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Submitted on 1 Jan 1988
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TWO-DIMENSIONAL MAGNETIC ORDER IN
HEXAGONAL LuFe2O4
J. Iida, Y. Nakagawa, S. Funahashi, S. Takekawa, N. Kimizuka
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
TWO-DIMENSIONAL MAGNETIC ORDER IN HEXAGONAL LuFe204
J. Iida (')
',
Y. Nakagawa (I), S. Funahashi (2), S. Takekawa (3) and N. Kimizuka (3) (') Institute for Materials research, Tohoku University, Sendai 980, Japan(2) Japan Atomic Energy Research Institute, Tokai 919-11, Japan
(3) National Institute for Researches in Inorganic Materials, Tsukuba 305, Japan
Abstract. - Neutron diffraction measurements have been performed on a LuFe204 single crystal with a hexagonal layered structure. At low temperatures a 2:l ferrimagntic spin arrangement of Fe ions is realized in each layer consisting of two triangular nets. The magnetic interaction between the different layers is so weak that the magnetic order is quasi-two-dimensional.
1. Introduction
LuFe204 belongs to hexagonal layered compounds RFe204 (R = Y, Ho, Er, Tm, Yb and Lu). As is shown in figure 1, the Fe202.5 layer (W layer) and the LuO1.5 layer (U layer) are stacked alternately along the c-axis [I]. Equal numbers of ~ e and ~ ~+ e ions are ~ + randomly distributed in a crystallographically equiva- lent site. Stoichiometric YFe204 shows a phase tran- sition of Verwey type at about 200 K; the magnetic susceptibility is small at lower temperatures [2]. The Verwey transition is not observed in an oxygen de- - -
ficient- specimen of YFe204-, (x= 0.05) which shows a large remanent magnetization induced by the field cooling along the *axis below about 200 K. It is re- vealed by neutron diffraction at 4.2 K that the stoichio- metric YFe204 is usual antiferromagnet while the spin structure of YFe204-, (x = 0.05) seems to be two- dimensional: magnetic Bragg lines such as (1/3, 1/3,
I) extend along the reciprocal c* axis 131. The remanent magnetization is thought t o be due t o the unbalanced moment in the triangular antiferromagnetic structure in the c-plane.
It has been known that the properties of LuFe204 are insensitive to the oxygen deficiency. Polycrys- talline specimens of LuFezO4-, with x = 0.00 0.065 do not show the Verwey transition, and the magne- tizations are almost independent of x [4]. Recently we succeeded to prepare a LuFe204 singlecrystal, the magnetization of which is easily saturated in high fields [5]. In the present work the magnetic structure was studied bv neutron diffraction measurements on this
sure using a C02-CO mixture. The oxygen deficiency
x has not been determined precesely since the struc- ture is thought t o be independent of x. The sample for neutron diffraction measurements is larger than that used for magnetization measurements, the size being
2 mm x 2 mm x 8 mm. The neutron experiment was
carried out at the JRR-2 reactor of JAERI, Tokai; a triple axis spectrometer was used for the elastic scat- tering of the neutrons with a wavelength of 0.2351 nm.
3. Results and discussion
The saturation magnetization of LuFezO4 is nearly equal to the thermoremanent magnetization along the c-axis obtained by the field cooling from 250 K to 4.2 K
in 140 kOe, being about 2.8 p~ per formula unit [5]. On the basis of the molecular field approximation, the Ising spins with antiferromagnetic nearest neighbor in- teractions in the triangular lattice are ordered in the 2:l ferrimagnetic arrangement when there exist small
single crystal.
1 : m g ~
Lu
2. Experimental
Fig. 1. - Crystal structure of LuFe204. Solid circles re The LuFezO4 single crystal was grown by the float- resent ~ e ~ + or ~ e ~ + ions and open circles represent Lu
&
ing zone method under controlled oxygen partial pres- ions. Oxygen ions are omitted in the figure.- -
' ~ r e s e n t address: Ochanornizu University, Bunkyo 112, Japan.
C8 - 1498 JOURNAL DE PHYSIQUE
ferromagnetic interactions between next nearest neigh- bors [6].
