Antiferromagnetic AF Research Articles

Dynamic phase transitions and the effects of frequency of oscillating magnetic field on the dynamic phase diagrams in the bilayer honeycomb lattice with AB stacking geometry

We study some dynamic properties of the bilayer honeycomb lattice with AB stacking geometry in the presence of a time-dependent oscillating external magnetic field. We employ the Glauber transition rates to construct the mean-field dynamical equations. First, we obtain dynamic phases in the system and observe the paramagnetic (p), ferromagnetic (f), compensated (c) antiferromagnetic (af), surface ferromagnetic (sf) and mixed (m) phases. Besides, coexistence phase regions also exist in the system. Second, we investigate the thermal behavior of the dynamic order parameters. From these study, the natures (first- or second-order) of the transitions are characterized and the dynamic phase transition (DPT) points are presented. The DPTs are obtained and the dynamic phase diagrams (DPD) are constructed plane of the temperature versus the amplitude of the magnetic field. We investigate the effect of the frequency of the oscillating external magnetic field on the DPD.

Spin dynamics in a Curie-switch

Ferromagnetic resonance properties of F1/f/F2/AF multilayers, where weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic layers F1 and F2, with F1 being magnetically soft and F2—magnetically hard due to exchange pinning to antiferromagnetic layer AF, are investigated. Spacer-mediated exchange coupling is shown to strongly affect the resonance fields of both F1 and F2 layers. Our theoretical calculations as well as measurements show that the key magnetic parameters of the spacer, which govern the ferromagnetic resonance in F1/f/F2/AF, are the magnetic exchange length (), effective saturation magnetization at T = 0 (m0) and effective Curie temperature (). The values of these key parameters are deduced from the experimental data for multilayers with f = NixCu100−x, for the key ranges in the Ni–concentration ( at. %) and spacer thickness ( nm). The results obtained provide a deeper insight into thermally-controlled spin precession and switching in magnetic nanostructures, with potential applications in spin-based oscillators and memory devices.

Magnetic exchange interactions inBaMn2As2: A case study of theJ1-J2-JcHeisenberg model

BaMn2As2 is unique among BaT2As2 compounds crystallizing in the body-centered-tetragonal ThCr2Si2 structure, which contain stacked square lattices of 3d transition metal T atoms, since it has an insulating large-moment (3.9 muB/Mn) G-type (checkerboard) antiferromagnetic AF ground state. We report measurements of the anisotropic magnetic susceptibility chi versus temperature T from 300 to 1000 K of single crystals of BaMn2As2, and magnetic inelastic neutron scattering measurements at 8 K and 75As NMR measurements from 4 to 300 K of polycrystalline samples. The Neel temperature determined from the chi(T) measurements is TN = 618(3) K. The measurements are analyzed using the J1-J2-Jc Heisenberg model. Linear spin wave theory for G-type AF ordering and classical and quantum Monte Carlo simulations and molecular field theory calculations of chi(T) and of the magnetic heat capacity Cmag(T) are presented versus J1, J2 and Jc. We also obtain band theoretical estimates of the exchange couplings in BaMn2As2. From analyses of our chi(T), NMR, neutron scattering, and previously published heat capacity data for BaMn2As2 on the basis of the above theories for the J1-J2-Jc Heisenberg model and our band-theoretical results, our best estimates of the exchange constants in BaMn2As2 are J1 = 13 meV, J2/J1 = 0.3 and Jc/J1 = 0.1, which are all antiferromagnetic. From our classical Monte Carlo simulations of the G-type AF ordering transition, these exchange parameters predict TN = 640 K for spin S = 5/2, in close agreement with experiment. Using spin wave theory, we also utilize these exchange constants to estimate the suppression of the ordered moment due to quantum fluctuations for comparison with the observed value and again obtain S = 5/2 for the Mn spin.

