PHYSICAL REVIEW B

VOLUME 58, NUMBER 17

1 NOVEMBER 1998-I

ESR study of Fe-doped Ca3Co2O6: Isolated spin state of Fe31 and anomalous D parameter
caused by exchange interaction
H. Kageyama,* K. Yoshimura, and K. Kosuge
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

H. Nojiri, K. Owari, and M. Motokawa
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan~Received 8 May 1998!
Impurity spin resonance of Fe31 ions doped in the one-dimensional ferromagnet Ca3Co2O6 was investigatedin a submillimeter wave region using pulsed magnetic fields at 1.7 K. As expected from the Mössbauer result,the isolated electron state of Fe31 with g52.00 was successfully observed. The D values of Fe ions are foundto be 0.66 cm21 in a ferrimagnetic region and 20.87 cm21 in a ferromagnetic region which is anomalously
large and would be caused by the exchange interaction between a Fe spin and surrounding Co spins.
@S0163-1829~98!01742-1#

In the field of Mössbauer spectroscopy, it has been widelybelieved that an isolated electron spin state of the 57Fe ion isobservable only for dilute magnetic materials, such as very
dilute solutions of Fe2O3 in Al2O3, where the relaxation of
an individual 57Fe31 electron spin is slow compared with theprecession time of the nuclear spin.1–3 On the other hand, inthe case of magnetically dense materials, an individual electron spin changes its z component at an approximate rate of
J/\ ~J: exchange interaction! which is fast enough to make anuclear spin feel a time averaged effective field.4
Recently, we measured the Mössbauer effect of 57Fe31ions doped in a one-dimensional ~1D! Ising ferromagnet
Ca3Co2O6 ~Ref. 5! and found that, although this material has
a high density of magnetic ions and exhibits a long-range
magnetic ~ferrimagnetic! ordering at T c 525 K,6–8 the observed spectra are similar to those in the case of magneticallydilute extreme: the spectra with hyperfine fields corresponding to the exchange-polarized Fe electron spins with m z 565/2, 63/2, and 61/2 are first observed below T c . It isconsidered that the key lies in the large difference betweenmagnetic characters of the host ~Ising! and impurity ~Heisenberg! spins. The spin wave band or the spin-cluster excitation energy of the host spin system is much higher than theimpurity spin energy so that the impurity spin can be regarded as extremely isolated, and can be treated as an ‘‘isolated paramagnetic moment.’’
The above interpretation on the Mössbauer result for
57 31
Fe -doped Ca3Co2O6 can be confirmed by ESR experiment since such a localized impurity state of a Heisenberg
spin in a host Ising-spin system has already been found bymeans of ESR. For example, Date et al. observed the impurity spin resonance for Mn21 and Fe31 in FeCl2, and Mn21
in CoCl22H2O. 9–11 The present paper reports on the experimental results of ESR for Fe-doped Ca3Co2O6, which is discussed from the viewpoint of localized spin resonance, using
a simple formula based on the molecular field approximation.
The 1% Fe-doped Ca3Co2O6, i.e., Ca3~Co0.99Fe0.01!2O6,
was prepared by a conventional solid state chemical reaction
0163-1829/98/58~17!/11150~3!/$15.00

PRB 58as described in Ref. 7. The powered sample was soaked into
an epoxy and then the magnetic field of H58 T was quickly
applied to align the sample along the hexagonal c axis ~parallel to the chain direction!. Submillimeter wave ESR has
been performed at 1.7 K by using pulsed magnetic fields up
to 30 T. The magnetic field is applied parallel to the c axis,
and the duration time of the pulsed magnetic field is about 4
ms. The optical pumped far-infrared laser ~about 40 lines up
to 1 THz!, backward traveling wave tubes ~220 and 370 GHzbands! and Gunn oscillators ~95, 110, 135, 190 GHz! havebeen employed. An InSb hot electron detector has been used.
Before presenting the ESR results, we will give a brief
review of the structural and magnetic properties of the hostmaterial. The structure of Ca3Co2O6 ~K4CdCl6 type; R3̄c! ischaracterized by 1D chains consisting of alternating facesharing CoO6 octahedra and CoO6 trigonal prisms along thec axis.12 These chains are widely separated from each otherby Ca ions. Another point of the structure is that the chainsform a triangular lattice in the c plane. It should be noted
here that the phase of each chain differs from that of threenearest-neighbor chains by 1/6 along the c axis and from thatof other three chains by 2/6 along the same axis.7 Concerning
its magnetism, Ca3Co2O6 is known as one of the typical Ising
spin systems due to a strong anisotropy of the g tensor.7,8
The intrachain interaction is ferromagnetic, whereas the interchain one is antiferromagnetic, which induces a ferrimagnetic ordering of the ferromagnetic chains below 25 K
(5T c ). Some anomalies have been observed at lower temperatures: the 1/3 plateau in magnetization curves disappears
below 10 K.8 The temperature dependence of the neutron
diffraction intensity of the magnetic ~100! reflection does notobey the Brillouin function.6 This implies the existence of
successive magnetic phase transitions. However, the resultsof our neutron diffraction experiment13 and the specific heatmeasurement by Michor and Hilscher14 show no evidence ofsuccessive phase transition. Therefore, we will discuss thefollowing ESR result by assuming that the lower field phaseat 1.7 K is a ferrimagnetic state, though further study isneeded to confirm this assumption.
11 150©1998 The American Physical Society

