T Jestädt

Muon-spin-rotation and magnetization study of metal-organic magnets based on the dicyanamide anion

We report the results of a study of the metal-organic magnets MII[N(CN)2]2, where MII = Ni, Co and Mn, using bulk magnetization and muon-spin relaxation (µSR). Implanted muons are sensitive to the onset of long-range magnetic order in each of these materials and strong muon-spin relaxation is observed in the paramagnetic state due to low-frequency fluctuations of the electronic moments in the 109-1010 Hz range. The size of the muon-spin relaxation in the paramagnetic state can be related to the magnitude of the transition-metal-ion moment. Very strongly damped oscillations are observed below the magnetic transition temperature in Co[N(CN)2]2.

Several Kinds of Aminoxyl Radicals and their Metal Ion Complexes

Abstract Recent results of magnetization, magnetic susceptibility and muon spin rotation/relaxation (μSR) measurements of some organic neutral radicals based on aminoxyl and their anion radical complexes with alkali and transition metal cations are reported. Ferromagnetic intermolecular interactions, coexistent with antiferromagnetic ones, in several carboxyaryl nitronyl nitroxide radicals are affected by inserting alkali metal ions, while the incorporation of transition metal ions result in complex behavior suggesting in part ferromagnetic interactions. Temperature dependence of spontaneous magnetization of TANOL suberate in the ordered state, obtained through the results of μSR measurements, is characteristic of a two-dimensional magnetic lattice, consistent with that of magnetic susceptibility above the ordering temperature.

Stability of the vortex lattice in ET superconductors studied by μSR

Abstract Muon-spin rotation (μSR) measurements have been used to study the vortex lattice and its instabilities in the organic superconductors κ-ET 2 Cu(SCN) 2 and β-ET 2 IBr 2 . We ascribe the field- and temperature-dependent destruction of the vortex lattice to the large anisotropy found in these materials.

Zero field μSR and QLCR in the molecular metal system (DMe-DCNQI)2Cu

We have carried out implanted positive muon studies on the molecular metal system dn- (DMe-DCNQI)2 Cu in order to understand better its novel magnetic properties. Examples of these salts at different levels of deuteration were studied. The fully deuterated ( d8) salt shows a metal–insulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate is enhanced below the MI transition, reflecting the localisation of spins along the Cu columns, however, the increase in muon spin relaxation rate occurs well above the metal–insulator (MI) transition and suggests a slowdown of the spin fluctuations around 120 K. At temperatures below 7 K a zero field precession signal was observed as a result of the 3D magnetic ordering of the Cu spins. For a muon site associated with the ring of the DCNQI molecule, the local field distribution was found to be consistent with the previously proposed magnetic structure. A sharp nuclear quadrupolar level crossing resonance (QLCR) was observed at 50 G which was assigned to resonance with the imine nitrogen on the DCNQI molecule.

Muon study of the spin dynamics in the organic spin-peierls compound MEM(TCNQ)2

Abstract We report a muon spin relaxation study of the organic spin-Peierls system MEM(TCNQ) 2 over the temperature range 39 mK to 200 K. Our results show a crossover from Gaussian to exponential relaxation below the spin-Peierls transition temperature. We attribute this change to the effect of a dilute set of static electronic defect spins resulting from the majority of spins freezing into a non-magnetic spin singlet state.

Layered transition metal molecular magnets studied with implanted muons

Abstract We have carried out magnetic measurements on the layered magnets Ni 2 (OH) 3 (N(CN) 2 ) ( NiAM ), Co 5 (OH) 8 (N(CN) 2 ) 2 ·6H 2 O ( CoAM ), Co 5 (OH) 8 (C 12 H 25 SO 4 ) 2 ·2NH 3 ( CoDOS ), and Co 5 (OH) 8 (C 7 H 15 CO 2 ) 2 ·4H 2 O ( CoCAP ), using various experimental methods, including muon spin relaxation (μSR). We find phase transitions at temperatures T m in all the samples, characterized by a change in the magnetic susceptibilty and loss of initial muon spin polarization.

Anisotropic polaron motion in conducting polymers studied by muon spin relaxation

Muon spin relaxation has been used to study the anisotropic motion of muon-generated negative polarons in polyaniline and polypyridine. Fast on-chain diffusion is found to become limited by 10 meV phenylene librations above about 150 K in polyaniline, whereas the rapid on-chain diffusion is maintained to much higher temperatures in polypyridine due to its more rigid structure.

Spin dynamics in the spin-gap system studied using muon-spin relaxation

We report a muon-spin relaxation study of the two-dimensional spin-gap system . We find that the form of the muon-spin relaxation is strongly temperature dependent and attribute this behaviour to the presence of a spin gap. At temperatures below 30 K a root-exponential behaviour is seen for the muon-spin relaxation function, which can be attributed to defect spins. At temperatures above 160 K we see the onset of muon hopping.

A μSR study of high oxidation state iron oxides displaying large magnetoresistance

The magnetic and conducting properties of many transition metal oxides may be drastically altered by doping. This study concerns a family of compounds with compositions intermediate between the itinerant helical antiferromagnet SrFeO3 and the ferromagnetic metal SrCoO3. We have measured the temperature-dependent muon-spin relaxation in five compounds in the family SrFe1−xCoxO3 where Co substitution varies from x=0 to 0.2 and investigated the relation between the relaxation rate and the magnetic susceptibility above the ordering temperature. These materials show large magnetoresistance which may be associated with the presence of competing interactions and non-equilibrium behaviour due to cluster formation. We find a quasistatic local field at the muon site below the magnetic transition temperature indicative of magnetic order in samples with x=0.15 and 0.2 but not in the sample with x=0.