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Onset of ferromagnetism in the 5f-band metamagnet UCoAl upon partial dilution of the U sublattice
Journal of Alloys and Compounds 307 (2000) 77–81
L
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Onset of ferromagnetism in the 5f-band metamagnet UCoAl upon partial dilution of the U sublattice ´ˇ ´ b , Y. Homma c , Y. Shiokawa c A.V. Andreev a , *, V. Sechovsky´ a,b , D. Rafaja b , L. Dobiasova a
Joint Laboratory for Magnetic Studies, Charles University and Institute of Physics of Academy of Sciences, Ke Karlovu 5, 12116 Prague 2, Czech Republic b Department of Electronic Structures, Ke Karlovu 5, 12116 Prague 2, Czech Republic c Institute for Materials Research, Tohoku University, Katahira 2 -1 -1, Aoba-ku, Sendai 980 -8577, Japan Received 15 March 2000; accepted 24 March 2000
Abstract UCoAl is a 5f-band metamagnet. The U 12x R x CoAl solid solutions with R5Lu and Y exhibit ferromagnetism for low R content. The spontaneous magnetic moment reaches a maximum value of 0.09 mB and 0.21 mB at x50.06 for R5Lu and Y, respectively, and vanishes at x$0.15. The evolution of crystal structure parameters together with magnetism are compared for U 12x Lu x CoAl, U 12xY x CoAl and also for UCoAl 12x X x (X5Ga, In, Sn). It is concluded that the linear changes of lattice dimensions rather that the unit cell volume play a principal role in determining the ground state properties of UCoAl related compounds. The decrease of the c /a ratio in these materials correlates with the transformation from the metamagnetic to a ferromagnetic state. 2000 Elsevier Science S.A. All rights reserved. Keywords: Uranium intermetallics; UCoAl; Metamagnetism
1. Introduction UCoAl (hexagonal ZrNiAl-type crystal structure) exhibits an unusual magnetic behavior. This compound shows no evidence of magnetic order at least down to 20 mK although the c-axis magnetic susceptibility as a function of temperature x (T ) exhibits a maximum around T max 520 K. However, if a magnetic field is applied along the c-axis for T ,T max , a metamagnetic transition (MT) with a critical filed Bc ¯0.7 T is observed on the magnetization curve M(B). The transition is between the lowfield paramagnetic state to the high-field ferromagnetic state with a U magnetic moments mU ¯0.3 mB (just above MT) aligned along the c-axis. This behavior has been attributed to 5f-band metamagnetism [1]. The magnetic moment associated with the Co 3 d-electrons is by one order of magnitude smaller [2]. The metamagnetic behavior of UCoAl is rather sensitive to alloying [3–11] and to external pressure [5,12–14], which can substantially modify the parameters Bc and T max . Substitution of a transition metal T for Co may lead *Corresponding author. Tel.: 1420-2-2191-1352; fax: 1420-2-21911351. E-mail address: [email protected] (A.V. Andreev)
to reduction of Bc and T max values towards ferromagnetism (T5Fe [3,4], Ru [3,5,6], Rh, Ir [7]). On the other hand, replacement of Co by Ni [3–5], Pt [8], Pd or Cu [9] yields an increase of both parameters, gradual smearing out of the characteristic M(B) and x (T ) anomalies and appearance of ‘conventional’ paramagnetism. Ga substitutions for Al in the UCoAl 12x Gax compounds also leads to gradual transformation of 5 f-band metamagnetism into ferromagnetism (for x$0.2) [10]. Application of pressure yields reentrance of metamagnetism, which is particularly manifest for compounds with x#0.4 [14]. It has been also demonstrated that ferromagnetism can be stabilized in a limited concentration range (x#0.15) of the U 12xY x CoAl system, where the U sublattice is diluted by the non-magnetic element Y [11]. The compounds with higher Y content exhibit a conventional paramagnetism. In this work we study the evolution of crystal structure and magnetic properties due to dilution of the U sublattice by Lu. A discussion of the results obtained for the U 12x Lu x CoAl compounds is given in comparison with data known for the U 12xY x CoAl system. 2. Experimental details The U 12x Lu x CoAl alloys (x#0.20) have been prepared
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00875-6
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by melting stoichiometric amounts of the elements (purity of U and Lu was 3N and better for the other metals) in an arc furnace with protective Ar atmosphere. The limiting of Lu content was chosen on the basis of the fact the compound with x50.20 is already a paramagnet without traces of metamagnetism [5]. The ingots of approximately 2 g mass were several times turned around and re-melted in order to achieve good homogeneity. Then they were wrapped in Ta foil and annealed at 7008C for a week. Conventional X-ray powder diffraction was applied to analyze the phase purity and the crystal structure of the U 12x Lu x CoAl samples. All diffraction patterns have been collected with Ni filtered CuKa radiation in the 2u range of 10–1508 with steps of 0.058. Analysis of the X-ray diffraction data was done using the Rietveld refinement [15,16]. Magnetic measurements were performed on isotropicpowder samples with powder particles fixed in random orientations by Stycast epoxy. Data obtained on such samples represent measurements on an ideal polycrystal. The magnetization was measured in a SQUID magnetometer with a superconducting magnet providing a maximum field of 5 T. Values of the Curie temperature T C and the spontaneous magnetic moment Ms have been determined from Arrott-plot analysis of magnetization isotherms.
