Magnetoresistance and high-field magnetization of Ce0.5La 0.5B6 and Ce0.7La 0.3B6 single crystals

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Journal of Magnetism and Magnetic Materials 177-181 (1998)429-430

Journal of magnetism and magnetic

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Magnetoresistance and high-field magnetization of Ceo.sLao.sB6 and Ceo.vLao.3B6 single crystals M. Hiroi a'*, M. Sera a, T. Sakon a, H. Nojiri a, N. Kobayashi a, M. Motokawa a, S. Kunii b alnstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendal 980-77, Japan bDepartment of Physics, Faculty of Science, Tohoku Universi~, Sendal 980-77, Japan

Abstract The magnetic phase diagrams of Ceo.vLao.3B 6 and Ceo.sLao.sB6 were studied in detail by magnetoresistance measurements in a temperature range between 0.4 and 4 K. Unusual magnetic phase diagrams were revealed. High-field magnetization measurements were also performed for Ceo.sLa0.sB6. @ 1998 Elsevier Science B.V. All rights reserved. Keywords: Kondo systems; Quadrupole ordering; Magnetic phase diagram

CexLat -xB6 is a typical Kondo system with TK ~ 1 K. In CeB6 three phases exist. Phase I: the paramagnetic phase above T o = 3.3 K. Phase II: the antiferro-quadrupolar (AFQ) ordered state between T o = 3.3 K and TN = 2.3 K. Phase III: the antiferro magnetic (AF) state below TN. Recently, unusual magnetic phase diagrams of CexLal xB6 (x = 0.5, 0.75) have been revealed [1-6]. In Ceo.75Lao.25B6 a new phase called Phase IV, which is considered to be an AF phase, was discovered besides these three phases. To study these peculiar magnetic phase diagrams, the magnetoresistance of CexLal_xB~ (x = 0.5, 0.7) for HI1(1 0 0 ) and (1 1 0) and the highfield magnetization of Ceo.sLao.sB6 were measured. The single crystals were prepared by the floating-zone method. The electrical resistivity was measured by the usual four-probe low-frequency AC method in the temperature range between 0.4 and 4 K in a magnetic field up to 5 T. High-field magnetization was measured with a pulsed magnet up to 30 T in a 3He refrigerator. Fig. 1 shows the temperature dependence of the resistivity of Ceo.TLao.3B6 for HII(1 1 0). In both cases for H(I 1 0) and HII(1 0 0) four phases I, II, III, and IV are recognized. The overall feature is similar to those of Ceo.vsLa0.25B6 and for both field directions. In H = 0 only one anomaly is observed in the p - T curve at TN~ = 1.45 K in a temperature range studied here and in

* Corresponding author. Fax: + 81 22 215 2026; e-mail: [email protected].

H = 0.8 T two anomalies appear at TN1 = 1.4 K, and TN2 = 1.05 K, where TNx and TN2 are transition temperatures from Phase IV to I and Phase III to IV, respectively. With further increase in the magnetic field, these two transition temperatures, TN1 and TN2 approach each other and they seem to coincide at H ~ 1.1 T and T ~ 1.3 K. This point looks like a tetra-critical point (TP). Above this field, two transition temperatures become observed, which separate from each other with increasing field. The higher and the lower transition

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Fig. 1. Temperature dependence of the electrical resistivity p of Ceo.TLa0.3B6 in the magnetic field along the (1 1 0> direction. The origins are shifted by 20 p f2 cm for clarity. Arrows indicate phase transitions.

0304-8853/98/$19+00 .~; 1998 Elsevier Science B.V. All rights reserved PI1 S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 9 7 3 - 6

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&L Hiroi et al. /'Journal oJ'Magnetism and Magnetic' Materials 177-181 (1998) 429 430 '

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Fig. 3. Magnetization of Ceo.sLao.sB6 for HII(1 0 0>. 1 \ 2 T ( K ) iv~,=o.:~ Fig. 2. Magnetic phase diagram of CexLal xB 6 (x - 0.5, 0.7, 0.75) for HII(1 1 0>. Transition points for x = 0.5 (~l,), 0.7 (O) and 0.75 (T) determined from resistivity measurements are shown. Phase boundary I IV for x = 0.5 is shown by hatched lines because it was not clearly defined. Phases I, II, IIl, and IV exist for x - 0.7 and 0.75, while for x = 0.5 Phase III does not exist in HII(I 1 0).

