Question

FeCo alloy thin film is a fundmental material for the STT-MRAM. It is a FCC structure when disordered and L10 structure when 100% ordered. For the perpendicular magnetic anisotropy, L10 structure with FeCo(100) perpendicular to the surface is preferred. I prepare the thin film by magnetron sputtering. Suppose I should prepare the FeCo alloy with atomic ratio of 1:1 in an ideal case. However, due to poor preparation of co-sputtering, the atom is 5:3 (iron rich FeCo thin film), the structure of Fe rich thin film is noy known. Anonalous X-ray scattering was perform to solve this issue. Three kind of models is proposed. In the first model, the addition Fe is segregated form the order FeCo (1:1) L10 structure, and fill in the grain boundaries. The second model is totally disorder FCC structure (order parameter is zero) and addition Fe is dissolved into a solution. The third model is Fe atom replaces 1/4 part of Co atom positions in the L10 structure.

(a) Please calculate the diffraction intensity ratio of FeCo(001)/FeCo(002) for these three models using 12 KeV X-rays. To simplify the question, we assume the atiomic structure factor is constant (usually, atomic structure factor is a function of q) $f_F_e=26$, $f_C_o=27$. In addition, all the FeCo alloys are in cubic structure and the lattice constants are all the same from 0-100% ordering

(b) In order to gain the sensitivity of determining the order parameter, I charge the energy of X-ray form 12 KeV to the K-edge of Fe (7.112 KeV). The atomic structure factor of Fe is changed by adding the dispersive factor of $f'$, which is around 5. Please also calculate the X-ray diffraction intensity ratios of FeCo(001)/FeCo(002) of the FeCo alloy for these three models, Hint: $f=f_0-f'$ and this method called anomalous X-ray scattering method.

(Please, answer All two question please , the type format is the best)

Answer 1

1. For a diffraction intensity the intensity is directly proportional to the structure factor ( F )

I $\alpha$ F²

so calculating different value of structure factors for all three models.

1 . for 1st model

F = f_{( FeCo)} e^{i [ 2$\pi$ ( h_x + k_y + l_z ]}

also f_{( FeCo)} = f_Fe + f_Co = 26+27 = 53

so F for FeCo ( 001 ) is

F = 53 e^{i [ 2$\pi$ ( h(0) + k(0) + l(1)]} = 53 e^{i [ 2$\pi$( l )]} ( h+k+l = odd ) so BCC

for FeCo ( 002 )

F = 53 e^{i [ 2$\pi$ ( h(0) + k( 0) +l (2)]} = F = 53 e^{i 2$\pi$(2l)}

2. for 2nd model ( FCC )

the structure value is given as F = f ( 1+e^{i$\pi$ ( h+k)} + e^{i$\pi$( k+ l )}+ e^{i$\pi$( l + h )}

for FeCo ( 001) F = 53 ( 1+ e^{i$\pi$(h + k)(0)}+ e^{i$\pi$(k+l)(0)}+e^{i$\pi$(l + h)(1)}

similarly find out for FeCo ( 002) coordinate

3. for third model the Fe atom is replaced by 1/4 th of Co atoms

so f_Fe = 1/4 x 27 =6.75 so f_FeCo = 6.75 + 27 = 33.75

Now the structure value of F can be calculated as the 1st model just by putting the value of f_FeCo = 33.75

2. as the value of f^' = 5 so,