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Submitted on 1 Jan 1978
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EXCITATIONS IN THE ISING COMPOUND
RbFeCl3-2H2O
Q. van Vlimmeren, C. Swüste, W.J.M. de Jonge
To cite this version:
JOURNAL D E PHYSIQUE
Colloque
C6,
supplPment au
no8, Tome 39, aoiit 1978, page C6-743
EXCITATIONS
IN THE
ISING COMPOUND RbFeC13.2 H20
Q.A.G. van Vlimmeren, C.H.W. Swiiste and W.J.M. de Jonge
Department of Physics, Eindhoven U n i v e r s i t y
ofTechnoZogy, Eindhoven, The N e t h e r Z a d s .
RBsum6.- La rlsonance de cluster de spin (SCR) a dt8 observ6e dans l'antiferromagn6tique
pseudo unidimensionnel d'Ising RbFeC13.2H20. L'inclinaison des moments conduit
2des
rlsultats caract8ristiques aussi bien d'une chalne ferromagnltique que d'une chalne anti-
ferromagnctique.
Abstract.- Spin cluster Resonance (SCR) is observed in the pseudo one-dimensional canted
Ising antiferromagnet RbFeC13.2H20. Due to the canting of the moments the SCR experiments
show characteristics of both a ferromagnetic and an antiferromagnetic chain.
Recent reports on RbFeC
13.2H20 (RFC)
/
1 ,
E
=-
2Ja
+m {aJb
+BJc
+
yJbc>
2
2mC.b for %//c
2,3/ have indicated that this compound can be con-
(1)
sidered as a good realization of an Ising system.
E = -
2Ja
+m {aJb
+
BJc
+
yJbc}
+
26mc.'it for'it//a
The specific heat
/ I /can be descirbed both above
and below the ordering temperature (TN
11.96 K)
with Onsagers rectangular S
= 1Ising model with
( ~ / k l
=39 K and ( ~ ' / k (
=0.3
K. At T
=4.2 K
application of a megnetic field along the c axis
results in two metamagnetic phase transisitons at
8.1 and 12.4 kOe /1,3/, respectively. The increase
in the magnetization is equal at both phase tran-
sitions. The basic excitations in Ising-like sys-
tems can be treated as localized spin reverseals.
In pseudo one-dimensional systems, like W C , the
excitation energy of a spin cluster depends mainly
on the intrachain interaction. For RFC these exci-
tations will be located in the far infrared (FIR)
region. Indirect excitations starting from a ther-
mally excited spin cluster, spin cluster resonance
(SCR), are much lower in energy since only inter-
chain and Zeeman interactions areinvolved. Given
the magnitude of
J'and the rather pronounced
Ising-like properties it was anticipated that SGR
should be observable in the
GHzregion. As a con-
sequence of the canting of the moments in the ac
plane the experimental SCR results exhibit charac-
teristics of both a ferromagnetic (H//c) and an
antiferromagnetic chain (Hlla).
We assume three different interchain in-
( 2 )
with a
=2,O or -2, 6
=2,O or -2,
y =4,2,0,
-2 or -4, 6
= 0for even m and
6 = 1for
m
m
odd m. The actual values of a, 6 and
ydepend on
the orientation of the spins in the neighbouring
chains. The expressions for the SCR frequencies
are obtained by taking the difference of spin clus-
ter energies with different
m.
The SCR data for H//c (Figure
1)can be
fitted very well on basis of this pure Ising
mo-del (Equation I), with Jb/k
-0.76 K, Jc/k
=0.21 K, Jbc/k
-
-0.13 K,
;.2/l3l
=1 . 5 0 ~ ~ .
Reson-
nances which involve an increase of the cluster-
size up till 9 spins are observed.
The SCR spectrum for H//a (Figure 2) shows
a nonlinear field dependence of the frequencies,
which is not predicted by the pure Ising model
(Equation 2). A detailed study of the SCR frequen-
cies shows that this curvature may be explained by
1
the splitting of the lowest doublet
( S = T )by the
crystal field and spin-orbit coupling.
