A. F. Gubkin

Magnetic order, field-induced phase transitions and magnetoresistance in the intercalated compound Fe0.5TiS2

Measurements of the magnetic susceptibility, magnetization, electrical resistivity and neutron diffraction have been performed for the compound Fe(0.5)TiS(2) in which Fe atoms are intercalated between S-Ti-S tri-layers. It has been shown that this compound with a monoclinic crystal structure exhibits an antiferromagnetic (AF) ground state below the Néel temperature T(N) ≈ 140 K. Small deviations from the stoichiometry and some disordering effects caused by the additional low-temperature heat treatment do not affect substantially the AF state in Fe(0.5)TiS(2). According to neutron diffraction data the magnetic structure at 2 K is described by the propagation vector k = (1/4,0,1/4). The Fe magnetic moments with a value of (2.9 ± 0.1) μ(B) are directed at an angle of (78.5 ± 1.8)° to the layers. Application of the magnetic field at T < T(N) induces a metamagnetic phase transition to the ferromagnetic (F) state, which is accompanied by the large magnetoresistance effect (|Δρ/ρ| up to 27%). Below 100 K, the field-induced AF-F transition is found to be irreversible, as evidenced by magnetoresistance and neutron diffraction measurements. The magnetization reversal in the metastable F state is accompanied at low temperatures by substantial hysteresis (ΔH ~ 100 kOe) which is associated with the Ising character of Fe ions.

A cluster-glass magnetic state in R5Pd2 (R?=?Ho, Tb) compounds evidenced by AC-susceptibility and neutron scattering measurements

AC- and DC-susceptibility, high-field magnetization and neutron diffraction measurements have been performed in order to study the magnetic state of R5Pd2 (R = Ho, Tb) compounds. The results show that both compounds undergo cluster-glass freezing upon cooling below Tf. According to the neutron diffraction a long-range magnetic order is absent down to 2 K and magnetic clusters with short-range incommensurate antiferromagnetic correlations exist not only below Tf but also in a wide temperature range above the freezing temperature (at least up to 2Tf). A complex cluster-glass magnetic state existing in Ho5Pd2 and Tb5Pd2 down to low temperatures results in rather complicated magnetization behavior in DC and AC magnetic fields. Such an unusual magnetic state in compounds with a high rare-earth concentration may be associated with the layered type of their crystal structure and with substantial atomic disorder, which results in frustrations in the magnetic subsystem.

Crystal and magnetic structures of Cr1∕3NbSe2 from neutron diffraction

Neutron diffraction measurements of the Cr intercalated niobium diselenide Cr1∕3NbSe2 together with magnetization measurements have revealed that this compound exhibits ferromagnetic ordering below TC = 96 K unlike a chiral helimagnetic order observed in the sulfide compound Cr1∕3NbS2. As derived from neutron diffraction data, the Cr magnetic moments μCr = 2.83 ± 0.03 μB in Cr1∕3NbSe2 are aligned within basal plane. The discrepancy in the magnetic states of Cr1∕3NbS2 and Cr1∕3NbSe2 is ascribed to the difference in the preferential site occupation of Cr ions in crystal lattices. In Cr1∕3NbSe2, the Cr ions are predominantly distributed over 2b Wyckoff site, which determines a centrosymmetric character of the crystal structure unlike Cr1∕3NbS2, where the Cr ions are mainly located in 2c position and the crystal structure is non-centrosymmetric.

Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe7-yMyX8 chalcogenides (X = S, Se; M = Ti, Co).

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe(7-y)M(y)X(8) (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe(7-y)M(y)X(8) compounds having a ferrimagnetic order at y = 0, the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around y = 1.0-1.5 in all the Fe(7-y)M(y)X(8) families except Fe(7-y)Co(y)Se(8). Suppression of a magnetically ordered state with substitutions in Fe(7-y)M(y)X(8) is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe(7-y)Ti(y)X(8) and Fe(7-y)Co(y)X(8) is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20-24 kOe) observed at low temperatures in the Ti-containing compounds Fe(7-y)Ti(y)X(8) with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution.