On the NMR linewidth divergence in the planar antiferromagnet K2NiF4

Volume 48A, number 3

PHYSICS LETTERS

17 June 1974

ON THE NMR LINEWIDTH DIVERGENCE IN THE PLANAR ANTIFERROMAGNET K2NiF4 C. BUCCI and G. GUIDI Physics Department, University of Parma, Parma, Italy Gruppo Nazionale di Struttura della Materia of the C.N.R. Received 1 May 1974 9F NMR linewidth as T—’~TN in K It is shown that the critical exponent for the ‘ 2NiF4 is significantly larger than the value of 0.54 reported and adopted in the literature.

K2NiF4 is one of theNMR. first planar antiferromagnets 19F Marschall et al. [1] to be studied with demonstrated the relevance of the information yielded by the linewidth measurements of both axial (F1) and planar (F11) fluorine nuclei in such systems. In particular they found that the F1 linewidth diverges as T -* T~and the experimental points could 54. be described by a power law of the type (T— TNY° The purpose of this letter is to show that the value of such exponent cannot be associated solely to the critical magnetic fluctuations and therefore the use made in the recent literature is not correct. In a recent work on K2MnF4 [2] the temperature dependence of F1 and F11 was carefully measured and analyzed. It was found that the divergence of the F1 linewidth as T-÷T~had to be corrected for the temperature independent inhomogeneous broadening due to nuclear interactions (gaussian likeshape) before obtaining the correct temperature dependence of the Lorentzian linewidth due to the critical spin fluctuations. In this system the critical broadening of the F1 linewidth was then described by the exponent

Thesewith values well as the linewidth agree theasprevious data [1]. As measurements for K 2MnF4, the analysis of the F1 linewidth requires the deconvolution of the inhomogeneous gaussian width from the experimental width. Using the same procedure as in ref. [4], we obtain, for the F1, a gaussian contribution of 2.2 for H0 along the a-axis, one considers the Oe interaction between similarifnuclei. In K only 2NiF4, at _________________________________________

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The discrepancy between the reported values of the exponents for these two quite similar systems created a considerable confusion and made the experiment on K2NiF4 worth of reexamination. The measurements are performed in a field H0 = 3.3kOewiththeexperimentalconditionsasinref.[2]. At room temperature, the lineshifts are: F1 nuclei: (HIIc-axis) 29.5 Oe, (HIla-axis) 10.5 Oe F11

nuclei: (HIIc-axis) 24.0 Oe, (HlI a-axis) 23.5 Oe and 60.0 Oe.

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TEMPERATURE

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Fig. 1. 1 linewidth as a function of temperature in K2NiF4. (.) experimental results; (.) corrected for similar nuclei dipolar broadening (A) corrected for dissimilar nuclei. The broken line represents the slope 0.54. 233

Volume 48A, number 3

PHYSICS LETTERS

T

17 June 1974

~ TN, dissimilar nuclei can contribute to the gaussian width since the broadening due to the hyperfine interaction is smaller than in K 2MnF4 [3]. By including such contributions, the total nuclear-nuclear term of the linewidth becomes for F1 nuclei 2.5 Oe ([lila). The experimental results for F1 linewidth together with the corrected linewidth are reported in fig. 1, for temperatures T> 1.2 TN. The experimental data agree quite well in this range with Marschall’s results. The remarkable fact, however, is that the correction for the nuclear dipolar linewidth increases drastically the slope of the data. In fig. 1, squares are the data corrected solely for the similar nuclei broadening and triangles are those corrected also for the dissimilar nuclei broadening.

to TN, the measurements are subjected to large errors and are rather difficult to obtain due to the incipient

In view of such indication it seems important to obtain data closer to TN. For temperatures closer

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overlapping of the broadened F1 line with the narrow and intense F11 lines. Accurate data in this range are in progress. In particular, for H01 c-axis, an improvement should be noticed in the precision of the linewidth measurement by orienting H0 in the direction (1, 1,0). In such configuration the splitting between F1 and F11 resonances is maximum.

References [1~ E.P. Marschall, AC. Botterman, S. Vega and A.R. Miedema, Physica 41(1969)473. C. Bucci and G. Guidi, Phys. Rev. B, to be published. [31 RE. Walstadt, Phys. Rev. B5 (1972) 41.