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We analyze coexistence of antiferromagnetic and ferromagnetic phases in bulk iron-rhodium and its alloys with palladium, Fe50,4Rh49,6, Fe49,7Rh47,4Pd2,9 and Fe48,3Rh46,8Pd4,9, using neutron diffraction, magnetization and scanning Hall probe imaging. Temperature dependencies of the lattice parameters, AFM and FM phase weight fractions, and Fe magnetic moment values were obtained on cooling and heating across the AFM-FM transition. Substantial thermomagnetic hysteresis for the phases’ weight fractions and a relatively narrow one for the unit cell volume has been observed on cooling-heating. A clear dependence of hysteretic behavior on Pd concentration has been traced. Additional direct magnetic measurements of the spatial distribution of the phase transition are acquired using scanning Hall probe microscopy, which reveals the length scale of the phase coexistence and the spatial progression of the transition in the presence of external magnetic field. Also, the magnetic phase diagram has been constructed for a series of Pd-doped FeRh alloys. © 2020 Elsevier B.V.

High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), magnetic methods, and density-functional theory (DFT) calculations were applied for the investigations of Co-doped anatase TiO2 nanoparticles (∼20 nm). It was found that high-spin Co2+ ions prefer to occupy the interstitial positions in the TiO2 lattice which are the most energetically favourable in compare to the substitutional those. A quantum mechanical model which operates mainly on two types of Co2+ – Co2+ dimers with different negative exchange interactions and the non-interacting paramagnetic Co2+ ions provides a satisfactorily description of magnetic properties for the TiO2:Co system. © 2020 Elsevier B.V., Russian Foundation for Basic Research. Ministry of Science and Higher Education of the Russian Federation

High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), magnetic methods, and density-functional theory (DFT) calculations were applied for the investigations of Co-doped anatase TiO2 nanoparticles (∼20 nm). It was found that high-spin Co2+ ions prefer to occupy the interstitial positions in the TiO2 lattice which are the most energetically favourable in compare to the substitutional those. A quantum mechanical model which operates mainly on two types of Co2+ – Co2+ dimers with different negative exchange interactions and the non-interacting paramagnetic Co2+ ions provides a satisfactorily description of magnetic properties for the TiO2:Co system. © 2020 Elsevier B.V., Russian Foundation for Basic Research. Ministry of Science and Higher Education of the Russian Federation

Magnetic properties and magnetic structures of layered GdMn

The report is devoted to an analysis of the structural and magnetic state of the nanocrystalline diluted magnetic semiconductors based on TiO2 doped with Fe and Co atoms. Structural and magnetic characterization of samples was carried out using X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR) spectroscopy, SQUID magnetometry, and the density functional theory (DFT) calculations. Analysis of the experimental data suggests the presence of non-interacting paramagnetic Fe3+ and Co2+ ions in the high-spin state and negative exchange interactions between them. The important conclusions is that the distribution of dopants in the TiO2 matrix, even at low concentrations of 3d-metal dopant (less than one percent), is not random, but the 3d ions localization and dimerization is observed both on the surface and in the nanoparticles core. Thus, in the paper the quantum mechanical model for describing the magnetic properties of TiO2:(Fe, Co) was suggested. The model operates only with two parameters: paramagnetic contribution of non-interacting 3d-ions and dimers having different exchange interactions between 3d magnetic carriers. © Published under licence by IOP Publishing Ltd.

In this work, we report the results of comprehensive experimental and theoretical study of magnetic properties of TiO 2 nanoparticles (20 nm) doped with Fe at various concentrations ranging from 0.1 to 4.6 at. %. Our electron paramagnetic resonance and magnetic measurements data evidence the mixed magnetic state, where paramagnetic Fe 3+ ions coexist with short-range antiferromagnetic correlations caused by negative exchange interaction between neighboring Fe 3+ ions. A quantum mechanical model of the Fe-based magnetic cluster represented as a set of dimers with strong a(100-300) K and weak (a1 K) exchange interactions has been proposed. Our model was found to provide a good description of magnetic properties of TiO 2 :Fe nanopowders. Density-functional theory (DFT) calculations revealed Fe 3+ oxidation state of the iron center in the vicinity of an oxygen vacancy in the crystal structure of anatase. DFT calculations confirmed that two types of Fe 3+ spin-pairs with weak and strong exchange interactions can be formed in the vicinity of an oxygen vacancy. Accumulation of magnetic moment carriers and formation of magnetic clusters in TiO 2 nanoparticles with anatase structure were found to be a general tendency for all studied TiO 2 :Fe nanopowders. © Copyright 2018 American Chemical Society., The research was supported by the Russian Science Foundation (project 16-12-10004). The authors are grateful for the contributions made by our colleagues G. S. Zakharova, A. S. Konev, K. I. Borodin, Z. A. Phatakhova, A. M. Murzakaev, and V. S. Gaviko. The X-ray investigation and magnetic measurements have been performed in the Collaborative Access Center of M. N. Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia.

