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Magnetic ordering in RPtX (R=Gd,Tb,Dy; X=Si,Ge) compounds
Journal of Alloys and Compounds 299 (2000) 79–87
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Magnetic ordering in RPtX (R5Gd,Tb,Dy; X5Si,Ge) compounds a a b c d a, ´ B. Penc , S. Baran , M. Hofmann , J. Leciejewicz , M. Slaski , A. Szytul«a * a
´ , Poland Institute of Physics, Jagiellonian University, Reymonta 4, 30 -059 Krakow Berlin Neutron Scattering Centre, Hahn-Meitner Institute Berlin-Wannsee, Berlin, Germany c Institute of Nuclear Chemistry and Technology, Warsaw, Poland School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, UK b
d
Received 2 November 1999; accepted 24 November 1999
Abstract GdPtX, TbPtX and DyPtX (X5Si, Ge) crystallize in the orthorhombic TiNiSi-type crystal structure. Magnetometric measurements show that all compounds (except GdPtSi) are antiferromagnetic at low temperatures. GdPtSi is probably ferromagnet below 16 K. The ´ temperature of GdPtGe is at 12.5 K. Neutron diffraction data indicate the presence of a complicated magnetic order in TbPtSi below Neel ´ point at 12.5 K. The magnetic structure of TbPtGe is of amplitude-modulated type along the b-axis with the wave vector the Neel ´ point of TbPtGe is at 15 K. A collinear antiferromagnetic ordering with the wave vector k5[]12 , 0, ]12 ] k5[0, 0.2840(3), 0]. The Neel ´ points of 8.2 and 8 K, respectively. 2000 Elsevier Science S.A. All rights is operating in DyPtSi and DyPtGe below the Neel reserved. Keywords: GdPtX, TbPtX and DyPtX compounds; Magnetic ordering; TiNiSi-type crystal structure
1. Introduction Our recent study of RPtX (R5Ho, Er; X5Si, Ge) ternaries has shown that all of them have TiNiSi-type crystal structure and order antiferromagnetically at low temperatures, with the exception of HoPtGe, which remains paramagnetic at 1.8 K [1]. Continuing our studies of the magnetic properties of TiNiSi-type lanthanide 24d (5d) transition metal silicides [2], germanides [3–7], stannides [8–10] and gallides [11,12] we report in this paper the results obtained for successive RPtX compounds: GdPtSi, GdPtGe, TbPtSi, TbPtGe, DyPtSi and DyPtGe.
2. Experimental Polycrystalline samples of the title compounds were obtained by arc melting of stoichiometric amounts of high
*Tel.: 148-12-6324-888 / 5546; fax: 148-12-633-7086. E-mail address: [email protected] (A. Szytul«a)
purity constituent elements. The samples were subsequently annealed in vacuum during 100 h at 8008C. The samples were identified by their X-ray powder diffractograms recorded using a Philips diffractometer (CoKa radiation). All reflections were indexed orthorhombic with TiNiSi-type crystal structure. The determined lattice parameters were found to be in fair agreement with those reported earlier [13]. A SQUID magnetometer was used to measure the DC magnetization and magnetic susceptibility in the temperature range from 4.2 to 300 K. Additional magnetization data were obtained at 4.2 K on free powder samples aligned in applied magnetic fields up to 120 kOe using Oxford Instruments VSM12T vibrating sample magnetometer. No demagnetization corrections were made in the process of obtaining the magnetization values. Neutron diffraction patterns were obtained for TbPtSi, TbPtGe, DyPtSi and DyPtGe compounds using the E6 diffractometer installed at the BERII reactor at the HahnMeitner Institute, Berlin. No neutron data could be obtained for GdPtSi and GdPtGe for the obvious reason, namely the very large absorption cross-section of Gd ˚ All nuclei. The used neutron wavelength was 2.438(3) A. data processing was done using the FULLPROF program
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00800-2
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Fig. 1. Temperature variation of the reciprocal magnetic susceptibility for (a) RPtSi and (b) RPtGe (R5Gd, Tb, Dy). The inset shows the x (T ) plot at low temperatures. Table 1 Crystal structure parameters of TbPtSi, TbPtGe, DyPtSi and DyPtGe derived from neutron diffraction data Compound T (K)
TbPtSi 25
DyPtSi 12
TbPtGe 25
DyPtGe 9
˚ a (A) ˚ b (A) ˚ c (A) xR zR x Pt z Pt xx zy R Bragg (%) R prof. (%)
7.013 (7) 4.271 (3) 7.402 (9) 20.002 (4) 0.671 (6) 0.205 (2) 0.143 (4) 0.305 (5) 0.413 (6) 14.6 12.5
6.948 (5) 4.251 (3) 7.419 (5) 0.006 (5) 0.6978 (15) 0.193 (4) 0.086 (9) 0.305 (10) 0.431 (19) 10.7 10.0
6.978 (2) 4.325 (1) 7.546 (2) 20.008 (4) 0.700 (1) 0.207 (1) 0.098 (2) 0.310 (1) 0.414 (2) 6.4 7.7
6.998 (11) 4.359 (6) 7.557 (11) 0.005 (8) 0.703 (2) 0.182 (7) 0.084 (5) 0.327 (10) 0.422 (7) 18.4 15.3
[14], which defines also the R factor as a criterion for the agreement between the observed and calculated neutron intensities. Neutron scattering lengths were taken from Ref. [15], magnetic form factors were adopted after Ref. [16]. The observed and calculated neutron intensities are obtainable from the corresponding author.
