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Pressure effect on electrical resistivity of Y1-xGdxCo2
ARTICLE IN PRESS
Physica B 378–380 (2006) 169–170 www.elsevier.com/locate/physb
Pressure effect on electrical resistivity of Y1xGdxCo2 T. Nakamaa,, Y. Takaesua, K. Yagasakia, E. Sakaia, N. Kuritab, M. Hedob, Y. Uwatokob, A.T. Burkovc a Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan c A. F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia b
Abstract Electrical resistivity of Y1x Gdx Co2 alloy system has been measured at temperatures from 2 to 300 K in magnetic field up to 15 T and under pressure up to 10 GPa. The compounds with the composition near to phase boundary between paramagnetic and ferromagnetic ground state (xc 0:12) show strong enhancement of electrical resistivity at low temperatures. Large positive magnetoresistance was observed in ferromagnetic alloys in composition range 0:15oxo0:3. The anomalous magnetoresistance as well as the composition dependence of the low-temperature resistivity were explained by an interplay of metamagnetic instability of Co 3d itinerant electrons and structural disorder within rare-earth sublattice. The pressure effect on electrical resistivity for Y1x Gdx Co2 at low temperatures is in agreement with the variation of magnetoresistance with the composition. r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.20.Lp; 71.27.+a Keywords: Magnetoresistance; Pressure effect; Itinerant electron metamagnetism
The Laves phase compound GdCo2 is a ferrimagnet with magnetic transition temperature T c 400 K. Due to 3d–4f exchange interaction the localized 4f magnetic moments of Gd induce in the ground state anti-parallel magnetization of the itinerant Co 3d subsystem. On the other hand, YCo2 is a strongly enhanced Pauli paramagnet, which itinerant 3d-electron subsystem undergoes metamagnetic transition into ferromagnetic ground state in the external magnetic field of B 70 T [1]. The ground state for the pseudobinary system of Y1x Gdx Co2 was categorized into three regions: paramagnetic phase (PM) for xo0:12, homogeneous ferrimagnet (FR) for x40:3 and two-component ferrimagnet (FM) for 0:12txt0:3 [2]. The FM Y1x Gdx Co2 compounds show anomalous large positive magnetoresistance (MR) at low temperatures, which is a sharp contradiction to theoretical predictions for magnetoresistance of ferromagnetic materials with respect to the sign as well as to the magnitude of the effect [3]. Additionally, a strong Corresponding author. Tel.: +81 98 895 8514; fax: +81 98 895 8509.
E-mail address: [email protected] (T. Nakama). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.001
enhancement of the low-temperature resistivity was observed for the compounds with the composition near to the phase boundary between the paramagnetic and ferromagnetic ground state. The anomalous positive magnetoresistivity and the composition dependence of the lowtemperature resistivity of the alloys were explained by an interplay of the metamagnetic instability of Co 3d itinerant electrons and structural disorder in R-sublattice. This interplay leads to the formation of two-component ferrimagnetic phase for xt0:3 with a random distribution of regions with high and low 3d magnetization. The lowtemperature resistivity rm due to the electron scattering on this random distribution of low and high 3d magnetizations can be expressed as: rm yð1 yÞ,
(1)
where y is the volume fraction of the high magnetization component of Co 3d subsystem [2]. According to the theoretical model parameter y, hence the electrical resistivity, is a function of the composition x, external magnetic field B and pressure P. Particularly, MR is related to the
ARTICLE IN PRESS T. Nakama et al. / Physica B 378–380 (2006) 169–170
170
0.9
1.5
Y1-xGdxCo2
1.0
PM 0.3 ∆ρ / ρ
∆ρ / ρ
0.6
FR 0.0
Y0.82Gd0.18Co2
FM -0.3 0.0
0.1
0.2
0.5
0.3 x
0.4
0.5
0.6
Fig. 1. Composition x dependence of PR (m) and MR () of Y1x Gdx Co2 system at 2 K. The dashed vertical lines indicate the phase boundaries. PM: paramagnet; FM: ferromagnet; FR: ferrimagnet.
