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Magnetic Properties of Coexistent System of Itinerant and Localized Electrons, (BEDT-TTF)2MCo(SCN)4
(M = K, Rb, Cs)
Hatsumi Mori, Shoji Tanaka, Takehiko Mori
To cite this version:
Hatsumi Mori, Shoji Tanaka, Takehiko Mori. Magnetic Properties of Coexistent System of Itinerant and Localized Electrons, (BEDT-TTF)2MCo(SCN)4 (M = K, Rb, Cs). Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1987-1996. �10.1051/jp1:1996194�. �jpa-00247293�
1Phys. I France 6 j1996) 1987-1996 DECEMBER1996, PAGE 1987
Magnetic Properties of Coexistent System of Itinerant and Localized Electrons, (BEDT-TTF)2MCo(SCN)4
(M = K, Rb, Cs)
Hatsumi Mon (~,*), Shoji Tanaka (~) and Takehiko Mori (2)
(~) International Superconductivity Technology Center, Shinonome. Tokyo 135, Japan
(~) Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152, Japan
(Received 30 April 1996, revised 8 July1996, accepted 19 August 1996)
PACS.71.20.Rv Polymers and organic compounds
PACS.11.30.+h Metal-insulator transitions and other electronic tran~itions PACS.71.70.Ch Crystal and Iigand fields
Abstract. A coexistent system of itinerant and Iocalized electrons was prepared and the
magnetic properties of bath electron~ were investigated. a-(BEDT-TTF)2CSCOjSCN)4 (abbre-
viated
as the Csco Salt), a-(BEDT-TTF)2RbCOjSCN)4 (the Rbco Salt), and a"-(BEDT-TTF)2
Ki 4Co(SCN)4 (the KCO Salt), undergo metal-insulator transitions at 10 K, 190 K, and 130 K.
respectively. The ESR measurement of the Rbco sait indicates that the electronic state below transition temperature, 190 K, is magnetic insulator due to the strong electron correlat,ion. The static magnetic susceptibility of the Csco Salt measured by SQUID is mostly originated from the Iocalized electron~, Co~+ (S = 3/2), and obeys the Curie-Weiss Iaw with 6 = -3 K above 20 K, wheie trie weak interaction between Co~+ might be caused by the superexchange through coordinated SCN~ amen. Below 10 K, the rapid decrease of XT is observed because of the
crystal field around Co~+
1. Introduction
The coexistent system of localized and itinerant electrons has aiforded us interesting topics.
In 1951, Zener di~cussed trie ferromagnetism of Fe in terms of trie interaction of unfilled d shell electrons of Fe through 4s conducting electrons [lj. His idea has developed by Ruderman, Kittel, Kasuya, and Yoshida. been called RKKY interaction [2j, and is discussed in rare earth metals, dilute alloys, etc. The negative s-d exchange interaction m a dilute alloy gives rise to a
resistance minimum with log T dependence below trie minimum temperature, so called Kondo temperature [3j. Not only s-d but ~-d interaction lias been eagerly investigated in morganic
and organic conductors. For trie intercalated graphite with a localized moment as C6Eu, Eu~+ spin which constructs a triangle lattice lias a RKKY-type interaction through ~-itinerant
electron m graphite and gives the curious magnetization curve by applying magnetic field [4j.
In Cu(DMeDCNQI)2 (DMeDCNQI; dimethyl-N,N-dicyanoqmnonediimine) system, the band of Cu+~/~ is mixed with that of ~-electron in DCNQI, but the couple of the Jahn-Teller eifect
(*) Author for correspondence
Q Les Éditions de Physique 1996
100 )° o a-ET~RbZn(SON)~
, A a-ET~RbCO(SON)~
~~
~ Î
14 a"
ào . a-ET~CSCO(SCN)~
~~ + '° + a-ET CsZn(SCN)
o
" ~ ~
E
fi
à
Î O.l ~+
É'a
O.Oi
O.Ooi
0 50 100 150 200 250 300
T/K
Fig. 1. Temperature dependences of electrical resistivities for a-(BEDT-TTF)2RbZn(SCN)4, o-(BEDT-TTF)2RbCOjSCN)4, a"-(BEDT-TTF)2Ki 4Co(SCN)4, a-(BEDT-TTF)2CsCo(SON)4, and
o-(BEDT-TTF)2CsZn(SCN)4.
around Cu cation and the instability of one-dimeiisional chain of DCNQI aifords
a variety of physical properties under pressure [5j. Recently it has been reported that À-(BETS)2GaC14 (BETS; bis(ethylenedithio)tetraselenafulvalene) shows trie superconductivit» at 8 K, while trie
isostructural À-(BETS)2FeC14 with localized spins of Fe~+ (S
= 5/2 indicates a metal-insulator transition at trie same temperature. This transition is explained by trie antiferromagnetic order of Fe~+ that introduces trie condensation of conduction electrons [6]. Under magnetic field, trie condensed conduction electrons are evaporated so that trie metal-insulator transition is
suppressed [7j.
