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Magnetic behavior in the CaMn3O6 nanoribbons
Chemical Physics Letters 570 (2013) 118–120
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Magnetic behavior in the CaMn3O6 nanoribbons X.K. Zhang a,⇑, S.L. Tang b,c, L.Q. Xu b,c, J.J. Yuan a, H.J. Yu a, H. Shen a, Y.M. Xie a a
School of Physics and Electronics and Institute of Optoelectronic Materials and Technology, Gannan Normal University, Ganzhou 341000, China Nanjing National Laboratory of Microstructures, Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing 210093, China c Department of Physics, Nanjing University, Nanjing 210093, China b
a r t i c l e
i n f o
Article history: Received 2 December 2012 In final form 22 March 2013 Available online 2 April 2013
a b s t r a c t We have presented a detailed study of the magnetic properties of single crystalline CaMn3O6 nanoribbons prepared via the molten salt synthesis method. Magnetization measurements show that these nanoribbons exhibit antiferromagnetic ordering with uncompensated spins at the surface due to spin canting at 125 K. By a careful analysis for the paramagnetic region in the magnetization curve, we find there is a tiny anomaly around 240 K, where the inverse magnetic susceptibility changes its slope apparently; the linear fits to the Curie–Weiss law indicate there exists the charge ordering state. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Mixed-valence manganites with the R1xAxMnO3 (R = rare earth ion, A = alkaline earth ion) form display a variety of magnetic and structural transitions, dramatic changes of electrical conductivity and magnetoresistance effects [1,2]. Recently, study on nanoscale manganites becomes a fascinating subject since these nanostructures, such as nanoparticles, nanowires and nanotubes, have been found to exhibit some anomalous phenomena compared with the bulk counterparts due to quantum confinement effects and high surface/volume ratio [3–10]. For example, in the Pr0.5Ca0.5MnO3 nanowires, the robust charge ordering (CO) state and antiferromagnetic (AFM) ground state were significantly suppressed whereas the ferromagnetic (FM) phase was formed [3]; furthermore, the complete ‘melting’ of the CO state was observed in the Pr0.57Ca0.41Ba0.02MnO3 nanowires [4]. In the nanoscale manganites, the suppression of CO along with the enhanced magnetization is mainly explained by the size effect and surface effect. However, Fan and co-workers have reported that the La0.18Ca0.82MnO3 nanowires have similar magnetic properties to the corresponding bulk and whether charge ordering is suppressed and ferromagnetism is developed do not depend on the nanodimensional effect or surface effect [6]. So, further investigating the source of anomalous magnetic phenomena and the evolutions of magnetic properties and CO is very necessary in mixed-valence manganites. From the view of crystal structure, the mixed-valence manganites can be classified into vertex-shared type compounds such as perovskite and edge-shared type compounds such as rutile or hollandite. The former have been widely studied owing to their ⇑ Corresponding author. E-mail address: [email protected] (X.K. Zhang).
possible technological applications in the field of magnetic devices. The later, such as Ba2xMn8O16, Ba6Mn24O48, CaMn3O6, and SrMn3O6x, CaMn4O8, and NaMn7O12, have been investigated by the different groups [11–16]. Moreover, the magnetism of one-dimensional (1D) BaMn8O16, Ba6Mn24O48, and SrMn3O6x have also been studied by our group [17,18]. However, the magnetic properties of 1D CaMn3O6 nanostructure have not been investigated till now. Since 1D nanostructure may show different properties in contrast to bulk counterparts, we will investigate the magnetism and surface effect of 1D CaMn3O6 nanoribbons in the current Letter. Our results show that these nanoribbons present AFM ordering with uncompensated spins due to spin canting at T = 125 K, which may originate from the surface effect. The linear fits to the Curie–Weiss law further support there exists the charge ordering state in CaMn3O6, as indicated in Ref. [13].
2. Experimental The synthesis of CaMn3O6 nanoribbons was reported in our previous study [19]. In brief, firstly, 1 mmol MnCO3 and 0.22 mmol Na2CO3 were mixed with 5.0 g NaCl, ground homogeneously, and then annealed at 870 °C for 5 h in a crucible furnace. Secondly, a mixture ground of 1 g CaCl2 and 1.5 g NaCl was added into the reactants at the end of 5 h. This process would continue for 4 h, and the crucible was subsequently naturally cooled to room temperature. Samples were washed several times with distilled water to remove the residual NaCl and CaCl2, and then dried at 90 °C in a drying oven. The crystallization of sample was characterized using an X-ray diffractometer (XRD, Rigaku, D/Max-RA) with Cu Ka radiation (k = 1.54 Å). Magnetic properties of the samples were measured using a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design).