PRB 58

BRIEF REPORTS

11 151

FIG. 1. Typical absorption lines observed at 1.7 K. The solid
circles, open circles, and triangles denote absorptions from branches
A, B, and C, respectively ~see Fig. 2!.

Many resonance lines having systematic frequency dependence were observed at 1.7 K in the frequency range 190 and717 GHz. Typical examples of the resonance are shown inFig. 1, where only the process under an increasing magnetic
field is displayed. Figure 2 shows the frequency-field diagram of the resonance points. These resonance points can beclassified into three branches, that is, A, B, and C, in theferrimagnetic state, the ferromagnetic state, and near theboundary of the two states, respectively. The linewidth forbranches A and B is about 1;2 kOe. However, the linewidthof branch C is unclear because of the very weak absorption
intensity, and the origin of the resonance has not been identified at the present stage.

FIG. 3. Models of the impurity Fe31 spin site in the hexagonal
ab plane. Note that there are two sites denoted by I, II in the ferrimagnetic region.

What is important is that both branches A and B have alinear field dependence of the resonance frequency with aneffective g valve of 2.00. It is clear that these come from theisolated impurity Fe31 spins in Ca3Co2O6 in accordance withthe expectation from the Mössbauer experiment.5 Eachbranch consists of several resonance lines equally separated,which is attributed to the uniaxial anisotropy D acting on theFe31 spin. The same kind of line separation is seen in thecase of Mn21-doped CoCl22H2O. 11
The effective spin Hamiltonian of the Fe31 spins S underan external magnetic field H 0 is expressed as
H5g m B ~ H 0 1H E ! S1DS 2z ,FIG. 2. Frequency-field diagram of the Fe31 impurity resonancein Ca3Co2O6 observed at 1.7 K (H 0 i c axis). The solid circles, opencircles, and triangles denote absorptions from branches A, B, and C,respectively. Full lines represent the theoretical ones.

~1!

where m B is the Bohr magneton, H 0 is an external magnetic
field, and H E is the resultant effective field coming from theexchange and dipolar interactions between the Fe and thesurrounding Co spins. In this case, the dipole field is negligible because it is much smaller than the exchange field. The
last term represents the uniaxial anisotropy. Considering that
the Fe31 spin is S55/2, each branch should consist of fiveresonance lines, though in reality the positions of thebranches A and B have four (A 1 ,A 2 ,A 3 ,A 4 ) and three lines(B 1 ,B 2 ,B 3 ), respectively. The remaining unobserved lines(A 5 ,B 4 ,B 5 ) as drawn in Fig. 2 were determined by comparing the relative absorption intensities: Since the m z 525/2state, where m z represents the expectation value of S z , is by
far most populated at low temperature 1.7 K, the lines

BRIEF REPORTS

11 152A 1 and B 1 having the highest relative intensity in eachbranch ~see Fig. 1! should correspond to the transition fromu m z u 55/2 to 3/2.
One can calculate the resonance conditions in the mannerused in the impurity Mn21 spin resonance in CoCl22H2O. 11For simplicity, we assume that the Fe31 ion is surrounded by
six Co31 ions in the c plane, that is to say, the phase difference between the chains as mentioned above is neglected
here. One can thus introduce the effective exchange fields,
H 1 and H 2 , corresponding to the intrachain and interchain
exchange interactions between Fe31 and Co31 spins, respectively. Consider the Fe31 impurity spins in the c plane as
illustrated in Fig. 3. We assume that the intrachain exchangeinteraction between Fe31 and Co31 spins is ferromagnetic
and the interchain one is antiferromagnetic. When H 0 is applied along the c axis, the resonance conditions of Fe31 spinsare expressed as follows.
~a! Ferrimagnetic state 0,H 0 ,H c (H c : the metamagnetic critical field!. Although only one branch has been observed experimentally, two inequivalent impurity sites are
considered theoretically as shown in Fig. 3~a!,v / g 52H 1 6H 0

for type I,

v / g 52H 1 16H 2 6H 0

for type II,~2!
~3!

where the D term is omitted or, in other words, only thetransition between m z 51/2 and 21/2 of the Fe31 ion is considered.
~b! Ferromagnetic state H c ,H 0 . As shown in Fig. 3~b!,
all impurity spins in this region are equivalent so that wehave only one resonance formula given byv / g 52H 1 26H 2 1H 0

for type III.

