Preparation of anisotropic magnetic FeNiPt2 films on MgO(0 0 1) : Atomistic mechanisms for the interdiffusion of two L10 phases
R.V.P. Montsouka, C. Goyhenex, G. Schmerber, C. Ulhaq-Bouillet, A. Derory, J. Faerber, J. Arabski, and V. Pierron-Bohnes
Phys. Rev. B 74, 144409 (2006)
L10 phase
L10-ordered FeNiPt2(0 0 1) thin films were prepared by the interdiffusion of FePt(0 0 1) and NiPt(0 0 1) layers co-deposited on MgO(0 0 1) substrates by MBE. A large uniaxial magnetic anisotropy (Ku = 9.105 J/m3) and a reduced magnetic transition temperature (Tc = 400 K) were obtained. Growth at 700K and a first annealing at 800K results in a large long-range order parameter reflecting the concentration modulation along the growth direction. This high long-range order parameter is conserved in the FeNiPt2 layers after interdiffusion at 900K, contrary to what is expected from a simple vacancy migration process. This experimental observation can be explained either by a 6-jump cycle mechanism or by the alternate diffusion of a double vacancy, which are both favored energetically over a 2nd-nearest-neighbor jump mechanism or the simultaneous diffusion of a double vacancy as shown by quenched molecular dynamics simulations in the CoPt system.
XRD θ/2θ spectra of a 25 nm NiPt/25 nm FePt/MgO(001) bilayer as deposited (a), after annealing at 800 K during 2 h 30 (b), and after further annealing at 900 K during 12 h ©. The intensities of curves © were divided by 10.
Magnetization hysteresis loops on 25 nm NiPt/25 nm FePt/MgO(001 ) before annealing (a) and after interdiffusion (b) at 900 K during 12 h (we obtain 50 nm NiFePt2/MgO(001)). The curves are recorded by SQUID at room temperature with the external magnetic field perpendicular (easy magnetization axis) or parallel to the film plane (hard magnetization axis). The inset shows the temperature dependence of the magnetization for H=0.11 T after interdiffusion.
6-jump cycle in a CoPt L10 structure (Co is the small atom). The global result of the cycle is the exchange of a Co atom and a 2nd-nn Co-vacancy through the Pt plane. This cycle allows the migration of Co atoms along the z-direction without destroying the L10 long-range order.
Total energy variation for a direct 2nd-nn jump (thin line) and for the 6 jump cycle (thick line). Configurations of the (1 0 0) plane containing the vacancy calculated by quenched molecular dynamics at several stages. The different energy levels are compared to EA = EM = : EB = EL = 1.08 eV ; EC = EK = 0.21 eV ; ED = EJ = 0.89 eV ; EE = EI = 0.37 eV ; EF = EH = 1.40 eV ; EG = 0.53 eV.