- Email: [email protected]
Co-existence of diamagnetism and ferromagnetism and possible superconductivity in Y8Ba5Zn4O21
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8– 2 2 2
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Co-existence of diamagnetism and ferromagnetism and possible superconductivity in Y8Ba5Zn4O21 Ugur Topal⁎ TUBITAK-UME, P.K. 54 41470 Gebze-Kocaeli/Turkey
AR TIC LE D ATA
ABSTR ACT
Article history:
In the present study, we investigate structural and magnetic properties of the Y8Ba5Zn4O21
Received 24 August 2010
structure, which was recently included in the catalog of the International Centre for
Received in revised form 1 December
Diffraction Data (ICDD) as a prototype structure. It was found that the Y8Ba5Zn4O21 bulk
2010
sample seems to be electrically insulating at temperatures of liquid nitrogen and the room
Accepted 7 December 2010
temperature. On the other hand, it has quite interesting magnetic properties. Diamagnetic phase transition was observed at ~ 90 K, which resembles that of the superconducting materials. The studied composition repels permanent magnets at the liquid nitrogen
Keywords:
temperature. Ferromagnetism also co-exists with diamagnetism in the temperature range
Diamagnetism
between 50 K and 90 K. Possible reasons for these behaviors are discussed.
Ferromagnetism
© 2010 Elsevier Inc. All rights reserved.
Superconductivity
1.
Introduction
Many of the well-studied materials, such as metals, semiconductors, ceramics or plastics cannot fulfill all technological demands for various new applications, such as; in the area of spintronics in which materials must possess both ferromagnetism and semiconductivity [1–4]. That is, multifunctional properties are required to increase the potential application areas of materials. This can be achieved by artificial methods. For instance, superconductor/ferromagnetic, semiconductor/ ferromagnetic and ferroelectric/ferromagnetic hetero-structures can be formed by thin film technologies [5–7]. Besides, a basic structural material (e.g. semiconducting material) can be incorporated by a second structural material showing different physical property (e.g. ferromagnetic material) as in the case of incorporation of magnetic particles into polymers, graphite, and cellulose and in the diluted magnetic semiconductors [8–11]. On the other hand, materials may also possess more than one useful property intrinsically. For instance, BiFeO3 is a fascinating material exhibiting both ferroelectricity and ferromagnetism, which has attracted much interest during the last decade
[12–15]. Co-existence of ferromagnetism and ferroelectricity can open the way for their potential applications in the memory devices, sensors, and optical filters [16]. Similarly, RuSr2RECu2O8 (RE= Eu, Gd, Ho,), which exhibits both superconductivity and ferromagnetism, is another typical example of coexistence of two physical properties in one crystallographic structure [17–21]. Recently, a prototype structure of Y8Ba5Zn4O21 material was announced in International Centre for Diffraction Data (ICDD) with PDF no 04-010-7590 (entry date: 11/15/2006). Its structure is similar to that of the well established Ho8Ba5Zn4O21 material [22]. This structure belongs to the I4/m space group and has a tetragonal symmetry. The lattice parameters are a = 13.782 and c = 5.715 with the unit cell volume V = 1085.50 Å3. To our best knowledge, the physical properties of this material have not been reported yet. Our preliminary measurements have shown that it exhibits an insulating-like behavior at room temperature and the liquid nitrogen temperature. On the other hand, we have observed diamagnetic-like repulsive interaction of this material with permanent magnets at liquid nitrogen temperature (77 K). That is, it becomes diamagnetic on cooling. Such an interesting
⁎ Tel.: +90 262 6795000; fax: + 90 262 6795001. E-mail address: [email protected]. 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.12.002
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8 –2 2 2
observation forced us to examine this compound more thoroughly. As a result, magnetization measurements carried out between 10 K and 350 K has shown that ferromagnetism and diamagnetism co-exist in the Y8Ba5Zn4O21 in addition to its insulating (dielectric) properties. This is going to be first publication reporting the synthesis and magnetic properties of Y8Ba5Zn4O21.
