Pseudobinary Fe4Ti3S8 compound with a NiAs-type structure: Effect of Ti for Fe substitution

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

Critical behavior study of magnetic transitions in Dy3Co single crystals

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.This work has been supported by Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU-GIU16/93) and also partially supported by FASO of Russia (themes No 01201463328 and 01201463334)

Peculiar magnetocaloric properties and critical behavior in antiferromagnetic Tb3Ni with complex magnetic structure

A study on the magnetocaloric properties of a Tb3Ni single crystal (which crystallizes in the orthorhombic Pnma space group) has been undertaken and combined with the study of the character and critical behavior of its magnetic transitions. It presents two important magnetocaloric effects in the temperature range 3–90 K due to the richness and variety of its temperature and magnetic field induced phase transitions. There is a conventional (direct) magnetocaloric effect with a maximum at 65 K and very competitive properties:  = 16.6 J/kgK, RCFWHM = 432 J/kg, with a 50 K span, for μ0ΔH = 5 T, which is due to the transition from a magnetically ordered state to the paramagnetic (PM) state with a combined antiferromagnetic to ferromagnetic (AFM-FM) metamagnetic transition. Besides, it also presents an inverse magnetocaloric effect at very low temperature for which the presence of metamagnetic transitions between AFM and FM states is responsible (=19.9 J/kgK, RCFWHM = 245 J/kg, with a 15 K span, for μ0ΔH = 5 T). At low field (<2 T), the character of the AFM-PM transition which takes place at ≈ 61 K has been well established to be second order and governed by short range order interactions, as the critical parameters α, A+/A- obtained from the specific heat at μ0H = 0 T point to the 3D-Heisenberg universality class. Conversely, the metamagnetic transitions between AFM and FM states, which appear for magnetic fields higher than 2 T, have a first order character, as proved by the magnetization behavior as a function of field and temperature. These properties make this material extremely interesting for magnetic refrigeration applications in the gas liquefaction range 4–77 K.This work has been supported by Universidad del País Vasco UPV/EHU (GIU16/93). A. Herrero thanks the Department of Education of the Basque Government as grantee of the programme “Programa Predoctoral de Formación de Personal Investigador No Doctor”. The authors thank for technical and human support provided by SGIker of UPV/EHU. This work was also supported by Russian Science Foundation (project No. 18-72-10022)

Comprehensive study of the magnetic phase transitions in Tb3Co combining thermal, magnetic and neutron diffraction measurements

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 Lt

Substitution Effects on the Magnetic Properties of Fe-Containing Chalcogenides with NiAs-Type Structures

The changes in the magnetization processes caused by Se for S substitutions in the layered chalcogenide compounds Fe_{x}TiS_{2-y}Se_{y} (x=0.5, 0.66) with antiferromagnetic (AF) and ferrimagnetic (FI) orderings, respectively, have been studied by using the magnetization and magnetoresistance measurements. Unusually high values of the coercive fields (H_{c} up to 56 kOe) have been observed at low temperatures in Fe_{0.5}TiS_{2-y}Se_{y} with the Se content y<0.5, which is ascribed to the presence of an unquenched orbital moment on Fe ions and to the formation of a magnetically heterogeneous state in the vicinity of H_{c}. The Se for S substitution in Fe_{0.66}TiS_{2-y}Se_{y} leads to the transition from the FI to AF state and to non-monotonous change of H_{c} with a maximal value 22 kOe an y=0.5. The changes in the magnetic state of Fe_{x}TiS_{2-y}Se_{y} are suggested to be strongly affected by the distribution of Fe ions and vacancies in cationic layers

Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

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&lt;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

Low frequency 1/f noise in doped manganite grain-boundary junctions

We have performed a systematic analysis of the low frequency 1/f-noise in single grain boundary junctions in the colossal magnetoresistance material La_{2/3}Ca_{1/3}MnO_{3-delta}. The grain boundary junctions were formed in epitaxial La_{2/3}Ca_{1/3}MnO_{3-delta} films deposited on SrTiO_3 bicrystal substrates and show a large tunneling magnetoresistance of up to 300% at 4.2 K as well as ideal, rectangular shaped resistance versus applied magnetic field curves. Below the Curie temperature T_C the measured 1/f noise is dominated by the grain boundary. The dependence of the noise on bias current, temperature and applied magnetic field gives clear evidence that the large amount of low frequency noise is caused by localized sites with fluctuating magnetic moments in a heavily disordered grain boundary region. At 4.2 K additional temporally unstable Lorentzian components show up in the noise spectra that are most likely caused by fluctuating clusters of interacting magnetic moments. Noise due to fluctuating domains in the junction electrodes is found to play no significant role.Comment: 9 pages, 7 figure

Peculiarities of the Phase Transformation Dynamics in Bulk FeRh Based Alloys from Magnetic and Structural Measurements

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.EL acknowledges funding from the UK EPSRC. LFC acknowledges funding from the EPSRC and InnovateUK: Project number: 105541. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. This work was partly supported by the state assignment of the Ministry of Science and Higher Education (themes “Flux” No. AAAA-A18-118020190112-8 and “Alloys” № AAAA-A19-119070890020-3)