Spin Moments Of Atoms Research Articles

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

Atomic relaxation around defects in magnetically disordered materials computed by atomic spin constraints within an efficient Lagrange formalism

Lattice and magnetic degrees of freedom are strongly coupled in magnetic materials. We propose a consistent first-principles framework to explore the joint configurational space. For this, we define atomic spin moments from the projector augmented-wave formalism of density-functional theory and control them via Lagrangian constraints. We demonstrate our approach for vacancy formation and migration in collinear paramagnetic bcc iron by implementing a relaxation scheme based on spin-space averaged forces (SSA relaxation). Based on these results we discuss the impact of the magnetic state on vacancy formation energies, migration barriers, and relaxations.

A giant enhancement of magnetic moment in a ternary three-shell icosahedral cluster: Fe@Mn12@Au20

ABSTRACTThe average magnetic moment per atom of Mn13 cluster is expected to be enhanced by doping or coating with a shell. Several ternary core–shell icosahedral clusters TM@Mn12@Au20 were constructed by combining substituting the central Mn with VIII elements (Fe, Co, Ni, Ru, Rh, Pd and Pt) and coating with a icosahedral Au20 shell, and systematically studied by using the first-principles density functional method. Compared to Mn13, Fe@Mn12@Au20 cluster shows a giant enhancement on total magnetic moment (52 µB) which can be greatly attributed to the ferromagnetic coupling between spin moments of atoms. Coating with Au20 shell enlarged the average distances of TM-Mn and Mn-Mn and is a useful way to change the magnetic coupling style. By analysis of density of states and electron localisation functional, we can conclude that the weak hybridisation between Fe and Mn in Fe@Mn12@Au20 is propitious to maintain their original direction of spin moments of atoms and then form ferromagnetic coupling.

First-principle based simulations of longitudinal spin-fluctuations in metals and integration in classical space with variable spin amplitude

The use of the classical Heisenberg model which incorporates only transverse spin degrees of freedom has only limited success for description of the metallic magnetism at finite temperature, since temperature and magnetic disorder induced longitudinal variations of the atomic spin moments might become large in the itinerant electron systems away from the limit of localized moments. In order to incorporate the longitudinal spin fluctuations in finite temperature simulation schemes a simple extended version of the Heisenberg model which allows for an on-site spin magnitude variation controlled by the one-site energy terms is widely used during the recent decade for ab-initio mapping and statistical simulations. Here, we apply and discuss such ab-initio based scheme for the canonical itinerant ferromagnetic metals (Fe, Co, Ni) and recently discovered high temperature antiferromagnet - V3Al, in conjunction with standard spherical integration metrics in classical spin state and the recently proposed linear one. We also examine the dependence of the results on the choice of the exchange and correlation potential in ab-initio total energy calculations. We compare the respective uncertainties in the calculated values of the magnetic ordering temperature and temperature dependent spin moment magnitude to the difference in the results which relate to the choice of the metrics.

Electronic structure, magnetism and thermoelectric properties of double perovskite Sr2HoNbO6

First principles calculations on structural, electronic, magnetic and thermoelectric properties of newly synthesized perovskite Sr2HoNbO6 are carried out at the behest of much reliable density functional theory (DFT). The equilibrium lattice parameters at the cost of structural optimizations are observed to agree with the available experimental data. Three different methods for exchange correlations are utilized to investigate the electronic structure of this oxide. Spin polarized band structure calculations predict the semiconducting nature of this compound along with a large energy gap of 3.6 eV. Ferromagnetic interactions among the constituent atomic spin moments determine the total magnetic moment μ = 4.0 μB for this material with a maximum contribution from μHo = 3.95 μB. In addition, the temperature dependent thermoelectric properties based on the electronic results of Sr2HoNbO6 in temperature range from 50 to 1000 K are investigated. The total negative Seebeck coefficient signifies the p-type conduction or holes as charge carriers along the selected temperature range. The outcome of this study specifies the maximum value of dimensionless figure of merit zT ≈ 0.97 at room temperature.

