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.

/pdf/the-emergence-of-spin-electronics-in-data-storage-1vrnm5549k.pdf

The emergence of spin electronics in data storage

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

Exchange bias in nanostructures

https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

The authors observed tunnel magnetoresistance (TMR) ratio of 604% at 300K in Ta∕Co20Fe60B20∕MgO∕Co20Fe60B20∕Ta pseudo-spin-valve magnetic tunnel junction annealed at 525°C. To obtain high TMR ratio, it was found critical to anneal the structure at high temperature above 500°C, while suppressing the Ta diffusion into CoFeB electrodes and in particular to the CoFeB∕MgO interface. X-ray diffraction measurement of MgO on SiO2 or Co20Fe60B20 shows that an improvement of MgO barrier quality, in terms of the degree of the (001) orientation and stress relaxation, takes place at annealing temperatures above 450°C. The highest TMR ratio observed at 5K was 1144%.

/pdf/tunnel-magnetoresistance-of-604-at-300k-by-suppression-of-ta-2yiy65l5s3.pdf

Tunnel magnetoresistance of 604% at 300K by suppression of Ta diffusion in CoFeB∕MgO∕CoFeB pseudo-spin-valves annealed at high temperature

All 2-terminal non-volatile memory devices based on resistance switching are memristors, regardless of the device material and physical operating mechanisms. They all exhibit a distinctive “fingerprint” characterized by a pinched hysteresis loop confined to the first and the third quadrants of the v–i plane whose contour shape in general changes with both the amplitude and frequency of any periodic “sine-wave-like” input voltage source, or current source. In particular, the pinched hysteresis loop shrinks and tends to a straight line as frequency increases. Though numerous examples of voltage vs. current pinched hysteresis loops have been published in many unrelated fields, such as biology, chemistry, physics, etc., and observed from many unrelated phenomena, such as gas discharge arcs, mercury lamps, power conversion devices, earthquake conductance variations, etc., we restrict our examples in this tutorial to solid-state and/or nano devices where copious examples of published pinched hysteresis loops abound. In particular, we sampled arbitrarily, one example from each year between the years 2000 and 2010, to demonstrate that the memristor is a device that does not depend on any particular material, or physical mechanism. For example, we have shown that spin-transfer magnetic tunnel junctions are examples of memristors. We have also demonstrated that both bipolar and unipolar resistance switching devices are memristors.

https://link.springer.com/content/pdf/10.1007%2Fs00339-011-6264-9.pdf

Resistance Switching Memories Are Memristors

The spin-transfer effect has been studied in magnetic tunnel junctions (PtMn/CoFe/Ru/CoFe/Al2O3/CoFe/NiFe) with dimensions down to 01×02 μm2 and resistance–area product RA in the range of 05–10 Ω μm2 (ΔR/R=1%–20%) Current-induced magnetization switching is observed with a critical current density of about 8×106 A/cm2 The attribution of the switching to the spin-transfer effect is supported by a current-induced ΔR/R value identical to the one obtained from the R versus H measurements Furthermore, the critical switching current density has clear dependence on the applied magnetic field, consistent with what has been observed previously in the case of spin-transfer-induced switching in metallic multilayer systems

Observation of spin-transfer switching in deep submicron-sized and low-resistance magnetic tunnel junctions

We present experimental and numerical results of current-driven magnetization switching in magnetic tunnel junctions. The experiments show that, for MgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratio is as large as 155% and the intrinsic switching current density is as low as 1.1 ? 106?A?cm?2. The thermal effect and current pulse width on spin-transfer magnetization switching are explored based on the analytical and numerical calculations. Three distinct switching modes, thermal activation, dynamic reversal, and precessional process, are identified within the experimental parameter space. The switching current distribution, write error, and read disturb are discussed based on device design considerations. The challenges and requirements for the successful application of spin-transfer torque as the write scheme in random access memory are addressed.

https://iopscience.iop.org/article/10.1088/0953-8984/19/16/165209/pdf

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

Spin-transfer torque (STT) switching demonstrated in submicron sized magnetic tunnel junctions (MTJs) has stimulated considerable interest for developments of STT switched magnetic random access memory (STT-MRAM). Remarkable progress in STT switching with MgO MTJs and increasing interest in STTMRAM in semiconductor industry have been witnessed in recent years. This paper will present a review on the progress in the intrinsic switching current density reduction and STT-MRAM prototype chip demonstration. Challenges to overcome in order for STT-MRAM to be a mainstream memory technology in future technology nodes will be discussed. Finally, potential applications of STT-MRAM in embedded and standalone memory markets will be outlined.

Spin-Transfer Torque MRAM (STT-MRAM): Challenges and Prospects

A method and system for providing a magnetic element that can be used in a magnetic memory is disclosed. The magnetic element includes pinned, nonmagnetic spacer, and free layers. The spacer layer resides between the pinned and free layers. The free layer can be switched using spin transfer when a write current is passed through the magnetic element. The free layer includes a first ferromagnetic layer and a second ferromagnetic layer. The second ferromagnetic layer has a very high perpendicular anisotropy and an out-of-plane demagnetization energy. The very high perpendicular anisotropy energy is greater than the out-of-plane demagnetization energy of the second layer.

Spin transfer magnetic element with free layers having high perpendicular anisotropy and in-plane equilibrium magnetization

A method and system for providing a magnetic element capable of being written using spin-transfer effect while generating a high output signal and a magnetic memory using the magnetic element are disclosed. The magnetic element includes a first ferromagnetic pinned layer, a nonmagnetic spacer layer, a ferromagnetic free layer, an insulating barrier layer and a second ferromagnetic pinned layer. The pinned layer has a magnetization pinned in a first direction. The nonmagnetic spacer layer is conductive and is between the first pinned layer and the free layer. The barrier layer resides between the free layer and the second pinned layer and is an insulator having a thickness allowing electron tunneling through the barrier layer. The second pinned layer has a magnetization pinned in a second direction. The magnetic element is configured to allow the magnetization of the free layer to change direction due to spin transfer when a write current is passed through the magnetic element.

Yiming Huai

Papers

Observation of spin-transfer switching in deep submicron-sized and low-resistance magnetic tunnel junctions

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

Spin-Transfer Torque MRAM (STT-MRAM): Challenges and Prospects

Spin transfer magnetic element with free layers having high perpendicular anisotropy and in-plane equilibrium magnetization

Magnetic element utilizing spin transfer and an mram device using the magnetic element