Electrons have a charge and a spin, but until recently these were considered separately. In classical electronics, charges are moved by electric fields to transmit information and are stored in a capacitor to save it. In magnetic recording, magnetic fields have been used to read or write the information stored on the magnetization, which 'measures' the local orientation of spins in ferromagnets. The picture started to change in 1988, when the discovery of giant magnetoresistance opened the way to efficient control of charge transport through magnetization. The recent expansion of hard-disk recording owes much to this development. We are starting to see a new paradigm where magnetization dynamics and charge currents act on each other in nanostructured artificial materials. Ultimately, 'spin currents' could even replace charge currents for the transfer and treatment of information, allowing faster, low-energy operations: spin electronics is on its way.

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The emergence of spin electronics in data storage

“Spintronics,” in which both the spin and charge of electrons are used for logic and memory operations, promises an alternate route to traditional semiconductor electronics. A complete logic architecture can be constructed, which uses planar magnetic wires that are less than a micrometer in width. Logical NOT, logical AND, signal fan-out, and signal cross-over elements each have a simple geometric design, and they can be integrated together into one circuit. An additional element for data input allows information to be written to domain-wall logic circuits.

http://science.sciencemag.org/content/sci/309/5741/1688.full.pdf

Magnetic Domain-Wall Logic

Antiferromagnetic materials are magnetic inside, however, the direction of their ordered microscopic moments alternates between individual atomic sites. The resulting zero net magnetic moment makes magnetism in antiferromagnets invisible on the outside. It also implies that if information was stored in antiferromagnetic moments it would be insensitive to disturbing external magnetic fields, and the antiferromagnetic element would not affect magnetically its neighbors no matter how densely the elements were arranged in a device. The intrinsic high frequencies of antiferromagnetic dynamics represent another property that makes antiferromagnets distinct from ferromagnets. The outstanding question is how to efficiently manipulate and detect the magnetic state of an antiferromagnet. In this article we give an overview of recent works addressing this question. We also review studies looking at merits of antiferromagnetic spintronics from a more general perspective of spin-ransport, magnetization dynamics, and materials research, and give a brief outlook of future research and applications of antiferromagnetic spintronics.

Antiferromagnetic spintronics

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Review on spintronics: Principles and device applications

A 4-Mb magnetoresistive random access memory (MRAM) with a novel magnetic bit cell and toggle switching mode is presented. The circuit was designed in a five level metal, 0.18-mum complementary metal-oxide-semiconductor process with a bit cell size of 1.55 mum2. The new bit cell uses a balanced synthetic antiferromagnetic free layer and a phased write pulse sequence to provide robust switching performance with immunity from half-select disturbs. This switching mode greatly improves the operational performance of the MRAM as compared to conventional MRAM. A detailed description of this 4-Mb toggle MRAM is presented

A 4-Mb toggle MRAM based on a novel bit and switching method

We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible.We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO.

MgO-based tunnel junction material for high-speed toggle magnetic random access memory

An amorphous layer of a cobalt iron-based (CoFe-based) magnetic alloy suitable for use in magnetoelectronic devices is disclosed. In the most preferred embodiments of the present invention, at least one amorphous layer is provided in an MTJ stack to increase the smoothness of the various layers in the MTJ stack while also enhancing the magnetic performance of the resulting device. Additionally, the alloys of the present invention are also useful in cladding applications to provide electrical flux containment for signal lines in magnetoelectronic devices and as a material for fabricating write heads.

Amorphous alloys for magnetic devices

A magnetic tunnel junction (MTJ), which is useful in magnetoresistive random access memories (MRAMs), has a free layer which is a synthetic antiferromagnet (SAF) structure. This SAF is composed of two ferromagnetic layers that are separated by a coupling layer. The coupling layer has a base material that is non-magnetic and also other materials that improve thermal endurance, control of the coupling strength of the SAF, and magnetoresistance ratio (MR). The preferred base material is ruthenium and the preferred other material is tantalum. Furthering these benefits, cobalt-iron is added at the interface between the tantalum and one of the ferromagnetic layers. Also the coupling layer can have even more layers and the materials used can vary. Also the coupling layer itself can be an alloy.

Synthetic antiferromagnet structures for use in MTJs in MRAM technology

Synthetic antiferromagnet (SAF) structures are a key element of TMR and GMR read heads and MRAM devices. Control of the SAF coupling strength and thermal endurance are key issues for these technologies. We find that the coupling strength increases with stronger crystalline texture in polycrystalline NiFe SAFs, and, surprisingly, we observe a strong dependence on seed layer in amorphous CoFeB SAFs. We also have developed an analysis method for evaluating thermal endurance of SAFs and show that failure of the SAF can be modeled as a thermally activated diffusion process. The analysis is used to predict the time to failure at any temperature, thus allowing accelerated failure analysis for SAF-based devices. The stability improves dramatically with increasing Ru spacer thickness. The time to failure for typical NiFe SAFs was found to be >10 years at 120/spl deg/C.

Srinivas V. Pietambaram

Papers

A 4-Mb toggle MRAM based on a novel bit and switching method

MgO-based tunnel junction material for high-speed toggle magnetic random access memory

Amorphous alloys for magnetic devices

Synthetic antiferromagnet structures for use in MTJs in MRAM technology

Exchange coupling control and thermal endurance of synthetic antiferromagnet structures for MRAM