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Magnetic properties of UFe5Sn single crystals
Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
Magnetic properties of UFe5Sn single crystals A.P. Gonc-alvesa,*, J.C. Waerenborgha, M. Almeidaa, M. Godinhob, I. Catarinoa,c, G. Bonfaita,c, H. No.eld a ! Departamento de Qu!ımica, Instituto Tecnologico e Nuclear, P-2686-953 Sacav!em, Portugal Departamento de F!ısica, Faculdade de Ci#encias da Universidade de Lisboa, Campo Grande ed. C1, P-1700 Lisboa, Portugal c Departamento de F!ısica, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, P2825-114 Monte de Caparica, Portugal d Laboratoire de Chimie du Solide et Inorganique Mol!eculaire, UMR CNRS 6511, Universit!e de Rennes 1, Avenue de G!en!eral Leclerc, F-35042 Rennes, France b
Received 29 August 2002; received in revised form 17 October 2002
Abstract Millimetre-size UFe5Sn single crystals were grown by the top seed solution growth method and characterized by . magnetization, 57Fe Mossbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower . temperature is associated with a reorientation towards b. Mossbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding g(0 K)D140 mJ/(mol K2) and yD290 K for UFe5Sn. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Cc Keywords: UFe5Sn; Uranium intermetallic; CeCu5Au-type structure; Magnetic intermetallic
1. Introduction Recently the U–Fe–Sn phase diagram has been the subject of a systematic investigation [1] and the new intermetallic compound UFe5Sn was reported [2]. This compound crystallizes in the orthorhombic space group Pnma with a CeCu5Au-type structure with Z ¼ 4 [3]. The unit cell of UFe5Sn, *Corresponding author. Tel.: +351-21-9946182; fax: +35121-9941455. E-mail address: [email protected] (A.P. Gonc-alves).
depicted in Fig. 1, contains four uranium atoms located in one 4c site, four tin atoms in one other 4c site and 20 iron atoms distributed in three 4c (Fe2, Fe3, Fe4) and one 8d (Fe1) positions. The analysis of nearest-neighbour distances in this structure suggests a reduced hybridization between uranium and the other atoms [2], and therefore the possibility for uranium to carry a magnetic moment. Due to the complexity of the structure, a diversity of interactions between the different iron atoms is expected.
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 4 0 5 - 1
A.P. Gonc-alves et al. / Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
474
Previous magnetic characterization of powder samples showed ferromagnetic behaviour below 248 K and a second anomaly around 178 K [2]. Low-temperature magnetization measurements in fixed powder indicated a saturation magnetization of MsE5 mB/fu [2]. These preliminary results from powder samples left unclear the nature of the lowest temperature transition, which could be intrinsic or assigned to the presence of small amounts of UFe2x phases. In view of the complexity and the low symmetry of the structure, the separate role of uranium and iron atoms for the magnetic properties has also remained unresolved. The lack of single crystals in the early studies prevented any detailed analysis of magnetic anisotropy. Although UFe5Sn does not melt congruently, millimetre-size single crystals were obtained more recently by the top seed solution growth method. In this paper, we report a 57Fe . Mossbauer spectroscopy, specific heat and single crystal magnetization study in UFe5Sn aimed at a better understanding of the magnetic behaviour of this compound.
samples were then used as bulk charges for the growth of millimetre-size single crystals by the Czochralski method. A tungsten needle was used as a seed, and a pulling rate of 1 cm/h and a rotation rate of 15 rpm were employed in the growth. In spite of the non-congruent melting of this composition, X-ray diffraction confirmed that after a small initial growing period, the pulled material becomes homogeneous, free from UFe2 and metallic Fe, and with the lattice parameters as previously determined for UFe5Sn. A small fragment taken out from the homogeneous part of the pulled material was crushed. The resulting powder was pressed together with lucite powder into a perspex holder, in order to . obtain a homogeneous and isotropic Mossbauer absorber containing B5 mg/cm2 of natural iron. . The 57Fe-Mossbauer spectra were obtained in transmission mode using a constant-acceleration spectrometer and a 25 mCi 57Co source in a rhodium matrix. The velocity scale was calibrated using an a-Fe foil at room temperature. Spectra were collected at several temperatures between 300 and 6 K. Low-temperature spectra were obtained using a flow cryostat with temperature stability of 70.5 K. The spectra were fitted to Lorentzian lines using a non-linear least-squares computer method [4]. Magnetization measurements were performed on oriented single crystals selected from the pulled material, with typical dimensions 1 1 1 mm3, using a SQUID magnetometer. Specific heat measurements between 15 K and room temperature were performed in a homemade system [5], using the so-called scanning adiabatic method. A sample of 68 mg was selected from a piece of the pulled material and used in these measurements.
