Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 47 (2014)

Magnetic ordering in Cu6O8TbCl probed during the SR measurement
K. Kawashima,＊1 H. Takeda,＊1 S. Igarashi,＊1 I. Watanabe,＊2 I. Kawasaki,＊2 and J. Akimitsu＊1Tb

100

150

200

250300( cm)

0.06
0.04
I = 1 mA

100150T (K)200250300Fig. 1 (Left panel) Crystal structure of Cu6O8MCl (program
VESTA was used. 4)). Solid line shows the unit cell. (Rightpanel) Temperature dependence of magnetic susceptibility
and electrical resistivity of Cu6O8TbCl. Inset shows theinversed magnetic susceptibility as a function oftemperature.
＊1＊20.5

11.5

2Time in microseconds ( s)

20.25
0.2
0.15
0.1
0.05

024681012050

100

150

200

250

300T (K)Fig. 2 (a) The time dependence of asymmetry of ZFSRspectra of Cu6O8TbCl measured at various temperatures.
Solid lines are fitting results using Eq. (1). (b) The
temperature dependence of A0 and  obtained by the fitting.

0.150

840Time in microseconds ( s)0.080A08T09161061010

H = 1 kOe

0.02

1811121050

14 (b)
1214200Asymmetry

2030016200K
300K

1240

035K100K
1350

1T = 10K30K

221/(emu/mol)

224 (a)Asymmetry

The copper-oxide family of Cu6O8MCl (M = cation) has
a caged structure (Cu6O8 cage) in its crystal structure(crystal system: cubic, space group: Fm-3m (No. 225)). The
form of the surface of the Cu6O8 cage is similar to the CuO2
plane in high-Tc cuprate superconductors. The Cu6O8 cage
forms a three-dimensional network, sharing their face (leftpanel of Fig. 1).1-3) The Cl- anions are located at the centerof the Cu6O8 cage, and the M cations, which have a valenceof 3+ or 4+, exist in the cuboid space located between
Cu6O8 cages. The average valence of Cu ions in the Cu6O8
cage is 2.33+ for M3+.
For the M = Tb3+ compound (Cu6O8TbCl), the magnetic
susceptibility data indicate a weak magnetic transition atapproximately 40 K. Moreover, the electrical resistivity data
exhibit a metal-insulator transition at approximately 40 K
(right panel of Fig. 1). The effective Bohr magneton Peff isestimated to be ~6.74, which is close to the theoretical value
for Tb3+, indicating that a localized magnetic moment exists
at the Tb3+ site. However, whether the origin of themagnetic ordering state is long range (static) or not and therelationship between the magnetic state and other physicalproperties is not yet clear. In order to clarify these points,
SR measurement was performed at the RIKEN-RAL
Muon Facility in U.K. using a polycrystalline sample ofCu6O8TbCl.
Figure 2 (a) shows the time dependence of asymmetryof the zero field (ZF) SR spectra, A(t), of Cu6O8TbCl
measured at various temperatures. These spectra wereobserved using the double-pulse muon beam. The spectra
show exponential-like depolarized behaviors, and themuon-spin depolarization becomes faster with decreasing
temperature above 40 K. Significant loss of initial

Department of physics and mathematics, Aoyama Gakuin University
RIKEN Nishina Center- 249 -asymmetry (at t = 0) is observed below 40 K. In order toobserve the change of the spin dynamics in detail, all of thetime spectra were analyzed using the following function:
A(t) = A0exp(-t)+ AB,(1)where A0 is the initial asymmetry,  is the depolarization
rate of the muon spins, and AB is the background. The
parameters obtained from the best fit of Eq. 1 to the data inFig. 2(a) (solid line) are shown in Fig. 2(b). From Fig. 2(b),A0 slightly decreases and  increases above 40 K. Moreover,A0 and  values rapidly change at approximately 40 K,
indicating that the spin state drastically changes at 40 K.
The inset of Fig. 2(a) shows an early time region of the ZF
SR spectra at 5 K (lowest temperature of thismeasurement) measured using the single-pulse muon beam.
Muon-spin precession behavior was observed, indicating
the existence of a long-range magnetic-ordered state.
Consequently, the weak magnetic transition confirmed in
the magnetic susceptibility data of Cu6O8TbCl is generated
by the development of long-range magnetic order of Tb3+.
Moreover, it is suggested that the conducting carrier istrapped and that the ground state changes from metallic tosemiconducting with the onset of long-range magneticorder.
References

1) I. Yazawa et al.: Jpn. J. Appl. Phys. 29 (1990) L1693.
2) R. Sugise et al.: J. Appl. Phys. 32 (1993) L940.
3) G. Zouganel et al.: Sol. St. Comm. 80 (1991) 709.
4) K. Momma et al.: Commission Crystallogr. Comput. IUCr
Newslett. (2006) No. 7, 106.