Antiferromagnetism in solid oxygen

Solid oxygen is known to have spontaneous antiferromagnetic order in its lowest temperature phase. Muons implanted in \alpha\mbox- O₂ in zero applied magnetic field take up positions where the local field is non‐zero and precess at a frequency proportional to the sublattice magnetisation. The reduction of the magnetisation as the temperature increases toward T_N is similar to that expected for the case of an anisotropic AFM coupling among spins of O_2 molecules. A fit to a simple non‐interacting, harmonic spin wave model yields an anisotropy‐induced gap in the spin wave density of states E_g/k_B=80.5(6) K, implying a large anisotropy constant K\approx 16J. This revised version was published online in July 2006 with corrections to the Cover Date.     

Antiferromagnetism and paramagnetism of some intermetallic compounds of rare-earth metals with silver

An investigation was made of the temperature dependence of the magnetic susceptibility of intermetallic compounds of Gd, Tb, Dy, and Ho with silver having CsC1 structure. It was established that in these compounds there exists an antiferromagnetic ordering with _N = 135°K for GdAg and _N = 105°K for TbAg. The paramagnetic Curie temperature _p has a negative value for all compounds. For the compound SmAg, the temperature dependence of the susceptibility is complicated. The results are explained on the basis of the theory of the Ruderman-Kittel-Kasuya-Yosida interaction and Van Vleck's theory of paramagnetism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 39–42, August, 1980.     

Antiferromagnetism in the Hubbard model

The energy spectrum of the two-sublattice Hubbard model is obtained in the static-fluctuation approximation. It is shown how the structure of the energy spectrum is modified as the parameters of the Hubbard model are varied. The ground state of the simple Hubbard model of dimension d=2 is the dielectric antiferromagnetic state. The author derives a consistency equation for the magnetization, which has an antiferromagnetic solution. Fiz. Tverd. Tela (St. Petersburg) 39, 1594–1599 (September 1997)     

Antiferromagnetism and high-temperature superconductivity

A model is presented describing superconductivity in close association with antiferromagnetism in a narrow-band system with electron correlations. We employ the Hubbard-Peierls Hamiltonian for weak to intermediate ratios between the on-site Coulomb repulsion and the electronic band width. Depending on the band structure and the electron number density antiferromagnetism arises, which for a nearly half-filled band gives rise to a Mott-Hubbard gap and resultant band splitting. The related density of energy states exhibits a singularity at the upper and lower edge of the lower and upper of the two split bands, respectively, if the lattice possesses the property of perfect nesting. The electron-phonon interaction is recast, by help of the method of canonical transformations, into a new form implying an attractive interaction between the quasiparticles in a Debye shell near the Fermi surface of each subband. A large effective interaction constant can be achieved and a BCS-type model for superconduction applies to each of the split bands if they are partially filled. Pairing arises between quasiparticles with parallel effective spin, i.e. in triplet states. The resulting gap equation is discussed in detail and the thermodynamic potential of the superconducting antiferromagnet is derived.     

Incommensurate Antiferromagnetism in the Extended t-J Model

The effect of the extra second neighbor hopping t‘ on the incommensurate spin correlation in the t-J modelin the underdoped regime is studied within the fermion-spin theory. It is shown that although the extra second neighborhopping t‘ is systematically accompanied with the increasing of the weight of the incommensurate peaks in the dynamicalspin structure factor, for the physical reasonable small value of t‘ the qualitative behavior of the incommensurate spincorrelation in the t-t‘-J model is the same as in the case of t-J model.     

Antiferromagnetism in 4f-systems with valence instabilities

An extendeds–f model is used for the study of intermediate-valence 4f-systems, which have antiferromagnetic ground states in the normal-valence phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate-valence phase. The local magnetic 4f-moment, the sublattice-magnetization as well as the Néel-temperatureT _N are discussed in terms of thes–f hybridizationV and thef-level positionE _f . Reduction of the energy gap betweenE _f and conduction band simulates the influence of external pressure or of alloying with suitable impurities. IncreasingV or decreasing4f-(5d, 6s)-gap intensifies the electronic fluctuations, which have two competitive consequences. They tend to quench the local moments, but simultaneously enhance the coupling between them. This results in a typical behaviour of the Néel-temperature. Conditions for the coexistence of antiferro-magnetism and intermediate valence are derived. Comparison with experimental data for systems like Eu-metal and Eu(Pd_1–x Au _x )₂Si₂ shows qualitative agreement.     