2. Experimental
3. Results and discussion
Samples with U:4Fe:Sn nominal composition were prepared by melting the elements (>99.9% purity) in an induction furnace, with a levitation cold crucible, under purified (99.999%) argon atmosphere. The melting process was repeated three times in order to ensure homogeneity. These
The magnetization versus temperature results are shown in Fig. 2. From the measurements along a, the lowest temperature anomaly (D178 K) is identified as a small maximum, followed by a continuous decrease of magnetization upon cooling, observed at low fields (up to 500 Oe), both in
UFe5Sn U Sn Fe1 Fe2 Fe3 Fe4
b c a
Fig. 1. Unit cell of UFe5Sn.
A.P. Gonc-alves et al. / Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
475
Fig. 2. Magnetization of UFe5Sn along the different crystallographic axes as a function of temperature and under a field of 500 G. Inset: expanded view of the magnetization of UFe5Sn under a field of 50 G (field cooled: open symbols; zero-field cooled: closed symbols).
zero field and field-cooled curves. A small irreversibility is detected, for temperatures lower than 100 K, as a slight separation between the two curves. The isothermal magnetization curves, MðHÞ (Fig. 3), show a sigmoidal shape type, characteristic of field-induced magnetic transitions, at temperatures below the low temperature anomaly. This behaviour evidences the fact that with increasing fields, the magnetic moments are first progressively turned towards the field direction, and then suddenly oriented along a. The critical field decreases with increasing temperature, from B20 kOe at 5 K to B5 kOe at 150 K. For temperatures between the two magnetic transitions, a regular ferromagnetic behaviour is observed. The two transitions previously reported in powder samples, respectively at 178 and 248 K, are clearly seen in measurements along the b and c directions. For measurements with the magnetic field applied parallel to b, only a small signature of the highest temperature transition is detected as a minor increase of magnetization in the MðTÞ curves, observed for temperatures between 250 and 240 K. Upon further cooling, the magnetiza-
Fig. 3. Magnetization of UFe5Sn along the different crystallographic axes as a function of the magnetic field at the different temperatures indicated.
tion increases continuously reaching a maximum slope at B178 K. The magnetization continues to increase with decreasing temperature but with a smaller variation. One can notice that, for a field of 500 Oe, the difference between magnetization values obtained respectively at 35 and 150 K is of the same order of magnitude as the magnetization decrease verified for measurements along a, in the same temperature range. The decrease in magnetization verified for H parallel to a can be associated to a reorientation of iron magnetic moments towards b. Evolution of the isothermal magnetization MðHÞ with temperature, along a and b, reinforces this idea. The MðH8bÞ curves are typical of a ferromagnet below the low-temperature transition, but a non-linear contribution appears for magnetic fields higher than 15000 Oe, indicating an increase of oriented moments along the field direction.
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For measurements along c, a sharp increase of magnetization appears at 248 K; below this temperature ferromagnetic behaviour is observed, with a large initial magnetization value, followed by a slow approach to saturation for fields >1500 Oe. Along this direction the second transition is seen as a small cusp (which becomes rounded in increasing applied fields) superimposed on the ferromagnetic MðTÞ curve; at the same time a slow increase of the high field magnetization as a function of applied field is observed for temperatures below 150 K. In spite of the different magnetic behaviour along the three crystallographic directions, the variation of the spontaneous magnetization with temperature revealed a similar behaviour for measurements with the magnetic field parallel to a and c, and a final value of B6.5 mB/f.u. for T0 K (Fig. 4). For measurements with the magnetic field parallel to b, the spontaneous magnetization values are significantly lower, reaching B3.8 mB/ f.u. at low temperatures. . The Mossbauer spectra of the UFe5Sn sample recorded at 260 and 295 K show that all the iron atoms are paramagnetic at those temperatures (Fig. 5), with only two peaks with similar relative areas and widths being observed. The spectra were
. Fig. 5. 57Fe Mossbauer spectra, obtained at different temperatures of a fragment taken out of the tip of the UFe5Sn single crystal, at different temperatures indicated in the figure. Fig. 4. Spontaneous magnetization versus temperature, along the three different crystallographic axes.
