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Spin glass behavior in hole- and electron-doped bismuth manganite single crystals
Physica B 407 (2012) 3457–3462
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Physica B journal homepage: www.elsevier.com/locate/physb
Spin glass behavior in hole- and electron-doped bismuth manganite single crystals B.C. Zhao a,n, Y.N. Huang a, C.Y. Hao b, G.L. Kuang b, Y.P. Sun a,b a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 14 February 2011 Received in revised form 18 April 2012 Accepted 1 May 2012 Available online 7 May 2012
The hole- and electron-doped bismuth manganites Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 single crystals were grown using the flux-growth method. Their structural, magnetic and electrical transport properties have been compared studied. All samples show spin-glass magnetic behavior at low temperatures. In the immediate temperature region, an antiferromagnetic transition at TA and a charge-ordering state at TCO are observed for Bi0.55Ca0.45MnO3 crystal, whereas only an antiferromagnetic transition exists in Bi0.95Ce0.05MnO3 and Bi0.55Sr0.45MnO3 crystals. Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples show semiconducting transport behavior in the whole studied temperature range, whereas Bi0.95Ce0.05MnO3 is an insulator at room temperature. In addition, near TCO a positive magnetoresistance as large as 70.7% is observed for Bi0.55Ca0.45MnO3 sample under 5 T applied magnetic field. The obtained results may originate from the rotation of the polarized Bi-6s2 lone pair electrons in the magnetic field. & 2012 Elsevier B.V. All rights reserved.
Keywords: Flux method Charge ordering
1. Introduction Since the discovery of the colossal magnetoresistance (CMR) effect in perovskite manganites with general formula R1 xAxMnO3 (where R is a trivalent cation and A a divalent or tetravalent cation), a great deal of attention has been focused on these compounds due to their unusual electronic states and physical properties [1–3]. The interest has both a fundamental and an applied perspective. In terms of the latter these system may have the possibility to fabricate sensor and storage devices based on CMR effect. The system also provide interesting possibilities for studying the complicated interplay between spin, charge, orbital, and lattice degrees in strong correlated systems. Spin-glass behavior is a key issue in the studying of manganite systems, in which short-range ferromagnetic (FM) clusters are considered to be embedded in an antiferromagnetic (AFM) matrix. The hypothesis has already been proved by various methods [4,5]. The spin glass behavior at low temperatures in manganite materials can be understood in terms of the competition between the FM ordering in the clusters and the AFM interactions present in the matrix. Under applied magnetic field, these FM clusters grow up and eventually coalesce, leading to the completing of the apparent FM ordering [6]. Charge-ordering phenomenon is another key issue in manganites, which is
n
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generally characterized by the direct space ordering of Mn3 þ and Mn4 þ ions [7]. At high temperatures these ions are randomly distributed within the MnO2 planes in the lattice and become ordered arrangement upon cooling through a certain temperature. This order is usually accompanied by orbital order (OO), an ordered occupation of the eg orbitals of the Mn ions. The CO state is expected to become stable when the repulsive Coulomb interaction between the carriers dominates over the kinetic energy of the carriers. Beyond this simple ionic model, ordered Zener polarons (ZP) were suggested as a more realistic scenario for the CO state in manganites [8]. In the ZP model, each eg electron is considered to be trapped in a Mn–O–Mn trio. The two Mn ions in the trio conserve an intermediate valence state and are ferromagnetically coupled due to the double-exchange interaction induced by the shared eg electrons. The family of bismuth-based manganites Bi1 xAxMnO3 has recently got much interest because of the physics lying behind the unusually high temperature charge ordering phase and the discovery of intrinsic phase separation [9,10]. The properties of bismuth-based oxides depend on the behavior of the highly polarizable 6s2 lone pair electrons of Bi3 þ ions. Bi3 þ ions have been found to push TCO up to 600 K in Bi0.75Sr0.25MnO3. This is the highest TCO reported yet in the manganites [11]. Bi1 xCaxMnO3 is insulating for 0.2oxo1.0, with CO at or above room temperature for 0.4rxr0.6 [9,12]. Its CO temperature is TCO ¼325 K for x¼0.5, it peaks at 335 K for x¼0.6, and drops to 210 K in the Mn4 þ rich region for x¼0.82 [13]. Except for the CO phenomenon, spin clusters and spin-glass behavior are also observed in the Bi1 xCaxMnO3 system with x 0.875 [14].