2.
Experimental
The precursors were prepared by the conventional solid-state sintering technique. High purity powders of Y2O3 (99.99%), BaCO3 (99.99%) and ZnO (99.99%) were weighed in the appropriate amounts to form a nominal composition of Y8Ba5Zn4Ox. Then the powders were mixed thoroughly using an agate mortar and pressed into pellets under a pressure of 2000 kg/cm2. Pellets were sintered at 1050 °C (20 h), 1080 °C (20 h), 1080 °C (20 h) and 1100 °C (20) with intermediate grinding and mixing. Finally, the samples were annealed under oxygen flow at 450 °C for 20 h to stabilize the oxygen content within the sample. Structural analysis was done using Shimadzu XRD-6000 diffractometer (CuKα). XRD patterns for the samples examined were recorded in the range of 2θ between 2º and 70º with a resolution of 0.02º and scan speed of 0.5 deg/min. The microstructural analysis was done using a scanning electron microscope (JEOL 6335 F Field Emission Gun). Magnetic measurements were done in a Quantum Design MPMS-5 SQUID magnetometer. M-H curves were obtained at 10 K and 77 K with a maximum magnetic field of 5 Tesla. M-T curves were obtained in the temperature range between 10 K and 350 K in zero-field-cooled and field-cooled modes in addition to AC-magnetic susceptibility measurements. Electrical resistivity measurements, on which four-probe technique was used with contact resistance being less than 0.1, was carried
219
out at room temperature and at liquid nitrogen temperature by using an electrometer HP 3458A.
3.
Results and Discussion
Fig. 1 shows the X-ray diffraction pattern of the sample. It is clearly seen that almost all the diffraction peaks are well indexed to the prototype of Y8Ba5Zn4O21 phase released by International Centre for Diffraction Data (ICDD). A few small diffraction peaks seen in the pattern were indexed to the Y2BaZnO5 phase. The amount of the impurity phase was determined to be 7.2% from the ratios of the most intense peaks. Well-defined sharp peaks indicate good crystallization of the samples. A SEM micrograph of the Y8Ba5Zn4O21 sample is shown in Fig. 2. The sample has heterogeneously distributed grains. It is also seen that the grains tend to have sharp edges and most of them are close to the tetragonal shape. In addition to the presence of elongated grains with the length of long side around 10 μm, there are also relatively small grains with average sizes less than 2 μm. Fig. 3 shows the magnetization vs. temperature curve in the temperature range of 10 K to 350 K and under applied fields of (a) 100 Oe and (b) 200 Oe, measured in both zero-field cooled (ZFC) and field-cooled (FC) modes. The M vs. T graphs for the applied fields of 100 Oe and 200 Oe exhibit convergence of the ZFC and the FC curves around 340 K (Tirr). This kind of irreversibility was reported for certain magnetic systems undergoing a transition to the ordered ferromagnetic, antiferromagnetic or ferrimagnetic states [23–31]. In these systems, the reason for the irreversibility can be explained in terms of magnetic frustrations, arising from the competing ferromagnetic and antiferromagnetic exchange interactions, deformed lattices, random distribution of the magnetic ions, etc. As the temperature decreases, a diamagnetic-like transition (Td) appears around 90 K and continues down to 65 K (inset to
Fig. 1 – The X-ray diffraction pattern of the Y8Ba5Zn4O21.
220
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8– 2 2 2
Fig. 2 – Scanning Electron Microscopy (SEM) picture of the Y8Ba5Zn4O21.
