THERMALLY ASSISTED MAGNETIZATION REVERSAL IN - SI UNS

THERMALLY ASSISTED MAGNETIZATION REVERSAL IN - SI UNS THERMALLY ASSISTED MAGNETIZATION REVERSAL IN PERPENDICULARLY MAGNETIZED THIN FILM Oleh: Dr. Eng. Budi Purnama, S.Si., M.Si Large magnetic anisotropy is necessary to ensure the practical nonvolatility of magnetization in the magnetic memory cell with a small volume. Ferromagnetic thin films with large perpendicular anisotropies are key materials to realize ultra-high density magnetic random access memories (MRAM) with an areal density exceeding several 22Gbits/cm where the size of magnetic memory cells is less than 100，100 nm. However, it is difficult to switch the magnetization by conventional bit-writing method using dc magnetic fields with the amplitude of the order of 100 Oe, because the switching field of such a perpendicular material exceeds 1 kOe. Thermally assisted magnetization reversal (TAMR), where the switching field is temporally reduced by heating the selected memory cell in the writing process, is considered to be a promising candidate for the successful switching of magnetic memory cells with large perpendicular anisotropy. The main objective of this study is to demonstrate the feasibility of the TAMR in the patterned magnetic thin films with a perpendicular anisotropy for the application to MRAMs. Firstly, the dynamics of TAMR was numerically investigated to evaluate the magnetic field required for the successful switching. After that, the TAMR operation was experimentally demonstrated for the patterned Co/Pd multilayer films. The perpendicular magnetic anisotropy of the Co/Pd multilayer is originated from the interfacial anisotropy between the Co and Pd layers. The total thickness of Co/Pd multilayer can be, therefore, decreased with keeping the sufficient perpendicular anisotropy. The critical thickness for the perpendicular anisotropy is significant to downsize the magnetic memory element because of the lithographic difficulties. This thesis consists of five chapters. Content of each chapter are briefly described below. Chapter 1 summarizes the background, the objective, as well as an outline of this study. Chapter 2 describes the details of numerical studies on the TAMR in the patterned magnetic thin films with a perpendicular magnetic anisotropy. The lateral size of the magnetic cell is ranged from 20 to 80 nm. In all calculations, the amplitude of the perpendicular anisotropy K was set so as to realize the height of energy barrier ,E for , the switching of magnetization being 60kT, which condition is generally required for B the sufficient thermal stability of the bit information. In this study, two different writing methods are demonstrated, i.e. (i) switching of magnetization at the temperature below the Cutie point (reduced barrier writing) and (ii) magnetic ordering via field cooling process from the temperature above the Curie point (Curie point writing).In the case of the reduced barrier writing scheme, it is confirmed that the switching field can be reduced by increasing the temperature of the magnetic cell. However, the magnetic field required for the switching is much higher than the threshold magnetic field H for the th Curie point writing scheme. The amplitude of H depends on not only the intrinsic th anisotropy field but also the magnetization reversal mechanism. From the temporal evolution of the Curie point writing scheme, it was found that a nonequilibrium multidomain configuration is formed during the field cooling process even though the cell size is much smaller than the critical size for the single domain configuration at room temperature. In Chapter 3, both the structural and the magnetic properties of Co/Pd multilayers were described. Temperature dependence of magnetic properties in [Co/Pd]multilayer have been systematically studied for lithographically patterned N samples with different Co thicknesses t and the bi-layer number N by extraordinary Co Hall effect measurements. The perpendicular coercive field H decreases with the c increase of texcepting a very thin thickness of 1 Å. While a threshold temperature (T) Co, th of H vanishing linearly increases with the increase of t. The H and T monotonously cCocth increases with the increase of N, ranged from 5 to 50. The superior crystalline structure for samples with the larger N was confirmed from enhanced Co/Pd(111) peak height in the XRD pattern. A practical thermal activation coefficient , of 174 was evaluated from a sweep rate dependence of H. A marked improvements for the heat assisted MRAM c application was attained by air annealing of [Co(1.7 Å)/Pd(8 Å )] at 220 ?C, that is, the 30 H was increased from 1.3 kOe to 2.9 kOe, while Treduced from 210 ?C to 190 ?C. cth The TAMR operation of the patterned Co/Pd multilayer film was demonstrated in chapter 4. The test device consists of square-shaped [Co(1.7Å)/Pd(8.0 Å)] (N = 7, 20) N2multilayers with a lateral size of 50，50 ,m and the Hall-bar shaped electrical leads. In these experiments, the electrical leads connected to the magnetic cell are used not only to measure the extraordinary Hall effect of the cell but also to apply the pulsed current for the Joule heating of the cell during the TAMR operation. The width of the pulsed current was changed in the range from 80 to 2000 ns. As the pulsed current with the 2electrical power of 0.2 mW/,m was applied, the amplitude of magnetic field required for the switching of magnetization could be suppressed to the 50 % of the coercive field at room temperature. Resultantly, the threshold values of the external field necessary for controlling the magnetization direction were 220 Oe for the N = 7 and 710 Oe for N = 20 samples. The partial switching of magnetization appeared for the condition with the insufficient amplitude of the magnetic field applied during the pulsed current application. This result suggests the formation of the nonequilibrium multidomain configuration during the TAMR operation which is discussed in chapter 2. To suppress the amplitude of magnetic field required for the successful writing in the TAMR, the depinning field of magnetic domain wall in the magnetic element should be smaller than the nucleation field. The concluding remarks are given in Chapter 5.