Acrystal
field study shows that the full splitting of the
spin quintet (S=2), may be described by the Harnil-
tonian
/ 7 / .K
=D(S~
z-
LS(S+I))
3
+E
(
S
:
-
):S(3)
teractions /2,3/ J in the b direction, Jc in the
b
when D<O an
(EI
I D /
a pseudo doublet is lowest
c direction, and J
diagonally in the bc plane.
bc
in energy with an energy splitting of
A
=12D(1-
When all the interactions are taken to be Ising,
JI
+
3(E/D)
)I
.
the excitation enargy of m neighbouring spins in
The data for H//a (Figure 2) can be explai-
a
chain can be expressed as
ned satisfactorily on basis of a S
=-
1model
2
+ - P
w i t h A = 0.76 K,
U.H/(%~
= 4 . 3 p and t h e sameB s e t o f i n t e r c h a i n exchange p a r a m e t e r s a s used f o r H//c. F i g . 1 : Observed r e s o n a n c e f r e q u e n c i e s p l o t t e d v e r s u s f i e l d H, f o r H a l o n g c . A l l t h e i n s e r t e d l i n e s a r e t h e o r e t i c a l p r e d i c t i o n s f o r t h e p u r e I s i n g model ( E q u a t i o n I ) . The l i n e s a r e l a b e l e d w i t h t h e number o f e x c i t e d s p i n s and t h e c o n f i - g u r a t i o n o f t h e c h a i n s i n t h e a n t i f e r r o m a g n e t i c , i n t e r m e d i a t e and f e r r o m a g n e t i c phase.The t h e o r e - t i c a l p r e d i c t i o n s o f SCR of o t h e r c o n f i g u r a t i o n s a r e r e p r e s e n t e d b y b r o k e n l i n e s . F i g . 2 : Observed r e s o n a n c e f r e q u e n c i e s p l o t t e d v e r s u s f i e l d H, f o r H a l o n g a . The c u r v e s a r e t h e t h e o r e t i c a l p r e d i c t i o n s f o r SCR when t h e e f f e c t of t h e pseudo d o u b l e t s p l i i t t i n g A i s t a k e n i n t o a c c o u n t . Open c i r c l e s r e p r e s e n t SCR d a t a b e l o n g i n g t o t h e a n t i f e r r o m a g n e t i c o r d e r i n g . Closed c i r c l e s r e p r e s e n t SCR i n o t h e r c o n f i g u r a t i o n s . o n l y t r a n s i s i t o n s between l a r g e c l u s t e r s a r e o b s e r v e d . The v a l u e of A c a n a l s o b e i n f e r r e d from t h e h i g h t e m p e r a t u r e s u s c e p t i b i l i t i e s . T h i s y i e l d D = -20
+
5 K and E-
2+
I K g i v i n g A = 0 . 6 K, which i s c o n s i s t e n t w i t h t h e v a l u e d e t e r m i n e d by SCR. P r e l i m i n a r y measurements i n t h e FIR r e g i o n show t h a t a l s o d i r e c t e x c i t a t i o n s o f s p i n c l u s t e r s c a n b e o b s e r v e d . These e x p e r i m e n t s y i e l d a n i n t r a - 1 c h a i n i n t e r a c t i o n J a / k = 35 K (S =?),
which i s i n r e a s o n a b l e agreement w i t h J J , / I C ~ = 39 K d e t e r m i n d from s p e c i f i c h e a t measurements. More d e t a i l e d e x p e r i m e n t s i n t h e FIR r e g i o n a r e i n p r o g r e s s .T h i s i n v e s t i g a t i o n i s p a r t o f t h e r e s e a r c h program o f t h e " S t i c h t i n g v o o r Fundamenteel Onder- zoek d e r M a t e r i e @OM)".
Re£ e r e n c e s
/ I / Kopinga, K . , Van Vlimmeren, Q . A . G . , B o n g a a r t s , A.L.M., and De J o n g e , W.J.M., P h y s i c a 86-88 B+C (1977) 803.
/ 2 / Van V l i m e r e n , Q . A . G . , De J o n g e , W.J.M., and S c h u i l w e r v e , M.S.J., 1 3 t h IUPAP Conf. S t a t . Phys. H a i f a (1977).
/ 3 / B a s t e n J . A . J . , Van Vlimmeren, Q.A.G. and D e
J o n g e , W.J.M., t o b e p u b l i s h e d .
/ 4 / T o r r a n c e , J.B. and Tinkham, M., Phys. Rev.= (1969) 587-595. N i c o l i , D.F. and Tinkham, M. Phys. Rev. B
2
(1974) 3126. T o r r a n c e , J.B. and Hay, K.A., Phys. Rev. L e t t .2
0 9 7 3 ) 163.151'
Date, M., and Motokawa, M., H. Phys. Soc. J a p n24 (1968) 41.
-
1 6 1 Thornton, D.D., Phys. Rev. B 1 (1970) 3193. / 7 / Schriempf, J.T. and F r i e d b e r g , S . A . Phys. Rev.
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