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Magnetic ordering in RPtIn (R=Dy and Ho) ternary intermetallics
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 305 (2006) 196–201 www.elsevier.com/locate/jmmm
Magnetic ordering in RPtIn (R ¼ Dy and Ho) ternary intermetallics S. Barana,, Ł. Gondekb, J. Herna´ndez-Velascoc, D. Kaczorowskid, A. Szytu"aa a
M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, PL-30 059 Krako´w, Poland Department of Physics, Cracow University of Agriculture, Mickiewicza 21, PL-30 120 Krako´w, Poland c BENSC, Hahn-Meitner Institute, Glienicker Str. 100, D-14 109 Berlin, Wannsee, Germany d Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, PL-50 950 Wroc!aw, Poland b
Received 14 July 2005; received in revised form 13 October 2005 Available online 25 January 2006
Abstract The magnetic ordering in RPtIn (R ¼ Dy and Ho) compounds, crystallizing in the hexagonal ZrNiAl-type structure, has been investigated by neutron diffractometry in the temperature range between 1.5 and 50.7 K. Moreover, bulk magnetic measurements have been carried out in the range 1.7–400 K. The rare-earth magnetic moments have been found to order ferromagnetically below 37.1 and 23.1 K in DyPtIn and HoPtIn, respectively (temperatures derived from the magnetization data). The analysis of neutron diffraction patterns collected below 2 K revealed for both compounds the presence of a ferromagnetic contribution along the c-axis and an antiferromagnetic one in the ab-plane. The antiferromagnetic contribution in HoPtIn disappears with increasing temperature while the ferromagnetic one remains up to the transition into the paramagnetic state. r 2006 Elsevier B.V. All rights reserved. PACS: 75.25.+z; 75.30.Cr; 75.50.Cc; 75.50.Ee Keywords: Intermetallics; Magnetically ordered materials; Neutron diffraction
1. Introduction The RPtIn (R ¼ Sc, Y, La-Nd, Sm, Gd-Lu) intermetallic compounds crystallize in the hexagonal ZrNiAl-type structure [1–4], whereas EuPtIn crystallizes in the orthorhombic TiNiSi-type [5,6] structure. Transport and magnetic studies of RPtIn intermetallics (R ¼ rare-earth element) revealed interesting physical properties in several members of this family of compounds. CePtIn [7–10] and YbPtIn [4,11,12] were found to be dense Kondo systems. No magnetic ordering was found down to 60 mK in CePtIn [7,8], whereas YbPtIn was reported to order antiferromagnetically below 3.4 K [11]. No magnetic ordering was found down to 1.7 K in PrPtIn. However, magnetic and resistivity data suggested possible ferromagnetic transition occurring at lower temperatures [2]. SmPtIn was found to order ferromagnetically below 25 K [2]. A ferromagnetic ordering was discovered in GdPtIn [13], DyPtIn [13] and ErPtIn [14] Corresponding author. Tel.: +48 12 6635686; fax: +48 12 6337086.
E-mail address: [email protected] (S. Baran). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.001
below 89, 38 and 13 K, respectively. An antiferromagnetic ordering was found in TbPtIn below 50 K [13] (46.0 K due to Ref. [15] and about 47 K due to Ref. [16]). The aim of the present work was to determine the magnetic behavior of the HoPtIn compound, which to the best of our knowledge has not been reported in the literature so far, as well as to resolve the magnetic structures of both DyPtIn and HoPtIn. For completeness, the bulk magnetic characteristics of DyPtIn have been reinvestigated. The results are compared here with the experimental data published for other members of the RPtIn family.
2. Experiment The samples were obtained by arc melting the respective elements (minimum purity 3 N) under argon atmosphere, and subsequent annealing at 800 1C for one week. X-ray powder diffraction (CuKa radiation) confirmed that the samples crystallize in the ZrNiAl-type structure. The
ARTICLE IN PRESS S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
determined lattice parameters were in reasonable agreement with the data published previously [1,3]. Magnetization measurements were done using a SQUID magnetometer (Quantum Design MPMS-5 type) in the temperature range from 1.7 to 400 K and in external magnetic fields up to 5.0 T. Neutron diffraction patterns were recorded with the E6 diffractometer at BENSC. The incident neutron wavelength was 2.44 A˚. The powder samples were enclosed in cylindrical vanadium containers with the diameter of 3 mm. The data were collected at several temperatures between 1.5 and 50.7 K, using an ILL-type cryostat. The neutron diffraction data were analyzed with the use of the Rietveld program FULLPROF (version 3.00— November 2004) [17]. 3. Bulk magnetic data
25
30
20
20
σ [emu/g]
15
FC ZFC
DyPtIn
10 100 0
10
80 0
10 20 30 40 Temperature [K]
50
40 T = 1.7 K
20 0
0
4. Crystal structure
60
5
0
50
100
0
150 200 250 Temperature [K]
1
300
2 3 Field [T]
4
350
5
σ [emu/g]
30
15
FC ZFC
3 R atoms at 3(g) site 3 In atoms at 3(f) site 2 Pt atoms at 2(c) site 1 Pt atom at 1(b) site
HoPtIn
20
100 B = 0.1 T 80 0
10 20 30 40 Temperature [K]
5
50
σ [emu/g]
0
40
50
100
x¯ R ; x¯ R ; 12 x¯ In ; x¯ In ; 0
5. Magnetic structure
T = 1.7 K
20
0
0; xR ; 12 0; xIn ; 0 2 1 3; 3; 0
60
0
0
xR ; 0; 12 xIn ; 0; 0 1 2 3; 3; 0 0; 0; 12
The neutron diffraction patterns recorded in the paramagnetic state are shown in Fig. 2. The lattice and positional parameters refined from the neutron data are listed in Table 1.