3. Results
3.1. Crystal structure. Neutron diffraction data recorded in the paramagnetic state confirmed the orthorhombic TiNiSi-type crystal struc-
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
Fig. 2. Field dependence of magnetization at different temperatures for RPtX (R5Gd, Tb, Dy; X5Si, Ge).
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B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
Fig. 3. Observed and calculated neutron diffractograms of TbPtSi at 1.5 and 25 K. Open squares represent the experimental points, solid lines the calculated profiles. The lower diffractogram shows the difference between the experimental and calculated patterns. Vertical ticks indicated the positions of magnetic and nuclear peaks. Parts of neutron diffractograms recorded in the temperature range from 1.5 to 14 K are shown in the inset.
ture (space group Pnma, 4R, 4Pt and 4X atoms in the 4(c) ¯ ]34 , z; ¯ ]12 2 x, ]34 , ]12 1 z; ]12 1 x, ]14 , ]12 2 z, each sites x, ]14 , z; x, with different x and z parameters). The crystal structure parameters are listed in Table 1.
3.2. Magnetic properties The temperature variation of the magnetic susceptibility x and reciprocal magnetic susceptibility x 21 recorded in
the presence of magnetic field of 10 Oe is displayed in Fig. 1. Maxima indicating the onset of long range antiferromagnetic order are observed at 12.5 K for GdPtGe, at 12.5 K for TbPtSi, at 15 K for TbPtGe, at 8.2 K for DyPtSi and at 8 K for DyPtGe. In the case of GdPtSi a broad maximum is observed at low temperatures. The values of x decrease above 16 K which is probably the phase transition from order to paramagnetic state. The additional small maximum is observed at about 6 K (see inset in Fig. 1).
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Fig. 4. Observed and calculated neutron patterns of TbPtGe at 1.5 and 25 K. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections. The temperature variation of the 100 6 magnetic peak intensity is illustrated in the inset.
The Curie–Weiss law is obeyed above the respective ´ temperatures in all title compounds (see Fig. 1). The Neel numerical fit to the experimental data yields magnitudes of the effective magnetic moments meff that are close to the R31 free ion values (see Table 2). The paramagnetic Curie temperatures are negative, thus indicating the dominance of antiferromagnetic interactions. Magnetization isotherms recorded at different temperatures are shown in Fig. 2. The curve of GdPtSi taken at 4.2
K has ferromagnetic character, however, even in the presence of magnetic field H5120 kOe the magnetization is not saturated. As the temperature rises to 10 K the curve attains metamagnetic character with a critical field at 11 kOe. The magnetization curves of TbPtSi and DyPtSi show also metamagnetic character with critical fields of 2.5 and 13.4 kOe, respectively. The magnetization curves of GdPtGe show linear increase as the magnetic field strength rises. The magnetization curves of TbPtGe and
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Fig. 5. Schematic representations of magnetic structures found in TbPtGe (a) and DyPtSi (b).
moment localized on the Tb 31 ion amounts to 6.92(16) mB at 1.5 K and is parallel to the c-axis. The temperature dependence of the neutron magnetic peak inten´ point at 15 K. sities locates the Neel
DyPtGe recorded at 4.2 K exhibit metamagnetic character with critical fields of 68 and 17 kOe, respectively. The values of magnetic moments for all title compounds determined at 4.2 K and 120 kOe are smaller than the free R31 ion values (see Table 2).
