Muons and magnets

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.

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.