10
10
The previous X-ray studies [1,3] revealed that the title compounds exhibit the hexagonal ZrNiAl-type crystal ¯ structure (space group P62m) with atoms located at the following sites:
400
20
χm-1 [mol/emu]
at 37.1 K (DyPtIn) and 23.1 K (HoPtIn), manifesting itself as an inflection point in the sðTÞ variation. In the paramagnetic region, the magnetic susceptibility obeys the Curie–Weiss law with the paramagnetic Curie temperature equal to 32.8 K (DyPtIn) and 25.0 K (HoPtIn). The effective magnetic moment is equal to 10.7 and 10:6 mB for DyPtIn and HoPtIn, respectively. Little deviation of the reciprocal susceptibility from a straight line behavior observed below ca. 100 K for the former and ca. 50 K for the latter compound presumably arise due to crystalline field effect. Apart from para- to ferromagnetic transitions described above, the zero field cooling (ZFC) magnetization curves have maxima at 17.9 and 7.3 K for DyPtIn and HoPtIn, respectively. The maxima are associated with anomalies in FC curves. These maxima may indicate the presence of additional transitions of magnetic origin. To confirm or reject this hypothesis, a verification by different experimental techniques is necessary. In high magnetic fields the magnetization measured at T ¼ 1:7 K does not saturate but increases nearly linearly up to the strongest field measured (see the bottom insets in Fig. 1). The magnetic moment per one rare-earth atom measured at B ¼ 5:0 T is equal to 6.8 and 8:2 mB for DyPtIn and HoPtIn, respectively. With decreasing field the magnetization shows a hysteresis effect with the remanent magnetic moment per one rare-earth atom being equal to 1.8 and 0:9 mB for DyPtIn and HoPtIn, respectively.
B = 0.1 T σ [emu/g]
χm-1 [mol/emu]
The temperature dependence of the magnetization, recorded in a magnetic field of 0.1 T (Fig. 1), indicates a phase transition from paramagnetic to ferromagnetic state
197
0
150 200 Temperature [K]
1
2 3 Field [T]
250
4
5
300
Fig. 1. Temperature dependencies of the reciprocal magnetic susceptibility of DyPtIn and HoPtIn measured in external magnetic field B ¼ 0:1 T. The solid lines represent the Curie–Weiss fits. The upper insets show lowtemperature part of the magnetization measured upon cooling the sample in zero (ZFC) and applied (FC) magnetic field. The lower insets show the magnetization versus applied magnetic field measured at T ¼ 1:7 K with increasing (filled symbols) and decreasing (open symbols) magnetic field.
The neutron diffraction patterns of RPtIn (R ¼ Dy, Ho) recorded below 2 K are shown in Fig. 3. There are three rare-earth atoms in the elementary unit cell. These atoms occupy the 3(g) site and are located at the following positions: R1 atom at R2 atom at R3 atom at
xR ; 0; 12 1 xR ; 1 xR ; 12 0; xR ; 12
ARTICLE IN PRESS S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
198
12000 10000
Yobs. Ycal. Yobs. Ycal.
DyPtIn T = 50.7 K
Counts
8000 6000 4000 2000 0 -2000 -4000
20
30
40
50
60
70
80
90
100
2θ [deg.] 25000
20000
Yobs. Ycal. Yobs. Ycal.
HoPtIn T = 39.8 K
Counts
15000
10000
5000
0
-5000 20
30
40
50
60
70
80
90
100
2θ [deg.]
Fig. 2. Neutron diffraction patterns of paramagnetic RPtIn (R ¼ Dy, Ho) together with Rietveld fits and difference plots. Vertical ticks indicate the positions of nuclear reflections. The bottom row of ticks in DyPtIn diffraction pattern corresponds to elemental aluminium originating from cryostat shielding. Table 1 Refined structural parameters of RPtIn (R ¼ Dy, Ho) and residuals for profile and integrated intensities Compound
DyPtIn
HoPtIn
T (K) a (A˚) c (A˚) c=a V (A˚3) xR xIn w2 Rprofile (%) RBragg (%)
50.7 7.580(4) 3.838(2) 0.5063(5) 191.0(3) 0.595(1) 0.253(4) 2.82 2.41 5.07
39.8 7.571(5) 3.812(3) 0.5035(7) 189.2(4) 0.588(2) 0.257(4) 4.37 2.92 7.83
For both compounds, the best fit of the neutron data was obtained for a magnetic structure which consists of two components: a simple ferromagnetic one with magnetic moments along the c-axis and an antiferromagnetic one
with magnetic moments in the ab-plane. The antiferromagnetic component corresponds to the propagation vector k~ ¼ ½12; 0; 12. The arrangement of magnetic moments for the antiferromagnetic component is shown in Fig. 4. The inset to Fig. 3 presents the temperature dependencies of two purely magnetic Bragg intensities observed at: 2y ¼ 21:7 which consists of both the ferromagnetic and antiferromagnetic components of the magnetic structure in HoPtIn, 2y ¼ 34:6 which consists of only the antiferromagnetic component of the magnetic structure in HoPtIn. Clearly, the ferromagnetic component in HoPtIn disappears at T C ¼ 27ð2Þ K while the antiferromagnetic one already at 8(1) K. High disprosium neutron absorption prevented measurements of the temperature evolution of the neutron diffraction pattern of DyPtIn.
