Effects of the stacking fault on free surface FePt L1 0
Katia HAMMAR
Université M. Mammeri de Tizi-Ouzou U.M.M.T.O (LPCQ), Algerie Email: [email protected]
Leila MESSAD Ziane Abdelhamid U.M.M.T.O U.M.M.T.O
(LPCQ), Algerie (LPCQ), Algerie Email: [email protected] Email: [email protected]
Abstract—Chemical ordered FePt L10, which is characterized by a tetragonal distortion of a few percent along the c-axis, accompanied by an alternating stacking of elemental layers along the [001] direction, is attractive candidate for many advanced magnetics material applications, such as high density magnetic recording media. The stacking fault effects on free surfaces was examined by first-principles calculations based on density functional theory (DFT), within the pseudopotential plane wave method as implemented in VASP (Vienna Ab initio Simulation Package). The projector augmented wave method with exchange correlation function is used for spin polarized generalized gradient approximation (GGA). The layer atomic relaxation for free surfaces show a compression on the relative displacement amount to about 1.35 %, 9.02 % for Fe and FeFe surfaces respectively, the magnetic moment decrease about 3.90 % for FeFe surfaces.
Keywords—L10 FePt; stacking fault; free surfaces
I. INTRODUCTION
Recently FePt alloys have attracted much interest in tech- nological applications, as future media for magnetic recording and other spintronic applications. L10 ordered FePt thin films with face centered tetragonal (fct) structure [1] (i.e; alternately Fe then Pt stacking along the (001) direction Fig. 1) are chosen as promising candidates of these applications. The high magnetic anisotropy (Ku~7x10-7 erg/cm3) is necessary to insure chemical stability [2].
The proprieties of FePt have been the subject of a number of theoretical and experimental studies[3-13]. However, at FePt nanoparticles the dislocations and the related generalized stacking faults (GSF) affect the magnetocrystalline anisotropy energy [14]. The effect of defects on L10 FePt indicates that the Fe-rich composition has higher saturation magnetization than Pt-rich composition [15]. The molecular dynamics simulations indicate that the vacancy formation energy of the Fe atom in L10 FePt is lower than that of the Pt atom [16]. The FM confi-guration in the (001) direction found as the most stable structure [17], due to the effect of intermixing of Fe and Pt atoms in the FePt layer, the spin accumulation and spin- orbit torques in L10-FePt/Pt bilayers was observed [18], although, structure defects play a significant role on
fluctuations of the saturation magnetization and anisotropy constant of the FePt/Fe bilayer [19].
In this work, we carried out the effect of stacking (planar) defect on the structural and magnetic moment of the free FePt thin film.
The self consistent calculations are carried out using the Vienna Ab initio Simulation Package (VASP) [20,21], based on density functional theory (DFT) [22, 23], within the Perdew Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) [24]. The electron–ion interactions are described using the projector-augmented wave (PAW) method [25]. The energy cut-off of 450 eV is used for the expansion of plane wave basis set.
In order to simulate surfaces, we adopted the symmetrical slab method, each slabs (9 or 11 atomic layers Fig. 2) are periodically reproduced in the three directions, with a vacuum region more than 15 Å in z-axis, chosen to avoid interferences between the neighboring images. The reciprocal space integration is carried out at the (14x14x1) Monkhorst-Pack K- points mesh, the lattice parameters at a'=b'= 2.74 Å and c'=3.74 Å. The structural relaxation was stopped until the forces are in practice smaller than 0.01 eV/Å.
Fig. 1: Structure of ordered L10 FePt: conventional cell (black), primitive cell (blue).
II. RESULTS
Preliminary work on the FePt bulk calculations shows the estimated lattice constants and the bulk properties. The different parameters obtained in this work are optimized:
lattice parameters at a'=b'= 2.74 Å, c' = 3.74 Å, so that c/a = 0.97, it is found experimentally [26] (a'= 2.73 Å, c' = 3.78 Å), the moments given by our calculation are μ(Fe) = 2.976 μB and μ(Pt)=0.353 μB in agreement with the value of Fe (Pt): 2.92μB
(0.33 μB) and 2.78μB (0.32 μB) obtained in [17], [27]
respectively, due to the strong hybridization between Fe (3d) and Pt (5d) states, a large induced magnetic moment in the Pt was obtained.
A. Structural properties
The structures were building by adding layer of the same atoms on perfect surface (ABABA to AABABAA) (Fig.2).
The interlayer separation distances along the z-axis were fully relaxed. the comparison of undefect and defect geometries relaxation (Fig.3) shows that alone the surface and subsurface interlayers distances are relaxed, the contraction of relative displacement is about -1.35 % and 9.02 % for the undefect and with defect surface layers respectively. The relaxation will be relatively large at the fault plane, because the structural arrangement of atom in the defect surface is different than that in the perfect surface.
B) Magnetic moment
The increase in the magnetic moments of the surfaces is related to the reduction in the coordination number. The calculated magnetic moment of perfect surface layer is 3.03 µB
and 2.92 µB for defect surface, in agreement with [18]. This enhanced surface can be explained that the overlap of the electronic states from two neighboring layers could still significantly modify the electronic structures, while the local magnetic moment of Pt do not change significantly in the presence of the stacking fault.
III. CONCLUSION
In this work, we performed the ab initio investigations of the L10 FePt perfect and defect surfaces in the (001) direction, using the slab approach. The relaxation geometries show a contraction in relative displacement. An increasing in the magnetic moment at surfaces layer was found, the magnetic moments of defect surface layers are decreased to about 3.90
% to the corresponding perfect surface, while the local
Fig. 4. Local magnetic moments of perfect (red color) and defect (black) surfaces.
Fig. 2. slabs used in the surfaces calculations in L10 Fept perfect and defect surfaces (Fe red color, Pt grey color).
Fig. 3. Relative displacement of thick layers of perfect and defect surfaces.
magnetic moment of Pt do not change significantly in the presence of the stacking fault.
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