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Magnetic phase diagram of UNi2Si2 under magnetic field and high-pressure
Journal of Magnetism and Magnetic Materials 226}230 (2001) 585}587
Magnetic phase diagram of UNi Si under magnetic "eld and high-pressure F. Honda *, G. Oomi , P. Svoboda, A. Syshchenko, V. SechovskyH , S. Khmelevski, M. Divis\ , A.V. Andreev, N. Takeshita, N. Mo( ri, A.A. Menovsky Department of Physics, Faculty of Science, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan JLMS, Charles University and Institute of Physics ASCR, Prague, Czech Republic ISSP, University of Tokyo, Tokyo, Japan University of Amsterdam, Amsterdam, Netherlands
Abstract Measurements of electrical resistance under high pressure and neutron di!raction in high-magnetic "eld of single crystalline UNi Si have been performed. We have found the analogy between the p}¹ and B}¹ magnetic phase diagrams. It is also found that the propagation vector q of incommensurate antiferromagnetic phase decreases with 8 increasing magnetic "eld. A new pronounced pressure-induced incommensurate}commensurate magnetic phase transition has been detected. 2001 Elsevier Science B.V. All rights reserved. Keywords: High pressure; Magnetic "eld; Neutron di!raction; Electrical resistance; Magnetic phase diagram
UNi Si crystallizes in the ThCr Si -type structure, which consists of alternating layers of U and Ni Si slabs along the c-axis [1]. The magnetic phase diagram contains in zero "eld, three magnetic phases below ¹ "124 K all of which consist of ferromagnetic basal , planes of U moments parallel to the c-axis. Their stacking or modulation along the c-axis can be described by a propagation vector q"(0 0 q ) with di!erent values of the q component [2]: uncompensated (UAF) with q " X for T(43 K"T ; pure antiferromagnetic stacking (AF-1) with q "1 for 43(T(103 K"T and incomX mensurate (IC) for 103 (T(124 K states [2,3]. The IC phase is characterized by temperature dependent q comX ponent. In the present work, measurement of electrical resistance under high pressure and neutron di!raction at am-
* Corresponding author. Tel.: #81-92-726-4602; fax. #8192-726-4841. E-mail address: [email protected] (F. Honda).
bient pressure in high-magnetic "eld have been performed. A single crystal of UNi Si was grown by the triarc Czochralski technique. Neutron di!raction measurement was carried out with the E4 spectrometer at HMI Berlin. A magnetic "eld up to 14.5 T along the c-axis was applied. Electrical resistance with the current ic was measured by a standard four-probe method. Hydrostatic pressure was generated using standard piston-cylinder and cubic anvil type high-pressure cells. A detailed explanation of the high-pressure apparatus was reported elsewhere [4,5]. The temperature dependence of the c-axis electrical resistivity (¹) (resistance, R(¹)) under high pressure is displayed in Fig. 1. The phase transition from the AF-I phase to the UAF structure at T is accompanied by a pronounced increase of the resistivity. Also, here the sudden change of magnetic periodicity (q ) plays the 8 principal role. This transition exhibits a large temperature hysteresis, suggesting a "rst-order phase transition. In the magnetic phase diagram presented below, we took T as an average of phase transition temperatures
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 8 9 - 2
586
F. Honda et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 585}587
Fig. 1. Temperature dependence of electrical resistance under high pressure.
