of nonvolatile ferroelectric random access memory. [11] Another example, given the optical anisotropy of ferroelectric crystals, the electro-optic performance of a ferroelectric-based modulator relays on the relationship between the domain orientation, the applied electric field, and the polarization of the optical signal. [12] Recent years, people have further found that the ferroelectric polarization can effectively tune the light emission from the doped metal ions, which offers an additional degree of freedom for designing luminescent materials and devices. [10,13] Control over ferroelectric polarization is of great importance from both scientific and technological viewpoints. To date, a static or pulsed electric field represents the most effective tuning knob. Electric field method, however, suffers from limitations associated with the requirement of circuitry access or the cumbersome and time-consuming process. [14] Furthermore, electrically controllable polarization modulation can be typically accomplished within nanoseconds. It is difficult to use electric fields for motivating and exploiting ultrafast phenomena on the picosecond time scale or less. [15] Strain engineering is now possible to render the polarization status and build artificial heterostructures engineered down to a unit cell. [16] Breakthroughs in synthesizing high-quality epitaxial ferroelectric thin films have provided more opportunities to explore strain effects on ferroelectric polarizations. [17] Practical implementations of strain control over ferroelectric polarization are limited to some particular systems. Ferroelectric oxides are generally brittle. However, their thin-film counterparts can tolerate lattice-mismatch-based biaxial strains within ±3%. [8,18] Also, mechanical strain cannot realize ultrafast modulation of ferroelectric polarization.Electromagnetic waves have offered an extremely versatile manner for controlling ferroelectric polarization. The interaction of light with ferroelectrics gives rise to several fascinating photosensitive and photoresponse phenomena such as anomalous photovoltaics, [19] photostriction, [20] and piezophototronics. [21] These fields were termed photoferroelectrics. [22] The recent renaissance of photoferroelectrics began with the interest on ferroelectric photovoltaics, especially after a giant photovoltaic effect observed in multiferroic BiFeO 3 (BFO) thin films. [23,24] The above bandgap open-circuit voltage represents the most unique feature of ferroelectric photovoltaics, [25] which is associated with remnant polarizations, domain walls, defects,

No abstract

This paper presents an Iterative Learning Controller (ILC) design for Self-Servowriting (SSW) process in Hard Disk Drives (HDDs). In SSW, the position and timing information are written onto the disk surface by referring to the previously written servo information. This process is repeated until the whole disk is completely written. The main issue in this process is Radial Error Propagation (REP), which refers to the accumulation of the written-in errors as tracks are written sequentially. In this paper, an ILC scheme is designed by using the position error information in the previous track to prevent REP and improve the quality of servowriting. Analytic conditions for stability and monotonic convergence are discussed. The learning filter design is formulated as an (or induced L ) optimal control problem, which is solved by optimization programming. Simulation results show that REP is contained and a good quality of servowriting is assured.

No abstract

No abstract