The magnetic properties of Mn1–xRhxSi solid solutions with a noncentrosymmetric structure B20 synthesized at a pressure of 8 GPa and temperatures of 1500–1770 K have been studied in detail in a wide temperature range of 2–300 K in magnetic fields up to 9 T. An anomalous increase in the Curie temperature TC of compounds with 0.15 ≤ x ≤ 0.8 by a factor of 8.4–11.5 compared to the pure helical magnet MnSi has been found. It has been established that the Curie temperature TC increases with the rhodium content to TC(x = 0.15) = (244 ± 4) K, TC(x = 0.4) = (299 ± 5) K, and TC(x = 0.8) = (334 ± 6) K. Uniquely high magnetic transition temperatures up to room values occur in the disordered ferromagnetic Griffiths phase and may be due to the spin-fluctuation mechanism of enhancement of the magnetic interaction.

The results of magnetic measurements, electron spin resonance, heat capacity, resistivity, and magnetocaloric effect as well as density functional theory (DFT), are presented for the compound NdRh2 (MgCu2-type structure). This compound was synthesized at a pressure of 8 GPa and a temperature of 1700 K. The magnetic properties of the material are shown to be determined by the high-temperature (over 400 K) spin polarization of the 4d Rh electrons and by the ferrimagnetic (FiM) interaction of Rh and Nd magnetic subsystems. Concurrently, the spontaneous magnetization of the spin polarization of 4d Rh electrons is approximate to 0.01 mu B/Rh, while the Nd magnetic moment is approximate to 1.7 mu B/Nd. This leads to complicated magnetic behavior with long range FiM order at TC less than or similar to 7 K at zero field and with wide temperature range of spin fluctuations TC<T less than or similar to 50 K. Specific heat and resistivity data provide corroboration of the spin fluctuation regime with the spin fluctuation temperature Theta sf approximate to 28.5 K. The optimal critical parameters were identified as beta=1.18 +/- 0.02, gamma=0.85 +/- 0.02, and TC approximate to 40 K from modified Arrott plotting, which are distinct from any conventional universality class. The maximum magnetocaloric effect, when the magnetic field changes from 0 to 9 T, Delta H=9 T occurs at T approximate to 11 K and the magnetic entropy change reaches -Delta Sm=7.0 J (kg K)(-1). In our DFT calculations, the FiM arrangement was obtained, with values of magnetic moments of Nd and Rh. Additionally, the Fermi surface was constructed.

Despite the 60-year history of research on band magnetism in MnSi, the field remains a vibrant area of study. This area is still of great interest although the physics of weak itinerant magnetism is complicated because of small magnetic moments and an uncertain role of local interactions. This work presents Rh-doped MnSi compounds in which a high-spin (HS) state of Mn magnetic moments has been detected in 55Mn nuclear magnetic resonance (NMR) measurements. The doping of MnSi with Rh results in a transition to the HS state for Mn1-xRhxSi at x = xc approximate to 0.025 with two Mn magnetic moments approximate to 1.3 and 2.2 mu B, which are ordered just below 200 K. This transition occurs only in part of the Mn atoms, while the other Mn atoms remain in a low-spin (LS) state. Concurrently, the Dzyaloshinsky-Moriya (DM) interaction for LS helical states of Mn moments is preserved up to x approximate to 0.13. Furthermore, variations in the Rh concentration result in discernible alterations in the magnetic field-temperature phase diagrams. In this case, it was observed that the temperature range of existence of the A phase, host skyrmion lattice, was markedly increased in presence of Rh doping, up to x = 0.025 at least. Small-angle neutron scattering has evidenced the existence of a skyrmion lattice in the helicoidal magnetic phase of Mn0.98Rh0.02Si. The Rh-doped MnSi compound thus demonstrates the coexistence of the HS and LS states of Mn. Our DFT calculations has indicated that this behavior can only be the case when Rh occupies not only Mn but also Si positions in the MnSi compound. Furthermore, our findings indicate that Ir doping of MnSi does not result in the formation of a high-temperature phase, but rather in the suppression of the DM interaction. Although Rh and Ir belong to the same column of Mendeleev's periodic table, they exhibit disparate behaviors upon MnSi doping.

The substitution of Bi by rare-earth ions is one of the common approaches for improving the electrical, magnetic, and multiferroic properties of the most studied multiferroic material BiFeO3. In this work, Bi1−xTbxFeO3 compounds with x = 0.05, 0.1, and 0.3 were synthesized using a two-step process: standard solid-state synthesis and high-pressure annealing. The obtained samples were studied by means of x-ray diffraction at normal pressure and neutron powder diffraction at high pressure. It was shown that high-pressure annealing could increase the Tb solubility limit to 10 at. %. It is proposed that the maximum solubility limit is even higher and could be achieved with high-pressure annealing in bulk samples. The transition from the R3c phase to the Pnma phase for the compounds with x = 0.05, 0.1 occurs through a two-phase region and starts at P≈4.4 and 1.7 GPa, respectively. The Pnma phase is stable in the compound with x = 0.3 up to P≈3.2 GPa. The values of Fe magnetic moments decrease with an increase in the Tb concentration or with external pressure for the compounds with x=0.05,0.3 in one-phase regions. The results will help to optimize the synthesis of multiferroic materials with improved magnetoelectric coupling for use in technological applications.