3.3. Magnetic structures 3.3.1. TbPtSi The neutron diffractogram of TbPtSi recorded at 1.5 K (Fig. 3) shows the presence of a number of magnetic peaks, of rather small intensities. The positions of the magnetic peaks indicate a coexistence of two magnetic phases: • a noncollinear antiferromagnetic corresponding to the G x C y mode (see Appendix A) with a component of the magnetic moment mx 52.57(5) mB and my 53.19(9) mB at 1.5 K, localized on the Tb 31 ion • a sine wave modulated structure described by the wave vector k5[0.345(2), 0.565(1), 0.455(3)]. The magnetic
3.3.2. TbPtGe Magnetic peaks observed in the neutron pattern of TbPtGe recorded at 1.5 K (Fig. 4) were indexed as satellites to the allowed and forbidden nuclear reflections. The absence of higher-order satellites suggested that the magnetic structure is either a flat spiral or sine wave modulated. The best fitted observed magnetic peaks intensities were obtained for a sine wave modulated structure characterized by the wave vector k5[0, 0.2840(3), 0] with Tb 31 moment of 9.33(10) mB at 1.5 K. The distribution of moments in the crystallographic unit cell corresponds to the C mode (see Appendix A). The moment direction makes an angle of 608 with the c-axis and 638 with the a-axis. The temperature variation of the intensities of two strongest sattellite reflections (100 6 and 001 6 see inset in Fig. 4) shows that this ordering scheme does not change in the temperature range from 1.5 K to 15 K, the
Table 2 Magnetic data for RPtX (R5Gd, Tb, Dy; X5Si, Ge) compounds obtained from magnetization and neutron diffraction experiments Compound
up (K)
T N (K) M
ND
meff ( mB / R atom) Experimental
m ( mB / R atom) Theoretical
Experimental M
GdPtSi TbPtSi DyPtSi GdPtGe TbPtGe DyPtGe
16.0 12.5 8.2 12.5 15 8
14.5 8 15 7
214.5 216.9 29.6 225 210.7 26.0
8.07 9.78 10.40 7.90 9.60 10.30
7.94 9.72 10.65 7.94 9.72 10.65
6.0 7.0 8.2 5.1 5.2 7.5
Theoretical
ND 7.12(20) 8.78(20) 9.33(10) 9.61(33)
7.0 9.0 10.0 7.0 9.0 10.0
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85
Fig. 6. Differential (1.5–12 K) and recorded at 12 K neutron diffraction patterns of DyPtSi. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections.
´ point of TbPtGe. This structure is schematically Neel illustrated in Fig. 5a.
3.3.3. DyPtSi and DyPtGe The magnetic peaks observed in the neutron diffraction patterns recorded at 1.5 K (Figs. 6 and 7) were readily indexed assuming a collinear antiferromagnetic structure
described by the wave vector k5[ ]12 , 0, ]12 ]. It corresponds to the C mode in terms of Bertaut’s analysis, see Appendix A. This structure is schematically illustrated in Fig. 5b. The magnetic moments are localized on Dy 31 ions and at 1.5 K amount to 8.8(2) mB in DyPtSi and 9.6(3) mB in DyPtGe. The moments are parallel to the b-axis in DyPtSi and the c-axis in DyPtGe. The above magnetic
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Fig. 7. Observed at 1.5 and 9 K and calculated neutron diffractograms of DyPtGe. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections.
