18:30 〜 18:45
▼ [21p-W241-20] Vortex Spin-Torque Oscillator using Co2(Fe,Mn)Si Heusler Alloys with various Fe-Mn Compositions
キーワード：spin torque oscillator,Heusler alloys,magnetic vortex
Introduction
A spin torque oscillator (STO) is a nanoscale oscillator consisting of a FM/NM/FM (FM: ferromagnetic metal, NM: nonmagnetic metal or insulator) junction. Since the output power (Pout) of a STO is determined by the magnetoresistance ratio of the junction, FMs exhibiting high spin polarization are strongly desired to develop high Pout STOs. Previously, our group achieved high Pout exceeding 1 nW by using Co2(Fe,Mn)Si (CFMS) Heusler alloys [1-3]. Also, in the recent work, we developed CFMS-based vortex STO exhibiting high Pout of 10.3 nW with an extremely high quality factor (Q) of 4000 [4]. Although these studies demonstrated the advantage of the use of CFMS alloys for achieving high Pout, systematic investigations on the relationships among the magnetic properties of CFMS films, the oscillation properties, e.g., Pout, and the threshold current (Ith) are still missing. In this study, we fabricated CFMS-based vortex STOs with various Fe-Mn compositions, and investigated the influences of the magnetic properties on the spin torque oscillations.
Experiments
CFMS (20)/Ag (5)/CFMS (30) (in nanometer) films were prepared on Cr/Ag-buffered MgO (001) substrates by using an ultrahigh-vacuum-compatible magnetron sputtering system with a base pressure Pbase < 1 × 10-7 Pa. The Fe-Mn composition of the CFMS films were controlled by co- sputtering a Co2FeSi and a Co2MnSi alloy targets. The CFMS films were deposited at room temperature, and were annealed at 500 degrees C to promote the L21 ordering. The structural analysis and the magnetization measurement were carried out by using X-ray diffraction and a vibrating sample magnetometer, respectively. The 30-nm-thick CFMS layer was formed into a 240-nm-diameter circular shape by employing electron-beam lithography and Ar-ion etching to stabilize a magnetic vortex. In order to excite the magnetization dynamics, dc current was fed into the device using a source meter, and the rf signal from the device was detected by using a spectrum analyzer.All the CFMS films showed high L21 ordering (> 60%) regardless of the Fe-Mn compositions whereas both the saturation magnetization (Ms) and the coercivity (Hc) of the CFMS films monotonically increased with increasing the Fe concentration. Pout of the device was increased by the Fe substitution and the maximum Pout of 15. 9 nW was obtained for Co2Fe0.4Mn0.6Si. Also, similar to the dependences of Ms and Hc on the Fe-Mn composition, almost linear increase of Ith was observed except around Co2Fe0.4Mn0.6Si where a remarkable reduction of Ith was observed. This result suggests that Ith of a vortex STO is determined not only by Ms, Hc and the spin polarization of FM electrodes but also by the Gilbert damping constant regardless of the strongly non-uniform magnetization configuration of a magnetic vortex. These knowledges would be profitable for the development of high Pout and highly efficient vortex STOs.
References
[1] R. Okura et al., APL 99, 052510 (2011). [2] T. Seki et al., APL 105, 092406 (2014). [3] T. Yamamoto et al., APL 106, 092406 (2015). [4] T. Yamamoto et al., The 77th JSAP Autumn Meeting, 2015.
A spin torque oscillator (STO) is a nanoscale oscillator consisting of a FM/NM/FM (FM: ferromagnetic metal, NM: nonmagnetic metal or insulator) junction. Since the output power (Pout) of a STO is determined by the magnetoresistance ratio of the junction, FMs exhibiting high spin polarization are strongly desired to develop high Pout STOs. Previously, our group achieved high Pout exceeding 1 nW by using Co2(Fe,Mn)Si (CFMS) Heusler alloys [1-3]. Also, in the recent work, we developed CFMS-based vortex STO exhibiting high Pout of 10.3 nW with an extremely high quality factor (Q) of 4000 [4]. Although these studies demonstrated the advantage of the use of CFMS alloys for achieving high Pout, systematic investigations on the relationships among the magnetic properties of CFMS films, the oscillation properties, e.g., Pout, and the threshold current (Ith) are still missing. In this study, we fabricated CFMS-based vortex STOs with various Fe-Mn compositions, and investigated the influences of the magnetic properties on the spin torque oscillations.
Experiments
CFMS (20)/Ag (5)/CFMS (30) (in nanometer) films were prepared on Cr/Ag-buffered MgO (001) substrates by using an ultrahigh-vacuum-compatible magnetron sputtering system with a base pressure Pbase < 1 × 10-7 Pa. The Fe-Mn composition of the CFMS films were controlled by co- sputtering a Co2FeSi and a Co2MnSi alloy targets. The CFMS films were deposited at room temperature, and were annealed at 500 degrees C to promote the L21 ordering. The structural analysis and the magnetization measurement were carried out by using X-ray diffraction and a vibrating sample magnetometer, respectively. The 30-nm-thick CFMS layer was formed into a 240-nm-diameter circular shape by employing electron-beam lithography and Ar-ion etching to stabilize a magnetic vortex. In order to excite the magnetization dynamics, dc current was fed into the device using a source meter, and the rf signal from the device was detected by using a spectrum analyzer.All the CFMS films showed high L21 ordering (> 60%) regardless of the Fe-Mn compositions whereas both the saturation magnetization (Ms) and the coercivity (Hc) of the CFMS films monotonically increased with increasing the Fe concentration. Pout of the device was increased by the Fe substitution and the maximum Pout of 15. 9 nW was obtained for Co2Fe0.4Mn0.6Si. Also, similar to the dependences of Ms and Hc on the Fe-Mn composition, almost linear increase of Ith was observed except around Co2Fe0.4Mn0.6Si where a remarkable reduction of Ith was observed. This result suggests that Ith of a vortex STO is determined not only by Ms, Hc and the spin polarization of FM electrodes but also by the Gilbert damping constant regardless of the strongly non-uniform magnetization configuration of a magnetic vortex. These knowledges would be profitable for the development of high Pout and highly efficient vortex STOs.
References
[1] R. Okura et al., APL 99, 052510 (2011). [2] T. Seki et al., APL 105, 092406 (2014). [3] T. Yamamoto et al., APL 106, 092406 (2015). [4] T. Yamamoto et al., The 77th JSAP Autumn Meeting, 2015.