- Email: [email protected]
Magnetic and Mössbauer study of U1−xSmxFe10Si2
July 1998
Materials Letters 36 Ž1998. 52–55
Magnetic and Mossbauer study of U1yx Sm x Fe 10 Si 2 ¨ M. Sorescu a
a,)
, M. Valeanu b, D. Tomuta
b
Duquesne UniÕersity, Physics Department, Mellon Hall, Pittsburgh, PA 15282, USA b Institute of Atomic Physics, Bucharest-Magurele, 76900, Romania Received 1 December 1997; accepted 8 December 1997
Abstract The ternary and pseudoternary compounds of the type U1yx Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. with the tetragonal ThMn 12-type structure were studied by 57 Fe Mossbauer spectroscopy, hysteresis loop and AC susceptibility measurements. ¨ Sm substitutions in UFe10 Si 2 were found to increase the magnetic hyperfine fields corresponding to the 8f, 8i and 8j inequivalent iron sites, with Si atoms distributed between the 8f and 8j sites. In the U1yx Sm x Fe10 Si 2 system, the Sm substitution was found to affect the magnetic moment and increase the hysteresis phenomenon. The magnetic moment increased for x s 0.5 and decreased for x s 0.75, reaching saturation at 4.2 K in an applied field of 30 kOe. Due to the three nonequivalent iron positions, this behavior can be attributed to a spin reorientation transition. The transition was observed at 180 K in the AC response Ž10 Oe, 10 Hz. of U0.25 Sm 0.75 Fe10 Si 2 . q 1998 Elsevier Science B.V. All rights reserved. PACS: 75.40 Gb; 75.50 Bb; 76.80 q y Keywords: Magnetically ordered materials; Spin dynamics; Uranium silicides; Ternary compounds; Intermetallics; Mossbauer spectroscopy ¨
1. Introduction Rare earth intermetallic compounds having the tetragonal crystal structure of the ThMn 12 -type form a new class of magnetic materials with very high iron content w1–5x. The iron subsystem in these compounds has a uniaxial magnetocrystalline anisotropy, which is of great interest for permanent magnet ) Corresponding author. Department of Physics, Duquesne University, Bayer School of Natural and Environmental Sciences, Pittsburgh, PA 15282-1503, USA. Tel.: q1-412-396-5277; fax: q1-412-396-4829; e-mail: [email protected].
applications. The crystal structure has only one high symmetry position for the rare earth atoms and three for iron and atoms of the stabilizing element. In this paper, new ternary and pseudoternary compounds of the type U1y x Sm x Fe10 Si 2 were synthesized to investigate the substitutional effects on the structural and magnetic properties. 57 Fe Mossbauer ¨ spectroscopy was used to clarify the influence of Sm substitutions on the hyperfine parameters. DC magnetic measurements were performed to evidence the relationship between structural changes and modifications in hysteresis and magnetic moment values. The concentration dependence of the magnetic mo-
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 0 7 - X
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
53
ments was correlated with AC susceptibility results on the occurrence of spin reorientation.
2. Experimental The U1y x Sm x Fe10 Si 2 samples were obtained by melting the components in an arc furnace under protective argon atmosphere. The ingots were turned several times to avoid inhomogeneities and annealed at 950–10508C for 10 days. Standard X-ray analysis was used for the determination of the phase composition of the alloys. Room temperature transmission Mossbauer spec¨ tra were recorded with a constant acceleration spectrometer using a 57Co source in Rh matrix. Least squares fitting of the Mossbauer spectra was per¨ formed using the NORMOS program w6x. The relative intensities of the outer:inner line pairs were correlated to be the same for all component subspectra. Magnetic measurements were performed using a susceptometer–magnetometer system. Hysteresis loops were recorded at 4.2 K in an applied magnetic field of 30 kOe. Magnetic susceptibility measurements were performed with an AC field of 10 Oe ŽRMS. at a frequency of 10 Hz in the temperature range 4.2–300 K.
