Total-energy Band Calculations Research Articles

Magnetic structure in FeRh from constrained total-energy calculations

The volume dependence of antiferromagnetic and ferromagnetic total-energy solutions derived from constrained total-energy band calculations is shown to explain the pressure and magnetic field effects on the antiferromagnetic to ferromagnetic transition in FeRh. We show that zero-point energy corrections are necessary to obtain reasonable agreement with experimental critical pressures and magnetic fields, and that the difference in electronic specific heats of the different magnetic states is small and does not play a role in the transition. Lattice constants of the magnetic states are inverted with respect to Invar, leading to enhanced rather than invariant thermal expansion near the transition, so that FeRh can be classified as an “anti-Invar” system.

Band Calculations for MnZn and MnHg Fixing Local Spin Directions

Both of the compounds of MnZn and MnHg have a cubic structure of CsCl type, while the proposed spin alignments are different each other: non-collinear and collinear ones for MnZn and MnHg, respectively. To investigate the mechanism that produces the different spin alignments, total-energy band calculations have been performed by the use of an exchange-correlation potentital that depends on each of local spin directions at atomic spheres. Our calculations give the results consistent with experiment, and suggest that the hybridization between majority- and minority-spin states of the Mn-atom d band benefits the non-collinear alignment.

Band Calculations for Non-Collinear Spin Arrangementsin Gamma-Phase Manganese-Iron Alloys

We perfomed total-energy band calculations for Mn, FeMn, Fe 3 Mn and Fe in gamma phase. In the calculations, we dealt with not only a collinear spin arrangement but also non-collinear spin arrangements in the local density approximation. Apart from gamma-Fe and FeMn, we obtained reasonable results in the ground-state spin arrangement and magnetic moments.

First-principles calculation of the Ag-Cu phase diagram.

Total-energy band calculations based on the augmented-spherical-wave method are used to determine the binding curves of the elements and of selected compounds in the Ag-Cu system. The calculated total energies, together with vibrational free energies determined by a Debye-Gr\"uneisen analysis of the binding curves and configurational entropies determined by the cluster variation method, are used to calculate the solid-state portion of the Ag-Cu phase diagram. The solubilities at both ends of the phase diagram, calculated with no adjustable parameters, are in excellent agreement with experiment. For the Ag-Cu system, it is shown that local volume relaxation effects are very important and that significant improvement in the calculated phase diagram is obtained by including vibrational free energies.

Magnetic States in bcc Manganese

We have carried out the total-energy band calculations of bcc Mn as a function of lattice constant for paramagnetic, ferromagnetic and antiferromagnetic states. We have found that bcc Mn undergoes a first-order transition from a low-spin state to a high-spin state, not only in the ferromagnetic state but also in the antiferromagnetic state. The obtained results of the total-energy calculations indicate that bcc Mn prefers a paramagnetic state under compression, a low-spin ferromagnetic state under zero pressure and a high-spin antiferromagnetic state under expansion.

Magnetic and metal-insulator transitions in metallic hydrogen.

Total-energy band calculations using a local-spin-density approximation and the fixed-spin-moment procedure are used to investigate the volume dependence of the magnetic and electronic properties of hydrogen constrained to an fcc unit cell over a range from below the equilibrium volume to the free-atom limit. With increasing volume, the system first undergoes a transition from nonmagnetic to ferromagnetic behavior and then undergoes a second transition from a ferromagnetic metal to a ferromagnetic insulator. The magnetic transition is shown to be second order, but the metal-insulator transition is shown to be higher than second order, in contradiction to previous work. The volume dependence of the zero-field total energy, magnetic moment, electron occupancy, magnetic modulus, band edges, and bandwidths are presented. The s- and p-electron occupancy is shown to vary linearly over the range of volumes considered.

Antiferromagnetism in 4d transition metals.

Total-energy band calculations employing a fixed-spin-moment procedure and the augmented-spherical-wave method are used to study the volume dependences and existence ranges of antiferromagnetic behavior in the 4d transition metals constrained to cubic lattices. At expanded volumes we find stable antiferromagnetic solutions of the Kohn-Sham equations for bcc niobium, molybdenum, and technetium, and for fcc technetium, and metastable antiferromagnetic solutions for fcc ruthenium. We find no stable antiferromagnetic solutions for fcc rhodium or palladium. Comparisons are made with the occurrence of antiferromagnetism in the 3d transition metals.

Antiferromagnetism in 3dtransition metals

Total-energy band calculations using the local-spin-density approximation, the augmented-spherical-wave method, and the fixed-spin-moment procedure are used to investigate the volume dependence of the magnetic and electronic properties of 3d transition metals constrained to cubic environments in one-atom or two-atom unit cells. The calculations cover a range of volumes spanning the nonmagnetic-ferromagnetic and the nonmagnetic-antiferromagnetic transitions. We find antiferromagnetism in bcc vanadium at expanded volumes, show that bcc chromium has a first-order transition from nonmagnetic to antiferromagnetic behavior at a 2% expanded lattice constant, and find that previously reported antiferromagnetic solutions for bcc iron are unstable. Comments are made on when antiferromagnetism occurs and on the many-atom ground states for chromium and manganese.

High-spin and low-spin states in Invar and related alloys.

