Solid State Phenomena Vol. 289

Abstract: We provide a discussion of our previously collected data (Chinotti et al., Phys. Rev. B 94, 245101 (2016)) on the electrodynamic response in YbMnBi₂, a representative Weyl semimetal, and in its gapped semimetal counterpart EuMnBi₂, which can be fairly well reproduced within a recent devoted theoretical treatment (S.P. Mukherjee and J.P. Carbotte, J. Phys.: Condens. Matter 29 (2017) 425301). This allows identifying and catching all the essential features of their peculiar electronic structure as imaged by the excitation spectrum.

Abstract: Substituting V/Nb in Fe₂VAl causes an initial increase of the unit cell volume as well as an increaseof the structural disorder. Although V and Nb are isoelectronically, slight changes of the electronicdensity of states N(E) right at the Fermi energy are obtained. While for a 10 % substitution of V/Nbthe absolute value of N(E_F) keeps almost constant, the slope of N(E) grows. As a result, the Seebeckeffect grows in comparsion to the starting material Fe₂VAl, reaching values of about 100 μ V/K at roomtemperature. The latter is one of the largest found so far for p-type substituted Fe₂VAl.

Abstract: We have carried out ⁷⁵As nuclear magnetic resonance (NMR) measurements to investigate a new antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC) in CaK(Fe_0.967Ni_0.033)₄As₄. The hedgehog SVC order is clearly demonstrated by the direct observation of internal magnetic induction along the c axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature of T_N ~ 45 K. In the superconducting (SC) state, the NMR signal intensity decreases suddenly just below T_c ~ 20 K due to Meissner effect, evidencing the coexistence of the hedgehog SVC AFM and SC states from a microscopic point of view.

Abstract: The work reports the results of neutron diffraction measurements of NiMnGe:T systems where T is Cr or Ti. All investigated compounds have the helicoidal magnetic structure with the propagation vector k = (k_a,0,0). The values of the k_a component decrease with increasing Cr content and increase with increasing Ti content. For all compounds, except the sample with x = 0.18 in Cr-system, the helicoidal order is stable up to the Néel temperature. The obtained data are analysed based on simple model in which the magnetic interactions are described by two exchange integrals J₁ > 0 for first and J₂ < 0 for second neighbouring moments. This model clears up different dependence of k_a component in different systems.

Abstract: We successfully synthesized polycrystalline samples of Y₂M₃Co₉P₇ (M = Cr, Mn, Fe, Co, Mo, and Ru), which has the Zr₂Fe₁₂P₇-type hexagonal structure with M and Co selectively occupy the pyramidal site and three tetrahedral ones, respectively. For the case of M = Co, or Y₂Co₁₂P₇, the itinerant electronic ferromagnetism was observed below the Curie temperature T_C = 153 K. For M = Cr, Mn, Fe, and Ru, the ferromagnetism was also observed below T_C = 167 K, 229 K, 233 K and 18 K, respectively, whereas for M = Mo, an exchange-enhanced Pauli paramagnetism tool the place of the ferromagnetism. Among compounds with M being 3d transition metals, T_C and spontaneous magnetization took the maximum around M = Fe and Mn. This tendency was also seen in the cases of M being 4d transition metals, i.e., M = Ru and Mo. Our result indicates that magnetic moments at the pyramidal site have rather localized nature and largely affect the stability of the ferromagnetism of Y₂M₃Co₉P₇ although the ferromagnetism is considered to be mainly derived from three Co sites.

Abstract: 0.85La_0.7Sr_0.3MnO₃/0.15GeO₂ composite material and pure La_0.7Sr_0.3MnO₃ were investigated by X-ray diffraction, scanning electron microscopy, magnetometry and magnetic resonance methods. It was observed that both samples demonstrate the ferromagnetic properties, while the absolute value of the magnetization, the magnetic entropy change and the magnetic ordering temperature decrease in composite in comparison with pure La_0.7Sr_0.3MnO₃. The magnetic resonance spectra of investigated (1-x)La_0.7Sr_0.3MnO₃/xGeO₂ (x=0, 0.15) can be attributed to the superposition of magnetic resonance spectra from magnetically anisotropic particles with different orientations.

Abstract: Metamagnetism observed in some heavy fermion compounds is an intriguing phenomena. To understand this phenomena, we explore the phenomenological mean-field theory for heavy fermion systems without microscopic mechanism. We write an extension of the Landau-like free energy density with the Bragg-Williams entropy. By minimising this free energy density with respect to the sub-lattice magnetizations, we can obtain thermal equilibrium quantities. As examples, the magnetic behaviors of CeRu₂ Si₂ (the metamagnetism with crossover) and CeTiGe (the first order metamagnetism) are explained qualitatively, where the scaling hypothesis is used to consult with experimental data under the constant pressure condition.

Abstract: The spin-disorder resistivity (SDR) of a broad range of magneticmaterials, both ordered and disordered, is reviewed.We identify the SDR at the critical temperature with the residualresistivity of the corresponding system evaluated in the frameworkof the disordered local moment (DLM) model.The underlying electronic structure is determined in the frameworkof the tight-binding linear muffin-tin orbital method which employsthe coherent potential approximation to describe the DLM stateand chemical disorder.The DLM fixed-spin moment method is used in the case when the DLMmoment collapses.The Kubo-Greenwood approach is employed to estimate the resistivityof the DLM state.Formalism is applied to Fe and Ni and its alloys, Heusler alloys,and ordered ferromagnetic and antiferromagnetic alloys.Finally, the SDR of the Earth's core will be studied using thesame formalism.Calculations are compared with available experimental data.

Abstract: In magnetically ordered metals the magnitude of the local atomic moment become temperature dependent. To deal with this problem on the ab-initio level one need to employ a specific methodology for calculation of the electronic structure that takes into the account the magnetic disorder effects. In addition one needs to setup a special statistical models allowing simultaneously for ab-initio mapping and for the variation of the local spin magnitude. To this end here we discuss and employ methodology that is based on the Disordered Local Moment (DLM) formalism, spin-constraint Local Spin Density Approximation (LSDA) and Lichtenstein theorem for calculation of the inter-site exchange interactions. An extended classical Heisenberg Hamiltonian used for mapping allows for the variation of the lattice site spin magnitude. We consider here three representative canonical transition metals ferromagnets hcp Gd, bcc Fe and fcc Ni with quite a different character of the magnetic moment localization and illustrate the relative importance of the longitudinal spin fluctuations and the magnetic disorder induced electronic structure reconstruction. We use recently introduced linear measure [1] for integration over the longitudinal spin component in the classical configurational spin space.

Abstract: Based on the spin-fluctuation theory and LSDA + U + SO-calculations of the electronic structure, the spin states arising in the region of the magnetic phase transition in the helicoidal ferromagnet MnSi are studied. Within the framework of the stated model of the electronic structure, a temperature dependence of the homogeneous magnetic susceptibility is obtained near the temperature of the magnetic phase transition, which is in good agreement with experiment. At the temperature of the transition to the paramagnetic state, the left chirality vanishes, and spin correlations arise with a radius equal to the length of the spin helix, which then decreases with temperature. The results were obtained within the framework of the assignment of the Ministry of Education and Science of the Russian Federation, contract 3.9521.2017 / 8.9