Ultra-thin L10-FePt films of 2 nm average thickness are prepared on (001) oriented MgO and VN underlayers epitaxially grown on base substrate of SrTiO3(001) single crystal. Detailed cross-sectional structures are observed by high-resolution transmission electron microscopy. Continuous L10-FePt(001) thin films with very flat surface are prepared on VN(001) underlayer whereas the films prepared on MgO(001) underlayer consist of isolated L10-FePt(001) crystal islands. Presence of misfit dislocation and lattice bending in L10-FePt material is reducing the effective lattice mismatch with respect to the underlayer to be less than 0.5 %. Formation of very flat and continuous FePt layer on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material at high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. An employment of underlayer or substrate material with the lattice constant and the surface energy larger than those of L10-FePt is important for the preparation of very thin FePt epitaxial thin continuous film with the c-axis controlled to be perpendicular to the substrate surface.

FePt alloy thin films with L10-ordered structure have been investigated for magnetic recording media and MRAM applications. For such applications, the easy magnetization axis, c-axis, must be controlled to be perpendicular to the substrate surface while achieving a high ordering degree and a very smooth flat surface. Epitaxial thin film growth on a substrate with the lattice constant slightly larger than that of L10-FePt crystal has been shown effective for fixing the c-axis perpendicular through an in-plane lattice strain caused in the FePt material.1,2 For L10-ordering, process temperatures higher than 500 °C are required, which enhance surface undulations through by faceting and de-wetting of FePt material on a substrate.3–5 The authors have shown that a two-step process consisting of low temperature film formation followed by high temperature annealing is effective for the preparation of L10-FePt thin film with an improved surface flatness.6,7 When an Fe-Pt alloy thin film is formed on an oxide substrate such as MgO(001) single-crystal at low temperatures where the impinging atom migration is limited, the film morphology tends to be continuous with small surface undulations. By heating the film at a higher temperature, migration of atoms within the film proceeds to form an L10-ordered structure keeping the film morphology. However, when film thickness is decreased to be less than several nanometers, de-wetting of FePt material takes place on MgO substrate forming isolated L10-ordered crystals islands. An employment of underlayer material which has a surface energy larger than that of MgO is effective in reducing the contact angle between an isolated FePt island-like crystal and the substrate. The authors have shown that VC and VN materials with lattice constants similar to that of MgO but with higher surface energies are useful for the preparation of continuous L10-ordered thin films with flat surfaces.8 

In the present study, film growth structures are investigated by employing a high-resolution transmission electron microscope (TEM) for c-axis perpendicularly oriented FePt ultra-thin films of 2 nm in average thickness formed on epitaxial underlayers of MgO(001) and VN(001). The two-step process consisting of low temperature deposition at 200 °C followed by high temperature annealing at 600 °C is employed for L10-FePt(001) film preparation. The morphology and the growth structure of L10-FePt crystal on the underlayers are studied for cross-sectional samples, and the technology for preparing very thin L10-FePt films with the easy magnetization axis controlled to be perpendicular to the substrate surface is discussed.

Table I shows the properties of materials used in the present study. A single-crystal substrate of SrTiO3(001) is employed for epitaxial growth of (001) oriented underlayers of MgO and VN. MgO and VN materials are refractory compounds of NaCl-type crystal structure and have similar lattice constants of 0.42 and 0.41 nm, but with different surface energies of 1.4 and 2.7 J/m2, respectively.9,10 The surface energy of VN is larger than that of FePt (2.1 J/m2)11 which is advantageous in reducing the contact angle between substrate and island formed on the substrate. Lower contact angle is a necessary condition in forming a very thin continuous film on the substrate. The Young’s relationship, γLGcosθ + γSL = γSG, determines the contact angle, θ, between island of deposited material and substrate where γLG and γSG are surface energies of island and substrate materials, and γSL is interfacial energy between the two materials.

TABLE I.

Structural and physical properties of materials used in the present study.