ARTICLE IN PRESS S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
12000
Yobs. Ycal. Yobs. Ycal.
DyPtIn
10000
199
T = 1.6 K
Counts
8000 6000 4000 2000 0 -2000 -4000
20
30
40
50
60
70
80
90
100
2θ [deg.] 25000
T = 1.5 K
15000
2θ=34.6° 0
Counts
Yobs. Ycal. Yobs. Ycal.
2θ=21.7°
Intensity [a.u.]
HoPtIn 20000
10000
5
10 15 20 25 Temperature [K]
30
5000
0
-5000 20
30
40
50
60
70
80
90
100
2θ [deg.] Fig. 3. Neutron diffraction patterns of RPtIn (R ¼ Dy, Ho) recorded below 2 K, together with Rietveld fits and difference plots. The upper row of vertical ticks indicates the positions of nuclear reflections. The next two rows indicate the positions of reflections originating from ferromagnetic and antiferromagnetic ordering, respectively. The bottom row of ticks in DyPtIn diffraction pattern corresponds to aluminum phase originating from cryostat shielding. The inset in the HoPtIn diffraction pattern presents the intensities of two magnetic reflections at different Bragg angles (see Section 5 for details).
b
a
1.5 K which remain unindexed. They are absent at 39.8 K and hence they probably originate from magnetic ordering in some unidentified impurity phase. Holmium has relatively high magnetic moment (10 mB for free Hoþ3 ion) and therefore the Bragg reflections of magnetic origin are often much stronger than those of nuclear origin. As a result, the nuclear Bragg reflections of impurity phase were probably too weak to be observed in the sample studied. The parameters of the magnetic structure of RPtIn (R ¼ Dy, Ho) below 2 K are summarized in Table 2.
Fig. 4. Antiferromagnetic component of the magnetic structure of DyPtIn and HoPtIn. The adjacent (0 0 1) planes are coupled antiferromagnetically.
6. Discussion
As seen in Fig. 3, there are some very weak Bragg reflections (for instance at 2y ¼ 36:3 or 2y ¼ 39:9 ) observed in the HoPtIn diffraction pattern recorded at
The magnetization vs. temperature data suggest ferromagnetic ordering below 37.1 and 23.1 K in DyPtIn and HoPtIn, respectively. The value found for DyPtIn agrees well with the transition temperature equal to 38 K which
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S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
Table 2 Parameters of the magnetic structure of RPtIn (R ¼ Dy, Ho) below 2 K Compound
DyPtIn
HoPtIn
T (K) Ferromagnetic component Direction of magnetic moment mF ðmB Þ Rmagn (%)
1.6
1.5
Antiferromagnetic component Propagation vector Direction of magnetic moment f1 a (deg.) f2 a (deg.) f3 a (deg.) mAF ðmB Þ Rmagn (%) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mTOTAL ¼ m2F þ m2AF ðmB Þ
kc 6.6(6) 5.57
7.4(2) 6.15 ½12; 0; 12 ?c 180 60 60
4.9(3) 19.0
3.5(2) 15.5
8.2(5)
8.2(2)
b
37(4)
25(2)
2
5.99 3.53
8.59 4.28
y (deg.) w Rprofile (%) a
f is an angle between the a-axis and the magnetic moment. y is an angle between the c-axis and the magnetic moment.
b
was reported in Ref. [13]. The paramagnetic Curie temperatures are positive (þ32:8 K for DyPtIn and þ25:0 for HoPtIn) and close to the ordering temperatures, as expected for ferromagnets. The effective magnetic moments (10:7 mB for DyPtIn and 10:6 mB for HoPtIn) are in very good agreement with the theoretical values for free ions (10:63 mB for Dyþ3 and 10:60 mB for Hoþ3 ). The temperature at which a maximum in the HoPtIn ZFC magnetization curve was found (7.3 K) coincides with the appearance of an antiferromagnetic component of the HoPtIn magnetic structure (8 K). There are no temperature dependence of DyPtIn diffraction pattern. However, assuming similar magnetic ordering schemes in both DyPtIn and HoPtIn, one may expect that an antiferromagnetic component of the DyPtIn magnetic structure should appear below 17.9 K (the temperature at which a maximum in DyPyIn ZFC magnetization curve was identified). The magnetic field dependence of magnetization is typical for a ferromagnet (Fig. 1). A distinct hysteresis effect is noticeable. The magnetic moment at T ¼ 1:7 K and applied magnetic field of 5.0 T was found to be 6.8 and 8:2 mB for DyPtIn and HoPtIn, respectively. These values are lower than the theoretical free ion magnetic moments (10 mB for both Dyþ3 and Hoþ3 ). The magnetic moments found from the neutron diffraction data below 2 K (8:2 mB for both DyPtIn and HoPtIn) are also lower than the free ion value. Such a reduction of rare-earth magnetic moment manifests an influence of crystal electric field. The best fits to the diffraction patterns collected below 2 K yielded a complex magnetic structure with a ferromagnetic component along the c-axis and an antiferromagnetic
