Magnetic Properties of Mn-based Heusler alloy films with Ru doping

Recently Mn-based Heusler alloys have attracted considerable attention for the potential spintronic applications due to their high spin polarization, low damping constant, high magnetic anisotropy, and low magnetization
^[1]–[3]
. Especially the tetragonal Mn-based Heusler alloys like Mn
3
Ga, Mn
3
Ge, Mn
2
RhSn have been found to be possible candidates for spin transfer-torque applications
^{[4], [5]}
. It is noted that most of the previous investigations are focused on Mn-based heusler alloys composed of $3 d$ transition metal elements like Cu, Co, Fe, and Ni. But there are few reports about Mn-based Heusler alloys composed of $4 d$ and $5 d$ elements
^[6]
. Here we report a series of Mn-based Heusler alloy films with Ru doping grown on MgO substrate with Cr buffer layer using co-sputtering method from Ru and a series of Mn
x
Ge $(\mathrm {x}=2,2.2,2.4,2.5)$ targets. All the films were deposited at room temperature and then $in -$situ annealed at $700 K$ for 1h. Finally, a 2 nm Ta cap layer was deposited at room temperature in order to prevent oxidation. X-ray diffraction measurements were carried out using Singapore Synchrotron Light Source (SSLS). The results indicate that all the films with the thickness of 35 nm exhibit ordered $L 2 _{1}$ structure and that the epitaxial relationship is Mn
3
xRu
x
Ge [110]//Cr [110]//MgO [100]. The magnetizations of the films were measured at room temperature using VSM in a magnetic field of up to ± 2T along, [100],[110], and directions of the film. The measurement results of the 35 nm Mn
2.2
Ru
0.8
Ge film are shown in Fig. 1, which indicates that the direction perpendicular to the film is a magnetic hard axis. Moreover, the magnetization shows very sharp switching at 25 Oe with applied filed along [110] direction of the film, and the residual magnetization almost equals the saturation magnetization value 135 emu/cc. Besides, the magnetization along [100] direction is a hard magnetic axis compared to the [110] direction one. For further comparison, we selected the easy axis $M- H$ loop measurements of all the films with different Ru concentration as shown in Fig.2. The results indicate that both residual magnetization $(M_{r})$ and saturation magnetization $(M_{S})$ increase with Ru concentration increasing, while the coercivity $(H_{C})$ becomes smaller. Considering of the different valence electrons number $(N_{v})$ between Ru $(N_{v}=8)$ and Mn $(N_{v}=7)$, the fact that the Ru substitution of Mn can alternate the total magnetic moment and ferrimagnetic magnetic structures of the Mn
3-x
Ru
x
Ge Heusler alloys. Our attempts provide alternative candidates for the Heusler alloys-based spintronic applications.