Possible realization of an antiferromagnetic Griffiths phase in Ba(Fe1−xMnx)2

Abstract

We investigate magnetic ordering in metallic Ba(Fe 1−x Mn x ) 2 As 2 and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations x > 10%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe 2 As 2 by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains equally sharp, in the x = 12% sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the x = 12% sample significantly above the Néel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba(Fe 1−x Mn x ) 2 As 2 could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…(b) The structural and magnetic phase diagram for Mndoped Ba(Fe1−xMnx)2As2 . (c) Similar phase diagram for Ba(Fe1−xMnx)2As2 obtained by another group (Inosov et al, 2013). Here the system is believed to form a disordered spin glass (Griffiths) phase for x > 0.1.…”

“…With increasing Mn doping in BaFe 2 As 2 , the structural and AF phase transitions are gradually suppressed as shown in Figs. 6(b) and 6(c) (Inosov et al, 2013;Kim et al, 2010). For Mn concentration x ≥ 0.1, the system goes into a mixed phase, possibly in the Griffiths regime, with coexisting short-range spin excitations at AF wave vectors similar to those in BaFe 2 As 2 [Q AF = Q stripe ] and BaMn 2 As 2 [Q = Q Néel rotated 45 • from Q AF ] (Inosov et al, 2013).…”

“…6(b) and 6(c) (Inosov et al, 2013;Kim et al, 2010). For Mn concentration x ≥ 0.1, the system goes into a mixed phase, possibly in the Griffiths regime, with coexisting short-range spin excitations at AF wave vectors similar to those in BaFe 2 As 2 [Q AF = Q stripe ] and BaMn 2 As 2 [Q = Q Néel rotated 45 • from Q AF ] (Inosov et al, 2013). In contrast to Cr and Mn doping, isoelectronic doping by replacing As with P in BaFe 2 (As 1−x P x ) 2 induces optimal superconductivity near x = 0.3 (Jiang et al, 2009).…”

“…While impurities such as Cr and Mn substituted for Fe in BaFe 2 As 2 suppress the static AF order in the parent compound without inducing superconductivity [Figs. 6(a), 6(b) and 6(c)] (Inosov et al, 2013;Kim et al, 2010;Marty et al, 2011), isoelectronic substitution by replacing As with P [ Fig. 6(d)] (Jiang et al, 2009;Shibauchi et al, 2014) or Fe with Ru [ Fig.…”

“…Since large sized single crystals can be grown by self-flux method in many of these cases , doped BaFe 2 As 2 materials provide a unique opportunity to study the evo-lution of the static AF order and spin excitations as a function of hole, electron, and isovalent doping throughout the entire AF order to superconductivity phase diagram, and determine their connection with superconductivity. These experiments, together with neutron scattering studies of related materials (Inosov et al, 2013;Ishikado et al, 2009;Johnston et al, 2011;Lamsal et al, 2013;Marty et al, 2011;Shamoto et al, 2010;Simonson et al, 2012;Taylor et al, 2013;Wakimoto et al, 2010), will establish the common features in the magnetic order and spin excitations in different families of iron-based superconductors. The outcome, together with the results from high-T c copper oxide and heavy fermion superconductors, and can form a basis to determine if magnetism is indeed responsible for superconductivity in these materials (Scalapino, 2012).…”