3. Results and discussion The diffraction patterns of all the alloys studied consist only of reflections compatible with the hexagonal crystal structure of the ZrNiAl type (space group P6¯m2 ). Among the structure parameters, lattice parameters and fractional coordinates of atoms located in general positions were refined. The concentration dependence of the lattice parameters a and c, the ratio c /a and the unit-cell volume V of U 12x Lu x CoAl is shown in Fig. 1 in comparison with data observed for U 12xY x CoAl [11]. A typical error of the lattice parameter a (c) was60.09 (60.04) pm. The lattice parameters behave rather similar with varying x in both systems. Whereas a increases with increasing x, c decreases. The rate of change is somewhat smaller in case of Lu substitutions which yields a considerable difference in the concentration dependence of the lattice volume V(x). The unit-cell volume is practically independent of Lu content, whereas a noticeable volume expansion is observed upon the Y substitution. The shortest inter-uranium distance d U – U , the important characteristic for U compounds, is in the basal plane ¯ y, ¯ between the U atoms with coordinates 0, y, 0.5 and y, 0.5, d U – U 5(123y13y 2 )1 / 2 a. No change in the atomic fractional coordinates was found within experimental accuracy using the Rietveld analysis of diffraction data. The value of y50.57960.001 and the nearest neighbor
Fig. 1. Concentration dependence of the lattice parameters a and c, ratio c /a and unit-cell volume V in U 12xY x CoAl and U 12x Lu x CoAl.
U–U distance d U – U 50.518 a were determined for all compounds studied. As seen in Fig. 1, the c /a ratio decreases at almost the same rate with increasing Lu and Y concentration. However, it stays considerably larger than 0.518, the value for which d U – U distance within the basal plane and along the c axis would become equal. Thus, the shortest inter-uranium distance remains between the U atoms in the basal plane. Considering the 5 f bonding anisotropy to be the main mechanism responsible for the magnetocrystalline anisotropy [1] we may assume that uniaxial magnetism with the c-axis as the easy magnetization direction prevails in our compounds. Fig. 2 shows the magnetization curves measured at 5 K for various U 12x Lu x CoAl compounds. For UCoAl (x50) one can see the metamagnetic transition from the paramagnetic to the ferromagnetic state. The transition field Bc 50.8 T determined as a field of maximum derivative dM / dB is close to that observed in a UCoAl single crystal at this temperature (0.7 T [13]). The compound with x50.20 is a rather weak paramagnet but the magnetization curve at 5 K is still strongly curved. Despite the composition invariant lattice volume in U 12x Lu x CoAl compounds, the evolution of magnetism strongly resembles that in U 12xYx CoAl. Note that in the latter case a considerable lattice expansion is induced with the substitution of Y for U. In both systems the value of Bc decreases to zero for compounds for x.0.04 and a spontaneous magnetization gradually emerges. It is especially well pronounced in the Arrott-plot representation shown in Fig.
A.V. Andreev et al. / Journal of Alloys and Compounds 307 (2000) 77 – 81
Fig. 2. Magnetization U 12x Lu x CoAl at 5 K.
curves
of
random-powder
samples
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of
3. At the lowest Lu content studied, ferromagnetism and metamagnetism coexist (see results for x50.02 in Figs. 2 and 3). In Fig. 4, the low-field part of magnetization curves at 5 K is compared for x50.02 and 0.06 in both, the Lu and Y doped system (the curve for UCoAl is also shown). The value of Bc seems to decrease with increasing x in both systems at approximately the same rate. The spontaneous magnetization of the Lu doped materials, however, is by a factor of two smaller than in the case of the analogous Y substituted compounds. The concentration dependence of the magnetic moment M in 2 T (at 5 K) observed in both systems is shown in Fig. 5. In this field, all the compounds with x,0.15 are in the ferromagnetic state. M(x) starts to decrease in the Lu system immediately from the lowest concentrations whereas there is a plateau for x#0.06 in the Y system. Fig. 6 shows the concentration dependence of the spontaneous magnetization Ms at 5 K and T C in both systems. Assuming the presence of strong uniaxial anisotropy in all compounds studied, we have calculated the values of Ms displayed in Fig. 6 by multiplying the values measured on random-powder samples by a factor of 2. The Ms values calculated in this way correspond a value that would be measured on a single crystal in a magnetic field along the easy magnetization direction. In both systems Ms exhibits a maximum around x¯0.06. The maximum Ms
Fig. 3. Arrott plots of random-powder samples of U 12x Lu x CoAl at 5 K.
value for U 12x Lux CoAl (0.09 mB ) is twice as small compared to U 12xY x CoAl (0.21 mB ). This difference is obviously in part due the fact that the temperature of measurement (T55 K) in the Lu containing system is 0.5 T C , whereas for the Y doped compounds it is 0.35 T C . This can be obviously deduced from Fig. 6, which shows that the T C values are considerably lower in the Lu containing compounds in comparison to Y doped materials. Despite this fact we believe that the U ferromagnetic moments in Lu compounds are also intrinsically smaller than in the other system. In Refs. [12,13], a lattice volume expansion of 0.3– 0.5% has been estimated as sufficient to stabilize ferromagnetism in UCoAl. This conclusion has been derived using linear extrapolation of the pressure change of the critical field for metamagnetism to Bc 50 T. Note that this estimate was made using the room-temperature compressibility. The consideration of volume expansion was successfully applied to explain the non-monotonous development of magnetism in the U 12xY x CoAl system [11]. Here the volume expansion DV/V reaches 0.3% for x5 0.06, where the transformation from metamagnetism to ferromagnetism is completed. As we have shown in this work also substitution of Lu for U leads to ferromagnetism in a limited concentration range. Also in the U 12x Lu x CoAl system the ferromagnetism becomes fully developed for x50.06 (the only difference is a much smaller ordered
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Fig. 4. Low-field part of magnetization curves at 5 K of random-powder samples of U 12x Lu x CoAl and U 12xY x CoAl with x50, 0.02 and 0.06.
Fig. 5. Concentration dependence of the magnetic moment M in 2 T field (at T55 K) of random-powder samples U 12x Lu x CoAl and U 12xY x CoAl.
Fig. 6. Concentration dependence of the spontaneous magnetic moment Ms (at T55 K) and the Curie temperature T C of U 12x Lu x CoAl and U 12xY x CoAl.
moment) although the unit cell volume of this compound is identical with that of UCoAl. This demonstrates that the volume expansion is not the only mechanism making UCoAl ferromagnetic and points to the importance of changes in linear dimensions. In this context it is worth to mention that the lattice parameter c decreases with increasing x not only in U 12x R x CoAl (for R5Y, Lu) but also in UCoAl 12x Ga x [10] where ferromagnetism gradually develops with substitutions without volume expansion. As concerns the c /a ratio, it decreases with increasing x in the U 12x R x CoAl (for R5Y, Lu) systems and in the UCoAl 12x X x (X5Ga, In) compounds [10,17] isoelectronic with UCoAl and also in the UCoAl 12x Sn x system [17]. These empirical findings compare well with the fact that the magnetostriction at the metamagnetic transition in UCoAl is positive along the a axis and negative along the c axis [18]. It means that both observed linear changes, not only expansion along the a axis but also contraction along the c axis lead to stabilization of ferromagnetism. In this respect, the volume change seems to loose its importance. Some effort of theorists is strongly desired to explain successfully the relation of these trends to variations in the electronic structure, the 5 f-ligand hybridization in particular and its impact on the hierarchy of exchange interactions in compounds of the UCoAl based family.
