FePt , and CoPt alloy epitaxial thin films with flat surface grown on MgO ( 111 ) substrate

FePd, FePt, and CoPt alloy epitaxial films of 40 nm thickness are prepared on MgO(111) singlecrystal substrates by employing two different methods. One is a one-step method consisting of hightemperature deposition at 600 °C and the other is a two-step method consisting of low-temperature deposition at 200 °C followed by annealing at 600 °C. Although the preparation method is different, similar final crystal structures are realized for all the film materials. FePd and FePt alloy films grow on the substrates with six L10(111) variants, whose c-axes are about 35° canted from the substrate surface and rotated around the film normal by 60° each other. The order degrees of FePd and FePt films prepared by oneand two-step methods, (Sone-step, Stwo-step), are estimated to be (0.25, 0.33) and (0.08, 0.15), respectively. On the contrary, L10 ordered phase formation is not recognized for CoPt alloy films. The films prepared by one-step method have rough surfaces surrounded by side facets, whereas the films prepared by two-step method have very flat surfaces with the arithmetical mean roughness of 0.3 nm. The two-step method is useful for preparation of L10 ordered films with flat surface.


Introduction
L1 0 ordered FePd, FePt, and CoPt alloys show uniaxial magnetocrystalline anisotropy energies greater than 10 7 erg/cm 3 along the c-axis and the thin films have been investigated for magnetic device applications like recording media, etc. Surface flatness is an important technological issue for practical applications.However in order to achieve a high order degree, it is necessary to employ a high temperature processing.Film deposition at a high substrate temperature tends to enhance the film surface roughness due to migration and clustering of deposited atoms [1].The control of c-axis distribution is also required for fabrication of magnetic film devices.In order to investigate the L1 0 crystal distribution, a welldefined epitaxial film is useful, since the crystallographic orientation can be controlled by single-crystal substrate.FePd [1][2][3][4], FePt [1,[5][6][7][8], and CoPt [1,[9][10][11][12] epitaxial films have been prepared on MgO substrates of (001), (110), and (111) orientations.Most of the films have been prepared by employing elevated substrate temperatures around 600 °C.
In our previous studies [13,14], L1 0 ordered FePd, FePt, and CoPt films were prepared on MgO substrates of (001) and (110) orientations by employing a two-step method; low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C.These films had very flat surface with the arithmetical mean roughness (R a ) less than 0.3 nm.The films formed on MgO(001) substrates consisted of L1 0 (001) crystal with the c-axis normal to the substrate surface and/or L1 0 (100) crystal with the c-axis lying in the film plane.The films formed on MgO(110) substrates involved L1 0 (110) crystal with the c-axis parallel to the substrate surface and L1 0 (011) crystal with the c-axis 45° canted from the perpendicular direction.The order degrees of films formed on MgO(001) and (110) substrates, (S FePd , S FePt , S CoPt ), were (0.63, 0.38, 0.16) and (0.29, 0.21, 0.14), respectively.The surface morphology, c-axis distribution, and order degree will be influenced by the film orientation, since the migration and clustering of deposited atoms vary depending on the surface free energy of crystallographic plane parallel to the substrate surface.In the present study, the two-step method is applied to preparation of FePd, FePt, and CoPt films on MgO(111) substrates.Films are also prepared by using a conventional one-step method consisting of high-temperature deposition at 600 °C.The structural and magnetic properties are compared.

Experimental procedure
A radio-frequency (RF) magnetron sputtering system equipped with a reflection high-energy electron diffraction (RHEED) facility was employed for film formation.The base pressures were lower than 4×10 -7  The surface morphology was observed by AFM.The magnetization curves were measured by using a vibrating sample magnetometer.The notations of crystallographic plane and direction are different between disordered A1 and ordered L1 0 structures.In the present study, A1-based notation is applied to the L1 0 structure for simple comparison with the A1 structure.

