Effect of microstructure on the magnetoresistive properties ofNiFe/Co(CoFe)/Al(Ta)–oxide/Co(CoFe) tunnel junctionsH. Kyung, H. S. Ahn, C. S. Yoon, C. K. Kim, Ohsung Song, T. Miyazaki, Y. Ando, and H. Kubota Citation: Journal of Applied Physics 89, 2752 (2001); doi: 10.1063/1.1343519 View online: http://dx.doi.org/10.1063/1.1343519 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Interface characterization and thermal stability of Co/Al–O/CoFe spin-dependent tunnel junctions J. Appl. Phys. 91, 7475 (2002); 10.1063/1.1452228 Magnetoresistance of spin-dependent tunnel junctions with composite electrodes J. Appl. Phys. 90, 6222 (2001); 10.1063/1.1419259 Deposition of [Ni–Fe/Al–O/Co–Fe] films with tunneling magnetoresistance effect using the interfacial modulationtechnique J. Appl. Phys. 89, 6647 (2001); 10.1063/1.1361043 Anomalous behavior of Co insertion to Al 2 O 3 in CoFe/Al 2 O 3 / NiFe tunnel junctions J. Appl. Phys. 88, 4764 (2000); 10.1063/1.1308871 Spin tunneling in Ni–Fe/Al 2 O 3 /Co junction devices (invited) J. Appl. Phys. 81, 3753 (1997); 10.1063/1.364957

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.174.255.116 On: Sun, 21 Dec 2014 16:52:43

Effect of microstructure on the magnetoresistive propertiesof NiFe ÕCo„CoFe…ÕAl „Ta…–oxide ÕCo„CoFe… tunnel junctions

H. Kyung,a) H. S. Ahn, C. S. Yoon, and C. K. KimHanyang University, Department of Materials Science and Engineering, Seoul, Korea

Ohsung SongThe University of Seoul, Department of Materials Science and Engineering, Korea

T. Miyazaki, Y. Ando, and H. KubotaTohoku University, Department of Applied Physics, Graduate School of Engineering, Japan

~Received 7 July 2000; accepted for publication 28 November 2000!

The microstructure of the NiFe/Co~CoFe!/Al ~Ta!-oxide/Co~CoFe! ferromagnetic tunnel junctionwas investigated using cross-sectional transmission electron microscopy~TEM!. The effect of theinsulating layer on the magnetoresistive~MR! properties of the junction was studied. The multilayerjunction was formed using magnetron sputtering and the insulating layer was created by plasmaoxidation of the deposited metal film. TEM analysis showed that the MR ratio was highly dependenton the insulating layer. For the NiFe/Co/Al-oxide/Co junction, when the Al2O3 layer was 13 Å, theoxide layer was flat and the highest MR ratio of 15% was attained. As the Al2O3 thickness increased,the interface roughness rapidly increased, and the MR ratio also markedly dropped. In contrast,NiFe/CoFe/Al-oxide/CoFe junction showed a comparatively flatter interface and recorded a higherMR ratio. The Ta-oxide insulating layer remained flat regardless of the thickness; however, thelargest MR ratio of only 9% was obtained within a narrow thickness range. We have demonstratedthat there exists a direct correlation between the microstructure of the oxide layer and the MR ratioof the junction, which could be utilized to optimize the electrical properties of the ferromagnetictunneling junction. ©2001 American Institute of Physics.@DOI: 10.1063/1.1343519#

I. INTRODUCTION

Tunneling magnetoresistive~TMR! junctions consist ofa ferromagnet~FM!/insulator/ferromagnet structure in whichTMR ratio changes as a function of an applied magneticfield. The resistance of the junction depends upon spin ar-rangement of the FM~parallel or antiparallel! layer separatedby an insulating layer. Such a junction can be potentiallyapplied to high density read head for the high-density storagemedia and Magnetic Random Access Memory~MRAM !application.

Recently, several groups have reported the TMR effectover 20% at room temperature,1–3 but the microstructuraleffect of the TMR junction was not studied extensively. Inorder to utilize the junction commercially, the thickness ofthe insulating layer needs to be controlled within a few ang-strom ranges to ensure reliable operation of the device.4 Inaddition, the defect structure and chemistry of the insulatinglayer between two FM layers plays an important role inachieving a large magnetoresistive effect.5 Thus, microstruc-tural evaluation of the interface is of crucial importance instudying the TMR junctions.

In spite of the importance of the barrier microstructure,there exists relatively little work on the structural character-ization using transmission electron microscope~TEM!.6,7 Inthis work, we have investigated the microstructural charac-teristics of Al and Ta oxide layers during as oxidation pro-

cess using cross-sectional TEM to establish reliable and re-producible processing conditions for TMR junctionfabrication.

