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Magnetic and transport properties of delafossite oxides CuCr1-x(Mg,Ca)xO2
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 890–892 www.elsevier.com/locate/jmmm
Magnetic and transport properties of delafossite oxides CuCr1xðMg; CaÞxO2 T. Okudaa,, T. Onoea, Y. Beppua, N. Teradaa, T. Doia, S. Miyasakab, Y. Tokurac a
Department of Nano-structures and Advanced Materials, Kagoshima University, Kagoshima 890-0065, Japan b Department of Physics, Osaka University, Osaka 560 0043, Japan c Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan Available online 4 December 2006
Abstract We investigated an effect of Mg2þ or Ca2þ substitution at the Cr sites on magnetic and transport properties in CuCrO2 . In the case of CuCr1x Mgx O2 polycrystals, the physical properties such as resistivity, magnetization, Seebeck coefficient and specific heat systematically varied with the increase of Mg content x, which indicated that holes were doped into them by substitution of Mg2þ ions at the Cr3þ sites. In the temperature dependence of resistivity of CuCr1x Mgx O2 for xX0:02, an anomaly around T N was observed and a negative magneto-resistance effect occurred around it. These facts strongly suggest the coupling between the doped holes and the local spins at the Cr-sites. On the other hand, in the case of CuCr1y Cay O2 polycrystals, the magnetization and resistivity almost did not change with the increase of Ca content y up to 0.04. It is necessary to take into account not only their valences but also their structures for explaining the difference of the substitution effects between the Mg-doped and Ca-doped compounds. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.th; 72.20.Pa Keywords: Delafossite oxides; CuCrO2 ; Substitution effect; Thermoelectric properties; Negative magneto-resistance
The hexagonal layered structure of delafossite oxides, CuBO2 (B being a trivalent cation), can be described as the alternate stacking of edge-shared BO2 octahedral layers and O2 2Cuþ 2O2 dumbbell-shaped layers perpendicular to c-axis (Fig. 1). Each Cuþ ion is linearly coordinated by two O2 ions, and the Cuþ and O2 ions are bonded covalently. If the antiferromagnetic correlation exists between the local spins at the B-sites, the geometrical frustration occurs because the B cations form triangular sublattices. Such frustration gives interesting magnetic properties [1]. If the carriers are introduced into such oxides, a fascinated phenomenon can be expected to occur because of the interaction between the frustrated local spin and anisotropic transport property originated in the layered structure. In this study, we synthesized polycrystalline CuCr1x Mgx O2 ð0pxp0:04Þ and CuCr1y Cay O2 ð0pyp0:04Þ and Corresponding author. Tel./fax: +81 099 285 7763.
E-mail address: [email protected] (T. Okuda). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1141
investigated their structural, transport, and magnetic properties to research an exotic novel phenomenon. We will report an effect of the Mg2þ and Ca2þ substitution at the Cr3þ sites on these properties and the negative magneto-resistance (MR) effect around T N in CuCr0:96 Mg0:04 O2 [2] indicating the coupling between the doped hole and the local spin at the Cr-site. Polycrystalline samples were prepared by using the standard solid-state reaction. The detailed preparation conditions were described in previous paper [2]. Powder X-ray diffraction measurements indicated that all obtained polycrystalline samples except for the Mg-doped compounds for x40:02 were single phase with a hexagonal structure. For CuCr0:97 Mg0:03 O2 and CuCr0:96 Mg0:04 O2 compounds, one tiny tetragonal distorted spinel of CuCr2 O4 appeared around 2y ¼ 35:7 . Fig. 2 shows the temperature ðTÞ dependence of magnetization ðMÞ under a magnetic field of 0.5 T in (a) CuCr1x Mgx O2 and (b) CuCr1y Cay O2 compounds for 0px; yp0:04. There is no difference between the FC and
ARTICLE IN PRESS T. Okuda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 890–892
ZFC magnetizations for both compounds. In the end compound, CuCrO2 , the antiferromagnetic (AF) transition occurred at T N of about 24.5 K. In the higher temperature
Fig. 1. Delafossite structure.
Fig. 2. Temperature dependence of magnetization under a magnetic field of 0.5 T in: (a) CuCr1x Mgx O2 and (b) CuCr1y Cay O2 0px; yp0:04.
