Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Perovskites of composition La_1?x Sr_x(Mn_1?x/2Nb _x/2)O₃ and La_0.49Sr_0.51(Mn_1?y Nb_y)O₃ have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb⁵⁺ to La_0.49Sr_0.51MnO₃ stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg²⁺ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn³⁺-O-Mn³⁺ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near T _C.     

Magnetic ordering on Yb3+ in YbBa2Cu3O7−x

The high critical temperature superconductor YbBa₂Cu₃O_7−x has been studied over the range 0.05 to 95K using ¹⁷⁰Yb Mössbauer absorption spectroscopy. Magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K. The saturated magnetic moments are 1.7μ_B. At all temperatures the hyperfine parameters show the presence of distributions attributed to some disordering of the oxygen vacancies.     

Magnetic behaviour of alloys in the series (Fe1−xCox)2P

Magnetization and Mössbauer spectroscopic measurements are reported on five alloys in the series (Fe_1−xCo_x)₂P (corresponding to x=0.03, 0.10, 0.30, 0.50 and 0.70). All the five alloys are ferromagnetically ordered. With increasing cobalt, the Curie temperature initially increases becoming maximum for x=0.30 at 480 K and then drops. In going from x=0.10 to 0.30, when the crystalline symmetry changes from Fe₂P-like hexagonal one to Co₂P-like orthorhombic one, the magnetic moment on the pyramidal metallic site tilts off the V_zz-axis. In all the compositions, substitution of cobalt reduces the internal magnetic field at the tetrahedral site whereas that at the pyramidal site shows little change. The system with x=0.03 exhibits temporal relaxation of magnetization over a long period of time. Mössbauer spectra of this alloy exhibit a reduction in linewidth with increasing temperature from 80 to 125 to 160 K. These observations are indicative of the existence of magnetic clusters.     

Kondo behaviour in magnetic (Ce−La)Pd2Si2

Susceptibility, specific heat and resistivity studies on CePd₂Si₂ and its pseudoternary system Ce_xLa_1–xPd₂Si₂ are reported. Replacement of Ce by La leads to a near dilution like decrease of the Néel temperaturesT _N (x): long range magnetic order in the ground state doublet exists in the wholex range explored (0.2x1). The present data indicate a concomitant Kondo effect, with Kondo temperaturesT _K decreasing linearly from 9 to 3 K, competing with the magnetic interaction. A close relationship between the value of the specific heat jump atT _N and the ratio between the two characteristic temperaturesT _N andT _K is demonstrated.     

Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni_2+x Mn_1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T _M and magnetic T _C (Curie point) phase transitions to converge. In the region where T _C =T _M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)     

Crystallography,magnetic properties and magnetocaloric effect in Gd4(BixSb1−x)3 alloys

A study of magnetic and thermal properties has been carried out on the alloys from the Gd₄(Bi_xSb_1−x)₃ series with x=0, 0.25, 0.5, 0.75, and 1. All of the alloys are ferromagnetic below their respective Curie temperatures which vary from 266 K for x=0 to 332 K for x=1.0. The magnetocaloric effect calculated from the temperature and magnetic field dependencies of the magnetization and heat capacity is moderate when compared to that of other materials, which order in the same temperature range. Both the magnetic ordering and the magnetocaloric effect peak temperatures increase nearly linearly with the increasing Bi content. Experimental magnetocaloric effect data obtained from two different measurement techniques are in excellent agreement.     

Effect of substitution of Ce by La in the CeNixPt1 − x dense Kondo ferromagnets

The effects of substitution of Ce by La in the orthorhombic CeNi_{1 − x}Pt_x dense Kondo ferromagnets are studied by means of magnetization and electrical resistivity measurements. A decrease of the exchange RKKY interactions leads to a decrease of the Curie temperature T_c as a function of the La content and hence to an enhancement of the Kondo character in the thermal dependence of the resisitivity. However, the Ce moment is almost independent of the La amount. The Kondo temperature being also independent, this surprising result seems in contradiction with the available Kondo lattice models.     

Magnetic ordering in Fe2−xZnxMoO4(X=0.1–1) spinel

We have studied the diluted Fe_2−xZn_xMoO₄ spinel ferrite which shows a frozen disordered magnetic state at low temperature. Magnetic properties are examined by DC magnetisation measurements as a function of temperature, field and time and AC susceptibility experiment. Our measurements show that this disordered magnetic system at low fields, shares many common features with spin glass or cluster glass like phases. Results suggest that the interaction gradually changes as the magnetic ion concentration decreases by the substitution of non-magnetic Zn on A site and causing a perturbation to the magnetically ordered spins and magnetic order decreases.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

Hexadecapolar Kondo effect in URu2Si2?

We derive the coupling of a localized hexadecapolar mode to conduction electrons in tetragonal symmetry. The derivation can be easily adapted to arbitrary multipoles in an arbitrary environment. We relate our model to the two-channel Kondo (2CK) model and show that for an f(2) configuration a relevant crystal field splitting in addition to the 2CK interaction is intrinsic to tetragonal symmetry. We discuss possible realizations of a hexadecapolar Kondo effect in URu(2)Si(2). Solving our model we find good agreement with susceptibility and specific heat measurements in Th(1-x)U(x)Ru(2)Si(2) (x?1).     

Antiferromagnetic ordering in the double perovskites La2−xSrxCoRuO6

The electric transport and magnetic susceptibility of double perovskites La_2?xSr_xCoRuO₆ have been studied over a temperature range up to 800–1000 K. The crystal and magnetic structure has been determined by neutron diffraction on two samples of the series, x=0.6 and 1.4, which represent the electron- and hole-doped systems with respect to “ideal” single-valent insulator x=1. The study shows that spins in both the Co and Ru f. c. c. like sublattices exhibit a long-range ordering of the antiferromagnetic type II (T_N=60 K for x=0.6 and T_N=60–80 K for x=1.4).     

Magnetic interactions in Pr1−xTbxMn2Ge2

The magnetic moments in the rare earth and Mn sublattices of RMn₂Ge₂, where R is a rare earth element, feature a variety of ordering configurations. In PrMn₂Ge₂ and TbMn₂Ge₂, the interlayer magnetic coupling in the Mn sublattice is, respectively, ferromagnetic and antiferromagnetic below about 350 K. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. In this study, we investigate the variations in the magnetic properties of Pr_1−xTb_xMn₂Ge₂ as a function of rare earth concentration by examining the evolution of the features in the temperature dependence of the magnetization. The results of earlier neutron diffraction and Mössbauer studies on samples with x=0 and 1 are also used for interpreting the magnetization data and to give an account of the competing effects between various magnetic structures in the Mn and rare earth sublattices. The results are summarized in the Pr_1−xTb_xMn₂Ge₂ magnetic phase diagram.     

Spiral Magnetic Order in the one—Dimensional Kondo Lattice

The effects of c-f(conduction-f electrons)hybridization on the spiral spin magnetism in the onedimensional Kondo Lattice are studied.By using the mean-field approximation,a close set of equations of the Green‘s functions with arbitrary wave vector Q for the spiral ordering of spins is deduced.The magnetic phase boundary between functions with arbitrary wave vector Q for the spiral ordering of spins is deduced.The magnetic phase boundary between the spiral magnetism and ferromagnetism has been calculated approximately.From our qualitative results,one can find that the ferromagnetic region is enlarged due to the c-f hybridization.Moreover,some new results reflecting the Kondo effect,such as the modified dispersion relation and the weakening of the localized magnetic moments are also obtained.effect,such as the modified dispersion relation and the weakening of the localized magnetic moments are also obtained.