order is stable in the temperature range from 1.5 K to the ´ points: 8 K (DyPtSi) and 7 K (DyPtGe). respective Neel
4. Comment Our X-ray and neutron diffraction data confirm that all title compounds exhibit the orthorhombic TiNiSi-type crystal structure. The magnetic moment carrying lantha-
nide ions R31 form wavy chains running along the a-axis ˚ with fairly large R31 –R31 separation of more than 3.5 A. Also the R31 –R31 distance between two nearest ions belonging to adjacent chains is large. This suggests that direct magnetic interactions are highly improbable. The stability of the observed magnetic ordering schemes may thus be considered as due to the interaction via conduction electrons (the RKKY model). However adoption of this model should lead to the proportionality of the values of
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Appendix A The moments localized on the lanthanide ions situated in the 4(c) position of the space group Pnma are denoted as:
S D S D S 1 1 1 S S] 1 x, ], ] 2 zD 2 4 2
D
1 3 1 3 1 ¯ ], z¯ ; S 3 ] 2 x, ], ] 1 z ; S 1 x, ], z ; S 2 x, 4 4 2 4 2 4
The analysis introduced by Bertaut [18] based on irreducible representations for spin transitions for the above group leads to three antiferromagnetic structures described by the following vectors: G5S 1 2S 2 1S 3 2S 4 ; C5S 1 1S 2 2S 3 2S 4 ; A5S 1 2S 2 2S 3 1S 4 . ´ temperatures of RPtX (R5Gd–Er; X5Si, Ge) compounds Fig. 8. Neel as compared with the de Gennes function. The dashed lines represent the de Gennes functions for RPtSi and RPtGe as normalized with respect to GdPtSi and GdPtGe.
´ or Curie points to the de Gennes function (( gJ 2 Neel 1)2 J(J 11) [17]. Fig. 8 shows that the de Gennes scaling for the title compounds is not obeyed. This discrepancy probably results from a strong influence of the crystalline electric field (CEF). The latter effect probably decreases the tendency to form oscillatory and incommensurate magnetic ordering. In fact, only in TbPtSi and TbPtGe such ordering schemes are observed, while DyPtSi, DyPtGe and HoPtSi [1] show collinear antiferromagnetic structures with the wave vector k5[ ]12 , 0, ]12 ]. At 1.5 K ErPtSi and ErPtGe exhibit also collinear antiferromagnetic structures, both characterized by the wave vector k5 ´ points a [0, ]12 , 0], however, close to the respective Neel transition to incommensurate structures is observed in these compounds [1].
Acknowledgements The kind hospitality and financial support by the HahnMeitner Institute to perform neutron diffraction measurements is gratefully acknowledged by three authors (J.L., B.P., A.S.). This work has been supported in part by the State Committee for Scientific Research in Poland with grant No. 270 / P03 / 98 / 15.
References ´ [1] B. Penc, S. Baran, M. Hofmann, J. Leciejewicz, M. Slaski, A. Szytul«a, J. Phys. Condens. Matter. 11 (1999) 5631. ~ [2] W. Bazela, J. Leciejewicz, A. Szytul«a, J. Magn. Magn. Mater. 50 (1985) 19. ~ [3] W. Bazela, A. Zygmunt, E. Ressouche, J. Leciejewicz, W. Sikora, J. Alloys Comp. 243 (1996) 106. ¨ [4] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, J. Magn. Magn. Mater. 149 (1995) 265. ´ [5] B. Penc, M. Hofmann, M. Kolenda, M. Slaski, A. Szytul«a, J. Alloys Comp. 267 (1997) L4. [6] B. Penc, S. Baran, M. Hofmann, A. Szytul«a, J. Alloys Comp. 287 (1999) 48. ´ [7] B. Penc, S. Baran, M. Hofmann, J. Leciejewicz, M. Slaski, A. Szytul«a, J. Alloys Comp. 287 (1999) 18. ¨ [8] M. Kolenda, J. Leciejewicz, N. Stusser, A. Szytul«a, A. Zygmunt, J. Magn. Magn. Mater. 145 (1995) 85. ¨ [9] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, A. Zygmunt, J. Magn. Magn. Mater. 163 (1996) 273. ¨ [10] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, J. Magn. Magn. Mater. 164 (1996) 377. ´ [11] B. Penc, M. Hofmann, M. Slaski, A. Zygmunt, Physica B, (1999) in press. [12] B. Penc, M. Hofmann, J. Leciejewicz, A. Szytul«a, A. Zygmunt, (1999) to be published. ´ J. Less [13] E. Hovestreydt, N. Engel, K. Klepp, B. Chabot, E. Parthe, Common. Methods 85 (1982) 247. [14] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [15] V.F. Sears, Neutron News 3 (1992) 26. [16] A.J. Freeman, J.P. Desclaux, J. Magn. Magn. Mater. 12 (1979) 11. [17] P.G. de Gennes, J. Phys Radium 23 (1962) 570. [18] E.F. Bertaut, Acta Crystallogr. A 24 (1968) 217.
