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Magnetic specific heat of a URhGe single crystal
Physica B 281&282 (2000) 223}225
Magnetic speci"c heat of a URhGe single crystal I.H. Hagmusa!, K. Prokes\ ",#,*, Y. Echizen$, T. Takabatake$, T. Fujita$, J.C.P. Klaasse!, E. BruK ck!, V. SechovskyH #, F.R. de Boer! !Van der Waals}Zeeman Instituut, Universiteit van Amsterdam 1018 XE, The Netherlands "Abteilung NE, Hahn-Meitner-Institute, Glienicker Strasse 100, 141 09 Berlin, Germany #Department of Electronic Structures, Charles University, 121 16 Praha 2, Czech Republic $ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Abstract URhGe crystallizes in the orthorhombic TiNiSi-type of structure and has been studied untill now only in polycrystalline form. Here, we report on the low-temperature speci"c heat of URhGe measured on a single crystal. The compound orders ferromagnetically at ¹ "9.6 K. Because of this low magnetic-ordering temperature, the speci"c heat c cannot C 1 be described by the usual expression c¹#a¹3, also at low temperatures. Up to 7 K, c is satisfactorily described by the 1 relationship c¹#b¹2, which must be associated with the temperature depenedence of the magnetization in this temperature interval. The value of c amounts to 164 mJ/mol K2. We have also studied the e!ect on c of an external U 1 magnetic "eld applied along the principal axes. The results are discussed in terms of the strong magnetic anisotropy of URhGe. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Speci"c heat; Ferromagnetism; Actinides; URhGe
URhGe belongs to the group of U¹X (T"late transition metal, X"Si or Ge) compounds crystallizing in the orthorhombic TiNiSi-type of structure [1]. In this structure, the U atoms form zig-zag chains running along the a-axis. The development of the magnetic properties of the UTX compounds when the d states of the transitionmetal atoms are increasingly "lled in a (3d, 4d or 5d) series of T elements, can be understood in terms of reduction of the 5f-ligand hybridization. URhGe is situated at the borderline between paramagnetic compounds (UCoGe, URuGe) [1] and magnetically ordered compounds (UIrGe and UPdGe) in which anisotropic hybridization between 5f states and other electronic states causes extremely large magnetic anisotropy [1,2]. All bulk properties of URhGe point to ferromagnetic (F) order below ¹ "9.5 K. Evidence for this comes C from the diverging susceptibility and the appearance of
* Corresponding author. Tel.: #49-30-8062-2804; fax: #4930-8062-3172. E-mail address: [email protected] (K. Prokes\ )
a spontaneous magnetization at lower temperatures [3}5], a peak in the c /¹ versus ¹ curve [5] and a drop 1 of the electrical resistivity below ¹ [4]. The physical C properties at low temperatures are reported to be very sensitive to applied magnetic "eld. A spontaneous moment of about 0.3 l /f.u. was derived from free-powder B high-"eld magnetization data measured at 4.2 K. [4]. A strong magnetic anisotropy in URhGe is documented by the clear di!erence between the free-powder and "xed-powder-magnetization curves. The neutron powder-di!raction experiments at low temperatures have been interpreted in terms of a canted ferromagnetic structure with a ferromagnetic component of 0.43 l along the B c-axis and an antiferromagnetic component of 0.26 l B along the a-axis [6]. In the present paper, we report on the speci"c heat of single-crystalline URhGe in magnetic-"elds up to 15 T. A single crystal of URhGe was grown from a stoichiometric melt by a modi"ed Czochralski technique in a continuously gettered Ar atmosphere. At least 99.95% pure materials were used in the preparation. No subsequent heat treatment was given to the crystal. The
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 1 9 0 - 4
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I.H. Hagmusa et al. / Physica B 281&282 (2000) 223}225
Fig. 1. Temperature dependence of the speci"c heat in c/¹ versus ¹ representation in various "elds up to 15 T applied along (a) the a-axis, (b) the b-axis and (c) the c-axis. In insets, the "eld dependence of c@ is given, determined by "tting the data to a c@#b¹ dependence.