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region, M obeyed the Curie–Weiss law ð1=w ¼ H=M ¼ C=ðT þ YCW ÞÞ. With the increase of the Mg content x to 0.04, T N slightly increased up to about 26 K, while the Curie–Weiss parameter YCW ð40Þ drastically decreased with the increase of x [2]. It is interesting that the magnetizations were much enhanced around T N by substitution of Mg2þ ions at the Cr3þ -sites as shown in Fig. 2(a). The resistivity ðrÞ drastically decreased by four orders magnitudes at 300 K with the increase of x to 0.04. In the higher T-region far above T N , the conductivity ðs ¼ 1=rÞ showed a thermal activation behavior, while it showed a variable range hopping behavior around T N . The sign of Seebeck coefficients was positive and its value also decreased with the increase of x [2]. These facts indicated that holes were introduced into the compounds by the substitutions of Mg2þ ions at the Cr3þ -sites. In the Mg-doped compounds, a phenomenon occurred revealing the coupling between the magnetic and electric transport properties. Fig. 3 shows the T-dependence of MR, ½rð8:1 TÞ rð0 TÞ=rð0 TÞ, for CuCr0:96 Mg0:04 O2 around T N . As shown in the Fig. 3, a negative MR was enhanced around T N , while a tiny positive MR effect was observed in the high temperature regime far above T N , which was a conventional MR effect due to the Lorenz force acting on holes. This negative MR effect indicated the coupling between the doped holes and the local spins at the Cr-sites. On the other hand, in the case of the Ca2þ -doped compound, CuCr1y Cay O2 , there was little change in its M and r with an increase of y up to 0.04. As shown in Fig. 2, the change of the M of the Ca-doped compound was much smaller than that of the Mg-doped compound. Corresponding to the y-dependence of M, r of all compounds showed an insulating behavior and a finite change was not observed in the T-dependence with the increase of y up to 0.04.
Fig. 3. T-dependence of magneto-resistance ½rð8:1 TÞ rð0 TÞ=rð0 TÞ for CuCr0:96 Mg0:04 O2 . The inset shows T-dependence of resistivity ðrÞ around T N with and without a magnetic field.
ARTICLE IN PRESS 892
T. Okuda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 890–892
However, there was a tiny but finite substitution effect on M. The inset of Fig. 2(b) shows the magnification of the low-T region of Fig. 2(b). The kink structure at T N for CuCrO2 became a peak just with the Ca-content y of 0.01. Furthermore, the peak became sharper as y increased, which was qualitatively similar to the behavior of the Mgdoped compound. Therefore, these results seemed to indicate that the carrier (hole) was also generated in the Ca-doped compound. If the hole-number is determined only by the averaged valence of Cu ion, the differences of the behaviors of the physical properties originate in those of their bandwidths. If so, we can imagine that there is a correlation between the sharpness of peak of M at T N and the motion of carrier (hole).
We would like to thank Z. Hiroi for his help with the Hall and MR measurements. This work was supported in part by a Grant-in-Aid for Science Research from the ministry of Education, Culture, Sports, Science and Technology. It was also supported in part by a research grant form the Iwatani Naoji Foundation, the Nissan Science Foundation, and the Sumitomo Foundation.
References [1] For example, N. Terada, et al., Phys. Rev. B 70 (2004) 174412. [2] T. Okuda, N. Jufuku, S. Hidaka, N. Terada, Phys. Rev. B 72 (2005) 144403.
Journal of Magnetism and Magnetic Materials 310 (2007) 890–892 www.elsevier.com/locate/jmmm
Magnetic and transport properties of delafossite oxides CuCr1xðMg; CaÞxO2 T. Okudaa,, T. Onoea, Y. Beppua, N. Teradaa, T. Doia, S. Miyasakab, Y. Tokurac a
Department of Nano-structures and Advanced Materials, Kagoshima University, Kagoshima 890-0065, Japan b Department of Physics, Osaka University, Osaka 560 0043, Japan c Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan Available online 4 December 2006
Abstract We investigated an effect of Mg2þ or Ca2þ substitution at the Cr sites on magnetic and transport properties in CuCrO2 . In the case of CuCr1x Mgx O2 polycrystals, the physical properties such as resistivity, magnetization, Seebeck coefficient and specific heat systematically varied with the increase of Mg content x, which indicated that holes were doped into them by substitution of Mg2þ ions at the Cr3þ sites. In the temperature dependence of resistivity of CuCr1x Mgx O2 for xX0:02, an anomaly around T N was observed and a negative magneto-resistance effect occurred around it. These facts strongly suggest the coupling between the doped holes and the local spins at the Cr-sites. On the other hand, in the case of CuCr1y Cay O2 polycrystals, the magnetization and resistivity almost did not change with the increase of Ca content y up to 0.04. It is necessary to take into account not only their valences but also their structures for explaining the difference of the substitution effects between the Mg-doped and Ca-doped compounds. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.th; 72.20.Pa Keywords: Delafossite oxides; CuCrO2 ; Substitution effect; Thermoelectric properties; Negative magneto-resistance
The hexagonal layered structure of delafossite oxides, CuBO2 (B being a trivalent cation), can be described as the alternate stacking of edge-shared BO2 octahedral layers and O2 2Cuþ 2O2 dumbbell-shaped layers perpendicular to c-axis (Fig. 1). Each Cuþ ion is linearly coordinated by two O2 ions, and the Cuþ and O2 ions are bonded covalently. If the antiferromagnetic correlation exists between the local spins at the B-sites, the geometrical frustration occurs because the B cations form triangular sublattices. Such frustration gives interesting magnetic properties [1]. If the carriers are introduced into such oxides, a fascinated phenomenon can be expected to occur because of the interaction between the frustrated local spin and anisotropic transport property originated in the layered structure. In this study, we synthesized polycrystalline CuCr1x Mgx O2 ð0pxp0:04Þ and CuCr1y Cay O2 ð0pyp0:04Þ and Corresponding author. Tel./fax: +81 099 285 7763.
