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In recent years, nanocrystalline magnets based on the critical single domain size theory have received increasing attention. However, nanocrystalline magnets are difficult to enhance the remanent magnetization by forming textures with conventional magnetic field orientation. Texturing of nanocrystalline magnets is usually done using the hot pressing and hot deformation technique, but there are some difficulties in applying this technique to form texture in SmCo systems. To discover and solve the difficulties of obtaining texturized nanocrystalline SmCo-based magnets during the deformation process, samarium cobalt magnets with different degrees of deformation were first experimentally prepared by low strain rate hot deformation method. Then, molecular dynamics simulations were utilized to study the microstructural changes and the multi-phase formation process during hot deformation. The process of deformation together with the reason why it is difficult to form textures in SmCo systems by low-temperature hot deformation is discussed, and the strain energy densities of SmCo multiphase in multiple directions were calculated, which gives a feasible method to form low-temperature hot deformation textures in SmCo systems experimentally.

Density functional theory (DFT) calculations are used to investigate the correlation between the uniaxial negative thermal expansion (NTE) and negative linear compressibility (NLC) behaviors in Ag3[Co(CN)6]. First, we reproduce the uniaxial-NTE and NLC behaviors under temperature- and pressure-field. And then the temperature dependence of elasticity is studied. The abnormal nature of elastic constants C33and C11+C12as the function of temperature is predicted. The hardening of phonon modes (below 568.7 cm–1) with increasing temperature can be as an indicative for abnormal physical properties of Ag3[Co(CN)6]. Through analyzing the vibration mode with the strongest phonon hardening, the deformation of wine-rack motif in anisotropic framework can be identified as the mechanism that leads to the coexistence of uniaxial-NTE and NLC in Ag3[Co(CN)6]. The response of phonon group velocity along c-axis is intense and quicker than that of a-axis on heating and on compression, facilitating c-axis itself to be the carrier of abnormal uniaxial-NTE and NLC properties.

Abstract: Ni2MnGa based ferromagnetic alloys are ideal candidates for applications such as actuators, magnetic refrigerators or magnetostrictive transducers due to their attractive properties such as magnetic field induced shape memory effect and large magnetocaloric effect. The properties of these alloys (e.g., the martensitic transformation temperature T M ) sensitively depend on the composition. Understanding the composition dependence of these properties so as to design the alloy as desired is one of the main research topics in this area. In recent years, we have investigated the composition dependent elastic modulus and phase stability of Ni2MnGa-based alloys by using a first-principles method, in hope of clarifying their connection to the properties of these alloys. In this article, we review the main results of our investigations. We show that the tetragonal shear modulus C′ is a better predictor of the composition dependent T M than the number of valence electrons per atom (e/a) since the general T M ∼C′ correlation works for some of the alloys for which the T M ∼e/a correlation fails, although there exist several cases for which both the general T M ∼C′ and T M ∼e/a correlations break down. Employing the experimentally determined modulation function, the complex 5-layer modulated (5M) structure of the martensite of Ni2MnGa and the Al-doping effect on it are studied. We find that the shuffle and shear of the 5M structure are linearly coupled. The relative stability of the austenite and the martensites is examined by comparing their total energies. The non-modulated martensite β″′ with the tetragonality of the unit cell c/a>1 is shown to be globally stable whereas the 5M martensite with c/a<1 is metastable. The critical Al atomic fraction over which the martensitic transformation between the 5M martensite and austenite cannot occur is predicted to be 0.26, in reasonable agreement with experimental findings.