Crystalline MgO Barriers Research Articles

Tunneling magnetoresistive devices as read heads in hard disk drives

Hard Disk Drive read heads moved from GMR to Tunneling Magnetoresistive (TMR) Devices as a way to enhance head SNR in order to keep up with advanced linear densities. TMR technology was further extended by the transition from amorphous alumina tunneling barriers to crystalline MgO barriers lattice matched to adjacent BCC magnetic layers resulting in much higher MR ratios. Enhanced side shielding using Junction Shield and reduced shield to shield spacing were additional enablers that increased track and linear density capabilities of TMR heads. TMR heads gained additional momentum and complexity by the introduction of two dimensional recording heads that boosted head SNR by up to 3 dB. As the RA of the tunneling barriers is reduced, overcoming spin torque noise presents itself as a further challenge to be addressed. As device dimensions are scaled down, magnetic thermal noise can only be overcome by increasing the moment of the free layer which may already start establishing the final density point achievable by TMR heads.

Low-frequency and shot noises in CoFeB/MgO/CoFeB magnetic tunneling junctions

The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic tunneling junctions were studied at low temperature. The measured $1/f$ noise around the magnetic hysteresis loops of the free layer indicates that the main origin of the $1/f$ noise is the magnetic fluctuation, which is discussed in terms of a fluctuation-dissipation relation. Random telegraph noise (RTN) is observed to be symmetrically enhanced in the hysteresis loop with regard to the two magnetic configurations. We found that this enhancement is caused by the fluctuation between two magnetic states in the free layer. Although the $1/f$ noise is almost independent of the magnetic configuration, the RTN is enhanced in the antiparallel configuration. These findings indicate the presence of spin-dependent activation of RTN. Shot noise reveals the spin-dependent coherent tunneling process via a crystalline MgO barrier.

Schottky and tunneling behavior of Fe/MgO/Ge(100) structures

We investigated interface and electrical properties of Ge-based Schottky and tunnel diodes with crystalline MgO barriers. Following a simple cleaning procedure not requiring a high-temperature anneal, x-ray data indicated smooth interfaces and that the MgO tunnel barrier was highly textured. Transport characteristics were fitted using a self-consistent field Simmons–Schottky current-voltage model, yielding the Schottky and tunnel barrier heights for the devices and the distribution of tunneling currents. Considering the Fermi-level depinning and the ratio of Schottky to tunneling currents for each barrier thickness, we find that a MgO thickness of 15 Å yields the best transport properties in this system.

Evidence of spin-polarized direct elastic tunneling and onset of superparamagnetism in MgO magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) with crystalline MgO barriers are currently being used in a variety of applications, namely forefront magnetic sensors and memories. In this work we probed the temperature $(T)$ dependence of the transport and magnetic properties of MgO-based MTJs with different CoFeB free layer thicknesses (${t}_{fl}=1.55$, 1.65, 1.95, and 3.0 nm). All samples have the same insulating MgO barrier with a nominal thickness of 1.35 nm. Our results show that the tunnel magnetoresistance (TMR) temperature behavior is mainly due to a strong variation of the conductance $(G)$ of the antiparallel state. Also, we provide evidence that direct elastic tunneling is the dominant mechanism determining the temperature dependence of the tunneling conductance and TMR in the studied MgO MTJs. Intrinsic to this mechanism is the thermal smearing of the electron energies near the Fermi level which then plays a key role in $G(T)$, especially in the parallel state where the overall change in $G$ is very small. Furthermore, we show a clear change in the MTJ properties as the free layer thickness is reduced. Besides the typical decrease of TMR related with the loss of spin polarization, we were able to probe the thickness dependence of the spin wave $\ensuremath{\alpha}$ parameter. MTJ with the thinner free layer show both absence of hysteresis in the room temperature TMR cycles and interesting freezing effects in the zero and field cool magnetization curves at low temperatures, revealing the discontinuous nature of thin free layers.

Spin-Polarized Transport and Dynamics in Magnetic Tunneling Structures

In the first part of this paper, we report a systematic study on the structural evolution under rapid thermal annealing and the corresponding transport properties in magnetic tunnel junctions (MTJs) with a crystalline MgO barrier. The results clearly indicate that high tunneling magnetic resistance can be achieved by annealing MTJs at a very short time, and it is directly related to the formation of (001) crystalline structures. In the second part, we report the spin dynamics in tunneling structure through direct electrical detection. A surprisingly large voltage generation in F/I/N and F/I/F junctions was observed, which is contradictory to the prediction from the standard spin-pumping theory. We proposed a theoretical formalism to study spin-pumping effects in ferromagnetic multilayer structures. The formalism can yield a remarkably clean physical picture of the spin and charge pumping in tunneling structures. The calculated values are consistent with experimental results.

Tunnel magnetoresistance in epitaxially grown magnetic tunnel junctions using Heusler alloy electrode and MgO barrier

Epitaxially grown magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi/MgO/CoFe were fabricated. Their tunnel magnetoresistance (TMR) effects were investigated. The TMR ratio and tunnelling conductance characteristics of MTJs were considerably different between those with an MgO barrier prepared using sputtering (SP-MTJ) and those prepared using EB evaporation (EB-MTJ). The EB-MTJ exhibited a very large TMR ratio of 217% at room temperature and 753% at 2 K. The bias voltage dependence of the tunnelling conductance in the parallel magnetic configuration for the EB-MTJ suggests that the observed large TMR ratio at RT results from the coherent tunnelling process through the crystalline MgO barrier. The tunnelling conductance in the anti-parallel magnetic configuration suggests that the large temperature dependence of the TMR ratio results from the inelastic spin–flip tunnelling process.