Ferromagnetism at the interfaces of antiferromagnetic FeRh epilayers

The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of 500 A were grown on MgO 001 substrates via molecular-beam epitaxy and capped with 20 A of MgO. The FeRh films show a clear transition from the antiferromagnetic AF state to the ferromagnetic FM state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within 60‐80 A of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness 100 A. Given the delicate energy balance between the AF and FM ground states we suggest a metastable FM state resides near to the interface within an AF matrix. The length scale over which the FM region resides is consistent with the strained regions of the film.

A Novel 1D-AF Hybrid Organic−Inorganic Chromium(II) Methyl Phosphonate Dihydrate: Synthesis, X-Ray Crystal and Molecular Structure, and Magnetic Properties

Light-blue crystals of chromium(II) methyl phosphonate dihydrate, [Cr(CH(3)PO(3))(H(2)O)].H(2)O, were obtained in water by mixing filtered solutions of methylphosphonic acid and chromium(II) chloride in the presence of urea in an inert atmosphere. The compound was characterized by elemental analysis, TGA-DSC, X-ray crystallography, magnetic measurements, and UV-visible and FT-IR spectroscopies. The crystal and molecular structures (orthorhombic Pnma (no. 62): a = 4.4714(5) A, b = 6.8762(7) A, c = 19.180(2) A, Z = 4) have been solved using single-crystal X-ray diffraction. The chromium(II) ion is six-coordinated by oxygens (4 + 2) to form an elongated octahedron, with the four equatorial oxygen atoms belonging to [-PO(3)](2-) phosphonate groups. This stereochemistry of the Cr(II) ion (high-spin d(4) electronic configuration) is ascribed to the Jahn-Teller effect. The [CrO(6)] chromophore, the [CH(3)PO(3)](2-) anions, and the water molecules build a novel one-dimensional (1D) metal(II) oxide chain, anchored to each other within the ab plane by two oxygens of the phosphonate ligand. Within the chain, each Cr(2+) ion is connected through double oxygen bridges to its two neighbors, forming edge-sharing octahedra running along the b axis. The chains are further connected with the adjacent chains by phosphonate [-PO(3)](2-) groups of the ligand, forming an inorganic layer that alternates along the c axis of the unit cell with bilayers, consisting of methyl groups and water of crystallization. The thermal variation of the magnetic susceptibility follows the Curie-Weiss law, with a large negative Weiss constant, theta = -60 K, indicating the presence of antiferromagnetic AF exchange interactions between neighboring Cr(II) ions. The magnetic behavior and the magnetic dimensionality have been analyzed in terms of Fisher's classical limiting form of the Heisenberg chain theory, and a value of J = -9.3 cm(-1) was found. The negative value of the intra-chain exchange constant coupling J confirms the presence of an AF coupling. No sign of long-range magnetic ordering down to 2 K (the lowest measured temperature) is observed, in agreement with the predominant one-dimensional character of the exchange interactions.

