Observation of spin filtering in magnetic insulator contacts to silicon

APPLIED PHYSICS LETTERS 98, 142503 共2011兲

Observation of spin filtering in magnetic insulator contacts to silicon Martina Müller,1,a兲 Martina Luysberg,2 and Claus M. Schneider1 1

Peter Grünberg Institut (PGI-6), Forschungszentrum Jülich, 52425 Jülich, Germany Peter Grünberg Institut, Ernst-Ruska Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, 52425 Jülich, Germany

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共Received 13 January 2011; accepted 7 March 2011; published online 5 April 2011兲 The magnetic insulator EuS is used to create a spin-selective and conductivity-matched tunnel contact to silicon, in analogy to a conventional ferromagnetic metal/semiconductor configuration employed for spin injection. The spin filter efficiency of such a magnetic “spin filter” tunnel barrier is quantified using an adjacent Co ferromagnetic electrode as spin detector in a spin valve-type structure. This previously unobserved magnetoresistance effect demonstrates the efficient spin-polarizing nature of magnetic semiconductors on silicon and its prospective functionality as spin injectors/detectors in hybrid semiconductor devices. © 2011 American Institute of Physics. 关doi:10.1063/1.3572016兴 In recent years, the field of spintronics has progressed rapidly, driven by the perspective to extend the functionality of conventional electronics by exploiting the electron’s spin degree of freedom.1 In semiconductor 共SC兲 spintronics, both information storage and processing may be joined into a single device, if a thorough control of the spin information 共i.e., injection, manipulation, and detection兲 in mainstream SCs becomes feasible. Devices envisioned for this purpose basically rely on dedicated ferromagnetic 共FM兲/SC contacts as the main building blocks for spin injection and detection.2,3 The key requirement is a small resistivity mismatch between FM and SC in order to realize a large spin transfer ratio.4 Present solutions combine a FM metal electrode with a tunneling barrier as spin-resistive injector/ detector contacts5,6 or employ half-metallic electrodes.7 So far, further development is still hampered by the lack of FM contact materials that are both highly spin-polarized and conductance-matched with a SC. The present study aims at bridging this gap, demonstrating the spin filter 共SF兲 functionality of a magnetic SC in direct contact to the technologically most relevant SC, silicon. Magnetic insulators 共MIs兲 represent an appealing class of materials that combine FM and insulating properties, with the magnetic ion being an intrinsic part of the crystal lattice.8 This configuration brings about the so-called SF effect, which describes a spin-selective tunnel transport of electrons across a magnetic barrier.9 If a MI is cooled below its Curie temperature TC, the conduction band spin-splits due to magnetic exchange interaction. This allows one to utilize MIs as “SF” tunnel barriers, in which two different barrier heights act for spin-up and-down electrons. Due to the exponential variation in the tunneling probability with barrier heights, highly spin-polarized tunnel currents result.10 Consequently, the SF effect in a magnetic barrier may replace FM metal electrodes as source of spin polarized carriers. SF tunneling has attracted considerable interest in the past few years, and fundamental phenomena have been studied in metal-based SF tunnel junctions,9 double SF tunnel devices11 and hybrid SF/FM spin valve structures.12 The potential of SF barriers as functional contacts for spin injection/detection into SCs, a兲

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however, remains basically unexplored to date. In this letter, we report the observation of spin filtering in the MI europium sulfide 共EuS兲 in direct contact to silicon. We present magnetotransport studies of EuS magnetic tunnel barriers on n-doped Si共001兲. We chose EuS as a well-known FM SC with a TC of 16.6 K, a magnetic moment of 7 ␮B and the capability to generate highly spin-polarized currents via tunneling.13 We demonstrate the SF capability of EuS magnetic tunnel barriers via the magnetoresistance 共MR兲 occurring in spin valve-type systems using FM Co counter electrodes as spin detector. This approach allows us to directly quantify the SF efficiency of EuS/Si共001兲 SC contacts. Crystalline structure and interface properties of EuS/ Si共001兲 were investigated using high-resolution transmission electron microscopy 共HRTEM兲. We note a polycrystalline growth of EuS on Si共001兲 substrate in Fig. 1共a兲, which also reveals the presence of textured nanocrystallites incorporated into the EuS film. The interplanar distances of 2.95 Å for selected crystallites closely match the 共001兲 planes of fcc EuS 共a = 5.96 Å兲. The HRTEM study also provides information on the structural quality of the EuS/Si interface, which is relevant for spin-selective tunnel transport: We observe a good structural separation of EuS and Si共001兲 as a consequence of optimized growth conditions. Any microroughness of the Si共001兲 substrate was most likely generated by the hydrofluoric 共HF兲 etching process prior to deposition.14 No