A comprehensive study of the magnetic phase transitions in Tb3Co has been undertaken combining different techniques. Using single crystal neutron diffraction in the paramagnetic state a weak crystal structure distortion from the room temperature orthorhombic structure of the Fe3C type described with the Pnma space group toward structure with lower symmetry has been observed with cooling below 100 K. At 81 K there is a second order phase transition to an antiferromagnetic incommensurate phase with the propagation vector k = (0.155, 0, 0). As derived from thermal diffusivity measurements, the critical exponents for this transition are very close to the 3D-Heisenberg universality class, proving that the magnetic interactions are short-range but with a deviation from perfect isotropy due to crystal field effects. At T2 ≈ 70 K there is another magnetic phase transition to a ferromagnetic state whose character is shown to be weakly first order. The low temperature magnetic state has a non-coplanar ferromagnetic structure with strong ferromagnetic components of Tb magnetic moments along the crystallographic c-axis. The application of an external magnetic field B = 2 T along the c crystallographic axis suppresses the incommensurate antiferromagnetic phase and gives rise to the ferromagnetic phase. The magnetic entropy peak change as well as the refrigerant capacity indicate that Tb3Co is a competitive magnetocaloric material in this temperature range. © 2019 Elsevier Ltd

The report is devoted to an analysis of the structural and magnetic state of the nanocrystalline diluted magnetic semiconductors based on TiO2 doped with Fe and Co atoms. Structural and magnetic characterization of samples was carried out using X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR) spectroscopy, SQUID magnetometry, and the density functional theory (DFT) calculations. Analysis of the experimental data suggests the presence of non-interacting paramagnetic Fe3+ and Co2+ ions in the high-spin state and negative exchange interactions between them. The important conclusions is that the distribution of dopants in the TiO2 matrix, even at low concentrations of 3d-metal dopant (less than one percent), is not random, but the 3d ions localization and dimerization is observed both on the surface and in the nanoparticles core. Thus, in the paper the quantum mechanical model for describing the magnetic properties of TiO2:(Fe, Co) was suggested. The model operates only with two parameters: paramagnetic contribution of non-interacting 3d-ions and dimers having different exchange interactions between 3d magnetic carriers. © Published under licence by IOP Publishing Ltd.

In this work, we report the results of comprehensive experimental and theoretical study of magnetic properties of TiO 2 nanoparticles (20 nm) doped with Fe at various concentrations ranging from 0.1 to 4.6 at. %. Our electron paramagnetic resonance and magnetic measurements data evidence the mixed magnetic state, where paramagnetic Fe 3+ ions coexist with short-range antiferromagnetic correlations caused by negative exchange interaction between neighboring Fe 3+ ions. A quantum mechanical model of the Fe-based magnetic cluster represented as a set of dimers with strong a(100-300) K and weak (a1 K) exchange interactions has been proposed. Our model was found to provide a good description of magnetic properties of TiO 2 :Fe nanopowders. Density-functional theory (DFT) calculations revealed Fe 3+ oxidation state of the iron center in the vicinity of an oxygen vacancy in the crystal structure of anatase. DFT calculations confirmed that two types of Fe 3+ spin-pairs with weak and strong exchange interactions can be formed in the vicinity of an oxygen vacancy. Accumulation of magnetic moment carriers and formation of magnetic clusters in TiO 2 nanoparticles with anatase structure were found to be a general tendency for all studied TiO 2 :Fe nanopowders. © Copyright 2018 American Chemical Society., The research was supported by the Russian Science Foundation (project 16-12-10004). The authors are grateful for the contributions made by our colleagues G. S. Zakharova, A. S. Konev, K. I. Borodin, Z. A. Phatakhova, A. M. Murzakaev, and V. S. Gaviko. The X-ray investigation and magnetic measurements have been performed in the Collaborative Access Center of M. N. Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia.

