Effect of vacancy doping on the Haldane spin-liquid state inPbNi2−xMgx

A. Zorko

¹

,

Denis Arčon

²

,

Alexandros Lappas

³

et al.

Abstract

The effect of spin-vacancy doping on the ground state of the quasi-one-dimensional Sϭ1 quantum antiferromagnet PbNi 2 V 2 O 8 is studied by X band as well as high-field ESR. A very broad ESR line is observed at low temperatures in powdered PbNi 2Ϫx Mg x V 2 O 8 for 0рxр0.24. The frequency dependence of the observed ESR line rules out the possibility that the low-temperature ESR signal comes from the isolated Sϭ 1 2 spins. Instead, it is proposed that a magnetic coupling ͑most likely of second order͒ between the chain-end spins on both sides of the Mg 2ϩ impurity plays an important role in this system. Impurity-induced spin states, due to the strong interchain coupling, also induce a staggered magnetization on the neighboring chains with a characteristic length . increases with decreasing temperature, as suggested by the additional broadening and the shift of the ESR signal at low temperatures. On the other hand the temperature dependence of the low temperature susceptibility suggests a much more complicated picture in which impurity-induced spins are exposed to coupling to thermal excitations on each chain as well as to three-dimensional antiferromagnetic correlations.

Magnetic ordering in the ultrapure site-diluted spin chain materialsSrCu1−xNixO2

“…In gapped spin chains and ladders, where spin correlations are intrinsically shortranged, defects may in some cases enhance magnetism by releasing new S = 1/2 degrees of freedom. 5,6 The latter may behave as paramagnetic spins, 7 bind into dimers across their connecting chain fragments, 8 or form complex interacting one-dimensional systems of their own.…”

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

“…For all investigated PbNi _xMgxV208 (0 < x < 0.24) samples, a very broad ESR resonance at around g ~ 2.2 has been observed between room temperature and 70 K and reported earlier [9]. The measured ESR line shape in this temperature regime is ranging between Lorentzian and Gaussian.…”

“…9 and here we summarize these observations. At high temperatures (i.e., at temperatures above 70 K) a nearly linear increase of the ESR line width with temperature has been observed, i.e., AHpp = a + bT.…”

“…In doped samples, a new line appears below 70 K which completely dominates the ESR spectra only at very low temperatures. We suggested that this line is due to the end-ofchain S = 1/2 spins released by the nonmagnetic doping [9].…”

“…Doping of PbNi2V20 s could be realized by a partial replacement of Ni 2 § (S = 1) with Mg 2+ (S = 0) resulting in a PbNi2_xMgxV20 s. On the basis of the valence-bond picture [6] it is known for a long time that the introduction of nonmagnetic impurities into the Haldane chain releases two S = 1/2 spins on each side of the impurity [7,8]. These spins have been indeed detected in PbNi2_xMgxV20 s by earlier electron spin resonance (ESR) measurements [9,10]. Specific heat [11] and neutron powder diffraction [12] in the PbNiajVlgxVaO s series find that nonmagnetic substitution leads to AF ordered phases for any x > 0, at temperatures Tr~ < 3.5 K. We thus have a unique system in which a transition from the nonmagnetic spin liquid state to the long-range ordered (LRO) AF state could be realized by minute changes of the chemical composition.…”