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Magnetic ordering in RPtX (R5Gd,Tb,Dy; X5Si,Ge) compounds a a b c d a, ´ B. Penc , S. Baran , M. Hofmann , J. Leciejewicz , M. Slaski , A. Szytul«a * a
´ , Poland Institute of Physics, Jagiellonian University, Reymonta 4, 30 -059 Krakow Berlin Neutron Scattering Centre, Hahn-Meitner Institute Berlin-Wannsee, Berlin, Germany c Institute of Nuclear Chemistry and Technology, Warsaw, Poland School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, UK b
d
Received 2 November 1999; accepted 24 November 1999
Abstract GdPtX, TbPtX and DyPtX (X5Si, Ge) crystallize in the orthorhombic TiNiSi-type crystal structure. Magnetometric measurements show that all compounds (except GdPtSi) are antiferromagnetic at low temperatures. GdPtSi is probably ferromagnet below 16 K. The ´ temperature of GdPtGe is at 12.5 K. Neutron diffraction data indicate the presence of a complicated magnetic order in TbPtSi below Neel ´ point at 12.5 K. The magnetic structure of TbPtGe is of amplitude-modulated type along the b-axis with the wave vector the Neel ´ point of TbPtGe is at 15 K. A collinear antiferromagnetic ordering with the wave vector k5[]12 , 0, ]12 ] k5[0, 0.2840(3), 0]. The Neel ´ points of 8.2 and 8 K, respectively. 2000 Elsevier Science S.A. All rights is operating in DyPtSi and DyPtGe below the Neel reserved. Keywords: GdPtX, TbPtX and DyPtX compounds; Magnetic ordering; TiNiSi-type crystal structure
1. Introduction Our recent study of RPtX (R5Ho, Er; X5Si, Ge) ternaries has shown that all of them have TiNiSi-type crystal structure and order antiferromagnetically at low temperatures, with the exception of HoPtGe, which remains paramagnetic at 1.8 K [1]. Continuing our studies of the magnetic properties of TiNiSi-type lanthanide 24d (5d) transition metal silicides [2], germanides [3–7], stannides [8–10] and gallides [11,12] we report in this paper the results obtained for successive RPtX compounds: GdPtSi, GdPtGe, TbPtSi, TbPtGe, DyPtSi and DyPtGe.
2. Experimental Polycrystalline samples of the title compounds were obtained by arc melting of stoichiometric amounts of high
*Tel.: 148-12-6324-888 / 5546; fax: 148-12-633-7086. E-mail address: [email protected] (A. Szytul«a)
purity constituent elements. The samples were subsequently annealed in vacuum during 100 h at 8008C. The samples were identified by their X-ray powder diffractograms recorded using a Philips diffractometer (CoKa radiation). All reflections were indexed orthorhombic with TiNiSi-type crystal structure. The determined lattice parameters were found to be in fair agreement with those reported earlier [13]. A SQUID magnetometer was used to measure the DC magnetization and magnetic susceptibility in the temperature range from 4.2 to 300 K. Additional magnetization data were obtained at 4.2 K on free powder samples aligned in applied magnetic fields up to 120 kOe using Oxford Instruments VSM12T vibrating sample magnetometer. No demagnetization corrections were made in the process of obtaining the magnetization values. Neutron diffraction patterns were obtained for TbPtSi, TbPtGe, DyPtSi and DyPtGe compounds using the E6 diffractometer installed at the BERII reactor at the HahnMeitner Institute, Berlin. No neutron data could be obtained for GdPtSi and GdPtGe for the obvious reason, namely the very large absorption cross-section of Gd ˚ All nuclei. The used neutron wavelength was 2.438(3) A. data processing was done using the FULLPROF program
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00800-2
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Fig. 1. Temperature variation of the reciprocal magnetic susceptibility for (a) RPtSi and (b) RPtGe (R5Gd, Tb, Dy). The inset shows the x (T ) plot at low temperatures. Table 1 Crystal structure parameters of TbPtSi, TbPtGe, DyPtSi and DyPtGe derived from neutron diffraction data Compound T (K)
TbPtSi 25
DyPtSi 12
TbPtGe 25
DyPtGe 9
˚ a (A) ˚ b (A) ˚ c (A) xR zR x Pt z Pt xx zy R Bragg (%) R prof. (%)
7.013 (7) 4.271 (3) 7.402 (9) 20.002 (4) 0.671 (6) 0.205 (2) 0.143 (4) 0.305 (5) 0.413 (6) 14.6 12.5
6.948 (5) 4.251 (3) 7.419 (5) 0.006 (5) 0.6978 (15) 0.193 (4) 0.086 (9) 0.305 (10) 0.431 (19) 10.7 10.0
6.978 (2) 4.325 (1) 7.546 (2) 20.008 (4) 0.700 (1) 0.207 (1) 0.098 (2) 0.310 (1) 0.414 (2) 6.4 7.7
6.998 (11) 4.359 (6) 7.557 (11) 0.005 (8) 0.703 (2) 0.182 (7) 0.084 (5) 0.327 (10) 0.422 (7) 18.4 15.3
[14], which defines also the R factor as a criterion for the agreement between the observed and calculated neutron intensities. Neutron scattering lengths were taken from Ref. [15], magnetic form factors were adopted after Ref. [16]. The observed and calculated neutron intensities are obtainable from the corresponding author.
