Muons and magnets

Abstract:

In order to study correlation between magnetic ordering and crystal structures of organic molecular magnets, zero-field muon spin rotation (mu SR), electron spin resonance (ESR) and a.c. susceptibility of the organic neutral radicals, 3-quinolyl nitronyl nitroxide (3-QNNN) and p-pyridyl nitronyl nitroxide (p-PYNN), were investigated. The onset of magnetic ordering is indicated at 0.21 K for 3-QNNN and 0.11 K for p-PYNN in the mu SR measurements as well as the a.c. susceptibility results. The magnetic structures are discussed in terms of canted ferromagnetic spin structures on the basis of molecular arrangements in the crystals. Preferred spin directions with respect to the molecular orientations of the radicals in the crystals are discussed.

Abstract:

Recently, ferromagnetism has been observed in a family of organic molecular crystals based on the nitronyl nitroxide radical group. We report zero-field mu SR measurements on compressed powder samples of the organic magnets p-NPNN and 3-QNNN which have been used to directly probe the temperature dependence of the spontaneous field and the relaxation rate in each system. In p-NPNN, we observe a temperature dependence of the spontaneous field which is very similar to that seen in an aligned single crystal experiment and yield the same Curie temperature (T-c=670 mK). However, the oscillations in the measured signal are very strongly damped, particularly so just below the transition temperature. In 3-QNNN, the oscillations indicate a lower spontaneous field and a smaller Curie temperature (T-c=210 mK).

Abstract:

Recently, ferromagnetism has been observed in a family of organic molecular crystals based on the nitronyl nitroxide radical group. We report zero-field μSR measurements on compressed powder samples of the organic magnets p-NPNN and 3-QNNN which have been used to directly probe the temperature dependence of the spontaneous field and the relaxation rate in each system. In p-NPNN, we observe a temperature dependence of the spontaneous field which is very similar to that seen in an aligned single crystal experiment and yield the same Curie temperature (TC = 670 mK). However, the oscillations in the measured signal are very strongly damped, particularly so just below the transition temperature. In 3-QNNN, the oscillations indicate a lower spontaneous field and a smaller Curie temperature (TC = 210 mK). © 1995.

Abstract:

Angle dependent magnetoresistance oscillations (AMRO) have been studied in the charge transfer salt α-(ET)2KHg(SCN)4 for magnetic fields in the range 0 - 30 T. This salt exhibits the onset of antiferromagnetic order at temperatures TN ∼8-10 K and the presence below this temperature of a region of sharp negative magnetoresistance at a field around 22 T known as the "kink". AMRO have been measured in this salt for a wide range of applied fields since the period, amplitude, and nature of the oscillations can be used to directly infer the character of the Fermi surface (FS) as a function of field. The data indicate that a profound change in the band structure occurs at this kink transition; the high field phase is characterised by quasi-2D oscillations from a closed cylindrical FS which is elongated in the c direction; the low field phase appears to be a spin density wave groundstate, with a FS consisting of a sheet (which is quasi-1D in character and tilted at an angle of ∼21° to the b*c plane) and small closed 2D pockets. It is suggested that the breakdown orbits between the pockets and the 1D sheets are able to account for the various Shubnikov-de Haas frequencies observed below the kink. © 1995.