Quantum Hall ferromagnetism in II–VI based alloys

Abstract

The article reviews our recent studies on quantum Hall ferromagnetism (QHF) in diluted magnetic semiconductors. We carried out magnetoresistance studies on modulation-doped, gated heterostructures of (Cd,Mn)Te/(Cd,Mg)Te:I. We put into evidence the formation of Ising quantum Hall ferromagnet with Curie temperature TC as high as 2 K. QHF is manifested by anomalous magnetoresistance maxima. Moreover, magnitude of these spikes depends dramatically on the history of the sample, shows hysteresis when either magnetic field or gate voltage are swept, stretched-exponential time evolution characteristic of glassy systems, and strong Barkhausen noise. Our study suggests that these metastabilities stem from the slow dynamics of ferromagnetic domains.1 Introduction Two-dimensional electron systems (2DES) under low temperatures (T ) and high magnetic field (B) offer a great opportunity for studying the fascinating strong-correlation physics. The carrier density can be controlled externally in 2DES, while tilted B allows one to discriminate between orbital and spin effects. Furthermore, owing to the quenching of the in-plane kinetic energy into quantized Landau levels (LL), correlation effects dominate over single particle potentials in a strong magnetic field leading to an unexpectedly rich variety of ground states and quasiparticle forms [1]. In particular, if LL corresponding to the opposite spin orientations of quasi-particles at Fermi level overlap, the spin degree of freedom is not frozen by the field so that a spontaneous spin order may appear at low temperatures [2,3], the resulting state being known as the quantum Hall ferromagnet (QHF). Importantly, the ground state is predicted to have the uniaxial anisotropy if the spin subbands involved originate from different LL [4]. The level arrangement corresponding to such Ising QHF has been realized in various III-V 2DES [5,6,7,8].In recent years we have witnessed a growing interest in diluted magnetic semiconductors [9] (DMS) because of their possible application in spintronics. Previous works demonstrate a strong influence of localized d-spins on electron transport of 1D [10, 11] and 2D [12] structures of DMS. In DMS the mean-field part of the sp−d exchange interaction between the electrons and d-spins leads to the giant s−d spin-splitting of the electronic bands. Since in DMS the spin-splitting of electronic states is not only giant but depends, in a nonlinear fashion, on the magnetic field B, many crossings of Landau spin sublevels occur in the QHE regime, so that in an independent-electron picture, energies come into "coincidence" for particular values of B. This worthwhile possibility was explored in the recent study [13], which demonstrated the existence of a quantum Hall ferromagnetism in a II-VI (Cd,Mn)Te/(Cd,Mg)Te modulation doped heterostructure with a quantum well (QW) containing magnetic ions. On either side of these coincidences, the magnetization of the electron system thus takes on distinct values, corresponding to QH ferromagnets (or "Ising"