A.P. Gonc-alves et al. / Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
therefore fitted to only one quadrupole doublet (Table 1). At 200 K and below, most of the iron atoms are magnetically ordered (Fig. 5) in agreement with the highest transition temperature, TordD248 K, deduced from the magnetization data. At 6 K (Fig. 5), three resolved peaks in the lower velocity range of the spectrum are observed. Each Table 1 . Estimated parameters from the Mossbauer spectra of UFe5Sn taken at different temperatures T T (K)
Site
z
295 260 200
187
175
100
6
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
d (mm/s)
D, e (mm/s)
Bhf (T)
G (mm/s)
I (%)
0.04 0.01
0.58 0.58
— —
0.33 0.34
100 100
0.07 0.07 0.01 0.01 0.01
0.37 0.35 0.02 0.27 0.06
— 2.8 8.9 4.9 10.0
0.28 0.28 0.28 0.28 0.28
8 13 39 20 20
0.08 0.08 0.03 0.03 0.03
0.38 0.35 0.02 0.31 0.01
— 3.2 9.6 5.5 10.9
0.28 0.28 0.28 0.28 0.28
6 14 40 20 20
0.10 0.09 0.04 0.04 0.04
0.35 0.26 0.02 0.28 0.00
— 3.9 10.3 6.2 11.6
0.28 0.28 0.28 0.28 0.28
6 14 40 20 20
0.15 0.15 0.09 0.09 0.09
0.92 0.13 0.21 0.35 0.27
1.3 7.9 12.0 12.1 14.6
0.28 0.28 0.28 0.28 0.28
9 12 39 20 20
0.15 0.15 0.09 0.09 0.09
1.11 0.38 0.24 0.35 0.27
2.0 8.8 13.4 13.5 15.9
0.28 0.28 0.28 0.28 0.28
7 13 40 20 20
z is the number of Fe nearest neighbours for each crystallographic site. d, is the isomer shift relative to metallic a-Fe at 295 K; e ¼ ðe2 VZZ Q=4Þð3 cos2 y 1Þ; quadrupole shift calculated from (f1 þ f6 f2 f5 Þ=2 where fn is the shift of the nth line of the magnetic sextet due to quadrupole coupling; G; line widths of the two inner peaks of a sextet; Bhf ; the magnetic hyperfine field; I; the relative area. Estimated errors are p0.1 T for Bhf, p0.02 mm/s for d, e, G and o2% for I.
477
of them may be attributed to the first peak of a sextet. Considering the absorption in the range between 1.5 and 1.5 mm/s, the presence of a fourth sextet is also obvious. Each sextet was attributed to iron atoms on each of the four crystallographic sites occupied by iron. Considering the results obtained for other intermetallic compounds, such as AFexAl12x (A=rare earth, uranium) [4,6,7], it is reasonable to assume that in UFe5Sn the relative values of the estimated magnetic hyperfine fields, Bhf, increase with the number of iron nearest neighbours (z). This assumption was found to be consistent with the estimated relative areas, I; and isomer shifts, d (Table 1). The sextet with the highest I; corresponding to iron atoms on 8d sites (Fe1), has the second highest Bhf in agreement with z of this site. Furthermore, d for the iron atoms with the lower Bhf, i.e. those on the 4c sites with z=5 (Fe2), are those with the highest d: Since this site is the one with the largest number of tin nearest neighbours, the present result agrees with the observation in Fe–Sn alloys of increasing d with increasing number of tin nearest neighbours [8]. Four sextets are however not enough to explain the strong absorption in the velocity range between –0.5 and 1 mm/s. If an additional sextet with Bhf as low as E2 T is considered, the quality of the final fitting is significantly improved. With this 5-sextet fitting, the sum of the relative areas of the two sextets with the lowest Bhf is consistent with the fraction of iron atoms on the crystallographic site with the lowest z (Fe2). Therefore . the present analysis of Mossbauer data suggest that iron atoms on this site, 4c with z=5, may have different Bhf but all of them significantly lower than those of the iron atoms on the other sites. This model was used for the 6 K spectrum and it was also found to give consistent results for the spectra obtained at higher temperatures below Tord. Up to 100 K and between 175 and 200 K only a small decrease of Bhf and increase of d are detected. These observations may be explained by the usual dependence with temperature of the iron magnetic moments mFe and the second-order Doppler shift, respectively [4].