MaterialLattice constant, crystal structureSurface energy9–11 Melting point
L10-FePt a = 0.3842 nm, c = 0.3702 nm 2.1 J/m2 1500 °C 
MgO a = 0.4212 nm, NaCl structure 1.5 J/m2 2800 °C 
SrTiO3 a = 0.3905 nm, Perovskite structure 2080 °C 
VN a = 0.4136 nm, NaCl structure 2.7 J/m2 2050 °C 
MaterialLattice constant, crystal structureSurface energy9–11 Melting point
L10-FePt a = 0.3842 nm, c = 0.3702 nm 2.1 J/m2 1500 °C 
MgO a = 0.4212 nm, NaCl structure 1.5 J/m2 2800 °C 
SrTiO3 a = 0.3905 nm, Perovskite structure 2080 °C 
VN a = 0.4136 nm, NaCl structure 2.7 J/m2 2050 °C 

Thin films were prepared on (001) single-crystal base substrate of SrTiO3 by using a radio-frequency magnetron sputtering system equipped with a reflection high-energy electron diffraction (RHEED) facility. The base pressures were lower than 4 x 10-7 Pa. SrTiO3(001) substrates were heated at 600 °C to clean the surfaces. Fe50Pt50 (at. %), MgO, and VN targets of 3-inch-diameter were employed and the RF powers were set at 43, 200, and 96 W where the deposition rates of FePt, MgO, and VN under an Ar gas pressure of 0.67 Pa were 0.020, 0.015, and 0.020 nm/s, respectively. MgO and VN underlayers of (001) orientation were prepared on SrTiO3(001) base substrates at 600 °C. The epitaxial orientation relationships of MgO(001)[100], VN(001)[100]//SrTiO3(001)[100] were confirmed by RHEED. FePt films were formed on the epitaxial underlayers at 200 °C and then annealed at 600 °C for 1 hour. The thickness of underlayer and FePt materials was fixed at 2 nm. RHEED confirmed that L10-FePt thin films with c-axis perpendicular were epitaxially grown on MgO(001) and VN(001) underlayers in agreement with our previous work.8 

Samples for cross-sectional structure observation were prepared by employing a focused ion-beam sampling technique. TEM observation was carried out by using a Hitachi TEM (HD-2700) equipped with an energy-dispersive X-ray spectroscopy (EDX) facility (AMETEK EDAX Octane T Ultra W) at an acceleration voltage of 200 kV.

Figure 1 compares the cross-sectional structure images of FePt films prepared on MgO(001) and VN(001) underlayers. Isolated L10-FePt crystals of 20 – 50 nm in diameter are formed on the MgO(001) underlayer, whereas a flat and continuous L10-FePt film is formed on the VN(001) underlayer. It is clear that de-wetting of FePt material has occurred on the MgO(001) underlayer during the high-temperature heating process at 600 °C. On the other hand, a continuous film morphology, which was realized by film deposition at 200 °C, has been kept even after the high temperature annealing at 600 °C for the case of VN(001) underlayer.

FIG. 1.

Cross-sectional TEM images of L10-FePt(001) films of 2 nm average thickness formed on 2 nm thick (a) MgO(001) and (b) VN(001) underlayers epitaxially grown on SrTiO3(001) substrates.

FIG. 1.

Cross-sectional TEM images of L10-FePt(001) films of 2 nm average thickness formed on 2 nm thick (a) MgO(001) and (b) VN(001) underlayers epitaxially grown on SrTiO3(001) substrates.

Close modal

Figure 2 shows a typical example of an isolated FePt crystal grown on MgO(001) underlayer. Contact angle of FePt crystal island with respect to MgO(001) surface can be measured from cross-sectional TEM images and the average contact angle is estimated to be 115 + 20 degrees. The high contact angle is due to the large surface energy of FePt material (2.1 J/m2)11 in comparison to that of MgO (1.4 J/m2).9 Using the Young’s relationship and assuming the surface energies of materials as listed in Table I, the interface energy between FePt and MgO is estimated to be 3.3 J/m2.

FIG. 2.

