Magnetic Anisotropy in LSMO Thin Films

Magnetic Anisotropy in LSMO Thin Films Rachel Luo Mentors: Jodi Iwata, Franklin Wong Suzuki Lab

Purpose and Applications • Important in information storage Bits High magnetic anisotropy – stable magnetic memory Sensing material

A Easy to measure current

Image from http://nylander.wordpress.com/ 2006/07/15/horseshoe-magnets/

Crystal Structure z

Perovskite: ABO3 (LaAlO3) 110 plane

x z

100 plane A (La)

y

B (Al) O x

Thin-film Growth • Substrate: LaAlO3 (LAO) • Film: La0.7Sr0.3MnO3 (LSMO) • Film’s inclination to conform to substrate’s parameters creates a strain on the film

Heteroepitaxial Strain • Strain on film leads to compression in plane and tension out of plane

substrate

X-ray Diffraction Data θ

d

(100) substrate

1000000 100000 10000 1000 100 10

40

45

50

2θ (degrees)

(110) substrate

10000

Intensity (Counts)

Intensity (Counts)

Bragg’s Law: nλ = 2d Sin(θ)

θ

55

1000

100

10

60

70

2θ (degrees)

80

Heteroepitaxial Strain Unstrained values (Å) 2.74

Obtained values (Å) 2.77

d110 substrate

2.68

2.68

d100 film

3.88

3.99

d100 substrate

3.79

3.79

d110 film

Ferromagnetism and Anisotropy Easy axis

M

High anisotropy H

M

Hard axis

MR

MR

H

MR: Remanent magnetization (Magnetization after the applied field is taken away)

H

Vibrating Sample Magnetometer

H

3

Remanent Magnetization (emu/cm )

3

Remanent Magnetization (emu/cm )

Remanent Magnetization (295 K) 140 120

110 out-of-plane orientation

100 80 60 40 20 0

[110]

20

40

60

80 100 120 140 160 180 200 220

140 120 100 80 60 40 20 0

[110] in-plane orientation 0

50

100

150

200

250

300

350

θ (degrees)

θ (degrees) [110]

[110]

[001]

[110]

[001]

400

SQUID • Measures ƒ Magnetic moment vs. applied magnetic field ƒ Magnetic moment vs. temperature

• High sensitivity • Can take measurements at low temperatures ƒ More obvious anisotropy

Magnetic Anisotropy at Low Temp In-plane T=5K

300

3

Magnetic Moment (emu/cm )

(100) LaAlO3

200

Out-of-plane T = 10 K

100

[100] out-of-plane x

z

0

[010] in-plane

-100

y

-200 -300 -60000 -40000 -20000

0

20000

Magnetic Field (Oe)

40000

60000

Magnetic Anisotropy at Low Temp

3

Magnetic Moment (emu/cm )

(110) LaAlO3 [110] direction T=5K

800 600

[001] direction T = 10 K

400 200 0 -200

y

-400

[110] x

[110]

[001]

-600

z

-800 -15000

-7500

0

7500

Magnetic Field (Oe)

15000

Sources of Magnetic Anisotropy • Shape anisotropy Easy plane

x

z

Easy axis [100] [010] y

• Crystalline anisotropy • Strain-induced anisotropy

[110] x y

Easy axis [110] [001] z

Magnetoresistance MR ( H , T ) =

ρ ( H , T ) − ρ (0, T ) ρ (0, T )

(110) LaAlO3 0.008

Resistivity (Ω−cm)

0.007

H = 0 Oe

0.006 0.005 0.004 0.003 0.002 0.001 0.000

0

50

100 150 200 250 300 350 400

Temperature (K)

Magnetoresistance (%)

0 -5 -10 -15 -20

H = 50000 Oe

-25 -30 0

50

100 150 200 250 300 350 400

Temperature (K)

Summary • Easy axis of (100) LaAlO3: out-of-plane [100] direction • Tension causes this direction to become the easy axis. x

z

[100] out-of-plane

y

• Easy axis of (110) LaAlO3: in-plane [110] direction • Compression allows this direction to become the easy axis. [110]

Conclusions • Strain is very important in the anisotropy of thin-films. • Overall strain on the LSMO thin-film: almost 2.5% • In the future: quantitative analysis of the sources of anisotropy

Acknowledgements Professor Chang-Hasnain, Avi, Evy, Amy, Franklin, Jodi, the Suzuki Lab, Dave, Pat

Double Exchange Mechanism behind negative magnetoresistance in La0.7Sr0.3MnO3. Mn3.3+

E

3d levels Mn3+

Mn4+

Ferromagnetism and better moment alignment enhance conductivity!