High-field ESR studies of the quantum spin magnet CaCu₂O₃

Abstract

We report an electron spin resonance (ESR) study of the s = 1/2-Heisenberg pseudo-ladder magnet CaCu 2 O 3 in pulsed magnetic fields up to 40 T. At sub-Terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few percent of "extra" spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic ordering at T N = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below T N of the order of ∼ 0.3 − 0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu 2 O 3 at weak magnetic fields µ 0 H sf ∼ 3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce the antiferromagnetic correlations and may even suppress the long-range magnetic order in CaCu 2 O 3 . ESR data support scenarios with a significant role of the "extra" spin states for the properties of lowdimensional quantum magnets.

Li nonstoichiometry and crystal growth of an untwinned one-dimensional quantum spin systemLixCu2O2

“…Such strong impact of impurity spins to the host ordered low dimensional system has been observed in quasi-1d AF CaCu 2 O 3 also. 9 In particular, similar incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering has been observed in the multiferroic DyMnO 3 .…”

The low-dimensional spin magnet CaCu2O3 probed by high-field ESR

“…perpendicular to the chains in the ladder ab plane, with spins rotating in the ac plane [2]. Our recent high-field electron spin resonance (ESR) results reveal an important role of socalled 'extra' Cu-spins for the low temperature magnetism of CaCu 2 O 3 [3]. In our model these 'extra' states arise presumably due to the occurrence of an appreciable amount of excess Cu which occupies interstitial positions in the structure [4].…”

“…The actual composition of the samples is Ca 0 not contribute to the magnetic susceptibility w ¼ M=H at To300 K. The Curie-like behavior of wðTÞ of the Zn-free sample ( Fig. 1) is determined by a few percent of interstitial quasi-free Cu-spins [3]. The fit to the wðTÞ data in Fig.…”

“…1) gives evidence that the 'extra' spins are involved in the bulk magnetic ordering. These spin states are also dominant in the ESR response [3]. The onset of the AF order yields a critical behavior of the linewidth DH and the shift of the resonance field H res around T N (Fig.…”

“…A two-leg antiferromagnetic (AF) s ¼ 1 2 ladder realized in SrCu 2 O 3 exhibits a nonmagnetic ground state and a strongly gapped spin excitation spectrum [1]. However, the structurally similar CaCu 2 O 3 compound having a strongly reduced Cu-O-Cu bond angle in the rungs as compared to an angle of 180 in the Sr-counterpart AF orders at T N ¼ 25 K [2] and only a small anisotropy (magnon) gap of the order of 0.3-0.8 meV has been found at ToT N [3]. Remarkably, possibly due to the anisotropic magnetic interactions the magnetic structure is incommensurate in the highly frustrated a direction, i.e.…”