Fixed Spin Moment Research Articles

Ab-initio study of quaternary Heusler alloys LiAEFeSb (AE = Be, Mg, Ca, Sr or Ba) and prediction of half-metallicity in LiSrFeSb and LiBaFeSb

The quaternary Heusler alloys with composition LiAEFeSb have been studied in this manuscript (AE = Alkaline Earth elements Be, Mg, Ca, Sr and Ba) for their potential applications in spintronics. All the investigations have been performed by employing density functional theory. A systematic comparison of results obtained within local density approximation (LDA) and the generalized gradient approximation (GGA) done. Our calculations reveal that all the studied alloys have lowest energy in type-I arrangement of basis and exhibit interesting electronic and magnetic properties. All the alloys exhibit a stable ferromagnetic ground state. Fixed spin moment calculations rule out existence of any other local magnetic minima. The phonon dispersions show that all the alloys except LiBeFeSb are dynamically stable. Notably, LiSrFeSb and LiBaFeSb exhibit half-metallic electronic band structure and 100 % spin polarization making them suitable choice for promising spintronic applications such as spin filter and detectors.

Spin-polarized linear dispersions, lattice dynamics and transport properties of quaternary Heusler alloys (LiMgXSb)

In this study, we are predicting a new class of materials obtained by substituting a magnetic transition metal ion in quaternary Heusler alloys with the formula LiMgXSb (X = Co, Cr, Fe, Mn). We found that all the studied alloys exhibit a ferromagnetic ground state except LiMgCoSb. The phonon dispersions of the studied alloys computed within spin-polarized density functional perturbation theory (DFPT) do not exhibit any imaginary frequencies indicating that the proposed alloys are dynamically stable. The band structure calculations show that the studied alloys are metallic and exhibit the states near Fermi level for both spins. The fixed spin moment (FSM) calculations have been employed to exclude the possibility of other ferromagnetic ground states. One of the most interesting aspects of the band structure of these compounds is the existence of multiple band crossings leading to multiple Weyl points in the Brillouin zone. We have also computed the spin-polarized transport properties of these alloys. Our results find that doping charge carriers in the unit cell can significantly enhance the power factor and ZT. Electron doping is more effective for improving the power factor. Our findings show that among all the alloys LiMgFeSb has the highest ZT value with electron doping. Our studied materials constitute a new family of three-dimensional materials exhibiting band degeneracies that lead to Weyl points.

DFT calculation of intrinsic properties of magnetically hard phase L10 FePt

Due to its strong magnetocrystalline anisotropy, FePt L1$\mathrm{_0}$ phase is considered as a promising magnetic recording media material. Although the magnetic properties of this phase have already been analyzed many times using density functional theory (DFT), we decided to study it again, emphasizing on full potential methods, including spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) and full-potential local-orbital (FPLO) scheme. In addition to the determination of exact values of the magnetocrystalline anisotropy constants K$\mathrm{_1}$ and K$\mathrm{_2}$, the magnetic moments (m), the Curie temperature, and the magnetostriction coefficient, we focused on the investigation of the magnetocrystalline anisotropy energy (MAE) dependence on the magnetic moment values using the fully relativistic fixed spin moment (FSM) method with various exchange-correlation potentials. We present nearly identical MAE(m) curves near the equilibrium point, along with different equilibrium values of MAE and magnetic moments. For a magnetic moment reduced by about 10%, we determined a theoretical MAE maximum in the ground state (0 K) equal to about 20.3 MJ m$\mathrm{^{-3}}$ and independent of the choice of the exchange-correlation potential form. These calculations allow us to understand the discrepancies between the previous MAE results for different exchange-correlation potentials.