The measurements of magnetic hyperfine fields (MHF), H hf , and isomer shift, δ, in Y(Fe 1 – x Ni x ) 2 intermetallic compounds (the MgCu 2 structure type) synthesized at high pressure are performed. The MHF values that appear on 57 Fe nuclei at a nickel concentration x below 20 at % practically do not change and are approximately equal to 22 T, and in the range from x = 0.4 to 0.98 they decrease linearly with an increase in the Ni concentration. However, linear extrapolation of the hyperfine field as a function of Ni concentration does not lead to its disappearance in YNi 2 . For YFe 2 , the rotation of the easy axis from the [101] direction to the [111] direction with increasing temperature is found. As the Ni concentration increases to x = 0.3 at a temperature of 5 K, the easy magnetization axis [101] is observed, and at x = 0.4 the axis changes direction to [100]. Based on the shape of the concentration dependence of the hyperfine field, it is assumed that during the crystallization of Y(Fe 1 – x Ni x ) 2 under high pressure conditions, a magnetic moment exists on Ni ions. First-principles calculations of magnetic properties and hyperfine interactions are performed, which are consistent with experiment.

We report on a comprehensive experimental and theoretical study of Fe 1 − x Rh x Ge compounds, within the entire concentration range x ∈ [0 . 0 − 1 . 0], using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions x ∈ [0 . 2 − 0 . 9]. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

The measurements of magnetic hyperfine fields (MHF), Hhf, and isomer shift, δ, in Y(Fe1 – xNix)2 intermetallic compounds (the MgCu2 structure type) synthesized at high pressure are performed. The MHF values that appear on 57Fe nuclei at a nickel concentration x below 20 at % practically do not change and are approximately equal to 22 T, and in the range from x = 0.4 to 0.98 they decrease linearly with an increase in the Ni concentration. However, linear extrapolation of the hyperfine field as a function of Ni concentration does not lead to its disappearance in YNi2. For YFe2, the rotation of the easy axis from the [101] direction to the [111] direction with increasing temperature is found. As the Ni concentration increases to x = 0.3 at a temperature of 5 K, the easy magnetization axis [101] is observed, and at x = 0.4 the axis changes direction to [100]. Based on the shape of the concentration dependence of the hyperfine field, it is assumed that during the crystallization of Y(Fe1 – xNix)2 under high pressure conditions, a magnetic moment exists on Ni ions. First-principles calculations of magnetic properties and hyperfine interactions are performed, which are consistent with experiment.

In the paper, the helical magnetic structure of the Mn 0.7 Fe 0.3 Ge compound under a high quasihydrostatic pressure of up to 1 GPa was investigated for the first time by small-angle neutron scattering (SANS) in a wide range of temperatures (5–300 K) and magnetic fields (0–5 T). It is shown that the wave vector of the magnetic spiral increases with pressure. The field-temperature (H–T) phase diagrams were plotted for the compound at different pressures up to P = 1 GPa. It was shown that the applied pressure leads to an increase of all the values of critical magnetic fields corresponding to the beginning of the process of the transition of the polycrystalline sample to the conical phase (H c1 ), the end of the process (H c1m ) and the transition to the ferromagnetic phase (H c2 ), at low temperatures, which may indicate the stabilization of the magnetic system under the external pressure. It was found that the region of existence of a skyrmion lattice (or a phase) decreases with the pressure increase both in the temperature and field ranges. This suggests that the influence of Dzyaloshinskii–Moriya exchange interaction is suppressed when the cell constant is decreased.

The YbCoC2 compound, which crystallizes in a base-centered orthorhombic unit cell in the Amm2 space group CeNiC2 structure, is unique among Yb-based compounds due to the highest magnetic ordering temperature of TN=27 K. Magnetization measurements have made it possible to plot the H-T magnetic phase diagram and determine the magnetocaloric effect of this recently discovered high-temperature heavy-fermion compound, YbCoC2. YbCoC2 undergoes spin transformation to the spin-polarized state through a metamagnetic transition in an external magnetic field. The transition is found to be of the first order. The dependencies of magnetic entropy change ΔSm(T)—have segments with positive and negative magnetocaloric effects for ΔH≤6 T. For ΔH=9 T, the magnetocaloric effect becomes positive, with a maximum ΔSm(T) value of 4.1 J (kg K)−1 at TN and a refrigerant capacity value of 56.6 J kg−1.

The positron annihilation lifetimes were measured using a 48V positron source in noncentrosymmetric cubic single crystals of CoSi, FeSi and MnSi. The following lifetimes were determined from the positron annihilation time spectra: 168(1) ps for CoSi, 114(1) ps for FeSi and 111(1) ps for MnSi. For single-crystal CoSi, the positron annihilation lifetime was also determined with a 22Na positron source. For CoSi, the lifetimes obtained from different positron sources are consistent. The differences in the positron annihilation lifetimes in MnSi and FeSi, on the one hand, and in the Weyl semimetal CoSi, on the other hand, are possibly caused by the formation of a positron + electron bound state (positronium).

