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 in hydrated 123 compounds

Copper nuclear quadrupole resonance and zero-field nuclear magnetic resonance (ZFNMR) studies of YBa₂Cu₃O_6.5 show that a magnetic phase appears in underdoped 123 superconductors treated in ambient moist air. The studies give convincing evidence that the “empty” CuO chains play the role of easy water insertion channels. The reaction occurs first in ordered regions of the crystallites. The final product of the reaction is a nonsuperconducting antiferromagnetic compound characterized by at least two types of magnetically ordered copper ions, with ZFNMR spectra in the frequency ranges 46–96 and 96–135 MHz, respectively. Even for powder samples fixed in an epoxy resin, this reaction is found to have partially occurred after a few years. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 739–744 (25 May 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

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)     

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 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 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 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.     

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 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 Repulsively Interacting Fermions in a Harmonic Trap

We propose a real-space Gutzwiller variational approach and apply it to a system of repulsively interacting ultracold fermions with spin-1/2 trapped in an optical lattice with a harmonic confinement. Using the realspace Gutzwiller variational approach, we find that in a system with balanced spin-mixtures on a square lattice, antiferromagnetism either appears in a checkerboard pattern or forms a ring, and the antiferromagnetie order is stable in the regions where the particle density is close to one, which is consistent with the recent results obtained by the real-space dynamical mean-field theory approach. We also investigate the imbalanced case and find that the antiferromagnetic order is suppressed there.