Recently we prepared new orgamc conductors which contain trie localized electron of Co~+
(S = 3/2), a-(BEDT-TTF)2CsCo(SCN)4 (BEDT-TTF: bis(ethylenedithio )tetrathiafulvalene),
abbreviated as (lie Csco salt), o-(BEDT-TTF)2RbCo(SCN)4 (trie Rbco salt), and a"-(BEDT- TTF)2Ki 4Co(SCN)4 (trie KCO salt) [8,9j. The Csco and Rbco salts are isostructural to
a-(BEDT-TTF)2CsZn(SCN)4 (trie CsZn salt) and o-(BEDT-TTF)2RbZn(SCN)4 (the RbZn
salt) which do not hâve a localized moment; a donor layer and a thick anion sheet of 8 À
stack altemately and trie arrangement of trie donors is a-type (strictly speaking Ù-type [10] ).
Trie donors stack regularly and trie side-by-side interaction, where a dihedral angle of donor planes is 120°, is langer than that of trie stacking direction. Trie interaction spreads in trie ac
plane to construct a closed Fermi surface. In an amon sheet. Co~+ (or Zn~+) is coordinated
distorted-tetrahedrally to N atom of SCN~ to form an anion polymer chair.
Trie metal-insulator transition occurs at 20 K for trie Csco and CsZn salts and at 190 K for trie Rbco and RbZn salts (Fig. 1). Since trie unit cells of two rubidium salts are shghtly
smaller thon those of two cesmm salts, trie rubidium salts seem to be trie chemically pressur-
ized system of trie cesmm salts. With applying pre+ure, overlap of molecular orbitals usually
N°12 MAGNETIC PROPERTIES OF (BEDT-TTF)2MCOjSCN)4 1989
mcreases, instability is suppressed, and transition temperature is decreased. Trie nietal- insulator transition for the rubidium pressurized system, however, occurs at higher tempera-
tures. The same eifect is observed by applying the extemal pressure in the Csco and CsZn
salts. That is, the metal-insulator transition temperatures increase with the pressure for the Csco and CsZn salts [8,9,11j. It might be due to the fact the transverse interaction reduces un- der pressure with changing the dihedral angles of donor planes [12j. Even though the Csco salt has a localized moment, magnetic field does trot influence the metal-insulator transition [11]
dissimilar to À-(BETS)2FeC14 (î].
In order to observe the magnetic properties of itinerant and localized spins for trie coexistent system of the Csco, Rbco, and KCO salts, ESR and SQUID measurements were carried out and a mechanism of a metal-insulator transition, the behavior of a localized moment, and the
interaction of both kind of spins are discussed iii this paper.
2. Experimental
Single crystals are prepared by electrochemical oxidation of BEDT-TTF by using MSCN (M
=
K, Rb, Cs), M'(SCN)2 (M'
= Co, Zn), and 18-crown-6 ether in 1,1,2-trichloroethane and 10%
ethanol. Electrical resistivities were measured by the conventional four-probe method with
applying a low ac current and the contacts were prepared by a gold point (Tokuriki 8560). The ESR spectra were obtained from 4 K to 300 K with utilizing a JEOL JES-RE3X (9.2 GHz.
X-band) spectrometer equipped with an Air Product and Chemicals inc. LTR-3 cryostat and
a Scientific Instrument model 5500 temperature controller. The temperature was controlled and monitored with chromel us. gold with 0.07 atomic % iron thermocouples. Li+TCNQ~
(g = 2.0026) was used as a standard. The magnetization data were collected by SQUID (Quantum Design model MPMS7) and the angle dependent measurements were carried out with an original quartz glass probe. The diamagnetic contribution was corrected using Pascal's
law: x~ore = 5.64 x10~~, 5.6i x 10~~, 5.75 x10~~, 5,î3 x 10~~, and 5.62 x 10~~ emumol~~
for the Rbco, RbZn, Csco, CsZn, and KCO salts, respectively.
