We report the angular dependence of three distinct de Haas-van Alphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3 K and magnetic fields up to 14 T. Comparison with the Fermi surface calculations considering an incommensurate cycloidal magnetic order suggests that two of the observed dHvA oscillations arise from electron-like Fermi surface sheets formed by bands with strong B-px,y character. The third orbit could correspond to a Cr-d derived Fermi surface sheet. The measured effective masses of these Fermi surface sheets display strong enhancements of a factor of about two over the calculated band masses which can be attributed to electron-phonon coupling and electronic correlations. Signatures of further heavy d-electron bands that are predicted by the calculations are not observed in the temperature and field range studied. In view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity.

We study the validity and limitations of a macrospin model to describe the voltage-controlled manipulation of ferromagnetic magnetization in nickel thin film/piezoelectric actuator hybrid structures. To this end, we correlate simultaneously measured spatially resolved magneto-optical Kerr effect imaging and integral magnetotransport measurements at room temperature. Our results show that a macrospin approach is adequate to model the magnetoresistance as a function of the voltage applied to the hybrid, except for a narrow region around the coercive field-where the magnetization reorientation evolves via domain effects. Thus, on length scales much larger than the typical magnetic domain size, the voltage control of magnetization is well reproduced by a simple Stoner-Wohlfarth type macrospin model. arXiv:1108.3521v1 [cond-mat.mtrl-sci]

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Torque magnetometry at low temperature and in high magnetic fields B is performed on a MgZnO/ZnO heterostructure incorporating a high-mobility two-dimensional electron system. We find a sawtooth-like quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, unexpected spike-like overshoots in M and non-equilibrium currents are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptor-like scatterers give rise to a magnetic thaw down effect which enhances the dHvA amplitude beyond the electron-electron interaction effects being present in the MgZnO/ZnO heterostructure. PACS numbers: Valid PACS appear hereOxide heterostructures have generated tremendous interest in recent years [1,2]. Two-dimensional electron systems (2DESs) formed therein exhibit remarkable properties such as superconductivity [3] and magnetism [4] or the fractional quantum Hall effect (QHE) [5,6]. MgZnO/ZnO-based heterostructures are outstanding in that 2DESs of small carrier density n s exhibit extremely high mobilities µ at low temperature T [5,6]. At the same time, the electron-electron interaction parameter r s ∝ n −0.5 s [7] is large allowing for electron correlation effects at oxide interfaces in an applied magnetic field B [6,8,9]. Still, the quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect reflecting the ground state properties of such 2DESs has not yet been explored. Since the discovery of the dHvA effect in Bi more than eight decades ago it has been argued that disorder reduces peak-to-peak amplitudes ∆M via broadening of the quantized Landau levels E j (j = 0, 1, 2, ...) [10][11][12]. In contrast, electron-electron interaction effects are known to enhance ∆M [13]. The two counteracting effects are however not easy to distinguish in a balancing situation. Sometimes the dHvA effect has been obscured in the QHE regime even completely by extremely large non-equilibrium currents (NECs) [14]. The NECs are induced near integer filling factors ν = hn s /(eB ⊥ ) at low T when the longitudinal resistivity ρ xx takes a vanishingly small value and are believed to be limited only by breakdown of the QHE [14]. Independent transport experiments on GaAs-based heterostructures have evidenced that, strikingly, minima in ρ xx and plateaus in the Hall resistivity ρ xy could be displaced away from integer ν towards smaller ν due to repulsive scatterers [15][16][17][18][19]. This phenomenon has not yet been resolved in M (B), and a clear experimental manifestation of the underlying asymmetric density of states (DOS) in a ground state property is still lacking.In this Letter, we report torque magnetometry on the equilibrium dHvA effect and NECs of a high-mobility 2DES residing at a MgZnO/ZnO heterointerface. We observe dHvA amplitudes ∆M at filling factors ν = 1 and 2 that are significantly enhanced over the expected values in the single-particle picture. Addressing a regime 0.28 K < T < 1.6 K we observe T -dependent shifts of ...