Effects of antidots on the transport properties of graphene nanoribbons

Effects of magnetic antidots on the transport properties of zigzag-edged graphene nanoribbons ZGNRs are investigated by spin-polarized first-principles calculations combined with a nonequilibrium Green’s-function technique. Specifically, the effects of antidots or holes with regular shapes rectangular and triangular are studied. It is found that rectangular holes with a zero total spin S0 and triangular holes with a finite spin S0 cause different effects on the equilibrium conductance of ZGNRs. A rectangular hole with zigzag edges parallel to the ribbon edges blocks the transmission of the band edges of both the valence band and the conduction band from both the spin-up channel and the spin-down channel. Thus a much wider transmission gap than the pristine ZGNRs can be observed. However, a triangular hole with zigzag edges blocks transmission from only one spin channel in either the valence-band edge or the conduction-band edge. Thus the gap width in the total conductance is not affected in this case. The difference originates from the different energy shift of the valence band and conduction band relative to Fermi energy as a result of two effects: finite-size effect and spin splitting from the antidot-induced effective internal magnetic field. Graphene nanostructures are supposed to be very important building blocks in future nanoelectronic devices due to their remarkable structural and electronic properties. Among them, graphene nanoribbons GNRs have attracted intensive attention 1–12 and it is found that the properties of GNRs are highly dependent on their sizes and edge shapes. Tightbinding calculations show that armchair-edged GNRs AGNRs can be either metallic or semiconducting depending on their widths 8,12 while first-principles calculations show that AGNRs are always semiconducting with an energy-gap scaling inversely to the GNR width. 8–10 More interestingly, in the zigzag-edged GNRs ZGNRs, magnetic ordering is formed due to the unpaired - and -edge electrons. Two most stable spin configurations have been observed: ferromagnetic FM and antiferromagnetic AF, 13 which mean that the localized edge states on the two sides are FM coupled or AF coupled. Tight-binding calculations show that the ZGNRs are always metallic 12 while first-principles calculations show that it is metallic only in the FM configuration. There is always an energy gap in the AF configuration. Calculations also show that the most stable configuration is the AF state with the states of different spins in the valence bands and conduction bands localized on different edges while the energy of the states with different spins is degenerate. 5,8 Meanwhile, quantum dots and antidots made by graphene are another kind of graphene nanostructures which initiate great interests. 14–20 The dots are just graphene molecules with finite size in all directions while antidots are holes in graphene with some kinds of dots cut away from it. Just like in ZGNRs, magnetism can also be formed in graphene molecules with zigzag edges. 15 In a graphene molecule with rectangular or hexagonal edges, magnetic moments with the same magnitude and different signs are always formed on the two opposite zigzag edges. Thus the total spin S0 in such systems is exactly zero. However, in a molecule with triangular zigzag edges, exactly the same magnetic moments are observed on all edges. So the total spin S0 is nonzero. Like in ZGNRs, the local magnetism in graphene molecules arises from the spin-polarized edge states localized on the zigzag edges. The local magnetism in graphene nanostructures may qualify graphene nanoribbons and molecules with zigzag edges as promising candidates for application in the spintronic devices. Therefore, the study of spin-polarized transport through these nanostructures are quite interesting. In this work, we build graphene-based devices by combining graphene nanoribbons and antidots with zigzag edges and investigate the electron transport in them. Specifically, we study the effects of graphene antidots or holes on the transport properties of ZGNRs. Two kinds of antidots will be considered: rectangular hole and triangular hole. The model structure is constructed as follows: the scatter

Magnetothermopower in Nd1−xEuxNiO3 compounds

We have measured magnetization M(T,H), thermal conductivity κ(T,H), and thermopower S(T,H) of polycrystalline samples of Nd1−xEuxNiO3, 0⩽x⩽0.35, as a function of temperature and external magnetic field. The data indicate a metal-insulator (MI) transition in a wide range of temperature (200<TMI<325K). The magnetic susceptibility χ(T) data, after the subtraction of the rare-earth contribution, exhibit a Curie-Weiss-like behavior at temperatures above TMI. Although a clear antiferromagnetic AF transition of the Ni sublattice is observed at TN⩽TMI, χ(T) still increases down to 5K, suggesting a heterogeneous ground state. The thermal conductivity of the NdNiO3 compound is not affected by an external magnetic field of 90kOe in a wide range of temperature, and its temperature dependence below 15K is approximately quadratic, strongly suggesting the presence of disorder. S(T) is negative above TMI and varies linearly with temperature. Below TMI, there is a minimum close to 120K, and S(T) changes its sign at T∼30K, indicating a competition between two types of charge carriers. A pronounced peak in S(T) at TS∼20K is observed and the peak remains unaltered under magnetic fields up to 90kOe. However, its magnitude is enhanced by ∼25% with applied magnetic field, exhibiting a clear magnetothermopower effect. The combined results indicate a coexistence of ordered and disordered phases below TN and that an applied magnetic field is suitable for enhancing the thermoelectric properties close to TS.