one in the ab-plane. In HoPtIn, the antiferromagnetic component disappears at 8(1) K, while the ferromagnetic one remains until 27(2) K. In the case of DyPtIn, no temperature dependence of neutron diffraction pattern was measured. For the neighboring members of the RPtIn family, an antiferromagnetic ordering was found in TbPtIn [13,15,16] while a ferromagnetic one in ErPtIn [14]. Thus, with increasing number of 4f electrons the magnetic ordering in the RPtIn family changes from an antiferromagnetic one with magnetic moments in the ab-plane (TbPtIn) through the coexistence of an antiferromagnetic ordering with magnetic moments in the ab-plane and a ferromagnetic one with magnetic moments along the c-axis (DyPtIn and HoPtIn) to purely ferromagnetic structure with magnetic moments along the c-axis (ErPtIn). The calculated angle between the c-axis and the direction of magnetic moment changes from 90 for TbPtIn through 37 for DyPtIn, 25 for HoPtIn to 0 for ErPtIn. The observed magnetic order results from the competition between exchange interactions of RKKY-type and crystalline electric field (CEF). The latter effect influences the direction of magnetic moment and is responsible for the magnetocrystalline anisotropy. In the hexagonal crystal structure of the ZrNiAl-type the rare-earth ion experiences an orthorhombic crystal field potential (point group C2v ) and the corresponding CEF Hamiltonian is given by the expression: H CEF ¼ B02 O^ 02 þ B22 O^ 22 þ B04 O^ 04 þ B24 O^ 24 þ B44 O^ 44 þ B0 O^ 0 þ B2 O^ 2 þ B4 O^ 4 þ B6 O^ 6 , 6
6
6
6
6
6
6
6
ð1Þ
m n m m where Bm n ¼ An hr iyn . In this formula Bn ðAn Þ are crystal m field parameters, On are the Stevens’ operator equivalents, hrn i are averages over the radial part of 4f wave functions and yn are multiplication factors (usually a notation y2 ¼ a, y4 ¼ b, y6 ¼ g is used) which are tabulated for all the rareearth ions [18]. In many cases, the n ¼ 2 term is dominant in Eq. (1) and the direction of magnetic moment depends on the sign of afactor of the corresponding Rþ3 ion. The change of direction of the magnetic moment between TbPtIn and ErPtIn may be connected with the change of the sign of afactor from negative for Tbþ3 to positive for Erþ3 . Since both the Dyþ3 and Hoþ3 ions have negative a-factors, the observed different directions of the magnetic moment indicate that higher-order terms with n ¼ 4 and 6 in Eq. (1) should be taken into account. With increasing temperature, the direction of the magnetic moment in HoPtIn changes: the component which is perpendicular to c-axis disappears and the magnetic moment becomes parallel to c-axis. Similar changes in the direction of magnetic moment were observed in isostructural DyPdIn [19], DyNiAl [20] and HoNiAl [20–22]. The change in spin arrangement across the RPtIn series is reminiscent to that observed for the pseudoternary systems RNi1x Cux Al (R ¼ Tb [23] and Er [24]). The
ARTICLE IN PRESS S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
RNiAl compounds are antiferromagnetic while those with Cu are ferromagnets. The solid solutions have complex magnetic structures with ferro- and antiferromagnetic components. The change of magnetic order type is associated with a change of the direction of magnetic moment, for example, from perpendicular to the c-axis in ErNiAl to parallel to the c-axis in ErCuAl [25]. Acknowledgements This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no: HII3-CT-2003-505925 (NMI3). References [1] R. Ferro, R. Marazza, G. Rambaldi, Z. Anorg. Allg. Chem. 410 (1974) 219. [2] V.I. Zaremba, Y.V. Galadzhun, B.D. Belan, A. Pikul, J. St˛epien´Damm, D. Kaczorowski, J. Alloys Comp. 316 (2001) 64. [3] Y.V. Galadzhun, V.I. Zaremba, H. Piotrowski, P. Mayer, R.-D. Hoffmann, R. Po¨ttgen, Z. Naturforsch. 55b (2000) 1025. [4] D. Kaczorowski, B. Andraka, R. Pietri, T. Cichorek, V.I. Zaremba, Phys. Rev. B 61 (2000) 15255. [5] R. Po¨ttgen, Z. Krist. 211 (1996) 884. [6] R. Mu¨llmann, B.D. Mosel, H. Eckert, G. Kotzyba, R. Po¨ttgen, J. Solid State Chem. 137 (1998) 174. [7] T. Fujita, K. Satoh, Y. Maeno, Y. Uwatoko, H. Fujii, J. Magn. Magn. Mater. 76–77 (1988) 133.
