[en] The magnetization of the aggregated cores of ferritin was measured as a function of the temperature and the external field. The blocking temperature, the susceptibility, the saturation magnetization, and the effective moments were deduced from the magnetization data and compared with those of natural ferritin. The blocking temperature decreased monotonically with increasing field in contrast to the anomalous field dependence of the blocking temperature of natural ferritin, which showed a broad peak around 0.3 T. The strong temperature dependence of the effective moment of the aggregated cores of ferritin was quite different from the weak temperature dependence of the effective moment of natural ferritin. The experimental results suggest that additional interparticle interactions among the cores may be important in understanding the magnetic behavior of aggregated ferritin cores.

[en] We present ⁵¹V and ²⁷Al nuclear magnetic resonance (NMR) and relaxation measurements in the compound Fe₂VAl. The asymmetric NMR line and its temperature dependence provide direct evidence for the presence of magnetic defects ascribed to antisite disorder. The concentration of magnetic defects is estimated from second-moment calculation, and is found to be c = 0.3 at% and c = 4 at% respectively, for two samples having different degrees of antisite disorder. The nuclear spin-lattice relaxation rate at low temperatures displays a maximum, which is attributed to slow superparamagnetic fluctuations of the magnetic clusters present in very low concentration, in addition to the majority of fast fluctuating paramagnetic centers.

[en] The effects of crystal imperfections on the magnetic properties of Mn12 acetate (Mn12-Ac) have been investigated. Crystal defects were introduced in the lattices of Mn12-Ac by using a novel heat-treatment process. In particular, the magnetization relaxation associated with quantum tunneling is found to be strongly influenced by the existence of crystal defects. Experimental data are explained qualitatively in terms of the distribution of the transverse anisotropy induced by the crystal imperfections.

[en] The magnetization of the protein shell (PepA) encapsulated CoPt nanoparticle (average diameter = 2.1 nm) specimen was investigated by implementing non-equilibrium magnetization calculation scheme and Curie-Weiss type formula. In doing that, the magnetization measurement sequence of SQUID (Superconducting Quantum Interference Device) magnetometer was modified to comply with the numerical analysis scheme. Applying the analysis methodology to the CoPt nanoparticles, we extract the values of zero-temperature coercive field and Bloch coefficient, which are in good agreement with the previously reported values. Remarkably, the distribution of the zero-field zero temperature anisotropy energy barrier follows an exponential type function, which is uncorrelated with the known size distribution of the particles. The lacking correlation between them was reported in ferritin, suggesting that it is a generic feature of certain magnetic nanoparticle systems having a range of disorders and imperfections. In addition, we analyze non-saturating behavior of a field dependent magnetization curve in terms of Curie-Weiss type formula, allowing a separation of the surface magnetic moments from the core moments.

[en] The magnetization of the protein shell (PepA) encapsulated CoPt nanoparticle (average diameter = 2.1 nm) specimen was investigated by implementing non-equilibrium magnetization calculation scheme and Curie-Weiss type formula. In doing that, the magnetization measurement sequence of SQUID (Superconducting Quantum Interference Device) magnetometer was modified to comply with the numerical analysis scheme. Applying the analysis methodology to the CoPt nanoparticles, we extract the values of zero-temperature coercive field and Bloch coefficient, which are in good agreement with the previously reported values. Remarkably, the distribution of the zero-field zero-temperature anisotropy energy barrier follows an exponential type function, which is uncorrelated with the known size distribution of the particles. The lacking correlation between them was reported in ferritin, suggesting that it is a generic feature of certain magnetic nanoparticle systems having a range of disorders and imperfections. In addition, we analyze non-saturating behavior of a field dependent magnetization curve in terms of Curie-Weiss type formula, allowing a separation of the surface magnetic moments from the core moments.

