Thermal Effects in High Coercivity Perpendicular Media

Transcription

1 Thermal Effects in High Coercivity Perpendicular Media J.W. Harrell Scott Brown MINT Center University of Alabama

2 Introduction Thermal stability of perpendicular media will be a limiting factor in future high density media. Stability is affected by exchange and dipolar (demagnetization) interactions. Relaxation studies in zero applied field as a function demagnetization state are useful in determining interactions. Here we present thermal stability measurements on 3 different types of high coercivity media: Co/Pd multilayer, CoPtCr-SiO, and CoPtCr. Preliminary Monte-Carlo calculations are also given.

3 Perpendicular Media Used in Study 1 CoPtCr-SiO - Hitachi Maxell 4 CoCrPt - Seagate 8 Co/Pd Multilayer - Hitachi Maxell Moment (µemu) 5-5 M S (emu/cc) - Moment (µemu) H (koe) H (koe) H (koe) CoPtCr-SiO : glass/ti(5nm)/ru(4nm)/coptcr-sio(16nm)/c(5nm) CoPtCr: glass/ta(5)/ru(1)/cocrpt(19)/c(5) Co/Pd: glass/ti(5nm)/pdb(8nm)/b[co/pd] (19.5nm)/C(8nm)

4 Time-dependent Remanent Coercivity Room-temperature measurements were analyzed using Sharrock formula: H n cr = ( t) = 3, f H = 1 k 1 K 1 Hz B u T V ln( f t) n H cr (koe) CoPtCr-SiO CoPtCr Co/Pd log(t) H C H K u V M S (koe) (koe) k B T (emu/cc) CoPtCr CoPtCr Co/Pd

5 Temperature-dependent Coercivity CoPtCr-SiO CoPtCr-SiO.7 1 Temperature-dependent H C measurements were analyzed using Sharrock formula with t eff ~ 1 sec. M s (T) ~ constant, suggesting H and K are constant. From temp. dependence of H C : (KV/kT) 3K = 88, H = 1. koe From time dependence of H CR : (KV/kT) 3K = 158, H = 7.5 koe M s (memu) M s H C Temperature K Reason for discrepancy is not understood. Note, however, that we are comparing ordinary and remanent coercivity Hc (koe)

6 Temperature-dependent Coercivity Co/Pd Ms (emu) Fit M s (T) to Hc (H, K = const) Hc (H ~ Ms, K ~ Ms ) Hc (H ~ Ms, K ~ Ms 3 ) 1 3 M M s s T (K) ( T ) () = 1 at Fit H C (T) to Sharrock equation with t eff ~ 1 sec. Ms / Hc (koe) Temperature dependence of M s suggests K and H may be temperature dependent. From temperature dependence of H C : (KV/kT) 3K = 54, if H, K = constant. (KV/kT) 3K = 146, if H M s,k Ms. (KV/kT) 3K = 5, if H M s,k M s3. From time dependence of H CR : (KV/kT) 3K = 17 Time and temp. measurements don t agree. We need a model to account for nucleation and domain wall motion.

7 Comparison of time and temperature dependent measurements For thermally activated reversal, we expect coercivity to have universal temperature-time dependence given by T ln(f t), if material parameters are independent of temperature. Measurements show stronger temperature dependence than time dependence for both CoPtCr-SiO and Co/Pd film (even though M s ~ constant for CoPtCr-SiO.) 1 CoPtCr-SiO 1 Co/Pd Hc, Hcr (koe) Hc (koe) Hcr (koe) Hc, Hcr (koe) Hc (koe) Hcr (koe) T*ln(fo*t) T*ln(fo*t)

8 Relaxation of remanence in zero applied field after dc demagnetization Measurement sequence Energy barrier distribution after partial dc demagnetization 4 1 G (E) 3 E C E

9 Effect of interactions on zero-field relaxation No interactions viscosity independent of M R over wide range. Exchange interactions stabilizes saturation remanence, enhances decay of reduced remanence. Demagnetization field destabilizes saturation remanence. Zero-Field Viscosity, S Remanence Moment, M r

10 Comparison of Zero Field Relaxation Measurements S (%/decade) CoPtCr-SiO CoPtCr Co/Pd M r /M rs The relaxation curves reflect competition between dipolar and exchange interactions. CoPtCr-SiO shows the largest relaxation at saturation remanence, although it has the largest KV/kT. Weak exchange (SiO grain isolation) and strong dipolar interactions (largest M s ). Co/Pd shows the lowest relaxation at saturation remanence due to strong exchange effects.

11 Measured and Monte-Carlo simulation of zero-field viscosity CoPtCr (in collaboration with Roy Chantrell) MC parameters taken from hysteresis fit includes moderate exchange (H ex =.4 H k ). Calculation gives correct shape but wrong magnitude. Parameters need further tuning to fit both hysteresis and zero-field relaxation S (%/dec)..1 measured S (%/dec) 1.5 calculated Mr Mr

12 Conclusions The time and temperature dependence of coercivities are not fully reconciled using the Sharrock formula. Measurements of H cr (t) as a function of temperature are needed. Zero-field viscosity measurements are sensitive to interactions. Although CoPtCr-SiO has the largest KV/kT, it also has the largest zero-field viscosity because of weak exchange interactions. Preliminary Monte-Carlo modeling on CoPtCr are in qualitative agreement with measurements.