The N$acute{rm e}$el temperature of the new frustrated family of Sremph{RE}$_2$O$_4$ (emph{RE} = rare earth) compounds is yet limited to $sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin i
nteractions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector textbf{Q}$^{rm 0.5 K}_{rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $mu_{rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the emph{bc} plane, 11.9(66)$^circ$ away from the emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations.
Inelastic neutron scattering from superconducting (SC) Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$ reveals anisotropic and quasi-two-dimensional (2D) magnetic excitations close to textbf{Q}$_{texttt{AFM}}=({1}{2}/{1}{2})$ - the 2D antiferromagnetic (AFM)
wave-vector of the parent BaFe$_2$As$_2$ compound. The correlation length anisotropy of these low energy fluctuations is consistent with spin nematic correlations in the $J_1$-$J_2$ model with $J_1/J_2 sim$ 1. The spin resonance at $sim$8.3 meV in the SC state displays the same anisotropy. The anisotropic fluctuations experimentally evolve into two distinct maxima only along the direction transverse to textbf{Q}$_{texttt{AFM}}$ above $sim$80 meV indicating unusual quasi-propagating excitations.
A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_texttt{O}$ = $T_texttt
{N}$ = (201.5 $pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than emph{T}$_texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.
