LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $4 - LT Properties of Solids 1: Magnetism (experiment)

Growth and anisotropic physical properties of metallic conductive oxide PdCoO 2 single crystal

Masayuki Tanaka', Masashi Hasegawa" and Humihiko Takei b

"Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106 Japan

bFaculty of Science, Osaka University, Machikaneyama, Toyonaka-shi, Osaka 560 Japan

Single crystals of PdCoO 2 have been successfully grown using the metathetical reaction. Crystals have a form of well-habited hexagonal platelet shape with maximum size 0.8 x 0.8 x 0.1 mm 3. Anisotropic electrical and magnetic properties have been cralified for the first time. Magnetic susceptibilitiy only in the direction of the c-plane is found to show a distinctive cusp at 24 K, suggesting a long-range magnetic ordering p~ and p. at 260K are 5 and 700

. . . . " / /

ltf2cm, respectively Residual res~stwmes p• and p. at 16K are 0.6 and 108 l.tflcm, respectively. Electrical . . . . . " . � 9

reslstwmes in both dlrecuons are found to show metalhc temperature dependence.

1. INTRODUCTION Noble metal compound PdCoO 2 crystallizes in the

delafossite-type structure which has alternative layers of cations and oxygens with two-dimensional triangular lattice [1]. Delafossite-type oxides have been mainly investigated in view of the frustrated magnetism in two-dimensional triangular lattice because each cation layer is completely separated by the adjacent oxygen layers. Magnetic states of the antiferromagnetic delafossite-type oxides have been extensively investigated to make sure the ground state of frustrated spin system by theoretica/[2] or experimental approach [3].

PdCoO 2 single crystals were first discovered and prepared by Shannon et al. [4], who reported some physical properties of the crystals, such as metallic resistivity in laf~cm order in the c-plane with large anisotropy at room temperature and weakly temperature dependent magnetic susceptibility [5]. However, they did not show any detailed data such as the resistivity- temperature relations or the magnetic susceptibility curves. No description on the crystal quality was also given. To the best of our knowledge, the reported electrical resistivity is the lowest among the normal state oxides. In view of the electronic transport properties, PdCoO 2 is regarded as an extremely attractive compound. Besides, it includes possible magnetic atoms of Co at the triangular sites. Therefore, it is interesting and necessary to investigate the electrical and magnetic properties in detail using large and high-quality single crystals. In this study, we will present the anisotropic temperature dependence of electrical resistivities and magnetic susceptibilities of single crystals. Anomalous physical

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $4

properties of PdCoO 2 clarified here will be discussed in view of the itinerant magnetism.

2. EXPERIMENTAL PdCoO 2 single crystals were grown using the

metathetical reaction, as reported in ref. [4]. Powders of reagent grade (99.9 % purity) PdCI 2 and CoO (Kokusan Chem. Co.) were well mi~ed in accordance with the chemical reaction, PdCI 2 + 2CoO --~ PdCoO 2 + CoCI r The mixed powder was sealed in an evacuated silica tube and then heated in a horizontal furnace at 700 "C for 40 hours. After cooling, single crystals were obtained by leaching out cobalt chloride of by-product with ethanol etc. The crystals were characterized using an optical microscope, a scanning electron microscope (SEM) with an electron probe microanalyzer (F.PMA), an X-ray powder diffractometer (XPD) and an X-ray precession camera. The chemical composition was determined by the inductively coupled plasma spectroscopy (ICP) and thermo-gravimetry (TG) with differential thermal analysis (DTA). TG-DTA was performed under H2/l-Ie (10/90 %) atmosphere in order to determine the oxygen content by reducing PdCoO 2 to metal Pd and Co.

The electrical resistivities were measured by a conventional direct current four-prove method from 12 K to room temperature. The magnetizations were measured by a superconducting quantum interference device (SQUID) magnetometer from 5 to 300 K.