Fig. 3a). It is more evident in the ZFC branch compared to the FC one. Such a curvature is mostly encountered in superconducting materials, in which magnetic field inside the specimen is excluded by the super currents formed on specimen surface below certain temperature, called critical temperature (Tc). In superconducting materials, magnitude of diamagnetic signals may be different on the ZFC and FC measurements, which is not explained by classical diamagnetism. The strength of diamagnetic signal is expected to be same for both the ZFC and the FC measurements in case the diamagnetism occurs due to the paired orbital electrons. On the other hand, if the superconductivity exists, the super currents may not be percolating through the specimen due to the weak links between grains or low portion of superconducting phase. Another possible reason of such a transition observed on the M–T curve may be Verwey-transition [32]. In this case, a significant increase on resistivity must be seen
below the transition temperature, which is contrary to our experiments. Notice that electrical resistivity of the sample at room temperature and liquid nitrogen temperature was measured to be 0.232 GΩ cm and 0.217 GΩ cm, respectively. At this stage, it is seen to be a complicated problem and so, it requires further experiments (such as; NMR analysis, etc.) for clarifying the real reason of the transition. Furthermore, an abrupt increase of magnetization occurs at temperatures below ~ 50 K. The hysteresis width, i.e. the difference between the ZFC and the FC branches, becomes larger below this temperature, suggesting the presence of long-range magnetic ordering. This may happen due to the spin-glass like behavior, which sets in at temperatures below 50 K [23]. In order to ensure from the M-T data, AC susceptibility measurements were also carried out as a function of temperature. Fig. 4 shows SQUID measurements of the AC susceptibility for the Y8Ba5Zn4O21 sample. While the diamagnetic-like transition temperature is seen almost at same temperature with that observed in zfc and fc measurements (taken at DC mode), ferromagnetic-like magnetic ordering seen below 50 K is invisible. Disappearance of ferromagnetic transition in AC-susceptibility curves is due to small amplitude of AC-field (4 Oe is max. applicable amplitude in SQUID magnetometer). The M–H curves measured at 10 K and 77 K are shown in Fig. 5. Insets show the magnified curves in the low field range, where one can clearly see the hysteresis. As seen, the M–H data are in good correlation with the M–T data. For example, while diamagnetism is apparent at 77 K, it is invisible at 10 K. The M–H curve at 77 K shows that both ferromagnetism and diamagnetism co-exist. In the low field region (≤1000 Oe), ferromagnetism is dominant. At 1000 Oe, the ferromagnetic signal reaches to the saturation. Above 1000 Oe, the ferromagnetic signal is dominated by the diamagnetic signal. The coercive field Hc was determined to be 400 Oe at 77 K. The remanence magnetization Mr and saturation magnetization M s are 0.13 × 10 − 2 emu/cm 3 and 0.27 × 10 − 2 emu/cm 3 ,
Fig. 3 – The M–T curve of Y8Ba5Zn4O21 samples under applied fields of (a) 100 Oe and (b) 200 Oe, measured in both zero-field cooled (ZFC) and field-cooled (FC) modes. Inset (a): The M–T curve between 50 K and 120 K.
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8 –2 2 2
221
is floating above the sample for a while. Later, the floating stops as the temperature reaches above the diamagnetic transition temperature. From these measurements, it is seen that both diamagnetism and ferromagnetism co-exist in the Y8Ba5Zn4O21 phase. If superconductivity is not the source of the diamagnetic signal (we could not observe a zero resistance phenomenon in the electrical measurements) then the paired orbital electrons must be responsible for the diamagnetism. At the atomic scale, it seems to be competing with the ferromagnetism, for which unpaired electrons are necessary. However, the exchange interactions between constituent atoms, as in the case of TiO2, may be the reason for the ferromagnetism [35].
Fig. 4 – SQUID measurements of the AC susceptibility for the Y8Ba5Zn4O21 sample. While the diamagnetic-like transition temperature is almost same as that observed in zfc and fc curves (taken at DC mode), ferromagnetic-like magnetic ordering seen below 50 K is invisible.
respectively. On the other hand, at 10 K, no traces of diamagnetism are observed. The M–H curve shows a typical ferromagnetic character. Magnetization is almost saturated at 50,000 Oe (M s = 8.8 × 10 − 2 emu/cm 3 ). The M r value is 0.17 × 10− 2 emu/cm3 and the Hc value is 250 Oe at 10 K. As mentioned before, the hysteresis width of the ZFC and the FC branches of the M–T data becomes larger below 50 K. It is also known that the magnitude of the hysteresis width in the M–T curve is related with the magnitude of the coercivity, which is a measure of the magnetic anisotropy [23]. Besides, the coercivity is known to decrease as the temperature rises [33,34]. So, the reason for the increase of the Hc value that takes place with increase of temperature from 10 K to 77 K, which is contrary to the common expectations, may be due to the possible interaction of ferromagnetic phase with diamagnetic phase. Fig. 6 shows a frame captured from the video file showing diamagnetic repulsion of a permanent magnet (black ring) by the Y8Ba5Zn4O21 sample cooled by liquid nitrogen. The magnet
4.
