Structure analysis of CoPt alloy film with metastable ordered phases of L 11 and B h formed on Ru ( 0001 ) underlayer

CoPt alloy films of 40 nm thickness are prepared on MgO(111) substrates with and without Ru(0001) underlayer at 300 °C by radio-frequency magnetron sputtering. CoPt films with the close-packed plane parallel to the substrate surface grow epitaxially on the Ru underlayer as well as on the MgO substrate. Flat surfaces with the arithmetical mean roughness value of 0.2 nm are realized for both films. The crystal structure is determined by considering the atomic stacking sequence of close-packed plane and the order degree. The film formed on MgO substrate consists of an fcc-based L11 ordered crystal, whereas the film grown on Ru underlayer involves an hcp-based Bh ordered crystal in addition to the L11 ordered crystal. The order degrees of films formed on MgO substrate and Ru underlayer are 0.30 and 0.34, respectively. The L11 crystal consists of two variants whose stacking sequences of close-packed plane are ABCABC... and ACBACB..., while the Bh crystal is a single-crystal with the stacking sequence of ABAB... Formation of Bh crystal is promoted on the Ru underlayer. The film formed on Ru underlayer shows a strong perpendicular magnetic anisotropy reflecting the magnetocrystalline anisotropies of L11 and Bh crystals.


Introduction
Magnetic thin films with the uniaxial magnetocrystalline anisotropy energy (K u ) greater than 10 7 erg/cm 3 and the easy magnetization axis perpendicular to the substrate surface have been investigated for applications such as recording media and magnetoresistive random access memory devices, etc. CoPt or FePt alloy around the equiatomic composition has an ordered phase of L1 0 .CoPt and FePt bulk crystals with L1 0 structure show K u of 4.9×10 7 [1] and 6.6×10 7  [2] erg/cm 3 , respectively.In order to prepare a CoPt or an FePt film with high K u value, a high degree of L1 0 ordering, which is realized by employing high-temperature processing around 600 °C, is necessary.However, film deposition at an elevated temperature enhances the film surface roughness [3].Surface flatness is an important technological issue for practical applications.High K u film materials which can be prepared at moderate temperatures are required.
Formation of metastable L1 1 ordered phase has been recognized for CoPt alloy films deposited on MgO(111) substrates around 300 °C [4][5][6][7][8].The L1 1 structure consists of alternate stacking of close-packed planes of Co and Pt, as shown in figure 1(a).In the present paper, the fcc-based notations of plane and direction are applied to the L1 1 and the L1 0 structures for simple comparison with the disordered A1 structure, though the accurate structures are rhombohedral and tetragonal, respectively.Furthermore, the unit cell shown in figure 1 the fcc-based structures.An L1 1 CoPt film with the longrange order degree (S) of 0.33 is reported to show K u of 1.7×10 7 erg/cm 3 along the direction normal to the closepacked plane [5].A theoretical calculation suggests that the K u value increases up to 10 8 erg/cm 3 with increasing the S value [9].Therefore, the L1 1 ordered CoPt alloy is one of the strong candidates for a high K u material which can be applicable to future magnetic thin film devices.
The fcc-and the hcp-based structures respectively consist of atomic stacking sequences of ABCABC… and ABAB… along the direction normal to the close-packed plane.The crystal structure is known to easily vary between the two structures through introduction of stacking faults parallel to the close-packed plane.There is thus a possibility that an hcp-based ordered phase of B h shown in figure 1  the substrate or underlayer material [11].Ru has been used as an underlayer to prepare Co-Pt-Cr films with hcpbased disordered A3 structure [11,12].Ru is also expected to stabilize the hcp-based structure for CoPt films.In the present study, a CoPt film is prepared on an Ru underlayer hetero-epitaxially grown on MgO(111) substrate at 300 °C.A CoPt film is also formed directly on an MgO substrate.The detailed film structure and the magnetic property are investigated.

