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Crystal structure effect of ferromagnetic electrode on tunneling magnetoresistance
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Acta Materialia 56 (2008) 1491–1495 www.elsevier.com/locate/actamat
Crystal structure effect of ferromagnetic electrode on tunneling magnetoresistance J. Joshua Yang, A.K. Bengtson, C.-X. Ji, D. Morgan, Y.A. Chang * Department of Materials Science and Engineering and Materials Science Program, University of Wisconsin-Madison, Madison, WI 53706, USA Received 22 October 2007; accepted 30 November 2007 Available online 29 January 2008
Abstract We show experimentally the effect of the crystal structure of a ferromagnetic (FM) electrode on tunneling magnetoresistance (TMR) by changing only the crystal structure of the bottom FM electrode in a magnetic tunnel junction (MTJ) and observing a significant TMR difference. Co87Fe13 was selected as the bottom FM electrode because the difference in stability between its face-centered cubic (fcc) and body-centered cubic (bcc) structures is very small. This enables us to compare the TMR of MTJs comprising ferromagnetic layers at the same composition but with different crystal structures. We find a significant increase in the TMR when changing from an fcc-Co87Fe13 to bcc-Co87Fe13 bottom FM electrode. The structurally induced TMR enhancement is attributed to a higher s-electron spin polarization for the bcc structure, which was confirmed for bulk Co and Co87.5Fe12.5 by ab initio calculations. These results unambiguously demonstrate the role of crystal structure and the associated electronic structure of FM electrodes in spin-dependent tunneling. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Crystal structure; Epitaxial growth; Phase stability; Transport properties
1. Introduction Magnetic tunnel junctions (MTJs), consisting of two ferromagnetic (FM) metal electrodes separated by a thin insulating barrier layer, have generated great interest in recent years. This interest is stimulated by the desire to better understand the spin-dependent tunneling (SDT) of MTJs as well as their application as sensors in advanced magnetic recording heads and as nonvolatile magnetic random access memory [1–4]. Different FM materials have been extensively studied experimentally and theoretically [3]. However, the mechanism of the SDT effect in MTJs is still a controversial issue [5,6]. This controversy is partially due to the complexity of MTJ systems, in which many factors influence the properties. Since some of the factors are coupled, very careful experimental design is required to separate these. One example is the effect of the FM crystal structure on the SDT, which is closely associated with the *
Corresponding author. Tel.: +1 608 262 0389; fax: +1 608 262 8353. E-mail address: [email protected] (Y.A. Chang).
electronic structure. Computational studies can clearly separate the role of the crystal structure in the SDT. However, experimental verification of the role of the crystal structure is much more challenging than in simulations because it is difficult to experimentally isolate the structural effects from those of composition and interfacial roughness. Here we take one of the most extensively studied MTJs, (Co, Fe)/ AlOx/(Co, Fe), as a model system to focus on a better understanding of the coupling of SDT to the crystal structure of the FM electrodes using both experimental and computational techniques. To the best of our knowledge, no definitive experimental evidence has yet been reported to support the claim that the body-centered cubic (bcc) (Co, Fe) FM electrode is indeed superior to the face-centered cubic (fcc) (Co, Fe) FM electrode in AlOx-based MTJs, although it appears that the tunneling magnetoresistance (TMR) is larger for MTJs with the bcc electrode than for MTJs with the fcc electrode [7]. However, the crystal structure of (Co, Fe) alloys in bulk or thin films depend on the composition of Fe. Since both the composition and the crystal structure may affect the
1359-6454/$34.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.11.045
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band structure, it is very difficult to identify the effect of crystal structure with FM electrodes exhibiting different crystal structures due to the compositional difference. In order to isolate the role of the crystal structure, it is necessary to overcome the difficulty of synthesizing two FM electrodes with the same composition but different crystal structures, i.e. fcc and bcc. The objective of the present study is to use a novel experimental approach based on thermodynamic considerations to identify a specific composition of FM alloy Co1aFea that can be induced to form either the bcc or the fcc structure. We synthesize two MTJs with one consisting of Co1aFea(bcc)/AlOx/Co75Fe25(bcc) and the other of Co1aFea(fcc)/AlOx/Co75Fe25(bcc). We demonstrate unambiguously that the TMR for the MTJ with the Co1aFea(bcc) FM bottom electrode is greater than that using the Co1aFea(fcc) FM bottom electrode. We have kept the interfacial smoothness in both junctions essentially the same, thereby ruling out the influence of interface roughness on TMR. We also provide computational evidence that bcc Co1aFea has a greater s-electron spin polarization than fcc Co1aFea. 2. Methodology and experimental approach On the basis of the phase stability of the Co1xFex alloys as a function of composition, we identified Co87Fe13 as a potential FM electrode which could exist either as bcc or fcc since the phase transition from fcc to bcc occurs at around a composition of 13 at.