temperatures are identified as TQ (transition I-II) and TN (transition II-III), respectively. TN decreases with increasing magnetic field for H H ( l l 0 > but for H[[(00 1> the unusual increase of TN up to ~ 2.3 T is found as in Ce0.vsLao.25B 6. The p - T curves around 1.1 T indicate rather complicated phase boundaries in the vicinity of TP. In Phase IV a characteristic flat magnetoresistance is observed, which is different from the small negative magnetoresistance in Phase I or a rapid drop where the system enters into Phase II or IIl. The temperature and magnetic field dependences of p of Ceo.5Lao.sB6 were also measured for Hit(0 0 1) and HI] (1 1 0). In the temperature dependence of p at H = 0, in contrast to Ceo.TLao 3B6, any clear anomaly is not seen down to the lowest temperature studied here. But the magnetoresistance below ~ 1 K in a low-field region becomes small, being different from the negative magnetoresistance above ~ 1 K. This contradicts the behavior expected for K o n d o effect and indicates that the A F phase, IV, is realized below ~ 1 K. The broad transition might be due to the large a m o u n t of La substitution. In Ceo.sLao.sB6 Phase III exists for H[I(00 1) but is rapidly suppressed as the field is tilted from (0 0 1 > and does not exist for HII [1, 2, 6]. The magnetic phase diagrams of Ceo.sLao.sB6, Ceo.TLao.3B6, and Ceo.TsLao.zsB6 for HII obtained from our magnetoresistance measurements are shown in Fig. 2 [5,6]. In Ceo.TLao.3B6, and Ceo.vsLao.25B6, the four phases exist but in Ceo.sLao.sB6 Phase III does not exist. While for HI[ the four phases exist in these three compounds. In Ceo.sLao sB6 the transition I - I V is broad, so the transition temperatures are shown by hatched lines. The phase boundary 1 IV does not depend on the field directions. The phase

boundary I - I I and I I I - I V slightly depend on the field direction, while a large dependence on the field directions of the phase boundary I I - I I I is observed. The anisotropy of the phase boundaries is c o m m o n for CexLal xB6 [ 5 ] . Htn 2.5 T, which is fairly larger c for x = 0.75 is than H¢<1 I l l - 1!1 O> 1.3 T for x = 1, but Hc f o r x = 0 . 7 and that for x = 0.75 are not so different. In Ceo.sLao.sB6 Phase IlI does not exist and H¢v n is ~ 1.7T for Hp[(1 1 0>, which indicates the instability of Phase III depending on the applied field direction. Phase IV exists in a narrow temperature and magnetic field region for x = 0.75 but extends to 0 K for x = 0.5 and 0.7. This is seen also for Hrl<100>. The T P seems to exist for x = 0.7 and 0.75 and it is almost fixed as the field direction changes. The present detailed studies, however, reveal that in Ceo.TLao.3B6 the phase boundaries for H[I <1 1 0> are rather complicated and the phase boundary I I - I I I seems to terminate at a little lower temperature than TP. The existence of T P is less clear for HIl than for HH < 1 0 0 > . The phase boundary I l l - I V takes a maximum at a temperature slightly lower than T P and the boundary I - I I takes a minimum at a temperature slightly higher than TP. The minimum in the phase boundary I(IV) II is also seen in Ceo.sLao.sB6. These results may reflect the competition between magnetic and quadrupolar interactions. F o r Ceo.sLa0.sB6 T o was found to increase with increasing field up to 15 T [5]. To study the phase boundary in a high-field region, the magnetization at 0.4 K up to 30 T for HII was measured in a pulsed magnet, as shown in Fig. 3. After entering Phase II above 4 T, no anomaly is found up to 30 T. This suggests that the critical field of Phase II is expected to be larger than 30 T, if it is closed. References [1] S. Nakamura et al., J. Phys. Soc. Japan 64 (1995) 3941. [2] T. Goto, private communication. I-3] T. Sakakibara et al., Physica B 230 232 (1997) 307. 1-4] T. Sakakibara, J. Phys. Soc. Japan 66 (1997) 2268. 1-5] M. Hiroi et al., Phys. Rev. B 55 (1997) 8339. [6] M. Hiroi et al., J. Phys. Soc. Japan 66 (1997) 1762.