A. A. Povzner

Electronic structure and magnetic susceptibility of nearly magnetic metals (palladium and platinum)

A self-consistent approach to calculations of the electronic structure and the magnetic susceptibility of nearly magnetic metals, such as palladium and platinum, has been developed in terms of the generalized s(p)d Hubbard model. The energy band structure has been calculated using the ab initio LDA + U + SO method with the additional inclusion of the interstitial s(p)d exchange interaction and spin-fluctuation renormalizations of the electronic spectra, which appear at finite temperatures. The developed approach makes it possible to quantitatively describe the density of states and unusual temperature dependences of the magnetic susceptibility of the nearly magnetic metals under consideration and to evaluate the basic parameters of the electron-electron interactions. The role of the spin-orbit interaction in the formation of the electronic and magnetic properties is enhanced when going from palladium (4d period) to platinum (5d period). The effects of the temperature redistribution of electrons between the s(p) and d states have been revealed.

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature TS (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.

Anomalous influence of spin fluctuations on the heat capacity and entropy in a strongly correlated helical ferromagnet MnSi

The influence of spin fluctuations on the thermodynamic properties of a helical ferromagnet MnSi has been investigated in the framework of the Hubbard model with the electronic spectrum determined from the first-principles LDA + U + SO calculation, which is extended taking into account the Hund coupling and the Dzyaloshinskii–Moriya antisymmetric exchange. It has been shown that the ground state of the magnetic material is characterized by large zero-point fluctuations, which disappear at the temperature T* (<Tc is the temperature of the magnetic phase transition). In this case, the entropy abruptly increases, and a lambdashaped anomaly appears in the temperature dependence of the heat capacity at constant volume (CV(T)). In the temperature range T* < T < Tc, thermal fluctuations lead to the disappearance of the inhomogeneous magnetization. The competition between the increase in the entropy due to paramagnon excitations and its decrease as a result of the reduction in the amplitude of local magnetic moments, under the conditions of strong Hund exchange, is responsible for in the appearance of a “shoulder” in the dependence CV(T)).

Electronic structure and spin fluctuations in the helical ferromagnet MnSi

The influence of spin fluctuations on the magnetic properties of the ferromagnetic helimagnet MnSi has been studied in the Hubbard model taking into account the antisymmetric relativistic Dzyaloshinskii–Moriya interaction for band electrons. The obtained equations of the magnetic state indicate the correlation between the fine structure of the density of electronic states and the magnetization and coefficient of mode–mode coupling. It has been shown that the position of the Fermi energy in the immediate proximity on the point of the local minimum of the density of electronic states leads to large zero spin fluctuations at low magnetization of the helimagnet. When approaching from down the Néel point (approximately, at 0.9TN), the zero fluctuation disappear, and the temperature rise of thermal spin fluctuation is accompanied by the change in the sign of the coefficient of mode–mode coupling. A magnetic field perpendicular to the helicoids plane brings about the formation and subsequent “collapse” of the helimagnetic cone. However, the condition of the change in the sign of the coefficient of mode–mode coupling divides the MnSi phase diagram into two parts, one of which corresponds to the ferromagnetic state induced by the field, and the other corresponding to the paramagnetic state. In this case, the h–T diagram has a specific region, inside which the paramagnetic and the ferromagnetic state are instable. The boundaries of the region agree with the experimental data on the boundaries of the anomalous phase (a phase). It has been found that the results of calculations of the temperature dependence of the magnetic susceptibility agree with the experimental data.

Electronic structure and magnetic susceptibility of monoclinic α-plutonium

The electronic structure of the α-phase of plutonium has been calculated by the band methods with allowance for the spin-orbit interaction and Coulomb correlations in the complete matrix form (the LDA + U + SO method). The strong spin-orbit interaction of the 5 f electrons is manifested in the splitting of the calculated density of the 5f states, which makes a small contribution at the Fermi level on the order of the contribution from the 6d states. Using the results of the ab initio calculations, the spin and orbit contributions to the magnetic susceptibility of α-plutonium have been determined. Along with the impurity contribution, they describe well the experimental data on the susceptibility of this plutonium phase to a temperature of 300 K.

The peculiarities of δ-plutonium electronic structure and magnetic susceptibility

The calculations of electronic density of states, exchange-enhanced spin and orbital magnetic susceptibility of plutonium in its δ-phase existing region are performed in terms of a generalized s(p)df-model, which enables to take the intra-site Hubbard ff-, dd-interactions and inter-site df-exchange interaction into account along with s,p,f and d-electrons band movement and orbital splitting of f-states. It is shown that the splitting of the electronic spectra induced by fluctuating exchange fields leads to appearing of the temperature-induced local magnetic moments, to changing of d,f-bands occupation numbers with temperature and to appearing of charge fluctuations induced by spin fluctuations. A good agreement with the experimental data on temperature dependencies of magnetic susceptibility of Pu0.95Ga0.05 and Pu0.957Ga0.043 alloys is obtained.

Thermodynamic simulation of the elastic and thermal properties of cobalt monosilicide

A self-consistent thermodynamic model is used to calculate the temperature dependences of the heat capacity, the thermal expansion coefficient, the bulk compression modulus, the density, Debye temperature, and the Grüneisen parameter of CoSi in the temperature range 0–1400 K. The calculation results agree well with the existing experimental data and can be used to predict the properties of CoSi in the temperature range that has not been experimentally studied. Cobalt monosilicide is shown to have a significant phonon anharmonicity, which can be caused by an electron–phonon interaction, and this anharmonicity should be taken into account in the simulation of its thermoelectric properties.

Phonon Anharmonicity and Thermodynamic Properties of Strongly Correlated Iron Monosilicide

Two computational approaches – a thermodynamic model based on results of ab initio calculations of the ground state and the self-consistent thermodynamic model have been applied to study thermal and elastic properties of iron monosilicide. It is shown that conventional DFT fails to reproduce experimental data for this strongly correlated compound. In addition, we have performed comparative analysis of anharmonicity of the acoustic and optical phonons in FeSi and their impact on the temperature dependencies of thermodynamic properties of FeSi.

Lattice anharmonicity and thermal properties of strongly correlated Fe1–xCoxSi alloys

The temperature dependences of the thermal and elastic properties of strongly correlated metal alloys Fe1–xCoxSi (x = 0.1, 0.3, 0.5) with different atomic chiralities have been calculated in the framework of the self-consistent thermodynamic model taking into account the influence of lattice anharmonicity. The lattice contributions to the heat capacity and thermal expansion coefficient of the alloys have been determined using the experimental data. It has been demonstrated that the invar effect in the thermal expansion of the lattice observed in the magnetically ordered region of Fe0.7Co0.3Si and Fe0.5Co0.5Si is not related to the lattice anharmonicity, even though its appearance correlates with variations in the atomic chirality.

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe 1 - x Co x Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe1 - xCoxSi ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature TC and chiral fluctuations of the local magnetization at T > TC. The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near TC have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature TS).