3. Results and discussion Room temperature transmission Mossbauer spec¨ tra of U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5; 0.75. are given in Fig. 1. Refined values of hyperfine parameters are listed in Table 1. All spectra could be analyzed by considering three sextets, corresponding to iron atoms in 8i, 8j and 8f positions. The sextet with the largest hyperfine field is related to iron atoms located on the 8i sites, whereas the sextet with the lowest value of hyperfine field corresponds to iron atoms on the 8f sites w7–10x. As in other RFe10 Si 2 compounds w7x, the 8i site is completely occupied by Fe, whereas the Si atoms share the 8j and 8f positions with the Fe atoms. In U1y x Sm x Fe10 Si 2 , the rare earth atoms
Fig. 1. The room-temperature transmission Mossbauer spectra of ¨ U1y x Sm x Fe10 Si 2 samples: Ža. x s 0; Žb. x s 0.5 and Žc. x s 0.75.
occupy the 2a crystallographic position. The uranium atom has a magnetic moment of 0.5 m B , coupled ferromagnetically with the iron sublattice w5x. It can be seen in Table 1 that magnetic Sm, which substitutes for uranium atoms, increases the values of the hyperfine magnetic fields at the iron sites. Fig. 2 shows the hysteresis loops of U1yx Sm x Fe 10 Si 2 Ž x s 0; 0.5; 0.75. samples, recorded at 4.2 K in an applied magnetic field of 30
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
54
Fig. 2. The hysteresis loop measurements recorded in a parallel orientation at 4.2 K with an applied magnetic field of 30 kOe for Ža. UFe10 Si 2 ; Žb. U0.5 Sm 0.5 Fe10 Si 2 and Žc. U0.25 Sm 0.75 Fe 10 Si 2 .
Fig. 3. The AC susceptibility of U0.25 Sm 0.75 Fe10 Si 2 recorded with a field of 10 Oe ŽRMS. at a frequency of 10 Hz in the temperature range 4.2–300 K. The arrow indicates the occurrence of a spin orientation transition at 180 K.
kOe. In agreement with literature data, the magnetization of UFe10 Si 2 has not reached saturation and shows a very weak hysteresis phenomenon w10–12x. Sm substitution increases the hysteresis and affects the magnetic moment. For x s 0.5, the magnetic moment increases as compared to the UFe10 Si 2 sample, whereas for x s 0.75 the magnetic moment decreases markedly and reaches saturation in the applied field of 30 kOe.
In order to explain the dependence of the magnetic moment values on the concentration x of Sm substitution in U1y x Sm x Fe10 Si 2 , AC susceptibility measurements were also performed and the results plotted in Fig. 3. The imaginary part x Y of the AC susceptibility of U0.25 Sm 0.75 Fe 10 Si 2 exhibits a maximum at about 180 K, which corresponds to a spin reorientation transition w12x. This transition originates in the three nonequivalent iron positions and the
Table 1 The hyperfine magnetic field H hf , quadrupole shift 2 ´ , isomer shift d Žrelative to a-Fe at 300 K., and relative areas corresponding to the component patterns in the transmission Mossbauer spectra of U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. ¨ Sample
H hf ŽkOe.
2 ´ Žmmrs.
d Žmmrs.
Rel. areas Ž%.
Assignment of sites
UFe10 Si 2
196.2 234.2 212.6 200.1 246.6 235.5 227.5 254.5 236.1 "0.5
0.08 0.21 0.04 0.11 0.46 0.10 0.08 0.15 0.38 "0.05
y0.009 0.041 y0.101 y0.110 0.079 y0.128 y0.173 y0.036 y0.099 "0.005
14.3 34.3 51.4 14.6 34.8 50.6 19.1 45.1 35.8 "0.5
8f 8i 8j 8f 8i 8j 8f 8i 8j
U0.5 Sm 0.5 Fe10 Si 2
U0.25 Sm 0.75 Fe10 Si 2
Errors
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
saturation is reached considerably more easily ŽFig. 2., but the magnetic moment value is low.
4. Conclusions 57
Fe Mossbauer spectroscopy, hysteresis loop and ¨ AC susceptibility measurements were employed to investigate ternary and pseudoternary compounds of the type U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. with the tetragonal ThMn 12 -type structure. U substitutions at the rare earth sublattice were found to affect the hyperfine parameters and relative occupancy of the three nonequivalent iron positions, cause modifications of the saturation magnetic moments and hysteresis phenomenon, and induce spin reorientation transitions.
