Muon spin rotation study of the magnetic structure in the tetragonal antiferromagnetic state of weakly underdoped Ba _{1− x} K _x Fe ₂ As ₂

Abstract

With muon spin rotation (µSR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba1−xKxFe2As2 single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-Q magnetic structure with c-axis oriented moments which has a vanishing magnetic moment on half of the Fe sites.The phase diagram of the iron arsenide superconductors is characterized by the close proximity of the antiferromagnetic (AF) and superconducting (SC) orders [1][2][3][4][5]. In the underdoped regime they coexist and compete for the same low-energy electronic states [6,7]. Moreover, the SC transition temperature, T c , reaches its maximum near the point where the AF order vanishes. It is therefore commonly assumed that the AF fluctuations are involved in the SC pairing mechanism [4,5]. Accordingly, it is important to obtain a full understanding of the AF ground state and also of the nearly degenerate states which can strongly impact the magnetic fluctuations.For the 122-type BaFe 2 As 2 system, the primary AF order is a so-called single-Q state with in-plane antiparallel spins along the (0, π) direction and parallel ones along (π, 0) [1][2][3]8]. The transition to this so-called "stripelike" AF state is coupled with a structural transition from a tetragonal paramagnetic to an orthorhombic AF (o-AF) state. The interactions underlying this magnetostructural transition are the subject of an ongoing discussion. The explanations range from the so-called itinerant models for which the Fermi-surface nesting governs the magnetic interactions [4,5,9] to orbital models which assume localized spins whose exchange interactions are determined by the orbital occupation [10,11]. An important role of the spin-lattice coupling [12] and a near degeneracy of different spin states of the Fe ions have also been discussed [13,14].In view of this ongoing discussion, the recent observation of a phase transition from the o-AF state to a new type of AF state with tetragonal lattice symmetry (t-AF) has attracted considerable attention. This t-AF state was first observed at ambient pressure in Ba 1−x Na x Fe 2 As 2 (BNFA) [15,16] and more recently in Ba 1−x K x Fe 2 As 2 (BKFA) [17,18] in a narrow doping range close to optimal doping. A state with very similar phenomenology was observed earlier in BKFA at elevated pressures [19]. In BKFA, even a reentrance of the o-AF state at lower temperature has been observed which seems to be induced (or at least supported) by superconductivity [17,20].Motivated by these observations, a range of so-called double-Q magnetic structures have been proposed for the t-AF state [21][22][23]. These correspond to different superpositions of the single-Q states with wave vectors (0, π) and (π, 0). A perpendicular magnetis...

Observation of the magnetic C4 phase in Ca1−xNaxFe2As2

“…This structural transition is coupled to a magnetic transition where the previously in-plane Fe-site moments rotate to be co-linear to the c-axis forming a double-Q magnetic structure with P C 4 2 /ncm magnetic space group. [31][32][33] Fig . 2(a) shows diffractograms of the 112 nuclear peak and the 1 2 1 2 1 and 1 2 1 2 3 magnetic peaks for Ca 0.58 Na 0.42 Fe 2 As 2 (all peaks indexed using the tetragonal symmetry of the room temperature structure).…”

Vestigial chiral and charge orders from bidirectional spin-density waves: Application to the iron-based superconductors

“…Because in the iron superconductors the hybridization with the puckered As atoms already doubles the unit cell of the Fe square lattice, the CDW and SVDW states are more rigorously classified as intra-unit-cell orders. Recent experiments on Sr 1−x Na x Fe 2 As 2 [53] and Ba 1−x K x Fe 2 As 2 [54] found direct evidence for a low-temperature CSDW phase, which can support a CDW vestigial phase. It remains to be seen whether the tetragonal magnetic phase can be reached in these compounds without first crossing the stripe magnetic state.…”

Anomalous Composition-Induced Crossover in the Magnetic Properties of the Itinerant-Electron AntiferromagnetCa1−xSrxCo2−yAs2Sangeetha1, Smetana2, Mudring3 et al. 2017Phys. Rev. Lett.213540View full textAdd to dashboardCite

“…A subsequent investigation by polarized and unpolarized neutron diffraction determined that the ordered moments in the new phase are aligned along the c axis instead of along the ab plane as in the stripe AFM structure [8]. The work on the Ba 1−x Na x Fe 2 As 2 system was followed by the observation of the same AFM C 4 phase in the Ba 1−x K x Fe 2 As 2 [9][10][11], Sr 1−x Na x Fe 2 As 2 [12,13], and Ca 1−x Na x Fe 2 As 2 [14] systems. These results are important to understanding the mechanism of superconductivity and other aspects of the hole-doped iron arsenides [15][16][17][18][19][20][21].…”