Conclusion
We have succeeded to synthesize nearly single phase Y8Ba5Zn4O21 samples using the conventional ceramic process. Our studies have shown that the present compound possess both ferromagnetic and diamagnetic properties. A superconducting-like diamagnetic transition takes place at ~ 90 K. However, our measurements are not enough to verify the existence of superconductivity. At 77 K, co-existence of ferromagnetism and diamagnetism is observed on the M-H curves. At 10 K, only ferromagnetism is existent and no traces of diamagnetism are detected. The reason for the ferromagnetism is assumed to be due to the exchange interactions between the constituent atoms. It was demonstrated that the studied samples repel permanent magnets in liquid nitrogen. All these properties will attract the attention of experimentalists and theoreticians to this material in order to clarify some complicated problems. For example, possibility of superconductivity in this material can be examined in more detail.
Acknowledgements The author thanks Dr. Kevser Gocmen Topal and Dr. Lev Dorosinskii for their valuable suggestions and technical checking.
Fig. 5 – The M–H curves measured at 10 K and 77 K. Insets: the M–H curve at low field interval.
222
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8– 2 2 2
Fig. 6 – A frame from the movie showing the floating of a permanent magnet (ring one) over the Y8Ba5Zn4O21sample.
Appendix A Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matchar.2010.12.002.
REFERENCES [1] Matsumoto Y, Murakami M, Shono T, Hasegawa H, Fukumura T, Kawasaki M, et al. Science 2001;291:854. [2] Ueda K, Tahata H, Kawai T. Appl Phys Lett 2001;79:988. [3] Ohno H. Science 1998;281:951. [4] Topal U, Ozkan H, Bakan HI, Cankur O, Topal K. J NonCryst Solids 2008;354:1678. [5] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Science 2003;299:1719. [6] Topal U, Birlikseven C, Yakıncı ME, Nurgaliev T. J Alloy Comp 2010;492:8. [7] Shevchenko P, Swierkowski L, Oitmaa J. J Magn Magn Mater 1998;177–181:1168. [8] Sigov AS, Evdokimov AA, Spichkin YI, Tishin AM. The Proceedings for the 27th ISTC Japan Workshop on Advanced Nanotechnologies in Russia / CIS, 2003, p. 158. [9] Chambers SA, Thevuthasan S, Farrow RFC, Marks RF, Thiele JU, Folks L, et al. Appl Phys Lett 2001;79:3467. [10] Kim DH, Yang JS, Lee KW, Bu SD, Noh TW, Oh S-J, et al. Appl Phys Lett 2002;81:2421. [11] Topal U. Unexpected transport and magnetic properties inYBa-Cu-O superconductors. Phys Stat Sol A 2010;207:1196. [12] Chu YH, Martin LW, Holcomb MB, Gajek M, Han SJ, He Q, et al. Nat Mater 2008;7:478. [13] Choi T, Lee S, Choi YJ, Kiryukhin V, Cheong SW. Science 2009;324:63. [14] Kim WS, Jun YK, Kim KH, Hong SH. J Magn Magn Mater 2009;321:3262.
