RESUMEN
Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite.
RESUMEN
By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.
RESUMEN
The free energies of the austenite, the (modulated) premartensite and the unmodulated martensite of Ni2MnGa are determined using density functional theory and including quasiharmonic phonons and fixed-spin-moment magnons. This approach very well reproduces the complete phase sequence (martensite<-->premartensite<-->austenite) of stoichiometric Ni2MnGa as a function of temperature. By analyzing the relevant free energy contributions, we also understand the delicate interplay of phonons and magnons driving both phase transitions.
RESUMEN
In the quest for ultra-high-density magnetic recording, new materials in the nanometre range have attracted much interest over the last decade involving intense studies of L1(0) phases of contemporary or future storage media materials like FePt or CoPt nanoparticles. Based on large-scale density functional theory calculations, we provide a systematic overview of the structural and magnetic properties of various morphologies of FePt and CoPt nanoclusters with diameters up to 3 nm. In this size range, the ordered multiply twinned morphologies are energetically favoured over the nanoclusters with the desired layer type L1(0) and high magnetocrystalline anisotropy. Other nanoparticles of interest, like FePd, also show a preference for multiply twinned structures or exhibit, as in the case of MnPt nanoclusters, a strong tendency for antiferromagnetic ordering instead of ferromagnetic order. The compositional trends of the various nanoparticles can be traced back to differences in the partial electronic density of states of the 3d element.