quality of the crystal was checked by Laue X-ray technique and by electron microprobe analysis (EPMA). The temperature dependence of the speci"c heat was measured by a semi-adiabatic heat-pulse method between 0.4 and 50 K in "elds up to 15 T applied along the principal axes. The temperature dependence of the speci"c heat divided by temperature c /¹ at various "elds up to 15 T 1 applied along the a-axis are shown in Fig. 1(a). As can be seen, the zero-"eld curve is dominated by a pronounced peak centered at 9.6 K. This anomaly agrees well with the temperature of the magnetic phase transition derived from the magnetic susceptibility [5]. The low-temperature part of c /¹ cannot be described by the usual 1 relation c /¹"c#a¹2, because at low temperatures 1 magnetic excitations strongly contribute to the speci"c heat of URhGe. Instead, we "nd a relationship c /¹"c@#b¹, from the lowest temperature measured 1 up to 7 K. The best "t of the zero-"eld data up to 5 K leads to c@"164.2$0.2 mJ/mol K2. As the applied U magnetic "eld is increased the anomaly smears out and shifts to higher temperatures in agreement with the fer-
romagnetic order in URhGe. For all applied magnetic "elds, the type of the temperature dependence remains the same as for zero "eld and the c@ coe$cient is slightly reduced with increasing "eld (c@"161.6 mJ/mol K2 at U 15 T). To obtain the magnetic entropy, we have determined the magnetic part of the speci"c heat c as the 1,. di!erence between the measured speci"c heat and an estimated contribution of the lattice to the speci"c heat which was approximated by a sum of two Debye functions (hU "140 K and hR)@G%"260 K). Together with D D a temperature-independent electronic-speci"c-heat coef"cient of about 20 mJ/mol K2 this gives good agreeU ment with the zero-"eld data at high temperatures (¹'20 K). By integrating c /¹ versus ¹ up to 20 K 1,. we obtain S "0.4 Rln2. Such a low value suggests . strongly delocalized U moments. In Fig. 1(b) we show the temperature dependence of the speci"c heat of URhGe measured in magnetic "elds up to 15 T applied along the b-axis. Comparison with the data for the a-axis (Fig. 1(a)) shows that the zero-"eld anomaly for the b-axis is much more sensitive to magnetic "elds. Already in a "eld of 1 T, it is signi"cantly smeared out and shifted to higher temperatures. Also the coe$cient c@ is more sensitive to the applied magnetic "eld and with increasing "eld it decreases exponentially, at 15 T amounting to 73% of its zero-"eld value. This supports the idea that a large part of the low-temperature speci"c heat is of magnetic origin. In Fig. 1(c) we show the c /¹ versus ¹ curves of P URhGe measured in "elds up to 15 T applied along the c-axis. It is clear that data taken in "elds along the b- and c-axis are quite similar. This is in agreement with magnetic measurements that show that the b- and c-axis form an easy-magnetization plane [2]. Also for the c-axis the anomaly shifts to higher temperatures and it is smeared out already in rather low "elds. The coe$cient c@ decreases exponentially with applied "eld and at 15 T it attains 81% of its zero-"eld value. On the basis of thermodynamics the speci"c heat of magnetization c is proportional to dM2/d¹. There1,. fore, the ¹2 dependence of c which is experimentally 1,. observed in URhGe up to &7 K, which is about 70% of the Curie temperature, implies that in this remarkably large temperature interval the temperature dependence of M2 of URhGe should follow an approximate ¹3 dependence. This will further be investigated. It is clear that for a more elaborate analysis of the temperature dependence of c in the various magnetic "elds applied along the 1 di!erent crystallographic directions, the complex spin con"guration of URhGe [6] should be taken into account.
Acknolwedgements This work has been supported in part by JSPS and by GACR (grant no. 202/99/0184).
I.H. Hagmusa et al. / Physica B 281&282 (2000) 223}225
References [1] V. SechovskyH , L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, North Holland, Amsterdam, 1998, p. 7.
[2] [3] [4] [5] [6]
225
K. Prokes et al., Phys. Rev. B. F.R. de Boer et al., Physica B 163 (1990) 175. V.H. Tran, R. Troc, Phys. Rev. B 57 (1998) 11592. K.H.J. Buschow et al., J. Appl. Phys. 67 (1990) 5215. V.H. Tran et al., J. Magn. Magn. Mater. 186 (1998) 81.
