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Formation of Magnetic Glass By Kinetic Arrest of First Order Magnetic Phase Transition In CaFeO3
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ScienceDirect Materials Today: Proceedings 5 (2018) 15426–15430
www.materialstoday.com/proceedings
ISCAS 2017
Formation of Magnetic Glass By Kinetic Arrest of First Order Magnetic Phase Transition In CaFeO3 B. Ghosh1,*, D.K. Mahato2,* 1
Surface Physics & Material Science Division, Saha Institute of Nuclear Physics, 1/AF-Bidhannagar, Kol-64 2 Department of Physics, National Institute of Technology, Patna
Abstract In this manuscript we have observed kinetic arrest leading glass like arrested state (GLAS) through first order magnetic phase transition associated with charge disproportion in CaFeO3 nanoparticles. CHUF protocol was used in magnetic study on this system to qualitatively determine the arrested phase as antiferromagnetic (AF)-insulating (I) state of that material which goes to equilibrium ferromagnetic (FM)-metallic (M) state upon devitrification. Field induced phase transition also shows thermal hysteresis depending upon cooling field.
© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 10th NATIONAL CONFERENCE ON SOLID STATE CHEMISTRY AND ALLIED AREAS (ISCAS – 2017).
Keywords: Oxides; Sol-gel growth; Electrical conductivity
*Corresponding author: e-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 10th NATIONAL CONFERENCE ON SOLID STATE CHEMISTRY AND ALLIED AREAS (ISCAS – 2017).
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
1.
15427
Introduction
Recently transition metal cations in perovskite oxides especially ferrites are highly researched topic for both theoretical study and wide range of applications ranging from conventional frequencies in computers, television, and magnetic recording systems to microwave systems as LOAD ISOLATORS, PHASE SHIFTERS, VARIABLE ATTENUATORS, MODULATORS, and SWITCHES etc. Though ferrites in principle are potential magnetic refrigerants, there was not much focus on these materials since room-temperature refrigeration demands materials with low Curie temperatures.Understanding electronic states and their unusual physical properties along with various kind of ordering and their dynamics e.g. charge ordering, metal insulator transition, magnetic phase transitions, magnetoresistance, magnetocaloric effect have made them promising materials for multi-functionality. In particular, an unusual high valence state of Fe4+ in oxide is interesting since a strong e--e- and e--phonon interactions are expected to play an important role. Among several oxide systems containing Fe4+ with simple structure, CaFeO3 has drawn a lot of attractions due to presence of multiple ordering phenomena like structural transition, metal-insulator transition (Verwey transition) and first order magnetic phase transition assisted by a strong charge disproportion or charge ordering. Despite of several phase transitions, the kinetics of these phase transitions is less studied. Recently a method is being well adapted to study the phase co-existence through macroscopic measurements which enable distinguishing metastable and stable phases along with the information about the co-existing phase fraction across a first order magnetic phase transition. This protocol namely CHUF (Cooling and Heating in Unequal Field) has been proven to be very useful tool to study in detail the kinetics and its arrest to give rise to a Glass Like Arrested State (GLAS) termed as magnetic glass. In this paper we present the CHUF measurements on CaFeO3 nanoparticles to study the AF-I ordered phase to FM-M state with temperature is reduced. 2.
Experimental details:
CaFeO3 nanoparticles were prepared by solvothermal method dissolving Ca(NO3)2.6H2O in Milli-Q water accompanied by slow addition of Citric Acid 2 gm/mol while stirring. The solution was stirred overnight at 600C to get precursor gel which was again dried at 900C giving as cast powders. This as-prepared sample was annealed at 9000C to get high pure nanocrystalline CaFeO3 samples. X-Ray Diffraction with high energy synchrotron source, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDAX) and High Resolution Transmission Electron Microsopy (HRTEM) were used to characterize the samples. Magnetization study was performed by SQUID magnetometer (MPMS7, Quantum Design). 3.