A.V. Andreev et al. / Journal of Alloys and Compounds 307 (2000) 77 – 81
Acknowledgements The work is supported by grant [202 / 99 / 0184 of Grant Agency of the Czech Republic and grant [A1010018 of Grant Agency of Academy of Sciences of the Czech Republic.
References [1] V. Sechovsky, L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, Elsevier Science, Amsterdam, 1998, p. 1, and references therein. ´ L. [2] M. Wulff, J.-M. Fournier, A. Delapalme, B. Gillon, V. Sechovsky, Havela, A.V. Andreev, Phys. B 163 (1990) 331. [3] A.V. Andreev, H. Aruga Katori, T. Goto, J. Alloys Comp. 224 (1995) 117. [4] V.H. Tran, R. Troc, A.J. Zaleski, F.G. Vagizov, H. Drulis, Phys. Rev. B 54 (1996) 15907. [5] A.V. Andreev, L. Havela, V. Sechovsky, M.I. Bartashevich, T. Goto, K. Kamishima, J. Magn. Magn. Mater. 169 (1997) 229. [6] A.V. Andreev, L. Havela, V. Sechovsky, M.I. Bartashevich, R.V. Dremov, I.K. Kozlovskaya, Phil. Mag. B 75 (1997) 827.
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[7] A.V. Andreev, I.K. Kozlovskaya, N.V. Mushnikov, T. Goto, V. ´ Y. Homma, Y. Shiokawa, J. Alloys Comp. 284 (1999) Sechovsky, 77. [8] A.V. Andreev, I.K. Kozlovskaya, V. Sechovsky, J. Alloys Comp. 265 (1997) 38. ´ N.V. Mushnikov, T. [9] A.V. Andreev, I.K. Kozlovskaya, V. Sechovsky, Goto, Y. Homma, Y. Shiokawa, J. Alloys Comp. 291 (1999) 11. ´ J. Alloys [10] A.V. Andreev, Y. Homma, Y. Shiokawa, V. Sechovsky, Comp. 269 (1998) 34. [11] A.V. Andreev, I.K. Kozlovskaya, N.V. Mushnikov, T. Goto, V. ´ L. Havela, Y. Homma, Y. Shiokawa, J. Magn. Magn. Sechovsky, Mater. 196–197 (1999) 658. [12] A.V. Andreev, M.I. Bartashevich, T. Goto, K. Kamishima, L. ´ Phys. Rev. B 55 (1997) 5847. Havela, V. Sechovsky, [13] N.V. Mushnikov, T. Goto, K. Kamishima, H. Yamada, A.V. Andreev, ´ Phys. Rev. B 59 (1999) 6877. Y. Shiokawa, A. Iwao, V. Sechovsky, ´ Phys. Rev. B [14] A.V. Andreev, N.V. Mushnikov, T. Goto, V. Sechovsky, 60 (1999) 1122. [15] H.M. Rietveld, Acta. Cryst. 22 (1967) 151. [16] H.M. Rietveld, J. Appl. Cryst. 22 (1969) 65. ´ N.V. Mushnikov, T. Goto, Y. Homma, Y. [17] A.V. Andreev, V. Sechovsky, Shiokawa, J. Alloys Comp. (2000), in press [18] A.V. Andreev, R.Z. Levitin, Yu.F. Popov, R.Yu. Yumaguzhin, Sov. Phys. Solid State 27 (1985) 1145.
