Ba1−xKxMn2As2: An Antiferromagnetic Local-Moment Metal

Abstract

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature TN = 625 K and a large ordered moment µ = 3.9 µB/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large µ. Ba1−xKxMn2As2 is thus the first metallic ThCr2Si2-type M As-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba1−xKxMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.Superconducting transition temperatures T c > 50 K have been observed for only two classes of materialslayered cuprates and iron arsenides [1,2]. Both classes contain stacked square lattice layers of the transition metal atoms. However, the parent compounds of the two families exhibit divergent physical properties. For example, La 2 CuO 4 is a local magnetic moment antiferromagnetic (AF) insulator [1] while BaFe 2 As 2 is metallic and its AF ordering is widely considered to be best characterized as a spin-density wave arising from conduction carriers [2]. These differences create barriers for a general and comprehensive understanding of the underlying mechanisms of high-T c superconductivity and related phenomena in a broad spectrum of materials. Thus, it is desirable to create a material that can bridge the gap between the cuprates and iron arsenides. Herein we report the synthesis and properties of such a material, Ba 1−x K x Mn 2 As 2 (x = 0.016, 0.05), which shares properties with both classes.The undoped parent compound BaMn 2 As 2 crystallizes in the same body-centered-tetragonal (bct) ThCr 2 Si 2 -type structure as the M Fe 2 As 2 (M = Ca, Sr, Ba) iron arsenide parent compounds do at room temperature [2][3][4]. It is a semiconductor with an activation energy of ∼ 30 meV determined from electrical resistivity ρ(T ) measurements [4,5], consistent with electronic structure calculations that indicate a band gap of ∼ 100-150 meV [5]. Heat capacity C p measurements at low-T yield an electronic linear heat capacity coefficient γ = 0 which is consistent with an insulating ground state [4]. BaMn 2 As 2 orders into a G-type (Néel-or checkerboard-type) AF structure below a Néel temperature T N = 625(1) K with an ordered moment at 10 K of µ = 3.88(4) µ B /Mn aligned along the crystallographic c axis [2, 4, 6]. Since BaMn 2 As 2 is an insulator at low temperatures, these results demonstrate that the antiferromagnetism arises from ordering of local Mn magnetic moments instead of from itinerant current carriers. Both the static and dynamic magnetic properties for T = 4-1000 K are welldescribed by the AF J 1 -J 2 -J c local moment Heisenberg model, with a Mn spin S = 5/2 as expected from the 3d 5 e...

Weakly ferromagnetic metallic state in heavily doped Ba1−xKxMn2Bao1, Jiang2, Sun3 et al. 2012Phys. Rev. B364141View full textAdd to dashboardCite

“…The room-temperature resistivity is 2∼3 mΩ·cm, which is over two orders of magnitude smaller than that of BaMn 2 As 2 , and about one order of magnitude smaller than that reported for the lightly K-doped samples with x=0.016 and 0.05. 21 This further confirms the insulator-to-metal transition in BKMA. Below 1.5 K, $A with x=0.19 shows a tiny drop in resistivity.…”

“…Although Cr and Fe (Mn's neighbors in the Periodic Table) can substitute for Mn significantly (at levels of 4.4% and 10%, respectively), these substitutions hardly change the electronic transport and magnetic properties. Very recently Pandey et al 21 succeeded in doping small amount of K (1.6% in single crystals and 5% in polycrystals) into BaMn 2 As 2 . They demonstrated that the K doping induced hole carriers, and changed the system into a metallic state.…”

“…It was thus advocated that the new class of materials bridged the gap between the iron pnictide and cuprate high T c materials. 21 Another relevant study showed a pressure-induced metallization in BaMn 2 As 2 . 22 Motivated by the hole-doping result in LaMnAsO, 5 we had also grown the K-doped BaMn 2 As 2 single crystals.…”

“…15 The unique insulating property of BaMn 2 As 2 makes it promising to explore novel states of matter via applying high pressures as well as various chemical doping just like those 7,16-19 employed in BaFe 2 As 2 . Some doping studies 20,21 were recently performed in BaMn 2 As 2 system. For the Mn-site doping, 20 unfortunately, many transition metals do not substitute for Mn at levels above 0.5%.…”

Synthesis and characterization of Ca-doped LaMnAsO

“…It has been shown that T N in the MnAs layer of Mn-122 compounds is robust against substitution of the layers adjacent to it. Indeed, substituting K for Ba in Ba 1−x K x Mn 2 As 2 shows that T N of the MnAs layers changes negligibly in the range 0 x 0.2 [19,20]. Figure 8 shows the linewidths of the (100) and (101) as a function of temperature that are extracted from fitting the diffraction patterns in scans similar to those in Fig.…”