Biaxial exchange anisotropy in PtMn/Ni80Fe20(110) bicrystal films

Biaxial exchange anisotropy in PtMn

Õ

Ni

Fe

„

110

…

bicrystal films

Physics Department, National Cheng-Kung University, Tainan, Taiwan, Republic of China C. H. Lee

Department of Engineering and System Science, National Tsing-Hua University, Hsinchu, Taiwan, Republic of China

By using molecular beam epitaxy PtMn/Ni80Fe20bilayers were prepared on Mo共100兲 seeding layer on MgO共100兲 substrate. X-ray diffraction showed that the PtMn/Ni80Fe20 bilayers were mainly grown as 共110兲 bicrystal films with 90° twin domains. The exchange anisotropy was studied by angular dependent magneto-optical Kerr effect. The results indicate that the PtMn/Ni80Fe20共110兲 bicrystal films possess biaxial exchange anisotropy with exchange field up to 130 Oe. © 2000

American Institute of Physics. 关S0021-8979共00兲35708-5兴

I. INTRODUCTION

The exchange bias effect caused by magnetic coupling across an antiferromagnetic–ferromagnetic interface has at-tracted much attention as it plays a key role in spin valve sensors.1,2Various Mn based antiferromagnetic共AF兲 alloys, such as FeMn, NiMn, and PtMn, have been used as exchange biasing layers in spin-valve structures.3–5 PtMn and NiMn exhibit higher blocking temperatures and are more corrosion resistant than FeMn. Since the AF phase of PtMn is the chemically ordered L1₀-type structure 共similar to NiMn兲,6 the exchange coupling of PtMn/Ni₈₀Fe₂₀ is expected to be sensitive to the crystal structure. In this article the exchange anisotropy of the as-deposited PtMn/Ni80Fe20共110兲 bicrystal films is studied. Biaxial exchange anisotropy across the PtMn/Ni80Fe20 interface was found. The result is explained

by the existence of bicrystal structure in the

PtMn/Ni80Fe20共110兲 films.

II. SAMPLE PREPARATION

The samples studied here were prepared by a molecular beam epitaxy system 共Vacuum Product made MBE-930兲.7 The PtMn/Ni80Fe20films were grown at 200 °C on epitaxial-grade MgO共100兲 substrates. Before initial deposition of the PtMn(100– 500 Å)/Ni₈₀Fe₂₀(100 Å) bilayers, about 100-Å-thick Mo共100兲 layer was established as a seeding layer.8The deposition rates of the Ni₈₀Fe₂₀and PtMn were controlled at about 0.1 and 0.3 Å/s, respectively. During deposition of the PtMn and Ni₈₀Fe₂₀layer, the growth pressure was controlled below 2⫻10⫺8Torr.

The crystal structure was studied by x-ray diffraction 共XRD兲 and reflection high-energy electron diffraction 共RHEED兲. The correlation between magnetic and crystal structure was investigated by magneto-optical Kerr effect 共MOKE兲. Because the penetration of the MOKE 共He–Ne兲 laser light is quite limited, only samples with thin 共less than 150 Å兲 PtMn layer can be probed by MOKE.

III. RESULTS AND DISCUSSIONS

On MgO共100兲 substrate the Mo seeding layer was grown as bcc 共100兲, and the subsequent Ni80Fe20 and PtMn layers were mainly grown as 共110兲 structure with,

however, relatively weaker PtMn共001兲 and 共002兲

peaks, as evidenced by XRD shown, for example, in Fig. 1. The main out-of-plane epitaxial relations are PtMn共110兲储Ni80Fe20共110兲储Mo共100兲储MgO共100兲. Note that the PtMn/Ni80Fe20共110兲 films were not single crystal, but were grown as a bicrystal structure, with two equal abundance domains 90° apart. The bicrystal structure was verified by detailed in-plane x-ray diffraction.9 The main in-plane epi-taxial relations of the epilayers and substrate were thus de-termined as the following:

PtMn关001兴储Ni₈₀Fe20关001兴储Mo共010兲储MgO共010兲, PtMn关1-10兴储Ni80Fe20关1-10兴储Mo共001兲储MgO共001兲, or

PtMn关1-10兴储Ni₈₀Fe20关1-10兴储Mo共010兲储MgO共010兲, PtMn关001兴储Ni80Fe20关001兴储Mo共001兲储MgO共001兲.

a兲_{Author to whom correspondence should be addressed; electronic mail:}

[email protected]

FIG. 1. X-ray diffraction scanned from PtMn(500 Å)/

Ni80Fe20(100 Å)/Mo(100 Å) films grown on MgO共100兲 substrate.

JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000

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0021-8979/2000/87(9)/4921/3/$17.00 © 2000 American Institute of Physics

Typical RHEED images of the Mo, Ni80Fe20and PtMn lay-ers are provided in Figs. 2共a兲–2共f兲. The Ni80Fe20共110兲 and PtMn共110兲 surfaces were rougher and more disordered than that of the Mo共100兲. This is likely due to the formation of twins in the PtMn/Ni80Fe20bilayers and high surface energy of the共110兲 surface. The splitting of the RHEED streaks in Figs. 2共d兲 and 2共f兲 together with the fourfold symmetry also indicate the existence of twinned structure 共bicrystal兲 in the PtMn/Ni80Fe20共110兲 films. Note that the growth of 90° twin structure 共bicrystal兲 in PtMn/Ni80Fe20共110兲 bilayer is due to the mismatch of crystal symmetry between the Mo共100兲 共fourfold兲 and PtMn/Ni80Fe20共110兲 共twofold兲 plane, as sche-matically illustrated in Fig. 2共g兲. X-ray diffraction studies further suggest that the PtMn layer was grown as a partially ordered, tetragonal structure with lattice parameters a ⫽3.97 Å and c⫽3.66 Å.9