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Between 100 and 175 K, however, significant changes in the values of the estimated parameters are observed. All the quadrupole shifts, e; change significantly. If no structural transition occurs, this may be attributed to a reorientation of mFe, which could be induced by the setting up of the uranium magnetic ordering due to U–Fe exchange interactions. Furthermore, as the temperature increases, a small fraction of iron atoms on Fe2 sites become paramagnetic and the Bhf estimated for the iron atoms on the Fe3 sites, which up to 100 K were similar to those of the iron atoms on 8d sites (Fe1), strongly decreases and becomes approximately half of the Bhf value estimated in Fe1. Of all the iron atoms in UFe5Sn, those on Fe3 sites have the largest number of Fe2 nearest neighbours [2]. This may explain why the iron atoms on Fe3 sites are the most sensitive to the changes observed in the Fe2 sublattice. The specific heat results clearly show (Fig. 6) a large anomaly associated with the magnetic transition at 248 K. At 178 K, only a slight change in the CðTÞ behaviour can be detected. This result indicates that the 178 K transition corresponds to a very small entropy change, being consistent with a spin reorientation. A similar behaviour (strong change in magnetic behaviour not detected by specific heat measurements) was also previously observed in the UFexAl12x series of intermetallics
Fig. 6. Specific heat of UFe5Sn as a function of temperature.
[9]. For temperatures above the Curie temperature, 248 K, and below 20 K, the specific heat can be well described by the weighted sum of the three Debye functions (of the three present elements, uranium, iron and tin). This adjustment yields g(0 K)D140 mJ/(mol K2), one order of magnitude higher than in a normal metal, and a Debye temperature of yD290 K.
4. Conclusion In spite of non-congruent melting, millimetresize UFe5Sn single crystals were prepared by the top seed solution growth method. . Magnetization and Mossbauer results indicate that the UFe5Sn compound presents two magnetic transitions at 248 and 178 K. The possible association of the last transition to UFe2, previously considered as a possibility, is ruled out by the present study on single crystals free from this phase. The first transition at 248 K is associated with magnetic ordering, essentially along c, of the Fe1, Fe2 and Fe3 sites. Upon cooling between 248 and 178 K there is a slow, progressive and partial ordering of the iron atoms in the Fe2 position along b, with almost 30% of the atoms in this position still remaining paramagnetic at the measuring temperature of 187 K. At 178 K, full ordering of the moments in the Fe2 sites along b occurs, although with a lower moment, as indicated by the small hyperfine field. This ordering of the Fe2 sites along b at 178 K induces a partial reorientation of spins in the remaining sites towards this direction. This reorientation is also seen as a change of the quadrupole shifts, as well as a strong increase of the magnetic hyperfine fields in Fe3, the position that has the largest number of Fe2 nearest neighbours. The specific heat results indicate that the 178 K transition corresponds to a very small entropy change, being consistent with a spin reorientation. Although no experimental evidence can be given, it is very probable that the uranium magnetic ordering is concomitant with the reorientation of the iron magnetic moments.
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Acknowledgements This work was partially supported by the FCT (Portugal) under contract no. POCTI/CTM/ 12068/98. The collaboration between Rennes and Lisbon benefited from the ICCTI/CNRS bilateral exchange programme.
References [1] H. Noel, A.P. Gonc-alves, Intermetallics 9 (2001) 473. [2] A.P. Gonc-alves, M. Godinho, H. Noel, J. Solid State Chem. 154 (2000) 551.
