LINEAR CHAIN ANTIFERROMAGNET KCuF3 AND FERROMAGNET K2CuF4

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LINEAR CHAIN ANTIFERROMAGNET KCuF3 AND FERROMAGNET K2CuF4

Kinshiro Hirakawa, Isao Yamada, Yukihisa Kurogi

To cite this version:

Kinshiro Hirakawa, Isao Yamada, Yukihisa Kurogi. LINEAR CHAIN ANTIFERROMAGNET KCuF3 AND FERROMAGNET K2CuF4. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-890-C1-891.

�10.1051/jphyscol:19711315�. �jpa-00214347�

JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 890

LINEAR CHAIN ANTIFERROMAGNET KC&, AND FERROMAGNET K2CuF4 (*)

KINSHIRO HIRAKAWA, ISAO YAMADA (**) and YUKIHISA KUROGI (***) Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo, Japan

Rksumk. - La distorsion partir de la structure ferovskite observe dans KCuF3 est causee par un effet Jahn-Teller coopkratif. Ceci produit un arrangement particulier des fonctions d'onde de Cuz+, qui conduit un antiferromagnktique en chaine linkaire d'intkgrale d'echange est - 190 OK le long de la chahe, mais seulement environ le centieme entre les chaines. La R. M. N. indique l'existence d'ordre a longue distance et une contraction du spin d'environ 50 % a tres basse tempkrature K z C U F ~ est ferromagnktique avec TC = 9,17 OK.

Abstract. - The distortion from the perovskite structure observed in KCuF3 is caused by the cooperative Jahn- Teller effect. This makes a peculiar arrangement of the C U ~ + wave functions resulting in a linear chain antiferromagnet.

The exchange integral is - 190 OK along the chain but only

-

-

I. Introduction. - KCuF, has the perovskite struc- ture with slight tetragonal distortion [I]. Nevertheless, it is one of the typical linear chain Heisenberg-like antiferromagnets [2, 31. This one dimensionality occurs because the cooperative Jahn-Teller effect among Cu2+ ions produces a special alignment of flat wave functions of the dy-orbitals. The wave functions over- lap strongly along the c-axis through the intervening fluorine ions F,, resulting in a strong antiferroma- gnetic coupling along the c-axis, but they overlap scarcely along the a-axes giving only weak (ferroma- gnetic) coupling in these directions 141. It has become clear now that there are two polymorphisms [5], i. e.

type (a) and type (d) according to the different manner of alignment of dy orbitals as shown in figure 1. Both types are linear chain antiferromagnets, but the type (d) has a better one-dimensionality, so we shall confine ourselves to the type (d) only hereafter. In the previous study, the Niel point TN could not be observed because there was no anomaly in susceptibility as well as specific heat curves. The anomalous point was found a t first by Date et al. [6] in the relaxation in ESR and then in neutron diffraction by Hutchings et al. [7].

The latter authors, however, could not observe reci- procal plane, which is direct evidence for one- dimensionality, instead they observed unusually large spin contraction of more than 50 % even far below TN. This may be an indirect evidence of its low dimen- sionality. It should be noted that, although a broad maximum of susceptibility appears at T,,, = 243 OK, TN is only 20 OK. Comparing with the theoretical calculations [a], this corresponds to the superexchange along the chain of - 190 OK, whereas the interaction between chains are estimated to be

-

(*) This work was supported by the R. C. A. grantt.

(**) Department of Electronics, Kyushu University, Fukuoka, Japan.

(***) Nippon Electric Co., Ltd, Kawasaki, Japan.

In this paper, an experimental study (mostly NMR) t o know how the spin correlation develops and how the slowing down of a special mode of spin fluctuation develops when TN is approached from high temperature side are presented. In the last part, newly found ferromagnetism in K2CuF4 [lo] with the K2NiF4 structure is reported.