Understanding the sp magnetism in substitutional doped graphene

Defect-induced magnetism in graphene has been predicted theoretically and observed experimentally. However, there are open questions about the origin of the magnetic behavior when substitutional impurities with $sp$ electrons are considered. The aim of this work is to contribute to the understanding of impurity-induced spin magnetism in doped graphene systems. Thus, the electronic structure and spin magnetic moments for substitutional doped graphene with impurities from groups IIIA (B, Al, and Ga) and VA (N, P, As, Sb, and Bi) of the periodic table were obtained within the framework of density functional theory. The nature of the magnetic ground state was determined from calculations of the total energy as a function of the spin magnetic moment using the fixed spin moment method. We show that the spontaneous magnetization in the studied systems arises from an electronic instability by the presence of a narrow impurity band at the Fermi level. Furthermore, we found that the emergence of spin polarization requires the impurity to introduce an extra electron to the graphene lattice and that the impurity-carbon hybridization is close to the $sp^3$ geometry. These features reveal that the charge doping sign and the hybridization degree play a fundamental role in the origin of $sp$ magnetism in substitutional doped graphene.

Effect of transition metal doping on magnetic hardness of CeFe[formula omitted]-based compounds

ThMn12-type ternary cerium alloys with tetragonal structure (Pearson symbol tI26, space group I4/mmm) are considered as promising materials for permanent magnets. In this work, compositions of CeFe11X (s.g. Pmmn, No. 59) and CeFe10X2 (s.g. P4/mmm, No. 123) with all 3d, 4d, and 5d transition metal substitutions are considered. Since many previous studies have focused on the CeFe11Ti compound, this particular case became the starting point of our considerations and we gave it special attention. We first determined the optimal symmetry of the simplest CeFe11Ti structure model. We then observed that the calculated magnetocrystalline anisotropy energy (MAE) correlates with the magnetic moment, which in turn strongly depends on the choice of the exchange–correlation potential. MAE, magnetic moments, and magnetic hardness were determined for all compositions considered. Moreover, the calculated dependence of the MAE on the spin magnetic moment allowed us to predict the upper limits of the MAE. We also showed that it does not depend on the choice of the exchange–correlation potential form. The economically justifiable compositions with the highest magnetic hardness values are CeFe11W, CeFe10W2, CeFe11Mn, CeFe10Mn2, CeFe11Mo, CeFe10Mo2, and CeFe10Nb2. However, calculations suggest that, like CeFe12, these compounds are not chemically stable and could require additional treatments to stabilize the composition. Further alloying of the selected compositions with light elements embedded in interstitial positions confirms the positive effect of such dopants on hard magnetic properties. Subsequent calculations performed for comparison for selected isostructural La-based compounds lead to similar MAE results as for Ce-based compounds, suggesting a secondary effect of 4f electrons. Our preliminary results obtained using the intra-atomic Hubbard repulsion term showed a relatively small difference for CeFe12 compared to the results without this correction. Calculations were performed using the full-potential local-orbital electronic structure code FPLO18, whose unique fully relativistic implementation of the fixed spin moment method allowed us to calculate the MAE dependence of the magnetic moment.

Structural, electronic and magnetic properties of weakly correlated metal Sr2CrTiO6: a first principles study

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6 has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t 2g density of states around the Fermi energy. The Cr4+ ion in Sr2CrTiO6, which is a d 2 system, shows a reverse splitting of the t 2g orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d xz and d yz orbitals, while the d xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t 2g bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner’s criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d xz /d yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr2CrTiO6 belongs to the class of weakly correlated metals, while the shifting of O 2p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr2CrTiO6.

Itinerant magnetism in the half-metallic Heusler compound Co2HfSn : Evidence from critical behavior combined with first-principles calculations

