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Magnetic study of the Pb2(FeMo)O6 perovskite
Journal of Magnetism and Magnetic Materials 219 (2000) 163}165
Magnetic study of the Pb (FeMo)O perovskite
I.O. Troyanchuk *, L.S. Lobanovsky , H. Szymczak, K. BaK rner Institute of Solids and Semiconductor Physics, NAS, P. Brovka str. 17, 220072 Minsk, Belarus Institute of Physics, PAS, Lotnikov str. 32/46, 02-668 Warsaw, Poland IV Physikalishes Institut, Bunenstra}e 13, D37073 Go( ttingen, Germany Received 5 January 2000; received in revised form 31 March 2000
Abstract Pb (FeMo)O has been received under high pressure (P"5 GPa, ¹"8003C). According to X-ray data this compound is pseudocubic perovskite with a"7.796 As . Magnetization study reveals a weak spontaneous magnetic moment around 0.35 l per formula unit. Spontaneous magnetization disappears at ¹ "280 K. As compared to , Sr (FeMo)O metallic ferromagnet the plumbum-based compound shows semiconductive behavior without appreciable magnetoresistance. It is suggested that magnetization behavior is strongly a!ected by the disordering of Fe> and Mo> ions in the B-sites of perovskite lattice ABO as well as reduction of negative Fe>}O}Mo> exchange interaction in comparison with Sr (FeMo)O . 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.Dd Keywords: High pressure; Perovskite; Magnetic phase transition; Magnetic clusters; Ionic ordering
1. Introduction Recently, low-"eld intergrain magnetoresistance at room temperature has been observed for polycrystalline samples of A (FeMo)O (A"Sr, Ba) [1]. These compounds have a perovskite structure with a NaCl-type ordering of Fe and Mo cations. The Curie point reaches 420 K for Sr (FeMo)O , 375 K for Ca (FeMo)O , and 340 K for Ba (FeMo)O [2]. Magnetic moments of Fe> and Mo> orient antiparallel to each other which is likely due to a strong negative superexchange magnetic interaction between these ions. The large * Corresponding author. Tel.: #7-375-0172-84-11-33; fax: #7-375-0172-84-08-88. E-mail address: [email protected] (I.O. Troyanchuk).
low-"eld magnetoresistance makes these compounds the most promising candidates for magnetic "eld sensors. In this paper we report on the synthesis of Pb (FeMo)O perovskite and its struc tural, magnetic and electrical transport properties.
2. Experiment All the attempts to prepare Pb (FeMo)O in #owing argon in a temperature range 600}10003C were unsuccessful. Hence, we undertook experiments under high-pressure conditions. The mixture of starting materials PbO, Fe O , MoO and MoO was placed in a nickel container and subjected to a high-pressure and temperature treatment. We have found that single-phase
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 4 0 4 - 2
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I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 219 (2000) 163}165
Pb (FeMo)O can be received in a relatively nar row window of temperature around 8003C whereas the pressure should be not less than P"4 GPa. The Sr (FeMo)O and Sr O were prepared in #owing argon at 12003C followed by cooling down to room temperature at a rate of 1003C/h. X-ray data were taken using Cu K radiation. Maga netization was measured with Foner vibrating sample magnetometer. Electrical measurements were done with the standard four-probe method.
3. Results and discussion X-ray di!raction pattern could be indexed in a cubic system; however, the width of the lines is rather large apparently due to local crystal structure distortions. The superstructure re#ections indicate an ordering of Fe and Mo ions; however their intensity is rather weak. The parameter of the unit cell a"7.976 As is in agreement with the expected magnitude considering an evolution of ionic sizes of Ca, Sr, Pb and Ba ions and unit-cell parameters of A (FeMo)O (A"Ca, Sr, Ga). We have found that magnetic and electrical transport properties di!er drastically from those for A (FeMo)O (A"Ca, Sr, Ba). Spontaneous magnetization at ¹"5 K is 3 emu/g or 0.35 l per formula unit (Fig. 1) whereas bulk samples of Sr (FeMo)O exhibit around 3.5 l per f.u. at the same temperature. Coercive "eld H "100 Oe is ! not large; however, in the case of Sr FeMoO coercive "eld is slightly less (H +50 Oe). ! The magnetization drops around 280 K thus indicating magnetic-phase transition (Fig. 2). According to electrical resistivity measurements the Pb (FeMo)O perovskite exhibits semiconductor like behavior in its resistivity (Fig. 2). We have not observed appreciable magnetoresistance e!ect below room temperature. The magnetic properties of A (FeMo)O (A"Pb, Sr, Ba) such as the saturation moment might be a!ected by the mis-site-type imperfection of the B-site order. In order to clarify this hypothesis we have prepared two samples of Sr (FeMo)O . The starting material was cut into bars. One sample was annealed at 6003C in evacuated silica tube during 48 h in order to improve Fe> and Mo>
Fig. 1. Magnetization of Pb (FeMo)O as a function of the magnetic "eld at ¹"6 K.