~4!

Let us compare Eqs. ~2!, ~3!, and ~4! with the experimental data. Extrapolating the lines A 3 and B 3 in Fig. 2 to thezero field, i.e., to the ordinate, the resonance frequencies in
the absence of the external magnetic field are given as 358.5
GHz ~5128.0 kOe! and 53.2 GHz ~519.0 kOe!, respectively. In the ferromagnetic state, one obtains H E 52H 1*Present address: Institute for Solid State Physics, University ofTokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan.
1
G. K. Wertheim and J. P. Remeika, Phys. Rev. Lett. 10, 14~1964!.
2
H. H. Wickman and G. K. Wertheim, Phys. Rev. 148, 211 ~1966!.
3
F. E. Obenshain, L. D. Roberts, C. F. Coleman, D. W. Forester,and J. O. Thomson, Phys. Rev. Lett. 14, 365 ~1965!.
4
R. S. Preston, S. S. Hanna, and J. Heberle, Phys. Rev. 128, 2207
~1962!.
5
H. Kageyama, S. Kawasaki, K. Mibu, M. Takano, K. Yoshimura,and K. Kosuge, Phys. Rev. Lett. 79, 3258 ~1997!.
6
S. Aasland, H. Fjellvåg, and B. Hauback, Solid State Commun.
101, 187 ~1997!.

PRB 58

26H2553.2 kOe by comparing with Eq. ~4!. In the ferrimagnetic state, the value of 128.0 kOe is comparable to 100 kOe
obtained by the Mössbauer effect measured at zero magnetic
field.5 Note that in the analysis of the Mössbauer result only
one branch was assumed. One cannot, however, determinewhether the observed resonance corresponds to the Fe31 spin
at a type-I or type-II site. If branch A comes from a type-Isite, the two exchange fields are calculated as H 1 564.0 kOe
and H 2 518.2 kOe, or H 1 536.8 kOe and H 2 59.1 kOe if A
comes from a type-II site. Since the low-field phase below 10
K is still not clear as mentioned above, the fact that only one
branch is observed implies the low-temperature phase is nota ferrimagnetic phase but an antiferromagnetic phase.
Finally, we discuss the fine structure splitting observed inbranches A and B. The D values of Fe31 are found to be 0.66
cm21 for branch B. Here, the magnitude of D is determinedby the line separation, and its sign is specified by the absorption intensity ratio of each line. The point we wish to stress isthat the magnitude of the D values is unusually large. So far
the largest D value was obtained for the Mn21 ion in
CoCl22H2O (D520.13 cm21). In addition, the sign of the Dvalues changes between the branches A and B. Obviously,the crystalline field contribution alone does not account for
the results. As proposed in the case of Mn21-doped
CoCl22H2O, the main reason should be the antisymmetricexchange
interactionoftheformd• @ S 1 3S 2 #~Dzyaloshinski-Moriya type!.11,15 Since the present material
is an oxide, the superexchange interaction through the oxygen ion would play an important role in giving rise to theanomalously large D value.
In conclusion, as expected from the previous Mössbauer
experiment, the impurity spin resonance of Fe31 in
Ca3Co2O6 has been successfully observed. The exchange
fields are evaluated to be 128.0 and 19.0 kOe for branches Aand B, respectively. The D values of 0.66 cm21 for branch Aand 20.87 cm21 for branch B are unusually large and thesign changes between these states, which would arise fromthe exchange interaction between Fe and Co spins.

7

H. Kageyama, K. Yoshimura, K. Kosuge, H. Mitamura, and T.
Goto, J. Phys. Soc. Jpn. 66, 1607 ~1997!.
8H. Kageyama, K. Yoshimura, K. Kosuge, M. Azuma, M. Takano,
H. Mitamura, and T. Goto, J. Phys. Soc. Jpn. 66, 3996 ~1997!.
9
M. Date and M. Motokawa, Phys. Rev. Lett. 15, 854 ~1965!.
10
M. Motokawa and M. Date, J. Phys. Soc. Jpn. 23, 1216 ~1967!.
11
N. Fujii, M. Motokawa, and M. Date, J. Phys. Soc. Jpn. 25, 700~1968!.
12
H. Fjellvåg, E. Gulbrandsen, S. Aasland, A. Olsen, and B. C.
Haucack, J. Solid State Chem. 124, 190 ~1996!.
13
H. Kageyama, K. Yoshimura, K. Kosuge, X. Xu, and S. Kawano,J. Phys. Soc. Jpn. 67, 357 ~1998!.
14H. Michor and G. Hilscher ~private communication!.
15
M. Tachiki, J. Phys. Soc. Jpn. 25, 686 ~1968!.