% Fe [8]. The near degeneracy between these phases at this composition indicates that it is possible to adapt an appropriate buffering layer to induce Co87Fe13 to be either fcc or bcc structure. In view of the close match of the lattice parameters of Cu (fcc, a = 0.36 nm) and Co87Fe13 (fcc) as well as Cr (bcc, a = 0.29 nm) and Co87Fe13 (bcc) (about 4% difference), Cu and Cr were selected as the respective buffers. Ag was used as a cap layer and the antiferromagnetic layer Ir22Mn78 was used to magnetically pin the top FM electrode. The top electrodes of the two samples consisting of Co75Fe25 (bcc, 4 nm)/Ir22Mn78 (18 nm)/Ag(50 nm) were synthesized by DC magnetron sputtering deposition on top of a tunneling barrier of AlOx. The AlOx barrier, about 1.6 nm thick, was formed by RF sputtering a thin Al metal layer followed by a RF plasma in situ oxidation in 100 mtorr O2 following the method used previously [9]. In the two types of junctions, we fabricated bottom electrodes of bcc Co87Fe13 and fcc Co87Fe13, using a 2 nm buffer layer of Cr(2 0 0) and Cu(2 0 0), respectively. These buffer layers were grown on top of a 30 nm Ag(2 0 0) layer on a Si(1 0 0) substrate. The wafers were in situ annealed at 400 °C for 1 h after the deposition of the 30 nm Ag to improve the crystal quality and surface smoothness, and then the rest of the layers were deposited at room temperature. The vacuum was broken once after the oxidation of the Al layer and two pieces of wafers were taken out of the chamber for four-circle X-ray diffraction (XRD) and
atomic force microscopy (AFM) characterization, respectively. This was done in order to ensure that the samples for AFM, XRD and TMR measurements were from the same experimental run and had the same qualities. The patterned MTJs were annealed at 250 °C for 1 h in a 1000 Oe magnetic field with a pressure of about 106 torr and the transport properties were then measured by a standard DC four-probe method. 3. Experimental results Four-circle XRD was used to verify the crystal structure of the bottom FM electrodes. As noted before, the bottom half-junction for AFM and XRD characterizations consisted of Si(1 0 0)/Ag(30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm). The XRD patterns for the two bottom half-junctions presented in Fig. 1a and b are the diffracted spectra collected with the scattering vector (normal to the diffraction plane) perpendicular to the surface of the multilayer. For both samples a Si(4 0 0) peak, a Ag(2 0 0) peak and a third peak can be seen. Since the Co87Fe13 layer (15 nm) is much thicker than the buffer (2 nm) layer, the third peak must come from the Co87Fe13 layer. The third peaks in Fig. 1a and b are 51.9° and 66.0°, corresponding to diffractions from fcc Co87Fe13 (2 0 0) (a = 0.35 nm) and bcc Co87Fe13 (2 0 0) (a = 0.28 nm), respectively. To further verify the symmetry of the crystal structure of the two epitaxial Co87Fe13 layers, the spectra of off-axis / scans of samples with Cu and Cr buffers were collected, as shown in Fig. 1c and d, respectively. Considering the angle between the (2 0 0) plane and the (1 1 1) plane in an fcc crystal is 54.74° and that between the (2 0 0) plane and the (1 1 0) plane in a bcc crystal is 45°, the samples were tilted to set either 54.74° or 45° (referred to as the off-axis angle) between their surface normal ([2 0 0] orientation) and the scattering vector during the off-axis / scans. Four Ag(1 1 1) and four Si(1 1 1) peaks (not shown here) were observed in both samples while the diffractions of Co87Fe13 layers are different for these two samples. For the Cu-buffered sample with an off-axis angle of 54.74°, four fcc (1 1 1) peaks with a 2h value of 44.15° but no bcc (1 1 0) peaks were observed as shown in Fig. 1c. This is opposite to what occurs in the Cr-buffered sample, for which four Co87Fe13 bcc (1 1 0) periodic peaks can be seen instead of Co87Fe13 fcc (1 1 1) peaks, as shown in Fig. 1d. All these XRD results suggest that the Co87Fe13 layers grew epitaxially along the [2 0 0] orientation into fcc and bcc structures in Cu-buffered and Cr-buffered samples, respectively. The interface roughness between the FM electrode and the tunnel barrier is an important factor that could significantly affect the TMR value [10]. Therefore, the roughness must be carefully controlled and identical within the uncertainty of measurement between samples to isolate the structural effect. The argon pressure during film deposition was lower than 2 mtorr in order to improve the smoothness of the multilayer. As mentioned earlier,
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Fig. 1. Four-circle XRD spectra for the bottom half of MTJs consisting of Si(1 0 0)/Ag(30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm). (a) and (b) are the spectra of normal h–2h scans of samples with Cu and Cr buffer, respectively. (c) and (d) are the spectra of off-axis / scans of Co87Fe13 layers in samples with Cu and Cr buffer, respectively.
a piece of the bottom half-junction was taken out of the chamber after the Al oxidation step for AFM characterization. The 1 lm 1 lm AFM images of these two samples are shown in Fig. 2. Given that the AlOx layers were ultrathin and oxidized under the same condition, we expect that the roughness measured at the AlOx surface will in large degree reflect the interfaces between the AlOx and the Co87Fe13 layers in these two samples. From Fig. 2, it can be seen that both interfaces of these two samples are quite smooth, with RMS values of about 0.36 nm and 0.40 nm for Cu-buffered and Cr-buffered samples, respectively. Considering that the Cu or Cr buffer layer was just 2 nm, these roughness values are reasonable and comparable to the commonly reported values of (Co, Fe) surfaces. Therefore, any significant difference in transport properties observed for these two samples
should not be caused by a difference in their interface roughnesses. The TMR value is one of the most important properties of MTJs and could be affected by many factors, including the FM electrode, tunnel barrier and the interfaces between them. A relatively small change in TMR values might be obscured by slight differences in the preparation condition, which poses some difficulty in correlating the TMR with the crystal and electronic structure [6]. However, a significant change in TMR values can be readily detected by varying the MTJs stack in a controlled way [11]. Fig. 3 shows the TMR curves of the two samples measured at room temperature. The TMR value of the Cr-buffered MTJ (bcc bottom electrode) reached 46.7% and that of the Cu-buffered MTJ (fcc bottom electrode) could only reach as high as 32.1% (Fig. 3). The TMR values measured from several junctions
Fig. 2. AFM images (1 lm 1 lm) and section analysis of (a) Cu-buffered and (b) Cr-buffered bottom electrodes.
