BUKU ABSTRAK - Universiti Putra Malaysia

Science, Technology & Engineering
Effect of Co Substitution on Magnetic and Magnetoresistance Effect in La0.67(Ba1xCox)0.33MnO3
System
Dr. Lim Kean Pah
Wong Jen Kuen, Abdul Halim Shaari, Chen Soo Kien and Ng Siau Wei
Faculty of Science, University Putra Malaysia,
43400 UPM Serdang, Selangor, Malaysia.
+603-8946 6660; kplim@science.upm.edu.my
Low and high-field magnetoresistance (LFMR, HFMR) in polycrystalline perovskite manganite of R1-
XAXMnO3 (R=rare-earth element, A=divalent alkaline-earth metal) have generated many interests due to
their potential technological applications in sensing device industries. Many efforts had been put in order to
improve the sensitivity towards different magnetic signal strength. Previous reports shown that magnetic
element doping in different site will manipulate their LFMR and HFMR effect which mainly influenced by their
extrinsic or intrinsic magnetotransport properties that governed by grain formation, grain size and spin alignment
corresponding to magnetic field respectively. Therefore, substitution of strong magnetic element such as cobalt
(Co) into Ba site of La0.67Ba0.33MnO3 could manipulate their magnetotransport behaviour. Therefore, a series
of polycrystalline perovskite manganite of La0.67(Ba1-XCoX)0.33MnO3 (x=0.00, 0.30 and 0.50) were prepared
through conventional solid-state route. XRD spectrum indicated that single phase rhombohedral perovskite
structure had been obtained for x=0.00 sample. When Co is introduced in the Ba site, its structure is distorted
from rhombohedral to pseudo-cubic. The SEM images show that the average grain sizes were found to be in
3-8µm (x=0.30) and 2-10µm (x=0.50) with less pore between the grain. For x=0.00, the grains are in melted
condition where no significant clear grain boundary can be observed. Pure sample had Curie temperature, TC of
343K. However, substitution of Co at Ba site brings down the TC below 293K. Pure sample shows LFMR effect
and the effect weakens when Co is introduced. The highest LFMR value is -13.0% for sample with x=0.00 in
0.1Tesla applied external magnetic field at 90K and the highest MR value of -22.5% is given by x=0.30 sample
at 1Tesla applied magnetic field at 90K. In summary, LFMR effect is observed in La0.67Ba0.33MnO3. However,
introduction of Co in Ba site can destroy LFMR effect but improved its HFMR effect.
Keywords: Magnetoresistance, low-field magnetoresistance, high-field magnetoresistance, ceramic
A New Two-step Gradient-type Method for Large-scale Unconstrained
Optimisation
Dr. Mahboubeh Farid
Wah June Leong and Malik Abu Hassan
Institute of Mathematical Research, University Putra Malaysia,
43400 UPM Serdang, Selangor, Malaysia.
+603-8946 6677; mahboubeh@inspem.upm.edu.my
In this paper we propose some improvements to a new gradient-type method for solving large-scale
unconstrained optimisation problems, in which we use data from two previous steps to revise the current
approximate Hessian. The new method which we have considered, resembles to that of Barzilai and Borwein
(BB) method. The innovation features of this approach consist in using approximation of the Hessian in diagonal
matrix form based on the modified weak secant equation rather than the multiple of the identity matrix in the BB
method. Using this approach, we can obtain a higher order accuracy of Hessian approximation when compares
to other existing BB-type method. By incorporating a simple monotone strategy, the global convergence of the
new method is achieved. Practical insights on the effectiveness of the proposed method are given by numerical
comparison with the BB method and its Variant.
Keywords: Diagonal updating, weak secant equation, two-step gradient method, barzilai and borwein method
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