Magnetic field sensing scheme using CoFeB/MgO/CoFeB tunneling junction with superparamagnetic CoFeB layer

Center for Frontier Materials, GIST, Gwangju 500-712, South Korea and Department of Materials Science and Engineering, GIST, Gwangju 500-712, South Korea
Applied Physics Letters (Impact Factor: 3.52). 11/2006; DOI: 10.1063/1.2370876
Source: IEEE Xplore

ABSTRACT 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Full Wheatstone Bridge incorporating a serially connected ensemble of Magnetic tunnel junctions was produced, targeting an application as a magnetic field compass. To that end, MTJs with RxA ~ 10kΩ μm2 TMR~150-200%, Hf = 5 Oe and Hf = 5 Oe were produced. In order to achieve a full bridge signal, two stacks with an asymmetric SAF reference structure where used to produce MTJs with opposite dR/dH upon annealing in the same substrate. The resulting Bridges exhibit sensitivities between 13.5-32 mV/V/Oe depending on the field range and provide a significantly advantageous alternative to AMR and GMR based bridges.
    IEEE Transactions on Magnetics 11/2012; 48(11):4107-4110. · 1.42 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: High perpendicular magnetic anisotropy (PMA) has been observed in MgO/CoFeB/Ta thin films if the thickness of CoFeB is in the range between 1.1 and 1.5 nm. However, both the coercivity and remanence vanish when the thickness of CoFeB is less than 1.1 nm, indicating a characteristic of superparamagnetic state. The magnetization versus external field of these thinner films shows a fit with a Langevin model. In addition, a temperature dependent study identifies a blocking temperature of 160 K by field cool and zero field cool measurements. The particle size of $sim23~hbox{nm}$ in diameter is estimated for a pancake shape cluster and the small particles are probably formed at the CoFeB/MgO interface at the beginning of the growth. Moreover, even the thicker films, which show ferromagnetic response at room temperature, also exhibit the temperature-dependent characteristic of superparamagnetic state due to the pinning at the edge of the small particles.
    IEEE Transactions on Magnetics 01/2014; 50(1):1-4. · 1.42 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Large area MTJs with linear response and very larger sensitivities are needed to fulfill the requirements for the detection of pT/Hz0.5 magnetic fields at low frequency and room temperature. MTJ stacks with soft-pinned sensing layers have been developed and the stack was optimized, providing a tunable soft-pinning field over a large range of values. Sensitivities corresponding to resistance changes of up to 7%/Oe are obtained in patterned MTJs with areas of 20 × 20 μm2 starting from films with high sensitivity soft-pinned layers made from 3.0Co40Fe40B20/0.21Ta/8.0Ni81Fe19/8.0Ir20Mn80.
    IEEE Transactions on Magnetics 11/2012; 48(11):3719-3722. · 1.42 Impact Factor