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Fig. 3. Different TMR ratios at room temperature arising from the different FM crystal structures induced by different buffers in the epitaxial growth of bottom electrodes of MTJs. MTJs used consist of Si(1 0 0)/Ag (30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm)/Co75Fe25 (4 nm)/ Ir22Mn78 (18 nm)/Ag (50 nm) and the buffer layers are Cu (open dots) or Cr (solid dots).
for these two samples were 45 ± 3% and 31 ± 2%, respectively, demonstrating that the difference is reproducible and statistically significant. The TMR value for the sample with Cr buffer is about 1.4 times that for the sample with Cu buffer. From a chemical point of view, the core parts of these two MTJs, are the same, i.e. Co87Fe13/AlOx/Co75Fe25. The only difference is the crystal structure of the bottom electrodes. According to Julliere’s model [12], the magnitude of the TMR is determined by the tunneling spin polarization P of the individual FM electrodes:MR ¼ 2P t P b =ð1 P t P b Þ; where P t=b ¼ ðn " n #Þ=ðn " þn #Þ, with n "; # the density of majority ð"Þ and minority ð#Þ states at the Fermi energy. The subscript t and b denote top electrode and bottom electrode, respectively. The top electrodes of both samples are bcc Co75Fe25, with a P t of about 50% at low temperatures (about 4.2 K). In order to estimate the tunneling spin polarization values at low temperatures, we assume that the TMR value is about 1.4 times that at room temperature for both samples, as usually observed for these junctions [13]. By applying Julliere’s model, the P b of the Cu-buffered (fcc) and Cr-buffered (bcc) Co87Fe13 layers can be extracted from the TMRs to be about 38% and 49%, respectively. These are close to the experimentally measured spin polarizations for fcc Co and bcc Co84Fe16, which are 42% and 52%, respectively. 4. Ab initio calculations and discussions The difference in the electronic structures resulting from the difference in the crystal structure must be responsible
for the observed TMR difference in the above two MTJs. The FM and insulator interfaces are well known to be crucial for TMR. However, some experiments have also demonstrated that TMR is sensitive to the bulk electronic states [6,14] and Zhu et al. reported a characteristic length of about 0.8 nm for the (Co, Fe) electrode [15]. In a (Co, Fe)-based MTJ with a thick (>1 nm) AlOx tunnel barrier, the predominant contribution of the tunneling current is from the s-like electrons [16,5,17]. It is almost impossible to perform ab initio calculations on an entire MTJ structure with an amorphous barrier, such as AlOx, even though such a calculation has proven to be very powerful for the investigation of fully epitaxial MTJs [5]. Tsymbal et al. have calculated the s-electron partial density of states (DOS) for the first two surface layers of fcc (2 0 0) Co and bcc (2 0 0) Fe [18]. They reported that the s-electron partial DOS at the surface of the bcc (2 0 0) Fe has a much larger spin asymmetry than that of the fcc (2 0 0) Co at the Fermi level, which results in the s-electron spin polarization of the former being higher than the latter by 32% [18]. This is consistent with bcc having larger s-electron polarization than fcc in (Co, Fe) alloys, although the structural effects cannot be separated from those of changing the element. In order to confirm the dependence of P values on structure, ab initio calculations were preformed to predict P for bulk bcc and fcc Co87.5Fe12.5 from the partial s-electron DOS and the equation for P given above. Calculations were performed using density functional theory and the projector-augmented plane-wave method [19–21], as implemented in the Vienna Ab-initio Simulation Package (VASP) [19]. Exchange-correlation was treated via the Perdew–Burke–Ernzerhof parametrization of the generalized gradient approximation [22]. The Brillouin zone was sampled by a Monkhorst–Pack k-point mesh of 26 26 26 for the fcc and bcc primitive cells and the k-point density was kept as constant as possible for different super-cells. Electron smearing for DOS calculations was done with Fermi-smearing for fcc and the tetrahedral method (with Blo¨chl corrections) for bcc (these correspond to ISMEAR = 1 and ISMEAR = 5 in the VASP code). The s-electron DOS at the Fermi surface was obtained by taking a bin at the Fermi energy of width 7 meV. Convergence of P with respect to k-points was extremely slow and the large k-point mesh and different electron smearing were necessary to obtain results that converged to within acceptable errors for P. A cutoff energy of 455 eV was used. Energies, volumes and P values were converged to within 1 meV/atom, 103 nm3 and 2%, respectively, with respect to k-points and energy cutoff. In order to avoid bias from a particular ordering we attempted to model a disordered system. To simulate results of a disordered material three different special quasirandom structures (SQSs) [23] were used for fcc and bcc and the results averaged. The SQSs are generated using the ATAT toolkit [24] to be as random as possible for pairs within the first three nearest-neighbor shells. They are all 16 atoms/unit cell with composition Co87.5Fe12.5. The mean P values for