Fig. 2. Magnetization and resistivity of Pb (FeMo)O as a function of the temperature.
ionic order. Another sample was treated at P" 5 GPa and ¹"13003C and then quenched down to room temperature in order to destroy B-site ionic order. X-ray data indicate that the quenched sample is much more disordered than the annealed one. Magnetic measurements indicate that both the samples exhibit similar Curie points ¹ (around ! 415 K); however, spontaneous magnetization of the quenched sample is slightly lower (3 l /f.u.) than for the annealed one. Hence, we cannot explain the magnetic behavior of Pb (FeMo)O taking into account only the e!ect of ionic disordering. Low spontaneous magnetic moment of Pb (FeMo)O may originate from Dzyaloshinsky} Moriya magnetic interaction [3]. However, in this
I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 219 (2000) 163}165
case true crystal symmetry of Pb (FeMo)O must not be cubic. Furthermore, the magnetization of the weak ferromagnets is, as a rule, saturated in a low-"eld regime, whereas a large magnetic paraprocess for Pr (FeMo)O could be attributed to magnetic nonhomogeneity of the sample. We suggest such a di!erence in the behavior between metallic ferrimagnet Sr (FeMo)O and in sulator Pb (FeMo)O is the result of a strong hy bridization of 5d orbital of molybdenum and 5p 6s orbital of plumbum. Apparently, this hybridization leads to decreasing superexchange interaction between Mo and Fe whereas a relative role of negative magnetic interaction between Fe> ions increases. It is known that Pb (Fe>Nb>)O cu bic perovskite is an antiferromagnetic insulator with ¹ "144 K [4]. The magnetic ordering in , this material is governed with negative superexchange interaction between Fe>}O}Fe> because Nb> is a diamagnetic ion. We have prepared a sample Sr (FeMo Nb )O in which part of magnetic Mo> ions is replaced with non-magnetic Nb> ions. According to magnetic data, Sr (FeMo Nb )O has an onset of the transition into magnetically ordered state at 370 K and spontaneous magnetic moment around 0.7 l /f.u. The magnetization vs. "eld dependences for Pb (FeMo)O , Sr (FeMo)O and Sr (FeMo Nb )O are presented in Fig. 3. It is clearly seen that both Nb-diluted and Pb (FeMo)O samples exhibit similar magnetiza tion behavior whereas Sr (FeMo)O shows mag netization saturation in a "eld below 5 kOe. Hence, the magnetic properties of Sr (FeMo Nb )O are in agreement with our assumption that Mo>}Fe> interaction in the Pb (FeMo)O becomes smaller in comparison with Sr (FeMo)O . Apparently, the decrease of
165
Fig. 3. Magnetization as a function of the magnetic "eld for Sr(FeMo)O , Sr (FeMo Nb )O and Pb (FeMo)O .
Mo>}Fe> negative interaction leads to a strong dependence of magnetic properties of Pb (FeMo)O on the ionic order between Mo> and Fe> ions. Substitution of the Mo> ions with Nb> leads to the transition from metallic to semiconductive behavior (Fig. 2).
Acknowledgements This work was supported by Belarus Fund for fundamental research (Grant F98-057) and NATO linkage grant PST.CLG 975703.
References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [2] T. Nakagawa, J. Phys. Soc. Japan 24 (1968) 806. [3] T. Moriya, Phys. Rev. 156 (1960) 562. [4] J.S. Jacobs, H.F. Burne, L.M. Levinson, J. Appl. Phys. 42 (1971) 1631.
