Thermal Effects in Magnetic Recording Media

Transcription

1 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. 21

2 Stability Problem in Granular Media Higher densities: smaller grain size magnetization decay and time dependent coercivity M M M

3 Projects Zero-field relaxation (NSF-MRSEC) Shoutao Wang Self-assembled L1 magnetic nanoparticles (NSF- MRSEC) Dave Nikles, Shishou Kang, Shoutao Wang, Min Chen Relaxation in perpendicular media (Hitachi Maxell) Scott Brown

4 Zero-Field Relaxation Measurements (S. Wang) Measure decay of remanence at zero field after partial dc demagnetization. Recently reported for longitudinal thin film media, granular films, and tapes.

5 Zero Field Relaxation in Longitudinal Media Relaxation curves for Fuji film (CoCrPtB) of 5 nm thickness (KV/k B T = 29).

6 Zero Field Relaxation in Perpendicular Media Fit Parameters: h = H/H k = h appl + αm r σ θ = σ K =, σ V =.17, KV/kT = 38, α=-.7 Note: (KV/kT) meas = 54 from H CR (t). S (%/dec) σ =.17 KV/kT = 38 α = -.7 measured calculated m r

7 Self-Assembled L1 FePt Nanoparticles (S. Kang, S. Wang, M. Chen, D. Nikles) (a) Before annealing: d ~ 3.4 nm, disordered fcc, superparamagnetic (b) After annealing above 5 o C: high anisotropy L1 phase (ordered fct)

8 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 Fractionation Ethanol was used to precipitate the nanoparticles from octyl ether. Redisperse in hexane Precipitate with ethanol Self-Assembly into Two Dimensional Arrays 1:1 mixture of hexane and octane were used to re-disperse the precipitated nanoparticles Place 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 2, 287,

9 M-H Loops of L1 FePt Nanoparticles 1 6 M x1-6 (emu) 5 FePt@5 o C DCD M x1-6 (emu) FePt@52 o C DCD H x1 3 (Oe) -1 1 H x1 3 (Oe) Mx1-6 (emu) FePt@55 o C DCD Mx1-6 (emu) o C DCD Hx1 3 (Oe) Hx1 3 (Oe)

10 Coercivity Ratio Large H CR /H C ratio in L1 FePt nanoparticles is related to large anisotropy distribution. Calculations: Log-normal distribution of anisotropy energies Mean-field interaction : h = h applied + αm No thermal effects H CR /H C D, α = 3-D, α =.2 3-D, α = D, α = distribution width

11 Magnetic Relaxation in Perpendicular Media (S. Brown (MINT), Hitachi Maxell).2 TbFeCo TbFeCo better understanding of nucleation and domain wall motion processes. Co/Pd mutilayers effect of seed layer, B addition, etc. Moment (memu) (a.) (b.) Time (s)

12 Moke Magnetometer for Perpendicular Media (Scott Brown)

13 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

14 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.