Influence of the number of periods on the magnetization reversal process of antiferromagnetically coupled amorphous CoxSi1−x/Si multilayers

The advantage of using amorphous materials for preparing complex magnetic structures, like magnetic/non-magnetic multilayers, is illustrated for the case of the study of the Co–Si system. In particular, the antiferromagnetic AF coupling recently reported for amorphous Co–Si alloys separated by Si spacers and the associated magnetization reversal processes have been studied in (Co0.75Si0.25/Si)n multilayers with different number of periods n. It has been found that the magnetic field corresponding to the onset of the reversal process increases with the number of periods in a linear way. For n>2, the magnetization stabilizes not only in the AF state but also in other intermediate configurations between both saturated states. At the same time, as n increases, all the processes of formation and breaking of these states get smoothed.

Phase diagrams of site-frustrated Heisenberg models on simple cubic, bcc, and fcc lattices

We have determined the phase diagrams of the site-frustrated Heisenberg model in three dimensions over the entire concentration regime of competing ferromagnetic F and antiferromagnetic A sites for the three basic lattice types: simple cubic sc , body-centered cubic bcc and face-centered cubic fcc using Monte Carlo methods. Long-range ferromagnetic FM or antiferromagnetic AF order is established at a finite temperature, second-order phase transition whenever F or A sites percolate. Since site percolation thresholds in three dimensions are less than 1 2 , FM and AF order coexist over a wide composition regime. The only apparent effect of frustration is to cause the AF and FM order to occur perpendicular to one another. Strong evidence is found to suggest that all of the transitions remain in the Heisenberg universality class and that there exists a tetracritical point at x= 1 2 where TC=TN.

Antiferromagnetic dipolar ordering in[Co2MnGe∕V]Nmultilayers

We have studied ${[{\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ge}∕\mathrm{V}]}_{N}$ multilayers with a thickness of the V layers ${t}_{\mathrm{V}}$ between 1.5 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and a fixed thickness of the Heusler layer ${t}_{{\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ge}}=3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ by x-ray scattering, neutron reflectivity, and magnetization measurements. In the thickness range ${t}_{\mathrm{V}}\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ neutron reflectivity results provide clear evidence for an antiferromagnetic (af) interlayer long-range order below a N\'eel temperature ${T}_{\mathrm{N}}$. The interlayer long range order does not show an oscillating character and is stabilized by a weak af coupling field ${H}_{\mathrm{af}}\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$. We attribute the af coupling to magnetic dipolar stray fields originating from magnetically rough surfaces of a granular ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Ge}$ microstructure. In the thickness range ${t}_{\mathrm{V}}\ensuremath{\geqslant}4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ the multilayers undergo a cluster glass transition at ${T}_{\mathrm{f}}\ensuremath{\approx}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At high temperatures above ${T}_{\mathrm{N}}$ or ${T}_{\mathrm{f}}$ the mutilayers are superparamagnetic with a huge cluster magnetic moment ${\ensuremath{\mu}}_{\mathrm{c}}\ensuremath{\geqslant}{10}^{5}{\ensuremath{\mu}}_{\mathrm{B}}$.

115In NQR study of [formula omitted] under high pressure

We have carried out 115In nuclear quadrupole resonance (NQR) measurements in CeCoIn 5 under pressure P. The obtained nuclear spin-lattice relaxation rate, 1 / T 1 , of 115In at ambient pressure varies in proportion to T 1 / 4 in the normal state, and T 3 below the superconducting transition temperature T C . The results show that the system locates just at the antiferromagnetic AF instability and the anisotropic superconductivity with line nodes appears. The value of 1 / T 1 is strongly suppressed with increasing P, indicating that P moves the system away from the AF instability. The Fermi-liquid behavior was observed at the high P region. The P dependence of 1 / T 1 in the superconducting state shows that the magnitude of the superconducting energy gap scales with its T C up to 3 GPa.