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[3] M. Ruck, G. Portisch, H.G. Schlager, M. Sieck, H.V. . Lohneysen, Acta Crystallogr. B 49 (1993) 936. [4] J.C. Waerenborgh, A.P. Gonc-alves, M. Almeida, Solid State Commun. 110 (1999) 369. [5] I. Catarino, G. Bonfait, Cryogenics 40 (2000) 425. [6] J.C. Waerenborgh, P. Salamakha, O. Sologub, A.P. Gonc-alves, C. Cardoso, S. S!erio, M. Godinho, M. Almeida, Chem. Mater. 12 (2000) 1743. [7] J.C. Waerenborgh, P. Salamakha, O. Sologub, A.P. Gonc-alves, S. S!erio, M. Godinho, M. Almeida, J. Alloys Compounds 317–318 (2001) 44. [8] H. Akai, S. Blugel, . R. Zeller, P.H. Dederichs, Phys. Rev. Lett. 56 (1986) 2407. [9] C. Cardoso, I. Catarino, A.P. Gonc-alves, J.C. Waerenborgh, M.M. Cruz, G. Bonfait, M. Kuznietz, M. Almeida, M. Godinho, Physica B 284288 (2000) 1339.
Magnetic properties of UFe5Sn single crystals A.P. Gonc-alvesa,*, J.C. Waerenborgha, M. Almeidaa, M. Godinhob, I. Catarinoa,c, G. Bonfaita,c, H. No.eld a ! Departamento de Qu!ımica, Instituto Tecnologico e Nuclear, P-2686-953 Sacav!em, Portugal Departamento de F!ısica, Faculdade de Ci#encias da Universidade de Lisboa, Campo Grande ed. C1, P-1700 Lisboa, Portugal c Departamento de F!ısica, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, P2825-114 Monte de Caparica, Portugal d Laboratoire de Chimie du Solide et Inorganique Mol!eculaire, UMR CNRS 6511, Universit!e de Rennes 1, Avenue de G!en!eral Leclerc, F-35042 Rennes, France b
Received 29 August 2002; received in revised form 17 October 2002
Abstract Millimetre-size UFe5Sn single crystals were grown by the top seed solution growth method and characterized by . magnetization, 57Fe Mossbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower . temperature is associated with a reorientation towards b. Mossbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding g(0 K)D140 mJ/(mol K2) and yD290 K for UFe5Sn. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Cc Keywords: UFe5Sn; Uranium intermetallic; CeCu5Au-type structure; Magnetic intermetallic
1. Introduction Recently the U–Fe–Sn phase diagram has been the subject of a systematic investigation [1] and the new intermetallic compound UFe5Sn was reported [2]. This compound crystallizes in the orthorhombic space group Pnma with a CeCu5Au-type structure with Z ¼ 4 [3]. The unit cell of UFe5Sn, *Corresponding author. Tel.: +351-21-9946182; fax: +35121-9941455. E-mail address: [email protected] (A.P. Gonc-alves).
depicted in Fig. 1, contains four uranium atoms located in one 4c site, four tin atoms in one other 4c site and 20 iron atoms distributed in three 4c (Fe2, Fe3, Fe4) and one 8d (Fe1) positions. The analysis of nearest-neighbour distances in this structure suggests a reduced hybridization between uranium and the other atoms [2], and therefore the possibility for uranium to carry a magnetic moment. Due to the complexity of the structure, a diversity of interactions between the different iron atoms is expected.
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 4 0 5 - 1
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474
Previous magnetic characterization of powder samples showed ferromagnetic behaviour below 248 K and a second anomaly around 178 K [2]. Low-temperature magnetization measurements in fixed powder indicated a saturation magnetization of MsE5 mB/fu [2]. These preliminary results from powder samples left unclear the nature of the lowest temperature transition, which could be intrinsic or assigned to the presence of small amounts of UFe2x phases. In view of the complexity and the low symmetry of the structure, the separate role of uranium and iron atoms for the magnetic properties has also remained unresolved. The lack of single crystals in the early studies prevented any detailed analysis of magnetic anisotropy. Although UFe5Sn does not melt congruently, millimetre-size single crystals were obtained more recently by the top seed solution growth method. In this paper, we report a 57Fe . Mossbauer spectroscopy, specific heat and single crystal magnetization study in UFe5Sn aimed at a better understanding of the magnetic behaviour of this compound.
