Tailoring tunnel magnetoresistance by ultrathin Cr and Co interlayers: A first-principles investigation of Fe/MgO/Fe junctions

Abstract

We report on systematic ab-initio investigations of Co and Cr interlayers embedded in Fe(001)/MgO/Fe(001) magnetic tunnel junctions, focusing on the changes of the electronic structure and the transport properties with interlayer thickness. The results of spin-dependent ballistic transport calculations reveal options to specifically manipulate the tunnel magnetoresistance ratio. The resistance area products and the tunnel magnetoresistance ratios show a monotonous trend with distinct oscillations as a function of the Cr thickness. These modulations are directly addressed and interpreted by means of magnetic structures in the Cr films and by complex band structure effects. The characteristics for embedded Co interlayers are considerably influenced by interface resonances which are analyzed by the local electronic structure.

First-principles study of Co concentration and interfacial resonance states in Fe1−xCoxmagnetic tunnel junctions

“…Because of this, the areas of higher transmission when using Co electrodes lead to a finite G AP comparable to Fe that decreases its TMR. Previous first-principles studies have also found a higher TMR using bcc Fe compared to bcc Co electrodes 27 , confirming that the k || = 0 are important. Previous theoretical and experimental work has found higher TMR when adding bcc Co interlayers between the bcc Fe electrode and MgO barrier 10,13 , however our present results suggest a pure bcc Co electrode will not necessarily improve TMR.…”

Quantitative Analysis of Coherent and Incoherent Tunneling Currents in MgO-Based Epitaxial Magnetic Tunnel Junctions

“…The observed is much longer than the theoretical value of 0.7 # A at t MgO ¼ 6 ML (%12:1 # A) and 2.1 # A at t MgO ¼ 10 ML. 23,25) The long could be partly due to the roughness of the Cr layer in type-A MTJs. However, a similar long was also obtained in type-B MTJs.…”

“…In the phenomenological model, we neglect the influence of the Cr/Fe interface because it should have little influence on the parallel conductance. 23) For simplicity, the model does not take the scattering at the bottom interface into account. Thus, the current through the transport channels 1), 4), and 5) are regarded as the coherent current (I coh ), and the current through the transport channels 2), 3), and 6) are regarded as the incoherent current (I inc ).…”

“…1, which should result in a rapid increase in tunneling resistance with respect to Cr thickness (t Cr ). 23) This implies that the Cr layer is useful for evaluating the contribution of specular tunneling current carried by Á 1 electrons. In type-B MTJs, however, the tunneling resistance only increased gradually (by a factor of only 2.5) with increasing t Cr .…”

Effect of boron in Fe/MgO interface on structural stability and state coupling

“…Additionally, small structural randomness at the Fe/MgO interface can also significantly reduce the TMR ratio [10]. Several methods such as introducing boron into the electrode or inserting a metal interlayer at the interface are used to improve the quality of the ferromagnet (FM)/MgO interface [11][12][13]. The introduction of B can effectively reduce the FM/MgO interface roughness [14], prevent interfacial oxidation [15], and reduce passive oxygen vacancies in MgO through B diffusion [16][17][18].…”