Physica B 194-196 (1994) 2179-2180 North-Holland PHYSICA

Cu/TI-NMR Study on Tl-based Antiferromagnetic Copper Oxide T1Ba2YCu2Or (T11212)

Takayuki Goto, Kazuya Miyagawa, Satoru Nakajima, Masae Kikuchi, Yasuhiko Syono and Tetsuo Fukase

IMR Tohoku Univ, 2-1-1 Katahira Sendai 980 JAPAN

Zero-field spectra of the antiferromagnetic Cu-NMR have been observed on the Tl-based copper oxide TIBa2YCu207 (Tl1212). The internal field and the electric field gradient at Cu site have been found to be comparable to other antiferromagnetic phase of high-To oxides. Hyperfine coupling constants of both Cu and TI sites have been obtained through the analysis of T1 and Cu spectra.

1. I N T R O D U C T I O N Since the discovery of high-To copper oxide

superconductors (HTSC), vast experiments per- formed on them seem to support the idea that the physical properties of all the hole-doped HTSC are universally characterized by a single param- eter, the carrier concentration. Namely, with in- creasing the hole concentration, HTSC undergoes the three regions: the antiferromagnet, the super- conductor, and finally, the over-doped metal.

However, for Tl-based HTSC so far, only the over-doped specimens have been synthesized and studied mainly. In particular, for the compound of T1Ba2CaCu~OT_~, which we are presenting here, it is not self evident whether or not the synthesis of the antiferromagnetic(AF) phase is possible. Actually, the small amount substitu- tion of Y with Ca, which is expected to act as hole-filling, does not affect the hole concentration. This is because the oxygen defect 6 automatically compensates the change in the hole concentra- tion due to the substitution[i]. For the compound T1Ba2YCu207 with the complete substitution of Ca with Y, the disappearance of the superconduc- tivity had been confirmed[l]. However, there have been few investigations carried out on whether or not it shows the. antiferromagnetism as observed in La2CuO4 or YBa2Cu306[2]. So far, only a very weak magnetic scattering is reported by the spin- polarized neutron experiment on this system[3].

The test whether or not the antiferromagnetic phase exists in Tl-based systems must be of help for understanding all the HTSC in a unified view. Furthermore, by the study of the antiferromag- netic phase of Tl-system, we expect to obtain di- rectly the hyperfine coupling constant (HFCC) of Cu and T1 site. So far, it has been impossi- ble to obtain HFCC of Tl-systems, because the Knight shift of the over-doped HTSC is temper-

ature independent, so that one cannot draw out K-X plot. The hyperfine coupling constant, which relates NMR observables such as T 1-1 with the dy- namical susceptibility x(q, ~), is a very important parameter in studying the Cu 3d-spin dynamics which is believed to play an important role in many anomalous properties of HTSC.

In this article, we report the first time obser- vation of the antiferromagnetic Cu-NMR signal in AF phase of Tl-based copper oxide, T1Ba2YCuO7 (Tl1212). Through the analysis of T1 and Cu spectra, we obtained the parameters of the inter- nal field, the electric field gradient(EFG) and the hyperfine coupling constants of Cu and T1 sites.

2. E X P E R I M E N T A L

A polycrystalline sample of Tl1212 was syn- thesized by the conventional solid state reaction of T1203, Y203, BaO and CuO. The method in detail is described elsewhere[1 ]. The powder X- ray diffraction showed that the crystal structure is tetragonal and that the sample is of single phase.

Zero-field spectra of Cu-NMR were obtained at 4.2K by averaging the spin-echo signal between 80 and 120MHz with a frequency step 0.1MHz. The frequency dependence of the spin-echo am- plitude was compensated by w -2. Field-swept spectra of T1-NMR were measured at the several fixed frequencies between 60 and 120MHz in the temperature range 4.2~100K.

3. R E S U L T S a n d D I S C U S S I O N

Figure 1 shows the zero-field spectrum of Cu- NMR. One can clearly see the six resonance lines, some of which are overlapped. These six lines were assigned to the quadrupolar-split Zeeman transition signals from the two isotopes of 63Cu and 65Cu. By the numerical diagonalization of

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the nuclear spin hamiltonian for •=3/2, we ob- tained the parameters of H0 the internal field, uO the quadruple frequency, and 0 the angle between H0 and the principal axis of EFG:

H0=8.62T, 0=81.1 °, 63uO=20.44MHz, 65uq=18.91MHz.

In this calculation, we assumed that EFG has the axial symmetry, considering the tetragonal crys- tal structure. These extracted parameters are almost the same as those reported on other AF phase of high-T¢ copper oxides, such as La2CuO4 or YBa2Cu306[2].

| • | |

o ,~ zero-field _', :~ 4.2K

t t

• ^ - ^ "

I I I I I I I | I 80 90 100 110 120

Frequency (MHz)

Fig. 1 Zero-field frequency spectrum of 63/65Cu in T1Ba2YCu207 at 4.2K. The arrows indicate the peak positions of the spectrum.

If one adopts the value of the 3d-spin mag- netic moment, J~/3d "~0.52pB reported by the neu- tron experiment on this system[3], the hyperfine coupling constant of Cu site is calculated to be,

[Aab -- 4BI=Ho/~3d ~ 166kOe//~B , where Aab and B are the on-site and the trans- ferred hyperfine coupling constants. This value is also comparable to those for other antiferromag- netic phase of high-To oxides[4].

The spectra of 2°5T1 was observed at the po- sition for which the internal field is almost zero. This is interpreted as the geometrical cancellation of the magnetic field induced by Cu 3d-spins, for the two Cu atoms with anti-spins locate symmet- rically above and below TI site. The existence of the geometrical cancellation at T1 site firmly assures that AF order in this system is three di- mensional.

The line width of 2°5T1 spectra was 6HT1 ~950

Oe in the gaussian approximation of the line shapes. This extremely large value is due to the inhomogeneity in the magnetic field at Cu site 6//0. While the mean value of the internal field at Cu site completely cancels out at T1 site, the inho- mogeneity resides at TI site to contribute to the line width. The contribution of the anisotropic Knight shift to the width is negligibly small, be- cause 6HTI was independent of the applied mag- netic field between 3 and 6T.

At higher temperatures up to 100K, 6HTI showed almost 10 % decrease, which may reflect the decreasing internal field of Cu site.

If we assume that 5//0 is originated from 6/~3d , the inhomogeneity in the magnetic moment of 3d- spin, the hyperfine coupling constant of T1 site AT1 can be obtained from the relation,

SHTI=V~ ' 6~3d ATh where the factor of v ~ is due to the fact that 6HTI is contributed from the two 3d-spins. The value of 6#3d is est imated to be 0.0104~u B from the line width of the zero-field Cu spectra 61-1o ~_l.72kOe. These allow one to reach the result of ATI~ 65kOe/~u B , the large value of which suggests the existence of the supertransferred hyperfine inter- action between Cu and T1.

4. S U M M A R Y In summary, we observed the zero-field Cu-

NMR signal on the Tl-based copper oxide Tl1212, which clearly demonstrated that the compound is antiferromagnet. The hyperfine coupling con- stants of both Cu and T1, as well as the param- eters of the internal field and EFG at Cu site were successfully obtained by analyzing the Cu/T1 spec- tra.

This work was part ly supported by Grand-in- Aid from the Ministry of Education, Science and Culture, Japan.

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