Cross-sectional TEM images of L10-FePt(001) films of 2 nm average thickness formed on epitaxial MgO(001) underlayer grown on SrTiO3(001) substrate. (a) High-resolution image of FePt isolated crystal on MgO(001) layer, (b) bright field TEM image and (c) inverse- FFT TEM image around the layer interfaces. The mark in (c) shows misfit dislocation.

FIG. 2.

Cross-sectional TEM images of L10-FePt(001) films of 2 nm average thickness formed on epitaxial MgO(001) underlayer grown on SrTiO3(001) substrate. (a) High-resolution image of FePt isolated crystal on MgO(001) layer, (b) bright field TEM image and (c) inverse- FFT TEM image around the layer interfaces. The mark in (c) shows misfit dislocation.

Close modal

Figures 2(b) and (c) show high-resolution TEM images around the material interfaces for the FePt/MgO/SrTiO3 sample. Continuous lattice images from the SrTiO3(001) crystal up to the FePt(001) layer indicate that the FePt and the MgO layers are epitaxially grown on the base substrate of SrTiO3(001). Figure 2(c) is an inverse first-Fourier-transformed (FFT) image of the same area shown as the bright field TEM image of Fig. 2(b) where misfit dislocations in L10-FePt(001) film are clearly visualized. Misfit dislocation is observed every 10 – 15 lattice lines of MgO(200), or the dislocation pitch of 0.32 – 0.47 nm-1. The effective lattice misfit is decreased to be less than 0.5 % from the nominal lattice mismatch of 9 % between the two materials calculated by using the bulk lattice constants shown in Table I. The slight lattice bending in MgO and FePt material is apparently reducing the lattice mismatch between the two materials. The crystal lattice (A1) of FePt film prepared on MgO(001) substrate at a lower temperature of 200 °C has been confirmed to be expanded in lateral direction and contracted in vertical direction with respect to the substrate surface.1 The in-plane stress in FePt film with disordered A1 phase, which is effective in enhancing nucleation of c-axis perpendicularly oriented L10-ordered crystal during high temperature annealing process, is almost relieved upon heating the sample at 600 °C through atomic migration within the film. Such atomic movements also cause de-wetting of FePt material from the MgO layer depending on a balance of surface and interface energies of FePt and MgO materials thus forming isolated L10-FePt crystal islands.

Figure 3 shows the elemental distributions measured by using the EDX facility for a cross-sectional FePt (2 nm)/VN (2 nm)/SrTiO3(001) sample. Very sharp compositional interfaces are observed between the stacked materials. The data clearly indicate that atomic diffusions crossing the interfaces are negligible. High-resolution images of bright-field TEM and inverse FFT-TEM are shown in Fig. 4. The diffraction patterns depicted in Fig. 4 are in agreement with those simulated from the respective crystal structures listed in Table I. The images clearly indicate that the 2-nm-thick VN(001) layer is epitaxially grown on the base substrate of SrTiO3(001) and that a 2-nm-thick L10-FePt layer with continuous morphology and with very flat surface is grown on the VN(001) layer. The (100) lattice planes of VN and FePt are slightly bended in some regions, which indicates that the crystal lattices of NaCl structure (VN) and L10-ordered structure (FePt) are flexible enough to absorb the lattice mismatches of +5.9 % and -7.1 % existing between these materials under conditions with the thickness as small as 2 nm. Furthermore, similar to the case of FePt/MgO interface, misfit dislocations are recognized in the FePt layer as well as in the VN layer around the FePt/VN interface. The effective lattice mismatch between FePt and VN layers is also decreased to be very small.

FIG. 3.

Elemental distributions measured for FePt (2 nm)/VN (2 nm)/SrTiO3 sample. (a) Annular dark field TEM image of cross-sectional sample, (a-1) - (a-6) elements (Fe, Pt, Sr, V, N, and O) distributions, and (b) combined image of Fe, V, and Sr elements distribution. By referring the scale bar of 2-nm, atomic diffusion crossing the interfaces is considered to be negligible.

FIG. 3.