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[8] K. Satoh, T. Fujita, Y. Maeno, Y. Uwatoko, H. Fujii, J. Phys. Soc. Japan 59 (1990) 692. [9] Y. Yamaguchi, J. Sakurai, F. Teshima, H. Kawanaka, T. Takabatake, H. Fujii, J. Phys. 2 (1990) 5715. [10] M. Kurisu, T. Takabatake, H. Fujii, J. Magn. Magn. Mater. 90–91 (1990) 469. [11] O. Trovarelli, C. Geibel, R. Cardoso, S. Mederle, R. Borth, B. Buschinger, F.M. Grosche, Y. Grin, G. Sparn, F. Steglich, Phys. Rev. B 61 (2000) 9467. [12] S. Yoshii, K. Kindo, K. Katoh, Y. Niide, A. Ochiai, J. Magn. Magn. Mater. 272–276 (2004) e99. [13] K.C. Watson, J. Crangle, K.-U. Neumann, K.R.A. Ziebeck, J. Magn. Magn. Mater. 140–144 (1995) 883. [14] S. Baran, Ł. Gondek, J. Herna´ndez-Velasco, D. Kaczorowski, A. Szytu"a, J. Magn. Magn. Mater., in press, doi:10.1016/j.jmmm.2005.05.036. [15] E. Morosan, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 71 (2005) 014445. [16] O. Garlea, E. Morosan, S.L. Bud’ko, J.L. Zarestky, P.C. Canfield, C. Stassis, J. Appl. Phys. 95 (2004) 6921. [17] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [18] M.T. Hutchings, Solid State Phys. 16 (1964) 227. [19] P. Javorsky´, S. Nishigori, B. Ouladdiaf, Physica B 276–278 (2000) 730. [20] G. Ehlers, H. Maletta, Physica B 234–236 (1997) 667. [21] P. Javorsky´, P. Burlet, E. Ressouche, V. Sechovsky´, G. Lapertot, J. Magn. Magn. Mater. 159 (1996) 324. [22] P. Javorsky´, H. Sugawara, D. Rafaja, F. Bourdarot, H. Sato, J. Alloys Comp. 323–324 (2001) 472. [23] G. Ehlers, D. Ahlert, C. Ritter, W. Miekeley, H. Maletta, Europhys. Lett. 37 (1997) 269. [24] J. Prchal, P. Javorsky´, J. Vejpravova´, M. Dopita, D. Rafaja, V. Sechovsky´, Acta Phys. Polonica B 34 (2003) 1485. [25] P. Javorsky´, P. Burlet, E. Ressouche, V. Sechovsky´, H. Michor, G. Lapertot, Physica B 225 (1996) 230.
Journal of Magnetism and Magnetic Materials 305 (2006) 196–201 www.elsevier.com/locate/jmmm
Magnetic ordering in RPtIn (R ¼ Dy and Ho) ternary intermetallics S. Barana,, Ł. Gondekb, J. Herna´ndez-Velascoc, D. Kaczorowskid, A. Szytu"aa a
M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, PL-30 059 Krako´w, Poland Department of Physics, Cracow University of Agriculture, Mickiewicza 21, PL-30 120 Krako´w, Poland c BENSC, Hahn-Meitner Institute, Glienicker Str. 100, D-14 109 Berlin, Wannsee, Germany d Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, PL-50 950 Wroc!aw, Poland b
Received 14 July 2005; received in revised form 13 October 2005 Available online 25 January 2006
Abstract The magnetic ordering in RPtIn (R ¼ Dy and Ho) compounds, crystallizing in the hexagonal ZrNiAl-type structure, has been investigated by neutron diffractometry in the temperature range between 1.5 and 50.7 K. Moreover, bulk magnetic measurements have been carried out in the range 1.7–400 K. The rare-earth magnetic moments have been found to order ferromagnetically below 37.1 and 23.1 K in DyPtIn and HoPtIn, respectively (temperatures derived from the magnetization data). The analysis of neutron diffraction patterns collected below 2 K revealed for both compounds the presence of a ferromagnetic contribution along the c-axis and an antiferromagnetic one in the ab-plane. The antiferromagnetic contribution in HoPtIn disappears with increasing temperature while the ferromagnetic one remains up to the transition into the paramagnetic state. r 2006 Elsevier B.V. All rights reserved. PACS: 75.25.+z; 75.30.Cr; 75.50.Cc; 75.50.Ee Keywords: Intermetallics; Magnetically ordered materials; Neutron diffraction
1. Introduction The RPtIn (R ¼ Sc, Y, La-Nd, Sm, Gd-Lu) intermetallic compounds crystallize in the hexagonal ZrNiAl-type structure [1–4], whereas EuPtIn crystallizes in the orthorhombic TiNiSi-type [5,6] structure. Transport and magnetic studies of RPtIn intermetallics (R ¼ rare-earth element) revealed interesting physical properties in several members of this family of compounds. CePtIn [7–10] and YbPtIn [4,11,12] were found to be dense Kondo systems. No magnetic ordering was found down to 60 mK in CePtIn [7,8], whereas YbPtIn was reported to order antiferromagnetically below 3.4 K [11]. No magnetic ordering was found down to 1.7 K in PrPtIn. However, magnetic and resistivity data suggested possible ferromagnetic transition occurring at lower temperatures [2]. SmPtIn was found to order ferromagnetically below 25 K [2]. A ferromagnetic ordering was discovered in GdPtIn [13], DyPtIn [13] and ErPtIn [14] Corresponding author. Tel.: +48 12 6635686; fax: +48 12 6337086.
E-mail address: [email protected] (S. Baran). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.001
below 89, 38 and 13 K, respectively. An antiferromagnetic ordering was found in TbPtIn below 50 K [13] (46.0 K due to Ref. [15] and about 47 K due to Ref. [16]). The aim of the present work was to determine the magnetic behavior of the HoPtIn compound, which to the best of our knowledge has not been reported in the literature so far, as well as to resolve the magnetic structures of both DyPtIn and HoPtIn. For completeness, the bulk magnetic characteristics of DyPtIn have been reinvestigated. The results are compared here with the experimental data published for other members of the RPtIn family.