Results and discussion
We show the XRD pattern and HRTEM image (inset) for our sample in fig.1. Powder X-Ray Diffraction with synchrotron radiation was carried out at Photon Factory, KEK, Japan. For XRD measurement a photon energy of 1.14 KeV (λ ~ 1.087 Å) was used. The average grain size was calculated using the Scherrer equation D=0.9λ/βCosθ, where β is the FWHM for corresponding XRD peak at angle 2θ. Diffraction planes matched from JCPDS data for CaFeO3-δ where δ is the deficiency of Oxygen from its stoichiometric ratio. We may assume this system as ideal perovskite polycrystalline sample with oxygen vacancy as defect site. The average particle size as seen from HRTEM image is 15 nm which agrees well with XRD data. Calculated d-value from number of lines vs. HRTEM resolution was 2.38 Å, typical for orthorhombic structure for CaFeO3 sample. We have measured standard M-T, M-H loop for zero fields cooled and field cooled condition along with CHUF study on this system to identify the phase co-existence. Cooling and Heating in Unequal Field (CHUF) protocol is recently being popular to probe this kinetic arrest, and the coexisting phase fraction at low temperature, at any measurement field H, can be tuned by cooling in an appropriate field and then changing the field isothermally to H.
15428
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
Figure1. XRD pattern for CaFeO3 with beam energy 1.14 KeV. (inset) HRTEM image of that sample
Cooling in some field may allow the first order transition to be completed and the equilibrium state to be established, while cooling in a very different field may totally inhibit (or arrest) the transition. Since the first order transition occurs over length scales of the correlation length, it was argued that the broadened transition will be completed only partially for cooling field lying between two values (H1 and H2) of magnetic field. If the cooling field is below H1 the transition will be completed (totally arrested) if the high-temperature phase was ferromagnetic (antiferromagnetic), and if the cooling field is above H2 the transition will be totally arrested (completed) if the hightemperature phase was ferromagnetic (antiferromagnetic). It was argued that by using unequal and appropriately chosen cooling and warming fields (HC and HW), de-arrest (or devitrification) of the kinetically arrested GLAS could be caused, and this de-arrest would be seen for only one sign of (HC - HW). Further heating would cause this de-arrested state to undergo the reverse magnetic transition, and this cooling and heating in unequal field (CHUF) protocol would show a re-entrant transition. These phenomenological predictions, where the re-entrant transition is seen only for positive (HC - HW) when the high temperature phase was ferromagnetic, apply for a generic ‘magnetic glass’. By similar arguments, the GLAS would show a re-entrant transition only for negative (HC - HW) when the high temperature phase was antiferromagnetic. Fig.2 shows the ZFC-FC measurement of that sample. The graph shows a strong phase transition in field cooled data whereas zero field cooled data gets prominent only in higher cooling field indicating a field induced phase transition. We have shown the results for CHUF protocol on that sample with fixed value of measuring field and varying the cooling field in fig.3. From this fig.3 we can clearly show that de-arrest is observed only for HC˂HW not for HC˃HW corresponding to an equilibrium FM-M phase at lower temperature. This de-arrest is initiated from higher temperature and typical for an FM equilibrium and AF arrested phase.
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
15429
Figure2. ZFC-FC magnetization measured at cooling field 100 Oe and 1 T.
Fig.4 shows the FCC-FCW measurement measured at different field which shows there are two temperature regions where thermal hysteresis appears. Hysteresis at lower temperature corresponds to the first order magnetic phase transition and devitrification of arrested phase whereas that in higher temperature corresponds to the structural phase transition taking place. In this system the structural phase transition does not depend upon the applied field but dependence of magnetic glassy state upon cooling field is evident from the fig shown in inset of fig. 4.
