Amit Keren

Searching for spontaneous magnetic order in an organic ferromagnet. μSR studies of β-phase p-NPNN

Abstract Zero-field muon spin rotation (ZF-μSR) studies in the recently synthesized organic ferromagnet β-phase p-NPNN are reported. Our measurements, using a microscopic magnetic probe, provide the first direct observation of spontaneous magnetic order in p-NPNN. The behaviour of the system is close to that of an isotropic three-dimensional (3D) Heisenberg spin system.

Short-lived charge-transfer excitons in organic photovoltaic cells studied by high-field magneto-photocurrent

The main route of charge photogeneration in efficient organic photovoltaic cells based on bulk hetero-junction donor-acceptor blends involves short-lived charge-transfer excitons at the donor-acceptor interfaces. The cell efficiency is critically affected by the charge-transfer exciton recombination and dissociation processes. By measuring the magneto-photocurrent under ambient conditions at room temperature, we show here that magnetic field-induced spin-mixing among the charge-transfer exciton spin sublevels occurs in fields up to at least 8.5 Tesla. The resulting magneto-photocurrent increases at high fields showing non-saturating behaviour up to the highest applied field. We attribute the observed high-field spin-mixing mechanism to the difference in the donor-acceptor g-factors. The non-saturating magneto-photocurrent response at high field indicates that there exist charge-transfer excitons with lifetime in the sub-nanosecond time domain. The non-Lorentzian high-field magneto-photocurrent response indicates a dispersive decay mechanism that originates due to a broad distribution of charge-transfer exciton lifetimes.

Muon spin relaxation investigation of the spin dynamics of geometrically frustrated antiferromagnets Y2Mo2O7 and Tb2Mo2O7.

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF=22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy.

Frustration Driven Lattice Distortion: An NMR Investigation of Y2Mo2O7

We have investigated the (89)Y NMR spectrum and spin lattice relaxation, T(1), in the magnetically frustrated pyrochlore Y(2)Mo(2)O(7). We find that upon cooling the spectrum shifts, and broadens asymmetrically. A detailed examination of the low T spectrum reveals that it is constructed from multiple peaks, each shifted by a different amount. We argue that this spectrum is due to discrete lattice distortions and speculate that these distortions relieve the frustration and reduce the systems energy.

Muon spin relaxation measurements of NaxCoO2.yH2O

Using the transverse field muon spin relaxation technique, we measure the temperature dependence of the magnetic field penetration depth lambda, in the NaxCoO2.yH(2)O system. We find that lambda, which is determined by the superfluid density n(s) and the effective mass m*, is very small and on the edge of the TF-microSR sensitivity. Nevertheless, the results indicate that this system obeys the Uemura relation. By comparing lambda with the normal state electron density, we conclude that m* of the superconductivity carrier is 70 times larger than the mass of bare electrons. Finally, the order parameter in this system cannot be described by a complete gap over the entire Fermi surface.

Magnetic dilution in the geometrically frustrated SrCr(9p)Ga(12-9p)O19 and the role of local dynamics: A muon spin relaxation study

We investigate the spin dynamics of SrCr(9p)Ga(12-9p)O19 for p below and above the percolation threshold p(c) using muon spin relaxation. Our major findings are as follows: (i) At T-->0 the relaxation rate is T independent and approximately p(3), (ii) the slowing down of spin fluctuation is activated with an energy U, which is also a linear function of p(3) and lim U as p-->0 = 8 K; this energy scale could stem only from a single ion anisotropy, and (iii) the p dependence of the dynamical properties is identical below and above p(c), indicating that they are controlled by local excitation.

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingμSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt3. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingTc. Our results provide evidence that the “lower superconducting phase” in theH-T phase diagram of UPt3 is characterised by broken time-reversal symmetry. Measurements of URu2Si2 and CeCu2.2Si2 indicate that the magnetically ordered volume fraction is temperature dependent in both systems.

