Spin Exchange Constants Research Articles

Effect of the surface anisotropy on the magnetic behavior of ultra-thin films single-domain on the spin reorientation transition regime by monte carlo simulation

In this work, Monte Carlo simulations allowed for studying the effect of the surface anisotropy on the Spin Reorientation Transition regime for single-domain ultra-thin films. The simulations included spin moments (1), exchange constant (10 meV), magnetocrystalline anisotropy (0.5 meV), and dipolar magnetic interaction (0.05 meV). We evaluated the surface anisotropy constant for K S = 0.5, 1, and 2 meV and thicknesses ranging from 3 to 11 cells. We determined the magnetothermal and hysteresis loop in- and out-plane at different angles to the plane. Analysis of Variance allows for modeling the coercivity as a function of the thickness and the applied field direction (in- and out-plane). Under these simulation conditions, the Spin Reorientation Transitions appear only at a thickness of 7 cells with K S =1 meV. The main contribution to magnetization rises out-plane at low thicknesses and in-plane at high values. However, the dominant magnetization contribution is in-plane for the thickness of 7 cells and low surface anisotropy. The magnetization is mainly oriented, out-plane for the high surface anisotropy case. However, hysteresis loops were observed independent of the applied magnetic field direction and thickness. Analysis of Variance results suggests that the thickness and the applied magnetic field direction allowed for tunning the coercivity (hard or soft magnetic materials). Sample simulated ignoring the magnetocrystalline and the surface anisotropies produce magnetic components in- and out-plane with slight differences as angle function. The combination of dipolar interactions and magnetocrystalline anisotropy showed two magnetic behaviors, a hard magnetic phase at low angles and a soft one at high angles in-plane direction. The inverse behavior results in the out-plane component. We observed the same behavior when surface anisotropy is considered. The differences are the transition angle of 50° in the first case and 30° in the second.

First-principles study on the electronic and magnetic properties of RCrWO6 (R=Y and Lu) and YFeWO6

Polar magnetic oxide materials have many interesting physical properties and been extensively studied in past two decades. Recently, two new Cr-based polar materials, i.e. YCrWO6 and LuCrWO6, are synthesized in the experiment. Based on density functional theory and Monte Carlo simulations, we present a systematic study on the electronic and magnetic properties of YCrWO6 and LuCrWO6. Our results indicate that their magnetic ground state is antiferromagnetic, where Cr atoms in the ab plane have the same spin orientation while they couple antiferromagnetically along the c axis. The calculated Néel temperatures of YCrWO6 and LuCrWO6 are 21.9 and 16.3 K, respectively, which are well consistent with the experimental results. Finally, we clarify the origin of the non-collinear magnetism in YFeWO6 by its spin-exchange constants. Our work reveals the magnetic interactions on these materials and can help further understand the magnetic behaviors of other polar magnetic oxide compounds.

Closing the spin gap of (NH4)xK1−xCuCl3 through chemical substitution

Unlike potassium trichlorocuprate ${\mathrm{KCuCl}}_{3}$, the isostructural ammonium trichlorocuprate ${\mathrm{NH}}_{4}{\mathrm{CuCl}}_{3}$ exhibits the rather unusual effect of orientation-independent steps in the magnetization $(M)$ as a function of the applied magnetic field $(H)$. However, replacement of ${\mathrm{NH}}_{4}^{+}$ by the isomeric ${\mathrm{K}}^{+}$ ion results in a smearing of these steps. With the view of probing this change, we carried out on the mixed system ${({\mathrm{NH}}_{4})}_{x}{\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{CuCl}}_{3}$, with $x=0.0, 0.3, 0.6, 0.9, \mathrm{and} 1.0:$ (1) High-field pulsed measurements of $M$ versus $H$ at 1.5 K, (2) susceptibility measurements from 1.8 to 295 K and (3) zero-field specific heat measurements from 1.8 to 295 K. The magnetization steps were found to be highly sensitive to the ${\mathrm{NH}}_{4}^{+}$ ion concentration, with the spin gap $\mathrm{\ensuremath{\Delta}}$ generally decreasing with increasing $x$. Analysis of the susceptibility and specific heat data with several theoretical schemes also supports this general trend. This includes data analysis using the Bleaney-Bowers, Ising chain, and the mean-field model. These results yielded intradimer spin-exchange constants of the order of 20--40 K, somewhat smaller than expected for ${\mathrm{Cu}}^{2+}\ensuremath{-}{\mathrm{Cu}}^{2+}$. This, in addition to the variation of $x$ with $\mathrm{\ensuremath{\Delta}}$, points to the need for additional theoretical modeling.