II. EXPERIMENTAL PROCEDURE

The magnetic tunnel junctions were deposited by dcmagnetron sputtering at room temperature on Si~100! waferswhich were cleaned with ethanol for 5 min to eliminate H2Oand oxidized thermally prior to the deposition. The followinglayered structure was deposited:

~i! Si/SiO2/NiFe(170Å)/Co(48 Å)/Al( x)-oxide/Co(750 Å),

~ii ! Si/SiO2/NiFe(170Å)/CoFe(48 Å)/Al( x)-oxide/CoFe(750 Å),

~iii ! Si/SiO2/NiFe(170Å)/Co(48 Å)/Ta(x)-oxide/Co(750 Å), and

~iv! Si/SiO2/NiFe(170Å)/Co(48 Å)/Ta(x)/Al(13 Å)-oxide/Co(750 Å).

The base pressure was,331026 Pa and the sputtering ofNiFe and Co were done at 0.074 Pa, 2.3 sccm Ar at thedeposition rate of 0.7 Å/s. The NiFe layers were depositedusing an alloying target. The Al–oxide was formed by firstdepositing Al film with a thickness ranging from 13 to 63 Åat 0.7 sccm Ar followed by exposure to an oxygen plasma.For the plasma oxidation, 3.0 sccm Ar at 0.3 Pa and 9.1 sccmO2 at 0.7 Pa was maintained for 5 mins.

The cross pattern for the junction was formed using aa!Electronic mail: he2@hymail.hanyang.ac.kr

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 5 1 MARCH 2001

27520021-8979/2001/89(5)/2752/4/$18.00 © 2001 American Institute of Physics

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.174.255.116 On: Sun, 21 Dec 2014 16:52:43

metal mask and the chamber was vented to change the metalmask. The total area of the TMR junction was 100mm3100mm.

Magnetoresistance loops and current–voltage curveswere measured at room temperature using a probing stationwith a dc current in four point geometry. Magnetic hysteresisloops were measured prior to the patterning, using a vibrat-ing sample magnetometer at room temperature. The chemi-cal composition of the oxide layers was examined by Augerelectron spectroscopy with sputter etch depth profiling. Aninterface study of TMR junctions was performed usingcross-sectional TEM~JEM2010, JEOL, 200 kV!.

III. RESULT AND DISCUSSION

A. Microstructure

Figure 1 shows the cross section and plan view of NiFe/Co/Al-oxide/Co junction with the 13 Å thick oxide layer.The bottom electrode, NiFe/Co, and the top Co layer had apolycrystalline columnar structure. The insulating Al-oxidelayer was found to be amorphous. Examining the plan viewin Fig. 1~b!, NiFe and Co appears to have formed a solidsolution as NiFe and face centered cubic Co have the samelattice constant of 3.54 Å, which agrees with the electrondiffraction shown in the inset.

In Fig. 2 are cross-sectional TEM images of the NiFe/Co/Al-oxide/Co junction with different oxide layer thick-nesses. The Al-oxide layer was flat~shown at higher magni-fication! at 13 Å. However, as the oxide layer becamethicker, although the NiFe, Co/Al-oxide interface remained

flat, the Al-oxide/Co became increasingly wavy. In someplaces, two electrodes were in contact, shorting the junction.To determine the cause of the wavy interface, the junctionwas created without the plasma oxidation process. The inter-face with the Al metal layer remained flat at 63 Å thickness,

FIG. 1. ~a! Cross-sectional transmission electron micrograph forSiO2 /NiFe/Co/Al(13 Å)-oxide/Co and~b! planar view and diffraction pat-tern for NiFe film.

FIG. 2. Cross-sectional transmission electron micrograph for~a!SiO2 /NiFe/Co/Al(13 Å)-oxide/Co,~b! SiO2 /NiFe/Co/Al(43 Å)-oxide/Co,and ~c! SiO2 /NiFe/Co/Al(63 Å)-oxide/Co.

FIG. 3. Cross-sectional transmission electron micrograph forSiO2 /Ta/NiFe/CoFe/Al(13 Å)-oxide/CoFe junction.

2753J. Appl. Phys., Vol. 89, No. 5, 1 March 2001 Kyung et al.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.174.255.116 On: Sun, 21 Dec 2014 16:52:43

which indicates that the plasma oxidation process led to theinterface instability when the oxide layer thickness exceededa critical point.

Shown in Fig. 3 is the cross-sectional view of the NiFe/CoFe/Al-oxide/CoFe junction with a 20 Å Al-oxide layer.Compared to the Co junction in Fig. 2, both interfaces at thetop and the bottom were wavy, but the thickness of the oxideremained relatively constant. As NiFe/CoFe grains grew incolumnar form, as can be seen in Fig. 3, grooves wereformed at the grain boundary to minimize the interfacial en-ergy. In contrast to the NiFe/Co/Al-oxide/Co junction, theAl-oxide grown on top of the grooves stayed comparativelyflat during plasma oxidation. Consequently, both interfacesat NiFe/Co/Al-oxide and Al-oxide/ CoFe were wavy, but re-mained parallel so that the oxide thickness stayed constantalong the junction.