Fig.3
Fig.4
Figure 3 & 4. Magnetization data for CHUF protocol on CaFeO3 nanoparticle sample with fixed value of measuring field 200 Oe and varying the cooling field from 50-5000 Oe (Left). FCC-FCW measured at different cooling field. (Inset) Cooling field dependence of thermal hysteresis (Right) Though it looks complicated but the above said results can be explained by taking account of all the phase transitions in CaFeO3. Firstly it goes through structural transition just below room temperature starting at around 290K which gives rise to thermal hysteresis at higher temperature. At 300K CaFeO3 adopts the GdFeO3 struture,
15430
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
space group Pbnm which is distorted from the ideal perovskite structure by tilting of the FeO6 octahedra about [110] and [001]. At 15 K the crystal structure belongs to space group monoclinic P21/n containing two distinct Fe sites in rock salt ordering of Fe3+ and Fe5+. This structural phase transition is associated with the charge ordering phenomena which is accompanied by valence electron delocalization giving rise to metal to insulator transition. Charge Disproportion (CD) which is responsible for order to order magnetic phase transition by introducing moment imbalance between two sub-lattices could be understood in terms of electronic properties of CaFeO3 discussed below. According to the Jahn-Teller theorem, the presence of a single localized electron in the doubly degenerate eg set of orbitals is not a stable situation. Hence perovskite with d4+ state is highly instable and unlike other materials CaFeO3 cannot go under Jahn-Teller distortion. The reason is Ca2+ being much smaller causes a tilting of the FeO6 octahedra to satisfy the valence requirements of calcium. Due to this seemingly subtle distortion the Fe-O-Fe bond is distorted away from the linear geometry observed in SrFeO3 (the Fe-O-Fe bond angle is 158° in CaFeO3). This reduces the spatial overlap of the Fe eg and O 2p orbitals, and the width of the σ* band decreases. To remove the degeneracy of t32ge1g configuration and minimizing the Culoumb repulsion it stabilizes through charge disproportion which can be written as 2Fe4+=Fe3++Fe5+ with inequal sublattices giving rise to FM-M state at lower temperature from high temperature AF-I state. From above CHUF measurements clearly shows from the sign of that the kinetic arrested state in this case is AF-I state which gives rises to FM-M equilibrium state upon devirtification at lower temperature. 4.
Conclusion:
CaFeO3 polycrystalline nano-powders were synthesized by sol-gel method. Strong electron localization was seen from magnetic measurements giving rise to field induced meta-magnetic first order transition. CHUF protocol was used to determine the phase co-existence between high temperature antiferromagnetic insulating state and ferromagnetic metallic equilibrium state upon devitrification at lower temperature. Acknowledgments We acknowledge DST-KEK for beamline facility, PF,Indian Beamline, Japan and Electron Microscope Facility, SINP for the necessary measurements. References [1] S. Morimoto, T. Yamanaka, M. Tanaka, Physica B 237-238 (1997) 66 67. [2] S. Nasu et al., Hyperfine Interaction 70 (1992) 1063. [3] P. Chaddah and A. Banerjee, arxiv:1201.0575v1 [4] Banerjee, Kranti Kumar and P. Chaddah, arxiv:0805.1514v1,11 May’2008 [5] P. Chaddah and A. Banerjee, arxiv:1004.3116v1, 19 April’2010 [6] P. M. Woodward, D. E. Cox, E. Moshopoulou, A. W. Sleight, and S. Morimoto, Phys. Rev. B 62, 844 (2000) [7] J. B. Goodenough, Magnetism and the Chemical Bond, Interscience, New York (1963) [8] M. Takano, N. Nakanishi, Y. Takeda, S. Naka and T. Takada, Mater. Res. Bull. 12, 923 (1977) [9] J. B. Yang et al., Journal of Applied Physics 97, 10A312 (2005) [10] T. Akao et al., Phys. Rev. Lett. 91,156405 (2003) [11] Y. Tomioka, A. Asamitsu, H. Kuwahara, Y. Moritomo, and Y. Tokura, Phys. Rev. B 53, R1689 (1996) [12] Y. Takeda, S. Naka, and M. Takano, J. Phys. Colloq. 40, C2-331 (1979)
ScienceDirect Materials Today: Proceedings 5 (2018) 15426–15430
www.materialstoday.com/proceedings
ISCAS 2017
Formation of Magnetic Glass By Kinetic Arrest of First Order Magnetic Phase Transition In CaFeO3 B. Ghosh1,*, D.K. Mahato2,* 1
Surface Physics & Material Science Division, Saha Institute of Nuclear Physics, 1/AF-Bidhannagar, Kol-64 2 Department of Physics, National Institute of Technology, Patna
Abstract In this manuscript we have observed kinetic arrest leading glass like arrested state (GLAS) through first order magnetic phase transition associated with charge disproportion in CaFeO3 nanoparticles. CHUF protocol was used in magnetic study on this system to qualitatively determine the arrested phase as antiferromagnetic (AF)-insulating (I) state of that material which goes to equilibrium ferromagnetic (FM)-metallic (M) state upon devitrification. Field induced phase transition also shows thermal hysteresis depending upon cooling field.