2. Experiment The samples were obtained by arc melting the respective elements (minimum purity 3 N) under argon atmosphere, and subsequent annealing at 800 1C for one week. X-ray powder diffraction (CuKa radiation) confirmed that the samples crystallize in the ZrNiAl-type structure. The
ARTICLE IN PRESS S. Baran et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 196–201
determined lattice parameters were in reasonable agreement with the data published previously [1,3]. Magnetization measurements were done using a SQUID magnetometer (Quantum Design MPMS-5 type) in the temperature range from 1.7 to 400 K and in external magnetic fields up to 5.0 T. Neutron diffraction patterns were recorded with the E6 diffractometer at BENSC. The incident neutron wavelength was 2.44 A˚. The powder samples were enclosed in cylindrical vanadium containers with the diameter of 3 mm. The data were collected at several temperatures between 1.5 and 50.7 K, using an ILL-type cryostat. The neutron diffraction data were analyzed with the use of the Rietveld program FULLPROF (version 3.00— November 2004) [17]. 3. Bulk magnetic data
25
30
20
20
σ [emu/g]
15
FC ZFC
DyPtIn
10 100 0
10
80 0
10 20 30 40 Temperature [K]
50
40 T = 1.7 K
20 0
0
4. Crystal structure
60
5
0
50
100
0
150 200 250 Temperature [K]
1
300
2 3 Field [T]
4
350
5
σ [emu/g]
30
15
FC ZFC
3 R atoms at 3(g) site 3 In atoms at 3(f) site 2 Pt atoms at 2(c) site 1 Pt atom at 1(b) site
HoPtIn
20
100 B = 0.1 T 80 0
10 20 30 40 Temperature [K]
5
50
σ [emu/g]
0
40
50
100
x¯ R ; x¯ R ; 12 x¯ In ; x¯ In ; 0
5. Magnetic structure
T = 1.7 K
20
0
0; xR ; 12 0; xIn ; 0 2 1 3; 3; 0
60
0
0
xR ; 0; 12 xIn ; 0; 0 1 2 3; 3; 0 0; 0; 12
The neutron diffraction patterns recorded in the paramagnetic state are shown in Fig. 2. The lattice and positional parameters refined from the neutron data are listed in Table 1.
10
10
The previous X-ray studies [1,3] revealed that the title compounds exhibit the hexagonal ZrNiAl-type crystal ¯ structure (space group P62m) with atoms located at the following sites:
400
20
χm-1 [mol/emu]
at 37.1 K (DyPtIn) and 23.1 K (HoPtIn), manifesting itself as an inflection point in the sðTÞ variation. In the paramagnetic region, the magnetic susceptibility obeys the Curie–Weiss law with the paramagnetic Curie temperature equal to 32.8 K (DyPtIn) and 25.0 K (HoPtIn). The effective magnetic moment is equal to 10.7 and 10:6 mB for DyPtIn and HoPtIn, respectively. Little deviation of the reciprocal susceptibility from a straight line behavior observed below ca. 100 K for the former and ca. 50 K for the latter compound presumably arise due to crystalline field effect. Apart from para- to ferromagnetic transitions described above, the zero field cooling (ZFC) magnetization curves have maxima at 17.9 and 7.3 K for DyPtIn and HoPtIn, respectively. The maxima are associated with anomalies in FC curves. These maxima may indicate the presence of additional transitions of magnetic origin. To confirm or reject this hypothesis, a verification by different experimental techniques is necessary. In high magnetic fields the magnetization measured at T ¼ 1:7 K does not saturate but increases nearly linearly up to the strongest field measured (see the bottom insets in Fig. 1). The magnetic moment per one rare-earth atom measured at B ¼ 5:0 T is equal to 6.8 and 8:2 mB for DyPtIn and HoPtIn, respectively. With decreasing field the magnetization shows a hysteresis effect with the remanent magnetic moment per one rare-earth atom being equal to 1.8 and 0:9 mB for DyPtIn and HoPtIn, respectively.
B = 0.1 T σ [emu/g]
χm-1 [mol/emu]
The temperature dependence of the magnetization, recorded in a magnetic field of 0.1 T (Fig. 1), indicates a phase transition from paramagnetic to ferromagnetic state
197
0
150 200 Temperature [K]
1
2 3 Field [T]
250
4
5
300
Fig. 1. Temperature dependencies of the reciprocal magnetic susceptibility of DyPtIn and HoPtIn measured in external magnetic field B ¼ 0:1 T. The solid lines represent the Curie–Weiss fits. The upper insets show lowtemperature part of the magnetization measured upon cooling the sample in zero (ZFC) and applied (FC) magnetic field. The lower insets show the magnetization versus applied magnetic field measured at T ¼ 1:7 K with increasing (filled symbols) and decreasing (open symbols) magnetic field.
The neutron diffraction patterns of RPtIn (R ¼ Dy, Ho) recorded below 2 K are shown in Fig. 3. There are three rare-earth atoms in the elementary unit cell. These atoms occupy the 3(g) site and are located at the following positions: R1 atom at R2 atom at R3 atom at
xR ; 0; 12 1 xR ; 1 xR ; 12 0; xR ; 12
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12000 10000
Yobs. Ycal. Yobs. Ycal.
DyPtIn T = 50.7 K
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20
30
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50
60
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2θ [deg.] 25000
20000
Yobs. Ycal. Yobs. Ycal.
HoPtIn T = 39.8 K
Counts
15000
10000
5000
0
-5000 20
30
40
50
60
70
80
90
100
2θ [deg.]
