Transport properties of magnetic tunnel junctions with ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ electrodes: The influence of temperature-dependent interface magnetization and electronic band structure

The transport properties of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Co}\text{−}\mathrm{Fe}$ magnetic tunnel junctions showing a tunnel magnetorestistance of 95% at low temperatures are discussed with respect to temperature-dependent magnetic moments at the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}∕\mathrm{Al}{\mathrm{O}}_x$ interface and electronic band structure effects. These junctions show a considerably larger temperature and bias voltage dependence of the tunneling magnetoresistance compared to $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ junctions, although the effective spin polarization of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ (66%) is larger than Co-Fe-B (60%). Especially, the tunnel magnetoresistance of the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ based junctions becomes inverse for large bias voltages. With increasing atomic disorder of the interfacial ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ its magnetic moments decrease and show a stronger temperature dependence. Even for the best atomic ordering achieved the corresponding spin-wave parameters of Mn and Co at the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}∕\mathrm{Al}{\mathrm{O}}_x$ interface are significantly larger than expected for ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ bulk and also larger than the spin-wave parameters of Co and Fe at a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}∕\mathrm{Al}{\mathrm{O}}_x$ interface. The influence of enhanced interfacial magnon excitation in the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Co}\text{−}\mathrm{Fe}$ junctions on their transport properties will be discussed as well as possible origins for the negative tunnel magnetoresistance at high bias voltages.

Figure 1

(a) TMR majorloops for “CMS100” (measured at $20\mathrm{K}∕1\mathrm{mV}$ bias) and “CMS100ag” $(16\mathrm{K}∕20\mathrm{mV})$ based MTJs. (b) Typical differential conductance $G_p=dj∕dU$ of both samples measured at RT in the parallel state. The inset shows the typical zero-bias anomaly in the differential conductance $G$ of a “CMS100” junction at low temperature for antiparallel (AP) and parallel (P) alignment of the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ and the Co-Fe electrodes.

Figure 2

(Color) (a) Typical bias voltage dependence at low temperature for “CMS100” (measured at $20\mathrm{K}$), and “CoFeB” $\left(30\mathrm{K}\right)$ and for “MTJ-NiFe” $\left(10\mathrm{K}\right)$. All data is normalized to the maximum TMR at low bias. The inset shows the TMR amplitude for “CMS100ag” $\left(16\mathrm{K}\right)$ (b) Typical normalized temperature dependence of “CMS100” (measured at $10\mathrm{mV}$), “CoFeB” $\left(10\mathrm{mV}\right)$, “MTJ-NiFe” $\left(10\mathrm{mV}\right)$ and “CMS100ag” $\left(20\mathrm{mV}\right)$.

Figure 3

Typical bias voltage dependence in the $\pm 1500\mathrm{mV}$ bias voltage range of a “CMS100” junction, measured at room temperaure. The insets show TMR minor loops measured at $+10\mathrm{mV}$ and $-1300\mathrm{mV}$, when only the magnetization of the soft ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ electrode is switched by the external magnetic field. The magnetizations of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ and Co-Fe are aligned parallel (antiparallel) for magnetic fields of $-60\mathrm{Oe}$ $(+60\mathrm{Oe})$.

Figure 4

XAS-TEY spectra at $\mathrm{Si}\text{−}K$ edge of the annealed sample “CMS100” (gray curve, measured at $15\mathrm{K}$) and of the as-grown sample “CMS100ag” (black curve, measured at $300\mathrm{K}$). The inset shows the according XMCD asymmetries of both samples and the XMCD asymmetry calculated by the SPR-KKR code (Ref. 11).

Figure 5

(Color) XAS-FY spectra at (a) Mn- and (b) $\mathrm{Co}\text{−}L$ edge of “CMS100ag” and “CMS100.” The spectra were measured at $15\mathrm{K}$. The XANES oscillations found for the annealed “CMS100” are marked by arrows and are also visible at $300\mathrm{K}$.

Figure 6

(Color) (a) Magnetization loop of the as-grown sample “CMS100ag” measured at $15\mathrm{K}$ (open circles). The black line is a fit by the Langevin function (see text). (b) X-ray absorption spectra and magnetic circular dichroism in TEY mode at the $\mathrm{Co}\text{−}L$ edge of “CMS100” measured at $15\mathrm{K}$. (c) X-ray absorption spectra and magnetic circular dichroism in TEY mode at the $\mathrm{Mn}\text{−}L$ edge of “CMS100” measured at $15\mathrm{K}$. The red curve $\delta \left(h\nu \right)$ is the difference of the Mn XMCD asymmetries of “CMS100” measured at $15\mathrm{K}$ and $300\mathrm{K}$, respectively. The inset shows differences of the Co XMCD asymmetry at $15\mathrm{K}$ between the ordered and disordered sample “CMS100” and “CMS100ag,” the photon energy is defined with respect to the energy position of maximum intensity of the $\mathrm{Co}\text{−}{L}_3$ resonance.

Figure 7

(Color) Valence band spectra of the disordered and ordered samples “CMS100ag” and “CMS100,” respectively. The valence band spectrum intensities of the $20\mathrm{nm}$ thick Au films covering one-half of the surfaces of both samples are scaled by a factor of 0.69 (Au on top of “CMS100”) and 0.71 (Au on top of “CMS100ag”), respectively. The measurements were performed at room temperature. The lowest curve “calculated” shows the calculated density of occupied states for bulk ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ (see text).

Figure 8

(a) Typical normalized TMR bias voltage dependence of an in situ annealed “CMS100” junction around zero bias, measured at $16\mathrm{K}$. (b) Typical temperature dependence of the normalized zero bias TMR, $R^P$ and $R^{AP}$ measured with two-probe ac technique($0.7\mathrm{mV}$ amplitude). The experimental data can be well reproduced by the magnon-assisted tunneling model (solid lines) proposed by Han et al. (Ref. 31).

Figure 9

Energy dependence of the effective spin polarization for $s$ electrons in perfectly ordered bulk ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ calculated by the SPR-KKR code (Ref. 11).