We have fabricated hybrid structures consisting of a metallic thin film and of a microstructured two-dimensional electron system in an InAs heterostructure. The devices are found to exhibit a huge magnetoresistance (MR) effect in magnetic fields ⩽1 T. At low temperature, a value of ΔR/R=[R(B=1 T)−R(B=0)]/R(B=0) as high as 115 000% is measured. The value of ΔR/R has been studied as a function of the electron mobility, the electron density and the lateral width of the semiconductor. We find that the MR effect can be tailored by these different parameters and technological relevant devices can be realized.

Using two independent magnetotransport experiments, i.e., thermal activation and the coincidence method in tilted fields, we determine the g factor in a two-dimensional electron system in a 4-nm-wide InAs quantum well. From these independent techniques we deduce consistently an absolute value |gexp|≅6. This is considerably smaller if compared to |g|=14.8 for bulk InAs. Nonparabolicity in InAs cannot fully explain the reduced g factor. We argue that the penetration of the wave function into the In0.75Ga0.25As barriers and into the In0.75Al0.25As spacer layer plays an additional role.

We report noise measurements on Au–InAs hybrid structures involving a high-mobility two-dimensional electron system. Such structures show the extraordinary magnetoresistance (EMR) effect. We find excellent noise performance at room temperature close to the Johnson noise, which is in particular important for a technical application. At 4.2 K and in a magnetic field of about 1 T the nonoptimized EMR device is found to exhibit a low magnetic flux noise, offering the perspective of sensor applications also in a high magnetic field and at cryogenic temperature.

No abstract

We review our recent work on ferromagnet/semiconductor hybrid structures. In particular we focus on magnetotransport experiments performed on Co/InAs/Co hybrid structures fabricated on the cleaved edge of an InAs/InGaAs heterostructure. By modulation doping we inserted a high-mobility twodimensional electron system (2DES) between the Co source and drain contacts which were separated by about 0.2 µm. This separation was smaller than the mean free path in the 2DES. Two additional metallic gate electrodes were integrated, thus forming a prototype spin field-effect transistor on a cleaved (110) surface of an InAs/InGaAs heterostructure. Intriguingly, we observe two characteristic magnetotransport signals at 4.2 K: (a) a hysteretic spin-valve-like signal and (b) a large positive magnetoresistance. We attribute the latter to the extraordinary magnetoresistance effect, i.e. the magnetic-field induced current redistribution between the Co contacts and the 2DES.