We investigated the dependence of transport properties of Fe4N/MgO/CoFeB-magnetic tunnel junctions (MTJs) on temperature. In antiparallel configurations of magnetizations of Fe4N and CoFeB layers, a convex shaped peak was observed on the differential conductance curve as a function of bias voltage and it became clearer with decreasing temperature. The bias voltage (VB) of ~ −200 mV, where the peak was observed, corresponded to the energy where a sharp peak exists in the density of states (DOS) of the minority spin band of Fe B 4N. The shape of the differential conductance curves suggested that the contribution of Δ1 electron tunneling is not significant to total conductance in the Fe4N/MgO/CoFeB-MTJs differently from that in CoFeB/MgO/CoFeB-MTJs, even though they have a similar crystallized MgO-barrier layer. The asymmetric shape of the dependence of the tunnel magnetoresistance (TMR) ratio on bias voltage did not change in a temperature range from 6 K to 300 K, but the absolute value of the maximum TMR ratio was monotonically increased from 76 % at 300 K to 103 % at 6K. Spin polarization of Fe4N DOS at the Fermi level was estimated to be −0.5 from the present TMR ratio near zero bias and this was close to the theoretical value of −0.6.

No abstract

No abstract

The unidirectional anisotropy constant, JK, of γ-Mn-Ir / FM (FM = Fe-Co-Ni) bilayers was investigated by systematically changing the composition of the Fe-Co-Ni layers. The FM layers were grown in the crystal structures corresponding to their composition on the (111)-oriented face centered cubic (fcc) Mn-Ir layer. JK was significantly changed around the phase boundary of the FM layer between the body-centered cubic (bcc) phase and fcc-phase. The experimental results revealed a general feature where bcc-structured FM layers were more likely to induce stronger exchange anisotropy than fcc-structured FM layers. The JK values and the lattice mismatch between the Mn-Ir layer and the FM layer were increased with increasing Co concentration in the bcc-FM region. The dependence of JK on FM composition have been explained qualitatively with lattice distortion at the interface between the Mn-Ir and FM layers.

Crystallographic orientation of the MgO barrier in sputter-deposited CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) and its effect on tunnel magnetoresistance (TMR) were investigated. The degree of MgO(001) orientation was estimated with the integral intensity ratio (I (200) /I (220)) of diffraction lines from MgO(200) and MgO(220) planes obtained in grazing incident x-ray diffraction profiles. The main results are stated as follows. (1) I (200) /I (220)~ 4, meaning the (001) orientation of MgO, is realized when the underlaid CoFeB maintains amorphous structure, meanwhile MgO on bcc(110)-oriented CoFe shows (111) orientation (I (200) /I (220) = 0). (2) The prevention of epitaxial growth on hcp(00.1)-oriented Ru layer is effective to maintain amorphous structure of CoFeB. (3) The achievable TMR ratio after high temperature (280 ºC 450 ºC) annealing is mainly dominated by the MgO orientation and giant TMR ratio exceeding 200% is only obtained with I (200) /I (220) 3.4, while the resistance area product is independent of I (200) /I (220). (4) Thin Mg layer inserted between CoFeB layer and MgO barrier is effective to obtain bcc(001)-oriented crystallization of CoFeB after high temperature annealing and results in a giant TMR ratio, because of its role to avoid surface oxidization of underlying ferromagnetic electrode during the deposition of MgO barrier.