Fig. 2. Neutron diffraction patterns of paramagnetic RPtIn (R ¼ Dy, Ho) together with Rietveld fits and difference plots. Vertical ticks indicate the positions of nuclear reflections. The bottom row of ticks in DyPtIn diffraction pattern corresponds to elemental aluminium originating from cryostat shielding. Table 1 Refined structural parameters of RPtIn (R ¼ Dy, Ho) and residuals for profile and integrated intensities Compound
DyPtIn
HoPtIn
T (K) a (A˚) c (A˚) c=a V (A˚3) xR xIn w2 Rprofile (%) RBragg (%)
50.7 7.580(4) 3.838(2) 0.5063(5) 191.0(3) 0.595(1) 0.253(4) 2.82 2.41 5.07
39.8 7.571(5) 3.812(3) 0.5035(7) 189.2(4) 0.588(2) 0.257(4) 4.37 2.92 7.83
For both compounds, the best fit of the neutron data was obtained for a magnetic structure which consists of two components: a simple ferromagnetic one with magnetic moments along the c-axis and an antiferromagnetic one
with magnetic moments in the ab-plane. The antiferromagnetic component corresponds to the propagation vector k~ ¼ ½12; 0; 12. The arrangement of magnetic moments for the antiferromagnetic component is shown in Fig. 4. The inset to Fig. 3 presents the temperature dependencies of two purely magnetic Bragg intensities observed at: 2y ¼ 21:7 which consists of both the ferromagnetic and antiferromagnetic components of the magnetic structure in HoPtIn, 2y ¼ 34:6 which consists of only the antiferromagnetic component of the magnetic structure in HoPtIn. Clearly, the ferromagnetic component in HoPtIn disappears at T C ¼ 27ð2Þ K while the antiferromagnetic one already at 8(1) K. High disprosium neutron absorption prevented measurements of the temperature evolution of the neutron diffraction pattern of DyPtIn.
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Yobs. Ycal. Yobs. Ycal.
DyPtIn
10000
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T = 1.6 K
Counts
8000 6000 4000 2000 0 -2000 -4000
20
30
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2θ [deg.] 25000
T = 1.5 K
15000
2θ=34.6° 0
Counts
Yobs. Ycal. Yobs. Ycal.
2θ=21.7°
Intensity [a.u.]
HoPtIn 20000
10000
5
10 15 20 25 Temperature [K]
30
5000
0
-5000 20
30
40
50
60
70
80
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100
2θ [deg.] Fig. 3. Neutron diffraction patterns of RPtIn (R ¼ Dy, Ho) recorded below 2 K, together with Rietveld fits and difference plots. The upper row of vertical ticks indicates the positions of nuclear reflections. The next two rows indicate the positions of reflections originating from ferromagnetic and antiferromagnetic ordering, respectively. The bottom row of ticks in DyPtIn diffraction pattern corresponds to aluminum phase originating from cryostat shielding. The inset in the HoPtIn diffraction pattern presents the intensities of two magnetic reflections at different Bragg angles (see Section 5 for details).
b
a
1.5 K which remain unindexed. They are absent at 39.8 K and hence they probably originate from magnetic ordering in some unidentified impurity phase. Holmium has relatively high magnetic moment (10 mB for free Hoþ3 ion) and therefore the Bragg reflections of magnetic origin are often much stronger than those of nuclear origin. As a result, the nuclear Bragg reflections of impurity phase were probably too weak to be observed in the sample studied. The parameters of the magnetic structure of RPtIn (R ¼ Dy, Ho) below 2 K are summarized in Table 2.
Fig. 4. Antiferromagnetic component of the magnetic structure of DyPtIn and HoPtIn. The adjacent (0 0 1) planes are coupled antiferromagnetically.
6. Discussion
As seen in Fig. 3, there are some very weak Bragg reflections (for instance at 2y ¼ 36:3 or 2y ¼ 39:9 ) observed in the HoPtIn diffraction pattern recorded at
The magnetization vs. temperature data suggest ferromagnetic ordering below 37.1 and 23.1 K in DyPtIn and HoPtIn, respectively. The value found for DyPtIn agrees well with the transition temperature equal to 38 K which
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Table 2 Parameters of the magnetic structure of RPtIn (R ¼ Dy, Ho) below 2 K Compound
DyPtIn
HoPtIn
T (K) Ferromagnetic component Direction of magnetic moment mF ðmB Þ Rmagn (%)
1.6
1.5
Antiferromagnetic component Propagation vector Direction of magnetic moment f1 a (deg.) f2 a (deg.) f3 a (deg.) mAF ðmB Þ Rmagn (%) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mTOTAL ¼ m2F þ m2AF ðmB Þ
kc 6.6(6) 5.57
7.4(2) 6.15 ½12; 0; 12 ?c 180 60 60
4.9(3) 19.0
3.5(2) 15.5
8.2(5)
8.2(2)
b
37(4)
25(2)
2
5.99 3.53
8.59 4.28
y (deg.) w Rprofile (%) a
f is an angle between the a-axis and the magnetic moment. y is an angle between the c-axis and the magnetic moment.