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www.elsevier.com / locate / jallcom
Onset of ferromagnetism in the 5f-band metamagnet UCoAl upon partial dilution of the U sublattice ´ˇ ´ b , Y. Homma c , Y. Shiokawa c A.V. Andreev a , *, V. Sechovsky´ a,b , D. Rafaja b , L. Dobiasova a
Joint Laboratory for Magnetic Studies, Charles University and Institute of Physics of Academy of Sciences, Ke Karlovu 5, 12116 Prague 2, Czech Republic b Department of Electronic Structures, Ke Karlovu 5, 12116 Prague 2, Czech Republic c Institute for Materials Research, Tohoku University, Katahira 2 -1 -1, Aoba-ku, Sendai 980 -8577, Japan Received 15 March 2000; accepted 24 March 2000
Abstract UCoAl is a 5f-band metamagnet. The U 12x R x CoAl solid solutions with R5Lu and Y exhibit ferromagnetism for low R content. The spontaneous magnetic moment reaches a maximum value of 0.09 mB and 0.21 mB at x50.06 for R5Lu and Y, respectively, and vanishes at x$0.15. The evolution of crystal structure parameters together with magnetism are compared for U 12x Lu x CoAl, U 12xY x CoAl and also for UCoAl 12x X x (X5Ga, In, Sn). It is concluded that the linear changes of lattice dimensions rather that the unit cell volume play a principal role in determining the ground state properties of UCoAl related compounds. The decrease of the c /a ratio in these materials correlates with the transformation from the metamagnetic to a ferromagnetic state. 2000 Elsevier Science S.A. All rights reserved. Keywords: Uranium intermetallics; UCoAl; Metamagnetism
1. Introduction UCoAl (hexagonal ZrNiAl-type crystal structure) exhibits an unusual magnetic behavior. This compound shows no evidence of magnetic order at least down to 20 mK although the c-axis magnetic susceptibility as a function of temperature x (T ) exhibits a maximum around T max 520 K. However, if a magnetic field is applied along the c-axis for T ,T max , a metamagnetic transition (MT) with a critical filed Bc ¯0.7 T is observed on the magnetization curve M(B). The transition is between the lowfield paramagnetic state to the high-field ferromagnetic state with a U magnetic moments mU ¯0.3 mB (just above MT) aligned along the c-axis. This behavior has been attributed to 5f-band metamagnetism [1]. The magnetic moment associated with the Co 3 d-electrons is by one order of magnitude smaller [2]. The metamagnetic behavior of UCoAl is rather sensitive to alloying [3–11] and to external pressure [5,12–14], which can substantially modify the parameters Bc and T max . Substitution of a transition metal T for Co may lead *Corresponding author. Tel.: 1420-2-2191-1352; fax: 1420-2-21911351. E-mail address: [email protected] (A.V. Andreev)
to reduction of Bc and T max values towards ferromagnetism (T5Fe [3,4], Ru [3,5,6], Rh, Ir [7]). On the other hand, replacement of Co by Ni [3–5], Pt [8], Pd or Cu [9] yields an increase of both parameters, gradual smearing out of the characteristic M(B) and x (T ) anomalies and appearance of ‘conventional’ paramagnetism. Ga substitutions for Al in the UCoAl 12x Gax compounds also leads to gradual transformation of 5 f-band metamagnetism into ferromagnetism (for x$0.2) [10]. Application of pressure yields reentrance of metamagnetism, which is particularly manifest for compounds with x#0.4 [14]. It has been also demonstrated that ferromagnetism can be stabilized in a limited concentration range (x#0.15) of the U 12xY x CoAl system, where the U sublattice is diluted by the non-magnetic element Y [11]. The compounds with higher Y content exhibit a conventional paramagnetism. In this work we study the evolution of crystal structure and magnetic properties due to dilution of the U sublattice by Lu. A discussion of the results obtained for the U 12x Lu x CoAl compounds is given in comparison with data known for the U 12xY x CoAl system. 2. Experimental details The U 12x Lu x CoAl alloys (x#0.20) have been prepared
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00875-6
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A.V. Andreev et al. / Journal of Alloys and Compounds 307 (2000) 77 – 81
by melting stoichiometric amounts of the elements (purity of U and Lu was 3N and better for the other metals) in an arc furnace with protective Ar atmosphere. The limiting of Lu content was chosen on the basis of the fact the compound with x50.20 is already a paramagnet without traces of metamagnetism [5]. The ingots of approximately 2 g mass were several times turned around and re-melted in order to achieve good homogeneity. Then they were wrapped in Ta foil and annealed at 7008C for a week. Conventional X-ray powder diffraction was applied to analyze the phase purity and the crystal structure of the U 12x Lu x CoAl samples. All diffraction patterns have been collected with Ni filtered CuKa radiation in the 2u range of 10–1508 with steps of 0.058. Analysis of the X-ray diffraction data was done using the Rietveld refinement [15,16]. Magnetic measurements were performed on isotropicpowder samples with powder particles fixed in random orientations by Stycast epoxy. Data obtained on such samples represent measurements on an ideal polycrystal. The magnetization was measured in a SQUID magnetometer with a superconducting magnet providing a maximum field of 5 T. Values of the Curie temperature T C and the spontaneous magnetic moment Ms have been determined from Arrott-plot analysis of magnetization isotherms.