Elemental distributions measured for FePt (2 nm)/VN (2 nm)/SrTiO3 sample. (a) Annular dark field TEM image of cross-sectional sample, (a-1) - (a-6) elements (Fe, Pt, Sr, V, N, and O) distributions, and (b) combined image of Fe, V, and Sr elements distribution. By referring the scale bar of 2-nm, atomic diffusion crossing the interfaces is considered to be negligible.

Close modal
FIG. 4.

High-resolution TEM image around the material interfaces observed for FePt (2 nm)/VN (2 nm)/SrTiO3 sample. (a) Bright field TEM image and (b) inverse-FFT-TEM image. The mark in (b) shows misfit dislocation.

FIG. 4.

High-resolution TEM image around the material interfaces observed for FePt (2 nm)/VN (2 nm)/SrTiO3 sample. (a) Bright field TEM image and (b) inverse-FFT-TEM image. The mark in (b) shows misfit dislocation.

Close modal

Magnetic measurements using a vibrating sample magnetometer indicated that the L10-FePt(001) films formed on VN(001) layer possessed strong perpendicular magnetizations of Mr/Ms = 0.8 - 1 even under an influence of strong demagnetization coming from a shape anisotropy with the demagnetization factor close to unity.8 Relatively low perpendicular coercivity, Hc = 1.2 - 2.0 kOe, indicates that magnetic domain wall motion is dominant in the FePt(001) thin films of extremely thin thickness (2 nm). The present results indicate that VN is a practical underlayer material for the preparation of L10-FePt film with c-axis perpendicular, and particularly for preparation of very thin continuous films of less than several nanometers.

Formation of L10-ordered structure with FePt material on a substrate involves atomic movements which influences the resulting film structure in both the crystallographic orientation and the film morphology. Although FePt material has a high melting point of 1500 °C, a substrate temperature around 600 °C is possible in forming films with L10-ordered structure. In order to fix the crystallographic orientation, c-axis, to be perpendicular and to keep the surface flatness, employment of material interaction with the substrate or the underlayer is necessary. An employment of substrate material where the in-plane lattice constant is slightly larger than that of in-plane lattice constant of L10-FePt crystal has been confirmed useful in enhancing the c-axis perpendicularly oriented L10-crystal nucleation under an influence of lateral stress in the film caused by the lattice mismatch with the substrate.1 In addition, the surface energy of substrate material needs to be larger than that of FePt material for reducing the contact angle, which is necessary for the preparation of continuous thin film, in particular for the thickness less than several nanometers, where a possibility of FePt material de-wetting from the substrate apparently increases. VN is a refractory compound with high melting temperature of 2050 °C, the lattice constant 7 % larger than that of in-plane lattice parameter of L10-FePt crystal, and with the surface energy larger than that of FePt. The crystallographic quality of VN thin film prepared by sputter deposition is good even for a very thin layer of 2-nm-thickness as revealed in the TEM images of Fig. 1(b) and Fig. 4. The present study has shown that VN is a practical substrate material in forming very thin L10-FePt films with the c-axis controlled to be perpendicular to the substrate surface and with continuous flat film morphology.

Morphology and crystal structure of very thin L10-FePt(001) film of an average thickness of 2 nm prepared on (001) oriented underlayer materials of MgO and VN are investigated by high-resolution TEM. The materials have a common NaCl-type crystal structure with similar lattice constants but with different surface energies. Isolated L10-FePt(001) crystals are formed on MgO(001) layer whereas a continuous L10-FePt(001) film is formed on VN(001) layer.

TEM observation has shown clearly that L10-ordered FePt(001) films are epitaxially grown on the (001) oriented underlayers involving misfit dislocations and slight lattice bending in the FePt and the underlayer materials. The effective lattice mismatches of L10-FePt(001) crystal with respect to MgO and VN underlayers are estimated to be very small, which is related to the introduction of misfit dislocation and the lattice bending. Formation of flat and continuous FePt film on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material during high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. The present study has shown that an employment of substrate material with the lattice constant and the surface energy larger than those of L10-FePt is very important for preparation of very thin L10-FePt epitaxial thin films with the c-axis controlled to be perpendicular to the substrate surface.

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