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Magnetic properties of PrRhSn: A single-crystal study
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760 www.elsevier.com/locate/jmmm
Magnetic properties of PrRhSn: A single-crystal study M. Mihalik, V. Sechovsky´ Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Available online 13 November 2006
Abstract A PrRhSn single crystal has been grown and investigated with respect to magnetic behavior. The magnetization and specific heat were measured as functions of temperature (range 2–300 K) and magnetic field (range 0–9 T) applied along the main crystallographic axes. A strong uniaxial magnetocrystalline anisotropy was observed with the c-axis as the easy magnetization direction. The transition temperature from paramagnetic to ferromagnetic state was determined to be T c ¼ 2:85ð3Þ K. The specific heat data exhibit a Schottky anomaly just above T c . The anomaly is lifted to higher temperatures with an applied magnetic field along the c-axis yielding a considerable magnetocaloric effect. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 75.30.Gw; 75.30.Sg Keywords: Rare earth; Specific heat; Magnetocrystalline anisotropy; Magnetic entropy
PrRhSn belongs to the group of the ternary RETX intermetallic compounds (RE-rare earth; T-transition metal; X -p-electron element), which crystallize in the hexagonal ZrNiAl-type structure. The lattice parameters of the compound are a ¼ 742:49ð7Þ pm and c ¼ 415:05ð5Þ pm [1]. Information on magnetism in this material available so far was limited to results of experiments performed on polycrystals [1,2]. Routsi et al. [2] presented results at temperatures only down to 4.2 K claiming, that compound obeys a Curie–Weiss (CW) law at temperatures above 40 K with Yp ¼ 10 K and with meff in good agreement the Pr3þ free ion value. Ła¸tka et al. [1] reports ferromagnetism of the PrRhSn with T c ¼ 3:0ð1Þ K. These authors also claim that the magnetization does not saturate up to 5 T, reaching only 1:85ð2Þ mB =f:u: at 5 T. To study intrinsic magnetic properties of PrRhSn we have grown a single crystal by Czochralski method and measured specific heat and magnetization as functions of temperature and magnetic field, and the temperature dependence of the AC susceptibility in field 0.001 T. All reported measurements were done using PPMS (quantum
Corresponding author. Tel.: +420 2 21911655; fax: +420 2 21911393.
E-mail address: [email protected] (M. Mihalik). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.585
design) apparatus in temperature range 2–300 K and in magnetic fields up to 9 T. The specific heat shows a peak at 2:85ð3Þ K (Fig. 1), which we associate with the Curie temperature T c . The peak shifts to higher temperatures and smears out with applying field along the c-axis to disappear already in 1 T. Above T c , we have observed a bump in the temperature dependence of the specific heat and attributed it to a Schottky contribution to specific heat. This bump broadens and shifts to higher temperatures in a magnetic field applied along the c-axis. To find and subtract the nonmagnetic contribution from the specific heat we have used expression C el ¼ gT for the electronic part of the specific heat and Debye theory for describing the phonon part of the specific heat. Also we have approximated the Debye integral with the expression published by Svoboda et al. [3]. From the fit in the temperature range 25–200 K we have obtained the parameters yDPr ¼ 205ð5Þ K and gPr ¼ 86ð5Þ mJ mol1 K2 . For comparison of the nonmagnetic contribution to the specific heat in PrRhSn we have grown a LaRhSn single crystal and measured its specific heat. Only for comparison we have used the same fitting procedure in the same temperature range also for LaRhSn and we have obtained YDLa ¼ 215ð5Þ K and gLa ¼ 43ð3Þ mJ mol1 K2 . We have to note that the
ARTICLE IN PRESS M. Mihalik, V. Sechovsky´ / Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760
Fig. 1. The temperature dependence of specific heat measured in applied field along the c-axis. Because there are no significant effects in temperatures above 30 K we present only low temperature detail to simplify the figure.