Theoretical models of spin-exchange optical pumping: Revisited and reconciled

Theoretical models for continuous-flow and stopped-flow spin-exchange optical pumping of 129Xe have long predicted much higher 129Xe polarization values than are measured experimentally, leading to a search for additional depolarization mechanisms. In this work, we show that a misapplication of the general theory of spin-exchange optical pumping along with the incorrect use of previously measured spin-exchange constants has been perpetuated in the past 20 years and is the main cause of the long-held discrepancy between theoretical and experimental 129Xe polarization values. Following the standard theory of spin-exchange optical pumping developed almost 40 years ago by Happer et al., we outline the common mistake made in the application of this theory in modern theoretical models and derive a simplified expression of the spin-exchange cross section that can be used to correctly predict 129Xe polarization values under any set of experimental conditions. We show that the complete expression of the spin-exchange cross section derived using the work of Happer et al. predicts spin-exchange rates tenfold higher than those previously assumed in theoretical models of continuous-flow and stopped-flow spin-exchange optical pumping and can fully rectify the long-standing discrepancy between theoretical and experimental polarization values.

Ferromagnetic Half-Semiconductor (HSC) gaps in co-doped CdS: Ab-initio study

In this work we have used the density functional theory (DFT) to study structural, magnetic and electronic properties of Co doped CdS in the zinc-blende (ZB) phase. Three approximations have been used to treat the exchange-correlation potential: the (PBE) GGA, (PBE) GGA+U and the model of Tran–Blaha modified Becke–Johnson potential (TB-mBJ). The on–site Coulomb interaction correction given by the Hubbard U has been calculated by the local density approximation constraint for the Co electronic orbitals. The theoretical results show that all the properties under study are affected by the doping of Co atom. Co doped CdS becomes a Ferromagnetic Half-Semiconductors (HSC). The total magnetic moment increases with increasing Co concentration; reaching the value of 9 (in the units of Bohr magneton) at high concentration (x = 18.75 %). The total magnetic moment value is an integer in the GGA+U and TB-mBJ approximations. Furthermore, to validate the effects resulting from the exchange splitting process, we have calculated the values of the spin-exchange constants N0α and N0β, respectively. All these changes make CdS:Co extremely interesting system for the spintronic applications.

A Systematic Study of Structural, Magneto-Electronic and Thermodynamic Properties of Mg1−xCrxSe DMS Alloys in the Rock–Salt Phase for Spintronic Applications

In this work, we use the full potential linearized augmented plane wave (FP-LAPW + lo) method under the framework of spin polarization density functional theory (SP-DFT) to study the structural properties of diluted magnetic semiconductors alloys, Mg1−xCrxSe (x = 0.25, 0.50 and 0.75), in the rock–salt ferromagnetic phase by using the WC-Cohen GGA approximation. To compute the electronic and magnetic properties of our compounds, we have used the Tran Blaha modified Becke–Johnson potential (TB-mBJ) combined with PBE approximation. The results of electronic properties show that our compounds have a half-metallic behavior with a spin polarization of 100% at the Fermi level. Moreover, at all concentrations, the magnetic moment has been estimated as equal to 4.0 (μB). Furthermore, to validate the effects resulting from the exchange splitting process, we calculate the values of the spin-exchange constants N0α and N0β, respectively. Finally, we present in detail the effects of the temperature and the pressure on some macroscopic thermodynamic parameters by using the quasi-harmonic Debye model.

