High- T C Ferromagnetic Semiconductors: Fake or Fact?
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igh-TC Ferromagnetic Semiconductors: Fake or Fact? A. Furrera, b, * a
bLaboratory
Swiss Neutronics AG, Klingnau, CH-5313 Switzerland for Neutron Scattering, Paul Scherrer Institute, Villigen, CH-5232 Switzerland *e-mail: [email protected] Received June 27, 2019; revised July 30, 2019; accepted August 10, 2019
Abstract—The long lasting interest in diluted magnetic semiconductors (DMS) relies on the combination of fundamental science and the potential for spintronics applications. Despite the large body of experimental and theoretical studies carried out in the past decades, there are ongoing controversies about high-TC ferromagnetic DMS. This paper summarizes the results of the few investigations performed by inelastic neutron scattering, which provides key information for some of the open questions. Emphasis is led on a complete study of the compound MnxGa1 – xN, as well as on novel DMS materials for which the spin injection is decoupled from carrier doping. Keywords: diluted magnetic semiconductors, exchange interaction, inelastic neutron scattering DOI: 10.1134/S1027451020070150
INTRODUCTION Traditional semiconductors are based on the control of charge carriers (n- or p-type). Magnetic-iondoped semiconductors allow, in addition, the control of the quantum spin state (up or down), which have attracted great interest because of their potential applications in spintronics and blue-light emitting diode (LED) technologies [1, 2]. The main challenge for practical applications is the attainment of a Curie temperature TC above room temperature to be compatible with junction temperatures. So far, the highest Curie temperatures were obtained for MnxGa1 – xAs (x = 0.13) with TC = 190 K [3], as well as for (KxBa1 – x)(MnyZn1 – y)2As2 (x = 0.3, y = 0.15) with TC = 230 K [4]. The observation of ferromagnetism above room temperature reported by several authors has to be considered with caution, as these findings neither resulted in a device working at room temperature nor were confirmed by other groups. Here we discuss how neutron scattering experiments can contribute to resolve the ongoing controversies about high-TC ferromagnetic semiconductors. The problems to be addressed are fourfold: (i) What is the solubility limit of the doped magnetic ions in the host compound? (ii) Where are the doped magnetic ions located in the host compound? (iii) What is the valence of the doped magnetic ions? (iv) What is the doping-induced enhancement of the ferromagnetic exchange interaction? The present paper provides answers to these questions by summarizing the few inelastic neutron scattering experiments performed for diluted magnetic semiconductors (DMS) in the past.
STRATEGY OF INELASTIC NEUTRON SCATTERING EXPERIMENTS ON DMS The content of magnetic ions in DMS compounds is rather low, thus, the magnetic excitations can be associated with monomers and dimers resulting from the random distribution of the magnetic ions. The spin Hamiltonian of magnetic dimers is given by
H D = –2Js1 · s2,
(1)
where J is the bilinear exch
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