[15] Zhang S, Luo W, Wang D, Ma Y. Mater Lett 2009;63:1820. [16] Yoneda Y, Yoshii K, Saitoh H, Mizuki J. Ferroelectrics 2007;348: 33. [17] Bauernfeind L, Widder W, Braun HF. Phys C 1995;254:151. [18] Klamut PW, Dabrowski B, Mini SM, Maxwell M, Kolesnik S, Mais J, et al. Phys C 2001;364–365:313. [19] Chmaissem O, Jorgensen JD, Shaked H, Dollar P, Tallon JL. Phys Rev B 2000;61:6401. [20] Tallon JL, Bernhard C, Bowden ME, Gilberd PW, Stoto TM, Pringle DJ. IEEE Trans Appl Supercond 1999;9:1696. [21] Topal U, Dorosinskii L. J Phys Chem Solids 2007;68:1969. [22] Muller-Buschbaum Hk, Rabbow Ch. Z Anorg Allg Chem 1993;619:529. [23] Joy PA, Anil Kumarz PS, Date SK. J Phys Condens Matter 1998;10:11049. [24] Greedan JE, Raju NP, Maignan A, Simon Ch, Pederson JS, Niraimathi AM, et al. Phys Rev B 1996;54:7189. [25] Maignan A, Varadaraju UV, Millange F, Raveau B. J Magn Magn Mater 1997;168:L237. [26] Maignan A, Sundaresan A, Varadaraju UV, Raveau B. J Magn Magn Mater 1998;184:83. [27] Wang XL, Horvat J, Liu HK, Dou SX. J Magn Magn Mater 1998;182:L1. [28] Perez J, Garcia J, Blasco J, Stankiewicz J. Phys Rev Lett 1998;80: 2401. [29] Troyanchuk IO, Samsonenko NV, Shapovalova EF, Szymczak H, Nabialek A. Mater Res Bull 1997;32:67. [30] Onodera H, Kobayashi H, Yamauchi H, Ohashi M, Yamaguchi Y. J Magn Magn Mater 1997;170:201. [31] Nishigori S, Hirooka Y, Ito T. J Magn Magn Mater 1998;177–181: 137. [32] Kosterov A, Fabian K. Geophys J Int 2008;174:93. [33] Joffe I. J Phys C Solid St Phys 1969;2:1537. [34] Tang W, Gabay AM, Zhang Y, Hadjipanayis GC, Kronmuller H. IEEE Trans Magn 2001;37:2515. [35] Rumaiz AK, Ali B, Ceylan A, Boggs M, Beebe T, Shah SI. Solid State Commun 2007;144:334.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Co-existence of diamagnetism and ferromagnetism and possible superconductivity in Y8Ba5Zn4O21 Ugur Topal⁎ TUBITAK-UME, P.K. 54 41470 Gebze-Kocaeli/Turkey
AR TIC LE D ATA
ABSTR ACT
Article history:
In the present study, we investigate structural and magnetic properties of the Y8Ba5Zn4O21
Received 24 August 2010
structure, which was recently included in the catalog of the International Centre for
Received in revised form 1 December
Diffraction Data (ICDD) as a prototype structure. It was found that the Y8Ba5Zn4O21 bulk
2010
sample seems to be electrically insulating at temperatures of liquid nitrogen and the room
Accepted 7 December 2010
temperature. On the other hand, it has quite interesting magnetic properties. Diamagnetic phase transition was observed at ~ 90 K, which resembles that of the superconducting materials. The studied composition repels permanent magnets at the liquid nitrogen
Keywords:
temperature. Ferromagnetism also co-exists with diamagnetism in the temperature range
Diamagnetism
between 50 K and 90 K. Possible reasons for these behaviors are discussed.
Ferromagnetism
© 2010 Elsevier Inc. All rights reserved.
Superconductivity
1.
