Non-collinear magnetism in iron at high pressures

We investigate the equation of state and elastic properties of hcp iron at high pressures and high temperatures using first principles linear response linear-muffin-tin-orbital method in the generalized-gradient approximation. We calculate the Helmholtz free energy as a function of volume, temperature, and volume-conserving strains, including the electronic excitation contributions from band structures and lattice vibrational contributions from quasi-harmonic lattice dynamics...

We have implemented non-collinear GGA and a generalized Bloch's theorem to simulate unconmensurate spiral arrangements of spins in a Density Functional Theory code based on localized wave functions. We have subsequently performed a thorough study of the different states of bulk Iron. We determine the minimal basis set required to obtain reliable orderings of ground and excited states. We find that the most stable fcc phase is a spiral with an equilibrium lattice constant 3.56...

We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron at high pressures using both first-principles linear response quasiharmonic calculations based on the full potential linear-muffin-tin-orbital (LMTO) method and the particle-in-cell (PIC) model for the vibrational partition function using a tight-binding total-energy method. The tight-binding model shows excellent agreement with the all-electron LMTO method. When hcp structure is stable, the calcula...

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of Iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with theore...

Electronic structure and magnetic properties of Fe$_3$Se$_4$ are calculated using the density functional approach. Due to the metallic properties, magnetic moments of the iron atoms in two nonequivalent positions in the unit cell are different from ionic values for Fe$^{3+}$ and Fe$^{2+}$ and are equal to $M_1=2.071 \mu_B$ and $M_2=-2.042 \mu_B$, making the system ferrimagnetic. The total magnetic moment for the unit cell is $2.135 \mu_B$. Under isotropic compression, the tot...

Experimental measurements of Raman spectra for hcp iron at high pressure show two modes over a considerable pressure range in contrast to the prediction of one doubly degenerate mode for the hcp lattice. We use density functional theory to investigate the influence of magnetic order on the Raman active modes of hcp iron. We find an antiferromagnetic state that lifts the degeneracy of the transverse optical mode, and yields stable antiferromagnetic moments up to approximately ...

Experimentally discovered compounds in the iron-nitrogen system belong to the low concentration part of the Fe-N phase diagram. In our paper, which is based on ab initio calculations we have studied formation and stability of high-pressure iron mono-nitride phases, and particularly the new magnetic phase with a NiAs-type structure. We have investigated the role of dynamic, thermodynamic and electronic properties, such as electronic correlations and pressure-induced phase stab...

The recent discovery of superconductivity in hexagonal iron under pressure poses a question about whether it is of conventional (phonon) or unconventional (magnetic?) origin. We present first-principles calculations of the electron-phonon coupling in iron at $P\agt 15$ GPa, and argue that a conventional mechanism can explain the appearance of superconductivity, but not its rapid disappearance at $P\agt 30$ GPa. We suggest that spin fluctuations, ferro- and/or antiferromagneti...

We present the magnetic Moment Tensor Potentials (mMTPs), a class of machine-learning interatomic potentials, accurately reproducing both vibrational and magnetic degrees of freedom as provided, e.g., from first-principles calculations. The accuracy is achieved by a two-step minimization scheme that coarse-grains the atomic and the spin space. The performance of the mMTPs is demonstrated for the prototype magnetic system bcc iron, with applications to phonon calculations for ...

The Atomic Cluster Expansion (ACE) provides a formally complete basis for the local atomic environment. ACE is not limited to representing energies as a function of atomic positions and chemical species, but can be generalized to vectorial or tensorial properties and to incorporate further degrees of freedom (DOF). This is crucial for magnetic materials with potential energy surfaces that depend on atomic positions and atomic magnetic moments simultaneously. In this work, we ...