[en] Unique properties of the Zr₃(Rh_(1-x)Pd_x)₄ alloy system are reported here. Contrary to most metallically bonded systems which are not quick to change magnetic behavior with alloy concentration, this system does. Pure Zr₃Rh₄ and pure Zr₃Pd₄ are each nonmagnetic, while in the limited range 0.02< x<0.08, there is a substantial increase in paramagnetism, attended by a similarly varying incommensurate density wave. At x=0.04 the system exhibits ferromagnetism and displays some superconductivity at x=0.06. The crystal structure involves disclination lines of Rh atoms, or channels, which are equivalent to those of the superconducting A15 structures and bear resemblances to those of the hard magnets. (c) 2000 American Institute of Physics

[en] We report ¹¹⁵In nuclear quadrupolar resonance measurements of the heavy fermion compound CeRhIn₅ in the paramagnetic and antiferromagnetic states. The magnetic order is consistent with a model of helical modulation of the Ce moments that is incommensurate with the lattice, and the magnetic dynamics indicate possible two-dimensional behavior. (c) 2000 The American Physical Society

[en] We study a realistic Kondo model for crystal field quartet ground states having magnetic and nonmagnetic (quadrupolar) exchange couplings with conduction electrons, using the numerical renormalization group method. We focus on a local effect dependent on singlet excited states coupled to the quartet, which reduces the nonmagnetic coupling significantly and drives non-Fermi-liquid behavior observed in the calculated quadrupolar susceptibility. A crossover from the non-Fermi-liquid state to the Fermi-liquid state is characterized by a small energy scale very sensitive to the nonmagnetic coupling. On the other hand, the Kondo temperature observed in the magnetic susceptibility is less sensitive. The different crystal-field dependence of the two exchange couplings may be related to the different x dependence of quadrupolar and magnetic ordering temperatures in Ce_xLa_1-xB₆. (c) 2000 The American Physical Society

[en] Moessbauer spectroscopy (⁵⁷Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe endash Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (10¹³ U/cm²); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfacial mixing cross section induced by electronic energy deposition suggests that a thermal diffusion process is mainly involved in addition to damage creation. copyright 1997 American Institute of Physics

[en] High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La_1.2Sr_1.8Mn₂O₇, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔH_pp(T)=[C/Tχ(T)]ΔH_pp(∞). The high temperature limit of the linewidth, ΔH_pp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔH_pp(∞)∝ω_p²/J where J is proportional to the Curie-Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔH_pp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La_1.2Sr_1.8Mn₂O₇, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for T_c=125 K< T< T_p≅450 K. (c) 2000 American Institute of Physics