Antiferromagnetic order in single crystals of theS=2quasi-one-dimensional chainMnCl3(bpy)

Abstract

A suite of experimental tools, including high-field magnetization and electron spin resonance (ESR) studies in magnetic fields of up to 50 T and heat capacity studies up to 9 T, have revealed antiferromagnetic order in single crystals of the Heisenberg S = 2 chain compound MnCl3(bpy), where bpy is 2,2 ′ -bipyridine. The Néel temperature, which depends on the strength of the applied magnetic field and its orientation with respect to the crystalline axes that was revealed by heat capacity measurements, is near 11.5 K in zero field. The spin-flop transition is identified in the magnetization curve acquired at 1.7 K and at µoH c SF = 24 T along the c-axis. The transition field HSF is lower than that expected from the previous antiferromagnetic resonance (AFMR) studies on a powder sample. The identification of the long-range antiferromagnetic order resolves an earlier report by Granroth et al. [Phys. Rev. Lett. 77, 1616(1996] that identified MnCl3(bpy) as an S = 2 Haldane system down to 40 mK. The ESR studies identify a wide-range of antiferromagnetic resonance modes that provide additional microscopic information about the g-values (ga * = 2.09, g b = 1.92, and gc = 2.07), the zero-field splitting constants, D = −1.5 K and E = −0.17 K when the nearest-neighbor spin interaction J/kB = 31.2 K, which is evaluated from fitting the susceptibility, and the anisotropy of this compound (easy-axis is the c-axis, the second easy-axis is the b-axis, and the hard axis is the a * -axis), when using a standard (two-sublattices) AFMR analysis that does not quantitatively reproduce the observed H

Long-range magnetic order and interchain interactions in theS=2chain systemMnCl3(bpy)

“…Using their values for the parameters (Table I), the caclulated mode frequencies in Fig. 2(a) reproduce the ones reported by Shinozaki et al [34]. In general, the experimental spectra are satisfactorily represented by those calculations, but the 10% overestimation of the spin-flop field B SF and the error in the mode frequencies for m = c and B > B SF are troubling issues.…”

“…Notice that the predicted values of M a * and M b are quite close and cross at J = −2.8 meV. The experimental value for the magnetization M exp b ≈ 0.68 µ B with field along b is also indicated in this figure [34]. Figure 4 shows earlier magnetization curves [34] at 1.7 K along the a * and b directions.…”

“…Our description also includes the easy-axis anisotropy K, which favors spin alignment along the chain axis c (K > 0) or in the a * − b plane (K < 0), and the easy-plane anisotropy E, which favors spin alignment along b (E > 0) or along a * (E < 0). As found earlier, the g tensor will be taken to be slightly anisotropic with eigenvalues g a * a * = 2.09, g bb = 1.92 and g cc = 2.07 [32,34]. Note that we have modified the previous notation [34], where D was used to represent the single-ion anisotropy along c, now defined as K.…”

“…As found earlier, the g tensor will be taken to be slightly anisotropic with eigenvalues g a * a * = 2.09, g bb = 1.92 and g cc = 2.07 [32,34]. Note that we have modified the previous notation [34], where D was used to represent the single-ion anisotropy along c, now defined as K.…”

“…However, weak signatures from randomly-arranged microcrystals hinted that longrange order might appear at low temperatures [31,32]. Recently, unambiguous long-range antiferromagnetic ordering was identified at T N = 11.5 K [33,34] in oriented single-crystals.…”