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the SQSs were calculated to be 50 ± 0.3% for fcc and 53 ± 0.8% for bcc, where the errors are the standard deviations of the mean obtained from the three SQS calculations. These values are somewhat larger and closer than those extracted from the experiment but confirm the trend of increasing P with the transformation from fcc to bcc. Quite similar values and trends (46% for fcc and 54% for bcc) were obtained for pure Co, further supporting the claim of higher P for bcc compared to fcc. It is also worth noting that the P values extracted from the experiment are for an epitaxial thin film with a specific crystal orientation, which may result in some discrepancy between the experimental results and the calculated bulk values [11]. 5. Conclusions We have taken advantage of the near degenerate fcc and bcc phase stability of Co87Fe13 to create a bottom FM electrode exhibiting either the fcc or bcc structure. The fcc and bcc phases are stabilized by using a buffer of 2 nm Cu(2 0 0) and Cr(2 0 0), respectively, on a layer of 30 nm Ag(2 0 0) epitaxially grown on Si(1 0 0) substrate. We observed that the TMR value increased from 31% to 45% when the crystal structure of the bottom electrode was changed from fcc to bcc in AlOx-based MTJs with the top electrode consisting of Co75Fe25 (4 nm)/Ir22Mn78 (18 nm)/Ag(50 nm). The tunneling spin polarizations were extracted to be about 38% and 49% for the fcc Co87Fe13 (2 0 0) and bcc Co87Fe13 (2 0 0) bottom electrodes, respectively. The increase in the TMR values is attributed to a higher s-electron spin polarization for the bcc structure and suggests that the bcc FM is the preferred structure for AlOx-based MTJ applications. Ab initio calculations on pure Co and disordered Co87.5Fe12.5 confirm that changing from fcc to bcc significantly increases the s-electron spin polarizations. This study clearly demonstrated the role of the electronic structure of the FM electrode on the SDT and how this can couple significantly to crystal structure. Acknowlegments J.J.Y., C.X.J. and Y.A.C. gratefully acknowledge the financial support from the Office of Basic Energy Research
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of DOE through Grant No. DE-FG02-99-ER45777. They especially thank Dr. Jane G. Zhu, Program Manager, Structure and Composition of Materials, Division of Materials Science and Engineering for her interest in this work. A.K. Bengtson gratefully acknowledges financial support from the Wisconsin Alumni Research Foundation (WARF). References [1] Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molna´r S, Roukes ML, Chtchelkanova AY, Treger DM. Science 2001;294: 1488. [2] Moodera JS, Kinder LR, Wong TM, Meservey R. Phys Rev Lett 1995;74:3273. [3] Zutic I, Fabian J, Sarma SD. Rev Mod Phys 2004;76:323. [4] Prinz GA. Science 1998;282:1660. [5] Nagahama T, Yuasa S, Tamura E, Suzuki Y. Phys Rev Lett 2005;95: 086602. [6] LeClair P, Kohlhepp JT, van de Vin CH, Wieldraaijer H, Swagten HJM, de Jonge WJM, Davis AH, MacLaren JM, Moodera JS, Jansen R. Phys Rev Lett 2002;88:107201. [7] Yang JJ, Bengtson AK, Ji CJ, Morgan D, Chang YA. J Appl Phys 2008 [Accepted for publication]. [8] Wojcik M, Jay JP, Panissod P, Jedryka E, Dekoster J, Langouche G. Z Phy B 1997;103:5. [9] Yang JJ, Ji CX, Ke X, Rzchowski MS, Chang YA. Appl Phys Lett 2006;89:202502. [10] Moodera JS, Nassar J, Mathon G. Annu Rev Mater Sci 1999;29:381. [11] Yuasa S, Sato T, Tamura E, Suzuki Y, Yamamori H, Ando K, Katayama T. Europhys Lett 2000;52:344. [12] Julliere M. Phys Lett 1975;54A:225. [13] Han XF, Oogane M, Kubota H, Ando Y, Miyazaki T. Appl Phys Lett 2000;77:283. [14] Nagahama T, Yuasa S, Suzuki Y. Appl Phys Lett 2001;79:4381. [15] Zhu T, Xiang X, Shen F, Zhang Z, Landry G, Dimitrov DV, Garcı´a N, Xiao JQ. Phys Rev B 2002;66:094423. [16] Mu¨nzenberg M, Moodera JS. Phys Rev B 2004;70:060402(R). [17] Thomas A, Meyners D, Ebke D, Liu N, Sacher MD, Schmalhorst J, Reiss G, Ebert H, Hu¨tten A. Appl Phys Lett 2006;89:012502. [18] Tsymbal EY, Pettifor DG. J Phys: Condens Mat 1997;9:L411. [19] Kresse G, Furthmuller J. Comput Mater Sci 1996;6:15. [20] Kresse G, Joubert D. Phys Rev B 1999;59:1758. [21] Blochl PE. Phys Rev B 1994;50:17953. [22] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865. [23] Wei SH, Ferreira LG, Bernard JE, Zunger A. Phys Rev B 1990;42: 9622. [24] van de Walle A, Asta M, Ceder G, Calphad. Comput Coupling Phase Diag Thermochem 2002;26:539.