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In this work, hole- and electron-doped bismuth-based crystals Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 have been synthesized. Detailed structural, magnetic, and transport characterization have been performed on the obtained crystals. The results show that spin-glass behavior is a common feature for these samples.
2. Experimental procedure Single crystals of Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 were grown by the flux melting technique using Bi2O3 as the flux. The precursors Bi2O3, CaCO3, SrCO3, CeO2 and MnO2 were weighed and mixed thoroughly in Bi:(Ce, Ca, Sr):Mn¼0.7:0.3:1 ratio and pre-heated twice at 900 1C for 24 h. The obtained powder with 5 times molar Bi2O3 was ground, pressed and then put into a platinum crucible. The growth was performed in a muffle furnace. The furnace was first heated to 1050 1C and kept at this temperature for 10 h to ensure sufficient melting, mixing and reaction of the raw materials. It was then cooled to 830 1C at a rate of 1 1C/h and finally cooled rapidly down to room temperature at a rate of about 200 1C/h in order to avoid possible twinning. Black tetrahedral-like Bi0.95Ce0.05MnO3 and rectangular-like Bi0.55(Ca,Sr)0.45MnO3 single-crystals could be extracted from the exposed surface and cavities within the solidified flux. The typical size of the crystals is 0.5 mm and 3 mm 2 mm 1.5 mm for Bi0.95Ce0.05MnO3 and Bi0.55(Ca, Sr)0.45MnO3, respectively. The structure and phase purity of the samples was checked by X-ray diffraction (XRD) using Cu Ka radiation at room temperature. The composition of the samples was determined by an energy dispersive spectroscopy (EDS) technique. The resistive measurements were performed with a Quantum Design physical properties measurement system (PPMS) using the Van der Pauw method. DC magnetization measurements were carried out with a vibrating sample magnetometer (VSM) attached to the PPMS system and ac susceptibility measurements were performed with a Quantum Design superconducting quantum interference device (SQUID) MPMS system.
Fig. 1. (a) XRD patterns of powdered single crystals and (b) XRD patterns of the Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 single crystals.
Fig. 2. DC magnetization as a function of temperature with applied magnetic field parallel (triangle) and perpendicular (circle) to the ab plane of the Bi0.55Sr0.45MnO3 single crystal.
3. Results and discussion The nominal composition Bi0.7A0.3MnO3 (A¼Ca, Sr and Ce) and the growth process of the three samples studied in this work are the same. However, the obtained crystals are quite different in their actual composition, crystal shape and size. From the EDS analysis, the actual composition of Ca and Sr in the resulting crystals is about 0.45, whereas that of Ce is about 0.05. The difference may originate from the much smaller ionic size of Ce4 þ ions compared to that of Sr2 þ /Ca2 þ (0.87, 1, and 1.18 for Ce4 þ , Ca2 þ , and Sr2 þ , respectively) in a perovskite structure. Fig. 1(a) shows the room-temperature powder X-ray diffraction (XRD) patterns for the three studied materials. All the reflection peaks in the XRD pattern of Bi0.55Sr0.45MnO3 can be indexed with the tetragonal perovskite structure, whereas the patterns of Bi0.55Ca0.45MnO3 and Bi0.95Ce0.05MnO3 can be indexed with the monoclinic and triclinic cell, respectively. The results are consistent with the earlier reported ones of similar compounds [15,16]. Moreover, y–2y X-ray diffraction was performed on one of the facets of the Ca- and Sr-doped crystals. As shown in Fig. 1(b), only sharp (1 0 0) family peaks are observed in the XRD patterns, indicating the single-crystal structure of the obtained samples. We performed magnetic measurement with applied fields parallel (Mab) and perpendicular (Mc) to the ab plane for both Bi0.55Sr0.45MnO3 and Bi0.55Ca0.45MnO3 samples. As shown in Fig. 2,
Fig. 3. The temperature dependence of DC magnetization for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples in an applied magnetic field of 0.01 T. The inset shows the results for Bi0.95Ce0.05MnO3 in a logarithmic scale.
