Improved Heat Assisted Magnetic Recording via Ultrathin Nonlocal Spin Valves and Three-Layer Composite Media

Abstract

This dissertation is structured into two main parts, dedicated to exploring potential optimizations in read head and recording media, two integral hard disk drive (HDD) components, for achieving reliable recording performance with high areal density.The first part delves into the complexity of transport within Al-based Nonlocal Spin Valves (NLSVs), particularly addressing spin relaxation induced by structural defects in the material. This can significantly enhance the spin relaxation and is likely respon- sible for the observed deteriorated performance of the devices, especially notable for metallic channels thinner than 10nm. To exclusively study the spin relaxation induced by surface and bulk defects, all the calculations are implemented at T=0K to eliminate temperature-dependent contributions such as phonon scattering. Resistivity, spin dif- fusion length and Elliott-Yafet constant β are determined in a simplified 3.6nm-thick Al-system where both leads and transport channel are made of Al and embedded in vacuum. Utilizing the Landauer-Bu ̈ttiker formalism and a recursive Green’s function tech- nique, predictions are made regarding the effects of surface and bulk scattering on elec- tronic and spin transport including surface roughness, grain boundaries, vacancies, and surface reconstruction. It is demonstrated that for thin sputtered films, point vacancies contribute dominantly to the momentum relaxation, and spin relaxation is dominated by the combined effect of surface reconstruction and point vacancies. This yields reason- able spin diffusion lengths and Elliott-Yafet constants. Further analysis reveals that the presence of surface corrugations leads to a clear departure from Matthiessen’s rule and the Elliott-Yafet prediction of β. However, this deviation is rectified in the presence of random surface corrugations with higher vacancy concentration, as the symmetry breaking is closely dependent on the characteristic scattering length and concentration of random defects. It is discovered that the spin diffusion length induced by surface roughness is pro- portional to the inverse square root of the ratio between the root mean square height (δh) and the lateral correlation length (ξ) of a given rough surface, i.e. (δh/ξ)−1/2, as opposed to (δh/ξ)−1 as is the mean free path. This phenomenon is attributed to the iv interference of extended surface features. Additionally, a pronounced anisotropy of spin relaxation is observed for spins parallel to the propagation direction, which is pertinent to surface corrugations. Overall, these findings can potentially facilitate the realization of magnetic recording read heads based on metallic nonlocal spin valves with sub-10nm shield-to-shield spacing, thereby improving head resolution. The second part of the dissertation shifts focus toward mitigating transition noise with a novel proposed heat-assisted magnetic recording (HAMR) media in order to achieve higher areal density of HAMR-based devices. Conventional two-layer thermally exchange coupled composition media (ECC) exhibit robust tolerance to noise induced by Tc variance and offer tunability of writing temperature but suffer from adjacent track interference issue with reduced writing temperature. To inherit these merits while addressing the thermal susceptibility, the recording performance of three-layer (3-ly) FePt-based ECC structure with reduced Tc = 500 K for FePt is evaluated, with pre- dictions for transition jitter, erasure-after-write (EAW), bit error rate and switching probability distribution. The original two-layer ECC structure features a magnetic soft writing layer with high Tc and a magnetic hard FePt-layer for long-term storage. In contrast, the proposed three-layer structure consists of a high-Ms and moderate-Ku writing layer, a middle layer with the same Ku as FePt but low-Ms and a FePt layer attached at the bottom, each layer being 3nm thick. The optimized parameters at 300K for the writing layer are Ms = 1300 emu/cm3 and Ku = 1.3 - 1.8 × 107 erg/cm 3 and Tc=600KandMs=∼350emu/cm3 andKu=3.3×107 erg/cm3 at300KandTc = 500 K for the middle layer. Compared to the conventional 2ly-ECC, the switching mechanism for proposed 3ly- ECC highlights the Zeeman effect that switches the writing layer and anisotropy field gradients to switch the middle layer. By decoupling the Zeeman effect and anisotropy field gradients, the proposed 3ly-ECCs effectively improve transition jitter by ∼ 15% and BER by ∼ 85% in the absence of intergranular exchange, compared to 2ly-ECC with the same total thickness. These improvements are attributed to large anisotropy and small magnetization in the middle layer, aligning with the analytical analysis of energy function based on a simple spin model. v In addition, the exploration of the switching rate at a constant temperature suggests that fast switching induced by the moderately soft writing layer can also potentially contribute to jitter improvement at the expense of enhanced EAW. It is also observed that the 3ly-ECC is more susceptible to EAW effect than 2ly-ECC. Calculations indicate that the suppression of EAW in 3ly-ECCs relies on the increase in the anisotropy of writing layer which adversely affects BER due to the loss of rapid switching. Overall, the proposed 3ly-ECCs effectively balance fast switching and EAW and thus exhibit superior jitters and BERs compared to the two-layer counterpart.