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High field magnetization of the new heavy fermion compound U1.2Fe4Si9.7
Physica B 246—247 (1998) 456—459
High field magnetization of the new heavy fermion compound U Fe Si 1.2 4 9.7 Satoru Noguchi!,*, Kiichi Okuda!, Melike Abliz", Kenji Goto", Koichi Kindo", Yoshinori Haga#, Etsuji Yamamoto#, Yoshichika O1 nuki$ ! Department of Physics and Electronics, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 593, Japan " Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka 560, Japan # Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai 319-11, Japan $ Department of Physics, Faculty of Science, Osaka University, Toyonaka 560, Japan
Abstract A newly synthesized compound U Fe Si is characterized by a layer structure of the disordered U-atoms. AC and 1.2 4 9.7 DC magnetization measurements reveal that the compound shows the double-peak spin-glass behavior at 19 and 23 K. High field magnetization measurements up to 51 T were performed for the compound at 4.2 K. It shows no tendency of saturation or metamagnetic transition, but a monotonic increase with increasing field. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Disordered layer structure; Spin glass behavior; High field magnetization; AC susceptibility; U Fe Si 1.2 4 9.7
Recently, a single crystal of a new ternary silicide U Fe Si was synthesized by our group [1]. 1.2 4 9.7 It crystallizes in the hexagonal layered structure with the lattice parameters of a"3.956(1) A_ and c"15.055(2) A_ . The structure is characterized by stacking of the Fe, Si(1), Si(2) and U—Si(3) layers where U and Si(3) atoms partially occupy 2c and 6h sites, respectively. Therefore, U atoms form a disordered two-dimensional network in the layer.
* Corresponding author. Fax:#81-722-59-3340; e-mail: [email protected].
The U Fe Si has an electronic specific heat 1.2 4 9.7 coefficient, c of 180 mJ/K2 mol U which suggests the compound belongs to the category of heavyfermion systems. Magnetic susceptibility follows the Curie—Weiss law down to 40 K with the effective magnetic moment of 2.4k /U which is B much smaller than the free ion values of 3.62 and 3.58k /U for U3` and U4`, respectively. A weak B ferromagnetic moment was observed in magnetization measurements below 25 K, while no clear peak due to a phase transition was observed down to 4.5 K in the specific heat measurements. This suggests that the magnetic order may be suppressed by the disordered structure of the U—Si layer and
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 9 6 0 - 5
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
the weak ferromagnetic behavior might be attributed to a frozen ferromagnetic moment in the spin glass state. The heavy fermion state comes from a mixed electronic band near the Fermi surface composed of the conduction and f-electrons giving quasiparticles with large effective mass. This means that the localized f-electron spins are strongly reduced and the Kondo effect plays an important role for the formation of the heavy fermion state. By applying a strong magnetic field, the heavy fermion state is destroyed and the exchange interactions between the recovered local moments become important. Indeed, metamagnetic transitions are observed in many heavy fermion compounds [2—4]. In the disordered system, however, there could be a broad distribution of Kondo temperatures, which leads to the destruction of coherence [5,6]. In this case, it is very interesting whether the itinerant state such as heavy fermion is destroyed by a strong field or not. In this paper, we report the observation of the spin-glass behavior in low fields and the high-field magnetization for the compound U Fe Si under fields up to 51 T. 1.2 4 9.7 The sample of the U Fe Si single crystal was 1.2 4 9.7 grown using the Czochralski method in a tetraarc furnace and cut along the a and c axes into a square shape of 2.3]2.3]1.0 mm3. An eddy current effect during AC susceptibility and pulsed field measurements was negligible because of the large resistivity of 0.3—0.6 m) cm in the compound [1]. Measurements of AC susceptibility and DC magnetization were performed with a conventional SQUID magnetometer between 4 and 300 K in magnetic fields up to 5 T. The high field magnetization measurement up to 51 T was performed at High Magnetic Field Laboratory, Research Center for Materials Science at Extreme Conditions, Osaka University. Temperature dependence of the inverse magnetization divided by the field, H/M, for HEc is shown in Fig. 1. Above 40 K the H/M shows no field dependence and follows the Curie—Weiss law as shown by a solid line in Fig. 1. This means that the magnetization shows a linear dependence on the applied field up to 5 T. The Curie— Weiss temperature of !29 K and the effective
457
Fig. 1. Temperature dependence of inverse magnetization divided by the measurement field for U Fe Si . Solid line is 1.2 4 9.7 the Curie—Weiss fitting in the temperature range between 40 and 300 K.