[en] We report a magnetization and electron spin resonance (ESR) study of the clinoatacamite Cu₂(OH)₃Cl, which is known to have a S = 1/2 distorted pyrochlore lattice. The static magnetic susceptibility shows a strong increase at temperatures below T_N2 = 6.4 K without any appreciable anomalies at T_N1 = 18.1 K. The magnetization vs. field curve exhibits the presence of a weak ferromagnetic moment of 0.1 μ_B/Cu at T = 2 K, possibly due to an antisymmetric Dzyaloshinsky-Moriya (DM) interaction and/or weak ferromagnetic interplane interactions. From the ESR linewidth, we estimate the magnitude of the DM interaction as 10% of the leading isotropic exchange interaction, J ∼ 170 K. The temperature dependence of the linewidth is characterized by three distinct temperature regions: (i) a high-temperature exchange narrowing region at temperatures above 170 K, (ii) a two-dimensional short-range ordered region at temperatures between 170 K and 36 K, and (iii) a low-temperature region at temperatures below 36 K where an antiferromagnetic-like resonance mode develops. The evolution of the ESR linewidth is comparable to that of the S = 1/2 kagome lattice in Cu₃Zn(OH)₆Cl₂ [Zorko et al., Phys. Rev. Lett. 101, 02640 (2008)]. This suggests that the magnetic properties of Cu₂(OH)₃Cl may be described in terms of a stack of kagome lattices.

[en] Here, we present muon spin relaxation (μSR) measurements of the extended kagome systems YBaCo₄O_7+δ (δ = 0,0.1), comprising two interpenetrating kagome sublattice of Co(I)³⁺ (S = 3/2) and a triangle sublattice of Co(II)²⁺ (S = 2). The zero- and longitudinal-field μ SR spectra of the stoichiometric compound YBaCo₄O₇ unveil that the triangular subsystem orders at TN = 101 K. In contrast, the muon spin relaxation rate pertaining to the kagome subsystem shows persistent spin dynamics down to T = 20 K and then a sublinear decrease λ(T) ~ T^0.66(5) on cooling towards T = 4 K. In addition, the introduction of interstitial oxygen (δ = 0.1) is found to drastically affect the magnetism. For the fast-cooling experiment (>10 K/min), YBaCo₄O_7.1 enters a regime characterized by persistent spin dynamics below 90 K. For the slow-cooling experiment (1 K/min), evidence is obtained for the phase separation into interstitial oxygen-poor and oxygen-rich regions with distinct correlation times. The observed temperature, cooling rate, and oxygen content dependencies of spin dynamics are discussed in terms of a broad range of spin-spin correlation times, relying on a different degree of frustration between the kagome and triangle sublattices as well as of oxygen migration.

[en] We have performed ¹¹B NMR measurements in ¹¹B enriched MgB₂ powder sample in the normal phase. The Knight shift was accurately determined by using the magic angle spinning technique. Results for ¹¹B and ²⁷Al Knight shifts (K) and relaxation rates (1/T₁) are also reported for AlB₂. The data show a dramatic decrease of both K and 1/T₁ for ¹¹B in AlB₂ with respect to MgB₂. We compare experimental results with ab initio calculated NMR relaxation rates and Knight shifts. The experimental values for 1/T₁ and K are in most cases in good agreement with the theoretical results. We show that the decrease of K and 1/T₁ for ¹¹B is consistent with a drastic drop of the density of states at the boron site in AlB₂ with respect to MgB₂

[en] We present ¹³⁹La and ⁶³Cu NMR relaxation measurements in single crystal La_1.67Eu_0.2Sr_0.13CuO₄. A strong peak in the ¹³⁹La spin-lattice relaxation rate observed in the spin ordered state is well described by the BPP mechanism [Bloembergen, Purcell, and Pound, Phys. Rev. 73, 679 (1948)] and arises from continuous slowing of electronic spin fluctuations with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy in the ordered state. The spin pseudogap is significantly enhanced by the static charge-stripe order in the LTT phase. (c) 2000 The American Physical Society

[en] We report ²³Na nuclear magnetic resonance (NMR) and zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (μSR) measurements of the depleted hyperkagome compounds Na_4−xIr₃O₈ (x  =  0.3 and 0.7), which undergo an insulator-semimetal transition as a function of x. The ²³Na spin-lattice relaxation rates, $T_1^{-<!--\; ???\; -->1}$, follow a T ^2.5 power law behavior at accessible temperatures of T  =  120–350 K. A substantial temperature dependence of $T_1^{-<!--\; ???\; -->1}$ indicates the presence of gapped excitations at elevated temperatures through the transition to a semimetallic phase. ZF-μSR results reveal that hole-doping leads to a melting of quasi-static order to a dynamically fluctuating state. The very slow muon depolarization rate which varies hardly with temperature indicates that spins are close to an itinerant limit in the largest doping x  =  0.7. The dynamic relaxation rates extracted from the LF-μSR spectra show a three-dimensional diffusive transport. Our combined NMR and μSR results suggest the occurrence of intriguing spin and charge excitations across the insulator-semimetal transition. (paper)