Johannes G.M. Sanderink

Temperature dependence of magnetocurrent in a magnetic tunnel transistor

The temperature dependence of magnetocurrent MC and transfer ratio has been investigated in a magnetic tunnel transistor MTT with a ferromagnetic FM emitter of Co or Ni80Fe20. MTT devices of sizes ranging from 10 to 100 m in diameter were fabricated using a standard photolithography process and predefined Si substrates. This reduces the edge leakage current across the collector Schottky diode and enables room-temperature operation. For the MTT with both Co and Ni80Fe20 emitter, we obtain a MC of about 80% at room temperature. This corresponds to a tunnel spin polarization of the FM emitter/Al2O3 interface of 29% at 1 V, demonstrating that the tunnel current is still spin-polarized at a high bias voltage.

Effect of orbital hybridization on spin-polarized tunneling across Co/C60 interfaces

The interaction between ferromagnetic surfaces and organic semiconductors leads to the formation of hybrid interfacial states. As a consequence, the local magnetic moment is altered, a hybrid interfacial density of states (DOS) is formed, and spin-dependent shifts of energy levels occur. Here, we show that this hybridization affects spin transport across the interface significantly. We report spin-dependent electronic transport measurements for tunnel junctions comprising C60 molecular thin films grown on top of face-centered-cubic (fcc) epitaxial Co electrodes, an AlOx tunnel barrier, and an Al counter electrode. Since only one ferromagnetic electrode (Co) is present, spin-polarized transport is due to tunneling anisotropic magnetoresistance (TAMR). An in-plane TAMR ratio of approximately 0.7% has been measured at 5 K under application of a magnetic field of 800 mT. The magnetic switching behavior shows some remarkable features, which are attributed to the rotation of interfacial magnetic moments. This behavior can be ascribed to the magnetic coupling between the Co thin films and the newly formed Co/C60 hybridized interfacial states. Using the Tedrow-Meservey technique, the tunnel spin polarization of the Co/C60 interface was found to be 43%.

Tunnelling anisotropic magnetoresistance due to antiferromagnetic CoO tunnel barriers

A new approach in spintronics is based on spin-polarized charge transport phenomena governed by antiferromagnetic (AFM) materials. Recent studies have demonstrated the feasibility of this approach for AFM metals and semiconductors. We report tunneling anisotropic magnetoresistance (TAMR) due to the rotation of antiferromagnetic moments of an insulating CoO layer, incorporated into a tunnel junction consisting of sapphire(substrate)/fcc-Co/CoO/AlOx/Al. The ferromagnetic Co layer is exchange coupled to the AFM CoO layer and drives rotation of the AFM moments in an external magnetic field. The results may help pave the way towards the development of spintronic devices based on AFM insulators.

Probing spin-polarized tunneling at high bias and temperature with a magnetic tunnel transistor

The magnetic tunnel transistor (MTT) is a three terminal hybrid device that consists of a tunnel emitter, a ferromagnetic (FM) base, and a semiconductor collector. In the MTT with a FM emitter and a single FM base, spin-polarized hot electrons are injected into the base by tunneling. After spin-dependent transmission through the ferromagnetic base they are collected in the conduction band of the semiconductor provided they have the right energy and momentum to overcome the Schottky barrier. Two factors determine the spin-sensitivity of the device: (i) spin-dependent tunneling from the emitter, and (ii) spin-dependent scattering of the hot electrons in the base. Since the magnetocurrent (MC) depends on the tunneling spin polarization, the MTT can be used to study the spin-polarization of ferromagnetic/insulator interfaces at high bias voltage. Moreover, the temperature dependence can be studied using a newly introduced lithographically defined MTT that allows us to probe the tunnel spin-polarization up to room temperature, removing a limitation of the standard technique of tunneling into a superconductor.