To understand quantum mechanical transport in a ferromagnetic semiconductor, the knowledge of basic material properties such as the phase coherence length and corresponding dephasing mechanism are indispensable ingredients. The lack of observable quantum phenomena has prevented experimental access to these quantities so far. Here we report the observations of universal conductance fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and temperature dependence of the fluctuations reveals a T(-1) dependence of the dephasing time.

We report on the observation of weak localization in arrays of (Ga,Mn)As nanowires at millikelvin temperatures. The corresponding phase coherence length L φ is typically between 100 nm and 200 nm at 20 mK. Strong spin-orbit interaction in the material is manifested by a weak anti-localization correction around zero magnetic field.PACS numbers: 73.43. Jn, 72.25.Dc, 73.43.Qt Quantum corrections to the resistance like weak localization are suppressed by a sufficiently strong perpendicular magnetic field B [1]. Hence the question arises whether such effects can be observed in ferromagnets which have an intrinsic magnetic induction. While few experimental works explored this problem [2,3], a definite experimental answer is still lacking. Hence, the advent of the new ferromagnetic semiconductor material (Ga,Mn)As with significantly smaller internal field compared to conventional ferromagnets offers a new opportunity to address such questions. Ferromagnetic semiconductors like (Ga,Mn)As [4] are interesting materials for spintronics as well, as they combine ferromagnetic properties with the versatility of semiconductors [5]. The spin 5 2 -Mn-ions on regular sites of the zinc-blende lattice of the GaAs host act as acceptors thus providing both holes and magnetic moments. The ferromagnetic order between the Mn-ions is mediated by these holes [6]. By now ferromagnetism in (Ga,Mn)As is well understood, allowing to predict Curie temperatures [6], magnetocrystalline anisotropies [7] as well as the anisotropic magnetoresistance effect [8]. In this respect (Ga,Mn)As is one of the best understood ferromagnetic materials at all [9] and hence suitable as a model system to study quantum corrections to the conductivity.

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Quantum interference effects and resulting quantum corrections of the conductivity have been intensively studied in disordered conductors over the last decades. The knowledge of phase coherence lengths and underlying dephasing mechanisms are crucial to understand quantum corrections to the resistivity in the different material systems. Due to the internal magnetic field and the associated breaking of timereversal symmetry quantum interference effects in ferromagnetic materials have been scarcely explored. Below we describe the investigation of phase coherent transport phenomena in the newly discovered ferromagnetic semiconductor (Ga,Mn)As. We explore universal conductance fluctuations in mesoscopic (Ga,Mn)As wires and rings, the Aharonov-Bohm effect in nanoscale rings and weak localization in arrays of wires, made of the ferromagnetic semiconductor material. The experiments allow to probe the phase coherence length L φ and the spin flip length L SO as well as the temperature dependence of dephasing.

We investigate magnetoresistance effects for transport across (Ga,Mn)As nanoislands, detached by nanoconstrictions from wider (Ga,Mn)As input leads. As in previous studies a huge magnetoresistance was found for nanoconstrictions in the tunnelling regime. For slightly wider junctions an enhanced anisotropic magnetoresistance effect was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)