Unique Phase Transition on Spin-2 Triangular Lattice of Ag₂MnO₂

Abstract

Ag2MnO2 is studied as a possible candidate compound for an antiferromagnetic XY spin model on a triangular lattice. In spite of the large Curie-Weiss temperature of -430 K found in magnetic susceptibi-lity, Mn3+ spins with S = 2 do not undergo a conventional long-range order down to 2 K probably owing to the geometrical frustration and two dimensionality in the system. Instead, a unique phase transition is found at 80 K, where specific heat exhibits a clear sign of a second-order phase transition, while magnetic susceptibility changes smoothly without a distinct anomaly. We think that this transition is related to the chirality degree of freedom associated with a short-range order, which has been expected for the classical XY spin model on a triangular lattice. On further cooling, spin-glass-like behavior is observed below 22 K, possibly corresponding to a quasi-long-range order.Comment: to appear in J. Phys. Soc. Jpn, Vol. 77, No.

Static magnetic order in metallic triangular antiferromagnetAg2MnO2detected by muon-spin spectroscopy

“…A more detailed description of the preparation and characterization of the powder is presented in Ref. 20.…”

“…This is probably due to the preferred orientation of the pressed powder sample. 20 On the contrary, since the c plane tends to align by pressing because of the 2D structural nature of Ag 2 MnO 2 , the smaller A tail for the pressed powder than that for a random powder ͑1/3͒ would imply that H int Ќ c as S ͑0͒ is perpendicular to the basal plane. Note that all the three ZF signals appear below T N end , at which A TF reaches 0.…”

“…Furthermore, metallic conduction in the Ag 2 plane [18][19][20] naturally hinders the interaction between the adjacent MO 2 planes through the superexchange interaction via Mn-O-͑Ag 2 ͒-O-Mn due to the absence of empty 5s orbital in Ag. [18][19][20] On the contrary, such interaction via Ni-O-Na-O-Ni is reported to induce interplane AF coupling in NaNiO 2 . 21 This implies that Ag 2 MO 2 is most likely to be a candidate for an ideal 2DTL system.…”

“…32͒ and Ag 2 NiO 2 , 17,18 the specific-heat ͑C p ͒ measurements indicated a transition at T m1 = 80 K, while the susceptibility ͑͒ measurements showed a spin-glass like transition at T m2 = 22 K under the magnetic field ͑H͒ of 1 kOe. 20 Also, both the paramagnetic ͑PM͒ Curie temperature ͑⌰ p ͒ and the effective magnetic moment ͑ eff ͒ of Mn ions are estimated as −370 K and 4.93 B , where the latter value is equivalent to the spin-only value for the Mn 3+ ions with S = 2 and g =2 20 Very recently, Yoshida et al 20 proposed the possibility of the chirality transition on the 2DTL MnO 2 plane below T m1 . However, the microscopic magnetic nature of Ag 2 MnO 2 , to authors' knowledge, has been less investigated by neutron, NMR, ESR, and + SR measurements thus far.…”

Highly Dynamic Chalcogen Chains in Silver(I) (Poly)Chalcogenide Halides: a New Concept for Thermoelectrics?

“…47 As stated by Prof. Jansen itself in a recent article, 48 the frustration of building blocks (like the present distortion phenomena here) is again a key ingredient for unconventional phenomena in solid materials.…”