Changes in the structural and magnetic properties of Ni?substituted hematite prepared from metal oxinates

Samples prepared by the novel method based on the thermal decomposition at 700 °C in air of mixed Fe and Ni oxinates were characterized by thermogravimetric analysis, X-ray diffraction, magnetization measurements, and variable-temperature Mossbauer spectroscopy. It is found that the combustion treatment produces Ni-hematite and trevorite, the fraction of the latter increasing with the increment of Ni in the metal oxinates. Results indicate that the substitution of Ni2+ for Fe3+ in the hematite structure, which was found to be less than 5.3 mol%, is accompanied by the presence of oxygen and structural vacancy sites. Both the metal replacement and the presence of defects cause the a and c cell hematite parameters to decrease. The Neel temperature and the difference between the saturation fields for the antiferromagnetic AF and the weakly ferromagnetic WF phases are also found to decrease with nickel content. These effects are due to the fact that the magnetic behavior of hematite depends on the presence of Ni, vacancy sites, and lattice distortion. The large decrease in the Neel temperature with Ni doping suggests that structural vacancies are also present On the contrary, Ni incorporation does not appreciably affect the Morin temperature and the temperature range in which both AF and WF phases coexist. It is suggested that these effects are probably due to the differing effects that Ni and defects can produce on the magnetic dipolar and the single ion anisotropies.

Macroscopically inhomogeneous state at the boundary between the superconducting, antiferromagnetic, and metallic phases in quasi-one-dimensional(TMTSF)2PF6

We report on experimental studies of the character of phase transitions in the quasi-1D organic compound (TMTSF)2PF6 in the close vicinity of the borders between the paramagnetic metal PM, antiferromagnetic insulator AF, and superconducting SC states. In order to drive the system through the phase border P_0(T_0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P_0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for tuning (P-P_0), and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured R(B) and its temperature dependence R(B,T) in the pressure range (0 - 1)GPa. Our studies focus on the features of the magnetoresistance at the phase transition between the PM and AF phases, in the close vicinity to the superconducting transition at T~1K. We found pronounced history effects arising when the AF/PM phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the PM to AF phase, the features of the PM phase extends well into the AF phase. At the opposite transition from the AF to PM phase, the features of the AF phase are observed in the PM phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the very phase boundary.

Easy axis dispersion in high-moment Fe-Co-Ni thin films electrodeposited on various underlayers

Soft high-moment Fe-Co-Ni films (50-1200-nm thick) were electrodeposited on nonmagnetic (100-nm Cu), ferromagnetic F (100-nm Fe/sub 20/Ni/sub 80/), antiferromagnetic AF (10-nm FeMn) and composite F/AF (AF on top) underlayers. The influence of the underlayers on magnetic properties - in particular, dispersion of the easy axis, coercivity, anisotropy field, and permeability was investigated. FeCoNi films on F and F/AF underlayers exhibit a well-defined uniaxial anisotropy with very small easy axis dispersion and low easy axis coercivity. Films grown on Cu or FeMn underlayers show a larger easy axis dispersion with relatively high coercivity. F/F or AF/F coupling contribute to the smaller easy axis dispersion of films grown on F or F/AF underlayers, respectively. The narrower easy axis dispersion also improves the high frequency permeability.

Effects of in situ magnetic field application and post-deposition magnetic annealing on sputtered Fe 50Mn 50/Ni 80Fe 20 bilayers

Effects of in situ magnetic field deposition and post-deposition magnetic annealing on the exchange field and coercivity of Fe 50Mn 50(200 Å)/Ni 80Fe 20 bilayers over a range of 40–120 Å Ni 80Fe 20 thicknesses were investigated. A large increase in coercivity and exchange field was observed in samples that were annealed at 230°C for 15 min in vacuum. The structure with 80 Å Ni 80Fe 20 thickness was found to exhibit maximum exchange field in the range of thickness investigated. The non-monotonic dependence of exchange bias with ferromagnetic layer thickness suggests the important role played by the microstructure of the Fe 50Mn 50 antiferromagnetic AF layer. These phenomena are discussed in relation to surface roughness, grain sizes, diffusion and magnetic domains.