0009-2614/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.03.066
X.K. Zhang et al. / Chemical Physics Letters 570 (2013) 118–120
119
Figure 1. The XRD pattern (a) and SEM image (b) of the as-synthesized CaMn3O6 nanoribbons.
a strong AFM character and shows weak FM or ferrimagnetic interactions at around 50 K [13]. A bulk antiferromagnet has zero net magnetic moment due to its two mutually compensating sublattices in a zero field. If the surface to volume becomes sufficiently large for an AFM particle then the surface always leads to a breaking of the sublattice pairing and thus leads to net magnetic moment. The increase of magnetization at 125 K in CaMn3O6 nanoribbons may originate from the net magnetic moment of the uncompensated surface spins as a result of breaking of sublattice pairing. This surface ferromagnetism has been extensively investigated in AFM nanoparticles or nanowires [20–23]. Furthermore, a clear difference between the FC and the ZFC curves at temperatures below 125 K and a pronounced maximum in the ZFC curve is observed in Figure 2. The bifurcation in ZFC and FC curves should originate from the weak ferromagnetism due to the surface effect. As we know, any weak ferromagnetic or ferromagnetic sample will show a bifurcation in ZFC and FC curves due to magnetic domains. Furthermore, we will see in next section that the Curie Weiss constant is negative, which shows the AFM interaction in the CaMn3O6 nanoribbons. This indicates that the magnetic transition at 125 K is due to AFM transition with surface spin canting (weak ferromagnetism). A careful analysis of the ZFC and FC magnetization curves between 125 and 300 K reveals there are two paramagnetic-like regions (Figure 3), which is different from that of the corresponding bulk [13]. The inset of Figure 3 shows the curves of inverse magnetic susceptibility (v1) as a function of temperature for the CaMn3O6 nanoribbons. As shown, there is a tiny anomaly around 240 K, where the inverse magnetic susceptibility changes its slope apparently. To make a quantitative analysis of the magnetic properties, the linear fits to the Curie–Weiss law v ¼ v0 þ C=ðT HW Þ in the two paramagnetic regions were performed. Effective moment leff and Weiss temperature hW are determined as leff ¼ 4:62 lB and hW = 596 below 240 K, and leff = 4.71 lB and hW = 296 above 240 K, respectively. Since the two effective magnetic moments are close to the expected value (4.54 lB) from the ideal formula CaMn3+2Mn4+O6 in the two paramagnetic regions, the Mn ions are considered to be a mixture of Mn3+ (S = 2, l = 4.90 lB) and Mn4+ (S = 3/2, l = 3.87 lB). The negative sign and the high absolute value of HW indicate very strong AFM exchange between the Mn ions. It is well known that the CO transition is characterized by a peak in the magnetization where the double exchange is suppressed due to the localization of the charge carriers, resulting in a large drop of the susceptibility. The jump of v1 at 240 K in CaMn3O6 nanoribbons may indicate
Figure 2. ZFC (squares) and FC (circles) magnetization curves for CaMn3O6 nanoribbons measured in an applied magnetic field of 1000 Oe as a function of temperature.
Figure 3. The ZFC and FC magnetization curves between 125 and 300 K. Inset: the inverse magnetic susceptibility (v1) as a function of temperature. The solid lines are a guide for the eyes.
3. Results and discussion The crystal structure of the as-synthesized samples was characterized by X-ray diffraction (XRD) pattern. Figure 1a gives the XRD pattern of the samples. All of the peaks can be easily indexed to that of a pure CaMn3O6. Figure 1b presents the low-magnification field-emission scanning electron microscope (FE-SEM) image of CaMn3O6. A large quantity of nanoribbons with diameters ranging from one hundred nanometers to a few hundred nanometers and length up to tens of microns were obtained. The magnetization curves, M(T), were taken in both zero-fieldcooled (ZFC) and field-cooled (FC) modes as a function of temperature. In Figure 2 we show the ZFC and FC curves taken in a field of 1000 Oe for CaMn3O6 nanoribbons. As temperature is decreased, FC magnetization seems to exhibit FM-like behavior below T = 125 K, which is different from the case of the bulk counterparts. According to the reported results [13], the bulk compound exhibits
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X.K. Zhang et al. / Chemical Physics Letters 570 (2013) 118–120
4. Conclusions In summary, we have investigated the magnetic properties of single crystalline CaMn3O6 nanoribbons. Magnetization measurements show that these nanoribbons present antiferromagnetic ordering with uncompensated spins at the surface at 125 K. In paramagnetic region between 125 and 300 K, there is a tiny anomaly around 240 K; the linear fits to the Curie–Weiss law indicate there exists the charge ordering state. Moreover, M–H curves show no obvious exchange bias effect in CaMn3O6 nanoribbons. Acknowledgements
Figure 4. Magnetization as a function of external magnetic field at T = 3 K for CaMn3O6 nanoribbons after ZFC and FC with the cooling field of 5 kOe. Inset: the central part of hysteresis loops.
