Spin-Polarized Tunneling Spectroscopy in Tunnel Junctions with Half-Metallic Electrodes

Abstract

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La 2/3 Sr 1/3 MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain a precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La 2/3 Sr 1/3 MnO3 and allows the determination of the gap δ between the Fermi level and the bottom of the t2g minority spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.PACS numbers: 75.47. Lx, 79.60.Jv A magnetic tunnel junction (MTJ) is composed of two conducting ferromagnetic electrodes separated by a thin insulating barrier. Its resistance depends on the relative orientation of the magnetizations of the electrodes, a property which is called TMR (Tunneling Magnetoresistance). The TMR ratio is defined aswhere R P and R AP are the junction resistances in the parallel (P) and antiparallel (AP) configuration respectively. The MTJs are extensively investigated for the interest of the TMR in spintronic devices such as MRAM (Magnetic Random Access Memory) or magnetic sensors [1], but they also raise interesting fundamental problems. In this Letter, we present an example of exploitation of the TMR to obtain a precise information on the spin and energy dependence of the density of states (DOS) of a ferromagnetic conductor. This shows the potential of MTJs for spin-resolved spectroscopic studies. Electron tunneling at different bias voltages (V DC ) probes different energy ranges of the DOS and this was first used to extract information on the electronic structure of a superconducting electrode by Giaever in 1960 [2] : the presence of a quasi-particle gap in the DOS of the superconductor is reflected in the voltage dependence of the current tunneling into the superconductor thus allowing a quantitative determination of this gap. More recently, Xiang et al [3] have also performed a numerical analysis of TMR vs V DC curves in transition metal based MTJs to estimate the spin-dependent DOS of a Co collecting electrode. However, the relatively narrow energy range (E F ±0.4 eV) probed by TMR is not well suited to investigate the DOS of a wide band transition metal. We will see that the technique is more appropriate for narrow band metallic oxides.Also, conceptually, to extract information on the DOS above the Fermi level of a ferromagnetic collecting electrode from the bias dependence of the TMR, it is highly desirable to use a fully spin-polarized emitting electrode, i.e. a half-metal [4]. Supposing, for example, a halfmetal (HM) electrode emitting only electrons of its majority spin direction, the tunneling will probe separately the majority-spin DOS of the collecting electrode in the P configuration (mainly at an energy eV DC above E F ) and the minority one in the AP configuration.The MTJs of...

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

“…As a result, an extra tunneling conductance and a decrease of TMR are obtained. These findings are also confirmed by experimental studies [55,65]. At a bias voltage of about 350 mV, a plateau in the TMR curve is generally observed, which is followed by another sharp decrease beyond 400 mV (Fig.…”

“…At a bias voltage of about 350 mV, a plateau in the TMR curve is generally observed, which is followed by another sharp decrease beyond 400 mV (Fig. 9) [65]. The point of inflection can be interpreted as the onset of electron tunneling into the minority conduction band of LSMO.…”

“…9) [63][64][65]. This strong bias dependence is accompanied by a zero-bias anomaly in the conductance curves.…”

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

“…We have provided and highlighted an explicit description of thermally activated defect-assisted magnetotransport by utilizing solid-state tunnelling spectroscopy 55 to probe the ground and excited states of oxygen vacancies 49,52 . Spin-polarized transport across MgObased MTJs with a 'low' density of M centres reproducibly proceeds with respect to F þ and F þ * centres for both P and AP channels, with a weighing at higher temperature (room temperature) in the AP channel towards the F þ * centre compared with the P channel due to symmetry mixing.…”

“…Spin-polarized solid-state tunnelling spectroscopy on lithographically processed stacks 8 can reveal the junction electrodes' electronic structure 12,55 , the junction's symmetry-resolved barrier heights 37 and inelastic tunnelling phenomena 19 that includes hopping onto defect states 24 . We note that, since the D 1 complex loop, which spans the band gap and defines the wavefunction decay through tunnelling, is rather flat within at least ± 1.1 eV about the band gap centre, both electron and hole tunnelling with respect to states above and below E F are considered 37 (rescaling the local density approximation (LDA)-derived 4.7 eV MgO band gap to its experimental value of 7.8 eV yields a flat D 1 band over 2.2 eV about the charge-neutrality level-pegged chemical potential 37 .…”