Fixed Spin Moment Research Articles

Magnetoelastic anomalies in Fe-Ni Invar alloys.

The Fe-Ni alloy is simulated by four ordered structures, whose total energies are obtained as a function of volume and magnetic moment by band-structure calculations employing the fixed-spin-moment (FSM) method. An analytic fit of these E(M,V) surfaces is made and permits an interpolation for varying Ni concentration. These parametrized surfaces allow the introduction of the thermodynamics of spin fluctuations. Among others, the following characteristic Invar properties are calculated: the magnetic contribution to the thermal expansion coefficient ${\mathrm{\ensuremath{\alpha}}}_{\mathit{m}}$, the critical pressure for the disappearance of magnetism ${\mathit{P}}_{\mathit{c}}$, the pressure dependence of the Curie temperature ${\mathit{dT}}_{\mathit{C}}$/dP, and the high-field susceptibility. These quantities agree well with experiment, especially their variation with Ni content. The key quantity is ${\mathrm{\ensuremath{\alpha}}}_{\mathit{m}}$, which shows a narrow minimum as a function of the Ni concentration before the \ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\gamma} (bcc\ensuremath{\rightarrow}fcc) phase transition occurs. Near the Invar composition the large negative contribution ${\mathrm{\ensuremath{\alpha}}}_{\mathit{m}}$ compensates the positive phonon part so that the total thermal expansion nearly vanishes near room temperature. The present combination of models provides new insight into the nature of the Invar effect.

Local-spin-density calculations of antiferromagnetic YBa 2Cu 3O 6 and La 2CuO 4

The two antiferromagnetic insulators are investigated by generalized Fixed-Spin-Moment (FSM) calculations using the Augmented-Spherical-Wave (ASW) method. In this scheme an antiferromagnetic (AF) moment on the copper atoms in the CuO 2 plane is enforced by a constraint. This procedure allows to search for a local minimum in the total energy, but this has not been found for a finite moment on Cu. Furthermore, no gap opens at the Fermi energy so that these two compounds remain metallic. Since the calculations are done for the experimentally observed structure and with high accuracy, it must be the present form of the local spin density approximation which causes this discrepancy with experimental findings.

On the coexistence of low- and high-spin states in transition metal systems

We discuss the unusual magnetic behaviour of itinerant magnetic systems close to magnetovolume instabilities. It is shown that the unusual properties result from transitions between, and coexistence of, high-spin (HS) and low spin (LS) states. We argue that the Invar effect is related to the coexistence of LS and HS states over a substantial temperature range. For the theoretical description an expansion of the free energy in terms of the magnetization and volume is used, which at zero temperature reproduces one-electron band structure results obtained with the fixed-spin-moment (FSM) method (1).

Metallic magnetism and magnetic volume collapse

We present a new description of possible magnetovolume instabilities in itenerant electron systems based on a generalized Ginzburg-Landau expression. The Landau coefficients are input data and are obtained by reproducing binding surfaces (total energies as function of volume and the magnetic moment), which are the result of spin-polarized band structure calculations performed with the fixed-spin-moment (FSM) method. Finite temperature results are obtained by constructing a classical field theory. It is shown that essentially two Landau parameters (the fourth-order coefficient and the critical volume) may serve to classify itinerant magnetic systems as stable or potentially instable systems. In the first category the magnetic phase transition is of second order while, in the latter, the transition can be of first order, associated with a large magnetic volume collapse. We show that all INVAR systems are potentially instable.

Antiferromagnetic and ferromagnetic gamma-manganese generalisation of the fixed-spin-moment method

We present ab initio band structure calculations for the magnetic and nonmagnetic γ and δ phases of manganese. Paramagnetic, ferromagnetic and antiferromagnetic spin ordering as well as the tetragonal distortion of the γ phase are taken into account. The relative stability of these different modifications is studied by total energy as a function of volume. The fixed spin moment (FSM) method is generalised to calculate the energy surface of the antiferromagnetic phase. γ-Mn is found to order antiferromagnetically, the tetragonal distortion of the lattice is driven by the formation of antiparallel spin alignment.

Magneto-volume effects in Fe-Ni invar: A first principles theory

A complete theory of the magnetic properties, the magneto-volume effect, and the thermal properties of weak itinerant ferromagnets is proposed in terms of spin- and volume fluctuations. This theory combines the results of band structure calculations (only valid at 0 K) with fluctuation models including temperature effects. We use the results of previous spin-polarised band structure calculations performed with the fixed-spin-moment (FSM) method using the local spin density approximation for exchange and correlation, where the total energy is obtained as a function of volume and the magnetic moment. The fluctuations are treated classically within the frame of a Ginzburg-Landau theory. Numerical results are given for the Fe-Ni invar system.

Generalized magnetic susceptibilities of itinerant magnetic systems

We use a multiband Hubbard model with intra-atomic Coulomb and exchange interactions for a microscopic description of the magnetic and nonmagnetic properties of fcc iron. High- and low-spin states are calculated by using a simplified version of the fixed-spin-moment (FSM) method of Moruzzi et al., Phys. Rev. B 34 (1986) 1784. We find that the wave-vector dependent spin susceptibility of the high-spin state differs remarkably from the susceptibility of the low-spin state.

Magnetic and electronic properties of CuRh alloys

The electronic properties of the disordered partly metastable alloy Cu xRh 1−x are studied by band theory assuming five ordered structures. The calculated total energies yield the heats of formation from which metastability is deduced in a certain range of composition in agreement with experiment. Using the fixed spin moment method the spin susceptibility is calculated. The experimentally observed maximum in the susceptibility as a function of composition can be explained by the present results. It is caused by a band filling connected with covalent interactions between Rh and Cu.

Hunting Magnetic Phases with Total-Energy Spin-Polarized Band Calculations

ABSTRACTComments are made on total energy band calculations as tools for exploring properties of solids; the importance of fixed spin moment calculations is noted. Use of energy - magnetisation curves to locate magnetic phases is described. Detailed results for fcc and bcc Co and Ni and phase diagrams on the magnetisation - volume plane exhibit two new phases for each metal and show that ferromagnetic fcc Co and bcc Ni break down at small volumes and make first order transitions to nonmagnetic phases in a metamagnetic volume range.

Itinerant metamagnetism in YCO2

The cubic Laves phase of YCo2 is near a transition from a paramagnetic to a magnetically ordered state. Self-consistent energy-band calculations yield the total energy and the magnetic moment as functions of volume. The new fixed spin-moment (FSM) method, which allows one to calculate the total energy as an explicit function of the magnetisation, is introduced. At the theoretical equilibrium volume YCo2 is found to be a strongly enhanced Pauli paramagnet, but at a slightly larger lattice constant a metamagnetic transition seems possible. Its occurrence can be understood on the basis of a double minimum in the total energy obtained from the present FSM calculations, which lead to an estimated critical field of about 350 T. In the magnetic state the Y moment is coupled antiferromagnetically to the Co moment.