Acta Materialia 56 (2008) 1491–1495 www.elsevier.com/locate/actamat
Crystal structure effect of ferromagnetic electrode on tunneling magnetoresistance J. Joshua Yang, A.K. Bengtson, C.-X. Ji, D. Morgan, Y.A. Chang * Department of Materials Science and Engineering and Materials Science Program, University of Wisconsin-Madison, Madison, WI 53706, USA Received 22 October 2007; accepted 30 November 2007 Available online 29 January 2008
Abstract We show experimentally the effect of the crystal structure of a ferromagnetic (FM) electrode on tunneling magnetoresistance (TMR) by changing only the crystal structure of the bottom FM electrode in a magnetic tunnel junction (MTJ) and observing a significant TMR difference. Co87Fe13 was selected as the bottom FM electrode because the difference in stability between its face-centered cubic (fcc) and body-centered cubic (bcc) structures is very small. This enables us to compare the TMR of MTJs comprising ferromagnetic layers at the same composition but with different crystal structures. We find a significant increase in the TMR when changing from an fcc-Co87Fe13 to bcc-Co87Fe13 bottom FM electrode. The structurally induced TMR enhancement is attributed to a higher s-electron spin polarization for the bcc structure, which was confirmed for bulk Co and Co87.5Fe12.5 by ab initio calculations. These results unambiguously demonstrate the role of crystal structure and the associated electronic structure of FM electrodes in spin-dependent tunneling. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Crystal structure; Epitaxial growth; Phase stability; Transport properties
1. Introduction Magnetic tunnel junctions (MTJs), consisting of two ferromagnetic (FM) metal electrodes separated by a thin insulating barrier layer, have generated great interest in recent years. This interest is stimulated by the desire to better understand the spin-dependent tunneling (SDT) of MTJs as well as their application as sensors in advanced magnetic recording heads and as nonvolatile magnetic random access memory [1–4]. Different FM materials have been extensively studied experimentally and theoretically [3]. However, the mechanism of the SDT effect in MTJs is still a controversial issue [5,6]. This controversy is partially due to the complexity of MTJ systems, in which many factors influence the properties. Since some of the factors are coupled, very careful experimental design is required to separate these. One example is the effect of the FM crystal structure on the SDT, which is closely associated with the *
Corresponding author. Tel.: +1 608 262 0389; fax: +1 608 262 8353. E-mail address: [email protected] (Y.A. Chang).
electronic structure. Computational studies can clearly separate the role of the crystal structure in the SDT. However, experimental verification of the role of the crystal structure is much more challenging than in simulations because it is difficult to experimentally isolate the structural effects from those of composition and interfacial roughness. Here we take one of the most extensively studied MTJs, (Co, Fe)/ AlOx/(Co, Fe), as a model system to focus on a better understanding of the coupling of SDT to the crystal structure of the FM electrodes using both experimental and computational techniques. To the best of our knowledge, no definitive experimental evidence has yet been reported to support the claim that the body-centered cubic (bcc) (Co, Fe) FM electrode is indeed superior to the face-centered cubic (fcc) (Co, Fe) FM electrode in AlOx-based MTJs, although it appears that the tunneling magnetoresistance (TMR) is larger for MTJs with the bcc electrode than for MTJs with the fcc electrode [7]. However, the crystal structure of (Co, Fe) alloys in bulk or thin films depend on the composition of Fe. Since both the composition and the crystal structure may affect the
1359-6454/$34.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.11.045
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band structure, it is very difficult to identify the effect of crystal structure with FM electrodes exhibiting different crystal structures due to the compositional difference. In order to isolate the role of the crystal structure, it is necessary to overcome the difficulty of synthesizing two FM electrodes with the same composition but different crystal structures, i.e. fcc and bcc. The objective of the present study is to use a novel experimental approach based on thermodynamic considerations to identify a specific composition of FM alloy Co1aFea that can be induced to form either the bcc or the fcc structure. We synthesize two MTJs with one consisting of Co1aFea(bcc)/AlOx/Co75Fe25(bcc) and the other of Co1aFea(fcc)/AlOx/Co75Fe25(bcc). We demonstrate unambiguously that the TMR for the MTJ with the Co1aFea(bcc) FM bottom electrode is greater than that using the Co1aFea(fcc) FM bottom electrode. We have kept the interfacial smoothness in both junctions essentially the same, thereby ruling out the influence of interface roughness on TMR. We also provide computational evidence that bcc Co1aFea has a greater s-electron spin polarization than fcc Co1aFea. 2. Methodology and experimental approach On the basis of the phase stability of the Co1xFex alloys as a function of composition, we identified Co87Fe13 as a potential FM electrode which could exist either as bcc or fcc since the phase transition from fcc to bcc occurs at around a composition of 13 at.% Fe [8]. The near degeneracy between these phases at this composition indicates that it is possible to adapt an appropriate buffering layer to induce Co87Fe13 to be either fcc or bcc structure. In view of the close match of the lattice parameters of Cu (fcc, a = 0.36 nm) and Co87Fe13 (fcc) as well as Cr (bcc, a = 0.29 nm) and Co87Fe13 (bcc) (about 4% difference), Cu and Cr were selected as the respective buffers. Ag was used as a cap layer and the antiferromagnetic layer Ir22Mn78 was used to magnetically pin the top FM electrode. The top electrodes of the two samples consisting of Co75Fe25 (bcc, 4 nm)/Ir22Mn78 (18 nm)/Ag(50 nm) were synthesized by DC magnetron sputtering deposition on top of a tunneling barrier of AlOx. The AlOx barrier, about 1.6 nm thick, was formed by RF sputtering a thin Al metal layer followed by a RF plasma in situ oxidation in 100 mtorr O2 following the method used previously [9]. In the two types of junctions, we fabricated bottom electrodes of bcc Co87Fe13 and fcc Co87Fe13, using a 2 nm buffer layer of Cr(2 0 0) and Cu(2 0 0), respectively. These buffer layers were grown on top of a 30 nm Ag(2 0 0) layer on a Si(1 0 0) substrate. The wafers were in situ annealed at 400 °C for 1 h after the deposition of the 30 nm Ag to improve the crystal quality and surface smoothness, and then the rest of the layers were deposited at room temperature. The vacuum was broken once after the oxidation of the Al layer and two pieces of wafers were taken out of the chamber for four-circle X-ray diffraction (XRD) and