© 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 10th NATIONAL CONFERENCE ON SOLID STATE CHEMISTRY AND ALLIED AREAS (ISCAS – 2017).
Keywords: Oxides; Sol-gel growth; Electrical conductivity
*Corresponding author: e-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 10th NATIONAL CONFERENCE ON SOLID STATE CHEMISTRY AND ALLIED AREAS (ISCAS – 2017).
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
1.
15427
Introduction
Recently transition metal cations in perovskite oxides especially ferrites are highly researched topic for both theoretical study and wide range of applications ranging from conventional frequencies in computers, television, and magnetic recording systems to microwave systems as LOAD ISOLATORS, PHASE SHIFTERS, VARIABLE ATTENUATORS, MODULATORS, and SWITCHES etc. Though ferrites in principle are potential magnetic refrigerants, there was not much focus on these materials since room-temperature refrigeration demands materials with low Curie temperatures.Understanding electronic states and their unusual physical properties along with various kind of ordering and their dynamics e.g. charge ordering, metal insulator transition, magnetic phase transitions, magnetoresistance, magnetocaloric effect have made them promising materials for multi-functionality. In particular, an unusual high valence state of Fe4+ in oxide is interesting since a strong e--e- and e--phonon interactions are expected to play an important role. Among several oxide systems containing Fe4+ with simple structure, CaFeO3 has drawn a lot of attractions due to presence of multiple ordering phenomena like structural transition, metal-insulator transition (Verwey transition) and first order magnetic phase transition assisted by a strong charge disproportion or charge ordering. Despite of several phase transitions, the kinetics of these phase transitions is less studied. Recently a method is being well adapted to study the phase co-existence through macroscopic measurements which enable distinguishing metastable and stable phases along with the information about the co-existing phase fraction across a first order magnetic phase transition. This protocol namely CHUF (Cooling and Heating in Unequal Field) has been proven to be very useful tool to study in detail the kinetics and its arrest to give rise to a Glass Like Arrested State (GLAS) termed as magnetic glass. In this paper we present the CHUF measurements on CaFeO3 nanoparticles to study the AF-I ordered phase to FM-M state with temperature is reduced. 2.
Experimental details:
CaFeO3 nanoparticles were prepared by solvothermal method dissolving Ca(NO3)2.6H2O in Milli-Q water accompanied by slow addition of Citric Acid 2 gm/mol while stirring. The solution was stirred overnight at 600C to get precursor gel which was again dried at 900C giving as cast powders. This as-prepared sample was annealed at 9000C to get high pure nanocrystalline CaFeO3 samples. X-Ray Diffraction with high energy synchrotron source, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDAX) and High Resolution Transmission Electron Microsopy (HRTEM) were used to characterize the samples. Magnetization study was performed by SQUID magnetometer (MPMS7, Quantum Design). 3.
