Magnetic Investigation Of CoNiGa, CoCr₂O₄/Ni Nanowires, And IrMn/FM ?CoFe, CoPt? Multilayers

Exchange bias originates from the exchange coupling effect of the ferromagnetic-antiferromagnetic materials,which is being widely used in various magneto-resistance devices.As the device size continue to decrease,the research on exchange-biased nanostructures have attained promising importance.Therefore,the first two projects in this thesis cover the fabrication of two types of exchange-coupled nanostructures on the backside nanodot arrays of anodized aluminum?AAO?template with different diameters:?i?Ir Mn/CoFe in-plane exchange bias multilayers system and?ii?[Pt/Co]₅/IrMn perpendicular exchange bias multilayers system.Herein,investigation of the size effect on the exchange bias field and coercivity in these two systems has been discussed in detail.In the meanwhile,two more projects related to the growth of magnetic nanowires with single-phase and composite core-shell structure within the nanopores of AAO templates using electrochemical deposition technique and sol-gel method have been focused in this dissertation.The magnetization reversal characteristics and thermal stability in these two nanowire systems have been studied in detail,providing a theoretical support for the development of high-density magnetic recording media or multifunctional magneto-electronic devices for the future technology.Specifically,this dissertation encloses the following four aspects:?1?IrMn/CoFe in-plane exchange bias multilayers were deposited on the backside nanodot arrays of AAO template with three different sizes?20 nm,70 nm,and 100 nm?,and the size effect of nanodot on the exchange bias field as well as coercivity in these structures have been investigated.Morphological analysis results show that the shadow effect during deposition process leads to the nonuniform distribution of magnetic film on the substrate,namely thicker on the surface of nanodots and thinner between nanodots?valley area?.Magnetic measurement indicates that the hysteresis loop becomes more tilted as the nanodot size decreases,which is caused by the magnetic easy-axis widening,due to the undulating morphology of the substrate,and the inconsistency of switching field in areas of different magnetic film thickness.This inconsistency of reversal process is also reflected in the first-order reversal curves,showing the expansion of main peak and the appearance of secondary peak.In addition,the local coercivity and exchange bias field of the nanostructured exchange bias multilayers are significantly enhanced compared to that of the continuous film.?2?[Pt/Co]₅/IrMn multilayers were deposited on the backside nanodot arrays of AAO templates with different diameters.The effect of nanodot size on the perpendicular exchange bias field and coercivity was investigated.Magnetic measurements show that the perpendicular exchange bias field increases with the decrease of nanodot size,which results from the increased random field caused by the decrease of antiferromagnetic domain size and the domain wall pinning in the valley district.Further reducing nanodot size,more magnetic moment orientations will deviate from the vertical direction as a result of curved morphology,leading to decline in the exchange bias field.Similarly,the coercivity also increases first and then decreases with the reduction of nanodot size.In addition,we found that the exchange bias field and blocking temperature in nanostructured exchange bias multilayer are much higher than that of continuous flat film.?3?CoNiGa alloy nanowires with different components were successfully prepared in AAO templates by DC electrochemical deposition.Firstly,the effects of deposition potential,electrolyte composition and pH of solution on the reduction of Co,Ni and Ga elements were systematically investigated to obtain the best deposition conditions.Then,magnetic properties of as-prepared CoNiGa alloy nanowires in different temperatures were studied in detail.Room temperature magnetic measurement results showed that the coercivity of CoNiGa nanowires increased by 37%when increasing Ga content by 10%,indicating that the magnetic properties of alloy nanowires can be well tuned by changing its components.For Co₅₅Ni₂₈Ga₁₇ nanowire,the angle-dependent coercivity curve reveals that magnetization reversal mode changes from curling to coherent rotation as the angle increases,while low-temperature magnetic measurement shows that coercivity decreases linearly with increasing temperature,satisfying the thermal activation model.?4?CoCr₂O₄/Ni core-shell nanowires with uniform morphology were synthesized in AAO templates by sol-gel method and electrochemical deposition technique.Crystal structure analysis shows that pure spinel structure CoCr₂O₄ phase can be obtained by annealing sample at 500°C.Temperature dependent magnetization curve shows that CoCr₂O₄ nanotubes undergo three magnetic phase transitions near 90K,21K,and 14K,corresponding to the ferrimagnetic transition,spiral magnetic order transition,and lock-in transition,respectively.In addition,the coercivity and remanence of CoCr₂O₄/Ni core-shell nanowires are significantly enhanced compared to single-phase Ni nanowires,and their magnetic properties are mainly determined by the relatively large Ni layer.Finally,micromagnetic simulations were conducted to explore the magnetization reversal properties of Co Cr₂O₄/Ni core-shell nanowires.Results show that temperature has little effect on the coercivity of CoCr₂O₄/Ni nanowires.Although the simulated coercivity are slightly different from the experimental ones,the shape and magnetization reversal characteristics of the hysteresis loop obtained by simulation are quite similar to the experimental results,proving that our theoretical simulation results are reliable.