Effect of pressure on the magnetic properties of MnRh 1− xCo xAs

The magnetic properties and their pressure dependence in Fe 2P-type quasi-ternary intermetallic compounds Mn(Rh 1− x Co x )As were investigated up to 8 GPa. The low temperature antiferromagnetic state AF(I) disappeared for x>0.2 and a new phase (Phase I) appeared with a small magnetic moment. The transition temperature T t2 from this phase to the canted state and the Curie temperature T c remained almost constant up to x=0.7. The pressure dependence of T t2 remained constant up to 3.5 GPa, and then Phase I disappeared above 4 GPa. On the other hand, T c increased with increasing pressure with a rate of 8 K/GPa up to 2–3 GPa and then with 22–25 K/GPa. This large increase in rate can be explained by considering an RKKY-type interaction between Mn–Mn atoms along the c-axis.

Appearance of anisotropic non s-wave superconductivity above the critical pressure of antiferromagnetic CeRhIn5

We have carried out 115In nuclear quadrupole resonance (NQR) measurements in CeRhIn5. At ambient pressure, CeRhIn5 undergoes an antiferromagnetic AF phase transition at \(\) K. The 115In NQR spectrum has shown the appearance of a small internal field in the direction perpendicular to the tetragonal c-axis. With application of a hydrostatic pressure, the AF state is suppressed and the superconductivity appears just above the critical pressure (P = 17 kbar). The nuclear spin lattice relaxation rate 1/T1 of 115In measured at P = 27 kbar indicates the occurrence of the superconductivity in the nearly AF region. In the superconducting state, 1/T1 has no Hebel-Slichter coherence peak just below \(\) of 2 K and has a power law T-dependence (T3) down to 300 mK. This is consistent with anisotropic superconductivity, with line nodes in the superconducting energy gap: non-s-wave superconductivity occurs in CeRhIn5.

Modulated magnetic structure of U(Pd1-xFex)2Ge2 studied byµSR

The magnetic ordering of the ternary intermetallic compoundU(Pd1-xFex)2Ge2 (x = 0.01, 0.02) was studied by µSR in the temperaturerange T = 15-300 K. The low temperature state changes from the modulatedlongitudinal spin density wave LSDW (x = 0.01) with a propagation vector k = [0 0 3/4] (TN = 140 K) to a simple antiferromagnetic structure AF (x = 0.02) with k = [0 0 1] (TN = 140 K). In the AF phase (sample with x = 0.02) the uranium magnetic moment shows a monotonic temperature dependence.In the LSDW phase with k = [0 0 3/4] (sample with x = 0.01) themagnetic moments of uranium atoms located in two magnetically nonequivalentpositions (0 0 0) and (0 0 1) have different temperature dependences. One momentassumes a maximum at Tm1 = 50 K, while the other one does not display anypeculiarities. The results are discussed within a model of the superposition ofa square and sine modulation and are compared with neutron diffraction data. Theobserved peculiar temperature behaviour of the moments is connected with thetemperature variation of the initial phase φ of the LSDW modulation byΔφ = 0.2 rad.

Magnetostriction and magnetization measurements on a DyNi 2B 2C single crystal in a magnetic field

In order to investigate the interactions between lattice properties, magnetic ordering and superconductivity of DyNi 2B 2C, thermal expansion, magnetostriction and magnetization measurements were performed for T=2–15 K and for μ 0 H=0–3 T on a single crystal in the crystallographic [1 1 0] direction. A magnetic phase diagram is derived that shows two phases (AF1 and AF2) in the narrow region between the zero-field antiferromagnetic AF and the induced ferromagnetic state FM. Moreover, it is characterized by a large-field hysteresis. This behaviour can be described by a two domain magnetic state. The metamagnetic structure AF1 with about a quarter of the saturated magnetization is responsible for suppressing the superconductivity in DyNi 2B 2C because of its ferromagnetic component.

Electronic structure of α-MnS (alabandite): an ab initio study

The structural, electronic and magnetic properties of α-MnS are investigated using an ab initio periodic linear combination of atomic orbitals self-consistent field approach. The geometry is optimized and the electronic structure discussed in terms of charge and spin density maps, band structure and density of states. The relative stability of the antiferromagnetic AF 2 phase with respect to the ferromagnetic one is discussed as a function of the metal-metal distance.