atomic force microscopy (AFM) characterization, respectively. This was done in order to ensure that the samples for AFM, XRD and TMR measurements were from the same experimental run and had the same qualities. The patterned MTJs were annealed at 250 °C for 1 h in a 1000 Oe magnetic field with a pressure of about 106 torr and the transport properties were then measured by a standard DC four-probe method. 3. Experimental results Four-circle XRD was used to verify the crystal structure of the bottom FM electrodes. As noted before, the bottom half-junction for AFM and XRD characterizations consisted of Si(1 0 0)/Ag(30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm). The XRD patterns for the two bottom half-junctions presented in Fig. 1a and b are the diffracted spectra collected with the scattering vector (normal to the diffraction plane) perpendicular to the surface of the multilayer. For both samples a Si(4 0 0) peak, a Ag(2 0 0) peak and a third peak can be seen. Since the Co87Fe13 layer (15 nm) is much thicker than the buffer (2 nm) layer, the third peak must come from the Co87Fe13 layer. The third peaks in Fig. 1a and b are 51.9° and 66.0°, corresponding to diffractions from fcc Co87Fe13 (2 0 0) (a = 0.35 nm) and bcc Co87Fe13 (2 0 0) (a = 0.28 nm), respectively. To further verify the symmetry of the crystal structure of the two epitaxial Co87Fe13 layers, the spectra of off-axis / scans of samples with Cu and Cr buffers were collected, as shown in Fig. 1c and d, respectively. Considering the angle between the (2 0 0) plane and the (1 1 1) plane in an fcc crystal is 54.74° and that between the (2 0 0) plane and the (1 1 0) plane in a bcc crystal is 45°, the samples were tilted to set either 54.74° or 45° (referred to as the off-axis angle) between their surface normal ([2 0 0] orientation) and the scattering vector during the off-axis / scans. Four Ag(1 1 1) and four Si(1 1 1) peaks (not shown here) were observed in both samples while the diffractions of Co87Fe13 layers are different for these two samples. For the Cu-buffered sample with an off-axis angle of 54.74°, four fcc (1 1 1) peaks with a 2h value of 44.15° but no bcc (1 1 0) peaks were observed as shown in Fig. 1c. This is opposite to what occurs in the Cr-buffered sample, for which four Co87Fe13 bcc (1 1 0) periodic peaks can be seen instead of Co87Fe13 fcc (1 1 1) peaks, as shown in Fig. 1d. All these XRD results suggest that the Co87Fe13 layers grew epitaxially along the [2 0 0] orientation into fcc and bcc structures in Cu-buffered and Cr-buffered samples, respectively. The interface roughness between the FM electrode and the tunnel barrier is an important factor that could significantly affect the TMR value [10]. Therefore, the roughness must be carefully controlled and identical within the uncertainty of measurement between samples to isolate the structural effect. The argon pressure during film deposition was lower than 2 mtorr in order to improve the smoothness of the multilayer. As mentioned earlier,
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Fig. 1. Four-circle XRD spectra for the bottom half of MTJs consisting of Si(1 0 0)/Ag(30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm). (a) and (b) are the spectra of normal h–2h scans of samples with Cu and Cr buffer, respectively. (c) and (d) are the spectra of off-axis / scans of Co87Fe13 layers in samples with Cu and Cr buffer, respectively.
a piece of the bottom half-junction was taken out of the chamber after the Al oxidation step for AFM characterization. The 1 lm 1 lm AFM images of these two samples are shown in Fig. 2. Given that the AlOx layers were ultrathin and oxidized under the same condition, we expect that the roughness measured at the AlOx surface will in large degree reflect the interfaces between the AlOx and the Co87Fe13 layers in these two samples. From Fig. 2, it can be seen that both interfaces of these two samples are quite smooth, with RMS values of about 0.36 nm and 0.40 nm for Cu-buffered and Cr-buffered samples, respectively. Considering that the Cu or Cr buffer layer was just 2 nm, these roughness values are reasonable and comparable to the commonly reported values of (Co, Fe) surfaces. Therefore, any significant difference in transport properties observed for these two samples
should not be caused by a difference in their interface roughnesses. The TMR value is one of the most important properties of MTJs and could be affected by many factors, including the FM electrode, tunnel barrier and the interfaces between them. A relatively small change in TMR values might be obscured by slight differences in the preparation condition, which poses some difficulty in correlating the TMR with the crystal and electronic structure [6]. However, a significant change in TMR values can be readily detected by varying the MTJs stack in a controlled way [11]. Fig. 3 shows the TMR curves of the two samples measured at room temperature. The TMR value of the Cr-buffered MTJ (bcc bottom electrode) reached 46.7% and that of the Cu-buffered MTJ (fcc bottom electrode) could only reach as high as 32.1% (Fig. 3). The TMR values measured from several junctions
Fig. 2. AFM images (1 lm 1 lm) and section analysis of (a) Cu-buffered and (b) Cr-buffered bottom electrodes.
