(left) STEM-HAADF (top) image of the reference Co2MnSi sample and STEM intensity (bottom) in the L21 and B2 regions of the sample. (Middle) Ferromagnetic absorption spectra at 19 GHz as a function of the He+ ion fluence. (right) SPRKKR calculation of damping coefficient as a function of disorder rate.

In the L2 ₁ crystal order, it is predicted that the Co ₂ MnSi Heusler alloy has a spin polarization of 100%, with a high magnetization of the order of 1.3 T, which must induce a very low dynamic relaxation coefficient α (of the order of 5 x 10 ^-4 ). Moreover, its high Curie temperature around 900K leads to a great stability of its magnetic properties at ambient temperature. It is therefore one of the most promising materials for spintronics applications, in particular for spin-transfer memories which require both high spin polarization and low α. However, these remarkable properties are rarely observed experimentally. One of the common reasons for this disagreement is the presence of structural defects in the material.

In this study, CEMES researchers used He ⁺ light ion irradiation to modify the atomic structure of an ultrathin Co ₂ MnSi film produced by UHV sputtering. To determine the different type of disorder and their rate, they performed macroscopic X-ray diffraction under normal and anomalous conditions, as well as a quantitative analysis of the atomic structure by electron microscopy in STEM-HAADF mode (Figure 1 left). From a reference material with a majority (75%) L2 ₁ order and a minority (25%) B2 order, they showed that ion irradiation favors on the one hand the disorder between the Mn-Si sites, thus increasing the B2 phase, and on the other hand induces random exchanges between Co and Mn atoms and therefore a D0 ₃ disorder type in the initial L2 ₁ matrix [1].

The magnetic properties of the alloy as a function of the fluence (nbr of ions / cm²) have been studied by ferromagnetic resonance. With this technique both the static (magnetization, crystal anisotropy, exchange constant) and dynamic (α and extrinsic contributions) parameters could be measured as a function of the disorder rate. Those ones were compared to calculations of electronic structure and α performed with the SPRKKR code which takes into account the intra- and inter-band spin transitions in the calculation of α. Although the experimental values ??are different from those calculated due to the contribution of two-magnon processes related to the structural inhomogeneity of the material, the general tendency of an increased α with disorder is recovered in the calculated phase diagram (Figure 1). However, in the case of D0 ₃ disorder type, the decrease of the magnetization and the increase of α are more important experimentally than by calculations [2, 3]. Thus, if a disorder between the Mn and Si sites does not seem to modify the intrinsic properties of the material with respect to its ordered phase, and in agreement with the theory, they have shown that the disorder between Co and Mn sites destroys the magnetic properties faster than predicted by usual ab-initio calculations. From a technological point of view, these results demonstrate that the partially disordered phase B2, which is easier to grow than a perfectly L2 ₁ phase, may be privileged, especially since this phase can be stabilized by ion irradiation. All these results were published in a series of articles [1,2,3].

References

• First-principles calculation of the effects of partial alloy disorder on the static and dynamic magnetic properties of Co2MnSi. B. Pradines, R. Arras, I. Abdallah, N. Biziere, and L. Calmels, Physical Review B. 95, 094425, 2017.
• Evolution of magnetic properties and damping coefficient of Co2MnSi Heusler alloy with Mn/Si and Co/Mn atomic disorder. I. Abdallah, B. Pradines, N. Ratel-Ramond, G. BenAssayag, R. Arras, L. Calmels, J.F. Bobo, E. Snoeck and N. Biziere, Journal of physics D, 50, 035003, 2017.
• Structural and magnetic properties of He+ irradiated Co2MnSi Heusler alloys. I. Abdallah, N. Ratel-Ramond, CMagen, B. Pecassou, R. Cours, A.Arnoult, M. Respaud, J.F Bobo, G. BenAssayag, E. Snoeck and N. Biziere, Material Research Express 3, 046101, 2016.

Contact

Nicolas Biziere – chercheur CNRS – CEMES-CNRS

Gerard BenAssayag – chercheur CNRS – CEMES-CNRS

Lionel Calmels – Professeur Université Paul Sabatier – CEMES-CNRS