Magnetic properties of a novel quasi-2D Cu(II)-trimer system

Abstract

We present structural and magnetic data of a new Cu(2+)(S = 1/2)-containing magnetic trimer system 2b·3CuCl(2)·2H(2)O (b = betaine, C(5)H(11)NO(2)). The trimers form a quasi-2D quantum spin system with an unusual intra-layer exchange coupling topology, which, in principle, supports diagonal four-spin exchange. To describe the magnetic properties, a 2D effective interacting-trimer model has been developed including an intra-trimer coupling J and two inter-trimer couplings J(a) and J(b). The low-energy description and effective parameters are obtained from numerical calculations based on four coupled trimers (with periodic boundary conditions). Fits to the experimental data using this model yield the magnetic coupling constants J/k(B) = -15 K and J(a)/k(B) = J(b)/k(B) = -4 K. These parameters describe the susceptibility and magnetization data very well over the whole temperature and field range investigated. Moreover, the model calculations indicate that, for certain ranges of the ratio J(b)/J(a), which might be accessible by either chemical substitution and/or hydrostatic pressure, the low-energy properties of 2b·3CuCl(2)·2H(2)O will be dominated by non-trivial four-spin exchange processes.

Magnetic Excitation in Interacting Spin Trimer Systems Investigated by Extended Spin-Wave Theory

“…18) From the crystal structure, it was discussed that (C 5 H 11 -NO 2 ) 2Á 3CuCl 2Á 2H 2 O can be regarded as a quasi twodimensional spin trimer system, where the spin trimer is formed by three Cu 2þ ions with S ¼ 1=2 spin with strong intratrimer exchange interactions. 19) It was also reported that there was no signature of a long-ranger magnetic order down to 2 K. 19) However, the recent detailed measurements of the specific head at low temperatures by Sanda et al revealed that (C 5 H 11 NO 2 ) 2Á 3CuCl 2Á 2H 2 O exhibits a long-range ordering at T N ¼ 1:38 K. 20) It indicates that this compound should be treated as a three-dimensional system with weak interlayer interactions.…”

“…(3.6)] as a y i0 a i0 ¼ 1 À 18,19) (a) Spin trimer configuration on the c ¼ 0 layer. The square represents the unit cell.…”

“…We used the set of exchange interaction parameters listed in Table II, with which the magnetization curve reproduces the observed result by Sanda et al 20) These values in Table II are consistent with those reported in ref. 19 obtained by analyzing thermodynamical quantities on the basis of a finite size calculation. The center spin aligns in the opposite direction of that of the left and right side spins at zero field.…”

“…In the simplest case, the trimer is formed by S ¼ 1=2 spins. [13][14][15][16][17][18][19][20] In addition, trimer systems consisting of S ¼ 1 21,22) and S ¼ 5=2 [23][24][25][26] spins are also reported. In these compounds, spins in a trimer are aligned in a straight line and there is no frustration inside the trimer.…”

“…Since experimental progress is made, we focus on (C 5 H 11 NO 2 ) 2Á 3CuCl 2Á 2H 2 O as a typical trimer compound. [18][19][20] Let us briefly summarize the fundamental properties of the interacting trimer system. Hamiltonian of an isolated spin trimer is given by…”