3. Results and Discussion
In order to investigate the behaviors of itinerant electrons. ESR measurements were carried out.
Àll ESR signals attributed to BEDT-TTF+ are Lorentzian and the angular dependences of linewidth and g-value at room temperature are shown in Figure 2. Trie g-values of the Csco and CsZn salts varies from 2.o135 to 2.oo33 when trie magnetic field applies parallel to trie b-axis (the
direction of molecular long axis) and to trie a-axis (thé direction tilted by 35° to trie molecular
plane), respectively. The KCO salt has the same tendency. These g-values are close to those of o-(BEDT-TTF)213, fl-(BEDT-TTF)213, (BEDT-TTF)2SbF6, and (BEDT-TTF)2ÀsF6 in which the principal values of the molecular g-tensor are gi " 2.oll 2.o12, g2 " 2.oo6 2.ooî,
and g3 " 2.oo2 1.oo3 for the directions of molecular long axis. short axis, and normal to the
molecular plane, respectively [13]. Since the g-values of the Rbco and Csco salts are similar to those of other BEDT-TTF salts without localized spms and do not shift to that of Co~+
(g = 2.2 2.3 [14]) in a distorted-tetrahedral crystal field, there is no interaction between the localized moment (Co~+) and the itinerant electron (BEDT-TTF+).
Figure 3 shows the temperature dependence of linewidth, intensity, and g-value. For the Csco and CsZn salts, the linewidth at room temperature are 80-72 G. slightly mcreases with
lowering temperature to 91-78 G around 70-110 K, which corresponds to the minimum tem- perature of the resistivity, and decreases rapidly below these temperatures. The intensities
are kept down to 20 K and the origm of the metal-msulator transitions at 10 K are under
. a-ET~CSCO(SCN)~
~
~ .
~
~ ~~~2~~~~~~~~~4
' ~ à ~ ~ ~~~~2~l.4~°~~~~~4
fl
X " 04 b4 04
~ 04
§4 ~ b4 "
§4 "
20
o
0 40 80 120 160
angle /deg
2.01
§4
, .
2.01 . ,
Q~ +
~ §4
g ~ ,
én '
. *
+
0 40 80 20 60
angle /deg
Fig. 2. Angular dependences of ESR hnewidths and g-values for a-(BEDT-TTF)2CsCo(SCN)4, a-(BEDT-TTF)2CsZn(SCN)4. and a"-(BEDT-TTF)2Ki 4Co(SCN)4. The data
are obtained by b
a b, b a b and a*
c a* rotation of the static magnetic field, respectively.
question due to the Curie impurity. The g-values are constant from room temperature to 50 K and reductions below that temperature for both Csco and CsZn softs suggests trie change
of the electromc state. For trie KCO salts, trie Iinewidth at room temperature is relatively
narrow, 32 G and sharpens with Iowering temperature. Trie intensity reduces rapidly below
trie metal-insulator transition temperature of 130 K with narrowmg of linewidth, which might
be attributed to trie CDW instability. Trie measurements of temperature dependence of ESR for trie RbZn and Rbco salts were carned out with applying the magnetic field nearly parallel
to the b-axis (the direction of the molecular long axis) and closely to the a-axis (the direction tilted to the molecular plane), respectively. The observed linewidth is 90 G and trie g-value is 2.01â4 for trie RbZn Salt and 65 G and 2.0064 for trie Rbco salt, which is similar behavior to the Csco and CsZn salts. The charactenstic feature is that the linewidth decreases suddenly from 69 G to 28 G for the RbZn salt and from 50 G to 24 G for the Rbco salt at the metal- msulator transition temperature around 190 K, but that the intensities for both salts remam.
The prelimmary result of magnetic measurement by SQUID for the RbZn salt shows that the
N°12 MAGNETIC PROPERTIES OF (BEDT-TTF)2MCo(SCN)4 1991
, a-ET~CSCO(SCN)~
+ a-ET CsZn(SCN)~
~ .
° °
, aET~RbCO(SCN)
. . g .. 2 4
80
~ + +
~ ~
8
o a-ET RbZn(SCN)
. * + + + + 2 ~
+ °°
w a"-ET~K~
, A
ç~~60 . +
, ' '
~ . A
X
~3 ~~ +
. ~4 mm
. o
~ 04
+ ~ o à ~4
20 o o ~
+ o m *
, @
1~4
0
0 100 200 300
T/K
1-O
"
O~Î i j 1
f 0.8 O
° °'
( ~
~
A A A ~
~
A o ~
+
fl 0,6 '~ . ~4 "
~
$ ~4
td .
