Possible realization of an antiferromagnetic Griffiths phase in Ba[Fe(1-x)Mn(x)](2)As(2)

TL;DR

This study explores the magnetic phase in Ba[Fe(1-x)Mn(x)](2)As(2), proposing it as a Griffiths-type phase with localized Mn moments causing unusual magnetic behavior above the quantum critical point.

Contribution

It provides evidence for a Griffiths-like magnetic phase in Ba[Fe(1-x)Mn(x)](2)As(2), highlighting the role of localized Mn moments and phase coexistence in this system.

Findings

Observation of smeared antiferromagnetic transition at x=12%

Detection of antiferromagnetic rare regions forming above Néel temperature

Identification of finite orthorhombic distortion in magnetic volume

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 BaFe2As2 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 N\'eel 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.