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Origin of Thickness Dependent Spin Reorientation Transition of B2 Type FeCo Alloy Films. Dongyoo Kim. Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden. Introduction. Chemical composition : 0.55 ≤ x ≤ 0.65.

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Origin of Thickness Dependent Spin Reorientation Transition of B2 Type FeCo Alloy Films

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Origin of Thickness Dependent

Spin Reorientation Transition

of

B2 Type FeCo Alloy Films

DongyooKim

Applied Materials Physics, Department of Materials Science and Engineering,

Royal Institute of Technology, Stockholm, Sweden.


Introduction

Chemical composition : 0.55 ≤ x ≤ 0.65

Tetragonal distortion : 1.2 ≤ c/a ≤ 1.25

KU = 700 ~ 800 µeV/atom

MS = 2.1 µB/atom


Introduction

about 15 MLs

SRT at 15 ML


Introduction


To account SRT of FeCo films at 15 ML

 We have considered film structures.

MCA of bulk FeCo cannot explain SRT of FeCo at 15 ML.

We calculated FeCo film with coverage of 6, 8, 10, 12, 14,

and 16 ML thickness

Purposes

Giant MCA Energy

Chemical

Composition

Tetragonal

distortion


Calculation Method

FLAPW method

(Full potential linearized augmented plane wave)

<Calculated Systems>

Exchange correlation potential : GGA

Spherical harmonics (lmax ) : 8

Energy cut-offs : 225 Ry and 13.7 Ry

Muffin-tin radius of Fe and Co atoms : 2.2 a.u.

Lattice Constant: 2.866 Å

(experimental value, [PRB, 64, 132102 (2001)])

K-points: 420 points

The optimized atomic structure in vertical

direction

 Force and total energy minimization procedure.


Structural Features

Calculated interlayer distance (in Å)

Thick

Thin

c/a = 1.07


Magnetic Moment

Calculated spin magnetic moment (in µB) in MT region.


Magnetization

Linearly increasing Ms


MAE in Thin Film Structures

Eㅗ

EMCA = E//– Eㅗ

E//

MAE = EMCA - ESh


Magnetic Anisotropy Energy

About 15 ML

EMCA = Torque method [1]

Esh = ½ µ0Ms2

Positive value: Perpendicular MCA

EMCA = E//– Eㅗ

Negative value: in-plane MCA

[1] X. D. Wang, R. Q. Wu, D. S. Wang, A. J. Freeman, Phys, Rev, B 54, 61 (1996)


MCA Energy in Thin Film Structures

Eㅗ

[J. Hong, et. al, PRL, 92, 147202 (2004)]

E//

K = KV + 2KS/d

KV : Volume contributions

KS : Surface contributions

d: Film thickness


Magnetic Anisotropy Energy

15 ML

Average Esh

=

~ 90 ML

Kv = 142.42 µeV/atom

Ks = 132.61 µeV/atom

K = KV + 2KS/d

Kv = 67.04 µeV/atom

Ks = 594.7 µeV/atom


MCA Energy – FeCo (3ML)

[FeCo(3ML) /Pt(7ML)]23


MCA Energy – FeCo (3ML)

K = KV + 2KS/d

K = KV + 2KS/d

Kv = 142.42 µeV/atom

Ks = 132.61 µeV/atom

Cal: EMCA = 227.82 µeV/atom at FeCo(3ML)

Exp: EMCA = (208±14) µeV/atom at FeCo(3ML)


We have investigated the thickness dependent magnetic anisotropy of B2 FeCo Films.

FeCo films show perpendicular MCA,

but MCA energy rapidly decrease as the film thickness increase.

The crossover of shape and MCA energies occurs at approximately 15 ML thickness.

 This agrees well with many experimental observations.

The competition of shape and MCA energies can nicely account for universal behavior

of thickness dependent SRT of FeCo alloy films

Summary


Thank You


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