~'
. + ~
~ 0.4 .. ~4
ce
~'
à .
lÎ 0.2
~4 O.O
0 100 200 300
T/K
2.
o o o o o o ooooo o o o O
~'°~ pi ii ii i~i ii ii
1.990
0 100 200 300
T/K
Fig. 3. Temperature dependences of ESR Iinewidths, intensities, and g-values for a-jBEDT- TTF)2CsCo(SCN)4 (H
(( a), a-(BEDT-TTF)2CsZn(SCN)4 (H ( a), o-(BEDT-TTF)2RbCo(SON)4 (H ( a), a-(BEDT-TTF)2RbZn(SCN)4 (H
(( b), and o"-(BEDT-TTF)~lli 4Co(SCN)4 (H
((c).
0.4 4 à expedmental filting X(lia)
o experimental filting x(1/b)
o experimental filting X(llc)
°.3 ÎÎÎÎÎÎrn~ÎÎÎÎ ÎItÎÎÎ ÎÎÎÎ 3~j
Ç . experimental fi#ng XT(llc)
~
o ~D
E 3
°~ : ..>;. : :- ~
~ ~Î
°
0.1 ~
O.o 0
0 20 40 60 80 100
T/K
Fig. 4. Temperature dependences of magnetic susceptibilities parallel to a-, b-, and c-axes for
a-(BEDT-TTF)2C~Co(SCN)4.
susceptibility with 6.1 x 10~~ emumol~~ at room temperature approaches to the calculated value based on two-dimensional Heisenberg mortel with J
= -100 K and follows it below 190 Il down to 50 K, suggesting the Mott transition at 190 K [12]. The sudden decrease of Iinewidth
around 190 K is related to the disappearance of the screening of itinerant electrons and the
elongation of the relaxation rate. On the other hand, the Iinewidth of the Csco and CsZn salts also decrease rapidly below the transition temperature (20 K), but the intrinsic intensity is not clear due to the Curie impurity component at Iow temperatures. Whether the metal-insulator
transition of the cesium softs are the same electron-correlated type as the rubidium softs or not is not clear by ESR measurements, but the other magnetic observation of NMR suggests
a similar electron-correlated type transition [15].
In order to observe the behavior of the Iocalized electron of the Csco sait. the temperature dependence of magnetic susceptibility parallel to the a-, b-, and c-axes are measured as shown in
Figure 4. Since the total susceptibility is 2 x 10~~ emu mol~~ at 100 K and 3 x 10~~
emu mol~~
at 2 Il, the contribution of the itinerant electron of around 6 x 10~~ emu mol~~ [16] is 3 0.2%.
so that most of the moment is originated from the Iocalized electron of Co~+ (S
= 3/2).
The specific feature is the decrease of xT below 20 K along the three axes. This deviation from the Curie-law behavior may be related to the zero field splitting due to the crystal field
around Co2+ with distorted-tetrahedral coordination of NCS~ Figure 5 indicates trie energy level diagram of Co~+ in
a tetrahedral ligand field [1î]. Subject to a crystal field by an exact tetrahedron of point charges, the ~F ground term of Co~+ as a free-ion is spht to make the lowest term in a cubic field as ~A2 of four-fold degeneracy. The ~T2 term also split from the free-ion ~F term usually lies at about 3000 cm~~ above the ~À2 term observed by spectroscopic
methods [18]. À distortion from the exact tetrahedral coordination, combined with second- order spin-orbit couplmg, split up this four-fold degeneracy into two Kramers doublets with
an eiiergy separation of 2D. Therefore, the susceptibility data have been fitted in the followiiig
manner. First, the energy levels m a magnetic field were calculated by using the Hamiltonian
N°12 MAGNETIC PROPERTIES OF (BEDT-TTF)2MCo(SCN)4 1993
4T
~~4T
~F(d7) ,[, ~
~~ ~3000cm"
4A Ms=±1/2
'~
il 2D
±3/2 )
free ion tetrahedral axial field field
Fig. 5. Energy Ievel di~gram of Co~* in
a tetrahedral Iigand field.
of equation il) [19].
H = DIS) S(S +1) /3] + ~tB ~ giH~S~ il)
D: zero field splitting; ~tBi Bohr magneton; S: spin quantum number as 3/2 for Co~+.
These energy levels are readily inserted into Van Vleck's equation [20] to yield equation (2) [19].
