Antiferromagnetic Order with Spatially Inhomogeneous Ordered Moment Size of Zn- and Si-DopedCuGeO3

Abstract

We report muon spin relaxation measurements of the doped spin-Peierls system ͑Cu 12x Zn x ͒-͑Ge 12y Si y ͒O 3 . Spontaneous muon spin precession in zero applied field was observed, confirming the presence of antiferromagnetic order in this series of compounds. In contrast to usual antiferromagnets, muon spin precession is accompanied by a relaxation signal indicating a large spatial inhomogeneity of the ordered moment size. Assuming an exponential decay of the moment size away from the doping centers, we estimated a decay length of j ϳ 10 lattice units along the chain. We suggest that both Zn and Si doping induces the same maximum moment size around the doping center. [S0031-9007(97)03590-4] PACS numbers: 75.40.Cx, 75.50.Ee, 76.75. + i One-dimensional quantum spin systems have received considerable attention recently because of the quantum mechanical nature of their ground states. One particularly interesting phenomenon is the spin-Peierls transition: a structural transition coupled with singlet pairing of neighboring spins along an S 1͞2 antiferromagnetic spin chain. This type of transition was first observed in several organic compounds in the 1970's [1-3]. The discovery of a new spin-Peierls material CuGeO 3 [4], however, changed the course of experimental investigations of spin-Peierls phenomena, because this material allows doping at the spin site (Zn ! Cu [5]) and at the side chain (Si ! Ge [6]). From several previous measurements, antiferromagnetic order of Cu moments has been identified in both the Zn-doped systems [7][8][9][10][11][12] and the Si-doped systems [6,13,14].To understand the ordering mechanism, recent theoretical investigations of the Si-doped system suggested that perturbations to the lattice may be important [15]. A silicon ion has a significantly smaller ionic radius than germanium and may cause local strain which prevents the spin-Peierls dimerization. The authors of Ref.[15] solved the spin-lattice Hamiltonian and showed that staggered moments are induced around the doping center, which leads to antiferromagnetic order. In the framework of this theory, both the lattice order parameter and the spin order parameter (ordered moment size) have a large spatial inhomogeneity; the size of the ordered moments will be maximum around the doping center, and decay exponentially along the chain (see Fig. 1). For the Zn-doped system, the mechanism responsible for the antiferromagnetic order is not yet clear; still, it has been proposed that a semi-infinite S 1͞2 spin chain will have staggered moments near the chain end [16,17]. If the lattice dimerization is somehow suppressed by Zn doping, the induced staggered moments may lead to an antiferromagnetic order. In this case as well, a large spatial inhomogeneity of the ordered moment size is expected [16].Experimentally, spatial inhomogeneity of the moment size has been suggested from neutron diffraction measurements of the Zn-doped system [10]; as indirect evidence, the absence of low-temperature saturation of the antiferromagnetic Bragg peak intensit...

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

“…Fits using just an exponentially damped cosine oscillation, exp(−λt) cos(ω µ t + φ), provides a phase angle φ ∼ −42 o , which is physically meaningless, [26] although φ ∼ 0 o for the bottom spectrum. [15] We therefore conclude that Na 0.9 CoO 2 undergoes a magnetic transition from a paramagnetic state to an IC-SDW state (i.e.…”

Bond-randomness-induced Néel order in weakly coupled antiferromagnetic spin chains

“…4 -10͒ or Zn, Ni, or Mg for Cu, 4,5,8,11 leads to the appearance of AF long range order ͑LRO͒ that may coexist with spinPeierls order ͑or dimerization͒ below a Néel ordering temperature T N . Increasing the doping level can drive the spinPeierls transition temperature T sP to zero, while T N remains finite; eventually strong enough doping suppresses both spinPeierls and Néel ordering in the system.…”

Absence of static magnetic order in lightly-doped Ti1−xScxOCl down to 1.7 K

“…In particular, it is possible to dope the magnetic Cu sites in CuGeO 3 with nonmagnetic Zn 2+ [5,16,[18][19][20][21], Mg 2+ [20], or Cd 2+ [22,23], as well as magnetic Ni 2+ [16,20], Co 2+ [24], or Mn 2+ [16]. Systems with Si 4+ [16,18] doped in for Ge 4+ have also been created. These materials have generally revealed phase diagrams with some common features, including the loss of SP order at a critical doping concentration x c , a "dimerized antiferromagnetic ground state" for the lightly-doped compounds with x < x c , and a uniform antiferromagnetic ground state for systems with x > x c [25].…”