---
abstract: 'Magnetic tunnel junctions with the layer sequence Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe were fabricated by magnetron sputtering at room temperature (RT). The samples exhibit a large inverse tunneling magnetoresistance (TMR) effect of up to $-66$% at RT. The largest value of $-84$% at 20K reflects a rather weak influence of temperature. The dependence on the voltage drop shows an unusual behavior with two almost symmetric peaks at $\pm600$mV with large inverse TMR ratios and small positive values around zero bias.'
author:
- 'A. D. Rata'
- 'H. Braak'
- 'D. E. Bürgler'
- 'C. M. Schneider'
title: 'Large inverse tunneling magnetoresistance in Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe magnetic tunnel junctions'
---

[^1]

The tunnel magnetoresistance effect (TMR), *i.e.* the resistance change upon application of a magnetic field in a structure consisting of two ferromagnetic (FM) layers separated by a thin insulating barrier, is subject of intense experimental research due to its potential for spintronic applications.[@wolf] The aim is to achieve a large TMR ratio for practical output voltages in combination with a weak temperature dependence. Recently, very high TMR ratios exceeding 200% at room temperature (RT) have been reported for magnetic tunnel junctions (MTJ) with fully epitaxial or highly oriented MgO barriers and Fe,[@yuasa] CoFe,[@parkin] and CoFeB [@djayaprawira] electrodes. Resonant tunneling and specific features in the band structures of MgO and Fe-based alloys with bcc structure give rise to a spin-filter effect, such that predominantly highly spin-polarized states of the FM contribute to the tunneling current.[@butler] Weak contributions from other states and any breaking of the translational invariance parallel to the interfaces (*e.g.* interface roughness, defects) limit the maximum achievable TMR ratio.[@zhang] Another strategy for obtaining large TMR effects is to use FM electrodes with an *intrinsically* high spin polarization, such as half-metallic ferromagnets featuring 100% spin polarization of the carriers.[@degroot] Among the vast family of the half-metallic ferromagnets, the Heusler alloys are promising candidates due to their high Curie temperatures well above 300 K.[@magprop]

In the last years, experimental efforts were concentrated on improving the structure of Heusler thin films and the properties of the interface between the Heusler electrode and the oxide barrier. Relatively high TMR ratios have been obtained using Co-based full-Heusler alloy thin films, *e.g.* Co$_2$MnSi [@hutten1; @sakuraba1] and Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al. [@inomata; @inomata1; @marukame] Marukame *et al.* [@marukame2] prepared fully epitaxial Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe MTJs with up to 90% TMR at RT. In this letter we report on large *inverse* TMR values obtained from Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe MTJ structures.

Details on the preparation and characterization of Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al (CCFA) Heusler thin films have been published elsewhere.[@rata] In contrast to previous reports about the growth of CCFA films on MgO(001) substrates,[@clemens1; @clemens2; @marukame] our CCFA films are grown at RT and adopt a different crystalline orientation with respect to the MgO(001) substrate.[@rata] The \[011\] direction of the CCFA films is parallel to \[001\] of the MgO(001) substrate. The CCFA films have the B2 structure, since the (111) reflection is absent in the X-ray diffraction patterns. Magnetization measurements of extended CCFA films reveal ferromagnetic ordering with Curie temperatures up to 630K after annealing in vacuum at 773K. The total magnetic moment found is about $2.5\mu_{B}$ per formula unit (f.u.). This value is small compared to the theoretical bulk value of $3.8\mu_{B}$/f.u.,[@galanakis3] but still comparable to the moments reported in Refs.  for films grown at elevated temperature.

