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Register a new userEvolution of the magnetism of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C
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The magnetic properties of polycrystalline Tb(Co_{x}Ni_{1-x})_{2}B_{2}C (x=0.2,0.4,0.6,0.8) samples were probed by magnetization, specific heat, ac susceptibility, and resistivity techniques. For x{ eq}0.4, the obtained curves are consistent with the features expected for the corresponding magnetic modes, namely k_{1}=(0.55,0,0) at x=0; k_{2}=([nicefrac] icefrac{1}{2}</LaTeX>,0,[nicefrac]<LaTeX> icefrac{1}{2}) at x= 0.2; k_{3}=(0,0,[nicefrac] icefrac{1}{3}) at x= 0.6, and k_{4}=(0,0,0) at x= 0.8 and 1. For x=0.4, even though the neutron diffraction indicates a k_{2} mode, but with a reduced magnetic moment, the magnetization, the ac susceptibility, and resistivity indicate two magnetic events; furthermore, deviation from Curie-Weiss behavior is observed below 150 K for this sample. These features, together with the evolution of both magnetic moment and critical temperature, are attributed to an interplay between competing magnetic couplings; for the particular x=0.4 case, additional factors such as crystalline electric field effects may be in operation.
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Neutron diffraction and thermodynamics techniques were used to probe the evolution of the magnetic properties of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C. A succession of magnetic modes was observed as x is varied: the longitudinal modulated k=(0.55,0,0) state at x=0 is transformed into a collinear k=([nicefrac]<LaTeX> icefrac{1}{2}</LaTeX>,0,[nicefrac]<LaTeX> icefrac{1}{2}</LaTeX>) antiferromagnetic state at x= 0.2, 0.4; then into a transverse c-axis modulated k=(0,0,[nicefrac]<LaTeX> icefrac{1}{3}</LaTeX>) mode at x= 0.6, and finally into a simple ferromagnetic structure at x= 0.8 and 1. Concomitantly, the low-temperature orthorhombic distortion of the tetragonal unit cell at x=0 is reduced smoothly such that for x >= 0.4 only a tetragonal unit cell is manifested. Though predicted theoretically earlier, this is the first observation of the k=(0,0,[nicefrac]<LaTeX> icefrac{1}{3}</LaTeX>) mode in borocarbides; our findings of a succession of magnetic modes upon increasing x also find support from a recently proposed theoretical model. The implication of these findings and their interpretation on the magnetic structure of the RM_{2}B_{2}C series are also discussed.
The evolution of the thermopower EuCu{2}(Ge{1-x}Si{x}){2} intermetallics, which is induced by the Si-Ge substitution, is explained by the Kondo scattering of conduction electrons on the Eu ions which fluctuate between the magnetic 2+ and non-magnetic 3+ Hunds rule configurations. The Si-Ge substitution is equivalent to chemical pressure which modifies the coupling and the relative occupation of the {it f} and conduction states.
The magnetism of the double perovskite compounds SLFCOx ($x$ = 0, 1, 2) are contrasted using magnetization, neutron diffraction and electron paramagnetic resonance with the support from density functional theory calculations. LFCO is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at $T_N$ = 270~K, accompanied by a low temperature structural phase transition at $T_S$ = 200~K. The structural phase transformation at $T_S$ occurs from $Roverline{3}c$ at 300~K to $Pnma$ at 200~K. The density functional theory calculations support an insulating non-compensated AFM structure. The long-range ordered magnetism of LFCO transforms to short-range glassy magnetism as La is replaced with Sr in the other two compounds. The magnetism of LFCO is differentiated from the non-equilibrium glassy features of SFCO and SLFCO using the {em cooling-and-heating-in-unequal-fields} (CHUF) magnetization protocols. This contransting magnetism in the SLFCOx series is evidenced in electron paramegnetic resonance studies. The electronic density-of-states estimated using the density functional theory calculations contrast the insulating feature of LFCO from the metallic nature of SFCO. From the present suite of experimental and computational results on SLFCOx, it emerges that the electronic degrees of freedom, along with antisite disorder, play an important role in controlling the magnetism observed in double perovskites.
We present a study of the evolution of magnetism from the quantum critical system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements, as well as photoemission spectroscopy. The results evidence a complex magnetic phase diagram, with a non-monotonic evolution of T_N and two successive transitions for some compositions resulting in two tricritical points. The strong similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co substitution basically corresponds to the application of positive chemical pressure. Analysis of the data proves a strong reduction of the Kondo temperature T_K with increasing Co content, T_K becoming smaller than T_N for x ~ 0.5, implying a strong localization of the 4f electrons. Furthermore, low-temperature susceptibility data confirm a competition between ferromagnetic and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an excellent experimental opportunity to gain a deeper understanding of the magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the antiferromagnetic phase disappears (T_N=>0).
The metamagnetic transitions in single-crystal rare-earth nickel borocarbide HoNi_{2}B_{2}C and ErNi_{2}B_{2}C have been studied at 1.9 K with a Quantum Design torque magnetometer. The critical fields of the transitions depend crucially on the angle between applied field and the easy axis [110] for HoNi_2B_2C and [100] for ErNi_2B_2C. Torque measurements have been made while changing angular direction of the magnetic field (parallel to basal tetragonal ab-planes) in a wide angular range (more than two quadrants). The results are used not only to check and refine the angular diagram for metamagnetic transitions in these compounnds, but also to find new features of the metamagnetic states. Among new results for the Ho borocarbide are the influence of a multidomain antiferromagnetic state, and ``frustrated behavior of the magnetic system for field directions close to the hard axis [100]. Torque measurements of the Er borocarbide clearly show that the sequence of metamagnetic transitions with increasing field (and the corresponding number of metamagnetic states) depends on the angular direction of the magnetic field relative to the easy axis.
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