The effect of magnetic annealing on barrier asymmetry in Co 40Fe 40B 20/MgO magnetic tunnel junctions

We demonstrate that high-quality magnetic tunnel junctions with Co 40Fe 40B 20 electrodes and crystalline MgO barriers, which show a 230% tunneling magnetoresistance ratio at room temperature, can be successfully fabricated by using a target-facing-target sputtering technique and present the results on the change of barrier asymmetry after the annealing at different temperatures. Bias voltage dependence of magnetoresistance ratio for Co 40Fe 40B 20/MgO magnetic tunnel junctions is highly asymmetrical in the as-deposited states and becomes more symmetrical after magnetic annealing. It also exhibits a shift of the tunneling magnetoresistance maximum to a positive bias voltage under the low-temperature annealing.

MgO-based double barrier magnetic tunnel junctions with thin free layers

The free layer thickness (tfree) in double barrier magnetic tunnel junctions (DMTJs) based on crystalline MgO barriers and CoFeB ferromagnetic layers has been varied from 0.5 to 3.0 nm in order to investigate its effect on the magnetic and electrical properties. One obvious feature of DMTJs with tfree≤1 nm is the absence of sharp free layer switching in the TMR curves, which can be explained by the superparamagnetic nature of discontinuous CoFeB layer, which breaks into nanodots when it is very thin. Normal free layer switch is observed when tfree=2.0 and 3.0 nm. Another difference is a rapid increase in junction resistance and tunnel magnetoresistance at low temperature for DMTJs with thin tfree, which is attributed to the Coulomb blockade effect. We also observed a small conductance peak in the dI/dV curve at low bias only in the parallel configuration and at temperatures below 100 K. This is related to the Kondo scattering process on the nanodots, which constitutes the discontinuous free layer. We found no Coulomb staircase existing in the I-V curves; this may be due to the microsize of the junctions.

A performance study of next generation's TMR heads beyond 200 gb/in/sup 2/

Practical level performance for /spl sim/200 Gb/in/sup 2/ has been verified by AlOx barrier tunneling magnetoresistive (TMR) heads, which resistance area product (RA) is more than 3 ohm/spl middot//spl mu/m/sup 2/, in perpendicular recording mode. In addition, improved AlOx barrier magnetic tunnel junctions (MTJs) formed on plated bottom shield with smoothed surface achieved TMR ratio of 25% and 16% with RA of 1.9 and 1.0 ohm/spl middot//spl mu/m/sup 2/, respectively, indicating over 200 Gb/in/sup 2/ is also possible by the AlOx barrier TMR heads with lower RA. Furthermore, TMR heads with crystalline MgO barrier were fabricated. The MgO barrier MTJs formed on plated bottom shield with smoothed surface achieved TMR ratio of 88% with RA of 2.0 ohm/spl middot//spl mu/m/sup 2/, which is 3.5 times higher than that of AlOx barrier MTJs under similar RA. Dynamic electrical test was also performed for TMR heads with the MgO barrier. As a result, good readback waveform with huge output was obtained. This is the first confirmation of readback waveform generated from TMR heads with crystalline MgO barrier. Our results indicate that the future of TMR heads technology is promising beyond 200 Gb/in/sup 2/ application.

Huge magnetoresistance and low junction resistance in magnetic tunnel junctions with crystalline MgO barrier

Inserting a 4 /spl Aring/-Mg metal layer between the amorphous CoFeB bottom electrode layer and the MgO barrier layer was found to be effective in realizing huge magnetoresistance effect in low-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). As a result, magnetoresistance (MR) ratio as high as 138% at resistance-area product (RA) of about 2.4 /spl Omega//spl middot//spl mu/m/sup 2/ was obtained. This value is about seven times that of state-of-the-art MTJs for magnetic sensor application. X-ray diffraction analysis clarified that crystal orientation of the poly-crystalline MgO(001) barrier layer was improved by the Mg layer. It is suggested that the higher crystalline orientation of the MgO(001) barrier layer could have enhanced the coherent tunneling of /spl Delta//sub 1/ electrons, resulting in an increase of MR ratio at the low RA (thin MgO thickness) region. The annealing temperature and free layer materials have also been optimized to satisfy the requirements for practical read head application. Although this optimization resulted in a reduction in the MR ratio to about 45%-53%, this value is still more than twice the highest MR ratio of conventional MTJs. The currently developed fabrication process will accelerate the development of highly sensitive read heads for ultrahigh-density hard-disk drives.

Fabrication and Characterization of Nb/MgO/Bi2Sr2CaCu2Ox Tunnel Junctions Using a Single Crystal

ABSTRACTNb/MgO/Bi2Sr2CaCu2Ox (BSCCO) tunnel junctions using both amorphous and crystalline MgO barriers were fabricated using a BSCCO single crystal in order to compare their superconducting states of interface. Both MgO thin films, 4–8 nm thick and with full coverage, were deposited onto BSCCO single crystals using the MBE method. The junctions using amorphous and crystalline MgO films were found to have gap opening structures at a bias voltage of about 38 mV and 25 mV at 5.2 K, respectively. The gap opening of the junction using an amorphous MgO barrier occurred below 86 K, corresponding very closely with the Tc for the BSCCO single crystal. Its characteristics showed a BCS-like temperature dependence.