A CEMES research team recently demonstrated for the first time the evolution of the magnetic properties of the Heusler Co2MnSi alloy as a function of the type and amount of atomic disorder. For this, a multi-scale magnetic and structural analysis was carried out on samples whose atomic structure was modified by ion irradiation and then compared to calculations of the electronic structure and relaxation coefficient as a function of the disorder type.
In this study, CEMES researchers used He+ light ion irradiation to modify the atomic structure of an ultrathin Co2MnSi 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%) L21 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 D03 disorder type in the initial L21 matrix .
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 D03 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 L21 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].
Nicolas Biziere – chercheur CNRS – CEMES-CNRS
Gerard BenAssayag – chercheur CNRS – CEMES-CNRS
Lionel Calmels – Professeur Université Paul Sabatier – CEMES-CNRS