Total-energy band calculations that show the coexistence of a high-spin and low-spin state in fcc transition metals and alloys are presented. The energy difference between the two states is shown to be a function of the electron concentration and to vanish at 8.6. At larger electron concentrations the low-temperature state is the high-spin state, and the thermal expansion is shown to pause at a system-dependent characteristic temperature. At lower electron concentrations the low-temperature state is the low-spin state, and enhanced thermal expansion is expected. An analysis that leads to a qualitative understanding of the thermal properties of Invar and that implies a connection with martensitic transformations and spin glasses in related alloys is presented. For Invar a magnetic collapse from the high-spin to the low-spin state at a pressure of 55 kbar is predicted.

Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese.

Total-energy band calculations, including an antiferromagnetic extension of the fixed-spin-moment procedure, are used to study magnetovolume effects in bulk fcc iron and maganese. By constraining these systems to have a fixed total magnetic moment in a single-atom fcc unit cell, we find magnetovolume instabilities in the form of first-order transitions from nonmagnetic to ferromagnetic behavior. Constraining the moments to have fixed values in a CuAu unit cell of two atoms to allow for antiferromagnetic (and field-induced ferrimagnetic) order alters these instabilities and yields second-order transitions from nonmagnetic to antiferromagnetic behavior at volumes coincident with the equilibrium volumes for both metals.

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001} c(2×2)- Au and Cu{001} c(2×2)- Pd, and on the bulk alloy Cu 3 Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu 3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A ̊ , c= 3.54 A ̊ and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 Å for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001}c(2×2)-Au and Cu{001}c(2×2)-Pd, and on the bulk alloy Cu3Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A, c= 3.54 A and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 A for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

Magnetism in fcc rhodium and palladium

First-principles total-energy band calculations using the fixed-spin-moment procedure are used to study the volume dependence of the magnetic behavior for fcc Rh and Pd. We calculate the total energy, the magnetic moment, and the spin-polarized l-decomposed electron occupancy from below the equilibrium volume to the free-atom limit, and show the magnetic susceptibility in the nonmagnetic range. We find that both metals are nonmagnetic at zero pressure, but undergo first-order transitions from nonmagnetic to magnetic behavior at expanded volumes. In both cases, the onset of magnetic behavior is accompanied by magnetic moments that exceed the Hund's-rule atomic limit. With increasing volume, we find a depletion of s and p states and a corresponding increase of d states with an approach to the 4${d}^{9}$ and 4${d}^{10}$ free-atom configurations for Rh and Pd, respectively.

Stoner model of ferromagnetism and total-energy band theory.

The Stoner model of ferromagnetism in metals is generalized to reproduce the results of spin-polarized total-energy band calculations with the fixed-spin-moment procedure. The Stoner parameter for the exchange field is made a function of both volume and magnetic moment and evaluated from the band calculations. A generalized Stoner condition for the occurrence of ferromagnetism is derived and applied to fcc Fe. Our previous results on the high-spin and low-spin phases of fcc Fe are reproduced and differences from a recent Stoner analysis of fcc Fe are explained.

Magnetism of metastable phases: Band theory and epitaxy (invited)

Total-energy band calculations are used to analyze the magnetic phases of metallic elements as functions of volume. The calculations utilize a fixed-spin-moment procedure, which is described and justified as a natural generalization of density-functional theory. This procedure finds the ground-state energies of electronic systems under two constraints, and hence determines the system energy as a function of two variables—volume and magnetic moment. The energy function is used to find the ferromagnetic phases and their ground-state properties, including bulk moduli and magnetic susceptibilities. The systems studied are fcc Fe, fcc Co, bcc Ni, fcc Pd, and bcc Mn, each of which undergoes a phase transition for small changes of the lattice constant from equilibrium (zero-pressure) values.

Epitaxial growth of a metastable modification of copper with body-centered-cubic structure.

Slow deposition of Cu onto a clean unheated Fe{001} surface produced films with large amounts of defects, but still detectable crystallinity. Low-energy electron diffraction showed that the films contained regions with a (somewhat distorted) body-centered-cubic structure and the same or nearly the same lattice parameter as the underlying substrate. Auger-electron spectroscopy suggested that the crystalline regions were essentially pure Cu, thus allowing the conclusion that a metastable bcc modification of Cu has been realized, consistent with the predictions of total-energy band calculations reported elsewhere.

A Study of Epitaxial Films of Bcc Nickel on FE{001} with leed Intensity analysis and Total Energy Band Calculations

ABSTRACTAnalysis of LEED intensities of thin films of Ni grown epitaxially on bcc Fe{001} establishes that the structure is bcc up to six layers, but with a contraction of 5% of the first layer spacing and expansion of 5% of the second layer spacing. At thicknesses above six layers there is a gradual change to a new surface and bulk structure with increasing disorder. Spin-polarized total-energy band calculations show that the bcc Ni film is stretched 1.4% from its equilibrium value and thereby changed from nonmagnetic to ferromagnetic. The calculated results on stable and metastable phases of Fe, Co, Ni and Cu show that the equilibrium lattice parameters and differences in binding energies favor mutual epitaxy for any bcc or fcc pair. There are six pairs in which a possible metastable phase is grown epitaxially on a stable substrate.

Hunting Magnetic Phases with Total-Energy Spin-Polarized Band Calculations

ABSTRACTComments are made on total energy band calculations as tools for exploring properties of solids; the importance of fixed spin moment calculations is noted. Use of energy - magnetisation curves to locate magnetic phases is described. Detailed results for fcc and bcc Co and Ni and phase diagrams on the magnetisation - volume plane exhibit two new phases for each metal and show that ferromagnetic fcc Co and bcc Ni break down at small volumes and make first order transitions to nonmagnetic phases in a metamagnetic volume range.