3. Results
3.1. Crystal structure. Neutron diffraction data recorded in the paramagnetic state confirmed the orthorhombic TiNiSi-type crystal struc-
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
Fig. 2. Field dependence of magnetization at different temperatures for RPtX (R5Gd, Tb, Dy; X5Si, Ge).
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Fig. 3. Observed and calculated neutron diffractograms of TbPtSi at 1.5 and 25 K. Open squares represent the experimental points, solid lines the calculated profiles. The lower diffractogram shows the difference between the experimental and calculated patterns. Vertical ticks indicated the positions of magnetic and nuclear peaks. Parts of neutron diffractograms recorded in the temperature range from 1.5 to 14 K are shown in the inset.
ture (space group Pnma, 4R, 4Pt and 4X atoms in the 4(c) ¯ ]34 , z; ¯ ]12 2 x, ]34 , ]12 1 z; ]12 1 x, ]14 , ]12 2 z, each sites x, ]14 , z; x, with different x and z parameters). The crystal structure parameters are listed in Table 1.
3.2. Magnetic properties The temperature variation of the magnetic susceptibility x and reciprocal magnetic susceptibility x 21 recorded in
the presence of magnetic field of 10 Oe is displayed in Fig. 1. Maxima indicating the onset of long range antiferromagnetic order are observed at 12.5 K for GdPtGe, at 12.5 K for TbPtSi, at 15 K for TbPtGe, at 8.2 K for DyPtSi and at 8 K for DyPtGe. In the case of GdPtSi a broad maximum is observed at low temperatures. The values of x decrease above 16 K which is probably the phase transition from order to paramagnetic state. The additional small maximum is observed at about 6 K (see inset in Fig. 1).
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Fig. 4. Observed and calculated neutron patterns of TbPtGe at 1.5 and 25 K. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections. The temperature variation of the 100 6 magnetic peak intensity is illustrated in the inset.
The Curie–Weiss law is obeyed above the respective ´ temperatures in all title compounds (see Fig. 1). The Neel numerical fit to the experimental data yields magnitudes of the effective magnetic moments meff that are close to the R31 free ion values (see Table 2). The paramagnetic Curie temperatures are negative, thus indicating the dominance of antiferromagnetic interactions. Magnetization isotherms recorded at different temperatures are shown in Fig. 2. The curve of GdPtSi taken at 4.2
K has ferromagnetic character, however, even in the presence of magnetic field H5120 kOe the magnetization is not saturated. As the temperature rises to 10 K the curve attains metamagnetic character with a critical field at 11 kOe. The magnetization curves of TbPtSi and DyPtSi show also metamagnetic character with critical fields of 2.5 and 13.4 kOe, respectively. The magnetization curves of GdPtGe show linear increase as the magnetic field strength rises. The magnetization curves of TbPtGe and
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
84
Fig. 5. Schematic representations of magnetic structures found in TbPtGe (a) and DyPtSi (b).
moment localized on the Tb 31 ion amounts to 6.92(16) mB at 1.5 K and is parallel to the c-axis. The temperature dependence of the neutron magnetic peak inten´ point at 15 K. sities locates the Neel
DyPtGe recorded at 4.2 K exhibit metamagnetic character with critical fields of 68 and 17 kOe, respectively. The values of magnetic moments for all title compounds determined at 4.2 K and 120 kOe are smaller than the free R31 ion values (see Table 2).