LaRhSn specific heat also obeys equation C ¼ gT þ bT 3 at temperature range 2.4–22 K with more accurate coefficients for normal metal: g ¼ 9:5ð3Þ mJ mol1 K2 and b ¼ 6:6ð4Þ 104 mJ mol1 K4 ; it means YD ¼ 206 K. From this analysis we can conclude that Debye temperatures for both compounds fit together. After subtracting the nonmagnetic part from the specific heat data we have calculated the magnetic entropy and its change with respect to the applied magnetic field (see Fig. 2). The maximum change of entropy occurs at 4.3 K for B ¼ 1 T applied along the c-axis. This maximum shifts to higher temperatures with applying a higher magnetic field, and reaches 9.9 K in 9 T. Such a large change of entropy at low temperatures one can use for example for magnetic cooling at temperatures near 10 K. The AC susceptibility reaches maximum at 2:9ð1Þ K for the a-axis. The c-axis AC susceptibility was found to be temperature independent at temperatures below 2:8ð2Þ K. These effects are consequences of magnetic ordering and are in good agreement with magnetization data published on polycrystals by Ła¸tka et al. [1]. The DC susceptibility obeys a CW law at temperatures higher than 50 K with parameters meffa ¼ 3:29 mB and Ypa ¼ 42:5 K for a-axis and with parameters meffc ¼ 3:5 mB and Ypc ¼ 38 K for caxis respectively. The values of the effective moment are in reasonable agreement with the Pr3þ free ion value. The striking difference of the Yp for the two perpendicular directions documents the strong anisotropy in the paramagnetic range due to a pronounced crystal field effect. The magnetization data measured at low temperatures are plotted in the Fig. 3. The a-axis magnetization is linear at all temperatures of measurement, reaching less than 0:7 mB =f:u: in 9 T (at the temperature of 2 K). The c-axis magnetization at 2 K (ordered state) shows an abrupt increase in low fields and then almost saturates in the field of 4.2 T to 3:5 mB =f:u: The magnetization at 10 K can be
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Fig. 2. The change of the magnetic entropy with applied magnetic field. The field was applied along the c-axis.
Fig. 3. Magnetization curves measured along the a-axis (full symbols) and along the c-axis (open symbols) together with the fit at 10 K using the Brollouin function. Because a-axis magnetization is linear and small compared to c-axis we present only 10 K curve to simplify the figure.
fitted by the Brillouin function. From this fit we have obtained a saturated magnetization M sat ¼ 3:6 mB =f:u:, which is somewhat higher than gJ ¼ 3:20 (theoretical moment of Pr3þ ion). This implies existence of an induced Rh moment of 0:4 mB besides the localized magnetic moment of the Pr3þ ion. Magnetic diffraction experiments (neutrons, synchrotron light) on a single crystal as well as ab initio electronic structure calculations are strongly desirable to test this scenario. To conclude, we have grown a single crystal of PrRhSn, confirmed existence of ferromagnetism in PrRhSn with T c ¼ 2:85ð3Þ K and found a strong magnetocrystalline anisotropy with the easy magnetization direction along the c-axis. Also we have found, that specific heat at temperatures below 10 K is strongly field-dependent (magnetic phase transition + Schottky anomaly) which
ARTICLE IN PRESS 1760
M. Mihalik, V. Sechovsky´ / Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760
can be used for magnetic cooling at temperatures below 10 K. This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic. Financial support by the Grant no. OC145 is also acknowledged.
References [1] K. Ła¸tka, R. Kmiec´, J. Gurgul, M. Rams, A.W. Pacyna, T. Schmidt, R. Po¨ttgen, J. Solid State Chem. 178 (2005) 3101. [2] Ch.D. Routsi, J.K. Yakinthos, H. Gamari-Seale, J. Magn. Magn. Mater. 117 (1992) 79. [3] P. Svoboda, P. Javorsky´, M. Divisˇ , V. Sechovsky´, F. Honda, G. Oomi, A.A. Menovsky, Phys. Rev. B 63 (2001) 212408.
Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760 www.elsevier.com/locate/jmmm
Magnetic properties of PrRhSn: A single-crystal study M. Mihalik, V. Sechovsky´ Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Available online 13 November 2006
Abstract A PrRhSn single crystal has been grown and investigated with respect to magnetic behavior. The magnetization and specific heat were measured as functions of temperature (range 2–300 K) and magnetic field (range 0–9 T) applied along the main crystallographic axes. A strong uniaxial magnetocrystalline anisotropy was observed with the c-axis as the easy magnetization direction. The transition temperature from paramagnetic to ferromagnetic state was determined to be T c ¼ 2:85ð3Þ K. The specific heat data exhibit a Schottky anomaly just above T c . The anomaly is lifted to higher temperatures with an applied magnetic field along the c-axis yielding a considerable magnetocaloric effect. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 75.30.Gw; 75.30.Sg Keywords: Rare earth; Specific heat; Magnetocrystalline anisotropy; Magnetic entropy
PrRhSn belongs to the group of the ternary RETX intermetallic compounds (RE-rare earth; T-transition metal; X -p-electron element), which crystallize in the hexagonal ZrNiAl-type structure. The lattice parameters of the compound are a ¼ 742:49ð7Þ pm and c ¼ 415:05ð5Þ pm [1]. Information on magnetism in this material available so far was limited to results of experiments performed on polycrystals [1,2]. Routsi et al. [2] presented results at temperatures only down to 4.2 K claiming, that compound obeys a Curie–Weiss (CW) law at temperatures above 40 K with Yp ¼ 10 K and with meff in good agreement the Pr3þ free ion value. Ła¸tka et al. [1] reports ferromagnetism of the PrRhSn with T c ¼ 3:0ð1Þ K. These authors also claim that the magnetization does not saturate up to 5 T, reaching only 1:85ð2Þ mB =f:u: at 5 T. To study intrinsic magnetic properties of PrRhSn we have grown a single crystal by Czochralski method and measured specific heat and magnetization as functions of temperature and magnetic field, and the temperature dependence of the AC susceptibility in field 0.001 T. All reported measurements were done using PPMS (quantum
Corresponding author. Tel.: +420 2 21911655; fax: +420 2 21911393.
E-mail address: [email protected] (M. Mihalik). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.585
design) apparatus in temperature range 2–300 K and in magnetic fields up to 9 T. The specific heat shows a peak at 2:85ð3Þ K (Fig. 1), which we associate with the Curie temperature T c . The peak shifts to higher temperatures and smears out with applying field along the c-axis to disappear already in 1 T. Above T c , we have observed a bump in the temperature dependence of the specific heat and attributed it to a Schottky contribution to specific heat. This bump broadens and shifts to higher temperatures in a magnetic field applied along the c-axis. To find and subtract the nonmagnetic contribution from the specific heat we have used expression C el ¼ gT for the electronic part of the specific heat and Debye theory for describing the phonon part of the specific heat. Also we have approximated the Debye integral with the expression published by Svoboda et al. [3]. From the fit in the temperature range 25–200 K we have obtained the parameters yDPr ¼ 205ð5Þ K and gPr ¼ 86ð5Þ mJ mol1 K2 . For comparison of the nonmagnetic contribution to the specific heat in PrRhSn we have grown a LaRhSn single crystal and measured its specific heat. Only for comparison we have used the same fitting procedure in the same temperature range also for LaRhSn and we have obtained YDLa ¼ 215ð5Þ K and gLa ¼ 43ð3Þ mJ mol1 K2 . We have to note that the
ARTICLE IN PRESS M. Mihalik, V. Sechovsky´ / Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760
Fig. 1. The temperature dependence of specific heat measured in applied field along the c-axis. Because there are no significant effects in temperatures above 30 K we present only low temperature detail to simplify the figure.