0.0 0
2
4
6
8
10
P [GPa]
decrease of the fraction of high magnetization component, y in Eq. (1), due to decrease of the effective magnetic field Beff ¼ ðBexc Ba ) acting on Co 3d itinerant subsystem. Here Bexc and Ba are the 4f–3d exchange field and the applied magnetic field, respectively. On the other hand, the metamagnetic critical field BC of Co 3d itinerant subsystem increases under external hydrostatic pressure P [4]. In a very rough approximation y / ðB¯ eff BC Þ þ 0:5, therefore y in alloy of a given composition decreases under hydrostatic pressure, i.e. the application of pressure is expected to have, to a first approximation, the same effect ¯ eff is the mean value of the as external magnetic field (B effective field). Therefore measuring the resistivity in dependence on pressure one can test the theory. In this paper we present the results on electrical resistivity r of the Y1x Gdx Co2 compounds (xt0:3), measured under pressure up to 10 GPa. The sample preparation procedure has been described in Ref. [2]. The clamp-type piston cylinder pressure cell for the measurements up to 2 GPa and the cubic anvil cell for the measurements up to 10 GPa were used for the resistivity measurements. Fig. 1 shows the pressure effect on electrical resistivity (PR ¼ Dr=r, DrðTÞ ¼ rðP; TÞ rð0; TÞ) and the magnetoresistivity (MR ¼ Dr=r, DrðTÞ ¼ rðB; TÞ rð0; TÞ) of Y1x Gdx Co2 alloys at 2 K, measured under pressure P ¼ 2 GPa and magnetic field B ¼ 15 T, respectively. In agreement with theoretical prediction the composition dependence of PR shows very good correlation with the dependence of magnetoresistivity on composition [2]. For the compound with the value of y just above 0.5, we expect that r increases with increasing P at first, and decreases with further increasing P. The pressure dependence of PR of Y0:82 Gd0:18 Co2 at 2 K measured by using the cubic anvil cell is shown in Fig. 2. r increases with
Fig. 2. Pressure dependence of PR of Y0:82 Gd0:18 Co2 . The dotted line is guide for the eye.
increasing P, having a maximum around 2 GPa and then decreases with increasing P. It means that y for the compound of x ¼ 0:18 decreases with increasing P, and crosses the value of y ¼ 0:5 around 2 GPa. As expected the pressure effect on r is qualitatively the same as the dependence of residual resistivity on alloy composition x [2]. In conclusion, both PR and MR at low temperatures are in agreement with the model predictions. The strong enhancement of r, the anomalous MR and the PR at low temperatures for the Y1x Gdx Co2 system are explained by means of the interplay of metamagnetic instability of Co 3d itinerant electrons and structural disorder within rare-earth subsystem. Acknowledgments We thank the Institute of Solid State Physics of Tokyo University, where part of this work was carried out under the joint research program. In part this work was supported by Grant no 05-02-17816a of Russian Foundation for Basic Research. References [1] T. Goto, K. Fukamichi, T. Sakakibara, H. Komatsu, Solid State Commun. 72 (1989) 945. [2] A.T. Burkov, A.Yu. Zyuzin, T. Nakama, K. Yagasaki, Phys. Rev. B 69 (2004) 144409. [3] T. Nakama, A.T. Burkov, M. Hedo, H. Niki, K. Yagasaki, J. Magn. Magn. Mater. 226–230 (2001) 671. [4] H. Yamada, J. Magn. Magn. Mater. 139 (1995) 162.
Physica B 378–380 (2006) 169–170 www.elsevier.com/locate/physb
Pressure effect on electrical resistivity of Y1xGdxCo2 T. Nakamaa,, Y. Takaesua, K. Yagasakia, E. Sakaia, N. Kuritab, M. Hedob, Y. Uwatokob, A.T. Burkovc a Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan c A. F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia b
Abstract Electrical resistivity of Y1x Gdx Co2 alloy system has been measured at temperatures from 2 to 300 K in magnetic field up to 15 T and under pressure up to 10 GPa. The compounds with the composition near to phase boundary between paramagnetic and ferromagnetic ground state (xc 0:12) show strong enhancement of electrical resistivity at low temperatures. Large positive magnetoresistance was observed in ferromagnetic alloys in composition range 0:15oxo0:3. The anomalous magnetoresistance as well as the composition dependence of the low-temperature resistivity were explained by an interplay of metamagnetic instability of Co 3d itinerant electrons and structural disorder within rare-earth sublattice. The pressure effect on electrical resistivity for Y1x Gdx Co2 at low temperatures is in agreement with the variation of magnetoresistance with the composition. r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.20.Lp; 71.27.+a Keywords: Magnetoresistance; Pressure effect; Itinerant electron metamagnetism
The Laves phase compound GdCo2 is a ferrimagnet with magnetic transition temperature T c 400 K. Due to 3d–4f exchange interaction the localized 4f magnetic moments of Gd induce in the ground state anti-parallel magnetization of the itinerant Co 3d subsystem. On the other hand, YCo2 is a strongly enhanced Pauli paramagnet, which itinerant 3d-electron subsystem undergoes metamagnetic transition into ferromagnetic ground state in the external magnetic field of B 70 T [1]. The ground state for the pseudobinary system of Y1x Gdx Co2 was categorized into three regions: paramagnetic phase (PM) for xo0:12, homogeneous ferrimagnet (FR) for x40:3 and two-component ferrimagnet (FM) for 0:12txt0:3 [2]. The FM Y1x Gdx Co2 compounds show anomalous large positive magnetoresistance (MR) at low temperatures, which is a sharp contradiction to theoretical predictions for magnetoresistance of ferromagnetic materials with respect to the sign as well as to the magnitude of the effect [3]. Additionally, a strong Corresponding author. Tel.: +81 98 895 8514; fax: +81 98 895 8509.
