N.V. Baranov

Electrical resistivity and magnetic phase transitions in modified FeRh compounds

Abstract The modified FeRh compounds with iron atoms partly substituted for nickel and with rhodium atoms substituted for palladium and iridium were systematically studied by means of magnetization, electrical resistivity, magnetoresistance and specific heat measurements. The magnetic state and electrical properties of FeRh-based compounds are strongly affected by the substitution owing to the electronic origin of magnetic phase transitions in these compounds. The field-induced first-order transitions from antiferromagnetic (AF) to ferromagnetic (F) state are accompanied by giant magnetoresistance (Δρ/ρ down to −88%) and by a change in the electronic specific heat coefficient γ from γ AF ≈ 35 mJ kg −1 K −2 up to γ F ≈ 79 mJ kg −1 K −2 . This behaviour is connected with the appearance or disappearance of superzones and the energy gap on superzone boundaries. The magnetic phase diagrams for (Fe 1− x Ni x )Rh and for Fe(Rh 1− x Pd x ) are determined.

Magnetic phase transitions in Gd3Co

Abstract Magnetization, electrical resistivity, thermal expansion, magnetostriction and sound velocity were measured for Gd2Co single crystals. The compound has an orthorhombic Fe3C-type crystal structure and exhibits an antiferromagnetic arrangement of Gd magnetic moments below TN = 131 K. First-order phase transitions from the antiferromagnetic to the forced ferromagnetic state are observed along the b and c axes at a critical field Hc ≈ 0.8 T. Along the a axis, Gd3Co also becomes a forced ferromagnet at approximately 1.5 T, but the magnetization process occurs by the rotation of Gd magnetic moments. An antiferromagnetic ordering of the Co sublattice appears below T ∗ = 3.7 K . Anisotropy of the electrical resistivity is found below TN. The field-induced antiferromagnetic to ferromagnetic transitions are accompanied by large magnetoresistance (Δρ/ρ0 ≈ −50%) and magnetostriction (Δl/l ≈ 3.8 × 10−4) as well as by a large change of the sound velocity of the shear wave in the bc plane (from 675 m s−1 down to 220 m s−1). Magnetic phase diagrams of Gd3Co are given for different directions of the applied field.

Magnetic properties of Cr-intercalated TiSe2

Abstract Using the measurements of the susceptibility and magnetization in steady and pulsed magnetic fields and under applied hydrostatic pressure we have investigated the magnetic properties of Cr-intercalated Cr x TiSe 2 compounds having a hexagonal crystal structure of a quasi-two-dimensional type. It has been found that the effective magnetic moment μ eff of Cr changes nonmonotonously with Cr-content. The μ eff ( x ) dependence correlates with the change in the interlayer distance at the intercalation which is attributed to the change of the localization degree of the chromium 3d-electrons. The intercalation leads to changes of the magnetic state of Cr x TiSe 2 from the Pauli paramagnetism at x =0 to the claster-glass behavior at 0.25≤ x ≤0.33 and then to the antiferromagnetic order in Cr 0.5 TiSe 2 . The last compound exhibits a field-induced spin-flip transition below the Neel temperature T N =38 K due to the very low ratio between the interlayer and intralayer exchange interaction ( w 2 / w 1 ≈5.4×10 −3 ). The applied hydrostatic pressure decreases the Neel temperature with a rate d T N /d p =−0.19 K/kbar. The interlayer exchange interaction was found to be more sensitive to the Cr–Cr distance than the intralayer one.

Magnetic hysteresis and domain wall dynamics in single chain magnets with antiferromagnetic interchain coupling.

Using Monte Carlo simulations we investigate magnetic hysteresis in two- and three-dimensional systems of weakly antiferromagnetically coupled spin chains based on a scenario of domain wall (kink) motion within the chains. By adapting the model of walkers to simulate the domain wall dynamics and using the Ising-like dipole-dipole model, we study the effects of interchain coupling, temperature and anisotropy axis direction on hysteresis curves.

Magnetic hysteresis in a molecular Ising ferrimagnet: Glauber dynamics approach

Motivated by recent experimental results reporting giant coercive fields in Co(II)-based molecular magnets we present a theory of hysteresis phenomena based on the Glauber stochastic dynamics. Unusual form of hysteresis loops is similar to those of found in Co-based quasi-one-dimensional ferrimagnet CoPhOMe at low temperatures. Temperature dependence of the coercive field has a characteristic form with an inflection that may serve as an indicator of the Glauber dynamics in real compounds. A relevance of the model for other Co-based molecular magnets is discussed.