the temperature dependence of Mab and Mc of Bi0.55Sr0.45MnO3 is almost the same, although Mab is a little bit larger than Mc. In the following, all the magnetic measurements on Bi0.55Sr0.45MnO3 and Bi0.55Ca0.45MnO3 were performed with magnetic field parallel to the ab plane. Fig. 3 shows the temperature dependence of the DC magnetization M(T) for the two samples. The M(T) curve for Bi0.95Ce0.05MnO3 is shown in its inset. The data are collected under both zero-field cooling (ZFC) and field cooling (FC) modes with an applied magnetic field of m0H¼0.01 T. A typical Hopkinson-type maximum exists at Tf in the ZFC curve and the low temperature ZFC and FC curves deviate substantially for all the samples. The phenomenon of the discrepancy between FC and ZFC magnetization
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mB/Mn. However, meff shows much higher value below the transi-
Fig. 4. The temperature dependence of the in-phase component of the ac susceptibility for Bi0.7Ce0.3MnO3 in an ac field of 1 Oe for the frequencies of 1, 10, 100 and 997 Hz.
curves is usually ascribed to the appearance of the spin glass (SG) or cluster glass (CG) state induced by the competition between FM and AFM exchange interactions. The peak temperatures Tf are 37.1 K, 25.2 K and 41.3 K for the Ca-, Sr- and Ce-doped samples, respectively. In order to clarify the characteristics of the glassy state, we also performed ac susceptibility measurements on a single crystal of Bi0.95Ce0.05MnO3. As shown in Fig. 4, the temperature dependence of the in-phase component w0ac ðTÞ of the ac susceptibility is plotted. The susceptibility data were collected in an ac field of 1 Oe at several frequencies of 1, 10, 100, and 997 Hz. w0ac ðTÞ presents a cusp around Tf ¼50 K, which is much higher than the peak temperature obtained from the DC magnetic measurement in an applied magnetic field of 0.01 T as shown in the inset of Fig. 3. With increasing measuring frequency, the cusp shifts toward a higher temperature and the magnitude of the peak magnetic susceptibility decreases, which is the typical behavior of a glassy state. The frequency dependence of the shift for the cusp temperature in w0ac ðTÞ, i.e. p ¼ dT f =ðT f d log10 f Þ 0:0285, is the typical value for the canonical spin glass systems in which p ranges from 0.0045 to 0.28 [17]. Therefore, the result suggests the present system is a SG rather than a CG. With increasing temperature, another clear broad maximum of AFM type at TN in the M vs T curve can be observed for all studied samples. The antiferromagnetic transition temperature is 132.8, 168.7 and 175.5 K for the Ca-, Sr- and Ce-doped samples, respectively. With further increasing temperature, no visible anomaly exists in the M(T) curve of the Ce- and Sr-doped samples. By fitting the magnetic curve above TN with a Curie–Weiss (CW) law w ¼C/(T yc), the effective paramagnetic moment meff and Weiss temperature yc can be obtained as follows: meff ¼ 3.594 mB/ Mn, yc ¼46.1 K, for Bi0.95Ce0.05MnO3 and meff ¼5.757 mB/Mn, yc ¼ 77.5 K for Bi0.55Sr0.45MnO3, respectively. The low effective magnetic moment of Bi0.95Ce0.05MnO3 may be related to the stronger AFM interaction