The authors investigated the tunneling magnetoresistance (TMR) of Co Fe B / Mg O / Co Fe B tunnel junctions by varying the thickness (t Co Fe B ) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with t Co Fe B ≤10 Å , while normal tunneling behavior occurred for t Co Fe B ≫10 Å . The field sensitivity and the sensing field range were found to be controlled by varying the thickness of the sensing layer. This finding means that the magnetic tunneling junction (MTJ) provides a scheme for magnetic field sensing, which has a simple sensor design and low power consumption. The magnetic properties of the sensing layer with t Co Fe B ≤10 Å were found to show the characteristics of superparamagnetism. Although the detailed mechanism of TMR in MTJs with a superparamagnetic layer is not fully understood at present, this phenomenon is observed repeatedly. Therefore, this sensing scheme would be an alternative method for overcoming the problems in magnetic sensors with a crossed magnetization pattern.
"Clear evidence is found for perpendicular magnetic anisotropy in the Pd/CoFeB/Pd bilayer system up to a CoFeB thickness of 0.6 nm. However, in the sputtered MgO/CoFeB/Pd system, where the CoFeB layer is amorphous, it may be superparamagnetic at room temperature despite the fact that it is a continuous film . High-temperature annealing does not enhance the perpendicular anisotropy, which is not the case in the MgO/CoFeB/Pt system. "
[Show abstract][Hide abstract] ABSTRACT: Perpendicular magnetic anisotropy is observed in ultrathin (~ 0.6 nm) amorphous Co<sub>40</sub>Fe<sub>40</sub>B<sub>20</sub> when sputtered on an MgO (001) buffer layer and capped with Pd. The layers are superparamagnetic with a blocking temperature of ~ 230 K, below which they show an exponential temperature dependence of coercivity. Perpendicular magnetic anisotropy is observed in the as-deposited state and the mechanism is different from that of CoFeB/Pt, which requires postannealing. These ultrathin layers could be a model system for studies of electric field effects on magnetic anisotropy.
[Show abstract][Hide abstract] ABSTRACT: Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe/Ru/CoFeB stack. Magnetoresistance values as high as -55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5 nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5 nm pinned CoFeB layer. (C) 2007 American Institute of Physics.
[Show abstract][Hide abstract] ABSTRACT: The magnetic moment of Co <sub>40</sub> Fe <sub>40</sub> B <sub>20</sub> electrodes has been investigated as a function of thickness for films prepared by magnetron sputtering on Si / Si O <sub>2</sub>/ Mg O and Si / Si O <sub>2</sub> substrates. On MgO, the metal film becomes discontinuous and superparamagnetic with no stable ferromagnetic moment below 1.0 nm , whereas on Si / Si O <sub>2</sub> , there appears to be a 0.7 nm dead region for all film thicknesses. The dead layer is attributed to an interdiffusion region at the interface with the substrate where there are weakly coupled noncollinear spins and a 0.2 nm dead layer associated with the Ta cap. The discontinuous ferromagnetic films maybe useful for the wide-range linear field sensors.
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