A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponent zv≈12.4, for x=0.5 composition.
Spin dynamics and magnetic frustration effects in La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds were systematically studied by means of resistivity, dc magnetization, ac susceptibility, and electron-spin-resonance spectra. The Fe substitution caused a metal-semiconductor transition for x⩽0.3 compositions whereas the only semiconducting state behaved for x>0.3 compositions. With increasing Fe-doping levels, the ferromagnetic–paramagnetic transition temperature, TC, decreased from 250 (x=0) to 166K (x=0.4). A spin-glass behavior was observed for x>0.4 compositions and is attributed to the frustration of random competing exchange interactions, namely, the ferromagnetic Co3+–Co4+ double-exchange interaction and the antiferromagnetic interactions such as Co–O–Fe and Fe–O–Fe. The internal dynamic properties of the samples were elucidated by the electron-paramagnetic-resonance spectra. An electronic and magnetic phase diagram of La0.5Sr0.5Co1−xFexO3 is drawn up.
A synthetic antiferromagnet (SAF) structure comprising of ferromagnetic amorphous Ni16Fe62Si8B14 layers has been devised and employed as a free layer of magnetic tunnel junctions (MTJs) to enhance cell switching performance. We observed −0.03erg∕cm2 of exchange coupling energy (Jex) by inserting a 0.5 nm Ru layer in between NiFeSiB layers. In Si∕SiO2∕Ta 45∕Ru 9.5∕IrMn 10∕CoFe 7∕AlOx1.5∕(single NiFeSiB 7) or [NiFeSiB(t)∕Ru 0.5∕NiFeSiB(7−t)]∕Ru 60(nm) MTJ structures, we found size dependence of the switching field originating from the lower Jex both experimentally and by simulation. The NiFeSiB SAF structure showed lower switching field than traditional CoFe and CoFeB SAF structures. This is because NiFeSiB possesses low saturation magnetization (Ms=800emu∕cm3) and high anisotropy constant (Ku=2,700erg∕cm3). These properties were proven beneficial for the switching characteristics such as reducing the coercivity (Hc) and increasing the sensitivity in micrometer to submicrometer sized elements.
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