Research Article | Open Access
Michihide Kitamura, Akinobu Irie, "Spin Flows in Magnetic Semiconductor/Insulator/Superconductor Tunneling Junction", International Journal of Superconductivity, vol. 2015, Article ID 273570, 9 pages, 2015. https://doi.org/10.1155/2015/273570
Spin Flows in Magnetic Semiconductor/Insulator/Superconductor Tunneling Junction
Tunneling currents along the c-axis of the majority and minority spin electrons have been studied for a magnetic semiconductor (MS)/insulator (I)/superconductor (S) tunneling junction consisting of a Ga1−xMnxAs with x = 1/32, a nonmagnetic with a realistic dimension, and a (Hg-1223) high- S. The normalized charge and spin currents, and , and the flows of the majority and minority spin electrons, and , have been calculated at a fixed external voltage , as a function of the magnetic moment μ′ per a Mn atom which is deduced from the band structure calculations. It is found that the tunneling due to the minority spin electron dominates when , but such a phenomenon is not found for . We have pointed out that the present tunneling junction seems to work as a switching device in which the ↑ and ↓ spin flows can be easily controlled by the external magnetic field.
Spintronics, in which both the charge and spin of an electron should be controlled, is one of the most attractive subjects in solid state physics and technology. Therefore, if one can make a device in which the spin flow can be easily controlled, then such a device may play an essential role in the field of the spintronics. By using ferromagnetic materials such as 3d-transition metal compounds, spin-polarized electrons can be easily injected into the other materials including superconductors. In the nonsuperconducting states, a phenomenon such as the tunneling magnetoresistance (TMR) has been clearly observed in the magnetic tunneling junction (MTJ) consisting of two ferromagnetic () electrodes separated by an insulating () barrier, that is, //-junction. Parkin et al.  and Yuasa et al.  have measured very large TMR values for Fe/MgO/Fe junctions, and Belashchenko et al.  have theoretically studied the electronic structure and spin-dependent tunneling in epitaxial Fe/MgO/Fe(001) tunnel junctions and found that interface resonant states in Fe/MgO/Fe(001) tunnel junctions contribute to the conductance in the antiparallel configuration and are responsible for the decrease of TMR at a small barrier thickness, which explains the experimental results of Yuasa et al. .
Many studies have been done for the superconductor ()/insulator ()/superconductor () tunneling junctions, that is, Josephson junction, from the experimental and theoretical points of view. Barone and Paterno  presented to us a guide principle to study the Josephson effect. We have also studied the current- (-) voltage () characteristics observed in the BSCCO intrinsic Josephson junctions from both the experimentally [5–8] and theoretically [9–11]. If a junction is made from and layers, the further interesting phenomena could be observed. For such junctions, there are two valuable review articles, one is by Golubov et al.  and the other is by Buzdin . The one of the interesting phenomena found in a junction consisting of the ferromagnetic and superconducting layers could be an occurrence of the -junction [14–21]. It has already been known that the -junction is caused to the damped oscillatory behavior of the Cooper pair (CP) wave function in the ferromagnetic layer.
Very recently, we have theoretically studied the -axis charge and spin currents in // tunneling junction , in which Hg- copper-oxides high- superconductor and a ferromagnetic Fe metal have been selected as the and layers. Our recent study  has showed that an interesting result such that the minority spin current exceeds the majority one is surely found in the junction consisting of the nonmagnetic insulating layer; however, more clear and remarkable result is found in the junction including the magnetic insulating layer. Magnetic insulator () can be made by doping the magnetic impurities into the nonmagnetic insulator, but it may not be so easy to make a tunneling device such as // junction whose magnetizations are in antiparallel configuration. In the present paper, therefore, we further study the magnetic semiconductor ()/insulator ()/superconductor () tunneling junction. As , the Hg- copper-oxides high- superconductor is selected again, and a As with is selected as . For the ferromagnetic III-V semiconductors, there is an excellent article written by Ohno , in which he presented the properties of III-V-based ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As. Some of the interesting results obtained for the As are that no ferromagnetism is observed below and the relation between and the ferromagnetic transition temperature is found as K up to . The in the present study is fixed to , so that the of the present As is calculated as K. It is well known that the As shows some phases such as magnetic semiconductor, half-metal, and ferromagnetic metal due to the change of the magnetization, that is, the change of the external magnetic field. Therefore, it is expected that an interesting phenomenon could be observed in the current- (-) voltage () characteristics of the present // tunneling junction. This is a motivation of the use of .
The transition temperature of Hg-based copper-oxides superconductors is fairly higher than the liquid nitrogen temperature (=77 K), so that the Hg- high- superconductor with , that is, whose is K, has been selected as a superconducting layer . As already stated, the ferromagnetic transition temperature of As with is calculated as about K. Therefore, it is certain that the superconductivity of Hg- high- superconductor is fairly well kept at the temperature region below K, since the of the Hg- high- superconductor is K. This is the reason why we have selected the Hg- high- superconductor as layer.
