Phonon softening by magnetic moment in γ‐Fe

Atomic forces at finite magnetic temperatures: Phonons in paramagnetic iron

“…2 (grey lines). The results are very similar to recent calculations employing ultrasoft pseudopotentials [6,21]. For both, bcc and fcc iron, unstable (imaginary) phonon modes appear.…”

“…Accurate and efficient computational schemes for the theoretical prediction of parameter-free atomic forces and force constants are an essential prerequisite for many applications within first-principles materials design, such as lattice vibrations (phonon spectra, vibronic entropies, phase transitions) [1][2][3][4][5][6][7][8][9][10][11][12][13], diffusion processes (vacancies, impurities, etc.) [14], strain fields (e.g.…”

“…Due to the lack of alternatives, current theoretical approaches for, e.g., vibronic contributions of magnetic materials such as bcc iron, rely on calculations performed in the magnetic groundstate (e.g., the ferromagnetically saturated state) [13,20], or take the missing data from experiment [7]. Approaches going beyond these simple approximations employ fixed-spin calculations [6,21] or GGA+U [5], but still employ magnetically fully ordered configurations. Although it is known that magnetism can have a substantial impact on vibrational properties [22,23], the basic fundamentals are still poorly understood [5,8,23].…”

“…As a material system we chose one of the best studied magnetic materials, pure iron. Although vibronic excitations in iron have been studied intensively both, experimentally as well as theoretically since decades [1,5,6,8,11,21,22], many basic questions remain still open. In particular the impact of magnetism on lattice vibrations and phase stability is poorly understood up to now [5,8,11,22,29].…”

Effects of Co doping on the metamagnetic states of the ferromagnetic fcc Fe–Co alloy

“…Magnetovolume instabilities which lead to anti-invar behavior have been observed experimentally in fcc Fe [32,33,17]. From the theoretical point of view, first-principles calculations on FM fcc Fe, under the local spin density approximation (LSDA) [16,34,18,35] and the generalized gradient approximation (GGA) [36,26,[37][38][39][40], have found two magnetic states. For the unambiguous description of the binding-energy curves in metamagnetic systems, the implementation of both LSDA and GGA has usually been accompanied by the fixed spin moment (FSM) method [15,16,41].…”