LaRhSn specific heat also obeys equation C ¼ gT þ bT 3 at temperature range 2.4–22 K with more accurate coefficients for normal metal: g ¼ 9:5ð3Þ mJ mol1 K2 and b ¼ 6:6ð4Þ 104 mJ mol1 K4 ; it means YD ¼ 206 K. From this analysis we can conclude that Debye temperatures for both compounds fit together. After subtracting the nonmagnetic part from the specific heat data we have calculated the magnetic entropy and its change with respect to the applied magnetic field (see Fig. 2). The maximum change of entropy occurs at 4.3 K for B ¼ 1 T applied along the c-axis. This maximum shifts to higher temperatures with applying a higher magnetic field, and reaches 9.9 K in 9 T. Such a large change of entropy at low temperatures one can use for example for magnetic cooling at temperatures near 10 K. The AC susceptibility reaches maximum at 2:9ð1Þ K for the a-axis. The c-axis AC susceptibility was found to be temperature independent at temperatures below 2:8ð2Þ K. These effects are consequences of magnetic ordering and are in good agreement with magnetization data published on polycrystals by Ła¸tka et al. [1]. The DC susceptibility obeys a CW law at temperatures higher than 50 K with parameters meffa ¼ 3:29 mB and Ypa ¼ 42:5 K for a-axis and with parameters meffc ¼ 3:5 mB and Ypc ¼ 38 K for caxis respectively. The values of the effective moment are in reasonable agreement with the Pr3þ free ion value. The striking difference of the Yp for the two perpendicular directions documents the strong anisotropy in the paramagnetic range due to a pronounced crystal field effect. The magnetization data measured at low temperatures are plotted in the Fig. 3. The a-axis magnetization is linear at all temperatures of measurement, reaching less than 0:7 mB =f:u: in 9 T (at the temperature of 2 K). The c-axis magnetization at 2 K (ordered state) shows an abrupt increase in low fields and then almost saturates in the field of 4.2 T to 3:5 mB =f:u: The magnetization at 10 K can be
1759
Fig. 2. The change of the magnetic entropy with applied magnetic field. The field was applied along the c-axis.
Fig. 3. Magnetization curves measured along the a-axis (full symbols) and along the c-axis (open symbols) together with the fit at 10 K using the Brollouin function. Because a-axis magnetization is linear and small compared to c-axis we present only 10 K curve to simplify the figure.
fitted by the Brillouin function. From this fit we have obtained a saturated magnetization M sat ¼ 3:6 mB =f:u:, which is somewhat higher than gJ ¼ 3:20 (theoretical moment of Pr3þ ion). This implies existence of an induced Rh moment of 0:4 mB besides the localized magnetic moment of the Pr3þ ion. Magnetic diffraction experiments (neutrons, synchrotron light) on a single crystal as well as ab initio electronic structure calculations are strongly desirable to test this scenario. To conclude, we have grown a single crystal of PrRhSn, confirmed existence of ferromagnetism in PrRhSn with T c ¼ 2:85ð3Þ K and found a strong magnetocrystalline anisotropy with the easy magnetization direction along the c-axis. Also we have found, that specific heat at temperatures below 10 K is strongly field-dependent (magnetic phase transition + Schottky anomaly) which
ARTICLE IN PRESS 1760
M. Mihalik, V. Sechovsky´ / Journal of Magnetism and Magnetic Materials 310 (2007) 1758–1760
can be used for magnetic cooling at temperatures below 10 K. This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic. Financial support by the Grant no. OC145 is also acknowledged.
References [1] K. Ła¸tka, R. Kmiec´, J. Gurgul, M. Rams, A.W. Pacyna, T. Schmidt, R. Po¨ttgen, J. Solid State Chem. 178 (2005) 3101. [2] Ch.D. Routsi, J.K. Yakinthos, H. Gamari-Seale, J. Magn. Magn. Mater. 117 (1992) 79. [3] P. Svoboda, P. Javorsky´, M. Divisˇ , V. Sechovsky´, F. Honda, G. Oomi, A.A. Menovsky, Phys. Rev. B 63 (2001) 212408.