${\mathrm{Co}}_{2}$-based Heusler alloys show diverse magnetic behavior, extending from the localized Heisenberg to the delocalized itinerant, and thus cannot be generalized with one specific model. Therefore, we bring together experiments and first-principles calculations to reveal the origin of long-range ferromagnetism in the ${\mathrm{Co}}_{2}$-based Heusler alloy ${\mathrm{Co}}_{2}\mathrm{HfSn}$. The precise value of the critical exponents $\ensuremath{\beta}=0.471(4), \ensuremath{\gamma}=1.02(4)$, and $\ensuremath{\delta}=3.273(5)$ with the Curie temperature ($T{}_{c}=430$ K) were determined experimentally by means of different analytical methods such as modified Arrott plot analysis, the Kouvel-Fisher method, and critical isotherm analysis. The deduced critical exponents belong to the theoretical prediction of the three-dimensional mean-field model. The magnetic exchange distance is found to decay as $J(r)\ensuremath{\sim}{r}^{\ensuremath{-}4.52}$, indicating the long-range magnetic interaction in ${\mathrm{Co}}_{2}\mathrm{HfSn}$. Moreover, the magnetic entropy change $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}$ features a maximum at ${T}_{c}$, i.e., $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{max}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 29.87(6) J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 5 T, whereas power law fitting of $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{max}$ with $H$ gives ${n}^{\ensuremath{'}}=0.663(5)$, also confirming the mean-field-type magnetic interaction in ${\mathrm{Co}}_{2}\mathrm{HfSn}$. Meanwhile, in order to explore the origin of long-range ferromagnetic ordering in ${\mathrm{Co}}_{2}\mathrm{HfSn}$, spin-polarized density functional theory calculations were employed in this study. No stable antiferromagnetic solution with stable local magnetic moments at the Co or Hf site was observed in our fixed spin moment simulation, revealing that ${\mathrm{Co}}_{2}\mathrm{HfSn}$ is a highly itinerant magnet with a long-range ferromagnetic solution together with robust half-metallic conduction stable against thermal excitation higher than room temperature. Furthermore, our theoretical analysis shows that ${\mathrm{Co}}_{2}\mathrm{HfSn}$ belongs to an unconventional family of itinerant magnetism. Both the 100% spin-polarized metallicity and ferromagnetism arise from the Co $d$ orbitals, causing it to be unique compared to the conventional itinerant family of Ni, Co, or Fe, where magnetism and metallicity originate from different orbitals characters.

Propensity of spin fluctuations in disordered NiCoCr alloys: A first principles study

A quantum critical behavior is reported in equi-atomic NiCoCr alloy, where the ferromagnetic transition temperature goes to zero. We perform a systematic study of (NiCo)1−xCrx alloys; 0 ≤ x ≤ 13, by means of density functional theory based Green’s function method, to understand the changes in the electronic structure and magnetic properties of this equi-atomic ternary alloy as a function of Cr at%. Chemical disorder is modeled by means of coherent-potential. approximation. To determine the propensity of spin fluctuations, we adopt to the fixed-spin moment method. A fit to the phenomenological Ginzburg Landau equation and estimation of the second derivative of the total energy with respect to magnetization shows that (NiCo)1−xCrx alloys display an increasing propensity of longitudinal spin fluctuations, proportional to the Cr at% in these alloys.

The effect of Hubbard-like interaction on molecular magnetism of TM-coronene complex (TM = Fe and Co)

The adsorption of two transition metal adatoms, Fe and Co added to a coronene molecule were studied by means of a full potential local orbital method in the framework of relativistic density functional theory. A sequence of fixed spin moment calculations based on the Hubbard-like interaction (U) were carried out on the basis of the generalized gradient approximation (GGA) of the exchange–correlation functional. We found a transition from low-spin to high-spin state upon increasing the transition metal-coronene distance. Furthermore, the magnetism of both Fe-coronene and Co-coronene complexes revealed sensitivity to the magnitude of the U values. In our GGA+U (U = 2 eV) calculations, a low-spin ground state was found with in-plane magnetic anisotropy for both Fe-coronene and Co-coronene, whereas the GGA+U (U = 4 eV) calculations resulted in high-spin state with out-of-plane and in-plane magnetic anisotropy for Fe-coronene and Co-coronene, respectively.

Electronic specific heat coefficient and magnetic properties of Y(Fe1−xCox)2 Laves phases: A combined experimental and first-principles study

We investigated experimentally and computationally the concentration dependence of electronic specific heat coefficient $\gamma$ in Y(Fe$_{1-x}$Co$_x$)$_2$ pseudobinary Laves phase system. The experimentally observed maximum in $\gamma$($x$) around the magnetic phase transition was interpreted within the local density approximation (LDA) combined with the virtual crystal approximation (VCA). To explain the formation of the observed maximum, we analyzed theoretically the dependence of the magnetic energy, magnetic moments, densities of states, and Fermi surfaces on the Co concentration. Furthermore, we carried out the calculations of density of states at the Fermi level as a function of fixed spin moment. The calculated Co concentration at which $\gamma$ takes the maximum value ($x_{\mathrm{max-LDA-VCA}}=0.91$) stays in good agreement with the measured value ($x_{\mathrm{max-expt}} = 0.925$). We conclude that the observed maximum in $\gamma(x)$ results from the presence of the sharp DOS peak in the vicinity of the Fermi level.