compared to the Bi0.55Sr0.45MnO3 crystal. It should be noted that the obtained yc are positive, signaling the local FM interaction in these samples. Similar phenomenon is also observed in Bi0.75Sr0.25MnO3 and Bi0.5Ca0.5MnO3 compounds [11,18]. The experimental value of meff for Bi0.55Sr0.45MnO3 is larger than the theoretically expected value mtheo ef f ¼4.467 mB/Mn, which is calculated for a mixture of Mn3 þ and Mn4 þ (in a ratio of 0.55:0.45). For the Bi0.55Ca0.45MnO3 sample, in the M(T) curve above TN a weak maximum exists at about 270 K. Charge ordering was suggested to be the physical origin for this transition. By fitting CW laws above and below this CO transition we have found a change in the effective paramagnetic moment and Weiss temperature from meff ¼4.451 mB/Mn and yc ¼ 66.4 K to meff ¼6.213 mB/Mn and yc ¼57.6 K. The value of meff above the transition is consistent with the theoretical value mtheo ef f ¼4.467
tion temperature. An increment of the effective moment in the low temperature phase is a characteristic of manganites exhibiting charge and orbital-ordering. A proper interpretation has been given in the framework of the ZP model [8]. The ZP model predicts a strong ferromagnetic coupling of Mn-pairs induced by the shared eg electron, which to align the magnetic moments of the Mn ions ferromagnetically in the paramagnetic region below TCO, leading to large fluctuating magnetic moments. Similar changes in the effective magnetic moment have also been reported in Bi0.75Sr0.25MnO3 manganite, where the transition temperature is as high as 600 K [11,19]. The Bi0.55Sr0.45MnO3 sample in present study may also be in a CO state above TN with a transition temperature well above 350 K, which induces the unexpected value of meff found in the CW fitting as aforementioned. The temperature dependence of the magnetization is also measured for all samples in fields of 0.01 T, 0.1 T, 0.5 T, 1 T and 5 T. The results for the Bi0.55Ca0.45MnO3 sample are shown in Fig. 5 as an example. The spin-glass transition temperature Tf decreases with increasing applied magnetic field. For the Ca- and Ce-doped samples, the FC curve almost superposes on the ZFC curve in applied fields above 1 T, indicating that a 1 T field is sufficient to melt the glass state in the two samples. However, the deviation between the FC and ZFC curves of Bi0.55Sr0.45MnO3 is still obvious even in a field of 3 T, which indicates that the glass state in the Sr-doped sample is much stronger than that in the Caand Ce-doped samples. The AFM transition in all samples and the CO transition in Bi0.55Ca0.45MnO3 exist for all the applied fields. TN of the Ce- and Sr-doped samples is almost not affected by the applied field. However, both TN and TCO in the Ca-doped sample increase monotonously with increasing field as shown in Table 1, implying that the applied magnetic field can stabilize the charge-
Fig. 5. The temperature dependence of the magnetization for Bi0.55Ca0.05MnO3 in magnetic fields of 0.01 T (a), 0.05 and 0.1 T (b), 0.5 and 1 T (c), and 5 T (d).
Table 1 Magnetic and transport properties of the studied crystals. The units of m0H and r are T and O cm, respectively.