moment of 2.4k are coincident with our previous B results [1]. The H/M at low field measurements deviated to small values below 30 K, which suggests that the weak ferromagnetic magnetization was induced, as mentioned below. On the other hand, the data at 4 or 5 T have no anomaly around 30 K and follow the Curie—Weiss law down to 4 K as fitted above 40 K. This suggests that the compound might be still paramagnetic under the field above 4 T. Fig. 2 shows a magnetization process of U Fe Si at 4 K. First the magnetization was 1.2 4 9.7 induced from zero with increasing magnetic field and then traced a hysteresis loop. The coercive field of 0.1 T and the remanence of 0.009k /U were obB tained. The hysteresis loop was reduced with increasing temperature and vanished at 25 K. Such a small magnetic moment, however, cannot be explained as a simple ferromagnetic ordering of localized spins but a frozen ferromagnetic moment in the spin-glass state. So, we measured the temperature dependence of the AC susceptibility as a parameter of the DC bias field in a field-cooled—warmed process for HEa and HEc. The AC driving field was 1 Oe in amplitude with the frequency of 115.7 Hz. As shown in Fig. 3, double peaks are clearly observed for both field directions; a small peak around 20 K and
458
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
Fig. 2. Low field magnetization process of U Fe Si for HEc 1.2 4 9.7 at 4 K.
Fig. 4. Frequency dependence of the real part of the AC susceptibility of U Fe Si for h Ea and h Ec. 1.2 4 9.7 !# !#
Fig. 3. The real part of the AC susceptibility of U Fe Si as 1.2 4 9.7 a parameter of DC bias field for HEa and HEc.
a large peak at 23 K denoted by ¹ and ¹ , 4'1 4'2 respectively. Under a few gauss of DC field, the peaks become much smaller and slightly shift to higher temperatures. This behavior is one of the
characteristic properties of spin-glass transition [7]. The double transition might be associated with the longitudinal and transverse spin freezing for a Heisenberg spin-glass with uniaxial local anisotropy [8]. A small shoulder was observed at 22 K only for HEc. The dependence of s@ (¹) on the frequency u of !# AC field was measured under zero bias field, as shown in Fig. 4. With increasing u, the magnitude of the peaks decreases and ¹ shifts to higher 4'2 temperatures. The shift of ¹ is very small. This 4'1 suggests that the characteristic relaxation time related to the spin-glass transition at ¹ is larger 4'2 than that at ¹ . 4'1 The high field magnetization of the U Fe Si 1.2 4 9.7 was measured at 4.2 K for HEc, as shown in Fig. 5. It appears no tendency of saturation or metamagnetic transition, but monotonic increase with increasing field. Induced magnetic moment at 51 T is 0.82k /U which is much smaller than the B free ion values of U3` or U4`. This magnetization curve is hardly fitted to the Brillouin function with
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
459
the AC susceptibility measurements. The high field magnetization measurements show no tendency of saturation or metamagnetic transition but monotonic increase with increasing field. The disordered structure of U—Si layer may suppress the localized character and may assist the itinerancy. This work was partially supported by a Grantin-Aid for Scientific Research on Priority Areas “Physical Properties of Strongly Correlated Electron Systems” from the Ministry of Education, Science, Sports and Culture of Japan. Fig. 5. High field magnetization of U Fe Si for HEc at 1.2 4 9.7 4.2 K.
the exchange parameter estimated from the Curie— Weiss temperature. Instead, it may be explained as the sum of weak ferromagnetic and Pauli paramagnetic contribution: the former shows a saturation of 0.2k /U while the latter shows a linear increase B with the gradient of 0.0125k /U/T which corresB ponds to a susceptibility of 0.107]10~4 cm3/g. Anyway, more experiments such as the temperature dependence of the high field magnetization are needed. In conclusion, the double spin-glass transitions were found at 19 and 23 K in the U Fe Si from 1.2 4 9.7
References [1] S. Noguchi, K. Okuda, T. Adachi, Y. Haga, E. Yamamoto, Y. O1 nuki, J. Phys. Soc. Japan 66 (1997) 2572. [2] K. Sugiyama, H. Fuke, K. Kindo, K. Shimohata, A. Menovsky, J.A. Mydosh, M. Date, J. Phys. Soc. Japan 59 (1990) 3331. [3] J.J.M. Franse, P.H. Frings, A. de Visser, A. Menovsky, Physica B 126 (1984) 116. [4] J.M. Mignot, J. Flouquet, P. Haen, F. Lapierre, L. Puech, J. Voiron, J. Magn. Magn. Mater. 76&77 (1988) 97. [5] O.O. Bernal, D.E. MacLaughlin, H.G. Lukefahr, B. Andraka, Phys. Rev. Lett. 75 (1995) 2023. [6] E. Miranda, V. Dobrosavljevic´, G. Kotliar, Phys. Rev. Lett. 78 (1997) 290. [7] V. Cannella, J.A. Mydosh, Phys. Rev. B 6 (1972) 4220. [8] D.M. Cragg, D. Sherrington, Phys. Rev. Lett. 49 (1982) 1190.