Also investigated is NiFe/Co/Ta-oxide/Co junctionshown in Fig. 4. Due to the slow oxidation rate, the Ta metalwas not fully oxidized and the residual Ta metal layer can beseen at the interface. No significant morphological distortionwas found at the Ta-oxide layer during oxidation process.

The insulation layer remained straight with increasing theinsulation thickness.

Examining the different junctions, depending on the ma-terial chosen for the ferromagnetic and insulation layer, themicrostructure of the interface at the junction was strikinglydifferent.

B. Electrical properties

Figure 5 plots the TMR ratio against the applied mag-netic field for the junctions shown in Figs. 2–4. For theNiFe/Co/Al-oxide/Co junction, as the thickness and rough-ness of the oxide layer increased, the TMR ratio markedlydropped. The maximum TMR ratio of 15% was recorded at13 Å thick Al oxide when the oxide layer was thin and flatwhile at 63 Å, the TMR ratio was only 3%.

In the case of the CoFe junction shown in Fig. 5~b!, theTMR ratio was considerably higher than that of the Co junc-tion. It has been reported that if CoFe is used as the ferro-magnetic material, TMR ratio could increase by 50% due tothe higher spin polarization of CoFe~CoFe,P550%, Co,

FIG. 4. ~a! Cross-sectional transmission electron micrograph forSiO2 /NiFe/Co/Ta(63 Å)-oxide/Co/Pt junction, and~b! high resolutiontransmission electron micrograph of the interface between the top electrode~Co! and Ta oxide. FIG. 5. Tunneling magnetoresistance ratio vs applied magnetic field for~a!

SiO2 /NiFe/Co/Al ~13, 43, and 63 Å!-oxide/Co and~b! SiO2 /Ta/NiFe/CoFe/Al(20 Å)-oxide/CoFe.

2754 J. Appl. Phys., Vol. 89, No. 5, 1 March 2001 Kyung et al.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.174.255.116 On: Sun, 21 Dec 2014 16:52:43

P545%!.8,9 In our case, a TMR ratio of 23% was measuredwhen the oxide layer was 20 Å thick. Similar to the NiFe/Co/Al-oxide/Co junction, the TMR ratio dropped with in-creasing thickness although the interface roughness did notchange significantly. Comparing the two junctions at differ-ent oxide thicknesses, TMR appears to be strongly dependenton the interface thickness and rather insensitive to the inter-face roughness.

A strong dependence of TMR on the insulation thicknessis further confirmed by the NiFe/Co/Ta-oxide/Co junction.Figure 6 shows the TMR ratio as a function of the Ta thick-ness deposited. As can be seen, the TMR ratio was verysensitive to the insulation thickness even though the junctioninterface remained flat as shown in Fig. 4. To maintain adetectable TMR ratio, the Ta thickness had to be maintained

within 11–13 Å. The narrow processing is partially attribut-able to the residual metal layer at the junction and non-stoichiometry of Ta2O5, which would lead to spin scattering.

IV. CONCLUSION

Using cross-sectional TEM, we have observed a differ-ent interface morphology of the TMR junction depending onthe material chosen for the ferromagnetic electrodes and theinsulation layer and demonstrated the strong dependence ofthe electrical properties on the microstructure of interface. Itis critical that the materials for the junction are carefullyselected and processed to ensure optimal microstructure ofthe junction in order to obtain desired performance from theTMR junction.

ACKNOWLEDGMENT

This work was supported by the Korea Science and En-gineering Foundation through the Research Center for Ad-vanced Magnetic Materials at Chungnam National Univer-sity.

1S. S. P. Parkinet al., J. Appl. Phys.85, 5828~1998!.2J. S. Moodera, J. Nowak, and J. M. Van De Veerdonk, Phys. Rev. Lett.80, 2941~1998!.

3R. C. Sousaet al., J. Appl. Phys.85, 5258~1999!.4J. S. Moodera, E. F. Gallagher, K. Robinson, and J. Nowak, Appl. Phys.Lett. 70, 3050~1997!.

5J. S. Moodera and G. Mathon, J. Magn. Magn. Mater.200, 248 ~1999!.6R. E. Dunin-Borkowskiet al., J. Appl. Phys.85, 4815~1999!.7D. J. Smith, M. R. McCartney, C. L. Platt, and A. E. Berkowitz, J. Appl.Phys.83, 5154~1998!.

8R. Meservey and P. M. Tedrow, Phys. Rep.238, 173 ~1994!.9R. J. M. van de Veerddonk, Ph.D. thesis, Eindhoven University of Tech-nology, 1999.

FIG. 6. Tunneling magnetoresistance ratio vs Ta thickness forSiO2 /NiFe/Co/Ta(x Å)-oxide/Co junction.

2755J. Appl. Phys., Vol. 89, No. 5, 1 March 2001 Kyung et al.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.174.255.116 On: Sun, 21 Dec 2014 16:52:43