Научная статья на тему 'NMR study of quasi-1D magnetic chain in cuprates LiCu2O2 and NaCu2O2'

NMR study of quasi-1D magnetic chain in cuprates LiCu2O2 and NaCu2O2 Текст научной статьи по специальности «Физика»

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NUCLEAR MAGNETIC RESONANCE / QUADRUPOLE RESONANCE / INCOMMENSURATE SPIN

Аннотация научной статьи по физике, автор научной работы — Gippius A. A., Morozova E. N., Okhotnikov K. S., Moskvin A. S.

NMR investigation of magnetic structure and phase transitions in two isostructural quasi-one-dimensional cuprates LiCu2O2 and NaCu2O2 has been performed. While LiCu2O2 exhibits a magnetic phase transition at Tc = 24 K, NaCu2O2 orders magnetically at around 13 K. 6,7Li and 23Na NMR spectra in LiCu2O2 and NaCu2O2, respectively, provide an unambiguous experimental evidence that below Tc an incommensurate in-chain helical spin structure is established in both compounds. However, the features of the observed low temperature NMR are different pointing to different properties of the helical magnetic structure.

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Текст научной работы на тему «NMR study of quasi-1D magnetic chain in cuprates LiCu2O2 and NaCu2O2»

NMR study of quasi-ID magnetic chain in cuprates LiCu2O2 and NaCu2O2

A.A. Gippius1'2, E.N. Morozova1’2, K.S. Okhotnikov1*, A.S. Moskvin3

1 Moscow State University, Faculty of Physics, Moscow 119992, Russia 2Institute of Crystallography RAS, Moscow 117333, Russia 3 Ural State University, Ekaterinburg 620083, Russia * E-mail: okhotkir@mail.ru

Received November 18, 2006 Revised December 7, 2006 Accepted December 8, 2006

Volume 8, No. 1, pages 28-32, 2006

http://mrsej.ksu.ru

NMR study of quasi-ID magnetic chain in cuprates LiCu2O2 and NaCu2O2

A.A. Gippius1,2, E.N. Morozova1,2, K.S. Okhotnikov1*, A.S. Moskvin3

1 Moscow State University, Faculty of Physics, Moscow 119992, Russia 2Institute of Crystallography RAS, Moscow 117333, Russia 3 Ural State University, Ekaterinburg 620083, Russia * E-mail: okhotkir@mail.ru

NMR investigation of magnetic structure and phase transitions in two isostructural quasi-one-dimensional cuprates LiCu2O2 and NaCu2O2 has been performed. While LiCu2O2 exhibits a magnetic phase transition at Tc = 24 K, NaCu2O2 orders magnetically at around 13 K. 6,7Li and 23Na NMR spectra in LiCu2O2 and NaCu2O2, respectively, provide an unambiguous experimental evidence that below Tc an incommensurate in-chain helical spin structure is established in both compounds. However, the features of the observed low temperature NMR are different pointing to different properties of the helical magnetic structure.

PACS: 76.60; 75.90

Keywords: Nuclear magnetic resonance, quadrupole resonance, incommensurate spin

1. Introduction

The quasi-ID spin chain cuprate LiCu2O2 exhibits a unique sequence of phase transitions at T = 24, 22.5, and 9 K [1,2] resembling the “Devil’s staircase” type behavior. Recently we have obtained the first NMR evidence for a low temperature incommensurate (IC) in-chain spin structure in LiCu2O2 [3]. It was shown that below the magnetic ordering temperature Tc = 24 K the 7Li NMR lineshape is determined by a IC static modulation of the local magnetic field caused by spin structure of Cu magnetic moments twisted along the chain axis [3]. This result was confirmed by NMR spectra measurements on the 6Li isotope as well as by neutron diffraction study [4].

The larger ionic radius of Na1+ (0.97 A against 0.68 A of Li1+) favors the higher degree of in-chain crystallographic order and hence increasing one-dimensionality of magnetic properties in NaCu2O2. This results in lower magnetic ordering temperature Tc = 12.6 K [5] and lower values of local magnetic fields in the ordered state. In this paper we report first 23Na NMR measurements which confirm the existence of incommensurate magnetic structure in NaCu2O2 at 3 K also seen by |aSR [5]. The unusual magnetic properties of 1D chain cuprates LiCu2O2 and NaCu2O2 are discussed in terms of strong in-chain frustration and intrinsic incommensurability.

2. Experiment

Single crystals of LiCu2O2 and NaCu2O2 were synthesized according to the procedure described in Ref. [4]. The quality of the new NaCu2O2 crystal was much better than that used in our preliminary NMR measurements reported in Ref. [3]. In contrast to LiCu2O2, the NaCu2O2 single crystal shows no twinning and has no deviation from the ideal stoichiometry as confirmed by X-ray. 23Na and 7Li NMR measurements were performed at several temperatures in the paramagnetic and in the ordered phases of both compounds. All three principal orientations of the external magnetic field with respect to the crystallographic axes: H || a, b, and c were used. The standard pulsed field-sweep NMR technique was applied at fixed frequency of 33.7 MHz for LiCu2O2 and 46.0 MHz for NaCu2O2.

3. Results and discussion

We present the characteristic NMR spectra of 7Li in LiCu2O2 and 23Na in NaCu2O2 measured both above and below magnetic ordering temperature Tc for orientation of the external magnetic field H II c (Fig.1). To make the comparison

more convenient we use the same scaling of magnetic field in both panels. Above Tc in LiCu2O2 and NaCu2O2 typical

first-order quadrupole perturbed NMR spectra for spin I = 3/2 nuclei are observed. The quadrupole splitting between the satellites in NaCu2O2 is about 200 mT, while in LiCu2O2 it is about 10 mT. Since the ratio of quadrupole moments

0(23Na)/0(7Li) = 2.7, this large quadrupole splitting ratio means that the electric field gradient (EFG) at Na site in

NaCu2O2 is almost an order of magnitude higher than the EFG at Li sites in LiCu2O2. This result reflects an enhanced role of EFG polarization effect on Na1+ as compared with Li1+, which has only weakly polarizable 1s2 shell.

