Magnetic monopole fingerprints in ferromagnetic semiconductors

D. LUCOT; M. GRANADA; R. GIRAUD; A. LEMAÎTRE; C. ULYSSE; X. WAINTAL; G. FAINI

Aussois

Conferencia; Réunion GDR 2426: Physique Quantique Mésoscopique; 2011

Mesoscopic transport measurements of GaMnAs diluted magnetic semiconductor (DMS) have been performed over the past years by very few groups. By choosing accurately material and experimental conditions, we have shown that phase coherent transport is preserved in ferromagnetic materials with a quite large coherence length [1]. Indeed, Universal Conductance Fluctuations (UCF) have been observed in nano Hall bars, and typical coherence lengths larger than 100nm at 100 mK were extracted from our data. Very strikingly, some intriguing faster fluctuations were observed in the low-field regime when the magnetic field is applied perpendicular to the magnetization easy axis and was discussed as probably due to some magnetic texture and/or presence of magnetic domain walls (DWs)[1]. Here we report on new transport measurements performed in another DMS system, namely GaMnAsP. Magnetic anisotropy easy axis in these systems is perpendicular to the sample plane. We performed magnetoconductance experiments, as a function of temperature and sweeping the external magnetic field at different angles from the anisotropy easy axis. Precise magnetic configuration of the sample was prepared in such a way that no DWs are present in the nano-Hall bar, so that it acts as a monodomain ferromagnet. Also in this case, two different oscillating regimes are observed: a conventional UCF one at magnetic fields higher than 200mT, and a faster one at low fields. We show here that these fast fluctuations are reproducible quasi-periodic oscillations due to the coherent rotation of the magnetization from the easy axis position down to the external magnetic field direction and are clearly not ascribed to the DWs presence. Moreover, contrary to the usual UCF temperature dependence of the high field regime, the correlation fields of these faster oscillations are temperature independent. This behaviour is coherent with what is expected for a mechanism induced by a simple geometric Berry phase effect.. Our experiments confort the Berry’s phase fingerprints of magnetic monopoles picture proposed very recently by Hals et al.,[2]. It offers a new way to manipulate momentum-space magnetic monopoles by an external magnetic field but, above all, it offers an experimental protocole using coherent phase interferences experiments, as for UCF in the present case, to directly reveal them. [1] L. Vila et al., Phys. Rev. Lett. 98, 027204 (2007). [2] K. Hals et al., Phys. Rev. Lett. 105, 207204 (2010).