FIG. 1. 共Color online兲共a兲 Cross-sectional HRTEM image of the Si共001兲/ EuS transport interface. Arrows indicate the lattice spacing of 共001兲 planes in the EuS layer, whereas 共001兲 and 共110兲 indicate the crystallographic axes of silicon. 共b兲 Device schematics of an embedded SF tunnel contact to silicon. A circular contact is predefined by optical lithography and the resulting cone is filled by MBE depositing EuS/ Al2O3 / Co/ CoO/ Au.

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Appl. Phys. Lett. 98, 142503 共2011兲

FIG. 2. 共Color online兲 Top: MR as function of magnetic field, taken at V = −150 mV and T = 4.2 K. Device schematics with arrows indicate the relative magnetizations of the FM electrode 共top electrode兲 and the SF barrier. Bottom: normalized magnetization M / M S of a CoO/ Co/ Al2O3 / EuS multilayer on Si共001兲, recorded at 2 and 30 K in a SQUID magnetometer.

amorphous intralayers are present at the EuS/Si interface and the possibility of SiOx formation can be ruled out. Embedded SF tunnel devices were fabricated on n-Si共001兲 substrates, as schematically depicted in Fig. 1共b兲. Circular contacts to Si共001兲 with diameters d = 320 ␮m were predefined into a 500 nm thick SiO2 layer by optical lithography and wet chemical etching. The resulting cone was filled by molecular beam epitaxy 共MBE兲 depositing the respective multilayers. Spin-polarized transport measurements were carried out in a two-probe configuration, as the EuS barrier resistance at 4.2 K was a factor of ⬃104 higher than the electrodes series resistances in the low-bias regime. Figure 2 shows a R共H兲 curve taken at 4.2 K which clearly demonstrates the MR effect in a CoO/ Co/ Al2O3 / EuS contact on Si: We find a well-defined change in R due to the change in the magnetic configuration between EuS and Co/CoO. The MR is shifted to negative magnetic fields because the Co magnetization is pinned by exchange bias 共EB兲 to CoO. This result is experimental evidence for the presence of spin filtering in the magnetic tunnel barrier with silicon electrode. The bottom panel of Fig. 2 shows the hysteresis loops M共H兲 of a similar unpatterned sample measured at 2 and 30 K using superconducting quantum interference device 共SQUID兲 magnetometry. On decreasing ␮0H from a large positive value, two gradual inflection points occur at ⫺50 and ⫺720 Oe, which mark the independent magnetic switching of EuS and Co. The clear shift in M共H兲 from zero field due to EB in the Co/CoO bilayer matches the shape of the MR curves shown in the upper panel of Fig. 2. The positive sign of MR indicates that the EuS SF and Co electrode are parallel polarized, which is consistent with the positive spin polarization P measured for EuS and Co in earlier tunnel experiments.10 The MR values were calculated using the relation MR= RAP − RP / RP, where RAP and RP represent the resistance values in the antiparallel 共AP兲 and parallel 共P兲 magnetic configurations of EuS and Co. The Al2O3 spacer layer thereby has a negligible influence on the MR

FIG. 3. 共Color online兲共a兲 I共V兲 characteristics for a magnetic tunnel contact consisting of CoO/ Co/ Al2O3 / EuS/ Si共001兲, recorded at 4.2 K under an applied magnetic field with P or AP configuration between the magnetic layers. 共b兲 Fitting the calculated d2I / dV2 curves linearly yields the direct tunneling regime.