The exchange anisotropy of the PtMn/Ni80Fe20共110兲 bi-layers was measured by angular dependent MOKE technique 共magnetic field fixed and sample rotated兲. For MOKE laser light focused on small sample area, bidirectional exchange anisotropy was observed as shown in Figs. 3共a兲–3共h兲. For magnetic field H directed parallel to the underlying as-defined Mo关010兴 or Mo关001兴 azimuth (␪⫽0°,90°), the

ex-change fields were of about⫺130 Oe, while for H along the Mo关0-10兴 or Mo关00-1兴 azimuth (␪⫽180°,270°), the ex-change fields were of about⫹130 Oe. For these four princi-pal azimuths (␪⫽0°,90°,180°,270°), the MOKE hysteresis loops were somewhat asymmetric共with respect to the loops’ center兲 because they contain both magnetic easy and hard components. The easy component 共shifted to left or right from H⫽0兲 is likely due to the magneto-crystalline anisot-ropy and exchange coupling effect, while the hard compo-nent 共symmetric to H⫽0兲 is caused by the easy axis in the perpendicular direction. For magnetic field directed between Mo关010兴 and 关001兴 azimuths 共␪⫽0° to 90°兲, the MOKE hys-teresis loops 关see, for example, Fig. 3共b兲 for ␪⫽45°兴 were somewhat similar to those of Figs. 3共a兲 and 3共c兲, where the

M-H loops of these azimuths shifted to the negative field. A

similar trend was found for magnetic field directed between Mo关0-10兴 and 关00-1兴 azimuths 共␪⫽180° to 270°兲. In these cases the M-H loops shifted to the positive field 共see, for example, Fig. 3共f兲 for␪⫽240°兲.

For field directed between Mo关001兴 and 关0-10兴 azimuths (␪⫽90° – 180°), on the other hand, the MOKE hysteresis loops were more symmetric like共with respect to H⫽0兲 with-out any significant exchange field 共see, for example, Fig. 3共d兲 for␪⫽135° or Fig. 3共b兲 for␪⫽315°兲. The same situa-tion was found for field directed between Mo关001兴 and 关0-10兴 directions (␪⫽270° – 360°).

As mentioned above, the bidirectional exchange anisot-ropy was found if the laser beam is well focused. For MOKE laser beam shined on a large sample area, biaxial exchange anisotropy, namely dual exchange bias loops, can be ob-served for field along any one of the principal easy axes (␪ ⫽0°,90°,180°,270°). The results suggest that there is an equivalent amount of 0°, 90° and 180° and 270° antiferro-magnetic domains in the PtMn/Ni80Fe20共110兲 bicrystal films. In addition, these antiferromagnetic domains are rather small compared to the unidirectional exchange bias sample.

Obviously, the bidirectional or biaxial magnetic ex-change anisotropy reported here is directly related to the

bi-FIG. 2. RHEED patterns for共a兲, 共b兲 100 Å Mo seeding layer on MgO共100兲 substrate, and subsequent共c兲, 共d兲 100 ÅNi80Fe20, and共e兲, 共f兲 120 Å PtMn

layers. The RHEED beam was directed parallel to the underlying Mo关010兴 for共a兲, 共c兲, and 共e兲, and along Mo关⫺110兴 for 共b兲, 共d兲, and 共f兲. The two-dimensional unit cells of Mo共100兲 共solid lines兲 and PtMn/Ni80Fe20共110兲 共dashed lines兲 planes and epitaxial relations are schematically illustrated in共g兲.

FIG. 3.共a兲–共h兲 The angular dependent MOKE hysteresis loops as a function of the azimuthal angle␪for PtMn(120 Å)/Ni80Fe20(100 Å) bilayers grown on Mo共100 Å兲 on MgO共100兲 substrate.␪is defined as the angle between the magnetic field and Mo关010兴 azimuth.

4922 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Huang, Yu, and Lee

crystal structure in the PtMn/Ni80Fe20共110兲 film. For com-parison we have indeed prepared PtMn/Ni₈₀Fe20共111兲 single crystal films 关on Mo共110兲 plane on sapphire共11–20兲 sub-strate兴. In this 共111兲 case, a relatively weak, unidirectional or uniaxial exchange anisotropy was observed.10

Finally we point out that the PtMn/Ni80Fe20共110兲 bicrystal films could be somewhat similar to the PtMn/Ni80Fe20共100兲 共uncompensated plane for spin align-ment兲 in terms of crystal symmetry and magnetic behavior. The fact that the exchange field is relatively small in the PtMn/Ni80Fe20共111兲 films compared to that of the 共110兲 bic-rystal samples implies that uncompensated plane关共100兲 like兴 may be beneficial to exchange coupling effect in this system. Note that the orientation dependent results for PtMn/Ni₈₀Fe₂₀ are very different from those of PtMn/Ni80Fe20 and CoMn/Ni80Fe20 cases, where for the latter two cases the 共111兲 oriented films show the best exchange coupling effect and the uncompensated plane seems not to result in better exchange coupling effect.11

ACKNOWLEDGMENT

The authors are grateful for the financial support of the ROC NSC under Grant Nos. 88-2112-M-006-009 and 89-2112-M-006-024.

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J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Huang, Yu, and Lee