arXiv subjects

G. J. Nieuwenhuys

Publications and source records attributed to G. J. Nieuwenhuys.

26 records · Page 2Linked to original sources

arXiv2000

Muon Spin Relaxation Study of (La, Ca)MnO3

We report predominantly zero field muon spin relaxation measurements in a series of Ca-doped LaMnO_3 compounds which includes the colossal magnetoresistive manganites. Our principal result is a systematic study of the spin-lattice relaxation rates 1/T_1 and magnetic order parameters in the series La_{1-x}Ca_xMnO_3, x = 0.0, 0.06, 0.18, 0.33, 0.67 and 1.0. In LaMnO_3 and CaMnO_3 we find very narrow critical regions near the Neel temperatures T_N and temperature independent 1/T_1 values above T_N. From the 1/T_1 in LaMnO_3 we derive an exchange integral J = 0.83 meV which is consistent with the mean field expression for T_N. All of the doped manganites except CaMnO_3 display anomalously slow, spatially inhomogeneous spin-lattice relaxation below their ordering temperatures. In the ferromagnetic (FM) insulating La_{0.82}Ca_{0.18}MnO_3 and ferromagnetic conducting La_{0.67}Ca_{0.33}MnO_3 systems we show that there exists a bi-modal distribution of \muSR rates λ_f and λ_s associated with relatively 'fast' and 'slow' Mn fluctuation rates, respectively. A physical picture is hypothesized for these FM phases in which the fast Mn rates are due to overdamped spin waves characteristic of a disordered FM, and the slower Mn relaxation rates derive from distinct, relatively insulating regions in the sample. Finally, likely muon sites are identified, and evidence for muon diffusion in these materials is discussed.

cond-mat.str-el↗

arXiv2000

Positive Muon Knight Shift Measurements in U_{0.965}Th_{0.035}Be_{13} Single Crystals

Muon spin rotation measurements of the temperature dependence of the Knight shift in single crystals of U_{0.965}Th_{0.035}Be_{13} have been used to study the static spin susceptibility below the transition temperatures T_{c1} and T_{c2}. While an abrupt reduction of the susceptibility with decreasing temperature is observed below T_{c1}, the susceptibility does not change below T_{c2} and remains at a value below the normal-state susceptibility. In the normal state we find an anomalous anisotropic temperature dependence of the transferred hyperfine coupling between the muon spin and the U 5f-electrons.

cond-mat.supr-con↗

arXiv1999

Disorder-to-order transition in the magnetic and electronic properties of URh_2Ge_2

We present a study of annealing effects on the physical properties of tetragonal single--crystalline URh_2Ge_2. This system, which in as-grown form was recently established as the first metallic 3D random-bond heavy-fermion spin glass, is transformed by an annealing treatment into a long-range antiferromagnetically (AFM) ordered heavy-fermion compound. The transport properties, which in the as-grown material were dominated by the structural disorder, exhibit in the annealed material signs of typical metallic behavior along the crystallographic a axis. From our study URh_2Ge_2 emerges as exemplary material highlighting the role and relevance of structural disorder for the properties of strongly correlated electron systems. We discuss the link between the magnetic and electronic behavior and how they are affected by the structural disorder.

cond-mat.str-el↗

arXiv1999

Observation of two time scales in the ferromagnetic manganite La(1-x)Ca(x)MnO(3), x = 0.3

We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two spatially separated regions in close proximity possessing very different Mn-ion spin dynamics. One region corresponds to an extended cluster which displays 'critical slowing down' near Tc and an increasing volume fraction below Tc. The second region possesses more slowly fluctuating spins and a decreasing volume fraction below Tc. These data are discussed in terms of the growth of small polarons into overlapping regions of correlated spins below Tc, resulting in a microscopically inhomogeneous FM transition.

cond-mat.str-el↗

arXiv1999

NMR Detection of Temperature-Dependent Magnetic Inhomogeneities in URu2Si2

We present 29Si-NMR relaxation and spectral data in URu2Si2. Our echo-decay experiments detect slowly fluctuating magnetic field gradients. In addition, we find that the echo-decay shape (time dependence) varies with temperature T and its rate behaves critically near the Neel temperature TN, indicating a correlation between the gradient fluctuations and the transition to small-moment order. T-dependent broadening contributions become visible below 100 Kelvin and saturate somewhat above TN, remaining saturated at lower temperatures. Together, the line width and shift suggest partial lattice distortions below TN. We propose an intrinsic minority phase below $T_{\rm N}$ and compare our results with one of the current theoretical models.

cond-mat.str-el↗

arXiv1998

Muon Spin Relaxation and Nonmagnetic Kondo State in PrInAg_2

Muon spin relaxation experiments have been carried out in the Kondo compound PrInAg_2. The zero-field muon relaxation rate is found to be independent of temperature between 0.1 and 10 K, which rules out a magnetic origin (spin freezing or a conventional Kondo effect) for the previously-observed specific heat anomaly at \sim0.5 K. The low-temperature muon relaxation is quantitatively consistent with nuclear magnetism including hyperfine enhancement of the ^{141}Pr nuclear moment. This is strong evidence against a Pr^{3+} electronic magnetic moment, and for the Γ_3 crystalline-electric-field-split ground state required for a nonmagnetic route to heavy-electron behavior. The data imply the existence of an exchange interaction between neighboring Pr^{3+} ions of the order of 0.2 K in temperature units, which should be taken into account in a complete theory of a nonmagnetic Kondo effect in PrInAg_2.

cond-mat.str-el↗

arXiv1998

Susceptibility inhomogeneity and non-Fermi liquid behavior in nominally ordered UCu_4Pd

Muon spin rotation experiments on a stoichiometric sample of the non-Fermi liquid (NFL) heavy-fermion compound UCu_4Pd, in which recent neutron Bragg scattering measurements are consistent with an ordered structure, indicate that the U-ion susceptibility is strongly inhomogeneous at low temperatures. This suggests that residual disorder dominates NFL behavior. The data also indicate a short correlation length (\lesssim 1 lattice spacing) for this inhomogeneity and a rapid low-temperature U-moment relaxation rate (\gtrsim 10^{12} s^{-1}), which constrain cluster-based models of NFL behavior.

cond-mat.str-el↗

arXiv1996

Spin glass behavior in URh_2Ge_2

URh_2Ge_2 occupies an extraordinary position among the heavy-electron 122-compounds, by exhibiting a previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetism. Here we present new results of ac and dc susceptibilities, specific heat and neutron diffraction on single-crystalline as-grown URh_2Ge_2. These data clearly indicate that crystallographic disorder on a local scale produces spin glass behavior in the sample. We therefore conclude that URh_2Ge_2 is a 3D Ising-like, random-bond, heavy-fermion spin glass.

cond-mat.stat-mech↗