Magnetocrystalline anisotropy of Fe5PB2 and its alloys with Co and 5d elements: A combined first-principles and experimental study

The Fe$_5$PB$_2$ compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P-sites. Here, we present a combined computational and experimental study of the magnetic properties of (Fe$_{1-x}$Co$_{x}$)$_5$PB$_2$. Computationally, we are able to explore the full concentration range, while the real samples were only obtained for 0 <= x <= 0.7. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of Fermi energy, the geometry of the Fermi surface of Fe$_5$PB$_2$ and the k-resolved contributions to the MAE are discussed. Low temperature measurements of an effective anisotropy constant for a series of (Fe$_{1-x}$Co$_{x}$)$_5$PB$_2$ samples determined the highest value of 0.94 MJ m$^{-3}$ for the terminal Fe$_5$PB$_2$ composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of Fe$_5$PB$_2$ is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of Fe$_5$PB$_2$, by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (Fe$_{0.95}$X$_{0.05}$)$_5$PB$_2$ (X = 5$d$ element) indicate that 5% doping of Fe$_5$PB$_2$ with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.

Stability of room temperature compensated half-metallicity in Cr-based Inverse-Heusler Compounds

Using three correlated band approaches, namely the conventional band approach plus on-site Coulomb repulsion U, the modified Becke-Johnson functional, and hybrid functional, we have investigated inverse-Heusler ferrimagnets Cr2CoZ (Z=Al, Ga, In). These approaches commonly indicate that the Cr2CoAl synthesized recently is a precise compensated half-metal (CHM), whereas Cr2CoGa and Cr2CoIn are ferrimagnets with a small moment. This is also confirmed by the fixed spin moment approach. Analysis of the Bader charge decomposition and the radial charge densities indicates that this contrast is due to chemical differences among the Z ions. Additionally, in Cr2CoAl, changing the volume by ±5% or the ratio of c/a by ±2% does not alter the CHM state, suggesting that this state is robust even under application of moderate pressure or strain. Considering the observed high Curie temperature of 750 K, our results suggest that Cr2CoAl is a promising candidate for robust high TC CHMs. Furthermore, the electronic structure of the CHM Cr2CoAl is discussed.

Spin-disorder resistivity of random fcc-NiFe alloys

The spin-disorder resistivity (SDR) of a disordered fcc-(${\mathrm{Ni}}_{1\ensuremath{-}x},{\mathrm{Fe}}_{x}$) alloy is determined from first principles. We identify the SDR at and above the critical temperature with the residual resistivity of the corresponding paramagnetic state evaluated in the framework of the disordered local moment (DLM) model. The underlying electronic structure is determined by means of the tight-binding linear muffin-tin orbital method, which employs the coherent potential approximation (CPA) to describe both the DLM state and the chemical disorder in alloys. An extension of the DLM fixed-spin moment method for two independent magnetic moments is used and combined with the paramagnetic lattice gas entropy to determine local moments by minimizing the corresponding free energy. The effect of phonon scattering is included through the mapping of static atomic displacements into a multicomponent random alloy which is then treated in the CPA. Finally, the Kubo-Greenwood-CPA approach is employed to estimate the SDR. We also address the problem of the validity of the Matthiessen rule at the Curie point. Good agreement of calculated and measured SDR is obtained over the whole studied concentration range; the results point to the importance of nonzero Ni magnetic moments in the limit of pure nickel.