since it does not distinguish spin-up and-down electrons. The maximum MR we have obtained in our contacts reaches +14%. Taking PCo ⬇ 35%, we may approximate the SF efficiency PEuS from Jullière’s formula, MR = 2PCoPEuS / 共1 − PCoPEuS兲. This gives a maximal EuS SF efficiency of PEuS ⬇ 20%. This calculated value is comparable with previous results from EuS-based tunnel junctions with metallic electrodes,15 and is consistent with the good magnetic properties of the EuS layer.16 Our observation of MR in CoO/ Co/ Al2O3 / EuS/ Si共001兲 directly proves a sizeable spin filtering in a magnetic SC in direct contact to silicon. The I共V兲 characteristics of the CoO/ Co/ Al2O3 / EuS/ Si共001兲 contact are shown in Fig. 3共a兲. By analyzing the second derivative of these tunneling measurements 关see Fig. 3共b兲兴, we obtain an estimate of the tunneling regime from the bias voltages, at which the slope deviates from linearity. d2I / dV2 is linear for −55 mVⱕ V ⱕ +55 mV, which indicates that direct tunneling dominates transport in this symmetrical bias regime. In addition, we determined the average barrier height ⌽ from fitting the I共V兲 curves to the Brinkman–Dynes–Rowell 共BRD兲 model for tunneling in the bias range ⫾150 mV.17 The fitting procedure yielded ⌽ = 50⫾ 5 mV, which is in very good agreement with the above estimation of the direct tunneling regime. It is slightly


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the Co electrode at positive bias. Further studies are underway to detail the effects of the SC band alignment on spinpolarized tunneling across the EuS/Si共001兲 interface. Summarizing, we have demonstrated spin filtering in a MI in direct contact to silicon, using EuS as SF tunnel barrier. MR values of up to 14% and SF efficiency of 20% were observed at 4.2 K for both bias polarities using Co electrodes as spin detector. Furthermore, the experimental MR ratio increased with bias voltage, as predicted for a model SF system. Our results not only prove the spin filtering capability of EuS/Si共001兲 magnetic tunnel contacts, but also highlight an exciting category of FM materials for spin injection/ detection in SCs. FIG. 4. 共Color online兲 MR as a function of bias voltage for a CoO/ Co/ Al2O3 / EuS/ Si共001兲 contact at 4.2 K. Device schematics refer to bias polarities of the spin injection and detection condition.

smaller compared to previously reported values of EuS barrier heights.15 Moving further, we took I-V curves in the P 共1500 Oe兲 and AP 共⫺130 Oe兲 states and extracted the MR bias dependence using the definition MR= IAP − IP / IP. The result, shown in Fig. 4, is an abrupt increase in absolute value of MR for small bias up to a maximum at about +150 meV 共⫺150 meV兲 for positive 共negative兲 bias, followed by a steady decrease at larger V. This observation is in line with previous work on SF barriers in metal-based devices and can be understood as Fowler–Nordheim tunneling through the spinpolarized EuS conduction band.15,18 The above data are an explicit confirmation of this mechanism in a Si-based tunnel contact. The increase in MR with V occurs because spin-up electrons possess a preferential tunnel probability, when the Fermi level approaches the spin-up conduction band. At sufficiently large bias, when the tunneling probability across the spin-down conduction band increases for the spin-down electrons as well, the MR ratio starts to decrease again. Due to the relatively small tunnel barrier height of Co/ Al2O3 / EuS/ Si, spin filtering persists up to a smaller bias value than in metal-based devices.15 The MR bias dependence is slightly asymmetrical and decreases more slowly for negative voltages. V1/2, at which the MR ratio was reduced to 50%, is around ⫺700 mV and +400 mV, respectively. We suggest this behavior to arise from an enhanced scattering in

The authors are grateful to R. Schreiber for sample preparation. M. M. acknowledges financial support by Deutsche Forschungsgemeinschaft 共Grant No. MU 3160/1-1兲. 1

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