3. RESULTS AND DISCUSSION The obtained crystals show metallic lustre and have

a form of weU-habited hexagonal plate with the maximum size 0.8 x 0.8 x 0.1 mm 3. Sharp, hexagonal growth

2109

steps are apparently observed on the surface of the crystals, indicating the lateral growth. The developed hexagonal crystal surface is found to correspond to the c-plane by the X-ray precession method. The sharp diffraction spots without those of any included phases indicate that the crystals have high quality with the space group R'3m. From the results of ICP and TG-DTA, the chemical composition of the crystal is determined as stoichiometric

Pdl.oooCOo.99sOi.99y Figure 1 shows the magnetic susceptibilities

perpendicular Z~ and parallel Zu to the c-axis. In both directions they show the Curie-Weiss-like temperature dependence. The effective moment estimated from the fitting parameter are 0.7 and 0.58 la 8 for Z• and Z~e respectively, which are apparently inconstant with the localized moment of Co ions about 5 I1B. It should be noted that a distinctive cusp is observed at 24 K only in case of ~., suggesting a magnetic long-range ordering. On the other hand, only slight shoulder in Xu was observed around 24 K. This is difficult to determine whether intrinsic or not because of the difficulty in setting the axis of the samples strictly parallel to the applied field. In addition, it is interesting that small hysteresis between the zero field cooled (ZFC) and field cooled (FC) measurements is observed in both cases below 24 K. The effective moment of Co ions is highly smaller than their localized moment. This may be attributable to the itinerant character of PdCoO 2, as mentioned below. Therefore, the magnetic ordering is thought to be originated from a remnant magnetic moment.

Figure 2 shows the temperature dependence of anisotropic electrical resistivities p of the crystal. The electrical resistivities in both directions 131 and pH decrease with decreasing temperature and show metallic temperature dependence. The resistivities at 260 K are 5 and 700 la f~cm and the anisotropy pulp• is estimated to be about 150 at 260 K. The extrapolated residual values at 0 K are p• = 0.51 ~t f~cm and Px = 97 I~ 12cm. Rogers et al. reported lower and higher values for p~ and pn, respectively, which resulted in a larger anisotropy at room temperature [5]. It should be also noted that the values begin to saturate down to the residual resistivity around 30K. This might be attributed to the correlation of the itinerant electrons and magnetic ordering.

4. CONCLUSIONS Anisotropic electrical and magnetic properties have

been clarified for the first time. Magnetic susceptibility only in the direction of the c-plane is found to show a distinctive cusp at 24 K, suggesting a long-range magnetic ordering. Electrical resistivities in both directions are

found to show metallic temperature dependence. From the present results, PdCoO 2 is thought to be the first material which is an itinerant magnet and has magnetically frustrated spins on the two dimensional triangular lattice.

REFERENCES

[1] C. T. Prewitt et al. Inorg. Chem. I0 (1971) 719.

[2] G. H. Wannier, Phys. Rev. 79 (1950) 357. [3] K. Hirakawa et al. J. Phys. Soc. Jpn. 52 (1983)

1814.

[4] R. D. Shannon et al. Inorg. Chem. 10 (1971) 713.

[5] D. B. Rogers et al. Inorg. Chem. 10 (1971) 723.

. . . . i . . . . ! . . . . i . . . . . . . . .

: �9 oa. ,Fcl �9 ~ 4 o o ca. F c l = , �9 //zvcl

= . o . . O . O 1 T

i l l ~ o

" ~ ~ I i l i l n i l n i i . . . ~ ~ �9 ] ] " ' l l l l i i l i g l i l n l l i i l l l ~ l l l e .

c / / i 0 10 20 30 40 50

T e m p e r a t u r e ( K ) Figure 1. Magnetic susceptibilities of single crystal

. ooo 800 [ ......................... 1

c//] 0LL- . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . u . . . . a . . . . i . . . . i . . . . ! . . . . . . I

O~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . g 50 100 150 200 250 I00

T e m p e r a t u r e ( K )

Figure 2. Electrical resistivities of single crystal

2110 Czech. J. Phys. 46 (1996), Suppl. S4