that the charge ordering is accompanied by an enhancement of the AFM interaction between Mn ions, which has been shown in CaMn3O6 bulk [13]. Indeed, the Weiss constant drops from 296 to 596, showing directly the increase of the AFM interaction. The magnetic field (H) dependence of the magnetization was measured at T = 3 K after cooling in zero field and after cooling in a field of 40 kOe. As shown in Figure 4, M–H curves present a nearly linear shape in the field range between 2 and 50 kOe, which is usually thought to be from a regular AFM behavior. Moreover, there is a coercivity of 1160 Oe observed in the central region of M–H curve, which confirm the occurrence of weak ferromagnetism due to surface spin canting in the CaMn3O6 nanoribbons. So, M– H curves indicate that, superposed on the AFM signal, there is a FM component. That is to say, the hysteresis loop may result from FM surface and AFM core. The phenomenological core–shell model was usually proposed to describe the magnetic structure of manganite nanoparticles. In the core–shell structure, the inner part of the particle (the core) has the same properties as the bulk materials, whereas the outer layer (the shell) contains most of the oxygen faults and vacancies in the crystallographic structure, which give rise to the relaxation of super-exchange interaction on the surface of AFM nanowires or nanoparticles. Then, a weak FM shell will form at the surface of these materials, resulting in natural AFM/ FM interface. The exchange bias effect originating from the coupling interaction between FM and AFM may be expected. In fact, exchange bias phenomenology in terms of core/shell model in magnetic nanoparticles has been recently reviewed in detail by Iglesias et al. [24]. Inset of Figure 4 presents the enlarged view of the low field region, there is no obvious exchange bias effect, which indicates the exchange coupling interaction between the FM shell and the AFM core in the CaMn3O6 nanoribbons is very weak.
This work was supported by the National Key Project of Fundamental Research of China (No. 2010CB923404) and the Natural Science Foundation of China (No. 11247305). This research was also supported by the Natural Science Foundation of Jiangxi (No. 20122BAB202011) and the Science and Technology Project of Jiangxi Provincial Department of Education (No. GJJ12574). References [1] M.B. Salamon, M. Jaime, Rev. Mod. Phys. 73 (2001) 583. [2] J.M.D. Coey, M. Viret, S.V. Molnár, Adv. Phys. 48 (1999) 167. [3] (a) S.S. Rao, K.N. Anuradha, S. Sarangi, S.V. Bhat, Appl. Phys. Lett. 87 (2005) 182503; (b) S.S. Rao, S. Tripathi, D. Pandey, S.V. Bhat, Phys. Rev. B 74 (2006) 144416. [4] K.N. Anuradha, S.S. Rao, S.V. Bhat, J. Nanosci. Nanotechnol. 7 (2007) 1775. [5] (a) T. Zhang, T.F. Zhou, T. Qian, X.G. Li, Phys. Rev. B 76 (2007) 174415; (b) T. Zhang, X.P. Wang, Q.F. Fang, J. Phys. Chem. C 114 (2010) 11796; (c) T. Zhang, M. Dressel, Phys. Rev. B 80 (2009) 014435. [6] (a) Y. Wang, H.J. Fan, Appl. Phys. Lett. 98 (2011) 142502; (b) Y. Wang, H.J. Fan, Phys. Rev. B 83 (2011) 224409. [7] T. Sarkar, B. Ghosh, A.K. Raychaudhuri, T. Chatterji, Phys. Rev. B 77 (2008) 235112. [8] S.K. Giri, A. Poddar, T.K. Nath, AIP Adv. 1 (2011) 032110. [9] S. Kundua, T.K. Nath, J. Phys.: Condens. Matter 23 (2011) 356001. [10] Z. Jirák, E. Hadová, O. Kaman, K. Knízˇek, M. Maryško, E. Pollert, Phys. Rev. B 81 (2010) 024403. [11] S. Ishiwata, J.W.G. Bos, Q. Huang, R.J. Cava, J. Phys.: Condens. Matter 18 (2006) 3745. [12] P. Boullay, M. Hervieu, B. Raveau, J. Solid State Chem. 132 (1997) 239. [13] J. Hadermann, A.M. Abakumov, L.J. Gillie, C. Martin, M. Hervieu, Chem. Mater. 18 (2006) 5530. [14] L.J. Gillie, J. Hadermann, O. Pérez, C. Martin, M. Hervieu, E. Suard, J. Solid State Chem. 177 (2004) 3383. [15] N. Barrier, C. Michel, A. Maignan, M. Hervieu, B. Raveau, J. Mater. Chem. 15 (2005) 386. [16] A. Prodi et al., Nat. Mater. 