Correlated excited states in the narrow band gap semiconductor FeSi and antiferromagnetic screening of local spin moments

The physical properties of the semiconductor FeSi with very narrow band gap, anomalous behavior of the magnetic susceptibility and metal-insulator transition at elevated temperatures attract gross interest due to the still controversial theoretical understanding of their origin. On one side the purely band like mechanism of the gap formation in FeSi at low temperature is well established, on other side a number of experiments and their theoretical interpretation suggest a rich physics of strong correlations at finite temperature. In this work we use an ab-initio scheme based on the Random Phase Approximation and Local Spin Density Approximation (RPA@LSDA) to reveal the role of the electron correlation effects in FeSi extending it by applying a fixed spin moment constraint. In the parameter free framework we show that correlation effects essentially alter the one-electron LSDA results leading to the formation of an additional state with finite magnetic moment on Fe, whose energy is almost degenerate with the non-magnetic ground state. This explains the results of high field experiments, which found a first-order meta-magnetic phase transition into a metallic ferromagnetic state. Our results suggest a strongly correlated nature of the low-energy excitations in FeSi. From our super-cells calculations we reveal that these excitations are local and exhibit a Kondo-like behavior since a strong antiferromagnetic screening is present.

Effect of Coulomb repulsion on spin and orbital magnetism of cobalt–benzene complex: A relativistic density functional study

We have demonstrated electronic configurations and magnetic properties of single Co adatom on benzene (Bz) molecule in the framework of relativistic density functional theory. A sequence of fixed spin moment (FSM) calculations were carried out with and without Coulomb repulsion (U). We have investigated that varying the strength of Coulomb repulsion results to different equilibrium positions for the Co adatom on benzene molecule. It was shown that inclusion of the on-site Coulomb repulsion in the Co 3d orbitals affects significantly the geometry of Co–Bz complex. We also found two stable low-spin and high-spin multiplicities for the complex. The nature of the high-spin configuration was explained according to the Hubbard electron–electron correlation in 3d shell of the Co adatom. Our FSM results indicate that the high-spin state is a global minimum in the presence of Hubbard parameter U. The relativistic spin–orbit coupling and using orbital polarization correction induce considerable orbital magnetism in both low and high spin states of the Co–Bz complex. We have also calculated magnetic anisotropy energies for two spin states and we found out that an out-of-plane magnetic orientation of Co adatom is more favorable in the low spin state whereas the Coulomb repulsion (U = 2 eV and U = 4 eV) predicts an in-plane magnetic orientation for Co adatom. Our findings can be implicitly taken into account for the extended system of added single Co atom on graphene.

Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering

Magnetocaloric ${\mathrm{LaFe}}_{13\ensuremath{-}x}{\mathrm{Si}}_{x}$-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent $^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering measurements (in a field ${\ensuremath{\mu}}_{0}H$ of $\ensuremath{\sim}0.7$ T) of the vibrational (phonon) density of states, VDOS, in ${\mathrm{LaFe}}_{11.6}{\mathrm{Si}}_{1.4}$ across the metamagnetic isostructural first-order phase transition at ${T}_{C}\ensuremath{\sim}192$ K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at ${T}_{C}$. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across ${T}_{C}$, occurring within a small temperature interval of $\mathrm{\ensuremath{\Delta}}T\ensuremath{\le}12$ K around ${T}_{C}$. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to ${T}_{C}$, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states $D({E}_{F}$) at the Fermi energy ${E}_{F}$. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}$ of $+6.9\ifmmode\pm\else\textpm\fi{}2.6$ J/(kg K) and in the vibrational specific heat $\mathrm{\ensuremath{\Delta}}{C}_{\mathrm{vib}}$ of $+2.7\ifmmode\pm\else\textpm\fi{}1.6$ J/(kg K) upon heating. The increase in magnitude of the vibrational entropy $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}|=6.9$ J/(kg K) of the Fe sublattice at ${T}_{C}$ upon heating is substantial, if compared with the magnitude of the isothermal entropy change $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{iso}}|$ of 14.2 J/(kg K) in a field change $\mathrm{\ensuremath{\Delta}}B$ from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. We demonstrate that $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}$ obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{iso}}$ at the phase transition of ${\mathrm{LaFe}}_{13\ensuremath{-}x}{\mathrm{Si}}_{x}$.