Antiferromagnetism of type II in nonstoichiometric YbP

A polycrystalline smaple of nonstoichiometric ytterbium phosphide, YbP_0.84, was investigated by neutron scattering, Mössbauer spectroscopy and bulk magnetic measurements. Neutron diffraction experiments prove the existence of antiferromagnetic type II ordering belowT _N =0.64 K, in contrast to the observed antiferromagnetic type III ordering in the stoichiometric Kondo-like compounds YbN and YbAs. The temperature dependence of the average ordered magnetic moment per Yb³⁺ ion with saturation value _Yb = 1.03(7) _B is similar to that of YbN. Mössbauer experiments prove the magnetic phase transition to be first order with different regions in the sample having slightly different transition temperatures. By means of inelastic neutron scattering the crystal-field level scheme was established to be ₆ – ₈(19meV) – ₇(43meV).     

Antiferromagnetism and lattice-distortion in the Peierls-Hubbard model

The ground state of the Peierls-Hubbard model is investigated. It is found to be dimerized or antiferromagnetic, depending on the strength of the effective electron-phonon coupling and the electron-electron interaction.Lattice distortion and magnetization do not coexist in the ground state.     

Antiferromagnetism of ThCr2Si2

Neutron diffraction measurements indicate that the magnetic moments of chromium atoms in the ThCr₂Si₂ compound show long-range order. The Cr magnetic moment equal to 1.20(25)μ_B at 1.5 K lie in the basal plane and form magnetic structure AFl-type.     

Antiferromagnetism of the α-Mn system

The variation of the Néel temperature (T_N) of antiferromagnetic alloys of Mn with 3-d elements has been studied by electrical resistivity measurements. Binary Mn-rich alloys Fe, Co, Ni, Ti, Rh, Os, Ir, and Ru were investigated. The results show a strong and systematic dependence on the number of d-electrons of the impurity element. Elements with more d electrons than Mn raise T_N, while those with fewer d electrons depress T_N. The change of T_N per atomic percent impurity also exhibits a striking, non-monotonic dependence on the impurity used. The dependence shows inversion symmetry with impurities around Mn.To further elucidate the properties of the Mn-rich antiferromagnetic system, a study was made of ternary Mn-rich alloys in the MnFeCr, MnCoNi, and MnVTi systems. The results from the MnFeCr system are consistent with the rigid-band model, the average number of d-electrons per atom being of primary importance determiningh the antiferromagnetism in this system.Our goal in these studies has been to provide a firm experimental data baseupon which future theoretical work can be built. A more detailed understanding of the very intriguing α-Mn magnetic system now awaits further theoretical investigation.     

Antiferromagnetism of quasi two-dimensional manganese stearate

The magnetic properties of manganese stearate, a highly two-dimensional magnetic material, are found to be changed considerably when it is synthesized by a new procedure. Whereas earlier samples behaved like weak-ferromagnets with ordering temperatures about 5 K, the new material is an antiferromagnet with T_n = 10 K. The reason may be a change in the crystal structure.     

Antiferromagnetism in half-filled S-band Hubbard model

The anitferromagnetic ground state in a half-filled narrow s-band is re-examined for the realistic case that the magnetic zone boundary does not coincide with a constant energy surface. We found no qualitative change of the characteristic features obtained previously, except that stronger correlation is required to produce similar results.     

Antiferromagnetism of strongly correlated electrons in narrow bands

We consider an antiferromagnetic solution of an effective narrow band hamiltonian within the moment-conserving decoupling for single-particle Green functions. In the limit of a half-filled band, corresponding to the Mott insulator, our results reduce analytically to the mean field approximation for the Heisenberg antiferromagnet, and in particular the band-width of quasiparticles states vanishes.