Effect of pressure on the distribution of hyperfine field about A Sn impurity in iron

The distribution of hyperfine field at the iron atoms in an FeSn alloy has been measured as a function of pressure to 15 kbar. The frequency of the main NMR line was found to change such that (? ln H_m/?P)_T = ? 1.7 × 10^?4 kbar^?1, which is close to the value in pure iron. The value for the satellite line, due to iron atoms which are third nearest neighbours of the impurity, was (? ln H₃/?P)_T = ? 2.4 × 10^?4 kbar^?1. The magnitude of the difference of the pressure derivatives of H_m and H₃ is not consistent with models in which the Sn atom is screened by conduction electrons but may be understood in terms of a perturbation of the 3d band of iron. The discrepancy between spin wave theory and the measured temperature dependence of the hyperfine fields at constant pressure is not removed by correction to constant volume.     

Electronic structure and magnetic coupling properties of EuFe2P2: First-principles calculations

We have investigated the electronic structure and magnetic properties of EuFe₂P₂ using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. Our calculated ground state magnetic configurations of EuFe₂P₂ is ferromagnetic which Eu²⁺ spins order along c axis. We argue that this kind of magnetic structure of Eu is determined by the indirect RKKY interactions between Eu and direct coupling interaction between Eu 4f with Fe 3d state by our spin-polarized density of states calculations. From the charge density and the Laplace charge density of EuFe₂P₂, we believe that the magnetic moment of Fe is determined by not only Fe-P coupling interactions but also Fe-Fe directly exchange interactions.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7 keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the pressure range 0–15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP=-1.8±0.3 meV kbar^-1. No change of the mean valence of Eu with pressure was observed.     

Pressure dependence of phase transitions in K2SnCl6 from NQR frequency measurements

The influence of hydrostatic pressure 0 ? p ? 4 kbar on the ³⁵Cl NQR in K₂SnCl₆ was studied in the temperature range 238 K ? T ? 300 K. The phase transition temperatures T_C1 and T_C2 were determined from changes in the NQR line pattern.The phase boundaries in the p-T diagram are straight lines in the region studied. The pressure coefficients are given by dT_C1/dP = 1.35 (10) K kbar^?1 and dT_C2/dP=?1.25 (20) K kbar^?1.     

The temperature and pressure dependence of the electric field gradient at tantalum in rhenium

The quadrupole interaction (QI) at substitutional Ta atoms in a Re matrix has been investigated by TDPAC between 1.2 K and 450 K. It was found to depend only weakly on temperature with a minium around 200 K. No change in the QI was observed when the sample was cooled into the superconducting state. The QI was found to increase with pressure with the coefficient (? lnq/?P)_T=+0.93(23)10^?3 kbar^?1 at 300 K. A discussion of the temperature and pressure variations in connection with published Mössbauer data reveals the role of the conduction electron contribution to the electric field gradient.     

The effect of high pressure on the electric field gradients at two inequivalent lattice sites of tantalum in ω-zirconium

The pressure variation of the electric field gradients at substitutional tantalum atoms on the two lattice sites (A and B) of ω-zirconium have been determined to be (? lnq ^A/?P)_T = ?1.05(26)10^?3 kbar^?1 and (? lnq ^B/?P)_T = +6.03(40)10^?3 kbar^?1. In addition the thermal expansion coefficients of the unit cell of ω-Zr were found to be α_‖ = +3.5(3)10^?6 K^?6 and α_⊥ = +7.9(4)10^?6 K^?1. A combined analysis of the pressure dependence and the previously determined temperature dependence of the electric field gradients supports the assumption of two different bonding types at the A and B sites.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7-keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the presence range 0 to 15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP = -1.8 ± 0.3 meV/kbar. No change of the mean valence of Eu with pressure was observed.     

Charge transport and pressure dependence of Tc of single crystal,ferromagnetic EuB6

We present Hall Effect and resistivity data which demonstrate that EuB₆ is a degenerate semiconductor transforming into a metal or semimetal below the ferromagnetic ordering temperature, T_c = 13.7K. We also report an anomalously large, positive pressure dependence of T_c, (1/T_c)(ΔT_c/ΔP) ? 4 × 10^?2 kbar^?1.     