Introduction
Many of the well-studied materials, such as metals, semiconductors, ceramics or plastics cannot fulfill all technological demands for various new applications, such as; in the area of spintronics in which materials must possess both ferromagnetism and semiconductivity [1–4]. That is, multifunctional properties are required to increase the potential application areas of materials. This can be achieved by artificial methods. For instance, superconductor/ferromagnetic, semiconductor/ ferromagnetic and ferroelectric/ferromagnetic hetero-structures can be formed by thin film technologies [5–7]. Besides, a basic structural material (e.g. semiconducting material) can be incorporated by a second structural material showing different physical property (e.g. ferromagnetic material) as in the case of incorporation of magnetic particles into polymers, graphite, and cellulose and in the diluted magnetic semiconductors [8–11]. On the other hand, materials may also possess more than one useful property intrinsically. For instance, BiFeO3 is a fascinating material exhibiting both ferroelectricity and ferromagnetism, which has attracted much interest during the last decade
[12–15]. Co-existence of ferromagnetism and ferroelectricity can open the way for their potential applications in the memory devices, sensors, and optical filters [16]. Similarly, RuSr2RECu2O8 (RE= Eu, Gd, Ho,), which exhibits both superconductivity and ferromagnetism, is another typical example of coexistence of two physical properties in one crystallographic structure [17–21]. Recently, a prototype structure of Y8Ba5Zn4O21 material was announced in International Centre for Diffraction Data (ICDD) with PDF no 04-010-7590 (entry date: 11/15/2006). Its structure is similar to that of the well established Ho8Ba5Zn4O21 material [22]. This structure belongs to the I4/m space group and has a tetragonal symmetry. The lattice parameters are a = 13.782 and c = 5.715 with the unit cell volume V = 1085.50 Å3. To our best knowledge, the physical properties of this material have not been reported yet. Our preliminary measurements have shown that it exhibits an insulating-like behavior at room temperature and the liquid nitrogen temperature. On the other hand, we have observed diamagnetic-like repulsive interaction of this material with permanent magnets at liquid nitrogen temperature (77 K). That is, it becomes diamagnetic on cooling. Such an interesting
⁎ Tel.: +90 262 6795000; fax: + 90 262 6795001. E-mail address: [email protected]. 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.12.002
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8 –2 2 2
observation forced us to examine this compound more thoroughly. As a result, magnetization measurements carried out between 10 K and 350 K has shown that ferromagnetism and diamagnetism co-exist in the Y8Ba5Zn4O21 in addition to its insulating (dielectric) properties. This is going to be first publication reporting the synthesis and magnetic properties of Y8Ba5Zn4O21.
2.
Experimental
The precursors were prepared by the conventional solid-state sintering technique. High purity powders of Y2O3 (99.99%), BaCO3 (99.99%) and ZnO (99.99%) were weighed in the appropriate amounts to form a nominal composition of Y8Ba5Zn4Ox. Then the powders were mixed thoroughly using an agate mortar and pressed into pellets under a pressure of 2000 kg/cm2. Pellets were sintered at 1050 °C (20 h), 1080 °C (20 h), 1080 °C (20 h) and 1100 °C (20) with intermediate grinding and mixing. Finally, the samples were annealed under oxygen flow at 450 °C for 20 h to stabilize the oxygen content within the sample. Structural analysis was done using Shimadzu XRD-6000 diffractometer (CuKα). XRD patterns for the samples examined were recorded in the range of 2θ between 2º and 70º with a resolution of 0.02º and scan speed of 0.5 deg/min. The microstructural analysis was done using a scanning electron microscope (JEOL 6335 F Field Emission Gun). Magnetic measurements were done in a Quantum Design MPMS-5 SQUID magnetometer. M-H curves were obtained at 10 K and 77 K with a maximum magnetic field of 5 Tesla. M-T curves were obtained in the temperature range between 10 K and 350 K in zero-field-cooled and field-cooled modes in addition to AC-magnetic susceptibility measurements. Electrical resistivity measurements, on which four-probe technique was used with contact resistance being less than 0.1, was carried
219
out at room temperature and at liquid nitrogen temperature by using an electrometer HP 3458A.
3.
