Ferromagnetic coupling in nonmetallic Cu2+ compounds

Continuous transformation between ferro and antiferro circular structures in [Formula: see text] frustrated Heisenberg model

Frustrated magnetic compounds, in particular low-dimensional, are topical research due to the persistent uncovering of novel nontrivial quantum states and potential applications. The problem of this field is that many important results are scattered over the localized parameter ranges, while areas in between still contain hidden interesting effects. We consider the [Formula: see text] Heisenberg model on the square lattice and use the spherically symmetric self-consistent approach for spin-spin Green's functions in 'quasielastic' approximation. We have found a new local order in spin liquids: antiferromagnetic isotropical helices. On the structure factor we see circular concentric dispersionless structures, while on any radial direction the excitation spectrum has 'roton' minima. That implies nontrivial magnetic excitations and consequences in magnetic susceptibility and thermodynamics. On the [Formula: see text] exchange parameters globe we discover a crossover between antiferromagnetic-like local order and ferromagnetic-like; we find stripe-like order in the middle. In fact, our 'quasielastic' approach allows investigation of the whole [Formula: see text] globe.

The crystal structure and magnetic properties of a new ferrimagnetic semiconductor: Ca21Mn4Sb18

Single crystals of the new transition metal Zintl phase, Ca(21)Mn(4)Sb(18), were prepared by high temperature melt synthesis. The crystal structure was determined by single crystal X-ray diffraction to be monoclinic in the space group C2/c. Crystal information was obtained at 90 K, and unit cell parameters were determined (a = 17.100(2) A, b = 17.073(2) A, c = 16.857(2) A, beta = 92.999(2) degrees, Z = 2, R1 = 0.0540, wR2 = 0.1437). The structure can be described as containing 4 discreet units per formula unit: 1 linear [Mn(4)Sb(10)](22-) anion, 2 dumbbell-shaped [Sb(2)](4-) anions, 4 individual Sb(3-) anions, and 21 Ca(2+) cations. The [Mn(4)Sb(10)](22-) anion contains four edge-shared MnSb(4) tetrahedra with distances between Mn ions of 3.388(4) A, 2.782(4) A, and 2.760(4) A. Electron counting suggests that the Mn are 2+. Temperature dependent magnetization shows a ferromagnetic-like transition temperature at approximately 52 K which is suppressed with increasing magnetic field. The paramagnetic regime is best fit to a ferrimagnetic model, providing a total effective moment of 4.04(2) mu(B), significantly less than that expected for 4 Mn(2+) ions (11.8 mu(B)). Temperature dependent resistivity shows that this compound is a semiconductor with an activation energy of 0.159(2) eV (100-300 K).

Insulating ferromagnetism in La4Ba2Cu2O10: an Ab initio wannier function analysis

Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.