Results and discussion
The FePd and the FePt films consist of six L1 0 (111) variants whose c-axes are about 35° canted from the substrate surface and rotated around the film normal by 60° each other.Figure 3 shows the E-scan pole-figure XRD patterns of FePd films prepared by one-and twostep methods measured by fixing the tile and diffraction angles of (D, 2T%) at (35°, 24°), where the scattering vector is 35° inclined from the in-plane and L1 0 (001) superlattice reflection is expected to be detectable.Six reflections which originate from six L1 0 variants are recognized with 60° separation for both films.The pole- The films involve two A1(111) variants whose atomic stacking sequences of close-packed plane along the perpendicular direction are ABCABC… and ACBACB… In ordered to characterize the degree of L1 0 ordering (S) by out-of-plane and in-plane XRDs, it is necessary to calculate the structure factors (F) of crystallographic planes expressed as island nucleation occurs and island-like surfaces involving side facets are observed.The orientations of facets are estimated from the three-dimensional AFM data shown, for example, in figure 6(a-3) to be {111}, {001}, and {011} which have low surface free energies in the L1 0 structure.The R a values of FePd, FePt, and CoPt films are 3.0, 1.2, and 1.4 nm, respectively.Figure 7 shows the AFM images observed for FePd, FePt, and CoPt films deposited at 200 °C.Figure 8 shows the AFM data of films deposited at 200 °C followed by annealing at 600 °C, that is, films prepared by two-step method.The R a values of films before and after annealing are below 0.4 nm and 0.3 nm, respectively.It is clearly shown that a very flat surface is obtained by employing the two-step method.
Figure 9 shows the magnetization curves.The hysteresis curves are almost isotropic in the in-plane measurements for all the films.The easy magnetization axis of L1 0 (111) film does not exist in the film plane, since the c-axis is 35° inclined from the in-plane.In addition, a complex L1 0 variant structure is formed in the film where the easy magnetization axes, [001], of six variant crystals are coexisting.This could be the reason why almost isotropic in-plane magnetization curves are observed for these films.

Conclusion
FePd, FePt, and CoPt alloy epitaxial films of 40 nm thickness are prepared on MgO(111) substrates by employing two different methods; one-step method consisting of deposition at 600 °C and two-step method consisting of deposition at 200 °C followed by annealing at 600 °C.Similar final crystal structures, order degrees, and magnetic properties are observed in the films prepared by both methods for respective film materials.FePd and FePt films consist of six L1 0 (111) variants with the c-axis 35° canted from the substrate surface, whereas ordered phase formation is not recognized in CoPt films.CoPt films consist of two A1(111) variants.The order degrees, (S one-step , S two-step ), of FePd and FePt films are (0.25, 0.33) and (0.08, 0.15), respectively.These films show in-plane magnetic properties.However, the surface flatness is quite different between the two cases.The films prepared by two-step method have very flat surfaces with the R a value lower than 0.3 nm, whereas the film prepared by the one-step method consist of islandlike surfaces involving side facets.The two-step method is found to be useful for preparations of very flat thin film with L1 0 ordered structure.

Figures 1 (
Figures 1(a) and (b), respectively, show the RHEED patterns observed for FePd films and FePt films prepared on MgO(111) substrates by employing the one-and the two-step methods.Clear diffraction patterns consisting of streaks are observed for the FePd and the FePt films prepared by both methods.The streaks indicate that the films have atomically flat terraces.The diffraction patterns are in agreement with the schematic diagrams of diffraction pattern simulated for A1(111) or L1 0 (111) surface shown in figure 2. Superlattice reflections are recognized as shown by the arrows in the intensity profiles of figure 1(a) and (b).The crystal structure is thus determined to be L1 0 .The epitaxial orientation relationship is determined by RHEED as follows,

Thus
or Pt + f Fe or Co ), (f Pd or Pt -f Fe or Co ), fundamental, superlattice, and forbidden, respectively.Figures 4(a, b) and 5(a, b) show the out-of-plane and in-plane XRD patterns of FePd and FePt films prepared by one-and two-step methods.Fundamental (111) and (are observed in the out-of-plane and the in-plane patterns, respectively.Superlattice (1 1 _ 0)+( 1 _ 10) reflections are recognized around 33° in the in-plane patterns.
Pa. FePd, FePt, and CoPt alloy films of 40 nm thickness were prepared on polished MgO(111) substrates by the oneand the two-step methods.Before film formation, substrates were heated at 600 °C for 1 h in the chamber to obtain clean surfaces.The R a value estimated by atomic force microscopy was 0.2 nm (not shown here).Fe 50 Pd 50 , Fe 50 Pt 50 , and Co 50 Pt 50 (at.%) alloy targets of 3 in diameter were employed.The Ar gas pressure during sputtering was kept constant at 0.67 Pa.