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Fig. 3. Different TMR ratios at room temperature arising from the different FM crystal structures induced by different buffers in the epitaxial growth of bottom electrodes of MTJs. MTJs used consist of Si(1 0 0)/Ag (30 nm)/buffer (2 nm)/Co87Fe13 (15 nm)/AlOx (1.6 nm)/Co75Fe25 (4 nm)/ Ir22Mn78 (18 nm)/Ag (50 nm) and the buffer layers are Cu (open dots) or Cr (solid dots).
for these two samples were 45 ± 3% and 31 ± 2%, respectively, demonstrating that the difference is reproducible and statistically significant. The TMR value for the sample with Cr buffer is about 1.4 times that for the sample with Cu buffer. From a chemical point of view, the core parts of these two MTJs, are the same, i.e. Co87Fe13/AlOx/Co75Fe25. The only difference is the crystal structure of the bottom electrodes. According to Julliere’s model [12], the magnitude of the TMR is determined by the tunneling spin polarization P of the individual FM electrodes:MR ¼ 2P t P b =ð1 P t P b Þ; where P t=b ¼ ðn " n #Þ=ðn " þn #Þ, with n "; # the density of majority ð"Þ and minority ð#Þ states at the Fermi energy. The subscript t and b denote top electrode and bottom electrode, respectively. The top electrodes of both samples are bcc Co75Fe25, with a P t of about 50% at low temperatures (about 4.2 K). In order to estimate the tunneling spin polarization values at low temperatures, we assume that the TMR value is about 1.4 times that at room temperature for both samples, as usually observed for these junctions [13]. By applying Julliere’s model, the P b of the Cu-buffered (fcc) and Cr-buffered (bcc) Co87Fe13 layers can be extracted from the TMRs to be about 38% and 49%, respectively. These are close to the experimentally measured spin polarizations for fcc Co and bcc Co84Fe16, which are 42% and 52%, respectively. 4. Ab initio calculations and discussions The difference in the electronic structures resulting from the difference in the crystal structure must be responsible
for the observed TMR difference in the above two MTJs. The FM and insulator interfaces are well known to be crucial for TMR. However, some experiments have also demonstrated that TMR is sensitive to the bulk electronic states [6,14] and Zhu et al. reported a characteristic length of about 0.8 nm for the (Co, Fe) electrode [15]. In a (Co, Fe)-based MTJ with a thick (>1 nm) AlOx tunnel barrier, the predominant contribution of the tunneling current is from the s-like electrons [16,5,17]. It is almost impossible to perform ab initio calculations on an entire MTJ structure with an amorphous barrier, such as AlOx, even though such a calculation has proven to be very powerful for the investigation of fully epitaxial MTJs [5]. Tsymbal et al. have calculated the s-electron partial density of states (DOS) for the first two surface layers of fcc (2 0 0) Co and bcc (2 0 0) Fe [18]. They reported that the s-electron partial DOS at the surface of the bcc (2 0 0) Fe has a much larger spin asymmetry than that of the fcc (2 0 0) Co at the Fermi level, which results in the s-electron spin polarization of the former being higher than the latter by 32% [18]. This is consistent with bcc having larger s-electron polarization than fcc in (Co, Fe) alloys, although the structural effects cannot be separated from those of changing the element. In order to confirm the dependence of P values on structure, ab initio calculations were preformed to predict P for bulk bcc and fcc Co87.5Fe12.5 from the partial s-electron DOS and the equation for P given above. Calculations were performed using density functional theory and the projector-augmented plane-wave method [19–21], as implemented in the Vienna Ab-initio Simulation Package (VASP) [19]. Exchange-correlation was treated via the Perdew–Burke–Ernzerhof parametrization of the generalized gradient approximation [22]. The Brillouin zone was sampled by a Monkhorst–Pack k-point mesh of 26 26 26 for the fcc and bcc primitive cells and the k-point density was kept as constant as possible for different super-cells. Electron smearing for DOS calculations was done with Fermi-smearing for fcc and the tetrahedral method (with Blo¨chl corrections) for bcc (these correspond to ISMEAR = 1 and ISMEAR = 5 in the VASP code). The s-electron DOS at the Fermi surface was obtained by taking a bin at the Fermi energy of width 7 meV. Convergence of P with respect to k-points was extremely slow and the large k-point mesh and different electron smearing were necessary to obtain results that converged to within acceptable errors for P. A cutoff energy of 455 eV was used. Energies, volumes and P values were converged to within 1 meV/atom, 103 nm3 and 2%, respectively, with respect to k-points and energy cutoff. In order to avoid bias from a particular ordering we attempted to model a disordered system. To simulate results of a disordered material three different special quasirandom structures (SQSs) [23] were used for fcc and bcc and the results averaged. The SQSs are generated using the ATAT toolkit [24] to be as random as possible for pairs within the first three nearest-neighbor shells. They are all 16 atoms/unit cell with composition Co87.5Fe12.5. The mean P values for