Results and discussion
We show the XRD pattern and HRTEM image (inset) for our sample in fig.1. Powder X-Ray Diffraction with synchrotron radiation was carried out at Photon Factory, KEK, Japan. For XRD measurement a photon energy of 1.14 KeV (λ ~ 1.087 Å) was used. The average grain size was calculated using the Scherrer equation D=0.9λ/βCosθ, where β is the FWHM for corresponding XRD peak at angle 2θ. Diffraction planes matched from JCPDS data for CaFeO3-δ where δ is the deficiency of Oxygen from its stoichiometric ratio. We may assume this system as ideal perovskite polycrystalline sample with oxygen vacancy as defect site. The average particle size as seen from HRTEM image is 15 nm which agrees well with XRD data. Calculated d-value from number of lines vs. HRTEM resolution was 2.38 Å, typical for orthorhombic structure for CaFeO3 sample. We have measured standard M-T, M-H loop for zero fields cooled and field cooled condition along with CHUF study on this system to identify the phase co-existence. Cooling and Heating in Unequal Field (CHUF) protocol is recently being popular to probe this kinetic arrest, and the coexisting phase fraction at low temperature, at any measurement field H, can be tuned by cooling in an appropriate field and then changing the field isothermally to H.
15428
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
Figure1. XRD pattern for CaFeO3 with beam energy 1.14 KeV. (inset) HRTEM image of that sample
Cooling in some field may allow the first order transition to be completed and the equilibrium state to be established, while cooling in a very different field may totally inhibit (or arrest) the transition. Since the first order transition occurs over length scales of the correlation length, it was argued that the broadened transition will be completed only partially for cooling field lying between two values (H1 and H2) of magnetic field. If the cooling field is below H1 the transition will be completed (totally arrested) if the high-temperature phase was ferromagnetic (antiferromagnetic), and if the cooling field is above H2 the transition will be totally arrested (completed) if the hightemperature phase was ferromagnetic (antiferromagnetic). It was argued that by using unequal and appropriately chosen cooling and warming fields (HC and HW), de-arrest (or devitrification) of the kinetically arrested GLAS could be caused, and this de-arrest would be seen for only one sign of (HC - HW). Further heating would cause this de-arrested state to undergo the reverse magnetic transition, and this cooling and heating in unequal field (CHUF) protocol would show a re-entrant transition. These phenomenological predictions, where the re-entrant transition is seen only for positive (HC - HW) when the high temperature phase was ferromagnetic, apply for a generic ‘magnetic glass’. By similar arguments, the GLAS would show a re-entrant transition only for negative (HC - HW) when the high temperature phase was antiferromagnetic. Fig.2 shows the ZFC-FC measurement of that sample. The graph shows a strong phase transition in field cooled data whereas zero field cooled data gets prominent only in higher cooling field indicating a field induced phase transition. We have shown the results for CHUF protocol on that sample with fixed value of measuring field and varying the cooling field in fig.3. From this fig.3 we can clearly show that de-arrest is observed only for HC˂HW not for HC˃HW corresponding to an equilibrium FM-M phase at lower temperature. This de-arrest is initiated from higher temperature and typical for an FM equilibrium and AF arrested phase.
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
15429
Figure2. ZFC-FC magnetization measured at cooling field 100 Oe and 1 T.
Fig.4 shows the FCC-FCW measurement measured at different field which shows there are two temperature regions where thermal hysteresis appears. Hysteresis at lower temperature corresponds to the first order magnetic phase transition and devitrification of arrested phase whereas that in higher temperature corresponds to the structural phase transition taking place. In this system the structural phase transition does not depend upon the applied field but dependence of magnetic glassy state upon cooling field is evident from the fig shown in inset of fig. 4.