samples were then used as bulk charges for the growth of millimetre-size single crystals by the Czochralski method. A tungsten needle was used as a seed, and a pulling rate of 1 cm/h and a rotation rate of 15 rpm were employed in the growth. In spite of the non-congruent melting of this composition, X-ray diffraction confirmed that after a small initial growing period, the pulled material becomes homogeneous, free from UFe2 and metallic Fe, and with the lattice parameters as previously determined for UFe5Sn. A small fragment taken out from the homogeneous part of the pulled material was crushed. The resulting powder was pressed together with lucite powder into a perspex holder, in order to . obtain a homogeneous and isotropic Mossbauer absorber containing B5 mg/cm2 of natural iron. . The 57Fe-Mossbauer spectra were obtained in transmission mode using a constant-acceleration spectrometer and a 25 mCi 57Co source in a rhodium matrix. The velocity scale was calibrated using an a-Fe foil at room temperature. Spectra were collected at several temperatures between 300 and 6 K. Low-temperature spectra were obtained using a flow cryostat with temperature stability of 70.5 K. The spectra were fitted to Lorentzian lines using a non-linear least-squares computer method [4]. Magnetization measurements were performed on oriented single crystals selected from the pulled material, with typical dimensions 1 1 1 mm3, using a SQUID magnetometer. Specific heat measurements between 15 K and room temperature were performed in a homemade system [5], using the so-called scanning adiabatic method. A sample of 68 mg was selected from a piece of the pulled material and used in these measurements.
2. Experimental
3. Results and discussion
Samples with U:4Fe:Sn nominal composition were prepared by melting the elements (>99.9% purity) in an induction furnace, with a levitation cold crucible, under purified (99.999%) argon atmosphere. The melting process was repeated three times in order to ensure homogeneity. These
The magnetization versus temperature results are shown in Fig. 2. From the measurements along a, the lowest temperature anomaly (D178 K) is identified as a small maximum, followed by a continuous decrease of magnetization upon cooling, observed at low fields (up to 500 Oe), both in
UFe5Sn U Sn Fe1 Fe2 Fe3 Fe4
b c a
Fig. 1. Unit cell of UFe5Sn.
A.P. Gonc-alves et al. / Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
475
Fig. 2. Magnetization of UFe5Sn along the different crystallographic axes as a function of temperature and under a field of 500 G. Inset: expanded view of the magnetization of UFe5Sn under a field of 50 G (field cooled: open symbols; zero-field cooled: closed symbols).
zero field and field-cooled curves. A small irreversibility is detected, for temperatures lower than 100 K, as a slight separation between the two curves. The isothermal magnetization curves, MðHÞ (Fig. 3), show a sigmoidal shape type, characteristic of field-induced magnetic transitions, at temperatures below the low temperature anomaly. This behaviour evidences the fact that with increasing fields, the magnetic moments are first progressively turned towards the field direction, and then suddenly oriented along a. The critical field decreases with increasing temperature, from B20 kOe at 5 K to B5 kOe at 150 K. For temperatures between the two magnetic transitions, a regular ferromagnetic behaviour is observed. The two transitions previously reported in powder samples, respectively at 178 and 248 K, are clearly seen in measurements along the b and c directions. For measurements with the magnetic field applied parallel to b, only a small signature of the highest temperature transition is detected as a minor increase of magnetization in the MðTÞ curves, observed for temperatures between 250 and 240 K. Upon further cooling, the magnetiza-
Fig. 3. Magnetization of UFe5Sn along the different crystallographic axes as a function of the magnetic field at the different temperatures indicated.
tion increases continuously reaching a maximum slope at B178 K. The magnetization continues to increase with decreasing temperature but with a smaller variation. One can notice that, for a field of 500 Oe, the difference between magnetization values obtained respectively at 35 and 150 K is of the same order of magnitude as the magnetization decrease verified for measurements along a, in the same temperature range. The decrease in magnetization verified for H parallel to a can be associated to a reorientation of iron magnetic moments towards b. Evolution of the isothermal magnetization MðHÞ with temperature, along a and b, reinforces this idea. The MðH8bÞ curves are typical of a ferromagnet below the low-temperature transition, but a non-linear contribution appears for magnetic fields higher than 15000 Oe, indicating an increase of oriented moments along the field direction.