3. Results and discussion The diffraction patterns of all the alloys studied consist only of reflections compatible with the hexagonal crystal structure of the ZrNiAl type (space group P6¯m2 ). Among the structure parameters, lattice parameters and fractional coordinates of atoms located in general positions were refined. The concentration dependence of the lattice parameters a and c, the ratio c /a and the unit-cell volume V of U 12x Lu x CoAl is shown in Fig. 1 in comparison with data observed for U 12xY x CoAl [11]. A typical error of the lattice parameter a (c) was60.09 (60.04) pm. The lattice parameters behave rather similar with varying x in both systems. Whereas a increases with increasing x, c decreases. The rate of change is somewhat smaller in case of Lu substitutions which yields a considerable difference in the concentration dependence of the lattice volume V(x). The unit-cell volume is practically independent of Lu content, whereas a noticeable volume expansion is observed upon the Y substitution. The shortest inter-uranium distance d U – U , the important characteristic for U compounds, is in the basal plane ¯ y, ¯ between the U atoms with coordinates 0, y, 0.5 and y, 0.5, d U – U 5(123y13y 2 )1 / 2 a. No change in the atomic fractional coordinates was found within experimental accuracy using the Rietveld analysis of diffraction data. The value of y50.57960.001 and the nearest neighbor
Fig. 1. Concentration dependence of the lattice parameters a and c, ratio c /a and unit-cell volume V in U 12xY x CoAl and U 12x Lu x CoAl.
U–U distance d U – U 50.518 a were determined for all compounds studied. As seen in Fig. 1, the c /a ratio decreases at almost the same rate with increasing Lu and Y concentration. However, it stays considerably larger than 0.518, the value for which d U – U distance within the basal plane and along the c axis would become equal. Thus, the shortest inter-uranium distance remains between the U atoms in the basal plane. Considering the 5 f bonding anisotropy to be the main mechanism responsible for the magnetocrystalline anisotropy [1] we may assume that uniaxial magnetism with the c-axis as the easy magnetization direction prevails in our compounds. Fig. 2 shows the magnetization curves measured at 5 K for various U 12x Lu x CoAl compounds. For UCoAl (x50) one can see the metamagnetic transition from the paramagnetic to the ferromagnetic state. The transition field Bc 50.8 T determined as a field of maximum derivative dM / dB is close to that observed in a UCoAl single crystal at this temperature (0.7 T [13]). The compound with x50.20 is a rather weak paramagnet but the magnetization curve at 5 K is still strongly curved. Despite the composition invariant lattice volume in U 12x Lu x CoAl compounds, the evolution of magnetism strongly resembles that in U 12xYx CoAl. Note that in the latter case a considerable lattice expansion is induced with the substitution of Y for U. In both systems the value of Bc decreases to zero for compounds for x.0.04 and a spontaneous magnetization gradually emerges. It is especially well pronounced in the Arrott-plot representation shown in Fig.
A.V. Andreev et al. / Journal of Alloys and Compounds 307 (2000) 77 – 81
Fig. 2. Magnetization U 12x Lu x CoAl at 5 K.
curves
of
random-powder
samples
79
of
3. At the lowest Lu content studied, ferromagnetism and metamagnetism coexist (see results for x50.02 in Figs. 2 and 3). In Fig. 4, the low-field part of magnetization curves at 5 K is compared for x50.02 and 0.06 in both, the Lu and Y doped system (the curve for UCoAl is also shown). The value of Bc seems to decrease with increasing x in both systems at approximately the same rate. The spontaneous magnetization of the Lu doped materials, however, is by a factor of two smaller than in the case of the analogous Y substituted compounds. The concentration dependence of the magnetic moment M in 2 T (at 5 K) observed in both systems is shown in Fig. 5. In this field, all the compounds with x,0.15 are in the ferromagnetic state. M(x) starts to decrease in the Lu system immediately from the lowest concentrations whereas there is a plateau for x#0.06 in the Y system. Fig. 6 shows the concentration dependence of the spontaneous magnetization Ms at 5 K and T C in both systems. Assuming the presence of strong uniaxial anisotropy in all compounds studied, we have calculated the values of Ms displayed in Fig. 6 by multiplying the values measured on random-powder samples by a factor of 2. The Ms values calculated in this way correspond a value that would be measured on a single crystal in a magnetic field along the easy magnetization direction. In both systems Ms exhibits a maximum around x¯0.06. The maximum Ms
Fig. 3. Arrott plots of random-powder samples of U 12x Lu x CoAl at 5 K.