MTJs are prepared by magnetron sputtering at RT without breaking the vacuum with the following layer sequence: MgO(100)/MgO(40 nm)/CCFA(25 nm)/MgO(3 nm)/CoFe(5 nm)/IrMn(15 nm). A 40nm-thick MgO seed layer is deposited on the MgO substrate to improve the texture of the CCFA electrode. CCFA and MgO are deposited by DC and RF stimulated discharge, respectively, from stoichiometric targets. The sputtering rates monitored with a quartz balance are calibrated by measuring the thicknesses of individual thin film layers by X-ray reflectivity. The completed stack is annealed *in-situ* for 1hour at 523 K in order to improve the interface quality. However, we cannot apply a magnetic field during the *in-situ* annealing. Thus, the CoFe electrode is not exchange-biased. Junctions with an area from $3\times3$ up to $15\times15$ $\mu$m$^{2}$ with crosse-bar electrodes are patterned for magnetotransport measurements in the current-perpendicular-plane (CPP) geometry by optical lithography. The transport measurements are performed with a DC setup in the standard 4-point geometry using a constant current source. $I-V$ characteristics measured at RT in zero field show nonlinear, *i.e.* non-Ohmic behavior.

In Fig. \[f-mr\] we present a magnetoresistance curve measured at RT on a CCFA/MgO/CoFe MTJ deposited on MgO(100) substrate with a 40nm-thick MgO seed layer. Obviously, we observe an *inverse* TMR effect. The TMR ratio defined as in Ref.  as TMR = $(R_{AP}-R_{P})/R_{AP}$, where $R_{AP}$ is the smallest resistance value in the antiparallel magnetization configuration and $R_{P}$ denotes the highest resistance in the saturated state, reaches $-66$%. The minimal resistance times area product is about 80k$\Omega\mu$m$^{2}$. The junction resistances are 1k$\Omega$ or larger in all samples and, thus, clearly exceed the lead and contact resistances of 10-20$\Omega$. Therefore, there is no geometrical enhancement in the TMR ratios. The TMR value obtained at RT is relatively large for structures comprising a FM Heusler electrode. The bell-shaped MR curve depicted in Fig. \[f-mr\] suggests a noncollinear orientation of the layers magnetizations for $H=0$ due to magnetic coupling of the two FM layers (arrows in Fig. \[f-mr\]). This is also confirmed by SQUID measurements, where independent switching of the two electrodes is found to be hindered. Antiferromagnetic, Néel-type and biquadratic coupling due to interface roughness are the most likely coupling mechanisms.

Figure \[f-voltage\] shows the typical dependence of the TMR ratio on the voltage drop $\Delta V$ across the MTJ measured in the parallel configuration at $H=\pm1$T. $\Delta V$ is experimentally controlled by varying the current bias supplied by the constant current source and is defined with respect to the CoFe electrode (see inset of Fig. \[f-voltage\]). The inverse TMR effect is dominant over a wide range of positive and negative $\Delta V$. Only for $|\Delta V|<300$mV we obtain a small positive TMR ratio of less than 1%. From $\Delta V=+300$ to +600mV the TMR ratio increases and abruptly decreases for larger $\Delta V$. A similar almost symmetric behavior is observed for negative $\Delta V$, except that the peak at $-600$mV is less pronounced.

In Fig. \[f-temp\] we plot the temperature dependence of the TMR ratio measured at $\Delta V=+600$mV. In contrast to MTJs with Co$_2$MnSi and Co$_2$MnGe Heusler electrodes,[@hutten1; @yamamoto; @sakuraba] our MTJs show only a moderate temperature dependence. The TMR ratio increases from $-66$% at RT to $-84$% upon cooling down to 20K. Similar behavior was reported in Ref.  for fully epitaxial CCFA/MgO/CoFe MTJs.