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For measurements along c, a sharp increase of magnetization appears at 248 K; below this temperature ferromagnetic behaviour is observed, with a large initial magnetization value, followed by a slow approach to saturation for fields >1500 Oe. Along this direction the second transition is seen as a small cusp (which becomes rounded in increasing applied fields) superimposed on the ferromagnetic MðTÞ curve; at the same time a slow increase of the high field magnetization as a function of applied field is observed for temperatures below 150 K. In spite of the different magnetic behaviour along the three crystallographic directions, the variation of the spontaneous magnetization with temperature revealed a similar behaviour for measurements with the magnetic field parallel to a and c, and a final value of B6.5 mB/f.u. for T0 K (Fig. 4). For measurements with the magnetic field parallel to b, the spontaneous magnetization values are significantly lower, reaching B3.8 mB/ f.u. at low temperatures. . The Mossbauer spectra of the UFe5Sn sample recorded at 260 and 295 K show that all the iron atoms are paramagnetic at those temperatures (Fig. 5), with only two peaks with similar relative areas and widths being observed. The spectra were
. Fig. 5. 57Fe Mossbauer spectra, obtained at different temperatures of a fragment taken out of the tip of the UFe5Sn single crystal, at different temperatures indicated in the figure. Fig. 4. Spontaneous magnetization versus temperature, along the three different crystallographic axes.
A.P. Gonc-alves et al. / Journal of Magnetism and Magnetic Materials 260 (2003) 473–479
therefore fitted to only one quadrupole doublet (Table 1). At 200 K and below, most of the iron atoms are magnetically ordered (Fig. 5) in agreement with the highest transition temperature, TordD248 K, deduced from the magnetization data. At 6 K (Fig. 5), three resolved peaks in the lower velocity range of the spectrum are observed. Each Table 1 . Estimated parameters from the Mossbauer spectra of UFe5Sn taken at different temperatures T T (K)
Site
z
295 260 200
187
175
100
6
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
Fe2
5
Fe1 Fe3 Fe4
7 7 8
d (mm/s)
D, e (mm/s)
Bhf (T)
G (mm/s)
I (%)
0.04 0.01
0.58 0.58
— —
0.33 0.34
100 100
0.07 0.07 0.01 0.01 0.01
0.37 0.35 0.02 0.27 0.06
— 2.8 8.9 4.9 10.0
0.28 0.28 0.28 0.28 0.28
8 13 39 20 20
0.08 0.08 0.03 0.03 0.03
0.38 0.35 0.02 0.31 0.01
— 3.2 9.6 5.5 10.9
0.28 0.28 0.28 0.28 0.28
6 14 40 20 20
0.10 0.09 0.04 0.04 0.04
0.35 0.26 0.02 0.28 0.00
— 3.9 10.3 6.2 11.6
0.28 0.28 0.28 0.28 0.28
6 14 40 20 20
0.15 0.15 0.09 0.09 0.09
0.92 0.13 0.21 0.35 0.27
1.3 7.9 12.0 12.1 14.6
0.28 0.28 0.28 0.28 0.28
9 12 39 20 20
0.15 0.15 0.09 0.09 0.09
1.11 0.38 0.24 0.35 0.27
2.0 8.8 13.4 13.5 15.9
0.28 0.28 0.28 0.28 0.28
7 13 40 20 20
z is the number of Fe nearest neighbours for each crystallographic site. d, is the isomer shift relative to metallic a-Fe at 295 K; e ¼ ðe2 VZZ Q=4Þð3 cos2 y 1Þ; quadrupole shift calculated from (f1 þ f6 f2 f5 Þ=2 where fn is the shift of the nth line of the magnetic sextet due to quadrupole coupling; G; line widths of the two inner peaks of a sextet; Bhf ; the magnetic hyperfine field; I; the relative area. Estimated errors are p0.1 T for Bhf, p0.02 mm/s for d, e, G and o2% for I.