Experimental procedure
Thin films were deposited on polished MgO(111) substrates at 300 °C by using a radio-frequency (RF) magnetron sputtering system equipped with a reflection high-energy electron diffraction (RHEED) facility.The base pressures were lower than 4×10 -7 Pa.Substrates were heated at 600 °C for 1 h in the chamber before film formation to obtain clean surfaces.Figures 2(a) and 3(a) show the RHEED pattern and the atomic force microscopy (AFM) image observed for a substrate after heating, respectively.The RHEED pattern exhibits a Kikuchi pattern, indicating that the surface is clean and smooth.The arithmetical mean surface roughness (R a ) is estimated from the AFM image to be 0.2 nm.Co 50 Pt 50 (at.%) alloy and Ru targets of 3 in diameter were employed.The Ar gas pressure and the distance between target and substrate were kept constant at 0.67 Pa and 150 mm, respectively.The RF powers for Co 50 Pt 50 and Ru targets were respectively fixed at 45 and 60 W, where the deposition rate was 0.02 nm/s for both materials.A 10-nm-thick Ru underlayer and a 40-nmthick CoPt film were sequentially deposited on the substrate.Figures 2(b  The R a value is 0.5 nm.A CoPt film of 40 nm thickness was also deposited directly on the substrate.The CoPt film compositions were confirmed by energy dispersive X-ray spectroscopy and the errors were less than 4 at.% from the target composition. The surface structure was observed by RHEED and the epitaxial orientation relationship was determined.The resulting film structure was investigated by 2ș/Ȧ-scan out-of-plane, 2șȤ/ĳ-scan in-plane, and ȕ-scan pole-figure X-ray diffractions (XRDs) with Cu-KĮ radiation (Ȝ = 0.15418 nm).The cross-sectional microstructure was observed by transmission electron microscopy (TEM).The TEM sample was first thinned mechanically and then ion-milled to be transparent for the electron beam accelerated at 300 kV.The magnetization curves were measured by using a vibrating sample magnetometer.In the present paper, an influence of temperature factor, which is often omitted when comparing intensities of two reflections, is not considered.The intensity ratio of superlattice to fundamental reflection is expressed as where the subscripts of s and f refer to the superlattice and fundamental reflections, respectively.The F values of L1 1 (111), L1 1 (222), B h (0001), and B h (0002) [7,8] are respectively calculated to be 16S(f Co -f Pt ), 16(f Co +f Pt ), 6S(f Co -f Pt ), and 6(f Co +f Pt ), where f is the atomic scattering factor of Co or Pt.Therefore, S is expressed as In order to investigate the atomic stacking sequence of close-packed plane, pole-figure XRD was performed.Figure 7 shows the schematic diagrams of pole-figure XRD patterns of CoPt film with (111) orientation of A1 and/or L1 1 or with (0001) orientation of A3 and/or B h epitaxially grown on MgO(111) substrate or Ru(0001) underlayer.When the rotation angle of ȕ is scanned by fixing the tilt and diffraction angles of (Į, 2șB) at (20°, 43°), six {222} reflections which originate from two variants of A1 and/or L1 1 with three-fold symmetry with respect to the perpendicular direction are recognized with 60° separation for a (111) film [figure 7(  Figures 3(c) and (d) show the AFM images observed for CoPt films deposited on MgO substrate and Ru underlayer, respectively.Flat surfaces are realized.The R a values are 0.2 nm for both films.Figure 10 shows the magnetization curves.These films show strong perpendicular magnetic anisotropies.The magnetic properties are reflecting the magnetocrystalline anisotropies of ordered crystals of L1 1 and/or B h .

Conclusion
CoPt epitaxial films with the close-packed plane parallel to the substrate surface are prepared on an MgO(111) substrate and an Ru(0001) underlayer at 300 °C.The film formed on MgO substrate consists of fcc-based atomic stacking sequence of close-packed plane along the direction, whereas film underlayer involves hcp-based stacking sequence in addition to fcc-based sequence.The order degrees of CoPt films deposited on MgO substrate and Ru underlayer are respectively 0.30 and 0.34.By considering the stacking sequence and the order degree, the crystal structures of CoPt films deposited on MgO substrate and Ru underlayer are determined to be L1 1 and L1 1 +B h .Flat surfaces with the R a value of 0.2 nm are realized for both films.The films show strong perpendicular magnetic anisotropies reflecting the magnetocrystalline anisotropies of L1 1 and/or B h phases.A CoPt alloy film with metastable ordered phase is useful as a high K u magnetic material which can be applicable to future magnetic thin film devices.

Fig. 2 .
Fig. 2. (a, b) RHEED patterns and the schematic diagrams of (a) an MgO(111) substrate and (b) an Ru underlayer deposited on MgO substrate.(c, d) RHEED patterns observed for CoPt films deposited on (c) MgO substrate and (d) Ru underlayer.The incident electron beam is parallel to MgO[11 _ 0].
) and3(b)  show the RHEED pattern and the AFM image observed for an Ru underlayer deposited on MgO substrate.A clear diffraction pattern corresponding to A3(0001) texture is recognized.An

Figures 2 (Fig. 5 .
Figures 2(c) and (d) show the RHEED patterns observed for CoPt films deposited on MgO(111) substrate and Ru(0001) underlayer, respectively.Clear patterns involving streaks are observed for both films.The RHEED patterns correspond to formations of A1(111),

L1 1 (
111), A3(0001), and/or B h (0001) crystals [figures 4(a)-(d)] and do not correspond to a diffraction pattern from L1 0 (111) surface since the streaks shown by the arrows in figure 4(e) are absent.When streaks are included in an RHEED pattern, assignment of crystal structure from a group of A1, L1 1 , A3, and B h is not easy because the RHEED patterns are very similar as shown in figures 4(a)-(d).The structure is thus determined by XRD described later.The epitaxial orientation relationships are estimated by RHEED as follows, {A1 or L1 1 }-CoPt(111)with the close-packed plane parallel to the substrate surface are obtained.The A1 and the L1 1 crystals consist of two (111) variants whose atomic stacking sequences of close-packed plane are ABCABC.. and ACBACB… On the contrary, the A3 and the B h crystals with the sequence of ABAB… are single-crystals.Figures5(a-1) and (b-1) show the out-of-plane XRD patterns of CoPt films deposited on MgO(111) substrate and Ru(0001) underlayer, respectively.CoPt(111) and/or CoPt(0001) superlattice reflections are recognized in addition to CoPt(222) and/or CoPt(0002) fundamental reflections for both films, indicating that L1 1 and/or B h phases are involved.The S value can be estimated by comparing the intensities (I) of fundamental and superlattice reflections.I is proportional to structure (F), Lorentz-polarization (L), and absorption (A) factors[13].

Figure 8 (
b) shows the ȕ-scan patterns of CoPt film deposited on Ru underlayer.Reflections are observed in both patterns.Therefore, the film consists of a mixture of ABCABC…+ACBACB… and ABAB… stacking sequences.hcp-based B h phase is included in the film in addition to L1 1 phase.The hcp-based stacking sequence is apparently promoted on the Ru underlayer.Figure 9(a) shows the cross-sectional high-resolution TEM image observed for the CoPt film grown on Ru underlayer.An atomically sharp boundary is recognized between the film and the underlayer.CoPt crystals with fcc-and hcp-based stacking sequences are apparently coexisting in the film [figures 9(b, c)].