μ ⁺ SR of the Organic Ferromagnet p -NPNN: Diamagnetic and Paramagnetic States

Abstract

We use zero-and longitudinal-field muon spin rotation and relaxation to study the muon states in the organic molecular ferromagnet: j3-phase para-nitrophenyl nitronyl nitroxide (p-NPNN). Below Tc = 0.65 K, oscillations in the relaxation function are observed in zero applied field which follow the magnetic order parameter, implying that the muon is bonded in an electronic spin-singlet state. We also study the behaviour of the oscillations in an applied longitudinal field and interpret the results by considering the magnetisation process and the demagnetising field. A significant field-induced repolarisation is also observed which is ascribed to the presence of spin-triplet states formed by a fraction of muons.Recently, there has been much interest in the search for a purely organic molecular ferromagnet which contains only light elements (like carbon, nitrogen, hydrogen, and oxygen) [l]. The frst such material to be found, the j3 crystal phase of para-nitrophenyl nitronyl nitroxide (p-NPNN, see fig. lb)), was reported to have a Curie temperature (T,) of 0.60K, a discovery which was made using heat capacity and magnetisation measurements [2,3]. The unpaired spin in this nitronyl nitroxide is associated with the N-0 groups. The role of the rest of the molecule is to ensure the appropriate overlap of the correct orbitals on neighbouring molecules to produce 3D ferromagnetism. A whole series of materials which incorporate this nitronyl nitroxide group are currently being synthesized in order to provide a systematic study of the effects of the variation of the rest of the molecule on the molecular packing and overlap and hence on the magnetic properties.Zero-field (ZF) muon spin rotation/relaxation (pSR) studies on aligned single crystals [4]and polycrystalline [51 samples have also demonstrated magnetic order with Tc -0.65 K; in these experiments, the muon spin precession rate directly yields the value of the internal magnetic field at the muon site. In a pSR experiment, a beam of spin-polarized muons is stopped in a target specimen. Each muon subsequently decays (with a mean lifetime of z p = = 2.2 ps), emitting a positron preferentially along the muon spin direction because of the parity violation of the weak interaction [6]. The time histograms N * (t) of positrons counted in the 8 Les Editions de Physique

Muon studies of organic ferromagnets and conductors

“…The resulting electronic spin-state of the muonated radical may be a singlet (S = 0, leading to a diamagnetic state) or a triplet (S = 1, leading to a paramagnetic state) [6]. Both states are found in experiments.…”

“…Small chemical changes to the rest of the molecule lead to significant changes in crystal structure, thereby altering the intermolecular overlaps and thus the magnetic interactions between unpaired spins on neighboring molecules. Thus, different compounds have greatly different magnetic ground states and these can be effectively studied using ~tSR [5][6][7][8][9][10][11][12].…”

Complémentarité de la diffraction de neutrons avec d'autres techniques spectroscopiques (dans l'étude du magnétisme)

“…Ils peuvent provenir aussi de moments électroniques très faibles, de l'ordre de 0.01 B , comme dans certains composés intermétalliques de terres rares ou d'actinides, ou bien dans des matériaux organiques [5,6]. De telles valeurs de moment sont difficilement décelables par diffraction de neutrons, et la spectroscopie de muons montre alors tout son intérêt.…”

“…Effectivement, des mesures macroscopiques ont mis en évidence dans p-NPNN une température de Curie très basse : T C = 0.6 K. On s'attend à ce que le moment magnétique spontané soit faible, du fait de la délocalisation de l'électron non-apparié. Des mesures SR [5] dans ce composé ont décelé une dépolarisation oscillante dans le temps pour T < T C ; à 0.045 K, le champ dipolaire responsable de cette oscillation vaut 150 G (voir Fig. 6 à gauche).…”

“…[7] ont réussi à observer une très faible augmentation de l'intensité des raies de Bragg nucléaires (1%) au-dessous de 0.7 K, ce qui démontre que la structure est ferromagnétique + dans le ferromagnétique organique p-NPNN à différentes températures, et variation thermique du champ local mesuré sur les spectres (d'après les Refs. [5,6]) ; À droite : variation thermique de l'intensité de la raie de Bragg (062) obtenue par diffraction de neutrons dans le même composé (d'après la Référence [7]). avec k = 0 (voir Fig.…”