3.3. Magnetic structures 3.3.1. TbPtSi The neutron diffractogram of TbPtSi recorded at 1.5 K (Fig. 3) shows the presence of a number of magnetic peaks, of rather small intensities. The positions of the magnetic peaks indicate a coexistence of two magnetic phases: • a noncollinear antiferromagnetic corresponding to the G x C y mode (see Appendix A) with a component of the magnetic moment mx 52.57(5) mB and my 53.19(9) mB at 1.5 K, localized on the Tb 31 ion • a sine wave modulated structure described by the wave vector k5[0.345(2), 0.565(1), 0.455(3)]. The magnetic
3.3.2. TbPtGe Magnetic peaks observed in the neutron pattern of TbPtGe recorded at 1.5 K (Fig. 4) were indexed as satellites to the allowed and forbidden nuclear reflections. The absence of higher-order satellites suggested that the magnetic structure is either a flat spiral or sine wave modulated. The best fitted observed magnetic peaks intensities were obtained for a sine wave modulated structure characterized by the wave vector k5[0, 0.2840(3), 0] with Tb 31 moment of 9.33(10) mB at 1.5 K. The distribution of moments in the crystallographic unit cell corresponds to the C mode (see Appendix A). The moment direction makes an angle of 608 with the c-axis and 638 with the a-axis. The temperature variation of the intensities of two strongest sattellite reflections (100 6 and 001 6 see inset in Fig. 4) shows that this ordering scheme does not change in the temperature range from 1.5 K to 15 K, the
Table 2 Magnetic data for RPtX (R5Gd, Tb, Dy; X5Si, Ge) compounds obtained from magnetization and neutron diffraction experiments Compound
up (K)
T N (K) M
ND
meff ( mB / R atom) Experimental
m ( mB / R atom) Theoretical
Experimental M
GdPtSi TbPtSi DyPtSi GdPtGe TbPtGe DyPtGe
16.0 12.5 8.2 12.5 15 8
14.5 8 15 7
214.5 216.9 29.6 225 210.7 26.0
8.07 9.78 10.40 7.90 9.60 10.30
7.94 9.72 10.65 7.94 9.72 10.65
6.0 7.0 8.2 5.1 5.2 7.5
Theoretical
ND 7.12(20) 8.78(20) 9.33(10) 9.61(33)
7.0 9.0 10.0 7.0 9.0 10.0
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Fig. 6. Differential (1.5–12 K) and recorded at 12 K neutron diffraction patterns of DyPtSi. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections.
´ point of TbPtGe. This structure is schematically Neel illustrated in Fig. 5a.
3.3.3. DyPtSi and DyPtGe The magnetic peaks observed in the neutron diffraction patterns recorded at 1.5 K (Figs. 6 and 7) were readily indexed assuming a collinear antiferromagnetic structure
described by the wave vector k5[ ]12 , 0, ]12 ]. It corresponds to the C mode in terms of Bertaut’s analysis, see Appendix A. This structure is schematically illustrated in Fig. 5b. The magnetic moments are localized on Dy 31 ions and at 1.5 K amount to 8.8(2) mB in DyPtSi and 9.6(3) mB in DyPtGe. The moments are parallel to the b-axis in DyPtSi and the c-axis in DyPtGe. The above magnetic
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Fig. 7. Observed at 1.5 and 9 K and calculated neutron diffractograms of DyPtGe. Open squares represent the experimental points, solid lines the calculated profiles. The lower pattern shows the difference between the experimental and calculated patterns. Vertical ticks indicate the positions of magnetic and nuclear reflections.