High field magnetization of the new heavy fermion compound U Fe Si 1.2 4 9.7 Satoru Noguchi!,*, Kiichi Okuda!, Melike Abliz", Kenji Goto", Koichi Kindo", Yoshinori Haga#, Etsuji Yamamoto#, Yoshichika O1 nuki$ ! Department of Physics and Electronics, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 593, Japan " Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka 560, Japan # Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai 319-11, Japan $ Department of Physics, Faculty of Science, Osaka University, Toyonaka 560, Japan
Abstract A newly synthesized compound U Fe Si is characterized by a layer structure of the disordered U-atoms. AC and 1.2 4 9.7 DC magnetization measurements reveal that the compound shows the double-peak spin-glass behavior at 19 and 23 K. High field magnetization measurements up to 51 T were performed for the compound at 4.2 K. It shows no tendency of saturation or metamagnetic transition, but a monotonic increase with increasing field. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Disordered layer structure; Spin glass behavior; High field magnetization; AC susceptibility; U Fe Si 1.2 4 9.7
Recently, a single crystal of a new ternary silicide U Fe Si was synthesized by our group [1]. 1.2 4 9.7 It crystallizes in the hexagonal layered structure with the lattice parameters of a"3.956(1) A_ and c"15.055(2) A_ . The structure is characterized by stacking of the Fe, Si(1), Si(2) and U—Si(3) layers where U and Si(3) atoms partially occupy 2c and 6h sites, respectively. Therefore, U atoms form a disordered two-dimensional network in the layer.
* Corresponding author. Fax:#81-722-59-3340; e-mail: [email protected].
The U Fe Si has an electronic specific heat 1.2 4 9.7 coefficient, c of 180 mJ/K2 mol U which suggests the compound belongs to the category of heavyfermion systems. Magnetic susceptibility follows the Curie—Weiss law down to 40 K with the effective magnetic moment of 2.4k /U which is B much smaller than the free ion values of 3.62 and 3.58k /U for U3` and U4`, respectively. A weak B ferromagnetic moment was observed in magnetization measurements below 25 K, while no clear peak due to a phase transition was observed down to 4.5 K in the specific heat measurements. This suggests that the magnetic order may be suppressed by the disordered structure of the U—Si layer and
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 9 6 0 - 5
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
the weak ferromagnetic behavior might be attributed to a frozen ferromagnetic moment in the spin glass state. The heavy fermion state comes from a mixed electronic band near the Fermi surface composed of the conduction and f-electrons giving quasiparticles with large effective mass. This means that the localized f-electron spins are strongly reduced and the Kondo effect plays an important role for the formation of the heavy fermion state. By applying a strong magnetic field, the heavy fermion state is destroyed and the exchange interactions between the recovered local moments become important. Indeed, metamagnetic transitions are observed in many heavy fermion compounds [2—4]. In the disordered system, however, there could be a broad distribution of Kondo temperatures, which leads to the destruction of coherence [5,6]. In this case, it is very interesting whether the itinerant state such as heavy fermion is destroyed by a strong field or not. In this paper, we report the observation of the spin-glass behavior in low fields and the high-field magnetization for the compound U Fe Si under fields up to 51 T. 1.2 4 9.7 The sample of the U Fe Si single crystal was 1.2 4 9.7 grown using the Czochralski method in a tetraarc furnace and cut along the a and c axes into a square shape of 2.3]2.3]1.0 mm3. An eddy current effect during AC susceptibility and pulsed field measurements was negligible because of the large resistivity of 0.3—0.6 m) cm in the compound [1]. Measurements of AC susceptibility and DC magnetization were performed with a conventional SQUID magnetometer between 4 and 300 K in magnetic fields up to 5 T. The high field magnetization measurement up to 51 T was performed at High Magnetic Field Laboratory, Research Center for Materials Science at Extreme Conditions, Osaka University. Temperature dependence of the inverse magnetization divided by the field, H/M, for HEc is shown in Fig. 1. Above 40 K the H/M shows no field dependence and follows the Curie—Weiss law as shown by a solid line in Fig. 1. This means that the magnetization shows a linear dependence on the applied field up to 5 T. The Curie— Weiss temperature of !29 K and the effective
457
Fig. 1. Temperature dependence of inverse magnetization divided by the measurement field for U Fe Si . Solid line is 1.2 4 9.7 the Curie—Weiss fitting in the temperature range between 40 and 300 K.
