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We present a combination of elastic neutron scattering measurements in zero and 14.5 T and magnetization measurements in zero and 14 T on underdoped superconducting Ba1-xKxFe2As2 (x = 0.17), and the same measurements in zero field on a nonsuperconducting crystal with x = 0.09. The data suggest that the underdoped materials may not be electronic phase separated but rather have slightly inhomogeneous potassium doping. The temperature dependence of the magnetic order parameter below the transition of the sample with x = 0.09 is more gradual than that for the case of the undoped BaFe2As2, suggesting that this doping may be in the vicinity of a tricritical point. We advance therefore the hypothesis that the tricritical point is a common feature of all superconducting 122s For the x = 0.17 sample, while Tc is suppressed from =≈17 to ≈8 K by a magnetic field of 14 T, the intensity of the magnetic Bragg peaks (1 0 3) at 1.2 K is enhanced by 10%, showing competition of superconductivity and antiferromagnetism. The intensity of the magnetic Bragg peaks (1 0 3) in the (Tc, TN) temperature interval remain practically unchanged in 14.5 T within a 10% statistical error. The present results are discussed in the context of the existing literature. [ABSTRACT FROM AUTHOR]

The superconducting system La2-xBaxCu04 is known to show a minimum in the transition temperature Tc at x = ⅛ where maximal stripe order is pinned by the anisotropy within the CuO2 planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x = 0.095, where Tc reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here, we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1% Zn substituted for Cu, which reduces Tc to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the Cu02 layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase. [ABSTRACT FROM AUTHOR]

Recent studies have reported the existence of an epitaxially stabilized tetragonal-like ("T-like") monoclinic phase in BiFe03 thin films with high levels of compressive strain. While their structural and ferroelectric properties are different from those of rhombohedral-like ("R-like") films with lower levels of strain, little information exists on magnetic properties. Here, we report on a detailed neutron scattering study of a nearly phase-pure film of T-like BiFe03. By tracking the temperature dependence and relative intensity of several superstructure peaks in the reciprocal lattice cell, we confirm antiferromagnetism with largely G-type character and TN = 324 K, significantly below a structural phase transition at 375 K, contrary to previous reports. Evidence for a second transition, possibly a minority magnetic phase with C-type character, is also reported with TN = 260 K. The coexistence of the two magnetic phases in T-like BiFe03 and the difference in ordering temperatures between /?-like and T-like systems is explained through simple Fe-O-Fe bond distance considerations. [ABSTRACT FROM AUTHOR]

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(Fe1-xCrx)2As2 as a function of Cr doping for 0 ⩽ x ⩽ 0.47. The average spin-density-wave moment is independent of concentration for x ⩽ 0.2 and decreases rapidly for x ⩾ 0.3. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials. [ABSTRACT FROM AUTHOR]

We present a systematic investigation of the antiferromagnetic ordering and structural distortion for the series of Ba(Fe1-xRux)2As2 compounds (0≤x≤0.246). Neutron and x-ray diffraction measurements demonstrate that, unlike for the electron-doped compounds, the structural and magnetic transitions remain coincident in temperature. Both the magnetic and structural transitions are gradually suppressed with increased Ru concentration and coexist with superconductivity. For samples that are superconducting, we find strong competition between superconductivity, the antiferromagnetic ordering, and the structural distortion. [ABSTRACT FROM AUTHOR]

The A-site spinel material CoAl2O4 is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which unique incommensurate or "spin-spiral-liquid" ground states are predicted. Our previous single-crystal neutron scattering study instead classified it as a "kinetically inhibited" antiferromagnet, where the long-ranged correlations of a collinear Néel ground state are blocked by the freezing of domain-wall motion below a first-order phase transition at T*=6.5 K. This paper provides new data sets from a number of experiments, which support and expand this work in several important ways. We show that the phenomenology leading to the kinetically inhibited order is unaffected by sample measured and instrument resolution, while new low-temperature measurements reveal spin correlations are unchanging between T=2 K and 250 mK, consistent with a frozen state. Polarized diffuse neutron measurements show several interesting magnetic features, which can be entirely explained by the existence of short-ranged Néel order. Finally, and crucially, this paper presents some neutron scattering studies of single crystalline MnAl2O4, which acts as an unfrustrated analog to CoAl2O4 and shows all the hallmarks of a classical antiferromagnet with a continuous phase transition to Néel order at TN=39 K. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T*. The higher level of cation inversion in the MnAl2O4 sample indicates that this behavior is primarily an effect of greater next-nearest-neighbor exchange. [ABSTRACT FROM AUTHOR]