Spin polarization and Gilbert damping of Co2Fe(GaxGe1−x) Heusler alloys

doi:10.1016/j.actamat.2012.07.045

Acta Materialia

Volume 60, Issue 18, October 2012, Pages 6257-6265

https://doi.org/10.1016/j.actamat.2012.07.045 Get rights and content

Abstract

The spin polarization (P) of ferromagnetic Heusler alloys, Co₂Fe(Ga_xGe_1−x) (x = 0, 0.25, 0.5, 0.75, and 1), is investigated by point contact Andreev reflection (PCAR) measurements. While the P values of the ternary Co₂FeGe and Co₂FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co₂Fe(Ge_0.5Ga_0.25) is as high as 0.69. Co₂Fe(Ge_0.5Ga_0.5) alloy shows a strong tendency for L2₁ ordering and a high Curie temperature of 807 °C. Ab initio calculations indicate that the band structures of the Co₂FeGe and Co₂Fe(Ge_0.5Ga_0.5) alloys with L2₁ or B2 structures are half-metallic. Thin films of the quaternary Co₂Fe(Ge_0.5Ga_0.5) alloy grown on MgO(0 0 1) substrates order to the L2₁ structure upon annealing at 500 °C, giving rise to a high P of 0.75. This is the highest P value reported for Heusler alloy thin films using the PCAR technique. Ferromagnetic resonance measurements show that the Gilbert damping constant of the film is ∼0.008. All these indicate that the Co₂Fe(Ge_0.5Ga_0.5) alloy is promising as a spin polarized current source for spintronics devices.

Introduction

Half-metallic ferromagnets (HFMs) were first proposed theoretically for half-Heusler NiMnSb and PtMnSb alloys by de Groot et al. [1]. Subsequently, several Co-based full Heusler alloys with the L2₁ structure were also predicted to be half-metal with high Curie temperatures [2]. The interest in Heusler alloys as spin polarized electron sources was boosted with the demonstration of the huge giant tunneling magnetoresistance (TMR) ratio of 570% at 2 K in the Co₂MnSi/Al–O/Co₂MnSi magnetic tunneling junction (MTJ) by Sakuraba et al. [3] The high TMR value was attributed to the half-metallic nature of the Co₂MnSi Heusler alloy; however, the TMR value of this MTJ is degraded at room temperature (RT) to only 67%. The reason for this large temperature degradation of TMR is considered to be due to the presence of quasi-particle states in the spin-down density of states at the Fermi level, which causes a large decrease in the spin polarization at room temperature. Fermi level tuning by alloying a fourth element to ternary Heusler alloys has been proposed to circumvent this problem and Co₂Fe(Al_1−xSi_x) was the first such material to be was investigated along these lines [4], [5], [6]. Tezuka et al. demonstrated the Fermi level tuning in Co₂Fe(Al_0.5Si_0.5) experimentally [5], a finding that was also supported by ab initio calculations of the density of states (DOS) [4] and by our previous point contact Andreev reflection (PCAR) measurements [6], [7]. Subsequently, several theoretical calculations showed the possibility of improving the spin polarization of Heusler alloys with site disorders by Fermi level tuning [8], [9].

We have systematically sought highly spin-polarized ferromagnetic (FM) alloys by PCAR measurements [6], [7], [10], [11]. Our search has led to the finding of highly spin-polarized Co-based quaternary Heusler alloys such as Co₂(Fe_1−xCr_x)Si [6], Co₂Mn(Ga_0.5Sn_0.5) [10], Co₂Mn(Ga_0.5Ge_0.5) [11] and Co₂Fe(Ga_0.5Ge_0.5) [12]. We also applied these alloys as FM layers of current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices and found that all of these gave rise to a rather high magnetoresistive area product change (ΔRA) of >6 mΩ μm², which is more than three times larger than can be obtained from the CPP-GMR devices using normal FM alloys, i.e. 1–2 mΩ μm². Among these alloys, the Co₂Fe(Ga_0.5Ge_0.5) alloy is of particular interest because its use as FM layers led to ΔRA of 9.5 mΩ μm² and an MR ratio exceeding 41% at room temperature [12]. The alloy composition used in the CPP-GMR device has been optimized using bulk alloys; thereafter films with an optimized composition have been prepared. In this paper, we report a systematic investigation on the structure, magnetic properties and intrinsic spin polarization of bulk Co₂Fe(Ga_xGe_1−x) alloys. The magnetic and transport properties of the films of optimized Co₂Fe(Ga_0.5Ge_0.5) alloy were also investigated.

Section snippets

Experimental

Bulk Co₂Fe(Ga_xGe_1−x) (x = 0, 0.25, 0.5, 0.75, and 1) alloys were prepared by arc melting high-purity elements in an Ar atmosphere. The alloys were annealed in evacuated fused silica tubes for 168 h at 673 K to attain phase equilibria. Curie temperatures (T_c), L2₁ ↔ B2 order–disorder and melting temperatures were measured by differential thermal analysis (DTA) at a heating rate of 20 °C min⁻¹. The bulk alloys were annealed at slightly below the L2₁ ↔ B2 order–disorder temperature. The constituent phases in

Bulk alloys

Fig. 1a shows the XRD patterns of Co₂Fe(Ga_xGe_1−x) alloy powders for x = 0, 0.25, 0.5, 0.75 and 1. It clearly shows that a certain degree of L2₁ order exists in all the Co₂Fe(Ga_xGe_1−x) samples. Note that Balke et al. [19] reported that the characteristic (1 1 1) and (2 0 0) reflections of the L2₁ structure were not observed in the XRD partterns of the Co₂FeGe and Co₂FeGa alloys and employed the extended X-ray absorption fine structure technique to show the existence of L2₁ order. However, our XRD results

Discussion

Earlier ab initio calculations on Co₂FeGe and Co₂FeGa did not predict these were half-metals [18], [19], [20]. Our results, which consider the on-site correlations of Fe in the Y site in the X₂YZ L2₁ lattice and the Hubbard parameter U, which were not considered by the earlier reports [18], [19], [20], suggest that Co₂FeGe could be half-metal. However, the current spin polarization of the Co₂FeGe alloy measured by PCAR was only 0.56, which is lower than that for Co₇₅Fe₂₅ (P ∼ 0.58) [29]. XRD and

Conclusion

The structure, magnetization and spin polarization of a new series of Co₂Fe-based Heusler alloys, Co₂Fe(Ga_xGe_1−x) (0 < x < 1), were investigated. The quaternary alloys exhibited L2₁ order in the as-cast as well in the annealed conditions. The intrinsic spin polarization of the quaternary alloys measured using the PCAR technique was found to be as high as 69% for x = 0.5, whereas the ternary alloy showed much lower spin polarization. Ab initio calculations performed with GGA + U potentials showed that

Acknowledgement

This work was supported by the Grant-in-Aid for Scientific Research (A) 22246091.

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Spin polarization and Gilbert damping of Co2Fe(GaxGe1−x) Heusler alloys

Abstract

Introduction

Section snippets

Experimental

Bulk alloys

Discussion

Conclusion

Acknowledgement

Acta Mater

J Phys Chem Solids

Phys Rev Lett

Phys Rev B

Appl Phys Lett

Phys Rev B

Appl Phys Lett

Appl Phys Lett

J Appl Phys

J Phys Condens Matter

J Phys D Appl Phys

Appl Phys Exp

Appl Phys Lett

Phys Rev B

J Appl Phys

Phys Rev Lett

J Phys Condens Matter

Phys Rev B

Appl Phys Lett

Spin polarization and Gilbert damping of Co₂Fe(Ga_xGe_1−x) Heusler alloys