Magnetic speci"c heat of a URhGe single crystal I.H. Hagmusa!, K. Prokes\ ",#,*, Y. Echizen$, T. Takabatake$, T. Fujita$, J.C.P. Klaasse!, E. BruK ck!, V. SechovskyH #, F.R. de Boer! !Van der Waals}Zeeman Instituut, Universiteit van Amsterdam 1018 XE, The Netherlands "Abteilung NE, Hahn-Meitner-Institute, Glienicker Strasse 100, 141 09 Berlin, Germany #Department of Electronic Structures, Charles University, 121 16 Praha 2, Czech Republic $ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Abstract URhGe crystallizes in the orthorhombic TiNiSi-type of structure and has been studied untill now only in polycrystalline form. Here, we report on the low-temperature speci"c heat of URhGe measured on a single crystal. The compound orders ferromagnetically at ¹ "9.6 K. Because of this low magnetic-ordering temperature, the speci"c heat c cannot C 1 be described by the usual expression c¹#a¹3, also at low temperatures. Up to 7 K, c is satisfactorily described by the 1 relationship c¹#b¹2, which must be associated with the temperature depenedence of the magnetization in this temperature interval. The value of c amounts to 164 mJ/mol K2. We have also studied the e!ect on c of an external U 1 magnetic "eld applied along the principal axes. The results are discussed in terms of the strong magnetic anisotropy of URhGe. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Speci"c heat; Ferromagnetism; Actinides; URhGe
URhGe belongs to the group of U¹X (T"late transition metal, X"Si or Ge) compounds crystallizing in the orthorhombic TiNiSi-type of structure [1]. In this structure, the U atoms form zig-zag chains running along the a-axis. The development of the magnetic properties of the UTX compounds when the d states of the transitionmetal atoms are increasingly "lled in a (3d, 4d or 5d) series of T elements, can be understood in terms of reduction of the 5f-ligand hybridization. URhGe is situated at the borderline between paramagnetic compounds (UCoGe, URuGe) [1] and magnetically ordered compounds (UIrGe and UPdGe) in which anisotropic hybridization between 5f states and other electronic states causes extremely large magnetic anisotropy [1,2]. All bulk properties of URhGe point to ferromagnetic (F) order below ¹ "9.5 K. Evidence for this comes C from the diverging susceptibility and the appearance of
* Corresponding author. Tel.: #49-30-8062-2804; fax: #4930-8062-3172. E-mail address: [email protected] (K. Prokes\ )
a spontaneous magnetization at lower temperatures [3}5], a peak in the c /¹ versus ¹ curve [5] and a drop 1 of the electrical resistivity below ¹ [4]. The physical C properties at low temperatures are reported to be very sensitive to applied magnetic "eld. A spontaneous moment of about 0.3 l /f.u. was derived from free-powder B high-"eld magnetization data measured at 4.2 K. [4]. A strong magnetic anisotropy in URhGe is documented by the clear di!erence between the free-powder and "xed-powder-magnetization curves. The neutron powder-di!raction experiments at low temperatures have been interpreted in terms of a canted ferromagnetic structure with a ferromagnetic component of 0.43 l along the B c-axis and an antiferromagnetic component of 0.26 l B along the a-axis [6]. In the present paper, we report on the speci"c heat of single-crystalline URhGe in magnetic-"elds up to 15 T. A single crystal of URhGe was grown from a stoichiometric melt by a modi"ed Czochralski technique in a continuously gettered Ar atmosphere. At least 99.95% pure materials were used in the preparation. No subsequent heat treatment was given to the crystal. The
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 1 9 0 - 4
224
I.H. Hagmusa et al. / Physica B 281&282 (2000) 223}225
Fig. 1. Temperature dependence of the speci"c heat in c/¹ versus ¹ representation in various "elds up to 15 T applied along (a) the a-axis, (b) the b-axis and (c) the c-axis. In insets, the "eld dependence of c@ is given, determined by "tting the data to a c@#b¹ dependence.