The new metastable high-pressure high-temperature synthesized hexagonal Laves phase compound YbZn2 was investigated. The unit cell parameters, low-temperature Sommerfeld coefficient and Debye temperature were determined. X-ray absorption studies and calculations based on dynamical mean-field theory (DMFT) showed the intermediate valence behavior of YbZn2 with mean Yb valence about 2.55. The values of hyperfine electric parameters in Zn (6h) sites were determined by means of 111Cd-time differential perturbed angular correlations and theoretical density functional calculations. DMFT calculations showed that the main contribution to the electron density of states at the Fermi level is due to Yb 4f electrons, while Zn spd electrons lie much lower in energy.

The magnetic H-T phase diagram and magnetocaloric effect in the recently discovered high-temperature heavy-fermion compound YbCoC_2 have been studied. With the increase in the external magnetic field YbCoC_2 experiences the metamagnetic transition and then transition to the ferromagnetic state. The dependencies of magnetic entropy change -Δ S_m (T) have segments with positive and negative magnetocaloric effects for Δ H ≤ 6 T. For Δ H = 9 T magnetocaloric effect becomes positive with a maximum value of -Δ S_m (T) is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg.

Positron annihilation lifetime spectroscopy (PALS) with the 48V isotope in Ti foil as a convenient positron source has been used to study the B20 single crystalsMnSi, FeSi and CoSi. The following positron lifetimes in single crystals were determined: 111(1) ps for MnSi, 114(1) for FeSi and 168(1) ps for CoSi. Italso has been established that the lifetimes in CoSi found with 22Na and 48V as positron sources are the same. The di erences in the lifetimes of MnSi, FeSi, onone side, and CoSi on the other, is attributed to the presence of Weyl fermions in single crystal CoSi.

We report on a comprehensive experimental and theoretical study of Fe_1-xRh_xGe compounds, within the entire concentration range x ∈[0.0 - 1.0], using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions x ∈[0.2 - 0.9]. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

We report on a comprehensive experimental and theoretical study of Fe$_{1-x}$Rh$_{x}$Ge compounds, within the entire concentration range $x \in \left[0.0 - 1.0\right]$, using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions $x \in \left[0.2 - 0.9\right]$. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

Hyperfine parameters and the pressure dependence of the magnetic transition temperatures of FeRhGe 2 have been investigated. Sample has been prepared using high pressure—high temperature synthesis technique. FeRhGe 2 consists of two B20 structure phases with close lattice constants. The phase separation stays constant in the temperature range 4–300 K. The magnetic transition temperatures T c 1 = 213 K and T c 2 = 135 K of FeRhGe 2 slightly increases with pressure in the range 0–4.5 GPa. We have compared this pressure dependence with some others compounds in the family Fe 1 − x Rh x Ge. The two phases in FeRhGe 2 have slightly different values of the hyperfine magnetic fields.

The helical magnetic structure of the Mn0.9Fe0.1Ge compound under a quasihydrostatic pressure of up to 1 GPa was investigated by small-angle neutron scattering in a wide range of temperatures (5-300 K) and magnetic fields (0-5 T). It is shown that the wave vector of the magnetic spiral increases with pressure. The field-temperature phase diagrams were plotted for a given compound at pressures up to P = 1 GPa. The temperature dependencies of the values of the magnetic fields corresponding to the beginning of the process of the transition of the polycrystalline sample to the conical phase, H-c1, the end of the process of transition to the conical phase, H-c1m, and the transition to the ferromagnetic phase, H-c2, are shown at different pressures. The applied pressure leads to an increase of all the values of critical magnetic fields at low temperatures, which may indicate the stabilization of the magnetic system under the external pressure. This might be caused by the tendency of the magnetic system to be in a commensurate state. Also, the decrease of the magnetic ordering temperature, T-c, with pressure increase is shown. This indicates the approach of the magnetic system to a quantum phase transition to a disordered state with increase of external pressure. (C) 2021 Elsevier B.V. All rights reserved.

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric YbNiC2. At normal pressure, YbNiC2 is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the CeNiC2-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional theory + dynamical mean field theory treatment. Our ab initio calculations also show that the valence of Yb in YbNiC2 at normal conditions is 2.85, which increases with temperature and pressure. The FS of YbNiC2 is anisotropic in shape in comparison with the 3D surface of YbCoC2.

We report on the study of the noncentrosymmetric ternary carbide YbCoC2. Our magnetization, specific heat, resistivity, and neutron diffraction measurements consistently show that the system behaves as a heavy-fermion compound, displaying an amplitude-modulated magnetic structure below the Neel temperature reaching T-N = 33 K under pressure. Such a large value, being the highest among the Yb-based systems, is explained in the light of our ab initio calculations, which show that the 4f electronic states of Yb have a dual nature, i.e., due to their strong hybridization with the 3d states of Co, 4f states expose both localized and itinerant properties.

Combining a precise ab initio electron band structure calculation of the TiO2 rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in TiO2, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density components (L = 2) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier component, in TiO2 due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter eta, which first decreases with temperature, goes to zero, and then increases.