Studies of spin-orbit coupling phenomena in magnetic semiconductors

Abstract

Hard disk drives (HDDs) have been the dominant secondary memory device in computing for over 50 years, while more recently magnetoresistive random access memory (MRAM) has emerged as a candidate for primary computing memory. Both HDDs and MRAM store information in the polarity of a magnetic layer, which is written and read by non relativistic mechanisms. There is now gathering interesting in using relativistic mechanisms whose origins lie with spin-orbit coupling (SOC) for MRAM writing because of potential benefits in terms of scalability, device design, and efficiency. This thesis investigates the fundamental physics of SOC phenomena that can write (spin-orbit torque (SOT), Neel order SOT) or read (anisotropic magnetoresistance (AMR), magnetic gating) the magnetic state by the application of electrical current. These phenomena are studied in ferromagnetic and antiferromagnetic semiconducting materials that offer a relevant electrical conductivity for integration into commercial electronic devices. Effective magnetic fields which parametrise the SOT phenomenon are measured in the diluted magnetic semiconductor (Ga,Mn)As using a technique based upon experimental planar Hall effect measurements and analytical fitting with a free energy equation for coherent magnetization rotation. It is found that effective magnetic fields which originate from Dresselhaus SOC increase in magnitude with increasing temperature, whereas those originating from Rashba SO have no significant temperature dependence within experimental uncertainty. The size of the measured effective fields per unit of current density, as well as the ratio of Dresselhaus to Rashba effective field magnitudes averaged over all temperatures are comparable to previous experimental measurements. Sb-based diluted magnetic semiconductors (Ga,Mn (As0.9,Sb0.1) and (Ga,Mn)Sb are characterised by magnetic and transport measurements. The Curie temperature (Tc) of (Ga,Mn)(As0.9,Sb0.1) increases from 28K to 55K upon sample annealing. The Tc of as-grown (Ga,Mn)Sb is found to be 34K, and in contrast to (Ga,Mn)(As0.9,Sb0.1) does not change upon annealing, indicating a lack of interstitial Mn in (Ga,Mn)Sb. Field rotation transport measurements for current along various crystalline directions reveal significant crystalline and non crystalline contributions to the AMR of both as-grown and annealed (Ga,Mn)(As0.9,Sb0.1). An anomalous temperature dependence of the AMR of the annealed (Ga,Mn (As0.9,Sb0.1) sample for current along the [110] crystalline direction is accounted for by considering the relative sizes of the individual AMR contributions as a function of temperature. Results are shown of an attempt to vary the current flow through a non-magnetic GaAs/AlGaAs 2D electron gas (2DEG) by changing the magnetization orientation of an electrically insulated Fe gate layer. Such magnetic gating of electrical current is based upon the principle that, as a result of SOC, the electrochemical potential of a ferromagnet is anisotropic with respect to its magnetization orientation. The magnetic gating experiment proved to be unsuccessful due to an AMR-like signal arising in field rotation measurements of 2DEG samples both with and without the gate layer. The origins of this AMR-like signal are unknown, and it cannot not be accounted for by fitting analysis