moment of 2.4k are coincident with our previous B results [1]. The H/M at low field measurements deviated to small values below 30 K, which suggests that the weak ferromagnetic magnetization was induced, as mentioned below. On the other hand, the data at 4 or 5 T have no anomaly around 30 K and follow the Curie—Weiss law down to 4 K as fitted above 40 K. This suggests that the compound might be still paramagnetic under the field above 4 T. Fig. 2 shows a magnetization process of U Fe Si at 4 K. First the magnetization was 1.2 4 9.7 induced from zero with increasing magnetic field and then traced a hysteresis loop. The coercive field of 0.1 T and the remanence of 0.009k /U were obB tained. The hysteresis loop was reduced with increasing temperature and vanished at 25 K. Such a small magnetic moment, however, cannot be explained as a simple ferromagnetic ordering of localized spins but a frozen ferromagnetic moment in the spin-glass state. So, we measured the temperature dependence of the AC susceptibility as a parameter of the DC bias field in a field-cooled—warmed process for HEa and HEc. The AC driving field was 1 Oe in amplitude with the frequency of 115.7 Hz. As shown in Fig. 3, double peaks are clearly observed for both field directions; a small peak around 20 K and
458
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
Fig. 2. Low field magnetization process of U Fe Si for HEc 1.2 4 9.7 at 4 K.
Fig. 4. Frequency dependence of the real part of the AC susceptibility of U Fe Si for h Ea and h Ec. 1.2 4 9.7 !# !#
Fig. 3. The real part of the AC susceptibility of U Fe Si as 1.2 4 9.7 a parameter of DC bias field for HEa and HEc.
a large peak at 23 K denoted by ¹ and ¹ , 4'1 4'2 respectively. Under a few gauss of DC field, the peaks become much smaller and slightly shift to higher temperatures. This behavior is one of the
characteristic properties of spin-glass transition [7]. The double transition might be associated with the longitudinal and transverse spin freezing for a Heisenberg spin-glass with uniaxial local anisotropy [8]. A small shoulder was observed at 22 K only for HEc. The dependence of s@ (¹) on the frequency u of !# AC field was measured under zero bias field, as shown in Fig. 4. With increasing u, the magnitude of the peaks decreases and ¹ shifts to higher 4'2 temperatures. The shift of ¹ is very small. This 4'1 suggests that the characteristic relaxation time related to the spin-glass transition at ¹ is larger 4'2 than that at ¹ . 4'1 The high field magnetization of the U Fe Si 1.2 4 9.7 was measured at 4.2 K for HEc, as shown in Fig. 5. It appears no tendency of saturation or metamagnetic transition, but monotonic increase with increasing field. Induced magnetic moment at 51 T is 0.82k /U which is much smaller than the B free ion values of U3` or U4`. This magnetization curve is hardly fitted to the Brillouin function with
S. Noguchi et al. / Physica B 246–247 (1998) 456—459
459
the AC susceptibility measurements. The high field magnetization measurements show no tendency of saturation or metamagnetic transition but monotonic increase with increasing field. The disordered structure of U—Si layer may suppress the localized character and may assist the itinerancy. This work was partially supported by a Grantin-Aid for Scientific Research on Priority Areas “Physical Properties of Strongly Correlated Electron Systems” from the Ministry of Education, Science, Sports and Culture of Japan. Fig. 5. High field magnetization of U Fe Si for HEc at 1.2 4 9.7 4.2 K.
the exchange parameter estimated from the Curie— Weiss temperature. Instead, it may be explained as the sum of weak ferromagnetic and Pauli paramagnetic contribution: the former shows a saturation of 0.2k /U while the latter shows a linear increase B with the gradient of 0.0125k /U/T which corresB ponds to a susceptibility of 0.107]10~4 cm3/g. Anyway, more experiments such as the temperature dependence of the high field magnetization are needed. In conclusion, the double spin-glass transitions were found at 19 and 23 K in the U Fe Si from 1.2 4 9.7
References [1] S. Noguchi, K. Okuda, T. Adachi, Y. Haga, E. Yamamoto, Y. O1 nuki, J. Phys. Soc. Japan 66 (1997) 2572. [2] K. Sugiyama, H. Fuke, K. Kindo, K. Shimohata, A. Menovsky, J.A. Mydosh, M. Date, J. Phys. Soc. Japan 59 (1990) 3331. [3] J.J.M. Franse, P.H. Frings, A. de Visser, A. Menovsky, Physica B 126 (1984) 116. [4] J.M. Mignot, J. Flouquet, P. Haen, F. Lapierre, L. Puech, J. Voiron, J. Magn. Magn. Mater. 76&77 (1988) 97. [5] O.O. Bernal, D.E. MacLaughlin, H.G. Lukefahr, B. Andraka, Phys. Rev. Lett. 75 (1995) 2023. [6] E. Miranda, V. Dobrosavljevic´, G. Kotliar, Phys. Rev. Lett. 78 (1997) 290. [7] V. Cannella, J.A. Mydosh, Phys. Rev. B 6 (1972) 4220. [8] D.M. Cragg, D. Sherrington, Phys. Rev. Lett. 49 (1982) 1190.