Spin-orbital model of stoichiometric LaMnO3 with tetragonal distortions

The model developed for LaMnO$_3$ addresses the spin-orbital order by superexchange and Jahn-Teller orbital interactions in the cubic (perovskite) symmetry up to now whereas real crystal structure is strongly deformed. We identify and explain three \textit{a priori} important physical effects arising from tetragonal deformation: (i) the splitting of $e_g$ orbitals $\propto E_z$, (ii) the directional renormalization of $d-p$ hybridization $t_{pd}$, and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO$_3$ crystal we evaluate their magnitude. It is found that the major effects of deformation are enhanced amplitude of $x^2-y^2$ orbitals induced in the orbital order by $E_z\simeq 300$ meV and anisotropic $t_{pd}\simeq 2.0$ (2.35) eV along the $ab$ ($c$) cubic axis, in very good agreement with the Harrison's law. We show that the tetragonal model analyzed within mean field approximation provides a surprisingly consistent picture of the ground state. Excellent agreement with the experimental data is obtained simultaneously for: (i) $e_g$ orbital mixing angle, (ii)~spin exchange constants, and (iii) the temperatures of spin and orbital phase transition.

A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties.

Four new manganese germanates and silicates, A2MnGeO4 (A = Li, Na) and A2MnSiO4 (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li2MnGeO4 is orthorhombic (Pmn21) layered, isostructural with Li2CdGeO4, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A2MXO4, Pna21 and Pbca AMO2), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO2. The sublattices of the magnetic Mn2+ ions in both structure types under study (Pmn21 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li2MnSiO4 were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag2MnSiO4 remains paramagnetic down to 2 K, but A2MnXO4 (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < TN for all AFM phases. We constructed the magnetic phase diagrams for A2MnXO4 (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.

Entangled quantum Otto and quantum Stirling heat engine based on two-spin systems with Dzyaloshinski-Moriya interaction

Recently, the influences of the Dzyaloshinski-Moriya (DM) interaction on the performances of the basic thermo-dynamical quantities have attracted a lot of attention. A large number of investigations on the quantum coupling systems with DM interaction have been carried out. However, the specific effects of spin-orbit coupling with the performance on the quantum heat engine have not been taken into account in previous studies. DM interaction is a special kind spin-orbit coupling. To enrich the research of the quantum heat engines, the investigation about the effect of DM interaction on its thermodynamic characteristics should be included. In this study, we construct two entangled quantum engines based on spin-1/2 systems with different DM interactions, with the spin exchange constant and magnetic field fixed. The quantum Otto engine and the quantum Stirling engine are discussed in this article. By numerical calculation, we obtain the expressions for several thermodynamic quantities and plot the isoline maps of the variation of the basic thermodynamic quantities such as heat transfer, work with D1 and D2 and their efficiency in the two engines. The results indicate that the DM interaction plays an important role in the thermodynamic quantities for the quantum Otto engine and the quantum Stirling engine. In addition, the positive work condition is discussed with different DM interactions, with the spin exchange constant and magnetic field. Furthermore fixed, it is found that the efficiency of quantum Otto engine cycle is smaller than the Carnot efficiency while the quantum Stirling cycle can exceed the Carnot efficiency by using the regenerator. Finally, the second law of thermodynamics is shown to be valid in the two entangled quantum systems.

Spin Exchange Between Charged Paramagnetic Particles in Dilute Solutions

Kinetic equations for the spin density matrix which take into account binary collisions and a method of calculating the spin exchange effective radius have been generalized to the case of dilute solutions of charged paramagnetic particles. The effective radius of the spin exchange and rate constant of the bimolecular spin exchange between charged paramagnetic particles in solutions have been calculated numerically. Calculations have been performed under the assumption that the exchange interaction is isotropic and decays exponentially with the increase in the distance between radicals, and the solution has a given dielectric permittivity and Debye screening radius. Dependences of the spin exchange rate constant on the mutual diffusion coefficient, exchange and electrostatic interactions parameters have been found numerically. The theory has been applied to experimental results taken from the literature. The rate constant of the spin exchange between radicals of like charge found from the experiment and calculated within the developed theory are in good qualitative agreement .

