Symmetry-filtering quantum tunneling

Symmetry-filtering tunneling in CoFeB/MgO/CoFeB junctions

The transmission probability of the Bloch electrons in an epitaxial ferromagnet /insulator/ferromagnet structure depends on the lateral symmetry of the wavefunctions. This symmetry filtering effect is converted into a large spin filtering effect if the wavefunction with the preferred symmetry only exists in one of the two spin channels in the ferromagnetic electrodes. Such a spin filtering effect was first theoretically predicted to give rise to a very large tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs), followed by the experimental realization in Fe/MgO/Fe, CoFe/MgO/CoFe and CoFeB/MgO/CoFeB MTJs. The TMR ratios in MgO-based junctions are much higher than those found in the Al2O­3-based junctions, where the TMR is only about 40-80%, mainly limited by the disordered Al2O3 barrier.

The giant TMR in CoFeB/MgO/CoFeB junctions is developed through the so-called solid state epitaxy process during annealing. The MgO barrier fabricated by RF sputtering has a very strong (001) texture when deposited on the amorphous CoFeB bottom electrode. During the post-growth thermal annealing, the top and bottom interfaces of the highly (001)-oriented MgO layer serve as templates for the crystallization of amorphous CoFeB layers in the (001) orientation, thus forming the out-of-plane epitaxial CoFeB(001)/MgO(001)/CoFeB(001) sandwich structure. Therefore the highly spin-polarized, slow-decaying Delta₁ band electrons gives rise to the giant TMR effect through the symmetry conserved coherent tunneling.

Dynamic evolution of TMR during thermal annealing

Thermal annealing is one of the most crucial steps to achieve high TMR ratios. It is during the post-annealing process that the TMR increases up to a few hundred percent, from typically 20-40% in the as-prepared state. Especially in CoFeB/MgO/CoFeB junctions, the matching of the Bloch waves in the electrodes to the corresponding evanescent waves in the barrier only occurs after initially amorphous CoFeB layers crystallize in the (001) orientation during thermal annealing. The annealing temperature dependence of TMR has been widely studied, with the usual time duration of 1-2 hours. However, the dependence of tunneling properties on annealing time has largely been neglected. Therefore there was little understanding about the evolution of magnetoresistance during annealing. Many questions of importance are still not answered, such as:

• What is the behavior of TMR during annealing? Is it linearly/exponentially increasing with time, or is there a more complicated dependence?

• How fast does the amorphous CoFeB crystallize? What are the different roles played by CoFeB and MgO during the evolution of TMR?

• What is the exact physical origin for such behaviors?

• Can we achieve higher TMR, build better MTJ devices and save energy if we understand better about the dynamic growth of coherent tunneling during annealing

We have carried out a systematic investigation to address these important questions. This research contains a series of comprehensive experiments including: i) an in-situ, time-resolved study to determine the fast crystallization rate of CoFeB with a synchrotron source(as shown in the figure below), ii) a high resolution TEM study to determine the solid state epitaxial growth mode of CoFeB and iii) an extensive transport study to directly compare the behaviors of coherent and non-coherent tunneling in hundreds of MTJs (including both MgO and Al2O3 barriers) processed by a unique rapid thermal anneal technique. Generally, the evolution of TMR consists of three distinct regions: a sharp increase in the 1st region at the beginning of annealing, a very gradual increase toward saturation in 2nd region and finally the decreasing of TMR in the 3rd region after prolonged annealing. The increase of TMR is so fast that a larger than 250% TMR can be achieved in merely a few seconds at high temperatures, in sharp contrast to the hour-long annealing used in conventional practice. From these studies we were able to describe the evolution of conductance and TMR in a simple phenomenological model:

From this model we can: 1) understand why the development of coherent tunneling occurs so fast, 2) estimate the annealing time needed to achieve a certain TMR at different temperature, and 3), more importantly, identify the different contributions of CoFeB and MgO to the coherent tunneling that leads to three very distinct regions in the evolution of TMR, e.g. the first region of initial fast increase of TMR is largely due to CoFeB crystallization, the second region (marked by the turning point of parallel conductance) of slow approaching to saturation of TMR is mostly due to the improvement of the MgO crystal structure, and finally the third region of decrease of TMR is due to increased impurity scattering at longer annealing. Application wise, these studies reveal critical information to achieve high TMR, low RA tunnel junctions for many devices such as hard disk read heads, ultra-low field sensors and spin-torque microwave oscillators

Perpendicular magnetic anisotropy and perpendicular magnetic tunnel junction

Previous studies have shown a rapid decrease in the TMR during post-growth thermal annealing, which was attributed to the loss of AP states due to decreased PMA at high temperatures. We have investigated the magnetic and transport properties of perpendicular magnetic tunnel junctions (pMTJs) with a heavy metal/ferromagnet/oxide (HM/FM/Oxide) structure, where the HM layer used was molybdenum. Consistent better performance was obtained compared to conventional pMTJ with Ta layers after samples were annealed at high temperature. Large TMR and PMA have been maintained after annealing for 2 hours at 400^oC in a wide range of samples with Mo layers, in sharp contrast to junctions with Ta layers where a superparamagnetic behavior with nearly vanishing magnetoresistance was observed. As a result of greatly improved thermal stability, TMR above 160% was obtained in junctions with Mo layers.

We further demonstrated a thin Mo dusting layer inserted at the interface of Ta/CoFeB can dramatically increase the TMR and PMA. Unlike thick Mo layers that exhibited a strong (110) crystalline texture, the inserted Mo layer between Ta/CoFeB had little negative influence on the crystallization of CoFe (001), therefore combining the advantages of Mo as a good thermal barrier and Ta as a good boron sink. For optimized Mo dusting thickness,a large TMR of 208% was achieved in junctions with superior thermal stability at 500C. This led to TMR above 250% (thermally stable at 500C ) to our best knowledge is the highest value so far in pMTJs with simple HM/FM/Oxide structures.

Related publications:

• "Effect of Mo insertion layers on the magnetoresistance and perpendicular magnetic anisotropy in Ta/CoFeB/MgO junctions". Appl. Phys. Lett. 109, 032401 (2016).

• "Conductive Atomic Force Microscopy of Small Magnetic Tunnel Junctions with Interface Anisotropy", IEEE Trans. Magn. 9464, 1 (2015)

• "Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo / CoFeB / MgO magnetic tunnel junctions" Appl. Phys. Lett. 106, 182406 (2015)

• "Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB", Appl. Phys. Lett. 99,102502 (2011)