Superconductivity and ferromagnetism in the orthorhombic system (TbxLu1−x)RuB2

The pseudoternary orthorhombic system Tb_xLu_1?xRuB₂) (0 ? x ? 0.1 andx = 1.0) has been investigated by static magnetization, ac susceptibility and resistivity measurements down to 1.5 K. Superconductivity occurs for 0 ? x ? 0.1, while ferromagnetic order occurs for x = 1.0. For each sample studied, the Tb ion maintains a configurationally stable trivalent state. The initial linear depression of the superconducting transition temperature yields a coupling constant value N(0)Γ² between conduction electrons and magnetic Tb³⁺ moments of 3.5 x 10^-4 ev-atom-states/spin-direction.     

The pressure dependence of the electric field gradient at tantalum in group IIIb metals: Sc,Y, and Lu

The electric field gradientq acting on¹⁸¹Ta nuclei embedded into Sc, Y, and Lu was found to increase with hydrostatic pressure with relative slopes of (dlnq/dP) _T=300K=+8.0(3)10^?3 kbar^?1, +9.2(6)10^?3 kbar^?1, +10.2(1.3)10^?3 kbar^?1 for Sc, Y, and Lu, respectively.     

High-temperature properties of the manganites: Manifestation of a paramagnetic-phase inhomogeneity?

An analysis was made of the magnetic susceptibility, electrical resistivity, and magnetoresistance of (La_1?yPr_y)_0.7Ca_0.3MnO₃ samples differing in Pr content and enriched in the oxygen isotope ¹⁸O. At high temperatures, all samples were paramagnetic insulators, while below 60 K, part of them transferred to a ferromagnetic metallic state. All the samples exhibit practically identical behavior of the susceptibility, resistivity, and magnetoresistance in the high-temperature region, despite a noticeable difference between their properties at low temperatures; more specifically, the magnetoresistance grows quadratically with magnetic field within a broad range of temperatures and magnetic fields and scales with increasing temperature close to 1/T⁵. A combined analysis of the magnetic susceptibility and magnetoresistance indicates the possible existence of an inhomogeneous state with considerable ferromagnetic correlations in the paramagnetic region.     

Magnetic structure of the random system near the ferro-antiferromagnetic transition

The magnetic structure of the disordered alloy Fe₆₅Ni₂₈Mn₇ was investigated in the temperature 4.2–300 K by the methods: small angle scattering of neutrons, Mössbauer effect, magnetization, magnetic contribution to the thermal coefficient of the thermal expansion, and resistivity. All measurements show that long-range ferromagnetic order appears below T_c ? 160 K. At the same time for T ? 100 K, a dramatic change of magnetic state takes place which is interpreted as the freezing of “spin glass”. An increase of the magnetic contribution to the resistivity with decreasing temperature was also found. This increase was attributed to the existence of poor-bonded magnetic moments of the Kondo-type. A model of the magnetic ground state is proposed which includes the details of magnetic behavior such as long-range ferromagnetic order, spin glass, finite ferro-and antiferromagnetic clusters, and Kondo-type states. A magnetic phase diagram of the system Fe₆₅(Ni_1?xMn_x)₃₅ is also proposed.     

Effect of Eu doping and partial oxygen isotope substitution on magnetic phase transitions in (Pr1 − y Eu y )0.7Ca0.3CoO3 cobaltites

We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr_{1 ? y} Eu _y )_0.7Ca_0.3CoO₃ cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope ¹⁸O, namely, with 90, 67, 43, 17, and 0% of ¹⁸O. Based on the measurements of the ac magnetic susceptibility χ(T) and electrical resistivity ρ(T), we analyze the evolution of the sample properties with a change of the Eu and ¹⁸O content. It is demonstrated that the effect of increasing the ¹⁸O content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t _2g and e _g bands including the renormalization of this gap due to the electron-phonon interaction reveals the physical mechanisms underlying this similarity.     