Results and Discussion
Fig. 1 shows the X-ray diffraction pattern of the sample. It is clearly seen that almost all the diffraction peaks are well indexed to the prototype of Y8Ba5Zn4O21 phase released by International Centre for Diffraction Data (ICDD). A few small diffraction peaks seen in the pattern were indexed to the Y2BaZnO5 phase. The amount of the impurity phase was determined to be 7.2% from the ratios of the most intense peaks. Well-defined sharp peaks indicate good crystallization of the samples. A SEM micrograph of the Y8Ba5Zn4O21 sample is shown in Fig. 2. The sample has heterogeneously distributed grains. It is also seen that the grains tend to have sharp edges and most of them are close to the tetragonal shape. In addition to the presence of elongated grains with the length of long side around 10 μm, there are also relatively small grains with average sizes less than 2 μm. Fig. 3 shows the magnetization vs. temperature curve in the temperature range of 10 K to 350 K and under applied fields of (a) 100 Oe and (b) 200 Oe, measured in both zero-field cooled (ZFC) and field-cooled (FC) modes. The M vs. T graphs for the applied fields of 100 Oe and 200 Oe exhibit convergence of the ZFC and the FC curves around 340 K (Tirr). This kind of irreversibility was reported for certain magnetic systems undergoing a transition to the ordered ferromagnetic, antiferromagnetic or ferrimagnetic states [23–31]. In these systems, the reason for the irreversibility can be explained in terms of magnetic frustrations, arising from the competing ferromagnetic and antiferromagnetic exchange interactions, deformed lattices, random distribution of the magnetic ions, etc. As the temperature decreases, a diamagnetic-like transition (Td) appears around 90 K and continues down to 65 K (inset to
Fig. 1 – The X-ray diffraction pattern of the Y8Ba5Zn4O21.
220
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8– 2 2 2
Fig. 2 – Scanning Electron Microscopy (SEM) picture of the Y8Ba5Zn4O21.
Fig. 3a). It is more evident in the ZFC branch compared to the FC one. Such a curvature is mostly encountered in superconducting materials, in which magnetic field inside the specimen is excluded by the super currents formed on specimen surface below certain temperature, called critical temperature (Tc). In superconducting materials, magnitude of diamagnetic signals may be different on the ZFC and FC measurements, which is not explained by classical diamagnetism. The strength of diamagnetic signal is expected to be same for both the ZFC and the FC measurements in case the diamagnetism occurs due to the paired orbital electrons. On the other hand, if the superconductivity exists, the super currents may not be percolating through the specimen due to the weak links between grains or low portion of superconducting phase. Another possible reason of such a transition observed on the M–T curve may be Verwey-transition [32]. In this case, a significant increase on resistivity must be seen
below the transition temperature, which is contrary to our experiments. Notice that electrical resistivity of the sample at room temperature and liquid nitrogen temperature was measured to be 0.232 GΩ cm and 0.217 GΩ cm, respectively. At this stage, it is seen to be a complicated problem and so, it requires further experiments (such as; NMR analysis, etc.) for clarifying the real reason of the transition. Furthermore, an abrupt increase of magnetization occurs at temperatures below ~ 50 K. The hysteresis width, i.e. the difference between the ZFC and the FC branches, becomes larger below this temperature, suggesting the presence of long-range magnetic ordering. This may happen due to the spin-glass like behavior, which sets in at temperatures below 50 K [23]. In order to ensure from the M-T data, AC susceptibility measurements were also carried out as a function of temperature. Fig. 4 shows SQUID measurements of the AC susceptibility for the Y8Ba5Zn4O21 sample. While the diamagnetic-like transition temperature is seen almost at same temperature with that observed in zfc and fc measurements (taken at DC mode), ferromagnetic-like magnetic ordering seen below 50 K is invisible. Disappearance of ferromagnetic transition in AC-susceptibility curves is due to small amplitude of AC-field (4 Oe is max. applicable amplitude in SQUID magnetometer). The M–H curves measured at 10 K and 77 K are shown in Fig. 5. Insets show the magnified curves in the low field range, where one can clearly see the hysteresis. As seen, the M–H data are in good correlation with the M–T data. For example, while diamagnetism is apparent at 77 K, it is invisible at 10 K. The M–H curve at 77 K shows that both ferromagnetism and diamagnetism co-exist. In the low field region (≤1000 Oe), ferromagnetism is dominant. At 1000 Oe, the ferromagnetic signal reaches to the saturation. Above 1000 Oe, the ferromagnetic signal is dominated by the diamagnetic signal. The coercive field Hc was determined to be 400 Oe at 77 K. The remanence magnetization Mr and saturation magnetization M s are 0.13 × 10 − 2 emu/cm 3 and 0.27 × 10 − 2 emu/cm 3 ,
Fig. 3 – The M–T curve of Y8Ba5Zn4O21 samples under applied fields of (a) 100 Oe and (b) 200 Oe, measured in both zero-field cooled (ZFC) and field-cooled (FC) modes. Inset (a): The M–T curve between 50 K and 120 K.