11. Experimental Results and Discussions. - As can be seen in figure 1, there are two different fluorine nuclei. One of which, named FE9, is located at the

(a) type (d t y p e )

FIG. 1. - TWO different alignments of the dy orbitals in KCuF3.

centre of symmetry of two adjacent Cu2+ ions with equal hyperfine coupling to both of them. Another one, named Fig, is located on the a-axis and couples only to one of the nearest neighbour spins via overlap of wave functions. The NMR shift which is proportional to the susceptibility of the chain is given by the circles in figure 2. The temperature dependence of the shift agrees very well with the curve calculated by Bonner and Fisher [8] for linear chain Heisenberg antiferro- magnet. The measured static susceptibility, however, goes up a t very low temperatures. The line width for FE9 decreases slightly when the temperature is reduced

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711315

LINEAR CHAIN ANTIFERROMAGNET KCuF3 AND FERROMAGNET K2CuF4 C 1 - 891

d type. From NMR shift o a type. "

FIG. 2. -Observed NMR shift of FE' (open circles), the theoretical curve for one-dimensional Heisenberg antiferro- magnet after Bonner and Fisher (full line) and the static suscep- tibility (dotted line) as a function of temperature. TN is shown

by the arrow.

beyond T,, untill TN is reached. There is no remar- kable change in width when the field direction is changed and the width is very narrow being -- 2.5 gauss, which is the order of nuclear-nuclear interaction.

So the effect of electron spins on the resonance width is almost completely eliminated. But the width for F," is quite different from that of F:'. At T z Tmax, the width for F i g depends weakly on the field direction.

As the temperature is reduced, the width for F,", when the field is applied parallel to the a-axis, increases prominently towards TN, but, when the field is applied parallel to the c-axis the width is independent of temperature untill the Nkel point is reached. So, near TN, the widths are extremely anisotropic as shown in figure 3. These changes in widths are readily explained. Suppose now that the system is very close to the critical point and the short range order within a chain is so developed that two adjacent Cu2+ spins are so tightly coupled antiparallel to each other though the spins can rotate freely and slowly around the c-axis. In this case can not see the slow motion of spin rotation any more because the hyperfine field is almost canceled out by symmetry. The local field

at F," can be produced only by twisting two Cu2+

spins to each other. But the frequency for such twisting is nearly 190 ktt-' -- 1013 Hz which is too high to give the broadening. This is the reason why the width for F:' is so narrow and independent of the field direction. Now we shall consider the width of F,".

When the field is applied along the c-axis, the fluc- tuation of field in this direction due to Cu2+ spin is very fast again as far as the out of plane anisotropy

Fa, H o 4 a

. P '/ ^a

"a*;;;.

^\

Tmax

OO 100 200 'K 300

FIG. 3. - Observed NMR line widths for F:' with different field directions.

is large. On the other hand, when the field is applied along the a-axis we can expect very large and slowly fluctuating field at F:' as appeared in our experiments.

So we think that, when T c Tmax, the SLO within a chain develops and at the same time, spin fluctuation is slowed down only in the c-plane and lead to the three-dimensional LRO. The motive force of this instability may possibly be the inter-chain dipole- dipole interaction which is only of the order of

-

but of sufficient magnitude to produce TN at 20 OK.

This instability is connected smoothly to the low temperature spin pattern. The nature of loosely coupled chains remains even at temperatures as low as 1.5 OK.

The T2 for F i g at these temperatures is of the order of 1

-

K2CuF4 has the same structure as K2NiF4, but it is ferromagnetic with Tc = 9.15 OK, 0 = 15 OK and the extrapolated spontaneous magnetization to OOK is 88 gauss/cm3 with g, = 2.30 and gc = 2.08, which correspond to S = 112 [lo]. No spin contraction as observed in KCuF, is observed. As this is nearly an Heisenberg ferromagnet with the easy plane, it mayLbe a good model for statistics.

References OKAZAKI (A.) and SUEMUNE (Y.), J. Phys. SOC. Japan,

1961, 16, 176.

SCAT~URIN (V.), CORLISS (L.), ELLIOTT (N.) and HASTINGS (J.), Acta Cryst., 1961, 14, 19.

KADOTA (S.), YAMADA (I.), YONEYAMA (S.) and HIRAKAWA (K.), J. Phys. Soc. Japan, 1967,23,751.

HIRAKAWA (K.) and KADOTA (S.), ibid., 1967, 23; 756.

OKAZAKI (A.) and TSUKUDA (N.), ibid., 1969, 27, 267, 518.

[6] IKEBE (M.) and DATE (M.), J. Phys. Soc. Japan (in press).

[7] HUTCHINGS (M. T.), SAMUELSEN (E. J.), SHIRANE (G.) and HIRAKAWA (K.), Phys. Rev., 1969, 188, 919.

[8] BONNER (J. C.) and FISHER ^{(M. E.),} Phys. Rev., -1964, 135, A 640.

[9] OGUCHI (T.), Phys. Rev. 1964, 113, A 1098.

[lo] YAMADA (I.), J. Phys. Soc. Japan, 1970, 28, 1585.