Electronic and magnetic properties of $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$ films embedded in vertical spin valve devices

We studied metastable $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_3\mathrm{Si}$ and ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.

Figure 1

(a) Schematic diagram of the device structure and the configuration of the magnetoresistance measurements for vertical transport. (b) Optical micrograph of the spin valve device. (c) Current-voltage ($I\text{−}V$) characteristics for devices D1 and D3 at temperatures of 295 K (solid lines) and $4.2\mathrm{K}$ (dashed lines).

Figure 2

(a) Change in resistance ($\mathrm{\Delta }R$) as a function of an external in-plane magnetic field (upward and downward sweeps) for devices D2 and D4 along the $\left[110\right]$ direction. The inset displays the typical anisotropic magnetoresistance (AMR) signals obtained for the lateral transport in the $\mathrm{Fe}{\mathrm{Si}}_3$ stripes. (b) Spin valve signal $\mathrm{\Delta }{R}_{\text{max}}$ as a function of the $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$ interlayer thickness $t$ for devices D1, D2, and D3. The solid line is a guide to the eye. (c) One-way normalized SQUID magnetization curves along the $\left[110\right]$ direction (applied field swept from negative to positive fields) for ${\mathrm{Fe}}_3\mathrm{Si}$/$\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$/${\mathrm{Fe}}_3\mathrm{Si}$ samples with different spacer thicknesses. (a–c) All the presented results were measured at room temperature.

Figure 3

Temperature dependence of the spin valve signal $\mathrm{\Delta }{R}_{\text{max}}$ and the relative magnetoresistance MR for device D4. Bottom right inset: Spin valve signals of device D4 at different temperatures. Top left inset: Spin valve signals (magnetoresistances) of device D2 at 295 K (red) and 4.2 K (blue).

Figure 4

(a) Calculated band structure of bulk $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$ for the experimentally determined strained lattice structure according to Ref. [12] (solid line) and for the fully relaxed lattice structure determined by DFT (dashed lines). (b) Resistance normalized to its room-temperature value ($R/R_{\text{RT}}$) for devices D1, D2, and D3 as a function of temperature. The inset displays the corresponding temperature derivatives of the normalized resistances $\left[d(R/R_{\text{RT}})/\left(dT\right)\right]$.