The authors investigated the tunneling magnetoresistance (TMR) of CoFeB∕MgO∕CoFeB tunnel junctions by varying the thickness (tCoFeB) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with tCoFeB⩽10Å, while normal tunneling behavior occurred for tCoFeB>10Å. 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 tCoFeB⩽10Å 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.
We successfully synthesized pure single crystals of GdB4, which reveals RRR=550, defined as RRR≡ρ(300K)∕ρ(2K). The temperature derivative of resistivity dρ(T)∕dT shows a transitionlike change of electronic property at Ta≈10K, in addition to the known antiferromagnetic transition at TN=42K. We also found that the ρ(T) shows an anomalous increase under the applied magnetic fields H below Ta, which leads to a positive magnetoresistance ratio MR≡[ρ(H,T)−ρ(0,T)]∕ρ(0,T)=58800% at T=2K and H=7T. Furthermore, the electronic feature near Ta manifests itself by a broad bump near T≈10K in a specific heat measurement, which indicates that the transition at Ta is a bulk property. We found that the positive and large magnetoresistance can be described by an empirical function, based on Kohler’s rule for the electron dynamics of a pure simple metal.
Temperature-dependent magnetization M(T), resistivity ρ(T), and specific heat Cp(T) were measured for single crystalline YbB4. M(T) showed a large anisotropy, that is M‖(T)>M⊥(T), for fields parallel and perpendicular, respectively, to the c axis. While M‖(T) followed Curie-Weiss behavior with slight crystalline electric field splittings, M⊥(T) showed a typical temperature dependence of intermediate valence (IV) fluctuation, which implies that YbB4 is a rare compound with two-dimensional IV fluctuation. ρ(T) followed ρ0+AT2 below Tcoh≈5K, indicating a low-temperature Fermi liquid state of YbB4. In addition, anomalies were also found at ≈Tcoh in M(T), ρ(T), and Cp(T).
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