Magnetic study of the Pb (FeMo)O perovskite
I.O. Troyanchuk *, L.S. Lobanovsky , H. Szymczak, K. BaK rner Institute of Solids and Semiconductor Physics, NAS, P. Brovka str. 17, 220072 Minsk, Belarus Institute of Physics, PAS, Lotnikov str. 32/46, 02-668 Warsaw, Poland IV Physikalishes Institut, Bunenstra}e 13, D37073 Go( ttingen, Germany Received 5 January 2000; received in revised form 31 March 2000
Abstract Pb (FeMo)O has been received under high pressure (P"5 GPa, ¹"8003C). According to X-ray data this compound is pseudocubic perovskite with a"7.796 As . Magnetization study reveals a weak spontaneous magnetic moment around 0.35 l per formula unit. Spontaneous magnetization disappears at ¹ "280 K. As compared to , Sr (FeMo)O metallic ferromagnet the plumbum-based compound shows semiconductive behavior without appreciable magnetoresistance. It is suggested that magnetization behavior is strongly a!ected by the disordering of Fe> and Mo> ions in the B-sites of perovskite lattice ABO as well as reduction of negative Fe>}O}Mo> exchange interaction in comparison with Sr (FeMo)O . 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.Dd Keywords: High pressure; Perovskite; Magnetic phase transition; Magnetic clusters; Ionic ordering
1. Introduction Recently, low-"eld intergrain magnetoresistance at room temperature has been observed for polycrystalline samples of A (FeMo)O (A"Sr, Ba) [1]. These compounds have a perovskite structure with a NaCl-type ordering of Fe and Mo cations. The Curie point reaches 420 K for Sr (FeMo)O , 375 K for Ca (FeMo)O , and 340 K for Ba (FeMo)O [2]. Magnetic moments of Fe> and Mo> orient antiparallel to each other which is likely due to a strong negative superexchange magnetic interaction between these ions. The large * Corresponding author. Tel.: #7-375-0172-84-11-33; fax: #7-375-0172-84-08-88. E-mail address: [email protected] (I.O. Troyanchuk).
low-"eld magnetoresistance makes these compounds the most promising candidates for magnetic "eld sensors. In this paper we report on the synthesis of Pb (FeMo)O perovskite and its struc tural, magnetic and electrical transport properties.
2. Experiment All the attempts to prepare Pb (FeMo)O in #owing argon in a temperature range 600}10003C were unsuccessful. Hence, we undertook experiments under high-pressure conditions. The mixture of starting materials PbO, Fe O , MoO and MoO was placed in a nickel container and subjected to a high-pressure and temperature treatment. We have found that single-phase
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 4 0 4 - 2
164
I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 219 (2000) 163}165
Pb (FeMo)O can be received in a relatively nar row window of temperature around 8003C whereas the pressure should be not less than P"4 GPa. The Sr (FeMo)O and Sr O were prepared in #owing argon at 12003C followed by cooling down to room temperature at a rate of 1003C/h. X-ray data were taken using Cu K radiation. Maga netization was measured with Foner vibrating sample magnetometer. Electrical measurements were done with the standard four-probe method.
3. Results and discussion X-ray di!raction pattern could be indexed in a cubic system; however, the width of the lines is rather large apparently due to local crystal structure distortions. The superstructure re#ections indicate an ordering of Fe and Mo ions; however their intensity is rather weak. The parameter of the unit cell a"7.976 As is in agreement with the expected magnitude considering an evolution of ionic sizes of Ca, Sr, Pb and Ba ions and unit-cell parameters of A (FeMo)O (A"Ca, Sr, Ga). We have found that magnetic and electrical transport properties di!er drastically from those for A (FeMo)O (A"Ca, Sr, Ba). Spontaneous magnetization at ¹"5 K is 3 emu/g or 0.35 l per formula unit (Fig. 1) whereas bulk samples of Sr (FeMo)O exhibit around 3.5 l per f.u. at the same temperature. Coercive "eld H "100 Oe is ! not large; however, in the case of Sr FeMoO coercive "eld is slightly less (H +50 Oe). ! The magnetization drops around 280 K thus indicating magnetic-phase transition (Fig. 2). According to electrical resistivity measurements the Pb (FeMo)O perovskite exhibits semiconductor like behavior in its resistivity (Fig. 2). We have not observed appreciable magnetoresistance e!ect below room temperature. The magnetic properties of A (FeMo)O (A"Pb, Sr, Ba) such as the saturation moment might be a!ected by the mis-site-type imperfection of the B-site order. In order to clarify this hypothesis we have prepared two samples of Sr (FeMo)O . The starting material was cut into bars. One sample was annealed at 6003C in evacuated silica tube during 48 h in order to improve Fe> and Mo>
Fig. 1. Magnetization of Pb (FeMo)O as a function of the magnetic "eld at ¹"6 K.