A comprehensive study of the magnetic phase transitions in Tb3Co has been undertaken combining different techniques. Using single crystal neutron diffraction in the paramagnetic state a weak crystal structure distortion from the room temperature orthorhombic structure of the Fe3C type described with the Pnma space group toward structure with lower symmetry has been observed with cooling below 100 K. At 81 K there is a second order phase transition to an antiferromagnetic incommensurate phase with the propagation vector k = (0.155, 0, 0). As derived from thermal diffusivity measurements, the critical exponents for this transition are very close to the 3D-Heisenberg universality class, proving that the magnetic interactions are short-range but with a deviation from perfect isotropy due to crystal field effects. At T2  70 K there is another magnetic phase transition to a ferromagnetic state whose character is shown to be weakly first order. The low temperature magnetic state has a non-coplanar ferromagnetic structure with strong ferromagnetic components of Tb magnetic moments along the crystallographic c-axis. The application of an external magnetic field B = 2 T along the c crystallographic axis suppresses the incommensurate antiferromagnetic phase and gives rise to the ferromagnetic phase. The magnetic entropy peak change as well as the refrigerant capacity indicate that Tb3Co is a competitive magnetocaloric material in this temperature range.

In this paper we report the detailed study of magnetic phase diagrams, low-temperature magnetic structures, and the magnetic field effect on the electrical resistivity of the binary intermetallic compound Tb3Ni. The incommensurate magnetic structure of the spin-density-wave type described with magnetic superspace group P1121/a1′(ab0)0ss and propagation vector kIC=0.506,0.299,0 was found to emerge just below Néel temperature TN=61 K. Further cooling below 58 K results in the appearance of multicomponent magnetic states: (i) a combination of k1=12,12,0 and kIC in the temperature range 51

An ac photopyroelectric calorimeter has been used to study the critical behaviour of the magnetic transitions in Dy3Co measuring thermal diffusivity, specific heat and thermal conductivity, at low temperature. There are two phase transitions, both of which present singularities in the three variables. The antiferromagnetic to paramagnetic phase transition at 42 K complies with the short range, isotropic universality class, 3D-Heisenberg (alfaexp = -0.133 for specific heat, bexp = -0.145 for thermal diffusivity, alfatheor = btheor = -0.13). In the case of the lower transition where there is a rearrangement of the antiferromagnetic spin ordering at  32 K the critical behavior shows a deviation from isotropy. These results are linked to magnetic measurements already found in literature.

Cobalt-doped anatase Ti0.97Co0.03O2 nanopowders with a particle size of about 100 °A were produced by a microwave-hydrothermal method. The obtained samples were characterized by means of X-ray diffraction, X-ray absorption (Ti L2,3, Co L2,3, and Co K), and 1s3p resonant inelastic X-ray scattering spectroscopies. Co2+ ions tetrahedrally coordinated by oxygen ions in nanoparticles were found to be located on the surface of agglomerates. Titanium ions were found to be in a tetravalent state (Ti4+). All the samples before the thermal treatment contain an amorphous phase of titanium dioxide on the surface. After annealing in vacuum or hydrogen anatase structure of Ti0.97Co0.03O2 remains and the amorphous phase disappears. Annealing the samples in vacuum or hydrogen leads to clustering metal cobalt at the particles from the bulk of agglomerates.

Cobalt-doped anatase Ti0.97Co0.03O2 nanopowders with a particle size of about 100 °A were produced by a microwave-hydrothermal method. The obtained samples were characterized by means of X-ray diffraction, X-ray absorption (Ti L2,3, Co L2,3, and Co K), and 1s3p resonant inelastic X-ray scattering spectroscopies. Co2+ ions tetrahedrally coordinated by oxygen ions in nanoparticles were found to be located on the surface of agglomerates. Titanium ions were found to be in a tetravalent state (Ti4+). All the samples before the thermal treatment contain an amorphous phase of titanium dioxide on the surface. After annealing in vacuum or hydrogen anatase structure of Ti0.97Co0.03O2 remains and the amorphous phase disappears. Annealing the samples in vacuum or hydrogen leads to clustering metal cobalt at the particles from the bulk of agglomerates.

Cobalt-doped anatase Ti1−xCoxO2 (0 < x ≤ 0.04) nanopowders (with a particle size of 30−40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti4+. In the as-prepared state, all investigated compounds of Ti1−xCoxO2 are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a g-factor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F+-center signals is reduced appreciably. Annealing of Ti0.96Co0.04O2 in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO2 nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core−shell model proposed to be the most adequate for describing the magnetic properties of TiO2:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is descri, This work was supported by the Russian Scientific Foundation, Grant No. 16-12-10004. We are grateful to Dr. N. N. Schegoleva for help and useful comments.