Enhanced exchange bias fields for CoO/Co bilayers: influence of antiferromagnetic grains and mechanisms

The emergence and optimization of devices that can be applied to spintronics have attracted considerable interest, and both experimental and theoretical approaches have been used in studies of exchange bias phenomena. A survey of the literature indicates that great efforts have been devoted to improving exchange bias fields, while only limited attempts have been made to controll the temperature dependence of exchange bias. In this study, the influence of antiferromagnetic grains on exchange bias phenomena in CoO/Co bilayers on a semiconductor surface was investigated. Based on an antiferromagnetic grain model, a correlation between grain size, grain density, blocking temperature, and the exchange bias field was established. For crystallites with a smaller median diameter, the dependence of the thickness of the CoO layer on blocking temperature showed a less pronounced variation. This is due to the larger thermal agitation of the atomic spin moments in the grain, which causes a weaker exchange coupling between atomic spin moments. The enhanced density of antiferromagnetic/ferromagnetic pinning sites resulting from an increased grain density is responsible for the enhancement in the exchange bias fields. The results reported herein provide insights into our knowledge related to controlling the temperature dependence of exchange bias and related mechanisms.

Improved Atoms-in-Molecule Charge Partitioning Functional for Simultaneously Reproducing the Electrostatic Potential and Chemical States in Periodic and Nonperiodic Materials.

We develop a nonempirical atoms-in-molecules (AIM) method for computing net atomic charges that simultaneously reproduce chemical states of atoms in a material and the electrostatic potential V(r) outside its electron distribution. This method gives accurate results for a variety of periodic and nonperiodic materials including molecular systems, solid surfaces, porous solids, and nonporous solids. This method, called DDEC/c3, improves upon our previously published DDEC/c2 method (Manz, T. A.; Sholl, D. S. J. Chem. Theory Comput. 2010, 6, 2455-2468) by accurately treating nonporous solids with short bond lengths. Starting with the theory all AIM charge partitioning functionals with spherically symmetric atomic weights must satisfy, the form of the DDEC/c3 functional is derived from first principles. The method is designed to converge robustly by avoiding conditions that lead to nearly flat optimization landscapes. In addition to net atomic charges, the method can also compute atomic multipoles and atomic spin moments. Calculations performed on a variety of systems demonstrate the method's accuracy, computational efficiency, and good agreement with available experimental data. Comparisons to a variety of other charge assignment methods (Bader, natural population analysis, electrostatic potential fitting, Hirshfeld, iterative Hirshfeld, and iterative stockholder atoms) show that the DDEC/c3 net atomic charges are well-suited for constructing flexible force-fields for atomistic simulations.

Methods for Computing Accurate Atomic Spin Moments for Collinear and Noncollinear Magnetism in Periodic and Nonperiodic Materials.

The partitioning of electron spin density among atoms in a material gives atomic spin moments (ASMs), which are important for understanding magnetic properties. We compare ASMs computed using different population analysis methods and introduce a method for computing density derived electrostatic and chemical (DDEC) ASMs. Bader and DDEC ASMs can be computed for periodic and nonperiodic materials with either collinear or noncollinear magnetism, while natural population analysis (NPA) ASMs can be computed for nonperiodic materials with collinear magnetism. Our results show Bader, DDEC, and (where applicable) NPA methods give similar ASMs, but different net atomic charges. Because they are optimized to reproduce both the magnetic field and the chemical states of atoms in a material, DDEC ASMs are especially suitable for constructing interaction potentials for atomistic simulations. We describe the computation of accurate ASMs for (a) a variety of systems using collinear and noncollinear spin DFT, (b) highly correlated materials (e.g., magnetite) using DFT+U, and (c) various spin states of ozone using coupled cluster expansions. The computed ASMs are in good agreement with available experimental results for a variety of periodic and nonperiodic materials. Examples considered include the antiferromagnetic metal organic framework Cu3(BTC)2, several ozone spin states, mono- and binuclear transition metal complexes, ferri- and ferro-magnetic solids (e.g., Fe3O4, Fe3Si), and simple molecular systems. We briefly discuss the theory of exchange-correlation functionals for studying noncollinear magnetism. A method for finding the ground state of systems with highly noncollinear magnetism is introduced. We use these methods to study the spin-orbit coupling potential energy surface of the single molecule magnet Fe4C40H52N4O12, which has highly noncollinear magnetism, and find that it contains unusual features that give a new interpretation to experimental data.

First-principles study of the onset of noncollinearity inMnnclusters: Magnetic arrangements inMn5andMn6

First-principles theoretical investigations of the noncollinearity of atomic spin moments in manganese clusters have been carried out within a gradient-corrected density-functional approach. Our studies on ${\mathrm{Mn}}_{5}$ and ${\mathrm{Mn}}_{6}$ include investigation of both collinear and noncollinear arrangements. It is shown that while the atomic structure of the ground state of ${\mathrm{Mn}}_{5}$ is a triangular bipyramid, the collinear and noncollinear arrangements have comparable energies and hence are degenerate. For ${\mathrm{Mn}}_{6}$, while the ground state has a square bipyramid arrangement, the noncollinear configuration is most stable making it the smallest cluster to feature a noncollinear ground state. The results are discussed in view of the recent experimental Stern-Gerlach profiles.

