Magnetic polarons in Eu-based films of magnetic semiconductors

Abstract

Various films of magnetic semiconductors EuO, EuS, EuSe, and EuTe have been studied by low-energy muon spin relaxation ͑LE-SR͒ as well as by magnetization measurement techniques. LE-SR allows measurements of the distribution of magnetic field on the subnanometer scale unaccessible to traditional macroscopic techniques. A bound state of an electron around a positive muon-a magnetic polaron-is formed in the paramagnetic phase of Eu-based films up to room temperature. The local environment around the muon in the magnetic polaron is shown to be ferromagnetic.Following a remarkable success of the metallic spintronics based on giant-magnetoresistive effect, 1 considerable efforts have been invested in recent years into the less developed semiconductor spintronics with the aim to produce three-terminal spintronic devices, in particular, spin transistors. 2,3 One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers from a ferromagnet ͑FM͒ into a nonmagnetic semiconductor, preferably one of those currently used in integrated circuits such as Si or GaAs. 4 Much work on metallic FM injectors proved them to be ineffective in either ohmic regime or tunneling injection. 5-7 Perhaps the best choice for the FM injector in such devices are magnetic semiconductors ͑MS͒ because of their compability with nonmagnetic semiconductors: the use of MS as a spin-polarized carrier injector avoids the so-called "conductivity mismatch" 8 which presents a fundamental difficulty for effective spin injection into a semiconductor from the FM 3d transition metals.The successful demonstration of the injection of polarized spins in spintronic devices 9,10 involves Mn-doped dilute magnetic semiconductors ͑DMS͒ as spin injectors. Current progress in spintronics is mainly connected to those DMS which become ferromagnetic by addition of several percent of magnetic impurities. 11 However, this addition significantly affects their homogeneity 12-14 which may cause an effective spin-flip scattering of spin-polarized carriers with subsequent reduction in the effectiveness of the spin injection. This unwanted feature is absent in concentrated MS such as Eu chalcogenides and EuO which are intrinsically magnetic.Being examples of strongly correlated electron systems, MS demonstrate strong dependence of the electrical and optical properties on the magnetization and spin fluctuations of the magnetic lattice 15 which stimulated their extensive studies in the 1960s and 1970s. Current renaissance in the study of MS is caused by the fact that they are relatives to DMS, high-temperature superconductors and colossalmagnetoresistive manganites. 15,16 Magnetic semiconductors offer several important advantages over DMS such as higher magnetization, spatial magnetic homogeneity and wider range of conductivity tunability by doping so that they can be used as spin filters in the insulating state and as spin injectors when doped. [17][18][19] When doped, however, these materials typically enter into dominant states th...

β-detected NMR spin relaxation in a thin film heterostructure of ferromagnetic EuO

“…This procedure reproducibly yielded stoichiometric EuO with T C = 69 K in zero magnetic field, and similar films were used in Ref. [10]. Gold capping is crucial as Eu 2+ is very easily oxidized and must be isolated from exposure to the atmosphere.…”

“…Here we study the magnetic dynamics in a thin film heterostructure based on europium monoxide. EuO is a prototypical semiconducting local moment Heisenberg ferromagnet with a simple rocksalt structure that has been studied for decades [5,6], including via 153 Eu NMR [7], zero field NMR [8], and μSR [9][10][11][12]. Below the Curie point, the elementary excitations of the FM state are well described as magnons [13,14], and thermally excited magnons account for the NMR spin relaxation rates.…”

Spin-polaron band in the ferromagnetic heavy-fermion superconductor UGe2

“…Although the possibility of SP formation in AFM and PM states has long been anticipated [46][47][48]52,53 and experimentally established [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]77,85,86 in a great variety of materials by many different techniques, its existence would seem to be incompatible with the FM state. The exchange contribution to SP stabilization amounts to the difference between the PM disorder (or AFM order) of the host and the FM order within the SP; in a fully saturated FM state the exchange contribution to the localization would be negligible, as the lattice spins are already aligned.…”

Direct Epitaxial Integration of the Ferromagnetic Semiconductor EuO with Silicon for Spintronic Applications

“…Concentrated MS offer several important advantages over DMS such as higher magnetisation, spatial magnetic homogeneity and wider range of conductivity tuning by doping, so that they can be used as spin filters in the insulating state and as spin injectors when doped [22,23]. Being doped, though, at high temperature, these materials typically enter into dominant states that are not spatially homogeneous due to formation of magnetic polaronsfew-body systems comprised of electron and local magnetic moments of the host [20,[24][25][26][27]. However, this unwanted formation does not take place when magnetisation of the lattice is significant, leaving the host material perfectly homogeneous in the region of its employment as a spin injector [25][26][27].…”