The most striking features of our results are (i) the dominant large *inverse* TMR, (ii) its strong dependence on the voltage drop $\Delta V$, and (iii) the weak temperature dependence. Thomas *et al.*[@hutten2] reported on inverted spin polarization of Co$_{2}$MnSi and Co$_{2}$FeSi Heusler alloys, which gave rise to a small inverse TMR ratio of about $-3$% at bias voltages exceeding $-1.3$V. For small bias, however, normal TMR of up to 100% was prevailing. Yamamoto *et al.* [@yamamoto] also observed a crossover to small negative TMR at negative bias voltage in MTJ structures comprising a Co$_2$MnGe electrode. These authors concluded from the conductance measurements that the inverse TMR was due to direct tunneling and thus reflected the bulk spin-dependent electronic density of states (DOS) of the Co$_{2}$MnGe electrode. In our case, the strong variation of the TMR ratio with the voltage drop, which clearly deviates from the usually found cusp-like behavior, and the weak temperature dependence also suggests a strong influence of the DOS on the TMR. In the framework of Jullière’s model, the TMR ratio can only be negative when the effective spin polarizations P$_L$ and P$_R$ on the left and right side of the barrier, respectively, are of opposite sign. Sharp features in the spin-split DOS of the electrodes give rise to a bias dependence of the effective polarizations and thus the TMR ratio.[@sharma] It was shown, see Refs. , that the nature of the bonding at the ferromagnet-insulator interface can influence the character of the tunnelling electrons and thus both size and sign of the effective polarization. Due to the preferential (110) orientation of our CCFA films, the bonding at the CCFA/MgO in our TMR structures is significantly different from the commonly found (100) orientation. The related differences in the band structures could be the reason for the inverse TMR ratios for certain $\Delta
V$ in our experiment. Assuming that CCFA shows much sharper features in the spin-split DOS than CoFe, *e.g.* due to a (pseudo-) gap and band edges, the data in Fig. \[f-voltage\] can be qualitatively explained by the schematic spin-split DOS of CCFA in the inset of Fig. \[f-temp\]. We have also assumed that the effective polarization of the CoFe/MgO interface is positive. If it is negative, minority and majority spin directions in the inset of Fig. \[f-temp\] must be exchanged. The large interval of 300-400mV between the Fermi level and the onsets of the peaks in the model DOS of CCFA explains the much weaker temperature dependence than found for other systems.[@sakuraba] Theoretical studies of the spin-split DOS for CCFA/MgO(110) interfaces and spin-dependent transport as finite voltages beyond the Jullière model could give more insight into the origin of the large inverse TMR effect. In particular, such calculations could also properly take into account the spin and bias dependence of the transmission probabilities, which have been shown to strongly affect the TMR effect.[@heiliger]

In conclusion, we observed a large inverse TMR effect in magnetron sputtered CCFA/MgO/CoFe MTJs at RT. The TMR ratio shows an unusual dependence on voltage drop $\Delta V$ across the structure with large negative values of up to $-66$% at $\Delta V=\pm600$mV and small positive values around zero bias. The temperature dependence is moderate with an increase from $-66$% to $-84$% ($\Delta
V=+600$mV) upon cooling from RT to 20K. We proposed that these findings are related to density-of-states effects of the CCFA electrode or the CCFA/MgO interface. Further investigations, both experimental and theoretical, are needed in order to understand and control the large inverse TMR as well as its bias and temperature characteristics. From the application point of view, our results are of high relevance, as they combine a large TMR ratio at a relatively high output voltage with a moderate temperature dependence.

[10]{}

S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science **294**, 1488 (2001).

S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Materials **3**, 868 (2004).

S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials **3**, 862 (2004).

D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, Appl. Phys. Lett. **86**, 092502 (2005).

W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. Rev. B **63**, 054416 (2001).

X.-G. Zhang and W. H. Butler, Rev. B **70**, 172407 (2004).

R. A. de Groot, F. M. Mueller, P. G. van Engen, and K. H. J. Buschow, Phys. Rev. Lett. **50**, 2024 (1983).

*Magnetic properties of metals*, edited by H. P. J. Wijn (Springer, Berlin, 1991).