Determination of the Zones of Action of Alternating Magnetic Field on the Patient's Body

Alternating magnetic fields (AMF) are widely used in modern medical practice [1, 3, 5-12, 14, 15, 1721, 23]. Magnetic fields (MF) used for physiotherapy vary considerably: from constant fields and alternating fields with a frequency of 50 Hz to pulsed fields with pulse duration of 0.2-2 msec and magnetic induction of 1.5 T [14, 15]. Many questions concerning the mechanisms of physiological action of magnetic field still remain open. This makes it difficult to state exact requirements for medical equipment and methods for MF treatment. Constant magnetic field acts only on moving electric charges. In tissues and plasma of the human body charges are carried by very slow ions. The action of a constant magnetic field on such ions is negligible. Magnetic field also acts on spin moments of atoms. The mechanism of this action is well studied. However, on the biological level this action is also very weak [2, 9, 20, 23]. Presumably, the biological effect of magnetic field is caused by currents induced in tissues rather than by direct action. The biological action of electric currents is well studied. They are used both for stimulation and physiotherapy. Currents exert an effect on the state and metabolism of cells. Within the range of electric field strength E > 0.2-2 V/cm (EMF U > 220 mV for typical linear size of cell structure) depolarization of cell membranes (including excitation of nerve pulses) is initiated. Within the range of electric field strength from 0.02 to 0.2 V/cm, natural depolarization is improved, and natural metabolic processes are activated and synchronized. Electric fields with significantly smaller strength presumably have no biological effect, or this effect is manifested only upon a very long exposure. Leaving aside the difficult problem of assessment of complete biological effect of MF, we consider in this work only the biological effect of electric currents induced by MF. It should be noted that induced currents cause heating of tissues, which also has a biological effect [23]. The effect of MP on the domain structure of water is also of interest because water is the main solvent of plasma ions. However, it should be taken into account that the time of relaxation of domain boundaries in water decreases sharply with increasing temperature. At body temperature this relaxation time is very small. The currents induced by AMF are calculated from the magnetic induction B(xyz) and the rate of changes in the induction dB/dt. It is rather difficult to assess the induction B(xyz) inside a certain organ. Special software was developed for this purpose (see Appendix). The magnetic field configuration is taken to be known, whereas the rate of field change is taken to be uniquely determined by the pulse duration T. It is required to determine the distribution of field strength E(xyz) and electric currents in a patient’s body from the known magnetic field B(xyz). Faraday law is used to determine EMF and currents induced in a closed circuit L [13, 16, 22]:

Time-dependent critical spin fluctuations in magnetic salts

A short perspective of dynamic critical scattering is given, together with an orientation to modern theories of static and dynamic critical phenomena. The materials discussed are magnetic salts that are believed to spontaneously order through correlations induced by a Heisenberg exchange interaction between atomic spin moments.

Epitaxial Layers of FCC FE on CU(001)

Metastable films of fcc Fe epitaxed to Cu(001) show unusual ferromagnetic properties(1). Changes in the lattice volume occur with film thickness in the 1 to 5 monolayer range, reflecting changes in the magnitude of the spin moments and the magnetic exchange coupling. The associated uniaxial strain is manifest in strong magnetic anisotropy, layer by layer lattice expansion, phonon softening, and surface layer atomic reconstructions. We present LEED results of a detailed structural investigation, neutron reflectance measurements of the spin moments, surface magneto-optic Kerr effect studies of the magnetic anisotropy, and angle resolved photoemission of the electronic states. The observed effects are due to the strain-related kinetic energy associated with the formation of the atomic spin moments, predicted by spin-density-functional calculations of bulk fcc Fe(2).

Magnetism of iron. II

A previously developed theory of the magnetism of iron, based upon the notion of a randomized exchange field, is modified and extended. It is shown that Coulomb fields arise in association with the randomized exchange field; the modification allows one to take into account these fields, which are found to change the details of the previous results without affecting the main conclusions. The theory has been extended to calculate the properties of the model at finite temperatures: the Curie temperature (1840 K), the magnetization curve, the paramagnetic susceptibility (a Curie-Weiss law), and the effective interatomic exchange coupling are calculated for iron. The magnitudes of the atomic spin moments were found to vary little up to 1.5 times the Curie temperature.