value for U 12x Lux CoAl (0.09 mB ) is twice as small compared to U 12xY x CoAl (0.21 mB ). This difference is obviously in part due the fact that the temperature of measurement (T55 K) in the Lu containing system is 0.5 T C , whereas for the Y doped compounds it is 0.35 T C . This can be obviously deduced from Fig. 6, which shows that the T C values are considerably lower in the Lu containing compounds in comparison to Y doped materials. Despite this fact we believe that the U ferromagnetic moments in Lu compounds are also intrinsically smaller than in the other system. In Refs. [12,13], a lattice volume expansion of 0.3– 0.5% has been estimated as sufficient to stabilize ferromagnetism in UCoAl. This conclusion has been derived using linear extrapolation of the pressure change of the critical field for metamagnetism to Bc 50 T. Note that this estimate was made using the room-temperature compressibility. The consideration of volume expansion was successfully applied to explain the non-monotonous development of magnetism in the U 12xY x CoAl system [11]. Here the volume expansion DV/V reaches 0.3% for x5 0.06, where the transformation from metamagnetism to ferromagnetism is completed. As we have shown in this work also substitution of Lu for U leads to ferromagnetism in a limited concentration range. Also in the U 12x Lu x CoAl system the ferromagnetism becomes fully developed for x50.06 (the only difference is a much smaller ordered
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Fig. 4. Low-field part of magnetization curves at 5 K of random-powder samples of U 12x Lu x CoAl and U 12xY x CoAl with x50, 0.02 and 0.06.
Fig. 5. Concentration dependence of the magnetic moment M in 2 T field (at T55 K) of random-powder samples U 12x Lu x CoAl and U 12xY x CoAl.
Fig. 6. Concentration dependence of the spontaneous magnetic moment Ms (at T55 K) and the Curie temperature T C of U 12x Lu x CoAl and U 12xY x CoAl.
moment) although the unit cell volume of this compound is identical with that of UCoAl. This demonstrates that the volume expansion is not the only mechanism making UCoAl ferromagnetic and points to the importance of changes in linear dimensions. In this context it is worth to mention that the lattice parameter c decreases with increasing x not only in U 12x R x CoAl (for R5Y, Lu) but also in UCoAl 12x Ga x [10] where ferromagnetism gradually develops with substitutions without volume expansion. As concerns the c /a ratio, it decreases with increasing x in the U 12x R x CoAl (for R5Y, Lu) systems and in the UCoAl 12x X x (X5Ga, In) compounds [10,17] isoelectronic with UCoAl and also in the UCoAl 12x Sn x system [17]. These empirical findings compare well with the fact that the magnetostriction at the metamagnetic transition in UCoAl is positive along the a axis and negative along the c axis [18]. It means that both observed linear changes, not only expansion along the a axis but also contraction along the c axis lead to stabilization of ferromagnetism. In this respect, the volume change seems to loose its importance. Some effort of theorists is strongly desired to explain successfully the relation of these trends to variations in the electronic structure, the 5 f-ligand hybridization in particular and its impact on the hierarchy of exchange interactions in compounds of the UCoAl based family.
A.V. Andreev et al. / Journal of Alloys and Compounds 307 (2000) 77 – 81
Acknowledgements The work is supported by grant [202 / 99 / 0184 of Grant Agency of the Czech Republic and grant [A1010018 of Grant Agency of Academy of Sciences of the Czech Republic.
References [1] V. Sechovsky, L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, Elsevier Science, Amsterdam, 1998, p. 1, and references therein. ´ L. [2] M. Wulff, J.-M. Fournier, A. Delapalme, B. Gillon, V. Sechovsky, Havela, A.V. Andreev, Phys. B 163 (1990) 331. [3] A.V. Andreev, H. Aruga Katori, T. Goto, J. Alloys Comp. 224 (1995) 117. [4] V.H. Tran, R. Troc, A.J. Zaleski, F.G. Vagizov, H. Drulis, Phys. Rev. B 54 (1996) 15907. [5] A.V. Andreev, L. Havela, V. Sechovsky, M.I. Bartashevich, T. Goto, K. Kamishima, J. Magn. Magn. Mater. 169 (1997) 229. [6] A.V. Andreev, L. Havela, V. Sechovsky, M.I. Bartashevich, R.V. Dremov, I.K. Kozlovskaya, Phil. Mag. B 75 (1997) 827.
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