order is stable in the temperature range from 1.5 K to the ´ points: 8 K (DyPtSi) and 7 K (DyPtGe). respective Neel
4. Comment Our X-ray and neutron diffraction data confirm that all title compounds exhibit the orthorhombic TiNiSi-type crystal structure. The magnetic moment carrying lantha-
nide ions R31 form wavy chains running along the a-axis ˚ with fairly large R31 –R31 separation of more than 3.5 A. Also the R31 –R31 distance between two nearest ions belonging to adjacent chains is large. This suggests that direct magnetic interactions are highly improbable. The stability of the observed magnetic ordering schemes may thus be considered as due to the interaction via conduction electrons (the RKKY model). However adoption of this model should lead to the proportionality of the values of
B. Penc et al. / Journal of Alloys and Compounds 299 (2000) 79 – 87
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Appendix A The moments localized on the lanthanide ions situated in the 4(c) position of the space group Pnma are denoted as:
S D S D S 1 1 1 S S] 1 x, ], ] 2 zD 2 4 2
D
1 3 1 3 1 ¯ ], z¯ ; S 3 ] 2 x, ], ] 1 z ; S 1 x, ], z ; S 2 x, 4 4 2 4 2 4
The analysis introduced by Bertaut [18] based on irreducible representations for spin transitions for the above group leads to three antiferromagnetic structures described by the following vectors: G5S 1 2S 2 1S 3 2S 4 ; C5S 1 1S 2 2S 3 2S 4 ; A5S 1 2S 2 2S 3 1S 4 . ´ temperatures of RPtX (R5Gd–Er; X5Si, Ge) compounds Fig. 8. Neel as compared with the de Gennes function. The dashed lines represent the de Gennes functions for RPtSi and RPtGe as normalized with respect to GdPtSi and GdPtGe.
´ or Curie points to the de Gennes function (( gJ 2 Neel 1)2 J(J 11) [17]. Fig. 8 shows that the de Gennes scaling for the title compounds is not obeyed. This discrepancy probably results from a strong influence of the crystalline electric field (CEF). The latter effect probably decreases the tendency to form oscillatory and incommensurate magnetic ordering. In fact, only in TbPtSi and TbPtGe such ordering schemes are observed, while DyPtSi, DyPtGe and HoPtSi [1] show collinear antiferromagnetic structures with the wave vector k5[ ]12 , 0, ]12 ]. At 1.5 K ErPtSi and ErPtGe exhibit also collinear antiferromagnetic structures, both characterized by the wave vector k5 ´ points a [0, ]12 , 0], however, close to the respective Neel transition to incommensurate structures is observed in these compounds [1].
Acknowledgements The kind hospitality and financial support by the HahnMeitner Institute to perform neutron diffraction measurements is gratefully acknowledged by three authors (J.L., B.P., A.S.). This work has been supported in part by the State Committee for Scientific Research in Poland with grant No. 270 / P03 / 98 / 15.
References ´ [1] B. Penc, S. Baran, M. Hofmann, J. Leciejewicz, M. Slaski, A. Szytul«a, J. Phys. Condens. Matter. 11 (1999) 5631. ~ [2] W. Bazela, J. Leciejewicz, A. Szytul«a, J. Magn. Magn. Mater. 50 (1985) 19. ~ [3] W. Bazela, A. Zygmunt, E. Ressouche, J. Leciejewicz, W. Sikora, J. Alloys Comp. 243 (1996) 106. ¨ [4] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, J. Magn. Magn. Mater. 149 (1995) 265. ´ [5] B. Penc, M. Hofmann, M. Kolenda, M. Slaski, A. Szytul«a, J. Alloys Comp. 267 (1997) L4. [6] B. Penc, S. Baran, M. Hofmann, A. Szytul«a, J. Alloys Comp. 287 (1999) 48. ´ [7] B. Penc, S. Baran, M. Hofmann, J. Leciejewicz, M. Slaski, A. Szytul«a, J. Alloys Comp. 287 (1999) 18. ¨ [8] M. Kolenda, J. Leciejewicz, N. Stusser, A. Szytul«a, A. Zygmunt, J. Magn. Magn. Mater. 145 (1995) 85. ¨ [9] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, A. Zygmunt, J. Magn. Magn. Mater. 163 (1996) 273. ¨ [10] A. Szytul«a, M. Kolenda, J. Leciejewicz, N. Stusser, J. Magn. Magn. Mater. 164 (1996) 377. ´ [11] B. Penc, M. Hofmann, M. Slaski, A. Zygmunt, Physica B, (1999) in press. [12] B. Penc, M. Hofmann, J. Leciejewicz, A. Szytul«a, A. Zygmunt, (1999) to be published. ´ J. Less [13] E. Hovestreydt, N. Engel, K. Klepp, B. Chabot, E. Parthe, Common. Methods 85 (1982) 247. [14] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [15] V.F. Sears, Neutron News 3 (1992) 26. [16] A.J. Freeman, J.P. Desclaux, J. Magn. Magn. Mater. 12 (1979) 11. [17] P.G. de Gennes, J. Phys Radium 23 (1962) 570. [18] E.F. Bertaut, Acta Crystallogr. A 24 (1968) 217.