Giant oscillations in spin-dependent tunneling resistances as a function of barrier thickness in fully epitaxial magnetic tunnel junctions with a MgO barrier

Takao Marukame, Takayuki Ishikawa, Tomoyuki Taira, Ken-ichi Matsuda, Tetsuya Uemura, and Masafumi Yamamoto

Phys. Rev. B 81, 134432 – Published 27 April 2010

Abstract

Giant oscillations in spin-dependent tunneling resistances as a function of MgO barrier thickness $(t_{MgO})$ were observed for fully epitaxial magnetic tunnel junctions with Heusler alloy ${Co}_{2} {Cr}_{0.6} {Fe}_{0.4} Al$ electrodes and a MgO barrier. The oscillations in tunneling resistances were well approximated by a superposition of an exponential function of $exp (a t_{MgO} + b)$ and a periodic function of $1 + C cos [(2 π / T) t_{MgO} + φ]$ with significantly large amplitudes $C \sim 0.16 \pm 0.01$ even at 293 K for both parallel and antiparallel magnetization orientations. The period was found to be almost independent of temperature and bias voltage $(V)$ . The amplitudes $C$ showed only weak dependence on $V$ at least up to 0.2 V. These features should be a key to understand the origin of the pronounced oscillations.

Received 13 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.134432

Authors & Affiliations

Takao Marukame^*, Takayuki Ishikawa, Tomoyuki Taira, Ken-ichi Matsuda, Tetsuya Uemura, and Masafumi Yamamoto^†

Division of Electronics for Informatics, Hokkaido University, Sapporo 060-0814, Japan

^*Present address: Corporate R&D Center, Toshiba Corp., Kawasaki 212-8582, Japan.
^†yamamoto@nano.ist.hokudai.ac.jp

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Vol. 81, Iss. 13 — 1 April 2010

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Images

Figure 1
Cross-sectional high-resolution transmission electron microscope lattice image of a fabricated CCFA/MgO (2.0 nm)/CCFA MTJ layer structure, along the [1–10] direction of the CCFA layers.Reuse & Permissions
Figure 2
(Color online) Closed and open circles represent $R_{AP} A$ and $R_{P} A$ , respectively, as a function of $t_{MgO}$ at (a) 4.2 K and (b) 293 K for fully epitaxial CCFA/MgO/CCFA MTJs, where $R_{P}$ and $R_{AP}$ are respective tunnel resistances for the P and AP magnetization orientations and $A$ is the nominal junction area of $10 \times 10 μ m^{2}$ . The bias voltage was 5 mV. Each dashed line is a curve fitted with Eq. (2) described in the text. (c) Closed and open circles represent corresponding experimental TMR ratios defined as $(R_{AP} - R_{P}) / R_{P}$ as a function of $t_{MgO}$ at 4.2 K and 293 K calculated from the experimental $R_{P}$ and $R_{AP}$ values shown in (a) and (b), respectively. The solid curves are the values of the TMR ratios calculated using the approximate functions for $R_{P}$ and $R_{AP}$ as a function of $t_{MgO}$ with Eq. (2).Reuse & Permissions
Figure 3
(Color online) Open and closed circles represent reduced tunnel resistances $r_{P}$ and $r_{AP}$ , respectively, at 4.2 and 293 K for CCFA MTJs (the same MTJs as for Fig. 2), where $r_{P}$ $(r_{AP})$ is the tunnel resistance $R_{P} A$ $(R_{AP} A)$ divided by each baseline function of $exp (a t_{MgO} + b)$ . The $R_{P} A$ and $R_{AP} A$ values are the same as for Fig. 2. (a) and (b) $r_{P}$ and $r_{AP}$ at 4.2 K. (c) and (d) $r_{P}$ and $r_{AP}$ at 293 K. Each solid line is a fitted curve with a form of $r = 1 + C cos [(2 π / T) t_{MgO} + φ]$ .Reuse & Permissions
Figure 4
(Color online) Closed circles, open triangles, and crosses represent reduced tunnel resistances $r_{P}$ and $r_{AP}$ as a function of $t_{MgO}$ at 293 K for various positive $V$ up to 0.2 V for a different set of CCFA MTJs from those shown in Fig. 2. (a) and (b) $r_{P}$ and $r_{AP}$ for positive $V$ up to 0.2 V, respectively. Solid lines are curves fitted with Eq. (1). (c) Oscillation amplitudes for P (open circles) and AP (closed circles) [ $C_{P}$ and $C_{AP}$ in Eq. (1)] as a function of $V$ from $- 0.2$ to 0.2 V. The bias voltage was defined with respect to the lower CCFA electrode. Here, the junction size was $8 \times 8 μ m^{2}$ .Reuse & Permissions
Figure 5
(Color online) (a) Closed and open circles represent $R_{AP} A$ and $R_{P} A$ , respectively, as a function of $t_{MgO}$ at 293 K for fully epitaxial CCFA/MgO/CoFe MTJs, where $R_{P}$ and $R_{AP}$ are respective tunnel resistances for the P and AP magnetization orientations, and $A$ is the nominal junction area of $10 \times 10 μ m^{2}$ . The bias voltage was 5 mV. Each dashed line is a curve fitted with Eq. (2) described in the text with respective parameters and without any background correction. (b) and (c) Open and closed circles represent reduced tunnel resistances $r_{P}$ and $r_{AP}$ , respectively, at 293 K, where $r_{P}$ $(r_{AP})$ is the tunnel resistance $R_{P} A$ $(R_{AP} A)$ divided by each baseline function of $exp (a t_{MgO} + b)$ . The $R_{P} A$ and $R_{AP} A$ values are the same as for Fig. 5a. Each solid line is a curve fitted with Eq. (1) with respective parameters.Reuse & Permissions

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