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8 –2 2 2
221
is floating above the sample for a while. Later, the floating stops as the temperature reaches above the diamagnetic transition temperature. From these measurements, it is seen that both diamagnetism and ferromagnetism co-exist in the Y8Ba5Zn4O21 phase. If superconductivity is not the source of the diamagnetic signal (we could not observe a zero resistance phenomenon in the electrical measurements) then the paired orbital electrons must be responsible for the diamagnetism. At the atomic scale, it seems to be competing with the ferromagnetism, for which unpaired electrons are necessary. However, the exchange interactions between constituent atoms, as in the case of TiO2, may be the reason for the ferromagnetism [35].
Fig. 4 – SQUID measurements of the AC susceptibility for the Y8Ba5Zn4O21 sample. While the diamagnetic-like transition temperature is almost same as that observed in zfc and fc curves (taken at DC mode), ferromagnetic-like magnetic ordering seen below 50 K is invisible.
respectively. On the other hand, at 10 K, no traces of diamagnetism are observed. The M–H curve shows a typical ferromagnetic character. Magnetization is almost saturated at 50,000 Oe (M s = 8.8 × 10 − 2 emu/cm 3 ). The M r value is 0.17 × 10− 2 emu/cm3 and the Hc value is 250 Oe at 10 K. As mentioned before, the hysteresis width of the ZFC and the FC branches of the M–T data becomes larger below 50 K. It is also known that the magnitude of the hysteresis width in the M–T curve is related with the magnitude of the coercivity, which is a measure of the magnetic anisotropy [23]. Besides, the coercivity is known to decrease as the temperature rises [33,34]. So, the reason for the increase of the Hc value that takes place with increase of temperature from 10 K to 77 K, which is contrary to the common expectations, may be due to the possible interaction of ferromagnetic phase with diamagnetic phase. Fig. 6 shows a frame captured from the video file showing diamagnetic repulsion of a permanent magnet (black ring) by the Y8Ba5Zn4O21 sample cooled by liquid nitrogen. The magnet
4.
Conclusion
We have succeeded to synthesize nearly single phase Y8Ba5Zn4O21 samples using the conventional ceramic process. Our studies have shown that the present compound possess both ferromagnetic and diamagnetic properties. A superconducting-like diamagnetic transition takes place at ~ 90 K. However, our measurements are not enough to verify the existence of superconductivity. At 77 K, co-existence of ferromagnetism and diamagnetism is observed on the M-H curves. At 10 K, only ferromagnetism is existent and no traces of diamagnetism are detected. The reason for the ferromagnetism is assumed to be due to the exchange interactions between the constituent atoms. It was demonstrated that the studied samples repel permanent magnets in liquid nitrogen. All these properties will attract the attention of experimentalists and theoreticians to this material in order to clarify some complicated problems. For example, possibility of superconductivity in this material can be examined in more detail.
Acknowledgements The author thanks Dr. Kevser Gocmen Topal and Dr. Lev Dorosinskii for their valuable suggestions and technical checking.
Fig. 5 – The M–H curves measured at 10 K and 77 K. Insets: the M–H curve at low field interval.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 2 1 8– 2 2 2
Fig. 6 – A frame from the movie showing the floating of a permanent magnet (ring one) over the Y8Ba5Zn4O21sample.
Appendix A Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matchar.2010.12.002.
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