3 (2004) 48. [17] X.K. Zhang, S.L. Tang, Y.W. Du, J. Phys. D Appl. Phys. 44 (2011) 185001. [18] (a) J.Y. Yu, S.L. Tang, X.K. Zhang, L. Zhai, Y.G. Shi, Y. Deng, Y.W. Du, Appl. Phys. Lett. 94 (2009) 182506; (b) J.Y. Yu, S.L. Tang, L. Wang, Y.W. Du, Chem. Phys. Lett. 496 (2010) 117. [19] X.K. Zhang, S.L. Tang, Z.B. Xu, Y.W. Du, Cryst. Growth Des. 11 (2011) 2852. [20] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393. [21] A. Sundaresan, C.N.R. Rao, Nanotoday 4 (2009) 96. [22] Y.Y. Xu, X.F. Rui, Y.Y. Fu, H. Zhang, Chem. Phys. Lett. 410 (2005) 36. [23] Q. Wei, T. Zhang, X.P. Wang, Q.F. Fang, Eur. Phys. J. Appl. Phys. 57 (2012) 30401. [24] Ó. Iglesias, A. Labarta, X. Batlle, J. Nanosci. Nanotechnol. 8 (2008) 2761.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Magnetic behavior in the CaMn3O6 nanoribbons X.K. Zhang a,⇑, S.L. Tang b,c, L.Q. Xu b,c, J.J. Yuan a, H.J. Yu a, H. Shen a, Y.M. Xie a a
School of Physics and Electronics and Institute of Optoelectronic Materials and Technology, Gannan Normal University, Ganzhou 341000, China Nanjing National Laboratory of Microstructures, Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing 210093, China c Department of Physics, Nanjing University, Nanjing 210093, China b
a r t i c l e
i n f o
Article history: Received 2 December 2012 In final form 22 March 2013 Available online 2 April 2013
a b s t r a c t We have presented a detailed study of the magnetic properties of single crystalline CaMn3O6 nanoribbons prepared via the molten salt synthesis method. Magnetization measurements show that these nanoribbons exhibit antiferromagnetic ordering with uncompensated spins at the surface due to spin canting at 125 K. By a careful analysis for the paramagnetic region in the magnetization curve, we find there is a tiny anomaly around 240 K, where the inverse magnetic susceptibility changes its slope apparently; the linear fits to the Curie–Weiss law indicate there exists the charge ordering state. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Mixed-valence manganites with the R1xAxMnO3 (R = rare earth ion, A = alkaline earth ion) form display a variety of magnetic and structural transitions, dramatic changes of electrical conductivity and magnetoresistance effects [1,2]. Recently, study on nanoscale manganites becomes a fascinating subject since these nanostructures, such as nanoparticles, nanowires and nanotubes, have been found to exhibit some anomalous phenomena compared with the bulk counterparts due to quantum confinement effects and high surface/volume ratio [3–10]. For example, in the Pr0.5Ca0.5MnO3 nanowires, the robust charge ordering (CO) state and antiferromagnetic (AFM) ground state were significantly suppressed whereas the ferromagnetic (FM) phase was formed [3]; furthermore, the complete ‘melting’ of the CO state was observed in the Pr0.57Ca0.41Ba0.02MnO3 nanowires [4]. In the nanoscale manganites, the suppression of CO along with the enhanced magnetization is mainly explained by the size effect and surface effect. However, Fan and co-workers have reported that the La0.18Ca0.82MnO3 nanowires have similar magnetic properties to the corresponding bulk and whether charge ordering is suppressed and ferromagnetism is developed do not depend on the nanodimensional effect or surface effect [6]. So, further investigating the source of anomalous magnetic phenomena and the evolutions of magnetic properties and CO is very necessary in mixed-valence manganites. From the view of crystal structure, the mixed-valence manganites can be classified into vertex-shared type compounds such as perovskite and edge-shared type compounds such as rutile or hollandite. The former have been widely studied owing to their ⇑ Corresponding author. E-mail address: [email protected] (X.K. Zhang).