E-mail address: [email protected] (T. Okuda). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.1141
investigated their structural, transport, and magnetic properties to research an exotic novel phenomenon. We will report an effect of the Mg2þ and Ca2þ substitution at the Cr3þ sites on these properties and the negative magneto-resistance (MR) effect around T N in CuCr0:96 Mg0:04 O2 [2] indicating the coupling between the doped hole and the local spin at the Cr-site. Polycrystalline samples were prepared by using the standard solid-state reaction. The detailed preparation conditions were described in previous paper [2]. Powder X-ray diffraction measurements indicated that all obtained polycrystalline samples except for the Mg-doped compounds for x40:02 were single phase with a hexagonal structure. For CuCr0:97 Mg0:03 O2 and CuCr0:96 Mg0:04 O2 compounds, one tiny tetragonal distorted spinel of CuCr2 O4 appeared around 2y ¼ 35:7 . Fig. 2 shows the temperature ðTÞ dependence of magnetization ðMÞ under a magnetic field of 0.5 T in (a) CuCr1x Mgx O2 and (b) CuCr1y Cay O2 compounds for 0px; yp0:04. There is no difference between the FC and
ARTICLE IN PRESS T. Okuda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 890–892
ZFC magnetizations for both compounds. In the end compound, CuCrO2 , the antiferromagnetic (AF) transition occurred at T N of about 24.5 K. In the higher temperature
Fig. 1. Delafossite structure.
Fig. 2. Temperature dependence of magnetization under a magnetic field of 0.5 T in: (a) CuCr1x Mgx O2 and (b) CuCr1y Cay O2 0px; yp0:04.
891
region, M obeyed the Curie–Weiss law ð1=w ¼ H=M ¼ C=ðT þ YCW ÞÞ. With the increase of the Mg content x to 0.04, T N slightly increased up to about 26 K, while the Curie–Weiss parameter YCW ð40Þ drastically decreased with the increase of x [2]. It is interesting that the magnetizations were much enhanced around T N by substitution of Mg2þ ions at the Cr3þ -sites as shown in Fig. 2(a). The resistivity ðrÞ drastically decreased by four orders magnitudes at 300 K with the increase of x to 0.04. In the higher T-region far above T N , the conductivity ðs ¼ 1=rÞ showed a thermal activation behavior, while it showed a variable range hopping behavior around T N . The sign of Seebeck coefficients was positive and its value also decreased with the increase of x [2]. These facts indicated that holes were introduced into the compounds by the substitutions of Mg2þ ions at the Cr3þ -sites. In the Mg-doped compounds, a phenomenon occurred revealing the coupling between the magnetic and electric transport properties. Fig. 3 shows the T-dependence of MR, ½rð8:1 TÞ rð0 TÞ=rð0 TÞ, for CuCr0:96 Mg0:04 O2 around T N . As shown in the Fig. 3, a negative MR was enhanced around T N , while a tiny positive MR effect was observed in the high temperature regime far above T N , which was a conventional MR effect due to the Lorenz force acting on holes. This negative MR effect indicated the coupling between the doped holes and the local spins at the Cr-sites. On the other hand, in the case of the Ca2þ -doped compound, CuCr1y Cay O2 , there was little change in its M and r with an increase of y up to 0.04. As shown in Fig. 2, the change of the M of the Ca-doped compound was much smaller than that of the Mg-doped compound. Corresponding to the y-dependence of M, r of all compounds showed an insulating behavior and a finite change was not observed in the T-dependence with the increase of y up to 0.04.
Fig. 3. T-dependence of magneto-resistance ½rð8:1 TÞ rð0 TÞ=rð0 TÞ for CuCr0:96 Mg0:04 O2 . The inset shows T-dependence of resistivity ðrÞ around T N with and without a magnetic field.
ARTICLE IN PRESS 892
T. Okuda et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 890–892
However, there was a tiny but finite substitution effect on M. The inset of Fig. 2(b) shows the magnification of the low-T region of Fig. 2(b). The kink structure at T N for CuCrO2 became a peak just with the Ca-content y of 0.01. Furthermore, the peak became sharper as y increased, which was qualitatively similar to the behavior of the Mgdoped compound. Therefore, these results seemed to indicate that the carrier (hole) was also generated in the Ca-doped compound. If the hole-number is determined only by the averaged valence of Cu ion, the differences of the behaviors of the physical properties originate in those of their bandwidths. If so, we can imagine that there is a correlation between the sharpness of peak of M at T N and the motion of carrier (hole).
We would like to thank Z. Hiroi for his help with the Hall and MR measurements. This work was supported in part by a Grant-in-Aid for Science Research from the ministry of Education, Culture, Sports, Science and Technology. It was also supported in part by a research grant form the Iwatani Naoji Foundation, the Nissan Science Foundation, and the Sumitomo Foundation.
References [1] For example, N. Terada, et al., Phys. Rev. B 70 (2004) 174412. [2] T. Okuda, N. Jufuku, S. Hidaka, N. Terada, Phys. Rev. B 72 (2005) 144403.