observed within cooling and heating processes, respectively. It can be seen that T shifts to higher temperature region with increasing pressure. In this pressure range, the rate of T increase is large, about 10 K/GPa. The hysteresis of R(¹) still exists at 3.0 GPa around T "77 K to disappear at 3.2 GPa. From these results we conclude that the intermediate AF-I phase is completely suppressed above 3.2 GPa. In order to show the e!ect of pressure on R(¹) more clearly, Fig. 2 displays the magni"ed R(¹) plots around ¹;¹ . ¹ decreases, and the shape of R(¹) around T becomes gradually more unclear with increasing pres sure. At 3.2 GPa, a small step-like decrease of resistivity with decreasing temperature appears (indicated by arrows in Fig. 2) around 100 K. This resistance anomaly can be attributed to the order}order magnetic phase transition between the IC and UAF phase (¹ ). '!@3 $ This point is supported by the fact that in the IC phase is larger than that in the UAF phase. At 0 GPa, no clear anomaly at ¹ is observed in R(¹) measurements '!@3 $ at high magnetic "eld (B'4 T) [6]. The UAF phase is further stabilized by external pressure also after the AF-I phase disappears. Here, results of the c-axis electrical resistance of UNi Si under high pressure up to 8 GPa, and neutron di!raction experiment in high magnetic "elds up to 15 T along the c-axis are compared and the p}¹ and B}¹ magnetic phase diagrams are constructed. From neutron di!raction experiment, it is found that the AF-I and IC phase become suppressed in a magnetic "eld of 4.5 and 15 T, respectively. Similarly, the resistivity measurements reveal that the AF-I phase disappears completely at 3.2 GPa (in zero "eld) and IC tends to be suppressed at higher pressure. This result is in qualitative agreement with the proposed p}¹ magnetic phase diagram, which is based on measurement up to 1.2 GPa [7]. The obtained magnetic phase diagrams of UNi Si in magnetic "eld at 0 GPa (left scale, full symbols) and under pressure (right
Fig. 2. R(¹) around T in expanded scale.
Fig. 3. Magnetic phase diagrams of UNi Si under pressure and magnetic "eld.
scale, open symbols) are shown in Fig. 3. The application of magnetic "eld and high pressure a!ect the electronic structure of UNi Si in a similar way. The analogy between the p}¹ and B}¹ diagrams is striking and motivates further analysis of "eld and pressure impact on the exchange integrals within suitable models, for example within the models of the ANNNI-type [8]. It is also found that the value of q in the IC phase 8 decreases and approaches to the value of , same as the one in the UAF phase. The q is changed not only by 8 temperature but also by magnetic "eld. The absence of any clear anomaly in R(¹) curve in high magnetic "elds at the UAF-IC transition probably re#ects the fact that the value of q in the IC phase is approximately equal to 8 that in UAF phase at ¹ . On the other hand, we '!@3 $ have clearly detected ¹ from the pronounced R(¹) '!@3 $ anomaly at high pressures which is expected to be due to a drastic change of q at ¹ . From this, one may 8 '!@3 $ deduce that the pressure dependence of q is small and 8 the q di!ers substantially from q at T under '! 3 $ '!@3 $ high pressure (+4 GPa). To con"rm this conclusion,
F. Honda et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 585}587
a neutron di!raction experiment under high pressure is strongly desirable. This work is supported "nancially by the grant GACR 106/99/0183. The author F.H. would like to thank the JSPS Research Fellowships for Young Scientists and G.O. also would like to thank the JSPS Japan#Europe Research Cooperative Program for "nancial supports.
References [1] L. Chelmicki, J. Leciejewicz, A. Zygmunt, J. Phys. Chem. Solids 46 (1985) 529.
587
[2] H. Lin, L. Rebelsky, M.F. Collins, J.D. Garrett, W.J.L. Buyers, Phys. Rev. B 43 (1991) 13232. [3] F. Honda, G. Oomi, T. Kagayama, A.V. Andreev, V. Sechovsky, L. Havela, M.I. Bartashevich, T. Goto, A.A. Menovsky, J. Magn. Magn. Mater. 177}181 (1998) 49. [4] G. Oomi, T. Kagayama, Physica B 239 (1997) 191. [5] N. Mo( ri, Y. Okayama, H. Takahashi, Y. Haga, and T. Suzuki, Jpn. J. Appl. Phys. Part 1 282 (1993) 182. [6] F. Honda, Ph.D. Thesis, Kumamoto University, Japan, 2000. [7] G. Quirion, F.S. Razavi, M.L. Plumer, J.D. Garrett, Phys. Rev. B 57 (1998) 5220. [8] A. Syshchenko et al., in: M.H. Manghnani et al. (eds.), Science and Technology of High Pressure 2 (2000) 832.