The transport problem in the // tunneling junction has already been studied by Tao and Hu  and Shokri and Negarestani . Here it should be noted that they have selected -symmetry low- superconductor as the and adopted the Blonder-Tinkham-Klapwijk (BTK) model , which is based on the effective mass approximation. In the present paper, we consider the -axis tunneling of the majority and minority spin electrons in As /insulator / high- superconductor tunneling junction within the framework of the tunneling Hamiltonian model. In the present junction, there are facts that the electron states in the vicinity of the Fermi level mainly come from orbitals of Mn and Cu atoms, the density of states (DOS) that originated from the orbital shows a pointed structure meaning the localized nature, on the contrary to the DOS from and orbitals which show a broadened structure, that is, the extended nature, therefore, the effective mass approximation, which is valid for the extended nature, may not be so good for the present system in which the electron states near the are fairly well localized, and the layer is not a delta-functional but in a real dimensional size, whose barrier strength is large enough, so it must be noted that the BTK model reaches the tunneling Hamiltonian model since the probability of Andreev reflection decreases with increasing the barrier strength of the layer. The above are just a reason why we have adopted the tunneling Hamiltonian model based on the electrons with the Bloch states which are decided from the band structure calculations. It must be noted here that we do not set here a realistic size such as a width of the insulating layer. We think that it may be enough to state that the insulating layer works well as a tunneling barrier so that the tunneling Hamiltonian model is valid.
Tunneling current as a function of an applied voltage of a ferromagnet- (-) insulator- (-) superconductor () tunneling junction is given by where is the first Brillouin zone of . The is the coefficient in the expansion by the Bloch orbitals of the total wave funtion of such thatwhere and are the site to be considered and the quantum state of atomic orbital of , respectively.
As already stated in our previous paper , the is the tunneling probability of a -spin electron in the // tunneling junction defined byso that the value of strongly depends on the magnetic nature of an insulating layer . It is clear that when the shows no magnetic nature, the tunneling probabilities of majority and minority spin electrons should be equal; that is, , and when the shows magnetic nature, those should differ from each other; that is, . In the present study, only the nonmagnetic layer is considered, so that the tunneling probabilities and of the majority and minority spin electrons are equal to each other; that is, only the case of is considered here. As a tunneling process, coherent, incoherent, and WKB cases can be considered. In the present paper, the incoherent tunneling is mainly studied. The reason is described later.
In the incoherent tunneling case, the in (2), which is written as , is given by where is a Fermi-Dirac distribution function and is the TDOS of the ferromagnetic layer, that is, As with layer, for spin state as a function of . For the spin symbol used in our studies, it is noted that and mean the majority and minority spin electrons, respectively. The is a quasiparticle excitation energy defined by , where the is one electron energy relative to the Fermi level and the is a superconducting energy gap given by .
The one electron energy is calculated on the basis of the band theory using a universal tight-binding parameters (UTBP) method proposed by Harrison . The energies of the atomic orbitals used in the band structure calculations have been calculated by using the spin-polarized self-consistent-field (SP-SCF) atomic structure calculations based on the Herman and Skillman prescription  using the Schwarz exchange correlation parameters . The calculation procedure of the present band structure calculation is the same as that of our previous calculation . Present band structure calculation for the As with has been done using a unit cell consisting of cubes such as a -structure by a primitive cube. The unit cell includes cations (Ga or Mn) and anions (As); therefore, the condition used in the present study means that the one of the Ga atoms is replaced by Mn atom.
3. Results and Discussion
3.1. Density of States
The densities of states (DOSs) of As magnetic semiconductor with have been calculated as a function of the -electron configuration of Mn atom. The electron configuration used in the spin-polarized self-consistent-field (SP-SCF) atomic structure calculation for the Mn atom is with , that for Ga atom is , and that for As atom is . The DOSs calculated by setting () to (), (), (), (), (), (), (), (), (), (), and () are shown in Figures 1(a), 1(b), 1(c), 1(d), 1(e), 1(f), 1(g), 1(h), 1(i), 1(j), and 1(k), respectively. Resultant magnetic moment calculated per Mn atom is , , , , , , , , , and for (b), (c), (d), (e), (f), (g), (h), (i), (j), and (k), respectively. Calculated DOSs clearly show that (a) is a nonmagnetic semiconductor, (b) is a ferromagnetic semiconductor, (c) is a ferromagnetic zero-gap semiconductor, (d) and (e) are ferromagnetic metals, (f) and (g) are half-metals, and (h), (i), (j), and (k) are ferromagnetic metals. The phase change mentioned above is closely related to the energy position of the -band of the minority spin electron denoted as -band. Actually, we can see that the -band of (d) locates below the Fermi level , that of (e) is very close to the , and that of (f) locates above the . The energy shift of the -band makes the rapid change of the magnetization of the As . Such a rapid change in the magnetic moment is really observed between (d) and (f).
Finally, it is noted that the DOS of Hg- high- superconductor with , that is, with K, has already been given in our recent paper .