Features of magnetic properties of La x MnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites La _x MnO_{3 + δ} (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to ⁵⁵Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the La _x MnO_{3 + δ} system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn⁴⁺ contents of 12–30%.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Magnetism and hyperfine interactions in EuM2Ge2 and GdM2Ge2(M = Mn,Fe, Co,Ni, Cu)

X-ray, magnetic susceptibility and ¹⁵¹Eu, ¹⁵⁵Gd Mössbauer effect studies of EuM₂Ge₂ and GdM₂Ge₂ were performed. All compounds crystallize in the ThCr₂Si₂ body centered tetragonal structure. In all compounds, except those with M = Mn and in EuM₂Ge₂, the M component carries no magnetic moment. All compounds except those with Mn are antiferromagnetic at low temperatures. In EuMn₂Ge₂ the Mn moments order ferromagnetically at 330 K and change to antiferromagnetic order when the Eu moments order ferromagnetically (9 K). This behaviour is different from that in GdMn₂Ge₂, where the Mn sublattice orders antiferromagnetically at 365 K and becomes ferromagnetic and antiparallel to the ferromagnetic Gd sublattice at 96 K. The Mössbauer studies of ¹⁵¹Eu and ¹⁵¹Gd provide values for the magnetic hyperfine fields, the quadrupole interactions and the orientation of the magnetic moments relative to the local fourfold axis (c-axis). It turns out that in the Eu compounds the easy axis of magnetization is close to the c-axis, while in the Gd compounds it is in the basal plane. In all systems, excluding those with Mn, the interatomic rare earth-rare earth distances have the dominant effect on the conduction electron charge density and polarization at the rare earth site and on the Curie point.     

Complex magnetism in a new alloy,Eu2PdSi3, with two crystallographically inequivalent sites

Lattice constants, electrical resistivity, heat capacity, AC and DC magnetic susceptibility and ¹⁵¹Eu Mössbauer effect studies on a new intermetallic compound, Eu₂PdSi₃, found to crystallize in an AlB₂-derived hexagonal crystal structure, are reported. The results establish that the Eu ions are in the divalent state down to low temperatures, undergoing two magnetic transitions, one at 40 K and the other at 10 K, pertaining to two different Eu sites. Interestingly, the minority Eu ions at the 2(b) sites order at a relatively high temperature (40 K) and this magnetic interaction is inferred to be ferromagnetic and quasi one-dimensional along the c-axis. The magnetic structure below 5 K following magnetic ordering (at 10 K) of the majority ions at 6(h) sites appears to be quite complex. In short, the crystallographic and (hence) the overall magnetic behaviors of this compound present an interesting situation.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Magnetic properties of electron-doped La_0.23Ca_0.77MnO₃ manganite nanoparticles, with average size of 12 and 60?nm, prepared by the glycine?Cnitrate method, have been investigated in the temperature range 5?C300?K and magnetic fields up to 90?kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T ^3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T?<?100?K.     

Switching from unusual to usual ferromagnetism in “112” LnBaCo2O5.50±δ: By calcium doping

The effect of calcium doping on the magnetic and transport properties of the intermediate size lanthanide cobaltites of the type Ln_1−xCa_xBaCo₂O_5.50±δ (Ln=Y, Gd, Eu and Sm) has been investigated for 0?x?0.2. The substitution of Ln by calcium induces a large expansion of the ferromagnetic state in the whole temperature range below T_C. The unusual trend of the decrease in T_C with the increase in Ln size in the undoped parent oxides becomes opposite in the calcium doped samples. Such an unusual behavior of the ferromagnetic T_C in the parent compounds is explained on the basis of thermally activated hole-mediated ferromagnetic coupling between the high-spin cobalt ions. The ferromagnetic state of the Ca-doped samples originates from the Co³⁺–O–Co⁴⁺ superexchange interaction, where Co⁴⁺ emerges from the disproportionation mechanism of the cobalt Co³⁺ into Co²⁺ and Co⁴⁺. However, the Ca-doping does not significantly affect the metal–insulator transition, which is associated with the structural change and not related to the spin state transition.