Fig. 2. Magnetization and resistivity of Pb (FeMo)O as a function of the temperature.
ionic order. Another sample was treated at P" 5 GPa and ¹"13003C and then quenched down to room temperature in order to destroy B-site ionic order. X-ray data indicate that the quenched sample is much more disordered than the annealed one. Magnetic measurements indicate that both the samples exhibit similar Curie points ¹ (around ! 415 K); however, spontaneous magnetization of the quenched sample is slightly lower (3 l /f.u.) than for the annealed one. Hence, we cannot explain the magnetic behavior of Pb (FeMo)O taking into account only the e!ect of ionic disordering. Low spontaneous magnetic moment of Pb (FeMo)O may originate from Dzyaloshinsky} Moriya magnetic interaction [3]. However, in this
I.O. Troyanchuk et al. / Journal of Magnetism and Magnetic Materials 219 (2000) 163}165
case true crystal symmetry of Pb (FeMo)O must not be cubic. Furthermore, the magnetization of the weak ferromagnets is, as a rule, saturated in a low-"eld regime, whereas a large magnetic paraprocess for Pr (FeMo)O could be attributed to magnetic nonhomogeneity of the sample. We suggest such a di!erence in the behavior between metallic ferrimagnet Sr (FeMo)O and in sulator Pb (FeMo)O is the result of a strong hy bridization of 5d orbital of molybdenum and 5p 6s orbital of plumbum. Apparently, this hybridization leads to decreasing superexchange interaction between Mo and Fe whereas a relative role of negative magnetic interaction between Fe> ions increases. It is known that Pb (Fe>Nb>)O cu bic perovskite is an antiferromagnetic insulator with ¹ "144 K [4]. The magnetic ordering in , this material is governed with negative superexchange interaction between Fe>}O}Fe> because Nb> is a diamagnetic ion. We have prepared a sample Sr (FeMo Nb )O in which part of magnetic Mo> ions is replaced with non-magnetic Nb> ions. According to magnetic data, Sr (FeMo Nb )O has an onset of the transition into magnetically ordered state at 370 K and spontaneous magnetic moment around 0.7 l /f.u. The magnetization vs. "eld dependences for Pb (FeMo)O , Sr (FeMo)O and Sr (FeMo Nb )O are presented in Fig. 3. It is clearly seen that both Nb-diluted and Pb (FeMo)O samples exhibit similar magnetiza tion behavior whereas Sr (FeMo)O shows mag netization saturation in a "eld below 5 kOe. Hence, the magnetic properties of Sr (FeMo Nb )O are in agreement with our assumption that Mo>}Fe> interaction in the Pb (FeMo)O becomes smaller in comparison with Sr (FeMo)O . Apparently, the decrease of
165
Fig. 3. Magnetization as a function of the magnetic "eld for Sr(FeMo)O , Sr (FeMo Nb )O and Pb (FeMo)O .
Mo>}Fe> negative interaction leads to a strong dependence of magnetic properties of Pb (FeMo)O on the ionic order between Mo> and Fe> ions. Substitution of the Mo> ions with Nb> leads to the transition from metallic to semiconductive behavior (Fig. 2).
Acknowledgements This work was supported by Belarus Fund for fundamental research (Grant F98-057) and NATO linkage grant PST.CLG 975703.
References [1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [2] T. Nakagawa, J. Phys. Soc. Japan 24 (1968) 806. [3] T. Moriya, Phys. Rev. 156 (1960) 562. [4] J.S. Jacobs, H.F. Burne, L.M. Levinson, J. Appl. Phys. 42 (1971) 1631.