Contrary to previous studies that identified the ground state crystal structure of the entire R3Co series (R is a rare earth) as orthorhombic Pnma, we show that Y3Co undergoes a structural phase transition at Ttв‰160 K. Single crystal neutron diffraction data reveal that at Tt the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the Pnma structure of Y3Co. In particular, a transversal acoustic phonon mode along the (00ξ) direction has imaginary frequencies at ξ<1/4. Employing inelastic neutron scattering measurements we find a strong damping of the (00ξ) phonon mode below a critical temperature Tt. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y3Co as a system with the charge density wave instability. © 2013 American Physical Society.

The transition metal sulfide Fe4Ti3S8 with 7:8 composition has been synthesized and studied by using X-ray diffraction, magnetization and electrical resistivity measurements. This compound exhibits a monoclinic crystal lattice (space group I12/m1). The substitution of Ti for Fe in Fe7S8 is found to result in a lowering of the Curie temperature (TC ≈ 205 K), in a larger value of the coercive field (Hc ∼ 9 kOe at low temperatures) and in a substantial growth of the resultant magnetic moment per formula unit (μFU) in comparison with Fe7S8. An enhanced value of μFU is attributed to the preferential substitution of Ti in alternating cation layers. From the paramagnetic susceptibility measured within temperature interval (250-350) K, a reduced value of the effective moment per iron (μFe ∼ 2.4μB) was determined. The electrical resistivity of Fe4Ti3S8 shows a non-metallic behavior and is affected by magnetic ordering. © 2013 Elsevier Masson SAS. All rights reserved.

The transition metal sulfide Fe4Ti3S8 with 7:8 composition has been synthesized and studied by using X-ray diffraction, magnetization and electrical resistivity measurements. This compound exhibits a monoclinic crystal lattice (space group I12/m1). The substitution of Ti for Fe in Fe7S8 is found to result in a lowering of the Curie temperature (TC ≈ 205 K), in a larger value of the coercive field (Hc ∼ 9 kOe at low temperatures) and in a substantial growth of the resultant magnetic moment per formula unit (μFU) in comparison with Fe7S8. An enhanced value of μFU is attributed to the preferential substitution of Ti in alternating cation layers. From the paramagnetic susceptibility measured within temperature interval (250-350) K, a reduced value of the effective moment per iron (μFe ∼ 2.4μB) was determined. The electrical resistivity of Fe4Ti3S8 shows a non-metallic behavior and is affected by magnetic ordering. © 2013 Elsevier Masson SAS. All rights reserved.

Elastic and inelastic neutron scattering and magnetization measurements have been used to study peculiarities of the magnetic state in R 3T compounds (R=Gd, Er, Tb; T=Ni, Co) below and above magnetic ordering temperatures. A pronounced non-Brillouin shape of the magnetization curves observed in the antiferromagnetic compounds Gd 3Ni and Tb 3Ni above their magnetic ordering temperatures together with earlier reported data about the retention of the magnetic contribution to the total specific heat of Gd 3T and anomalous behavior of the electrical resistivity above magnetic ordering temperatures are ascribed to the existence of short-range magnetic correlations in the wide temperature range in the paramagnetic state. The persistence of short-range magnetic order up to temperatures greater than 56 times the Néel temperature has been revealed by powder neutron diffraction measurements performed for Tb 3Ni and Tb 3Co. On the other hand, results from inelastic neutron scattering show that the low temperature magnetic excitations are strongly suppressed in both Tb 3Co and Er 3Co. It is suggested that the extended short-range magnetic correlations, which turn out to be an inherent feature of R 3T type compounds, are due to the layered crystal structure and to the difference between geometrically frustrated intra-layer exchange interactions and inter-layer exchange. © 2012 Elsevier B.V. All rights reserved.

The crystal structure and magnetic properties of ErFe 2D 3.1 have been studied by means of magnetic measurements and neutron powder diffraction in a wide temperature range from the highest temperature T = 450 K down to the lowest one T = 12 K. The crystal structure of ErFe 2D 3.1 is found to be strongly modified by temperature: the high-temperature cubic structure decomposes to the deuterium-deficient cubic and deuterium-rich distorted phases which coexist in a wide temperature range below 400 K and exhibit successive structural transitions. The neutron powder diffraction patterns analysis revealed ordering of deuterium atoms over preferential A 2B 2 interstitial sites upon cooling. Below the Curie temperature ErFe 2D 3.1 is a ferrimagnet. Two separate magnetic ordering temperatures for Er and Fe magnetic sublattices were observed. A large magnetic anisotropy has been found in ErFe 2D 3.1 at low temperature. © 2012 Elsevier B.V. All rights reserved.