N ~[(E[°)~/kT 2E[~)j exp(-E(°1/kT)
,i n
~ ~jexp(-E(°1/kT)
Np(g([1 + 9 exp(-2D /kT)j
" 4kT[1 + exp(-2D /kT)j
En = H°E(°1 + H~Ef) + H~E(~l +
~i N~tlg[
~
3N/~lg[lexvl2D/kT) il
~ kT[1 + exp(-2D /kT)] 4D[exp(2D /kT) + lj j~~
2D: energy separation of Kramers doublet; k: Boltzmann constant; gjj, gi g-values parallel
and perpendicular to the z axis.
Then the macroscopic susceptibilities were corrected for the presence of exchange by means of
a molecular-field approxiination with zJ to aiford equation (3) [18].
~i
~~ 21JXÎ ~~~
l jç~2~2
1: ((, 1; z: nearest neighbor; J: exchange parameter.
The fitting of the data using these equations resulted in 2D/k
= -î.0 K, ga
= 2.2, gb " 2.1,
g~ = 1.95, and cJ/k
= -1.3 K as shown m Figure 4 as solid lines. Horrocks and Burlone calculated the relation between the distortion of the tetrahedral coordination and the separation
of Kramers doublet [21]. By applying their calculation, the N-Co-N angle contaimng dj (= a)
axis (lo4.5°) obtained by crystal structure analysis of the Csco salt [8] computed the separation
3
O x, for a-ET Csco(SON)~
a x,üor a-ET Îbco(SCN)~
x x'üora"-EÉK
Co(SON)~
~
. xTüora.ET)~Éo(SON)~
~1°0 ~à
~~~~~~~Î~Î~)~
E Î f
mm oex, fora.ET~CSCO(SON)~
~ Î~
C m,, o,x üora-ET Rbco(SON)
f
, 2l ~ ~
~
~'
~ 3
Ç 1
w
o o
SO IOC ISO 200 250 300
T/K
Fig. 6. Temperature dependences of magnetic susceptibilities for a-(BEDT-TTF)2CsCo(SCN)4, o-(BEDT-TTF)2RbCo(SCN)4, and a"-(BEDT-TTF)2Ki 4Co(SCN)4.
Table I. The Weiss temperatttres (Ù) and g-vailles of (BEDT-TTF)2fifCo(SCN)4 (M
=
K, Rb, Cs).
a"-(ET)2K14Co(SCN)4 o-(ET)2RbCo(SCN)4 a-(ET)2CsCo(SCN)4
Ù/1( -4.6 -3.7 -3.1
g 2.23 2.38 2.48
of two Ilramers doublets to be 2D/k
= -î K. This is m good agreement with our fitting data of 2D/k
= -î.0 K. When D < 0, the level of J~Is
= +3/2 is lower than that of Ms
= +1/2
and an antiferromagnetic order following the Ising mortel (S
= 1/2) is anticipated at a Iow temperature and the deviation ofXT from the fitting hne along the O.axis suggests its precursor of antiferromagnetism (Fig. 4).
In order to find out the averaged magnetic properties, the temperature dependences of magnetic susceptibilities at 5 T for powder sainples of (BEDT-TTF)2MCo(SCN)4 (M
= Ii,
Rb, Cs) ~~ere measured by a SQUID (Fig. 6). From 300 ii to 20 K, the magnetic susceptibility
obeys the Curie-Weiss Iaw and the fitted Weiss temperature (Ù) and g-value are Iisted in Table I. The Weiss temperatures are relatively large compared with other BEDT-TTF salts like (BEDT-TTF)4IlFe(C204)3(C6HSCN) (Ù
= -0.25 K) [12], (BEDT-TTF)4(H20)Fe(C204)3 (C6HSCN) (Ù
= -0.21() [23], (BEDT-TTF)[MoOC14(H20)] (Ù
= -0.23 Ii) [21j. In the present sait, the Co~+ locahzed spms mteract weakly with each other through the coordinated SCN~
anions.
In conclusion, the locahzed moments (Co~+) are mtroduced into the anion layers of the
orgamc conductors as a-(BEDT-TTF)2CsCo(SCN)4, a-(BEDT-TTF)2RbCo(SCN)4, and a"-
(BEDT-TTF)2Ki_4Co(SCN)4. According to the ESR measurement, the hnewidth decreases
rapidly at 190 K aiid trie intensity remained down to 50 K for the Rbco and RbZn salts.