Muon spin relaxation and susceptibility studies of the pure and diluted spin 1/2 kagomé-like lattice system (CuxZn(1-x))3V2O7(OH2) 2H2O.

Muon spin relaxation and magnetic susceptibility measurements have been performed on the pure and diluted spin 1/2 kagomé system (CuxZn(1-x))3V2O7(OH)2 2H2O. In the pure x=1 system we found a slowing down of Cu spin fluctuations with decreasing temperature towards T approximately 1 K, followed by slow and nearly temperature-independent spin fluctuations persisting down to T=50 mK, indicative of quantum fluctuations. No indication of static spin freezing was detected in either of the pure (x=1.0) or diluted samples. The observed magnitude of fluctuating fields indicates that the slow spin fluctuations represent an intrinsic property of kagomé network rather than impurity spins.

Common energy scale for magnetism and superconductivity in underdoped cuprates : A muon spin resonance investigation of (CaxLa1-x) (Ba1.75-xLa0.25+x)Cu3Oy

We characterize the spontaneous magnetic field, and determine the associated temperature T(g), in the superconducting state of (Ca(x)La(1-x)) (Ba(1.75-x)La(0.25+x)) Cu(3)O(y) using zero and longitudinal field muon spin resonance measurements for various values of x and y. Our major findings are (i) T(g) and T(c) are controlled by the same energy scale, (ii) the phase separation between hole poor and hole rich regions is a microscopic one, and (iii) spontaneous magnetic fields appear gradually with no moment size evolution.

Synthesis, structural, and magnetic characterization of substituted benzoimidazole-l-yl N,N′-dioxides

The crystal structures, EPR spectra and magnetic properties of the novel halogen- and cyano-substituted nitronyl nitroxide radicals 2-(2,6-dichlorophenyl)benzimidazolyl N,N′-dioxide, 6, 2-(2,6-difluorophenyl)benzimidazolyl N,N′-dioxide, 7, 2-(2-chloro-6-fluorophenyl)benzimidazolyl N,N′-dioxide, 8, 2-(2,3,6-trichlorophenyl)benzimidazolyl N,N′-dioxide, 9, 2-(2,3,4,5,6-pentafluorophenyl)benzimidazolyl N,N′-dioxide, 10, and 2-(3-cyanophenyl)benzimidazolyl N,N′-dioxide, 11, are reported. Compound 6 crystallizes in the triclinic crystal system in space group P. The molecules of 6 are arranged in pairs with short intermolecular distances between the NO groups. 7 crystallizes in two different modifications: polymorph α is orthorhombic, space group Pbca; polymorph β is monoclinic, space group P21/c. 8 crystallizes in two modifications: the α polymorph is monoclinic, space group P21/c; and the β polymorph is monoclinic, space group P21/n. 9 crystallizes in the monoclinic system, space group P21/c. 10 crystallizes in the monoclinic system, space group C2/c. The molecules of 10 are packed in pairs of two types that form a chain perpendicular to the c-axis. 11 crystallizes in the monoclinic crystal system in space group P21/c. The rotation angle between the two rings in compounds 6–10 is 54.2–76.7°. The rotation angle between the two rings is only 21.0° in 11 and it strongly affects the packing of the molecules that adopt the stacking mode. The magnetic measurements show that 6, 7, 10 and 11 exhibit large magnetic coupling. The best fitting with the experimental data for 6 and 11 was obtained using the Bleaney–Bowers singlet–triplet model plus the Curie–Weiss spin impurity (S = 1/2; H = −2JS1·S2) J/kB = −84.2 K and θimp = 0.3 K and J/kB = −95.3 K, θimp = 1.8 K, respectively. A Pade expression for 7 revealed Jintra/kB = 66.0 K and zJinter/kB = −14.0 K. Compound 10 shows evidence for large antiferromagnetic spin coupling (θ = −37.0 K Curie–Weiss model).