Evaluating the Curie-Weiss Temperature of a Magnetic System Composed of Nonequivalent Magnetic Ions in Terms of Spin Exchange Constants

E-mail: mike_whangbo@ncsu.eduReceived January 18, 2014, Accepted February 5, 2014Key Words : Nonequivalent magnetic ions, Curie-Weiss temperature, Mean field theory, Spin exchange pa-rameters, Density functional theory Magnetic properties of a system containing magnetic ionsare related to its electronic structure,

Strongly correlated one-dimensional magnetic behavior ofNiTa2O6

The magnetic properties of NiTa$_2$O$_6$ were investigated by magnetic susceptibility, specific heat, electron paramagnetic resonance, neutron powder diffraction and pulse field magnetization measurements. Accompanying \textit{ab initio} DFT calculations of the spin-exchange constants complemented and supported our experimental findings that NiTa$_2$O$_6$ must be described as a quasi-1D Heisenberg $S$ = 1 spin chain system with a nearest-neighbor only anti-ferromagnetic spin-exchange interaction of 18.92(2) K. Inter-chain coupling is by about two orders of magnitude smaller. Electron paramagnetic resonance measurements on Mg$_{1-x}$Ni$_x$Ta$_2$O$_6$ ($x \approx$ $1\%$) polycrystalline samples enabled us to estimate the single-ion zero-field splitting of the $S$ = 1 states which amounts to less than $4\%$ of the nearest-neighbor spin-exchange interaction. At 0 T NiTa$_2$O$_6$ undergoes long-range anti-ferromagnetic ordering at 10.3(1) K evidenced by a $\lambda$-type anomaly in the specific heat capacity. On application of a magnetic field the specific heat anomaly is smeared out. We confirmed the magnetic structure by neutron powder diffraction measurements and at 2.00(1) K refined a magnetic moment of 1.93(5) $\mu_{\rm{B}}$ per Ni$^{2+}$ ion. Additionally, we followed the magnetic order parameter as a function of temperature. Lastly we found saturation of the magnetic moment at 55.5(5) T with a $g$-factor of 2.14(1), with an additional high field phase above 12.8(1) T. The onset of the new high field phase is not greatly effected by temperature, but rather smears out as one approaches the long-range ordering temperature.

Structural, electronic, and magnetic properties of double perovskite Pb2CrMO6 (M=Mo, W and Re) from first-principles investigation

We use first-principles methods to study double perovskite Pb2CrMO6 (M: some 4d and 5d transition metals), and find that a magnetic I4/m structure is lowest in total energy per formula unit for M=Mo, W, and Re. Our electronic structure investigation shows that the three double perovskite compounds are half-metallic ferrimagnets with total magnetic moments of 2μB, 2μB and 1μB per formula unit, respectively. With the spin–orbit coupling included, our calculations show the high magnetocrystalline anisotropy (MCA) in Pb2CrReO6. Fortunately, the spin polarization of Pb2CrMO6 at the Fermi level remains 90% ~99%. The orbital moments of 3d Cr and 4d Mo are small, but the 5d ions of W and Re have large unquenched orbital moments, which can be attributed stronger spin–orbit coupling in the 5d orbitals than in 3d/4d ones. In addition, these results are confirmed by using improved exchange–correlation functionals. Furthermore, we calculate the spin exchange constants of the three Pb2CrMO6, and our Monte Carlo simulated Curie temperatures for them are at least 480K. These materials, when synthesized, should be useful in spintronics.

Electronic structure and magnetic and optical properties of double perovskite Bi2FeCrO6 from first-principles investigation

Double perovskite Bi2FeCrO6, related with multiferroic BiFeO3, is very interesting because of its strong ferroelectricity and high magnetic Curie temperature beyond room temperature. We investigate its electronic structure and magnetic and optical properties by using a full-potential density-functional method. Our optimization shows that it is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to previous studies. Spin exchange constants and optical properties are calculated. Our Monte Carlo magnetic Curie temperature is 450 K, much higher than any previously calculated value and consistent with experimental results. Our study and analysis reveal that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful in understanding such perovskite materials and exploring their potential applications.

Synthesis and Characterization of the Crystal Structure and Magnetic Properties of the New Fluorophosphate LiNaCo[PO4

The new compound LiNaCo[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO(4)]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) Å, and Z = 8. The structure consists of edge-sharing CoO(4)F(2) octahedra forming CoFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie-Weiss behavior above 60 K with θ = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO(4)]F undergoes a three-dimensional magnetic ordering at T(mag) = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO(3) chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations.