Longitudinal integration measure in classical spin space and its application to first-principle based simulations of ferromagnetic metals

The classical Heisenberg type spin Hamiltonian is widely used for simulations of finite temperature properties of magnetic metals often using parameters derived from first principles calculations. In itinerant electron systems, however, the atomic magnetic moments vary their magnitude with temperature and the spin Hamiltonian should thus be extended to incorporate the effects of longitudinal spin fluctuations (LSF). Although the simple phenomenological spin Hamiltonians describing LSF can be efficiently parameterized in the framework of the constrained Local Spin Density Approximation (LSDA) and its extensions, the fundamental problem concerning the integration in classical spin space remains. It is generally unknown how to integrate over the spin magnitude. Two intuitive choices of integration measure have been used up to date – the Murata-Doniach scalar measure and the simple three dimensional vector measure. Here we derive the integration measure by considering a classical limit of the quantum Heisenberg spin Hamiltonian under conditions leading to the proper classical limit of the commutation relations for all values of the classical spin magnitude and calculate the corresponding ratio of the number of quantum states. We show that the number of quantum states corresponding to the considered classical spin magnitude is proportional to this magnitude and thus a non-trivial integration measure must be used. We apply our results to the first-principles simulation of the Curie temperatures of the two canonical ferromagnets bcc Fe and fcc Ni using a single-site LSF Hamiltonian with parameters calculated in the LSDA framework in the Disordered Local Moment approximation and a fixed spin moment constraint. In the same framework we compare our results with those obtained from the scalar and vector measures.

Pressure effect on magnetic susceptibility of LaCoO3

The effect of pressure on magnetic properties of LaCoO$_3$ is studied experimentally and theoretically. The pressure dependence of magnetic susceptibility $\chi$ of LaCoO$_3$ is obtained by precise measurements of $\chi$ as a function of the hydrostatic pressure $P$ up to 2 kbar in the temperature range from 78 K to 300 K. A pronounced magnitude of the pressure effect is found to be negative in sign and strongly temperature dependent. The obtained experimental data are analysed by using a two-level model and DFT+U calculations of the electronic structure of LaCoO$_3$. In particular, the fixed spin moment method was employed to obtain a volume dependence of the total energy difference $\Delta$ between the low spin and the intermediate spin states of LaCoO$_3$. Analysis of the obtained experimental $\chi(P)$ dependence within the two-level model, as well as our DFT+U calculations, have revealed the anomalous large decrease in the energy difference $\Delta$ with increasing of the unit cell volume. This effect, taking into account a thermal expansion, can be responsible for the temperatures dependence of $\Delta$, predicting its vanishing near room temperature.

Competing magnetic instabilities in the weak itinerant antiferromagnetic TiAu

The binary intermetallic compound TiAu is distinguished by an extremely sharp and narrow density of states peak at the Fermi level that has been proposed, via scattering between mirrored van Hove singularities, to be the mechanism for antiferromagnetic ordering versus the more fundamental ferromagnetic Stoner instability. Here we study, using density functional theory methods, magnetic tendencies and effects of doping, the latter within the virtual crystal approximation (VCA). Ferromagnetic tendencies are quantified using the fixed spin moment approach, illustrating the strong Stoner instability that does not, however, provide the ground state. Use of VCA results allows the identification of the value of the Stoner exchange constant $\mathcal{I}=0.74\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for Ti. Magnetic fluctuations not included with semilocal density functionals are quantified with the procedure provided by Ortenzi and coauthors, with alloy concentrations corresponding to the quantum critical points reduced by a factor of three to five. Our results provide useful guidelines for experimental doping studies of TiAu.

Possible Correlation Between Stripe-Like Antiferromagnetic Spin Fluctuations and Superconductivity in ThFeAsN

The recently invented ThFeAsN superconductor, belonging to the family of Fe-based superconductors, has been studied using ab initio density functional theory. Our calculations demonstrate that the electronic structure of this superconductor is similar to that of other Fe-based superconductors with computed magnetic ground state showing strong dependence on atomic positions. Using relaxed atomic positions, the single- and double-stripe magnetic states are found to be energetically close to each other, which may lead to stripe-like antiferromagnetic spin fluctuations. We speculate that such energetic degeneracy among two antiferromagnetic structures may be an ideal condition for strong stripe-like antiferromagnetic spin fluctuations, which may be playing a key role in obtaining high transition temperature of 30 K. Fixed spin moment calculations demonstrate the possibility of local minima for magnetic states in ThFeAsN. The absence of long-range ordering in ThFeAsN may be due to presence of such local minima.