m0H/r
0.01 0.05 0.1 0.5 1 5
r300 K r150 K
Bi0.95Ce0.05MnO3
Bi0.95Ca0.45MnO3
Bi0.95Sr0.45MnO3
Tf (K)
TN (K)
Tf (K)
TN (K)
TCO (K)
Tf (K)
TN (K)
41.3 32.1 17.9 – – – – –
175.5 175.5 175.4 176.1 176.1 176.1
37.1 33.1 27.2 11.2 7.2 – 2.82 3202.2
132.8 134.8 136.9 138.8 142.7 142.8
264.4 266.4 266.5 268.3 274.3 274.3
25.2 – 11.3 – 7.2 – 3.02 780.6
168.7 – 168.6 – 168.6 –
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ordering state in the present sample. The enhancement of TCO under applied magnetic field may be attributed to the highly polarizable 6s2 character of Bi3 þ ions [20]. The applied field can put the orientation of the 6s2 lone pair toward a surrounding anion (O2 ), which can produce a local distortion or even hybridization between Bi-6s orbits and O-2p orbits [21]. This hybridization would avoid the movement of eg electrons from Mn to Mn through the Mn–O–Mn bridges, thus localizing these electrons and favoring charge ordering. The supposition above can be used to explain the positive magnetoresistance observed in the sample as discussed below. The isothermal M vs H curves obtained at 5 K after a zero-field cooling process are shown in Figs. 6 and 7. Almost linear M–H evolution of Bi0.55Sr0.45MnO3 in the figure can be observed below 3 T, which is considered to originate from a progressive spin canting in the AFM structure. Interestingly, a weak field-induced transition starts at about 3.5 T. A similar transition is also observed in Bi0.5Sr0.5MnO3 and Bi0.75Sr0.25MnO3, but the required field is as high as 37 T. In contrast with the linear M–H evolution of Bi0.55Sr0.45MnO3, the isothermal magnetization curves for Bi0.55Ca0.45MnO3 show a nonlinear increase at low fields. The residual magnetization and coercive field for the latter sample are Mr ¼0.0088 mB/Mn and HC ¼ 453 Oe, respectively. The marked
Fig. 6. Isothermal magnetization hysteresis loops at 5 K for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples. The inset shows the magnified M(H) curve for Bi0.55Sr0.45MnO3.
Fig. 7. Isothermal magnetization hysteresis loops of Bi0.95Ce0.05MnO3 at 5 K (a), 15 K (b), 40 K (c), 60 and 80 K.
curvature in the M(H) isotherms indicates the induction of ferromagnetism and the ferromagnetic component increases with increasing magnetic field. Compared to the Ca- and Sr-doped samples, the 5 K hysteresis loop of Bi0.95Ce0.05MnO3 displays a large coercivity of about 0.6 T and a much larger magnetic moment of about 20 emu/g under 4 T magnetic field, which is about twice compared to that of the Caand Sr-doped samples. In order to understand the magnetic properties deeply, we also measured the field dependence of the magnetization behavior at 15, 40, 60, and 80 K (as shown in Fig. 7). Before each hysteresis loop measurement, the sample was first warmed to 300 K and then cooled to the settled temperature at zero magnetic field. From Fig. 7(b), we can see that the hysteresis loop at 15 K looks like a dumbbell and the coercive field is about 0.26 T. The result shows that the AFM interaction in the crystal plays a more important role as temperature increases. The hysteresis loop at 40 K is similar to that of 15 K, while its coercive field decreases considerably to about 0.11 T. The variation of the magnetic phenomena above indicates that the crystal system changes gradually from the FM state to the AFM state with increasing temperature. As temperature increases to 60 K, the M(H) curve shows no hysteresis and nonlinear behavior. In the temperature range above 80 K, all M(H) curves obey stringent linear behavior, indicating the whole system are in the PM state. One possible explanation for the observed M(H) behavior is as follows. In BiMnO3 system, all Mn ions are in the form of Mn3 þ and the interaction between adjacent Mn ions is FM at low temperatures [21]. A slight of Ce is doped into the system only replace the Mn3 þ ions in a very small regions and should not change the FM arrangement in the parent compound BiMnO3. In general, the valence of the doped Ce in manganite is considered to þ4 and bring Mn2 þ ions in the sample as predicted in previous works [22,23]. Accordingly, a mixture of Mn2 þ and Mn3 þ ions are present in the region around the doped Ce4 þ ions and the