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of them may be attributed to the first peak of a sextet. Considering the absorption in the range between 1.5 and 1.5 mm/s, the presence of a fourth sextet is also obvious. Each sextet was attributed to iron atoms on each of the four crystallographic sites occupied by iron. Considering the results obtained for other intermetallic compounds, such as AFexAl12x (A=rare earth, uranium) [4,6,7], it is reasonable to assume that in UFe5Sn the relative values of the estimated magnetic hyperfine fields, Bhf, increase with the number of iron nearest neighbours (z). This assumption was found to be consistent with the estimated relative areas, I; and isomer shifts, d (Table 1). The sextet with the highest I; corresponding to iron atoms on 8d sites (Fe1), has the second highest Bhf in agreement with z of this site. Furthermore, d for the iron atoms with the lower Bhf, i.e. those on the 4c sites with z=5 (Fe2), are those with the highest d: Since this site is the one with the largest number of tin nearest neighbours, the present result agrees with the observation in Fe–Sn alloys of increasing d with increasing number of tin nearest neighbours [8]. Four sextets are however not enough to explain the strong absorption in the velocity range between –0.5 and 1 mm/s. If an additional sextet with Bhf as low as E2 T is considered, the quality of the final fitting is significantly improved. With this 5-sextet fitting, the sum of the relative areas of the two sextets with the lowest Bhf is consistent with the fraction of iron atoms on the crystallographic site with the lowest z (Fe2). Therefore . the present analysis of Mossbauer data suggest that iron atoms on this site, 4c with z=5, may have different Bhf but all of them significantly lower than those of the iron atoms on the other sites. This model was used for the 6 K spectrum and it was also found to give consistent results for the spectra obtained at higher temperatures below Tord. Up to 100 K and between 175 and 200 K only a small decrease of Bhf and increase of d are detected. These observations may be explained by the usual dependence with temperature of the iron magnetic moments mFe and the second-order Doppler shift, respectively [4].
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Between 100 and 175 K, however, significant changes in the values of the estimated parameters are observed. All the quadrupole shifts, e; change significantly. If no structural transition occurs, this may be attributed to a reorientation of mFe, which could be induced by the setting up of the uranium magnetic ordering due to U–Fe exchange interactions. Furthermore, as the temperature increases, a small fraction of iron atoms on Fe2 sites become paramagnetic and the Bhf estimated for the iron atoms on the Fe3 sites, which up to 100 K were similar to those of the iron atoms on 8d sites (Fe1), strongly decreases and becomes approximately half of the Bhf value estimated in Fe1. Of all the iron atoms in UFe5Sn, those on Fe3 sites have the largest number of Fe2 nearest neighbours [2]. This may explain why the iron atoms on Fe3 sites are the most sensitive to the changes observed in the Fe2 sublattice. The specific heat results clearly show (Fig. 6) a large anomaly associated with the magnetic transition at 248 K. At 178 K, only a slight change in the CðTÞ behaviour can be detected. This result indicates that the 178 K transition corresponds to a very small entropy change, being consistent with a spin reorientation. A similar behaviour (strong change in magnetic behaviour not detected by specific heat measurements) was also previously observed in the UFexAl12x series of intermetallics
Fig. 6. Specific heat of UFe5Sn as a function of temperature.
[9]. For temperatures above the Curie temperature, 248 K, and below 20 K, the specific heat can be well described by the weighted sum of the three Debye functions (of the three present elements, uranium, iron and tin). This adjustment yields g(0 K)D140 mJ/(mol K2), one order of magnitude higher than in a normal metal, and a Debye temperature of yD290 K.
4. Conclusion In spite of non-congruent melting, millimetresize UFe5Sn single crystals were prepared by the top seed solution growth method. . Magnetization and Mossbauer results indicate that the UFe5Sn compound presents two magnetic transitions at 248 and 178 K. The possible association of the last transition to UFe2, previously considered as a possibility, is ruled out by the present study on single crystals free from this phase. The first transition at 248 K is associated with magnetic ordering, essentially along c, of the Fe1, Fe2 and Fe3 sites. Upon cooling between 248 and 178 K there is a slow, progressive and partial ordering of the iron atoms in the Fe2 position along b, with almost 30% of the atoms in this position still remaining paramagnetic at the measuring temperature of 187 K. At 178 K, full ordering of the moments in the Fe2 sites along b occurs, although with a lower moment, as indicated by the small hyperfine field. This ordering of the Fe2 sites along b at 178 K induces a partial reorientation of spins in the remaining sites towards this direction. This reorientation is also seen as a change of the quadrupole shifts, as well as a strong increase of the magnetic hyperfine fields in Fe3, the position that has the largest number of Fe2 nearest neighbours. The specific heat results indicate that the 178 K transition corresponds to a very small entropy change, being consistent with a spin reorientation. Although no experimental evidence can be given, it is very probable that the uranium magnetic ordering is concomitant with the reorientation of the iron magnetic moments.
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Acknowledgements This work was partially supported by the FCT (Portugal) under contract no. POCTI/CTM/ 12068/98. The collaboration between Rennes and Lisbon benefited from the ICCTI/CNRS bilateral exchange programme.
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