Fig.3
Fig.4
Figure 3 & 4. Magnetization data for CHUF protocol on CaFeO3 nanoparticle sample with fixed value of measuring field 200 Oe and varying the cooling field from 50-5000 Oe (Left). FCC-FCW measured at different cooling field. (Inset) Cooling field dependence of thermal hysteresis (Right) Though it looks complicated but the above said results can be explained by taking account of all the phase transitions in CaFeO3. Firstly it goes through structural transition just below room temperature starting at around 290K which gives rise to thermal hysteresis at higher temperature. At 300K CaFeO3 adopts the GdFeO3 struture,
15430
B. Ghosh and D.K. Mahato / Materials Today: Proceedings 5 (2018) 15426–15430
space group Pbnm which is distorted from the ideal perovskite structure by tilting of the FeO6 octahedra about [110] and [001]. At 15 K the crystal structure belongs to space group monoclinic P21/n containing two distinct Fe sites in rock salt ordering of Fe3+ and Fe5+. This structural phase transition is associated with the charge ordering phenomena which is accompanied by valence electron delocalization giving rise to metal to insulator transition. Charge Disproportion (CD) which is responsible for order to order magnetic phase transition by introducing moment imbalance between two sub-lattices could be understood in terms of electronic properties of CaFeO3 discussed below. According to the Jahn-Teller theorem, the presence of a single localized electron in the doubly degenerate eg set of orbitals is not a stable situation. Hence perovskite with d4+ state is highly instable and unlike other materials CaFeO3 cannot go under Jahn-Teller distortion. The reason is Ca2+ being much smaller causes a tilting of the FeO6 octahedra to satisfy the valence requirements of calcium. Due to this seemingly subtle distortion the Fe-O-Fe bond is distorted away from the linear geometry observed in SrFeO3 (the Fe-O-Fe bond angle is 158° in CaFeO3). This reduces the spatial overlap of the Fe eg and O 2p orbitals, and the width of the σ* band decreases. To remove the degeneracy of t32ge1g configuration and minimizing the Culoumb repulsion it stabilizes through charge disproportion which can be written as 2Fe4+=Fe3++Fe5+ with inequal sublattices giving rise to FM-M state at lower temperature from high temperature AF-I state. From above CHUF measurements clearly shows from the sign of that the kinetic arrested state in this case is AF-I state which gives rises to FM-M equilibrium state upon devirtification at lower temperature. 4.
Conclusion:
CaFeO3 polycrystalline nano-powders were synthesized by sol-gel method. Strong electron localization was seen from magnetic measurements giving rise to field induced meta-magnetic first order transition. CHUF protocol was used to determine the phase co-existence between high temperature antiferromagnetic insulating state and ferromagnetic metallic equilibrium state upon devitrification at lower temperature. Acknowledgments We acknowledge DST-KEK for beamline facility, PF,Indian Beamline, Japan and Electron Microscope Facility, SINP for the necessary measurements. References [1] S. Morimoto, T. Yamanaka, M. Tanaka, Physica B 237-238 (1997) 66 67. [2] S. Nasu et al., Hyperfine Interaction 70 (1992) 1063. [3] P. Chaddah and A. Banerjee, arxiv:1201.0575v1 [4] Banerjee, Kranti Kumar and P. Chaddah, arxiv:0805.1514v1,11 May’2008 [5] P. Chaddah and A. Banerjee, arxiv:1004.3116v1, 19 April’2010 [6] P. M. Woodward, D. E. Cox, E. Moshopoulou, A. W. Sleight, and S. Morimoto, Phys. Rev. B 62, 844 (2000) [7] J. B. Goodenough, Magnetism and the Chemical Bond, Interscience, New York (1963) [8] M. Takano, N. Nakanishi, Y. Takeda, S. Naka and T. Takada, Mater. Res. Bull. 12, 923 (1977) [9] J. B. Yang et al., Journal of Applied Physics 97, 10A312 (2005) [10] T. Akao et al., Phys. Rev. Lett. 91,156405 (2003) [11] Y. Tomioka, A. Asamitsu, H. Kuwahara, Y. Moritomo, and Y. Tokura, Phys. Rev. B 53, R1689 (1996) [12] Y. Takeda, S. Naka, and M. Takano, J. Phys. Colloq. 40, C2-331 (1979)