Alignment of <i>c</i>-Axis Orientation Perpendicular to the Substrate Surface in FePt Alloy Thin Film with <i>L</i>1₀ Structure

Abstract

An FePt alloy thin film with L10 structure formed on a (001)-oriented polycrystalline underlayer or on a (001)-oriented single-crystal substrate may involve (100) variant with the c-axis lying in the film plane in addition to (001) variant with the c-axis normal to the substrate surface. In the present study, FePt(2-40 nm) films with and without MgO(2 nm) cap layers are prepared on MgO(001) single-crystal substrates by employing a two-step formation method consisting of low-temperature deposition followed by high-temperature annealing. Disordered FePt(001) single-crystal films with and without MgO(001) single-crystal cap layers epitaxially grow on the substrates at 200 °C. For the films thinner than 10 nm, the in-plane and out-of-plane lattices are respectively expanded and shrunk due to accommodation of the lattice misfit of film with respect to the substrate. With increasing the thickness up to 40 nm, the strain is relaxed. The phase transformation from A1 to L10 occurs when the films are annealed at 600 °C. The cap layer seems to prevent the strain release and to promote the phase transformation along the perpendicular direction. The c-axis orientation is controlled to be perpendicular by using the two-step method for the FePt films of thicknesses less than 10 nm with MgO cap layers. On the contrary, the film of 40 nm thickness with cap layer and the films of 2-40 nm thicknesses without cap layers involve small volumes of L10(100) variant. The order degrees of films with cap layers are higher than those of films without cap layers. The films with cap layers have flat surfaces with the arithmetical mean roughness values lower than 0.3 nm for all the investigated thicknesses, whereas an island-like surface is formed for the 2-nm-thick film without cap layer. The film consisting of only L10(001) variant shows a strong perpendicular magnetic anisotropy and a very low in-plane coercivity. Introduction of MgO cap layer is useful in aligning the c-axis perpendicular to the substrate surface, enhancing the L10 ordering, forming a flat surface, and achieving a strong perpendicular magnetic anisotropy.

Growth of <i>L</i>1₀-ordered Crystal in FePt Epitaxial Magnetic Thin Films on (001) Oriented Substrates

“…8,15,16,19) The magnetic properties of these samples are also reported in the references. 16,20) Cross-sectional samples were prepared by employing a focused ion-beam sampling technique.…”

“…The result that L10-FePt(010) variant with the c-axis lying in in-plane is observed away from the FePt/MgO interface (A region) is in agreement with the previous works carried out by using XRD techniques. 7,15) Figure 10 shows the inverse-FFT high resolution TEM image of FePt/VN/SrTiO3(001) sample which is filtered in a similar way to that of Fig. 9.…”

“…12,13) The authors have confirmed that the two-step process is effective for not only the preparation of c-axis perpendicularly oriented thin films with good crystallographic quality but also for the preparation of L10-ordered films with very flat surfaces. 14,15) However, when the film thickness was decreased to be less than several nanometers, which would be the thickness range of L10-ordered FePt film for future device applications, de-wetting of FePt material took place on MgO substrate and the film morphology became discontinuous consisting of L10-ordered crystal islands. 7,15) The volume fraction of c-axis in-plane oriented L10-crystal increased when de-wetting took place.…”

“…14,15) However, when the film thickness was decreased to be less than several nanometers, which would be the thickness range of L10-ordered FePt film for future device applications, de-wetting of FePt material took place on MgO substrate and the film morphology became discontinuous consisting of L10-ordered crystal islands. 7,15) The volume fraction of c-axis in-plane oriented L10-crystal increased when de-wetting took place. In order to suppress de-wetting, an employment of underlayer material with the surface energy larger than that of MgO was considered effective in reducing the contact angle between an isolated FePt island-like crystal and the substrate.…”

Development of Media Nanostructure for Perpendicular Magnetic Recording

“…Figure 21 An another possibility to prepare L10(001) oriented magnetic thin films with flat surfaces is an employment of a thin overcoat on top of the magnetic thin film prepared by using the two-step method 191,192) . A thin overcoat, 2 nm thick MgO layer was formed at 200 °C on FePt/MgO(001) samples with different FePt film thicknesses before annealing at 600 °C.…”