Acknowledgements This work has been supported by a Cottrell College Science Award of Research.
55
References w1x D.B. de Mooij, K.H.J. Buschow, Philips J. Res. 42 Ž1987. 246. w2x K.H.J. Buschow, J. Appl. Phys. 63 Ž1988. 3130. w3x B. Ping Hu, H.-S. Li, J.P. Cavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 Ž1989. 755. w4x A.V. Andreev, V. Sechovsky, N.V. Kudrevatykh, S.S. Sigaev, E.N. Tarasov, J. Less-Common Met. 144 Ž1988. L21. w5x A.V. Andreev, V.A.N. Bogatkin, N.V. Kudrevatykh, S.S. Sigaev, E.N. Tarasov, Phys. Met. Metallogr. 68 Ž1989. 68. w6x R.A. Brand, J. Lauer, D.M. Herlach, J. Phys. F 13 Ž1983. 675. w7x T. Sinnemann, M.U. Wisniewski, M. Rosenberg, K.H.J. Buschow, J. Magn. Magn. Mater. 83 Ž1990. 259. w8x F.R. de Boer, Z.G. Zhao, K.H.J. Buschow, J. Magn. Magn. Mater. 157 Ž1996. 504. w9x A.V. Andreev, F.G. Vagizov, W. Suski, H. Drulis, J. Alloys Comp. 187 Ž1992. 401. w10x T. Berlureau, B. Chevalier, P. Gravereau, L. Fournes, J. Etourneau, J. Magn. Magn. Mater. 102 Ž1991. 166. w11x H.S. Li, J.D.M. Coey, Handbook of Magnetic Materials. North Holland, Amsterdam, 1991. w12x W. Suski, A. Baran, T. Mydlarz, Phys. Lett. A 136 Ž1989. 89.
Materials Letters 36 Ž1998. 52–55
Magnetic and Mossbauer study of U1yx Sm x Fe 10 Si 2 ¨ M. Sorescu a
a,)
, M. Valeanu b, D. Tomuta
b
Duquesne UniÕersity, Physics Department, Mellon Hall, Pittsburgh, PA 15282, USA b Institute of Atomic Physics, Bucharest-Magurele, 76900, Romania Received 1 December 1997; accepted 8 December 1997
Abstract The ternary and pseudoternary compounds of the type U1yx Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. with the tetragonal ThMn 12-type structure were studied by 57 Fe Mossbauer spectroscopy, hysteresis loop and AC susceptibility measurements. ¨ Sm substitutions in UFe10 Si 2 were found to increase the magnetic hyperfine fields corresponding to the 8f, 8i and 8j inequivalent iron sites, with Si atoms distributed between the 8f and 8j sites. In the U1yx Sm x Fe10 Si 2 system, the Sm substitution was found to affect the magnetic moment and increase the hysteresis phenomenon. The magnetic moment increased for x s 0.5 and decreased for x s 0.75, reaching saturation at 4.2 K in an applied field of 30 kOe. Due to the three nonequivalent iron positions, this behavior can be attributed to a spin reorientation transition. The transition was observed at 180 K in the AC response Ž10 Oe, 10 Hz. of U0.25 Sm 0.75 Fe10 Si 2 . q 1998 Elsevier Science B.V. All rights reserved. PACS: 75.40 Gb; 75.50 Bb; 76.80 q y Keywords: Magnetically ordered materials; Spin dynamics; Uranium silicides; Ternary compounds; Intermetallics; Mossbauer spectroscopy ¨
1. Introduction Rare earth intermetallic compounds having the tetragonal crystal structure of the ThMn 12 -type form a new class of magnetic materials with very high iron content w1–5x. The iron subsystem in these compounds has a uniaxial magnetocrystalline anisotropy, which is of great interest for permanent magnet ) Corresponding author. Department of Physics, Duquesne University, Bayer School of Natural and Environmental Sciences, Pittsburgh, PA 15282-1503, USA. Tel.: q1-412-396-5277; fax: q1-412-396-4829; e-mail: [email protected].
