Nuclear quadrupole resonance in two sublattice 2-D antiferromagnetic cuprate Sr2Cu3O4Cl2
Introduction
It is established that the magnetic properties of low-dimensional cuprates are determined by the magnetic interactions in CuO2 planes with identical Cu-sites. There exist, however, special class of compounds called oxychlorides (Ba, Sr)2Cu3O4Cl2 with two types of Cu-site in Cu3O4 plane [1]. Two-thirds of the Cu-sites (Cu1) are surrounded by four in-plane oxygen atoms and two apical Cl atoms to form CuO4Cl2 octahedra, one-thirds of the sites (Cu2) are located in the center of the Cu-square (Fig. 1). This creates two interpenetrating square lattices of Cu ions. Neighboring Cu atoms interact magnetically by superexchange interactions through O between them, Cu1–O–Cu1 180° interaction is antiferromagnetic (AF) and Cu1–O–Cu2 90° one is ferromagnetic (FM) or AF, which would cause a magnetic frustration. Under this situation, one can expect magnetic properties different from those observed in the single Cu-site cuptares, and the magnetic ground state of this material is the point of great interest.
Magnetization measurements [1], [2] as well as neutron-diffraction study [3] indicate that the Cu1 sublattice orders antiferromagnetically (AF) at TN1=380 K. This transition is accompanied by the appearance of the in-plane FM moment due to spin canting within the Cu3O4 plane in Sr2Cu3O4Cl2 [2]. The Cu2 sublattice exhibits the AF transition at TN2=40 K. Since the magnetic unit cell is identical to the chemical one, the nuclear reflections superpose on the magnetic reflections in the neutron diffraction experiment. Consequently, it is not possible to determine the exact value and the direction of the Cu1 local magnetic moment from the neutron diffraction data due to the large thermal factor of the nuclear reflections [3]. Therefore it is of great interest to use another microscopic tool to obtain the information about the magnetic structure of both Cu sublattices in the ground state of Sr2Cu3O4Cl2 (T<TN2) as well as in the temperature range TN1<T<TN2 where only Cu1 sublattice is ordered. The Nuclear quadrupole resonance (NQR) method provides such a possibility. In this paper, we present for the first time the results of NQR experiments, performed on 63,65Cu and 35Cl nuclei in the powder sample Sr2Cu3O4Cl2 at various temperatures.
Section snippets
Experiment
Homogeneous powder sample Sr2Cu3O4Cl2 was synthesized from SrCO3 (99.9%), SrCl2 (99.9%) and CuO (99.9%) by using a conventional solid-state reaction.
NQR spectra were obtained with the standard coherent pulsed spectrometer using a point-by-point technique at 4.2 and 45 K for 63,65Cu spectra, and at 12 and 45 K for 35Cl NQR spectra.
Results and discussion
The NQR spectrum (zero external magnetic field) of Sr2Cu3O4Cl2 consists of two parts. The high-frequency part of NQR spectra of Sr2Cu3O4Cl2 at temperatures 4.2 and 45 K are shown in Fig. 2. The resonance lines are distributed over a wide frequency range from 90 upto 125 MHz. These lines are attributed to 63,65Cu Zeeman spectrum of the Cu1 nuclei in the local in-plane magnetic field of 93 kOe perturbed by quadrupole splitting in the first order (antiferromagnetic resonance (AFMR) spectrum).
Acknowledgements
We thank A.M. Abakumov and M.G. Rozova for providing us with the Sr2Cu3O4Cl4 sample and fruitful discussions. This work was supported by RFBR Grants 04-03-32876, 05-02-17719. E.M. thanks for support Grant MK-1212.2005.2.
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Magnetic structure of the Sr<inf>2</inf>Cu<inf>3</inf>O<inf>4</inf>Cl <inf>2</inf> two-subsystem antiferromagnet according to nuclear quadrupole resonance data
2007, Journal of Experimental and Theoretical Physics