interaction between Mn2 þ and Mn3 þ is FM due to the doubleexchange mechanism. That is to say, there are two magnetic domains of the Mn2 þ /Mn3 þ and Mn3 þ /Mn3 þ coexist around the doped Ce ions and they are both FM, but they are antiparallel with each other. Similar two antiparallel magnetic domains is also observed in the slight Ce-doping NdMnO3 samples [24]. The two magnetic domains will be locked randomly in the ZFC process. When the magnetic field is applied, the two magnetic domains will rotate subsequently and then result into the dumbbell-like hysteresis loop as aforementioned. We tried to measure the electrical resistivity of all studied samples but the resistivity of Bi0.95Ce0.05MnO3 is too high to measure. The temperature dependence of the electrical resistivity r(T) is shown for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 in Figs. 8 and 9, respectively. Both samples show semiconducting transport behavior throughout the range of our investigation ( 120–380 K). For Bi0.55Ca0.45MnO3, the onset of the chargeordered phase is characterized by a clear increase in the resistivity at about 280 K. This is consistent with the suppression of the carrier hopping associated with the charge-ordering phase. No clear resistivity anomalies are detected at the Ne´el temperature in the r vs T curves for both samples. It should be noted that the resistivity of Bi0.55Sr0.45MnO3 is larger than that of Bi0.55Ca0.45MnO3 at 300 K, whereas it has a smaller value at 150 K. For comparison, the resistivities at 300 K and 150 K are listed in Table 1. The ionic radius and the charge-ordering state may contribute to the variation of the resistivity of the two samples. In the left inset of Fig. 8 we compare the logarithmic variations of the resistivity for Bi0.55Ca0.45MnO3 vs (1/T)1/4 and (1/T). The resistivity above TCO of Bi0.55Ca0.45MnO3 can be well approximated by a r ¼ r0 expðE0 =kB TÞ form, indicating an activated variation nature of the carriers in the crystal [25]. Similar to most
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where r0 is the resistivity at zero field and rH is the resistivity at m0H¼5 T. It is interesting to note that the applied magnetic field suppresses the conductivity considerably and induces a positive MR as large as 70.7% at applying a 5 T field near TCO in Bi0.55Ca0.45MnO3. The enhancement of the charge-ordered state in a magnetic field as aforementioned may attribute to the observed large positive MR. MR decreases with decreasing temperature from TCO while a weak negative MR appears in the temperature range below 127 K. Similarly low temperature negative MR is also observed in the r(T) curve of Bi0.55Sr0.45MnO3. However, no obvious MR effect is detected in the high temperature region.
4. Conclusion
Fig. 8. The temperature dependence of the resistivity for Bi0.55Ca0.45MnO3 at 0 T. The right inset shows the MR vs T plot at an applied field of 5 T. The left inset shows ln r vs T 1/4 and ln r vs T 1 dependences.
Structural, magnetic and transport measurements of the bismuth-based manganites Bi0.95Ce0.05MnO3, Bi0.55Ca0.45MnO3, and Bi0.55Sr0.45MnO3 single crystals have been compared studied. DC magnetization and ac susceptibility measurements evidence spinglass behavior in the low-temperature region for all studied samples. With increasing temperature, a AFM transition at TA and a charge-order transition at TCO are observed in Bi0.55Ca0.45MnO3, whereas only a AFM transition exists in Bi0.95Ce0.05MnO3 and Bi0.55Sr0.45MnO3 crystals in the studied temperature range. Both TN and TCO increase monotonically with increasing magnetic field. The temperature dependence of the resistivity r(T) for Bi0.55(Ca,Sr)0.45MnO3 displays semiconducting behavior over the whole measured temperature range. The transport mechanism in Bi0.55Ca0.45MnO3 changes from the high-temperature thermal-active model to low-temperature variable-range-hopping (VRH) model at TCO, whereas the r(T) curve of Bi0.55Sr0.45MnO3 fits the VRH model in the whole studied temperature range. In addition, positive magnetoresistance as large as 70.7% near TCO is observed in the Bi0.55Ca0.45MnO3 crystal. The results may be caused by the rotation of the polarized Bi-6s2 lone pair in a magnetic field.
