture of the transversal fluctuations of thesublattice A and B spins. As a result, the magnons’ fluctuations suppress in a different way themagnetic orders of the A and B sublattices and one obtains two phases. At low temperature(0,T*) the magnetic orders of the A and B spins contribute to the magnetization of the system, while at hightemperature (T*,TN), the magnetic order of the spins with a weaker intra-sublattice exchangeis suppressed by magnon fluctuations, and only the spins with strongerintra-sublattice exchange have non-zero spontaneous magnetization. TheT* transition is a transition between two spin-ordered phases in contrastto the transition from the spin-ordered state to the disordered state(TN-transition). There is no additional symmetry breaking, and the Goldstoneboson has a ferromagnetic dispersion in both phases. A modified spin-wavetheory is developed to describe the two phases. All known Neel’s anomalousM(T) curves are reproduced, in particular that with ‘compensation point’. Thetheoretical curves are compared with experimental ones for sulfo-spinelMnCr2S4−xSex and rare earth iron garnets.

Spin States of Zigzag-Edged Möbius Graphene Nanoribbons from First Principles

Mobius graphene nanoribbons have only one edge topologically. How the magnetic structures, previously associated with the two edges of zigzag-edged flat nanoribbons or cyclic nanorings, would change for their Mobius counterparts is an intriguing question. Using spin-polarized density functional theory, we shed light on this question. We examine spin states of zigzag-edged Mobius graphene nanoribbons (ZMGNRs) with different widths and lengths. We find a triplet ground state for a Mobius cyclacene, while the corresponding two-edged cyclacene has an open-shell singlet ground state. For wider ZMGNRs, the total magnetization of the ground state is found to increase with the ribbon length. For example, a quintet ground state is found for a ZMGNR. Local magnetic moments on the edge carbon atoms form domains of majority and minor spins along the edge. Spins at the domain boundaries are found to be frustrated. Our findings show that the Mobius topology (i.e., only one edge) causes ZMGNRs to favor one spin over the other, leading to a ground state with non-zero total magnetization.

Magnetization Anisotropy of Semiconductor Quantum Wires with Magnetoelectric Quantization

The carrier magnetization of diluted magnetic semiconductor quantum wires having hybrid magnetoelectric quantization has been investigated. The magnetization as a function of the chemical potential shows that magnetic ions in semiconductor quantum wires enhance the magnetization and induce nonzero magnetization in a low-chemical-potential region. Magnetization anisotropy is not predominant in quantum wires while it is manifested in quantum wells because this anisotropy originates from changes in the density of states. The magnetization of quantum wires shows superimposed oscillations due to two hybrid eigenfrequencies.

Nonequilibrium Tricritical Point in a System with Long-Range Interactions

Systems with long-range interactions display a short-time relaxation towards quasistationary states whose lifetime increases with system size. With reference to the Hamiltonian mean field model, we here show that a maximum entropy principle, based on Lynden-Bell's pioneering idea of "violent relaxation," predicts the presence of out-of-equilibrium phase transitions separating the relaxation towards homogeneous (zero magnetization) or inhomogeneous (nonzero magnetization) quasistationary states. When varying the initial condition within a family of "water bags" with different initial magnetization and energy, first- and second-order phase transition lines are found that merge at an out-of-equilibrium tricritical point. Metastability is theoretically predicted and numerically checked around the first-order phase transition line.