quality of the crystal was checked by Laue X-ray technique and by electron microprobe analysis (EPMA). The temperature dependence of the speci"c heat was measured by a semi-adiabatic heat-pulse method between 0.4 and 50 K in "elds up to 15 T applied along the principal axes. The temperature dependence of the speci"c heat divided by temperature c /¹ at various "elds up to 15 T 1 applied along the a-axis are shown in Fig. 1(a). As can be seen, the zero-"eld curve is dominated by a pronounced peak centered at 9.6 K. This anomaly agrees well with the temperature of the magnetic phase transition derived from the magnetic susceptibility [5]. The low-temperature part of c /¹ cannot be described by the usual 1 relation c /¹"c#a¹2, because at low temperatures 1 magnetic excitations strongly contribute to the speci"c heat of URhGe. Instead, we "nd a relationship c /¹"c@#b¹, from the lowest temperature measured 1 up to 7 K. The best "t of the zero-"eld data up to 5 K leads to c@"164.2$0.2 mJ/mol K2. As the applied U magnetic "eld is increased the anomaly smears out and shifts to higher temperatures in agreement with the fer-
romagnetic order in URhGe. For all applied magnetic "elds, the type of the temperature dependence remains the same as for zero "eld and the c@ coe$cient is slightly reduced with increasing "eld (c@"161.6 mJ/mol K2 at U 15 T). To obtain the magnetic entropy, we have determined the magnetic part of the speci"c heat c as the 1,. di!erence between the measured speci"c heat and an estimated contribution of the lattice to the speci"c heat which was approximated by a sum of two Debye functions (hU "140 K and hR)@G%"260 K). Together with D D a temperature-independent electronic-speci"c-heat coef"cient of about 20 mJ/mol K2 this gives good agreeU ment with the zero-"eld data at high temperatures (¹'20 K). By integrating c /¹ versus ¹ up to 20 K 1,. we obtain S "0.4 Rln2. Such a low value suggests . strongly delocalized U moments. In Fig. 1(b) we show the temperature dependence of the speci"c heat of URhGe measured in magnetic "elds up to 15 T applied along the b-axis. Comparison with the data for the a-axis (Fig. 1(a)) shows that the zero-"eld anomaly for the b-axis is much more sensitive to magnetic "elds. Already in a "eld of 1 T, it is signi"cantly smeared out and shifted to higher temperatures. Also the coe$cient c@ is more sensitive to the applied magnetic "eld and with increasing "eld it decreases exponentially, at 15 T amounting to 73% of its zero-"eld value. This supports the idea that a large part of the low-temperature speci"c heat is of magnetic origin. In Fig. 1(c) we show the c /¹ versus ¹ curves of P URhGe measured in "elds up to 15 T applied along the c-axis. It is clear that data taken in "elds along the b- and c-axis are quite similar. This is in agreement with magnetic measurements that show that the b- and c-axis form an easy-magnetization plane [2]. Also for the c-axis the anomaly shifts to higher temperatures and it is smeared out already in rather low "elds. The coe$cient c@ decreases exponentially with applied "eld and at 15 T it attains 81% of its zero-"eld value. On the basis of thermodynamics the speci"c heat of magnetization c is proportional to dM2/d¹. There1,. fore, the ¹2 dependence of c which is experimentally 1,. observed in URhGe up to &7 K, which is about 70% of the Curie temperature, implies that in this remarkably large temperature interval the temperature dependence of M2 of URhGe should follow an approximate ¹3 dependence. This will further be investigated. It is clear that for a more elaborate analysis of the temperature dependence of c in the various magnetic "elds applied along the 1 di!erent crystallographic directions, the complex spin con"guration of URhGe [6] should be taken into account.
Acknolwedgements This work has been supported in part by JSPS and by GACR (grant no. 202/99/0184).
I.H. Hagmusa et al. / Physica B 281&282 (2000) 223}225
References [1] V. SechovskyH , L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, North Holland, Amsterdam, 1998, p. 7.
[2] [3] [4] [5] [6]
225
K. Prokes et al., Phys. Rev. B. F.R. de Boer et al., Physica B 163 (1990) 175. V.H. Tran, R. Troc, Phys. Rev. B 57 (1998) 11592. K.H.J. Buschow et al., J. Appl. Phys. 67 (1990) 5215. V.H. Tran et al., J. Magn. Magn. Mater. 186 (1998) 81.