An n-type tunable two-dimensional ferromagnetic semiconductor
Abstract
When a high concentration of magnetic impurities is incorporated in non-magnetic semiconductors, ferromagnetic behavior is observed. Magnetism, in these systems, is produced when a carrier-mediated interaction with the magnetic impurities takes place. This category of semiconductors is known as diluted magnetic semiconductors (DMS). So far, all the efforts in this field have been focused in growing crystals that would exhibit a ferromagnetic transition temperature above room temperature, as well as demonstrating the possibility of external control (i.e. via electric field) of the magnetic properties. Several type of structure with high transition temperature have been engineered [1], [2] and [3]. In some cases electrical [4], [5] and optical control [4] has also been achieved by means of relatively high gate voltage and light irradiation. In these systems, the origin of ferromagnetism has been proven to be hole-mediated through the magnetic Mn atoms [6]. To date, the only systems that have provided indications of the possibility of an n-type carrier mediated ferromagnetism are the large bandgap GaMnN system [8], [2] and the ZnMnAlO system [3]. However no direct evidence, such as external control or magnetotransport data, has been provided. In this work we will present data for a novel MBE (Molecular Beam Epitaxy) grown heterostructure that provides evidence of electron-mediated ferromagnetism with good gating capabilities. To our knowledge this is the first observation of n-type electron-mediated ferromagnetism in DMSs. The ferromagnetism is controllable via tuning of the n-carrier density by means of a low gate voltage. Our conclusions are based on magneto-transport data, which exhibit hysteretic behavior in an applied magnetic field when the GaMnAs quantum well is filled with electrons. In contrast, when electrons only occupy a heterojunction situated far away from the Mn atoms, no hysteresis is observed. (The quantum well filling behavior as a function of gate voltage is also in agreement with simulations). Moreover, we have seen that in this case there appears to be no presence of the Anomalous Hall Effect (AHE) -an effect rising from spin-orbit scattering-, either above or below TC, in contrast to what has been widely observed in all other GaMnAs systems. We will also discuss a novel technique to form Ohmic contact to a buried two-dimensional electron gas without altering the magnetic properties of the sample.
Degree
Ph.D.
Advisors
Giordano, Purdue University.
Subject Area
Condensed matter physics
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