applications. The crystal structure has only one high symmetry position for the rare earth atoms and three for iron and atoms of the stabilizing element. In this paper, new ternary and pseudoternary compounds of the type U1y x Sm x Fe10 Si 2 were synthesized to investigate the substitutional effects on the structural and magnetic properties. 57 Fe Mossbauer ¨ spectroscopy was used to clarify the influence of Sm substitutions on the hyperfine parameters. DC magnetic measurements were performed to evidence the relationship between structural changes and modifications in hysteresis and magnetic moment values. The concentration dependence of the magnetic mo-
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 0 7 - X
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
53
ments was correlated with AC susceptibility results on the occurrence of spin reorientation.
2. Experimental The U1y x Sm x Fe10 Si 2 samples were obtained by melting the components in an arc furnace under protective argon atmosphere. The ingots were turned several times to avoid inhomogeneities and annealed at 950–10508C for 10 days. Standard X-ray analysis was used for the determination of the phase composition of the alloys. Room temperature transmission Mossbauer spec¨ tra were recorded with a constant acceleration spectrometer using a 57Co source in Rh matrix. Least squares fitting of the Mossbauer spectra was per¨ formed using the NORMOS program w6x. The relative intensities of the outer:inner line pairs were correlated to be the same for all component subspectra. Magnetic measurements were performed using a susceptometer–magnetometer system. Hysteresis loops were recorded at 4.2 K in an applied magnetic field of 30 kOe. Magnetic susceptibility measurements were performed with an AC field of 10 Oe ŽRMS. at a frequency of 10 Hz in the temperature range 4.2–300 K.
3. Results and discussion Room temperature transmission Mossbauer spec¨ tra of U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5; 0.75. are given in Fig. 1. Refined values of hyperfine parameters are listed in Table 1. All spectra could be analyzed by considering three sextets, corresponding to iron atoms in 8i, 8j and 8f positions. The sextet with the largest hyperfine field is related to iron atoms located on the 8i sites, whereas the sextet with the lowest value of hyperfine field corresponds to iron atoms on the 8f sites w7–10x. As in other RFe10 Si 2 compounds w7x, the 8i site is completely occupied by Fe, whereas the Si atoms share the 8j and 8f positions with the Fe atoms. In U1y x Sm x Fe10 Si 2 , the rare earth atoms
Fig. 1. The room-temperature transmission Mossbauer spectra of ¨ U1y x Sm x Fe10 Si 2 samples: Ža. x s 0; Žb. x s 0.5 and Žc. x s 0.75.
occupy the 2a crystallographic position. The uranium atom has a magnetic moment of 0.5 m B , coupled ferromagnetically with the iron sublattice w5x. It can be seen in Table 1 that magnetic Sm, which substitutes for uranium atoms, increases the values of the hyperfine magnetic fields at the iron sites. Fig. 2 shows the hysteresis loops of U1yx Sm x Fe 10 Si 2 Ž x s 0; 0.5; 0.75. samples, recorded at 4.2 K in an applied magnetic field of 30
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
54
Fig. 2. The hysteresis loop measurements recorded in a parallel orientation at 4.2 K with an applied magnetic field of 30 kOe for Ža. UFe10 Si 2 ; Žb. U0.5 Sm 0.5 Fe10 Si 2 and Žc. U0.25 Sm 0.75 Fe 10 Si 2 .
Fig. 3. The AC susceptibility of U0.25 Sm 0.75 Fe10 Si 2 recorded with a field of 10 Oe ŽRMS. at a frequency of 10 Hz in the temperature range 4.2–300 K. The arrow indicates the occurrence of a spin orientation transition at 180 K.
kOe. In agreement with literature data, the magnetization of UFe10 Si 2 has not reached saturation and shows a very weak hysteresis phenomenon w10–12x. Sm substitution increases the hysteresis and affects the magnetic moment. For x s 0.5, the magnetic moment increases as compared to the UFe10 Si 2 sample, whereas for x s 0.75 the magnetic moment decreases markedly and reaches saturation in the applied field of 30 kOe.