Below magnetic ordering temperature Tc the spectra of both compounds exhibit a dramatic change. The spectra are characteristic of IC static modulation of the local magnetic field caused by helical spin structure of Cu moments [3,6,7]. It is worth to mention, that the phase transition at Tc is much narrower in NaCu2O2 than in LiCu2O2. The formation of IC field modulation in NaCu2O2 occurs within 0.6 K while in LiCu2O2 it takes more than 2 K [3]. The asymmetric van Hove singularities of 23Na NMR central transition line are very sharp and are clearly visible also on satellite transitions, which in contrast to LiCu2O2 are well separated due to the larger EFG. The lineshape of the satellite transitions follows

the distribution of the Larmor frequency caused by IC local field modulation. Therefore, in the first-order quadrupole

perturbation the satellite lineshape is almost

the exact copy of the central transition profile.

The most striking difference between 7Li and 23Na spectra is that in NaCu2O2 the doublets (or degenerated quartets) are observed for all three principal orientations of external magnetic field in the magnetically ordered state. For H II a the intensities of both parts of the doublets are equal while for H II c the high field component of the doublet is more intensive. This anisotropy becomes more significant with decreasing temperature. The splitting is almost symmetric with respect to the central field determined as the resonance field of the central transition above Tc. These results are completely unlike the situation in LiCu2O2 where the 7Li NMR quartet and sextet Fig.1. 7Li and 23Na NMR spectra measured above Tc (left spectra) and are observed for H 11 c (Fig.1) and H II

(a,b) [3]. The possible reason for such dissimilar behavior could be the influence of

1,9

2,0

2,1

2,2

1,9

2,0

2,1

2,2

Magnetic Field, T

7Li and 23Na NMR spectra measured above Tc (left spectra) and below Tc (right spectra) for H II c in LiCu2O2 and NaCu2O2 single crystals at 33.7 and 46.0 MHz, respectively.

Fig.2. Crystal structure of LiC^Ü2 (left panel) and NaCu2Û2 (right panel).

non-magnetic Li defects in CuO2 chains of LiCu2O2. Due to AF character of NNN interaction the both helix phase angles 6 and p exhibit a step-like change on n in the vicinity of Li defect. This phenomenon will be analyzed in more detail elsewhere.

The value of the local magnetic field on Na site estimated as the linewidth at the base of the central transition for H II c is only 80 mT, which is a factor of 3 less than that for Li (250 mT). This is quite reasonable since the transition temperature in NaCu2O2 is lower than in LiCu2O2 pointing to a weaker inter-chain interaction in NaCu2O2.

The difference in crystal structure peculiarities of the two compounds (Fig. 2) is reflected in their NQR properties. Fig.3 shows the NQR spectra of NaCu2O2 and LiCu2O2 measured above Tc on 63 65Cu nuclei in Cu(2) site. The right and left lines of each

pair are assigned to the 63Cu and the 65Cu isotope,

Fig.3.

NQR spectra of NaCu2O2 (black open circles) and LiCu2O2 (red filled squares) measured above Tc on 63,65Cu nuclei in Cu(2) site.

F (MHz)

respectively. The observed frequency and intensity ratios correspond to ratios of isotope quadrupole moments and natural abundances, respectively. In the following, we will consider only the 63Cu isotope NQR lines for convenience. From Fig. 3 it is clearly seen that in NaCu2O2 the NQR linewidth is a factor of 3 smaller than that in LiCu2O2. This result reflects more homogeneous EFG distribution as a consequence of higher degree of structural order in NaCu2O2. Probably for the same reason, we succeeded to find another 63Cu line at 26.8 MHz originating from the Cu(1) position. It should be noted that the 63Cu line at around 27 MHz has been observed earlier on the polycrystalline LiCu2O2 sample in [8], but it was falsely assigned to the Cu(1) position. In this case, the line should exist also below Tc which contradicts to our experimental findings described below.

All lines shown in Fig. 3 completely disappear below Tc. Instead, in LiCu2O2 we observed very complicated 63,65Cu antiferromagnetic resonance (AFMR) spectrum at 4.2 K. This effect is caused by the space modulated internal magnetic field at the Cu(2) site in the ordered state of LiCu2O2. For yet unknown reason we did not find any copper AFMR spectrum in NaCu2O2. At the same time, the Cu(1) NQR line at 26.8 MHz exists even below Tc, as expected for non-magnetic Cu+ ion at the Cu(1) site which is symmetric with respect to magnetic Cu2+ ions in the CuO2 chains. Therefore, the complete cancellation of local magnetic field occurs at Cu(1) site in the ordered state of NaCu2O2.

In conclusion, 7Li and 23Na NMR spectra measured in the magnetically ordered state of the isostructural quasi-1D oxides LiCu2O2 and NaCu2O2 give unambiguously evidence for static IC modulation of local magnetic fields at the Li and the Na site, respectively. This modulation is caused by a helical spin structure of Cu moments below Tc. Due to the crystal structure peculiarities the character of the magnetic helix is dissimilar in both compounds reflected both in NMR and NQR spectra.

Acknowledgement

We appreciate support by the Grants RFBR 04-03-32876, MK-1212.2005.2.

References

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