Static magnetic order in metallic triangular antiferromagnet Ag 2 MnO 2 Jun Sugiyama a,Ã , Hiroshi Nozaki a , Yutaka Ikedo a , Kazuhiko Mukai a , Peter L. Russo b , Daniel Andreica c , Alex Amato d , Hiroyuki Yoshida e , Zenji Hiroi e a Toyota Central Research and Development Laboratories Inc., Nagakute, Aichi 480-1192, Japan b TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2A3 c Faculty of Physics, Babes-Bolyai University, 3400 Cluj-Napoca, Romania d Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland e ISSP, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8581, Japan article info Keywords: Layered manganese dioxides Triangular lattice Metallic antiferromagnet abstract The magnetic nature of the triangular antiferromagnet Ag 2 MnO 2 , which exhibits two magnetic transitions at T m1 80 K and T m2 30 K, has been studied with m þ SR spectroscopy using a polycrystalline sample in the temperature (T ) range between 300 and 1.8 K. Weak transverse field muon-spin rotation and relaxation (m þ SR) measurements suggest the appearance of a random internal magnetic field at T below T m1 , while zero field (ZF) m þ SR measurements indicate the existence of static internal magnetic fields below T m2 . Furthermore, two components with 10 times different precession frequencies but almost equivalent amplitudes in the ZF-spectrum indicate the formation of a static but complex antiferromagnetic order below T m2 . The overall magnetic behavior is therefore clarified in that the static disordered moments appear below T m1 , whereas the short-range order completes below T m2 ¼ T N . & 2008 Elsevier B.V. All rights reserved. 1. Introduction Layered transition-metal dioxides AMO 2 , which consist of alternating stacks of A and MO 2 planes, where A þ stands for an alkali (Li þ , Na þ ,K þ , Rb þ , and Cs þ ), Ag þ or ðAg 2 Þ þ ion and M 3þ for a transition metal ion, and in which M ions form a two-dimensional triangular lattice (2DTL) by connection of edge-sharing MO 6 octahedra, have been heavily investigated due to their complex magnetic behavior [1–4]. Although the 2D interaction is thought to play an essential role for determining the magnetic nature of AMO 2 , the inter-plane (3D) interaction sometimes contributes to form long-range order in the AMO 2 compounds. Here, we call the AMO 2 with relevant interactions as ‘‘quasi-2D’’ com- pounds. In order to reduce the 3D interaction and to make the quasi-2D compounds close to an ideal 2D system, one could increase the inter-plane distance (d IP ) of AMO 2 by using A ions with larger ionic radius. In particular, the largest d IP is achieved for A ¼ Ag 2 for AMO 2 , when M is Ni; that is, d IP ¼ 0:473 nm for LiNiO 2 [5], 0.52 nm for NaNiO 2 [6], 0.612 nm for AgNiO 2 [7], and 0.801 nm for Ag 2 NiO 2 [8], but KNiO 2 , RbNiO 2 , and CsNiO 2 are currently unavailable. This implies that Ag 2 MO 2 is most likely to be a candidate for an ideal 2DTL system. Indeed, according to our recent muon-spin rotation and relaxation (m þ SR) experiment [9], Ag 2 NiO 2 was found to exhibit an incommensurate (IC) AF order below 56 K, although LiNiO 2 was reported to enter into a spin- glass-like state below 10 K [5,10–13]. NaNiO 2 displays an A-type commensurate (C) AF state with T N ¼ 20 K that is stabilized by a cooperative Jahn–Teller distortion of the NiO 6 octahedra [6,14], and AgNiO 2 a C-AF state with T N ¼ 21 K caused by slight spatial deviation of the O 2 ions [15]. Note that, since the Ni 3þ ions are in a low spin state with S ¼ 1 2 ðt 6 2g e 1 g Þ, ANiO 2 has been thought to be a typical 2DTL with half-filling. However, the IC-AF order, which is predicted for the 2DTL with half-filling [16], is only observed for Ag 2 NiO 2 [9]. Larger interplane distance in ANiO 2 thus seems to induce more complex magnetic behavior in the NiO 2 plane. Among several combinations between Ag 2 and M for AMO 2 , only Ag 2 NiO 2 and Ag 2 MnO 2 have, to authors’ knowledge, been prepared thus far. Disilver manganese dioxide Ag 2 MnO 2 belongs to a monoclinic system with a ¼ 0:5178 nm, b ¼ 0:2875 nm, c ¼ 0:8815 nm, and b ¼ 102:3  at ambient T [17] (see Fig. 1). In spite of its metallic conductivity down to 2K due to a quarter- filled Ag 5s band, as in the case of Ag 2 F [18] and Ag 2 NiO 2 [8,19], the specific heat (C p ) measurements indicated a transition at T m1 ¼ 80 K, while the susceptibility (w) measurements showed a spin-glass-like transition at T m2 ¼ 22 K under the magnetic field (H) of 1 kOe [17]. Also, both the paramagnetic Curie temperature (Y p ) and the effective magnetic moment (m eff ) of Mn ions are estimated as 429 K and 4:90m B , where the latter value is equivalent to the spin-only value for the Mn 3þ ions with S ¼ 2 and g ¼ 2(m eff ¼ g ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi SðS þ 1Þ p m B ) [17]. Based on the lack of a clear ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.175 Ã Corresponding author. Tel.: +81561718029; fax: +8156163 6137. E-mail address: e0589@mosk.tytlabs.co.jp (J. Sugiyama). Physica B 404 (2009) 777–780