Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Thermal Effects in Magnetic Recording Media

J.W. Harrell

MINT Center and Dept. of Physics & Astronomy

University of Alabama

Work supported by NSF-MRSEC

MINT Fall Review, Nov. 2001

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Stability Problem in Granular Media

M M M

Higher densities:

⇒ smaller grain size

⇒ magnetization decay and time dependent coercivity

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Projects

• Zero-field relaxation (NSF-MRSEC) – Shoutao Wang

• Self-assembled L10 magnetic nanoparticles (NSF-MRSEC) – Dave Nikles, Shishou Kang, Shoutao Wang, Min Chen

• Relaxation in perpendicular media (Hitachi Maxell) – Scott Brown

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Zero-Field Relaxation Measurements

Measure decay of remanence at zero field after partial dc demagnetization.

Recently reported for longitudinal thin film media, granular films, and tapes.

(S. Wang)

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Zero Field Relaxation in Longitudinal Media

Relaxation curves for Fuji film (CoCrPtB) of 5 nm thickness (KV/kBT = 29).

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Zero Field Relaxation in Perpendicular Media

-6

-4

-2

0

2

4

6

-1 -0.5 0 0.5 1

measuredcalculated

S0 (

%/d

ec)

mr0

σ = 0.17KV/kT = 38α = -0.07

Fit Parameters:

h = H/Hk = happl+ αmr

σθ= σK=0, σV=0.17,

KV/kT = 38, α=-0.07

Note: (KV/kT)meas = 54 from HCR(t).

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Self-Assembled L10 FePt Nanoparticles

(a) Before annealing: d ~ 3.4 nm, disordered fcc, superparamagnetic

(b) After annealing above 500oC: high anisotropy L10 phase (ordered fct)

(S. Kang, S. Wang, M. Chen, D. Nikles)

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Preparation of FePt Nanoparticles *

(D. Nikles and M. Chen)Particle Synthesis

Reduction of platinum acetylacetonate with 1,2-hexadecanediol and decomposition of iron pentacarbonyl in octyl ether with oleic acid and oleyl amine.

Particle Purification by FractionationEthanol was used to precipitate the nanoparticles from octyl ether.Redisperse in hexanePrecipitate with ethanol

Self-Assembly into Two Dimensional Arrays1:1 mixture of hexane and octane were used to re-disperse the precipitated nanoparticlesPlace a drop on a coated Cu TEM grid (either carbon-coated or silicon oxide coated)Allow to slowly evaporate

* Modification of the procedures reported in S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser Science 2000, 287, 1989-1992

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

M-H Loops of L10 FePt Nanoparticles100

50

0

-50

M x1

0-6

(em

u)

-10 0 10H x10

3 (Oe)

DCD

FePt@500oC

60

40

20

0

-20

-40

-60

M x1

0-6

(em

u)

-10 0 10H x10

3 (Oe)

DCDFePt@520

oC

80

60

40

20

0

-20

-40

-60

-80

Mx1

0-6

(em

u)

-10 0 10Hx10

3 (Oe)

DCDFePt@550

oC

100

50

0

-50

-100

Mx1

0-6

(em

u)

-10 0 10Hx10

3 (Oe)

DCD

FePt @580oC

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Coercivity Ratio

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 0.2 0.4 0.6 0.8 1 1.2

3-D, α = 03-D, α = 0.23-D, α = -0.22-D, α = 0

HC

R/H

C

distribution width

Calculations:

• Log-normal distribution of anisotropy energies

• Mean-field interaction : h = happlied + αm

• No thermal effects

Large HCR/HC ratio in L10 FePt nanoparticles is related to large anisotropy distribution.

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Magnetic Relaxation in Perpendicular Media

1 10 100Time (s)

0

0.1

-0.1

-0.2(b.)

0

0.1

-0.1

0.2

-0.2(a.)

Mom

ent (

mem

u)

TbFeCo – better under-standing of nucleation and domain wall motion processes.

Co/Pd mutilayers – effect of seed layer, B addition, etc.

(S. Brown (MINT), Hitachi Maxell)TbFeCo

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Moke Magnetometer for Perpendicular Media

(Scott Brown)

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Future Work

Zero Field Relaxation:

- Additional measurements and modeling to understand the role of interactions.

Self-Assembled Nanoparticles:

- Reduction of ordering temperature

- Alignment of easy axes

- Origin of anisotropy distribution

Center for Materials for Information Technologyan NSF Materials Science and Engineering Center

Future Work (cont.)

Perpendicular Media:

- Continued investigation of TbFeCo system to better understand the reversal processes.

- Magnetization reversal in Co/Pd multilayers. Effect of seed layer, boron addition.

- Zero field relaxation.