Magnetic phase diagram of UNi Si under magnetic "eld and high-pressure F. Honda *, G. Oomi , P. Svoboda, A. Syshchenko, V. SechovskyH , S. Khmelevski, M. Divis\ , A.V. Andreev, N. Takeshita, N. Mo( ri, A.A. Menovsky Department of Physics, Faculty of Science, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan JLMS, Charles University and Institute of Physics ASCR, Prague, Czech Republic ISSP, University of Tokyo, Tokyo, Japan University of Amsterdam, Amsterdam, Netherlands
Abstract Measurements of electrical resistance under high pressure and neutron di!raction in high-magnetic "eld of single crystalline UNi Si have been performed. We have found the analogy between the p}¹ and B}¹ magnetic phase diagrams. It is also found that the propagation vector q of incommensurate antiferromagnetic phase decreases with 8 increasing magnetic "eld. A new pronounced pressure-induced incommensurate}commensurate magnetic phase transition has been detected. 2001 Elsevier Science B.V. All rights reserved. Keywords: High pressure; Magnetic "eld; Neutron di!raction; Electrical resistance; Magnetic phase diagram
UNi Si crystallizes in the ThCr Si -type structure, which consists of alternating layers of U and Ni Si slabs along the c-axis [1]. The magnetic phase diagram contains in zero "eld, three magnetic phases below ¹ "124 K all of which consist of ferromagnetic basal , planes of U moments parallel to the c-axis. Their stacking or modulation along the c-axis can be described by a propagation vector q"(0 0 q ) with di!erent values of the q component [2]: uncompensated (UAF) with q " X for T(43 K"T ; pure antiferromagnetic stacking (AF-1) with q "1 for 43(T(103 K"T and incomX mensurate (IC) for 103 (T(124 K states [2,3]. The IC phase is characterized by temperature dependent q comX ponent. In the present work, measurement of electrical resistance under high pressure and neutron di!raction at am-
* Corresponding author. Tel.: #81-92-726-4602; fax. #8192-726-4841. E-mail address: [email protected] (F. Honda).
bient pressure in high-magnetic "eld have been performed. A single crystal of UNi Si was grown by the triarc Czochralski technique. Neutron di!raction measurement was carried out with the E4 spectrometer at HMI Berlin. A magnetic "eld up to 14.5 T along the c-axis was applied. Electrical resistance with the current ic was measured by a standard four-probe method. Hydrostatic pressure was generated using standard piston-cylinder and cubic anvil type high-pressure cells. A detailed explanation of the high-pressure apparatus was reported elsewhere [4,5]. The temperature dependence of the c-axis electrical resistivity (¹) (resistance, R(¹)) under high pressure is displayed in Fig. 1. The phase transition from the AF-I phase to the UAF structure at T is accompanied by a pronounced increase of the resistivity. Also, here the sudden change of magnetic periodicity (q ) plays the 8 principal role. This transition exhibits a large temperature hysteresis, suggesting a "rst-order phase transition. In the magnetic phase diagram presented below, we took T as an average of phase transition temperatures
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 8 9 - 2
586
F. Honda et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 585}587
Fig. 1. Temperature dependence of electrical resistance under high pressure.