possible technological applications in the field of magnetic devices. The later, such as Ba2xMn8O16, Ba6Mn24O48, CaMn3O6, and SrMn3O6x, CaMn4O8, and NaMn7O12, have been investigated by the different groups [11–16]. Moreover, the magnetism of one-dimensional (1D) BaMn8O16, Ba6Mn24O48, and SrMn3O6x have also been studied by our group [17,18]. However, the magnetic properties of 1D CaMn3O6 nanostructure have not been investigated till now. Since 1D nanostructure may show different properties in contrast to bulk counterparts, we will investigate the magnetism and surface effect of 1D CaMn3O6 nanoribbons in the current Letter. Our results show that these nanoribbons present AFM ordering with uncompensated spins due to spin canting at T = 125 K, which may originate from the surface effect. The linear fits to the Curie–Weiss law further support there exists the charge ordering state in CaMn3O6, as indicated in Ref. [13].
2. Experimental The synthesis of CaMn3O6 nanoribbons was reported in our previous study [19]. In brief, firstly, 1 mmol MnCO3 and 0.22 mmol Na2CO3 were mixed with 5.0 g NaCl, ground homogeneously, and then annealed at 870 °C for 5 h in a crucible furnace. Secondly, a mixture ground of 1 g CaCl2 and 1.5 g NaCl was added into the reactants at the end of 5 h. This process would continue for 4 h, and the crucible was subsequently naturally cooled to room temperature. Samples were washed several times with distilled water to remove the residual NaCl and CaCl2, and then dried at 90 °C in a drying oven. The crystallization of sample was characterized using an X-ray diffractometer (XRD, Rigaku, D/Max-RA) with Cu Ka radiation (k = 1.54 Å). Magnetic properties of the samples were measured using a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design).
0009-2614/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.03.066
X.K. Zhang et al. / Chemical Physics Letters 570 (2013) 118–120
119
Figure 1. The XRD pattern (a) and SEM image (b) of the as-synthesized CaMn3O6 nanoribbons.
a strong AFM character and shows weak FM or ferrimagnetic interactions at around 50 K [13]. A bulk antiferromagnet has zero net magnetic moment due to its two mutually compensating sublattices in a zero field. If the surface to volume becomes sufficiently large for an AFM particle then the surface always leads to a breaking of the sublattice pairing and thus leads to net magnetic moment. The increase of magnetization at 125 K in CaMn3O6 nanoribbons may originate from the net magnetic moment of the uncompensated surface spins as a result of breaking of sublattice pairing. This surface ferromagnetism has been extensively investigated in AFM nanoparticles or nanowires [20–23]. Furthermore, a clear difference between the FC and the ZFC curves at temperatures below 125 K and a pronounced maximum in the ZFC curve is observed in Figure 2. The bifurcation in ZFC and FC curves should originate from the weak ferromagnetism due to the surface effect. As we know, any weak ferromagnetic or ferromagnetic sample will show a bifurcation in ZFC and FC curves due to magnetic domains. Furthermore, we will see in next section that the Curie Weiss constant is negative, which shows the AFM interaction in the CaMn3O6 nanoribbons. This indicates that the magnetic transition at 125 K is due to AFM transition with surface spin canting (weak ferromagnetism). A careful analysis of the ZFC and FC magnetization curves between 125 and 300 K reveals there are two paramagnetic-like regions (Figure 3), which is different from that of the corresponding bulk [13]. The inset of Figure 3 shows the curves of inverse magnetic susceptibility (v1) as a function of temperature for the CaMn3O6 nanoribbons. As shown, there is a tiny anomaly around 240 K, where the inverse magnetic susceptibility changes its slope apparently. To make a quantitative analysis of the magnetic properties, the linear fits to the Curie–Weiss law v ¼ v0 þ C=ðT HW Þ in the two paramagnetic regions were performed. Effective moment leff and Weiss temperature hW are determined as leff ¼ 4:62 lB and hW = 596 below 240 K, and leff = 4.71 lB and hW = 296 above 240 K, respectively. Since the two effective magnetic moments are close to the expected value (4.54 lB) from the ideal formula CaMn3+2Mn4+O6 in the two paramagnetic regions, the Mn ions are considered to be a mixture of Mn3+ (S = 2, l = 4.90 lB) and Mn4+ (S = 3/2, l = 3.87 lB). The negative sign and the high absolute value of HW indicate very strong AFM exchange between the Mn ions. It is well known that the CO transition is characterized by a peak in the magnetization where the double exchange is suppressed due to the localization of the charge carriers, resulting in a large drop of the susceptibility. The jump of v1 at 240 K in CaMn3O6 nanoribbons may indicate
Figure 2. ZFC (squares) and FC (circles) magnetization curves for CaMn3O6 nanoribbons measured in an applied magnetic field of 1000 Oe as a function of temperature.