Magnetic state of Dy3Co

Abstract Using data on magnetization, magnetoresistance, a.c. susceptibility and specific heat, magnetic phase diagrams of the compound Dy 3 Co were obtained for the main crystallographic axis. The different antiferromagnetic (AF) structures exist below T N = 44 K: AF 1 exists at T T t = 32 K and AF 2 exists at the temperatures T t T T N . The field-induced magnetic phase transitions observed along the three main axes are associated with a complex non-collinear antiferromagnetic structure and are accompanied by a significant change in the electrical resistivity (Δ ρ/ρ up to −40%) owing to the appearance or disappearance of the energy gaps on superzone boundaries. At the phase transitions along a - and c -axes, large hysteresis loops are observed at low temperatures. At T 3 Co remains in a magnetized state after the application and removal of a magnetic field along the c -axis.

AF–FRI metamagnetic transition in itinerant Mn2−xCoxSb system: high-field and high-pressure effects

Abstract The magnetization of Mn2−xCoxSb system with 0⩽x⩽0.35 has been measured under high magnetic fields up to 100 T and high pressure up to 12 kbar . Mn 2 Sb is a ferrimagnet and substitution of Co for Mn at 0.18⩽x⩽0.35 results in the appearance of the spontaneous first-order magnetic phase transition from ferrimagnetic (FRI) to antiferromagnetic (AF) state at T=Tt with decreasing temperature at a critical Co concentration. Below Tt a first-order field-induced AF–FRI transition is observed at a critical field Bc. The critical field Bc is found to decrease with increasing pressure applied at T=4.2 K . The AF state which appeared for the compound with the critical concentration x=0.18 can be transformed to the FRI one by the application of the high pressure of about 16 kbar by increasing the amount of FRI phase. Using the experimental data, the magnetic phase diagrams Bc–T–x and Bc–p–x are determined. The observed magnetic properties and magnetic phase diagrams of Mn2−xCoxSb are explained by the itinerant magnetism of d-electrons and by peculiarities of electronic structure. The itinerant nature of AF–FRI transition is confirmed by specific heat measurements in AF- and field-induced FRI states.

Magnetic state of R1−xYxCo2 compounds near the critical concentration

Abstract The substitution of R ions in RCo2 compounds for non-magnetic yttrium leads to the disappearance of the magnetic moment in the itinerant d-electron subsystem at a critical Y concentration. The magnetic state of the Ho1−xYxCo2 and Er1−xYxCo2 compounds near the corresponding critical concentrations xc=0.58 and xc=0.47 has been studied by means of neutron diffraction, electrical resistivity and a.c. susceptibility measurements. It is found that a long-range magnetic order coexists with a short-range order in a narrow concentration range near xc (1 at.% for (Ho,Y)Co2). The change in magnetic order with Y concentration in both systems occurs through a magnetic phase transition of first order. The properties of the (Ho,Y)Co2 and (Er,Y)Co2 compounds near xc are strongly affected by the applied magnetic field.

Effect of magnetic interphase boundaries on the electrical resistivity in metallic metamagnets

The magnetoresistance of single crystals of ErGa2, Tb3Co and NdCu2 compounds has been studied in the regions of the field-induced spin-flip transitions. An anomalous increase was found in the electrical resistivity in the intermediate state where the magnetic phases with the different magnetic structures coexist. A theoretical model to explain this anomaly is proposed. The peaks of the electrical resistivity in this model are connected with the s electron reflection from the potential barrier at the interphase boundary separating the phases with different magnetic structures. This effect is the magnetic analogy of so-called Andreevs reflection in superconductors.

Evidence of uranium magnetic ordering in UFe10Si2

Abstract The magnetocrystalline anisotropy of UFe10Si2 has been studied on aligned powder samples at 4.2 K. The enhancement of uniaxial anisotropy in comparison with that of YFe10Si2 and the field-induced phase transition in hard direction have been observed. The conclusion about a strong uranium contribution to the anisotropy energy has been deduced from comparison with the isostructural compounds YFe10Si2, HoFe10Si2 and SmFe11Ti.