In order to explain the dependence of the magnetic moment values on the concentration x of Sm substitution in U1y x Sm x Fe10 Si 2 , AC susceptibility measurements were also performed and the results plotted in Fig. 3. The imaginary part x Y of the AC susceptibility of U0.25 Sm 0.75 Fe 10 Si 2 exhibits a maximum at about 180 K, which corresponds to a spin reorientation transition w12x. This transition originates in the three nonequivalent iron positions and the
Table 1 The hyperfine magnetic field H hf , quadrupole shift 2 ´ , isomer shift d Žrelative to a-Fe at 300 K., and relative areas corresponding to the component patterns in the transmission Mossbauer spectra of U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. ¨ Sample
H hf ŽkOe.
2 ´ Žmmrs.
d Žmmrs.
Rel. areas Ž%.
Assignment of sites
UFe10 Si 2
196.2 234.2 212.6 200.1 246.6 235.5 227.5 254.5 236.1 "0.5
0.08 0.21 0.04 0.11 0.46 0.10 0.08 0.15 0.38 "0.05
y0.009 0.041 y0.101 y0.110 0.079 y0.128 y0.173 y0.036 y0.099 "0.005
14.3 34.3 51.4 14.6 34.8 50.6 19.1 45.1 35.8 "0.5
8f 8i 8j 8f 8i 8j 8f 8i 8j
U0.5 Sm 0.5 Fe10 Si 2
U0.25 Sm 0.75 Fe10 Si 2
Errors
M. Sorescu et al.r Materials Letters 36 (1998) 52–55
saturation is reached considerably more easily ŽFig. 2., but the magnetic moment value is low.
4. Conclusions 57
Fe Mossbauer spectroscopy, hysteresis loop and ¨ AC susceptibility measurements were employed to investigate ternary and pseudoternary compounds of the type U1y x Sm x Fe10 Si 2 Ž x s 0; 0.5 and 0.75. with the tetragonal ThMn 12 -type structure. U substitutions at the rare earth sublattice were found to affect the hyperfine parameters and relative occupancy of the three nonequivalent iron positions, cause modifications of the saturation magnetic moments and hysteresis phenomenon, and induce spin reorientation transitions.
Acknowledgements This work has been supported by a Cottrell College Science Award of Research.
55
References w1x D.B. de Mooij, K.H.J. Buschow, Philips J. Res. 42 Ž1987. 246. w2x K.H.J. Buschow, J. Appl. Phys. 63 Ž1988. 3130. w3x B. Ping Hu, H.-S. Li, J.P. Cavigan, J.M.D. Coey, J. Phys.: Condens. Matter 1 Ž1989. 755. w4x A.V. Andreev, V. Sechovsky, N.V. Kudrevatykh, S.S. Sigaev, E.N. Tarasov, J. Less-Common Met. 144 Ž1988. L21. w5x A.V. Andreev, V.A.N. Bogatkin, N.V. Kudrevatykh, S.S. Sigaev, E.N. Tarasov, Phys. Met. Metallogr. 68 Ž1989. 68. w6x R.A. Brand, J. Lauer, D.M. Herlach, J. Phys. F 13 Ž1983. 675. w7x T. Sinnemann, M.U. Wisniewski, M. Rosenberg, K.H.J. Buschow, J. Magn. Magn. Mater. 83 Ž1990. 259. w8x F.R. de Boer, Z.G. Zhao, K.H.J. Buschow, J. Magn. Magn. Mater. 157 Ž1996. 504. w9x A.V. Andreev, F.G. Vagizov, W. Suski, H. Drulis, J. Alloys Comp. 187 Ž1992. 401. w10x T. Berlureau, B. Chevalier, P. Gravereau, L. Fournes, J. Etourneau, J. Magn. Magn. Mater. 102 Ž1991. 166. w11x H.S. Li, J.D.M. Coey, Handbook of Magnetic Materials. North Holland, Amsterdam, 1991. w12x W. Suski, A. Baran, T. Mydlarz, Phys. Lett. A 136 Ž1989. 89.