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the SQSs were calculated to be 50 ± 0.3% for fcc and 53 ± 0.8% for bcc, where the errors are the standard deviations of the mean obtained from the three SQS calculations. These values are somewhat larger and closer than those extracted from the experiment but confirm the trend of increasing P with the transformation from fcc to bcc. Quite similar values and trends (46% for fcc and 54% for bcc) were obtained for pure Co, further supporting the claim of higher P for bcc compared to fcc. It is also worth noting that the P values extracted from the experiment are for an epitaxial thin film with a specific crystal orientation, which may result in some discrepancy between the experimental results and the calculated bulk values [11]. 5. Conclusions We have taken advantage of the near degenerate fcc and bcc phase stability of Co87Fe13 to create a bottom FM electrode exhibiting either the fcc or bcc structure. The fcc and bcc phases are stabilized by using a buffer of 2 nm Cu(2 0 0) and Cr(2 0 0), respectively, on a layer of 30 nm Ag(2 0 0) epitaxially grown on Si(1 0 0) substrate. We observed that the TMR value increased from 31% to 45% when the crystal structure of the bottom electrode was changed from fcc to bcc in AlOx-based MTJs with the top electrode consisting of Co75Fe25 (4 nm)/Ir22Mn78 (18 nm)/Ag(50 nm). The tunneling spin polarizations were extracted to be about 38% and 49% for the fcc Co87Fe13 (2 0 0) and bcc Co87Fe13 (2 0 0) bottom electrodes, respectively. The increase in the TMR values is attributed to a higher s-electron spin polarization for the bcc structure and suggests that the bcc FM is the preferred structure for AlOx-based MTJ applications. Ab initio calculations on pure Co and disordered Co87.5Fe12.5 confirm that changing from fcc to bcc significantly increases the s-electron spin polarizations. This study clearly demonstrated the role of the electronic structure of the FM electrode on the SDT and how this can couple significantly to crystal structure. Acknowlegments J.J.Y., C.X.J. and Y.A.C. gratefully acknowledge the financial support from the Office of Basic Energy Research
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of DOE through Grant No. DE-FG02-99-ER45777. They especially thank Dr. Jane G. Zhu, Program Manager, Structure and Composition of Materials, Division of Materials Science and Engineering for her interest in this work. A.K. Bengtson gratefully acknowledges financial support from the Wisconsin Alumni Research Foundation (WARF). References [1] Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molna´r S, Roukes ML, Chtchelkanova AY, Treger DM. Science 2001;294: 1488. [2] Moodera JS, Kinder LR, Wong TM, Meservey R. Phys Rev Lett 1995;74:3273. [3] Zutic I, Fabian J, Sarma SD. Rev Mod Phys 2004;76:323. [4] Prinz GA. Science 1998;282:1660. [5] Nagahama T, Yuasa S, Tamura E, Suzuki Y. Phys Rev Lett 2005;95: 086602. [6] LeClair P, Kohlhepp JT, van de Vin CH, Wieldraaijer H, Swagten HJM, de Jonge WJM, Davis AH, MacLaren JM, Moodera JS, Jansen R. Phys Rev Lett 2002;88:107201. [7] Yang JJ, Bengtson AK, Ji CJ, Morgan D, Chang YA. J Appl Phys 2008 [Accepted for publication]. [8] Wojcik M, Jay JP, Panissod P, Jedryka E, Dekoster J, Langouche G. Z Phy B 1997;103:5. [9] Yang JJ, Ji CX, Ke X, Rzchowski MS, Chang YA. Appl Phys Lett 2006;89:202502. [10] Moodera JS, Nassar J, Mathon G. Annu Rev Mater Sci 1999;29:381. [11] Yuasa S, Sato T, Tamura E, Suzuki Y, Yamamori H, Ando K, Katayama T. Europhys Lett 2000;52:344. [12] Julliere M. Phys Lett 1975;54A:225. [13] Han XF, Oogane M, Kubota H, Ando Y, Miyazaki T. Appl Phys Lett 2000;77:283. [14] Nagahama T, Yuasa S, Suzuki Y. Appl Phys Lett 2001;79:4381. [15] Zhu T, Xiang X, Shen F, Zhang Z, Landry G, Dimitrov DV, Garcı´a N, Xiao JQ. Phys Rev B 2002;66:094423. [16] Mu¨nzenberg M, Moodera JS. Phys Rev B 2004;70:060402(R). [17] Thomas A, Meyners D, Ebke D, Liu N, Sacher MD, Schmalhorst J, Reiss G, Ebert H, Hu¨tten A. Appl Phys Lett 2006;89:012502. [18] Tsymbal EY, Pettifor DG. J Phys: Condens Mat 1997;9:L411. [19] Kresse G, Furthmuller J. Comput Mater Sci 1996;6:15. [20] Kresse G, Joubert D. Phys Rev B 1999;59:1758. [21] Blochl PE. Phys Rev B 1994;50:17953. [22] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865. [23] Wei SH, Ferreira LG, Bernard JE, Zunger A. Phys Rev B 1990;42: 9622. [24] van de Walle A, Asta M, Ceder G, Calphad. Comput Coupling Phase Diag Thermochem 2002;26:539.