Since equal numbers of ~ eand F'e3+ ions coexist ~ + in LuFez04, the expected average moment is 4.5 p~
per Fe ion. When these moments are ordered in the 2:l ferrimagnetic arrangement, a saturation moment of 3 p~ per formula unit should be induced, which is close to the experimental value [5]. We ignore the possible existence of charge ordering of Fez+ and Fe3+ since the Verwey transition is not observed in LuFezO4.
The neutron diffraction measurements reveal the magnetic peaks (113, 113, 0) and (0, 1, 0) in the re- ciprocal a*
-
b* plane; the width of (113, 113, 0) is not larger than the instrumental resolution observed on the nuclear peaks, while that of (0, 1, 0) is about twice larger. It is also revealed that these peaks ex- tend along the reciprocal c*-axis. The line profiles of (113, 1/3, I) and (0, 1, I) at 14 K are shown in figure 2.No sharp magnetic peaks are observed in both profiles, suggesting a lack of three-dimensional magnetic order. The nuclear Bragg peaks (0,1, 3m
-
1) appear in the (0, 1, I) scan, where rn is an integer; the peak intensity is about 100 times larger than the intensity of mag- netic scattering, and the width Al is only about 0.02. The gradual decrease in intensity of (1/3, l j 3 , I) with increasing I corresponds to the decrease in magneticform factor. The existence of broad maximum in (0, 1, I) is explained as follows.
It is assumed that the 2:l ferrimagnetic arrangement is realized in each triangular net. Two parameters s
and q are introduced, which represent the strength of magnetic correlation between two triangular nets in a
W layer and between different W layers, respectively. If the two nets have the 2:l (or 1:2) arrangement, then
s = +l; if one net is 2:l and the other net is 1:2, then s = -1. An intermediate value of s represents the
probability of occurrence of the two cases. The situ- ation is the same about q. The calculation is similar t o that for the X-ray diffraction intensity of a layered structure with a stacking disorder. The calculated in- tensity of (0, 1, I) magnetic scattering strongly depends on s and q. The best fit to the experiment is obtained for s = 0.52 and q = -0.11, as shown by the full line in figure 2. It turns out that there is ferromagnetic correlation between two triangular nets in a W layer and there is very weak antiferromagnetic correlation between different W layers.
In usual quasi-two-dimensional magnet, the corre- lation length becomes infinite at the transition point, resulting in a three-dimensional magnetic order. Our
Fig. 2.
-
Neutron diffraction intensitia of (113, 113, I) and (0,1, I) at 14 K. Arrows represent positi.ons of nuclear Bragg peaks (0, 1, 3 m-
1) which exceed106
counts/3 min. Full lines are calculated curves for the magnetic scattering.observations indicate that, down to the temperature of 14 K, long-range three-dimensional order does not ex- ist in LuFezO4. The correlation I e ~ ~ g t h in the c-plane should be also finite. More detailed analysis will be
made after more detailed experiments of the neutron diffraiction including inelastic scattering.
Acknowledgments
The authors would like to express their thanks to Professor M. Tanaka of 0chanomiz:u University, Pro- fessor K. Siratori of Osaka University and Professor J. Akimitsu of Aoyama Gakuin Univei:sity for their help- ful discussion. Thanks are also due to Mr. K. Nemoto for his technical assistance in neutron diffraction mea- surements.
[I] Kato, K., Iwada, I., Kimizuka., N. and Katsura, T., 2. Kristallogr. 1 4 (1975) 314.
[2] Nakagawa, Y., Inazumi, M. a n c l Kimizuka, N. and Siratori, K., J. Phys. Soc. Jpn 47 (1979) 1369.
[3] Funahashi, S., Akimitsu, J., Siratori, K.,
Kimizuka, N., Tanaka, M. and Fujishita, H., J.
Phys. Soc. J p n 53 (1984) 2688.
[4] Nakagawa, Y., Kishi, M., Hiroyoshi, H., Kimizuka, N. and Siratori, K., Proc. 3rd Int. Conf. on Ferrites (CAPJ) 1981, p. 115.
[5] Iida, J., Nakagawa, Y., Takekawa, S. and Kimizuka, N., J. Phys. Soc. Jpn 56 (1987) 3746.