b
was reported in Ref. [13]. The paramagnetic Curie temperatures are positive (þ32:8 K for DyPtIn and þ25:0 for HoPtIn) and close to the ordering temperatures, as expected for ferromagnets. The effective magnetic moments (10:7 mB for DyPtIn and 10:6 mB for HoPtIn) are in very good agreement with the theoretical values for free ions (10:63 mB for Dyþ3 and 10:60 mB for Hoþ3 ). The temperature at which a maximum in the HoPtIn ZFC magnetization curve was found (7.3 K) coincides with the appearance of an antiferromagnetic component of the HoPtIn magnetic structure (8 K). There are no temperature dependence of DyPtIn diffraction pattern. However, assuming similar magnetic ordering schemes in both DyPtIn and HoPtIn, one may expect that an antiferromagnetic component of the DyPtIn magnetic structure should appear below 17.9 K (the temperature at which a maximum in DyPyIn ZFC magnetization curve was identified). The magnetic field dependence of magnetization is typical for a ferromagnet (Fig. 1). A distinct hysteresis effect is noticeable. The magnetic moment at T ¼ 1:7 K and applied magnetic field of 5.0 T was found to be 6.8 and 8:2 mB for DyPtIn and HoPtIn, respectively. These values are lower than the theoretical free ion magnetic moments (10 mB for both Dyþ3 and Hoþ3 ). The magnetic moments found from the neutron diffraction data below 2 K (8:2 mB for both DyPtIn and HoPtIn) are also lower than the free ion value. Such a reduction of rare-earth magnetic moment manifests an influence of crystal electric field. The best fits to the diffraction patterns collected below 2 K yielded a complex magnetic structure with a ferromagnetic component along the c-axis and an antiferromagnetic
one in the ab-plane. In HoPtIn, the antiferromagnetic component disappears at 8(1) K, while the ferromagnetic one remains until 27(2) K. In the case of DyPtIn, no temperature dependence of neutron diffraction pattern was measured. For the neighboring members of the RPtIn family, an antiferromagnetic ordering was found in TbPtIn [13,15,16] while a ferromagnetic one in ErPtIn [14]. Thus, with increasing number of 4f electrons the magnetic ordering in the RPtIn family changes from an antiferromagnetic one with magnetic moments in the ab-plane (TbPtIn) through the coexistence of an antiferromagnetic ordering with magnetic moments in the ab-plane and a ferromagnetic one with magnetic moments along the c-axis (DyPtIn and HoPtIn) to purely ferromagnetic structure with magnetic moments along the c-axis (ErPtIn). The calculated angle between the c-axis and the direction of magnetic moment changes from 90 for TbPtIn through 37 for DyPtIn, 25 for HoPtIn to 0 for ErPtIn. The observed magnetic order results from the competition between exchange interactions of RKKY-type and crystalline electric field (CEF). The latter effect influences the direction of magnetic moment and is responsible for the magnetocrystalline anisotropy. In the hexagonal crystal structure of the ZrNiAl-type the rare-earth ion experiences an orthorhombic crystal field potential (point group C2v ) and the corresponding CEF Hamiltonian is given by the expression: H CEF ¼ B02 O^ 02 þ B22 O^ 22 þ B04 O^ 04 þ B24 O^ 24 þ B44 O^ 44 þ B0 O^ 0 þ B2 O^ 2 þ B4 O^ 4 þ B6 O^ 6 , 6
6
6
6
6
6
6
6
ð1Þ
m n m m where Bm n ¼ An hr iyn . In this formula Bn ðAn Þ are crystal m field parameters, On are the Stevens’ operator equivalents, hrn i are averages over the radial part of 4f wave functions and yn are multiplication factors (usually a notation y2 ¼ a, y4 ¼ b, y6 ¼ g is used) which are tabulated for all the rareearth ions [18]. In many cases, the n ¼ 2 term is dominant in Eq. (1) and the direction of magnetic moment depends on the sign of afactor of the corresponding Rþ3 ion. The change of direction of the magnetic moment between TbPtIn and ErPtIn may be connected with the change of the sign of afactor from negative for Tbþ3 to positive for Erþ3 . Since both the Dyþ3 and Hoþ3 ions have negative a-factors, the observed different directions of the magnetic moment indicate that higher-order terms with n ¼ 4 and 6 in Eq. (1) should be taken into account. With increasing temperature, the direction of the magnetic moment in HoPtIn changes: the component which is perpendicular to c-axis disappears and the magnetic moment becomes parallel to c-axis. Similar changes in the direction of magnetic moment were observed in isostructural DyPdIn [19], DyNiAl [20] and HoNiAl [20–22]. The change in spin arrangement across the RPtIn series is reminiscent to that observed for the pseudoternary systems RNi1x Cux Al (R ¼ Tb [23] and Er [24]). The
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RNiAl compounds are antiferromagnetic while those with Cu are ferromagnets. The solid solutions have complex magnetic structures with ferro- and antiferromagnetic components. The change of magnetic order type is associated with a change of the direction of magnetic moment, for example, from perpendicular to the c-axis in ErNiAl to parallel to the c-axis in ErCuAl [25]. Acknowledgements This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no: HII3-CT-2003-505925 (NMI3). References [1] R. Ferro, R. Marazza, G. Rambaldi, Z. Anorg. Allg. Chem. 410 (1974) 219. [2] V.I. Zaremba, Y.V. Galadzhun, B.D. Belan, A. Pikul, J. St˛epien´Damm, D. Kaczorowski, J. Alloys Comp. 316 (2001) 64. [3] Y.V. Galadzhun, V.I. Zaremba, H. Piotrowski, P. Mayer, R.-D. Hoffmann, R. Po¨ttgen, Z. Naturforsch. 55b (2000) 1025. [4] D. Kaczorowski, B. Andraka, R. Pietri, T. Cichorek, V.I. Zaremba, Phys. Rev. B 61 (2000) 15255. [5] R. Po¨ttgen, Z. Krist. 211 (1996) 884. [6] R. Mu¨llmann, B.D. Mosel, H. Eckert, G. Kotzyba, R. Po¨ttgen, J. Solid State Chem. 137 (1998) 174. [7] T. Fujita, K. Satoh, Y. Maeno, Y. Uwatoko, H. Fujii, J. Magn. Magn. Mater. 76–77 (1988) 133.
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