Figure 3. The ZFC and FC magnetization curves between 125 and 300 K. Inset: the inverse magnetic susceptibility (v1) as a function of temperature. The solid lines are a guide for the eyes.
3. Results and discussion The crystal structure of the as-synthesized samples was characterized by X-ray diffraction (XRD) pattern. Figure 1a gives the XRD pattern of the samples. All of the peaks can be easily indexed to that of a pure CaMn3O6. Figure 1b presents the low-magnification field-emission scanning electron microscope (FE-SEM) image of CaMn3O6. A large quantity of nanoribbons with diameters ranging from one hundred nanometers to a few hundred nanometers and length up to tens of microns were obtained. The magnetization curves, M(T), were taken in both zero-fieldcooled (ZFC) and field-cooled (FC) modes as a function of temperature. In Figure 2 we show the ZFC and FC curves taken in a field of 1000 Oe for CaMn3O6 nanoribbons. As temperature is decreased, FC magnetization seems to exhibit FM-like behavior below T = 125 K, which is different from the case of the bulk counterparts. According to the reported results [13], the bulk compound exhibits
120
X.K. Zhang et al. / Chemical Physics Letters 570 (2013) 118–120
4. Conclusions In summary, we have investigated the magnetic properties of single crystalline CaMn3O6 nanoribbons. Magnetization measurements show that these nanoribbons present antiferromagnetic ordering with uncompensated spins at the surface at 125 K. In paramagnetic region between 125 and 300 K, there is a tiny anomaly around 240 K; the linear fits to the Curie–Weiss law indicate there exists the charge ordering state. Moreover, M–H curves show no obvious exchange bias effect in CaMn3O6 nanoribbons. Acknowledgements
Figure 4. Magnetization as a function of external magnetic field at T = 3 K for CaMn3O6 nanoribbons after ZFC and FC with the cooling field of 5 kOe. Inset: the central part of hysteresis loops.
that the charge ordering is accompanied by an enhancement of the AFM interaction between Mn ions, which has been shown in CaMn3O6 bulk [13]. Indeed, the Weiss constant drops from 296 to 596, showing directly the increase of the AFM interaction. The magnetic field (H) dependence of the magnetization was measured at T = 3 K after cooling in zero field and after cooling in a field of 40 kOe. As shown in Figure 4, M–H curves present a nearly linear shape in the field range between 2 and 50 kOe, which is usually thought to be from a regular AFM behavior. Moreover, there is a coercivity of 1160 Oe observed in the central region of M–H curve, which confirm the occurrence of weak ferromagnetism due to surface spin canting in the CaMn3O6 nanoribbons. So, M– H curves indicate that, superposed on the AFM signal, there is a FM component. That is to say, the hysteresis loop may result from FM surface and AFM core. The phenomenological core–shell model was usually proposed to describe the magnetic structure of manganite nanoparticles. In the core–shell structure, the inner part of the particle (the core) has the same properties as the bulk materials, whereas the outer layer (the shell) contains most of the oxygen faults and vacancies in the crystallographic structure, which give rise to the relaxation of super-exchange interaction on the surface of AFM nanowires or nanoparticles. Then, a weak FM shell will form at the surface of these materials, resulting in natural AFM/ FM interface. The exchange bias effect originating from the coupling interaction between FM and AFM may be expected. In fact, exchange bias phenomenology in terms of core/shell model in magnetic nanoparticles has been recently reviewed in detail by Iglesias et al. [24]. Inset of Figure 4 presents the enlarged view of the low field region, there is no obvious exchange bias effect, which indicates the exchange coupling interaction between the FM shell and the AFM core in the CaMn3O6 nanoribbons is very weak.
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