E-mail address: [email protected] (T. Nakama). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.001
enhancement of the low-temperature resistivity was observed for the compounds with the composition near to the phase boundary between the paramagnetic and ferromagnetic ground state. The anomalous positive magnetoresistivity and the composition dependence of the lowtemperature resistivity of the alloys were explained by an interplay of the metamagnetic instability of Co 3d itinerant electrons and structural disorder in R-sublattice. This interplay leads to the formation of two-component ferrimagnetic phase for xt0:3 with a random distribution of regions with high and low 3d magnetization. The lowtemperature resistivity rm due to the electron scattering on this random distribution of low and high 3d magnetizations can be expressed as: rm yð1 yÞ,
(1)
where y is the volume fraction of the high magnetization component of Co 3d subsystem [2]. According to the theoretical model parameter y, hence the electrical resistivity, is a function of the composition x, external magnetic field B and pressure P. Particularly, MR is related to the
ARTICLE IN PRESS T. Nakama et al. / Physica B 378–380 (2006) 169–170
170
0.9
1.5
Y1-xGdxCo2
1.0
PM 0.3 ∆ρ / ρ
∆ρ / ρ
0.6
FR 0.0
Y0.82Gd0.18Co2
FM -0.3 0.0
0.1
0.2
0.5
0.3 x
0.4
0.5
0.6
Fig. 1. Composition x dependence of PR (m) and MR () of Y1x Gdx Co2 system at 2 K. The dashed vertical lines indicate the phase boundaries. PM: paramagnet; FM: ferromagnet; FR: ferrimagnet.
0.0 0
2
4
6
8
10
P [GPa]
decrease of the fraction of high magnetization component, y in Eq. (1), due to decrease of the effective magnetic field Beff ¼ ðBexc Ba ) acting on Co 3d itinerant subsystem. Here Bexc and Ba are the 4f–3d exchange field and the applied magnetic field, respectively. On the other hand, the metamagnetic critical field BC of Co 3d itinerant subsystem increases under external hydrostatic pressure P [4]. In a very rough approximation y / ðB¯ eff BC Þ þ 0:5, therefore y in alloy of a given composition decreases under hydrostatic pressure, i.e. the application of pressure is expected to have, to a first approximation, the same effect ¯ eff is the mean value of the as external magnetic field (B effective field). Therefore measuring the resistivity in dependence on pressure one can test the theory. In this paper we present the results on electrical resistivity r of the Y1x Gdx Co2 compounds (xt0:3), measured under pressure up to 10 GPa. The sample preparation procedure has been described in Ref. [2]. The clamp-type piston cylinder pressure cell for the measurements up to 2 GPa and the cubic anvil cell for the measurements up to 10 GPa were used for the resistivity measurements. Fig. 1 shows the pressure effect on electrical resistivity (PR ¼ Dr=r, DrðTÞ ¼ rðP; TÞ rð0; TÞ) and the magnetoresistivity (MR ¼ Dr=r, DrðTÞ ¼ rðB; TÞ rð0; TÞ) of Y1x Gdx Co2 alloys at 2 K, measured under pressure P ¼ 2 GPa and magnetic field B ¼ 15 T, respectively. In agreement with theoretical prediction the composition dependence of PR shows very good correlation with the dependence of magnetoresistivity on composition [2]. For the compound with the value of y just above 0.5, we expect that r increases with increasing P at first, and decreases with further increasing P. The pressure dependence of PR of Y0:82 Gd0:18 Co2 at 2 K measured by using the cubic anvil cell is shown in Fig. 2. r increases with
Fig. 2. Pressure dependence of PR of Y0:82 Gd0:18 Co2 . The dotted line is guide for the eye.
increasing P, having a maximum around 2 GPa and then decreases with increasing P. It means that y for the compound of x ¼ 0:18 decreases with increasing P, and crosses the value of y ¼ 0:5 around 2 GPa. As expected the pressure effect on r is qualitatively the same as the dependence of residual resistivity on alloy composition x [2]. In conclusion, both PR and MR at low temperatures are in agreement with the model predictions. The strong enhancement of r, the anomalous MR and the PR at low temperatures for the Y1x Gdx Co2 system are explained by means of the interplay of metamagnetic instability of Co 3d itinerant electrons and structural disorder within rare-earth subsystem. Acknowledgments We thank the Institute of Solid State Physics of Tokyo University, where part of this work was carried out under the joint research program. In part this work was supported by Grant no 05-02-17816a of Russian Foundation for Basic Research. References [1] T. Goto, K. Fukamichi, T. Sakakibara, H. Komatsu, Solid State Commun. 72 (1989) 945. [2] A.T. Burkov, A.Yu. Zyuzin, T. Nakama, K. Yagasaki, Phys. Rev. B 69 (2004) 144409. [3] T. Nakama, A.T. Burkov, M. Hedo, H. Niki, K. Yagasaki, J. Magn. Magn. Mater. 226–230 (2001) 671. [4] H. Yamada, J. Magn. Magn. Mater. 139 (1995) 162.