Non-Idle-Spin Behavior and Field-Induced Magnetic Transitions of the Triple Chain Magnet Cu3(OH)4SO4

The magnetic properties of the triple chain magnet system Cu 3 (OH) 4 SO 4 were examined by evaluating the spin exchange constants, the Dzyaloshinskii–Moriya (DM) vectors, and the magnetic anisotropy energies of the Cu(1) and Cu(2) sites on the basis of density functional calculations. Our work shows that Cu 3 (OH) 4 SO 4 is not an idle-spin magnet, the Cu(1) spins have a strong antiferromagnetic coupling in the Cu(1) chain, and the Cu(2) spin arrangements of a Cu(2)–Cu(1)–Cu(2) triple chain result from the presence of two non-equivalent sets of antiferromagnetic interactions between the Cu(1) and Cu(2) chains. The probable spin arrangements of a triple chain as well as the magnetic energy terms responsible for the field-induced successive magnetic transitions were identified.

First-principles study on the electronic structure and magnetism of layered oxyselenide La2Mn2Se2O3

The electronic structure and magnetism of layered oxyselenide La2Mn2Se2O3 have been studied by using first-principles calculations within the generalized gradient approximation (GGA) and GGA + U methods. The G-type antiferromagnetic (AF) state is calculated to be the most stable phase among the various magnetic configurations of interest, irrespective of the choice of the functional used, which is in good agreement with the experiments. In contrast to La2Fe2Se2O3 and La2Co2Se2O3, in which the AF states show metallic behavior under the GGA method, we predict the ground state of La2Mn2Se2O3 is a semiconductor with an indirect band gap of ∼0.52 eV via the GGA calculations. This is closely related to a closed shell configuration and large exchange splitting (∼3.5 eV) in the Mn 3d states. Moreover, the magnetic properties are also discussed in terms of the calculated Heisenberg spin exchange constants, suggesting that La2Mn2Se2O3 is a strong two-dimensional magnetically frustrated system.

Spin-Orbital Liquid on a Triangular Lattice

Using Lanczos exact diagonalization of finite clusters we demonstrate that the spin-orbital $d^1$ model for triply degenerate $t_{2g}$ orbitals on a triangular lattice provides an example of a spin-orbital liquid ground state. We also show that the spin-orbital liquid involves entangled valence bond states which violate the Goodenough-Kanamori rules, and modify effective spin exchange constants.

ChemInform Abstract: Nature of the Spin Frustration in the CuO2 Ribbon Chains of LiCuVO4: Crystal Structure Determination at 1.6 K, Magnetic Susceptibility Analysis, and Density Functional Evaluation of the Spin Exchange Constants.

AbstractLiCuVO4 crystallizes in the space group Imma and the structure at 1.6 K is very close to that determined by powder XRD at room temperature.

On the Nature of the Spin Frustration in the CuO2 Ribbon Chains of LiCuVO4: Crystal Structure Determination at 1.6 K, Magnetic Susceptibility Analysis, and Density Functional Evaluation of the Spin Exchange Constants

The spin-1/2 Cu(2+) ions of LiCuVO(4) form one-dimensional chains along the b direction, and the spin frustration in LiCuVO(4) is described in terms of the nearest-neighbor ferromagnetic exchange J(1) and the next-nearest-neighbor antiferromagnetic exchange J(2) in these chains. Recently, it has become controversial whether or not J(1) is stronger in magnitude than J(2). To resolve this controversy, we determined the crystal structure of LiCuVO(4) at 1.6 K by neutron diffraction, analyzed the magnetic susceptibility of LiCuVO(4) to deduce the Curie-Weiss temperature θ and the J(2)/J(1) ratio, and finally extracted the spin exchange constants of LiCuVO(4) on the basis of density functional calculations. Our work shows unambiguously that the Curie-Weiss temperature θ of LiCuVO(4) is negative in the range of -20 K, so that J(2) is substantially stronger in magnitude than J(1).