observed within cooling and heating processes, respectively. It can be seen that T shifts to higher temperature region with increasing pressure. In this pressure range, the rate of T increase is large, about 10 K/GPa. The hysteresis of R(¹) still exists at 3.0 GPa around T "77 K to disappear at 3.2 GPa. From these results we conclude that the intermediate AF-I phase is completely suppressed above 3.2 GPa. In order to show the e!ect of pressure on R(¹) more clearly, Fig. 2 displays the magni"ed R(¹) plots around ¹;¹ . ¹ decreases, and the shape of R(¹) around T becomes gradually more unclear with increasing pres sure. At 3.2 GPa, a small step-like decrease of resistivity with decreasing temperature appears (indicated by arrows in Fig. 2) around 100 K. This resistance anomaly can be attributed to the order}order magnetic phase transition between the IC and UAF phase (¹ ). '!@3 $ This point is supported by the fact that in the IC phase is larger than that in the UAF phase. At 0 GPa, no clear anomaly at ¹ is observed in R(¹) measurements '!@3 $ at high magnetic "eld (B'4 T) [6]. The UAF phase is further stabilized by external pressure also after the AF-I phase disappears. Here, results of the c-axis electrical resistance of UNi Si under high pressure up to 8 GPa, and neutron di!raction experiment in high magnetic "elds up to 15 T along the c-axis are compared and the p}¹ and B}¹ magnetic phase diagrams are constructed. From neutron di!raction experiment, it is found that the AF-I and IC phase become suppressed in a magnetic "eld of 4.5 and 15 T, respectively. Similarly, the resistivity measurements reveal that the AF-I phase disappears completely at 3.2 GPa (in zero "eld) and IC tends to be suppressed at higher pressure. This result is in qualitative agreement with the proposed p}¹ magnetic phase diagram, which is based on measurement up to 1.2 GPa [7]. The obtained magnetic phase diagrams of UNi Si in magnetic "eld at 0 GPa (left scale, full symbols) and under pressure (right
Fig. 2. R(¹) around T in expanded scale.
Fig. 3. Magnetic phase diagrams of UNi Si under pressure and magnetic "eld.
scale, open symbols) are shown in Fig. 3. The application of magnetic "eld and high pressure a!ect the electronic structure of UNi Si in a similar way. The analogy between the p}¹ and B}¹ diagrams is striking and motivates further analysis of "eld and pressure impact on the exchange integrals within suitable models, for example within the models of the ANNNI-type [8]. It is also found that the value of q in the IC phase 8 decreases and approaches to the value of , same as the one in the UAF phase. The q is changed not only by 8 temperature but also by magnetic "eld. The absence of any clear anomaly in R(¹) curve in high magnetic "elds at the UAF-IC transition probably re#ects the fact that the value of q in the IC phase is approximately equal to 8 that in UAF phase at ¹ . On the other hand, we '!@3 $ have clearly detected ¹ from the pronounced R(¹) '!@3 $ anomaly at high pressures which is expected to be due to a drastic change of q at ¹ . From this, one may 8 '!@3 $ deduce that the pressure dependence of q is small and 8 the q di!ers substantially from q at T under '! 3 $ '!@3 $ high pressure (+4 GPa). To con"rm this conclusion,
F. Honda et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 585}587
a neutron di!raction experiment under high pressure is strongly desirable. This work is supported "nancially by the grant GACR 106/99/0183. The author F.H. would like to thank the JSPS Research Fellowships for Young Scientists and G.O. also would like to thank the JSPS Japan#Europe Research Cooperative Program for "nancial supports.
References [1] L. Chelmicki, J. Leciejewicz, A. Zygmunt, J. Phys. Chem. Solids 46 (1985) 529.
587
[2] H. Lin, L. Rebelsky, M.F. Collins, J.D. Garrett, W.J.L. Buyers, Phys. Rev. B 43 (1991) 13232. [3] F. Honda, G. Oomi, T. Kagayama, A.V. Andreev, V. Sechovsky, L. Havela, M.I. Bartashevich, T. Goto, A.A. Menovsky, J. Magn. Magn. Mater. 177}181 (1998) 49. [4] G. Oomi, T. Kagayama, Physica B 239 (1997) 191. [5] N. Mo( ri, Y. Okayama, H. Takahashi, Y. Haga, and T. Suzuki, Jpn. J. Appl. Phys. Part 1 282 (1993) 182. [6] F. Honda, Ph.D. Thesis, Kumamoto University, Japan, 2000. [7] G. Quirion, F.S. Razavi, M.L. Plumer, J.D. Garrett, Phys. Rev. B 57 (1998) 5220. [8] A. Syshchenko et al., in: M.H. Manghnani et al. (eds.), Science and Technology of High Pressure 2 (2000) 832.