3.2. Spin Flow
First of all, we did calculations for two cases in which the sample temperature has been set to and K. The BCS curve gives the values of and meV as the amplitudes of superconducting gap at and K, respectively. The difference between these two values is small, so we have found that there is no significant difference between the current- (-) voltage () characteristics calculated for these two temperatures, as is expected. In the following, therefore, the is set to K. Here note that the magnetic field dependence of the magnetization of As with has already been measured at K by Ohno .
The calculations for the coherent and WKB cases need a very large CPU time as compared with the incoherent one . Therefore, first, the - characteristics for given electron configuration, that is, resultant magnetic moment, have been calculated for the coherent, incoherent, and WKB cases. As a result, we have found that the results calculated for three cases are fairly similar to each other. In the following, therefore, only the incoherent tunneling case is considered because of the CPU times in the numerical calculations.
The normalized charge and spin currents, and , calculated for the present // tunneling junction are shown in Figure 2. Here note that the and have already been defined by in our recent paper  and that the used in (a) to (k) in Figure 2 is the same as that in (a) to (k) in Figure 1. Figure 2 clearly shows that the charge and spin currents are changed due to the change of the magnetization of . In order to directly see the currents due to the majority () and minority () spin electrons, we have drawn in Figure 3 the normalized currents calculated for the and spin electrons, and . Here note that the normalized current is equal to defined by in our previous paper , so that a relation is satisfied. (a) to (k) in Figure 3 correspond to those in Figure 2. Figure 3 shows that the tunneling nature changes due to the change of the magnetic moment, that is, the magnetization of As . For example, if the normalized voltage is fixed to , then we can see in (b), (c), (d), and (e) an interesting result such that the tunneling current due to the spin electron is larger than the one, but such a result is not found in (f), (g), (h), (i), (j), and (k). It is clear that the result is closely related to the electronic structural change of which causes the change of the magnetization.
Experimentally, it may be possible to observe the external magnetic field dependence of the tunneling current at a fixed external voltage . In order to reproduce such an experimental situation, we have calculated the magnetic moment dependence of charge and spin currents. The normalized charge current for the present purpose is defined byHere means the nonmagnetic phase, so that the magnetic moment () per Mn atom is . means the ferromagnetic phase; therefore, in the following, the symbol is replaced by the symbol . Using (6), we can calculate the normalized charge current as a function of . The raw values of and have already been calculated numerically, so that the ratio of those raw values is easily given. As a result, we can get the normalized spin current as a function of . The calculated normalized charge and spin currents, and , are shown in Figure 4(a) as a function of the calculated . By using the above and , we can easily get the flows and of the majority () and minority () spin electrons. Those are shown in Figure 4(b) as a function of . Figure 4 shows that the nature of the spin flow is changed at the with the value around . Namely, the tunneling due to the minority spin electron dominantly occurs when , but for the case of , such a tunneling phenomenon is not found. It is certain that such a change is closely related to the variation of the -band of the As . The value of the magnetization can be easily controlled by the external magnetic field . The - curve at K of As with has already been drawn in Figure in Ohno’s paper , which clearly shows that the T is a large external magnetic field enough to get the saturation of the magnetization. Here note that we have checked that the magnetic induction with the value of T has no considerable effect on the present superconductor.
3.3. Effect of Nonequilibrium
We are now considering the superconductors consisting of Cooper pairs (CPs) with a spin-singlet state. In the junctions involving the ferromagnetic materials and the superconductors, therefore, it is easily supposed that the unbalance in the numbers of the and spin electrons makes a decrease in the number of the CPs. This is just a nonequilibrium effect. The decrease in the number of CPs makes a decrease in the amplitude of the superconducting gap. Therefore, in order to take into account the influence of such a nonequilibrium effect, we have introduced a parameter with a range of , by which the is reduced to . It is clear that the case of means no consideration for the nonequilibrium effect.
Figure 1(e) shows that the Fermi level just locates on the -band with a pointed shape, so that a sizable unbalance in the numbers of the and spin electrons could be found in this case. Nevertheless, the nonequilibrium effect should not be so large; therefore, as an attempt we have calculated the - characteristics by setting to . The normalized currents calculated for and are shown in Figures 5(a) and 5(b), respectively. Apart from the reliability of the value of , the calculated results have showed that there is no significant difference between them. The above result tells us that a considerable nonequilibrium effect could not be found in the - characteristics of the // tunneling junction studied here. This means that the present // tunneling junction stably works as a device to switch the and spin flows by varying the within the range of T.
The -axis tunneling of the majority and minority spin electrons has been studied for the // tunneling junction consisting of As magnetic semiconductor with , an insulator with a realistic dimension, and (Hg-) high- superconductor . We have deduced the magnetic moment () per Mn atom from the band structure calculations for the As and calculated the normalized charge and spin tunneling currents, and , and the flows of the majority () and minority () spin electrons, and , as a function of at a given external voltage . We have found that the tunneling due to the minority spin electron dominantly occurs when , but such a phenomenon is not found in the case of . We have pointed out that the present // tunneling junction seems to work as a switching device in which the and spin flows can be easily controlled by varying slightly the external magnetic field.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
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