Novel Techniques For Ultra-high-density Magnetic Data Storage

Magnetic recording, especially hard disk drive (HDD), is the most important way for data storage due to its economy and reliability. Since the birth of HDD, the recording areal density keeps increasing at a non-stop pace, sometimes a drastic one. With new technology in head and media, scaling in recording bit makes it possible to pack more information into one disk. Nowadays the recording areal density is approaching 400 Gbits/in2 while the upsurge of digital information worldwide requires more. Thus ultra-high density magnetic recording over 1 Tbit/in2 is the hit of research.However the method of simply scaling a bit does not apply anymore due to the physical limit called superparamagnetism. Novel magnetic recording techniques are proposed, among which heat assisted magnetic recording (HAMR) is a very promising approach. Exchange coupled composite (ECC) media, is another attempt to separate the thermal stability and writability of the media. Apart from the traditional way of magnetic recording, racetrack memory is a novel concept based on the motion of domain wall (DW) along the magnetic nanowires, which requires a sophisticate control of DW. This thesis is dedicated to the study of such novel techniques of magnetic recording which may serve the future demand of data storage. The thesis consists of there parts:1. A test platform of HAMR is built on a modified Guzik Spinstand with the aid of far field optics, which could test the dynamic performance of different combinations of ring heads and media, with or without laser heating. Dynamic performance of heat assisted magnetic recording (HAMR) on different media is investigated. Signal and signal-to-noise ratio enhancement is observed in high coercivity perpendicular media with the aid of laser heating. Linear recording density is increased while saturation write current is lowered. Trailing field partial erasure is observed in lower coercivity media with ring head, which results in signal reduction with increase of write current or application of laser. Precautions should be taken against partial erasure in overall recording system optimization of HAMR in order to achieve ultra high recording density.2. Instead of altering the writing procedure of recording head, the concept of ECC media solves the problems between thermal stability and writability by engineering the media. Both theoretical work and experiments predict the reduction of switching field due to the exchange coupling of soft layer and hard layer. With L10-FePt as the hard layer, we've proposed a perpendicular ECC media with [Co/Ni]N multilayer on top. Substantial reduction of coercivity is observed. With a thin layer of Pt inserted in between the soft/hard layer interface, the exchange coupling strength could be tuned to obtain the best performance. The results show that with thin soft layer, a slight decoupling between the [Co/Ni]N multilayer and FePt is benificial for the switching field reduction due to the freedom of magnetization rotation or nucleation in the soft layer. However in the composite with thick soft layer, decoupling gives rise to the increase of switching field no matter how. Furthermore, [Co/Pt]N multilayers are also used as the soft layer, with different thickness of Co and Pt in one period, i.e. [Co(0.2 nm)/Pt(0.6 nm)]N and [Co(0.2 nm)/Pt(0.3 nm)]N respectively. At last, the concept of graded media is achieved by inserting a layer of Co with moderate anisotropy in between the interface of FePt/Fe. The triple-layer structure of FePt/Co/Fe shows better performance than the double-layer of FePt/Fe.3. Individual event of domain wall (DW) depinning has been investigated in planar magnetic nanowires with an asymmetric notch by means of focused magneto-optical Kerr effect (fMOKE) magnetometer and micromagnetic simulation. Two types of field-driven depinning process with distinctly different depinning fields have been observed directly by single-shot fMOKE measurements. In combining with micromagnetic simulations, we have shown that the chirality of the vortex DW can control the depinning field at the asymmetric notch in a magnetic nanowire. In addition, due to the asymmetric shape of notch the depinning field to different direction is different. The depinning field of DW is determined by not only the wall spin structure, but the local geometry which will create an energy landscape for the moving domain wall.