R. Vlutters

Transfer ratio of the spin-valve transistor

We describe the factors that control the transfer ratio of the spin-valve transistor. An increase in transfer ratio is obtained by a systematic variation of the height of emitter and collector Schottky barrier, and of the nonmagnetic metals. Next, we found that in some cases, a thicker base leads to a higher transfer ratio. Finally, the thickness of the magnetic layers in the Ni 80 Fe 20 /Au/Co spin-valve base can be optimized for a maximum absolute change of collector current. An overall increase by a factor of 24 was achieved, without loss of the magnetocurrent.

Low-field magnetocurrent above 200% in a spin-valve transistor at room temperature

A spin-valve transistor (SVT) that employs hot electrons is shown to exhibit a huge magnetotransport effect at room temperature in small magnetic fields. The SVT is a ferromagnet-semiconductor hybrid structure in which hot electrons are injected into a NiFe/Au/Co spin valve, and collected on the other side with energy and momentum selection. This makes the collector current extremely sensitive to spin-dependent scattering. The hot-electron current output of the device changes by more than a factor of three in magnetic fields of only a few Oe, corresponding to a magnetocurrent above 200% at room temperature.

The spin-valve transistor: Fabrication, characterization and physics

An overview is given of the fabrication, basic properties, and physics of the spin-valve transistor. We describe the layout of this three-terminal ferromagnet/semiconductor hybrid device, as well as the operating principle. Fabrication technologies are discussed, including vacuum metal bonding. We characterize properties of the device relevant for possible applications in magneto-electronics, such as relative magnetic response, output current, and noise behavior. Furthermore, we illustrate the unique possibilities of the spin-valve transistor for fundamental studies of the physics of hot-electron spin transport in magnetic thin film structures.

The spinvalve transistor: technologies and progress

The paper describes the necessary technologies needed for realising a RT operating spin-valve transistor (SVT) which is in fact a magnetic controlled metal base transistor. The preparation of a 350×350 μm2 SVT consisting of an Si emitter and collector and Co/Cu/Co GMR multilayer are described. The metal bonding technology in vacuum is described, which is essential for preparing small SVTs with photolithography and etching technologies. The quality of the bonding interfaces as well as the interface between GMR layer and semiconductor are important for the electrical properties. In more general terms the SVT research also establishes the feasibility of various hybrid structures combining semiconductor technology and spin electronics.

300% magnetocurrent in a room temperature operating spin-valve transistor

Here we present the realization of a room temperature operating spin-valve transistor with huge magnetocurrent (MC=300%) at low fields. This spin-valve transistor employs hot-electron transport across a Ni81Fe19/Au/Co spin valve. Hot electrons are injected into the spin valve across a Si–Pt Schottky barrier. After traversing the spin valve, these hot electrons are collected using a second Schottky barrier (Si–Au), which provides energy and momentum selection. The collector current is found to be extremely sensitive to the spin-dependent scattering of hot electrons in the spin valve, and therefore on the applied magnetic field. We also illustrate the role of the collector diode characteristics in determining the magnetocurrent under collector bias.

Hot-electron transport through Ni80Fe20 in a spin-valve transistor

Hot-electron transport in Ni80Fe20 thin films was studied using a spin-valve transistor. By varying the NiFe thickness from 10 to 100 A we obtain an attenuation length of 43 A for majority-spin hot electrons at 0.9 eV above the Fermi level. Based on such relatively long bulk attentuation lengths, one would expect a current transfer ratio that is much larger than the measured value. We propose that the discrepancy can be accounted for by considering interfacial scattering. Increasing the growth quality should thus provide a means to improve the current transfer ratio.

Giant tunnel magnetoresistance in multilayered metaloxide structures comprising multiple quantum wells

From a theoretical point of view, we have investigated the transport properties in metal/oxide multilayer structures of the form M/O/M/O/M/O/M where M represent ferromagnetic layers alternating with three insulating barriers (O = oxide). The two inner magnetic layers form two quantum wells, the depths of which are spin dependent. For particular thicknesses of these magnetic layers, resonances occur in the quantum wells which lead to strong increase in the electron transmission through the insulating barriers. We show that if the magnetization in the successive magnetic layers can be changed from parallel to antiparallel as in spin-valves, then, for particular values of the thicknesses of the two inner magnetic layers, very large magnetoresistance effects can be expected due to the interplay of resonance effects in the two neighbouring quantum wells. The conductivity and magnetoconductivity are calculated within a quantum theory of linear response (Kubo formalism) taking into account the scattering in the magnetic layers. We show that in such a structure, giant tunnel magnetoconductivity can arise not only from a difference between spin up and spin down Fermi wave-vectors in the magnetic layers but also from spin-dependent mean free paths. In the latter case, the effect of the scattering is to induce a spin-dependent broadening of the resonances in the quantum wells.

Noise properties of the spin-valve transistor

Noise measurements have been performed on a spin-valve transistor. This transistor consists of a Pt/NiFe/Au/Co/Au multilayer sandwiched between two semiconductors. For comparison, we also studied metal base transistors with a Pt/Au or Pt/NiFe/Au base. All samples show full shot noise in the collector current. The inclusion of a spin-valve in the base layer decreases the absolute value of the collector current and with it the noise level but it does not change the nature of the noise in this device. Similarly, the collector current, and therefore, the noise changes as a function of magnetic field for the spin-valve transistor, but no additional noise of magnetic origin is observed.

Fabrication technology for miniaturization of the spin-valve transistor

A new fabrication technology that allows miniaturization of the spin-valve transistor is presented. The spin-valve transistor consists of a spin-valve base (Pt 2 nm/NiFe 3 nm/Au 3.5 nm/Co 3 nm/Au 4 nm) sandwiched between a Si emitter and collector. With the use of a silicon-on-insulator wafer and vacuum metal bonding, spin-valve transistors down to a few tens of micron size are realized through conventional photolithography and etching processes. These spin-valve transistors show 275% magnetocurrent at 87 K and 170% at room temperature in small magnetic fields.

Size dependence of the magnetic and electrical properties of the spin-valve transistor

The electrical and magnetic properties of the spin-valve transistor (SVT) are investigated as a function of transistor size. A new fabrication process, designed to study the size dependence of the SVT properties, uses: silicon-on-insulator (SOI) wafers, a combination of ion beam and wet etching and a negative tone photoresist (SU8) as an insulating layer. The Si/Pt emitter and Si/Au collector Schottky barrier height do not depend on the transistor dimensions. The parasitic leakage current of the Si/Au collector is, however, proportional to its area. The relative collector current change with magnetic field is 240%, independent of size, while the transfer ratio starts to decrease for SVTs with an emitter area below 25 /spl times/ 25 /spl mu/m/sup 2/. The maximum input current is found to be limited by the maximum current density allowed in the base (1.7 /spl times/ 10/sup 7/ A/cm/sup 2/), which is in agreement with the maximum current density for spin valves.