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OPTICAL DETECTION OF 1D FERROMAGNETISM
IN CsNiF3
T. Tsuboi, R. Laiho
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
OPTICAL DETECTION OF
1DFERROMAGNETISM IN CsNiF3
T. ~ s u b o i ' and R. Laiho
Wihuri Physical Laboratory, University of Turku, 120500 Zbrku, Finland
Abstract. - Hot magnon sidebands associated with 3 ~ 2 g --+I Tzg(a) and 3 ~ 2 g -+ ' ~ ~ ~ ( b ) absorption bands of Ni2+ are investigated for 3D antiferromagnet CsNiF3 (TN= 2.61 K). Presence of 1D ferromagnetic spin coupling along hexagonal c-axis is confirmed from temperature dependence of these intensities.
In CsNiF3, the ferromagnetic spin coupling be- tween the ~ i ' + ions along oaxis is much stronger than the antiferromagnetic coupling along a and b axes of the hexagonal lattice. Thus CsNiF3 behaves as an easy plane one-dimensional (ID) ferromagnet with a ferro- magnetic ochain above 2.61 K. Below TN (= 2.61 K) , the weak interchain coupling gives rise to a 3D anti- ferromagnetic spin ordering [l, 21. The present work was undertaken to clarify the influence of 1D ferromag- netism on the optical absorption measurements.
Magnon sidebands are known to reflect directly the dimensionality of magnetic coupling: for example, hot magnon sideband (called hot band hereafter) shows a different temperature dependence among ID, 2D and 3D magnets [3]. A hot band in CsNiF3 has been ob- served a t 484.3 nm on a ~ i ' + absorption band due to spin forbidden 3 ~ 2 g + ' T Z ~ (a) transition. Previously
we showed that the 484.3 nm band (called A band) increases its intensity linearly with increasing temper- ature in the 2.6-8.0 K region but the detailed investi- gation has not been done above 8 K [4]. In addition to the A band, a series of weak temperature (T) sensitive hot bands, which are located close to each other, are observed in another spin forbidden 3 ~ 2 g -) 'TZ~ ( b ) ab- sorption band 15, 61. The spin forbidden band appears between two broad, strong and non-structured double- exciton bands as shown in figure 1: one is a band due t o 3 ~ 2 g +3 Azg + ' E ~
+'
Eg transition with a peak around 310 nm, while the other due to 3 ~+3 ~Azg g -) 'E,+'
Tz9 (a) transition with a broad peak around 260 nm.At least five weak, T-sensitive bands are observed (Fig. 1). Of them, the peaks of relatively strong bands are at 291.1, 194.4, 297.8 and 301.6 nm. Their T- dependence is quite similar to each other. In figure 2 is shown the T-dependence of the 297.8 nm band to- gether with that of the A band. The A band increases linearly with increasing T in the low T region but it slightly deviates from linearity from about 10 K. As the case of the A band, the 297.8 nm band grows rapidly in the low T region, approximately in proportion to
Fig. 1. - Absorption spectrum, measured with unpolari- zed light at 21 K, of CsNiF3 in 250-360 nm region. Inset shows temperature variation of absorption bands appearing in 285-310 nm region. Arrow shows a peak position of the
297.8 nm band.
T, but its growth becomes slow from about 30 K and tends to saturate in the high T region. Our data on the 297.8 nm band are consistent with a Petit's resuIt on the 294.4 nm band [6].
Ebara and Tanabe ([7], hereafter referred t o as ET) obtained the T-dependence of hot band in 1D ferro- magnets and antiferromagnets using a classical spin approximation for the calculation of spin correlation functions. We calculated a theoretical T-dependence of hot band in 1D ferromagnet for CsNiF3 using the ET method, which is shown in figures 2a and 2b, and compared it with the experiments. In the calculation we used S = 1 and J = 7.99 cm-' as the magnitude of spin and the ferromagnetic spin exchange constant, respectively.
P ~ e a e n t addi-ess: Physics Department, Kyoto Sangyo University, Kyoto 603, Japan.
C8 - 1448 JOURNAL DE PHYSIQUE
other and being superimposed on the strong double exciton bands.
The intensity of hot band has been observed to devi- ate from the T-linearity below
TN
because the appear- ance of 3D antiferromagnetic spin order 141. Taking into account this point, it is suggested that the agree- ment between calculated and experimental results for the hot band aboveTN
confirms the nature of the o p tical transitions and the validity of the ET calculation which is done for 1D spin system ,with ferromagnetic short range interaction, i.e. the presence of 1D ferro- magnetic spin coupling in CsNiF3.Fig. 2. - (a) Temperature (T) dependence of the A band area (open circles) and a theoretical T-dependence curve of hot band in 1D ferromagnet. Both are normalized at 20 K. (b) T-dependence of the 297.8 nm band area (open circles). Continuous line is a theoretical curve, while closed circles and broken line (drawn by eyes) data obtained by Petit [6]. All intensities are normalized at 40 K.
A good agreement between the experiment and the- oretical curve is obtained both for the A and high- energy bands in a wide T region as seen in figure 2. A slight disagreement, however, is observed below 30 K in figure 2b, probably because the exact intensities are difficult to estimate, the bands overlapping with each
[I] Steiner, M. and Kjems, J. K., J. Phys.
C
10(1979) 2665.
[2] Tsuboi, T. and Laiho, R., Phys.
Rev.
B 32 (1985) 1673.[3] Tsuboi, T., Phys. Lett. A 102; (1984) 138. [4] Laiho, R. and Tsuboi, T., J. Magn. Magn. Mater.
54-57 (1986) 1265.
[5] Petit, R. H., Ferre, J. and IVouet, J., Physica B 86-88 (1977) 1213.
[6] Petit, R. H., Thesis, Univ. Pierre et Marie Curie (1974).
