Field-induced effects of anisotropic magnetic interactions in SrCu₂(BO₃)₂

Abstract

Abstract. We observed a field-induced staggered magnetization in the 2D frustrated dimer singlet spin system SrCu 2 (BO 3 ) 2 by 11 B NMR, from which the magnitudes of the intradimer Dzyaloshinsky-Moriya interaction and the staggered g-tensor were determined. These anisotropic interactions cause singlet-triplet mixing and eliminate a quantum phase transition at the expected critical field H c for gap closing. They provide a quantitative account for some puzzling phenomena such as the onset of a uniform magnetization below H c and the persistence of the excitation gap above H c . The gap was accurately determined from the activation energy of the nuclear relaxation rate.Spin systems with singlet ground states exhibit a variety of quantum phase transitions in magnetic field [1]. A generic example is the Bose-Einstein condensation of triplets when the field exceeds the critical value at which the excitation energy vanishes. This results in an antiferromagnetic order with the staggered moment perpendicular to the field, as has been observed, e.g., in TlCuCl 3 [2]. Another possibility is the formation of a superlattice of localized triplets due to repulsive interactions, which translates into magnetization plateaus at fractional values of the saturated magnetization. The best known example is SrCu 2 (BO 3 ) 2 with its two-dimensional network of orthogonal dimers of Cu 2+ ions (spin 1/2). This material shows an excitation gap ∆ 0 =35 K and plateaus at 1/8, 1/4, and 1/3 of the saturated magnetization [3,4,5]. A magnetic superlattice at the 1/8-plateau has actually been observed by NMR experiments [6].

Magnetic fluctuations above the Néel temperature in κ‐(BEDT‐TTF)2Cu[N(CN)2]Cl, a quasi‐2D Heisenberg antiferromagnet with Dzyaloshinskii–Moriya interaction

“…The magnetic-field-induced change in the excitation spectrum of 1D Heisenberg chains [3,4] agrees with theory [5,6]. Field-induced magnetism has also been studied in a quasi 2D S = 1 dimer system [7]. The present experiments are on a quasi 2D, S = 1/2 magnet where the continuous symmetry is broken by a very small anisotropy and the applied magnetic field, H. κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl (κ-ET 2 -Cl) is a layered spin 1/2 Heisenberg antiferromagnet with a magnetic ordering transition [8] at T N = 23 K measured [9] in H = 0.…”

Effect of Dzyaloshinskii-Moriya interactions on the phase diagram and magnetic excitations of SrCu2(BO3)Romhányi1, Totsuka2, Penc3 2011Phys. Rev. B

“…[20], and g s = 0.023 in Ref. [25]. The value of the intra-dimer DM coupling D = 0.034J ≈ 60 GHz is obtained in Ref.…”

Magnon dispersion and anisotropies inSrCu2(BO3)2

“…29 It is interesting to note that the sign of D Ќ Ј extracted by analyzing 29 far-infrared experiments 8,30 is opposite to that suggested to explain avoided crossing at higher fields. 14,15 The apparent contradiction may be resolved because the avoided crossing is not determined solely by D, as suggested earlier, [14][15][16] …”