Development of technology of nanostructures with a spin-tunnel magnetoresistive effect
The results of experimental studies of multilayer nanostructures producing with spin-tunnel magnetoresistive (STMR) effect are presented. When STMR nanostructures were formed using integrated technology, the STMR effect increased up to 157.5%.
Thin-film multilayer nanostructures with a spin-tunnel magnetoresistive (STMR) effect are used in a certain number of modern devices, such as highly sensitive magnetic field transducers, reading heads, non-volatile memory, biosensor and logic elements of spintronic devices. Developing the technology of forming nanostructures with the STMR-effect opens a wide range of opportunities for such sectors of the economy as instrumentation engineering, medicine, transport, aerospace technology and security systems.
There are two most actual applications of STMR-nanostructures: non-volatile memory, highly sensitive sensors and transducers of magnetic field. For example, Everspin was one of the first companies to implement magnetoresistive random-access memory (MRAM) and is currently a leader in producing advanced non – volatile storage devices. MRAM-technology is based on the sequential formation of a CMOS-circuit and an array of spin-tunnel magnetoresistive elements. As a rule, the MRAM-cell consists of a MOSFET and a STMR-element. The main feature of MRAM is the use of quantum and magnetic effects in STMR-elements. The main advantage of MRAM is a combination of properties that no other standard of non – volatile memory technology has: the complete non-volatility, high read/write speed and an unlimited number of read/write cycles .
The American-Chinese company "MultiDimension Technology" has a wide range of products (highly sensitive magnetic field transdusers, switches, angle sensors, current sensors, etc.) based on STMR-nanostructures. The product line of company includes demo boards for mastering their products in various applications .
A magnetic tunnel junction (MTJ) consists of a tunnel barrier layer enclosed between two ferromagnetic (FM) films of different coercivity . Due to the fact that FM-layers can be oxidized, Ta-, Ti-, Ru-films are used as a protection. Figure 1 shows a scheme of the MTJ-structure.
The MgO dielectric is mainly used as a tunnel barrier, it provides a higher STMR-effect. However, a number of companies make research on MTJ based on Al2O3 , NaCl , ZnO  and Mg3B2O6 . Co-based alloys are mainly used for FM1 and FM2 films (CoFe, FeNiCo, CoFeB and several others), Geisler alloys and rare earth metals are actively studied .
When the spin orientation of the FM-layers coincides, the conductivity of the structure increases; in condition of the antiparallel configuration, the conductivity decreases significantly . The difference in resistances for parallel and antiparallel configurations of the FM-layer magnetization determines the STMR-effect of the nanostructure. When one of the FM-layers is fixed in the MTJ with an antiferromagnet, the magnetic characteristic of the Fe/MgO/Fe/IrMn nanostructure has the form shown in Fig.2 . The FM-layer without fixation is called "free" and is remagnetized at significantly lower fields in comparison with the "fixed" FM-layer, which is remagnetized by a stronger magnetic field. The operation of spintronics devices is based on a change in the resistance in the region of small fields, that is, the remagnetization of the free FM-layer.
Creating an effective MTJ is the basis for designing a wide range of devices based on STMR-effect. To minimize time and material costs in the process of manufacturing the MTJ nanostructure, the mask method was used . Silicon wafers with through holes in the form of strips and polygons can be used as stencil masks (Figure 3). Arrays of these figures are formed orthogonally, so that when the stencil and wafer are combined, a cross-shaped element is formed, containing MTJ-layers in the area of intersections of top and bottom FM-electrodes (Fig.4).
According to studies  nanostructure CoFeB/MgO/CoFeB has a high STMR-effect even at room temperature. The significant STMR-effect is explained by coherent spin-dependent tunneling through the MgO tunnel barrier. Coherent tunneling is possible, for example, in a single-crystal nanostructure Fe/MgO/Fe(001). ■