J. Schmalhorst, S. Kämmerer, M. Sacher, G. Reiss, A. Hütten, and A. Scholl, Phys. Rev. B **70**, 024426 (2004).

Y. Sakuraba, J. Nakata, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota, Appl. Phys. Lett. **88**, 022503 (2006).

K. Inomata, S. Okamura, R. Goto, and N. Tezuka, Jpn. J. Appl. Phys. **42**, L419, (2003).

K. Inomata, S. Okamura, and N. Tezuka, J. Magn. Magn. Mater. **282**, 269 (2004).

T. Marukame, T. Kasahara, K. Matsuda, T. Uemura, and M. Yamamoto, Jap. J. Appl. Phys. **44**, L521 (2005).

T. Marukame, T. Ishikawa, K.-I. Matsuda, T. Uemura, and M. Yamamoto, Appl. Phys. Lett. **88**, 262503 (2006).

A. D. Rata, H. Braak, D. E. Bürgler, S. Cramm, and C. M. Schneider, Euro. Phys. J. B, **52**, 445 (2006).

R. Kelekar and B. M. Clemens, J. Appl. Phys. **96**, 540 (2004).

R. Kelekar and B. M. Clemens, Appl. Phys. Lett. **86**, 232501 (2005).

I. Galanakis, J. Phys.: Condens. Matter **16**, 3089 (2004).

J. M. De Teresa, A. Barthélémy, A. Fert, J. P. Contour, R. Lyonnet, F. Montaigne, P. Seneor, and A. Vaurès, Phys. Rev. Lett. **82**, 4288 (1999).

Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota, Appl. Phys. Lett. **88**, 192508 (2006).

M. Yamamoto, T. Marukame, T. Ishikawa, K. Matsuda, T. Uemura, and M. Arita, J. Phys. D: Appl. Phys. **39**, 824 (2006).

A. Thomas, D. Meyners, D. Ebke, N.-N. Liu, M. D. Sacher, J. Schmalhorst, G. Reiss, H. Ebert, and A. Hütten, Appl. Phys. Lett. **89**, 012502 (2006).

M. Sharma, S. X. Wang, and J. H. Nickel, Phys. Rev. Lett. **82**, 616 (1999).

J. M. De Teresa, A. Barthélémy, A. Fert, J. P. Contour, F. Montaigne, and P. Seneor, Science **286**, 507 (1999).

E. Y. Tsymbal and D. G. Pettifor, J. Phys.: Condens. Matter **9**, L411 (1997).

C. Heiliger, P. Zahn, B. Y. Yavorsky, and I. Mertig, Phys. Rev. B **73**, 214441 (2006).

![Magnetoresistance curve of a $10\times 10$ $\mu$m$^{2}$ MTJ with layer sequence MgO/MgO(40 nm)/CCFA(25 nm)/MgO(3 nm)/CoFe(5 nm)/IrMn(15 nm). The measurement is performed at RT and yields an *inverse* magnetoresistance of $-66$% for $\Delta V=+600$mV.[]{data-label="f-mr"}](Fig1){width="7cm"}

![Normalized TMR ratio of a MTJ with layer sequence MgO/MgO(40 nm)/CCFA(25 nm)/MgO(3 nm)/CoFe(5 nm)/IrMn(15 nm) as a function of the voltage drop $\Delta V$ across the junction measured at RT. Inset: $\Delta V$ is defined with respect to the CoFe electrode.[]{data-label="f-voltage"}](Fig2){width="7cm"}

![Temperature dependence of the TMR ratio for a MgO/MgO(40 nm)/CCFA(25 nm)/MgO(3 nm)/CoFe(5 nm)/IrMn(15 nm) MTJ measured at a voltage drop $\Delta V=+600$mV. Inset: Schematic spin-split DOS of CCFA, see text.[]{data-label="f-temp"}](Fig3){width="7cm"}

[^1]: Corresponding author. Email: d.buergler@fz-juelich.de, phone: +49 2461 614 214, FAX: +49 2461 614 443