Acknowledgment Fig. 9. Resistivity as a function of temperature for a Bi0.55Sr0.45MnO3 single crystal. The inset shows the fitting results according to the VRH model.
oxide semiconductors, low temperature resistivity data below TCO in Bi0.55Ca0.45MnO3 can be well fitted with Mott’s variable range hopping (VRH) model, r ¼ r0 expðT 0 =TÞ1=4 , where T0 is characteristic temperature related to the localization length x and the density of state N (EF) in the vicinity of the Fermi energy level as kBTE21/[x3N(EF)]. It is suggested that the onset of the chargeordered phase plays a main role in determining the evolution of the transport mechanism in the crystal. As for the sample of Bi0.55Sr0.45MnO3, the r(T) curve can be well fitted according to the VRH model for T4230 K, with (T0)1 ¼1.43 108 K, and for To230 K, with (T0)2 ¼2.52 108 K, as shown in the inset of Fig. 9. It is found that T0 increases obviously with decreasing temperature, implying that the decrease of the localization length and the carrier mobility is intimately related to the localization of the carriers. The resistivity data obtained in an applied magnetic field of 5 T for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 single crystals are also recorded. The temperature dependence of MR for Bi0.55Ca0.45MnO3 is shown in the right inset of Fig. 8. Here MR is defined as
Dr=r0 ¼
rH r0 100% r0
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Physica B journal homepage: www.elsevier.com/locate/physb
Spin glass behavior in hole- and electron-doped bismuth manganite single crystals B.C. Zhao a,n, Y.N. Huang a, C.Y. Hao b, G.L. Kuang b, Y.P. Sun a,b a b
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 14 February 2011 Received in revised form 18 April 2012 Accepted 1 May 2012 Available online 7 May 2012
The hole- and electron-doped bismuth manganites Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 single crystals were grown using the flux-growth method. Their structural, magnetic and electrical transport properties have been compared studied. All samples show spin-glass magnetic behavior at low temperatures. In the immediate temperature region, an antiferromagnetic transition at TA and a charge-ordering state at TCO are observed for Bi0.55Ca0.45MnO3 crystal, whereas only an antiferromagnetic transition exists in Bi0.95Ce0.05MnO3 and Bi0.55Sr0.45MnO3 crystals. Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples show semiconducting transport behavior in the whole studied temperature range, whereas Bi0.95Ce0.05MnO3 is an insulator at room temperature. In addition, near TCO a positive magnetoresistance as large as 70.7% is observed for Bi0.55Ca0.45MnO3 sample under 5 T applied magnetic field. The obtained results may originate from the rotation of the polarized Bi-6s2 lone pair electrons in the magnetic field. & 2012 Elsevier B.V. All rights reserved.
Keywords: Flux method Charge ordering
1. Introduction Since the discovery of the colossal magnetoresistance (CMR) effect in perovskite manganites with general formula R1 xAxMnO3 (where R is a trivalent cation and A a divalent or tetravalent cation), a great deal of attention has been focused on these compounds due to their unusual electronic states and physical properties [1–3]. The interest has both a fundamental and an applied perspective. In terms of the latter these system may have the possibility to fabricate sensor and storage devices based on CMR effect. The system also provide interesting possibilities for studying the complicated interplay between spin, charge, orbital, and lattice degrees in strong correlated systems. Spin-glass behavior is a key issue in the studying of manganite systems, in which short-range ferromagnetic (FM) clusters are considered to be embedded in an antiferromagnetic (AFM) matrix. The hypothesis has already been proved by various methods [4,5]. The spin glass behavior at low temperatures in manganite materials can be understood in terms of the competition between the FM ordering in the clusters and the AFM interactions present in the matrix. Under applied magnetic field, these FM clusters grow up and eventually coalesce, leading to the completing of the apparent FM ordering [6]. Charge-ordering phenomenon is another key issue in manganites, which is
n
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generally characterized by the direct space ordering of Mn3 þ and Mn4 þ ions [7]. At high temperatures these ions are randomly distributed within the MnO2 planes in the lattice and become ordered arrangement upon cooling through a certain temperature. This order is usually accompanied by orbital order (OO), an ordered occupation of the eg orbitals of the Mn ions. The CO state is expected to become stable when the repulsive Coulomb interaction between the carriers dominates over the kinetic energy of the carriers. Beyond this simple ionic model, ordered Zener polarons (ZP) were suggested as a more realistic scenario for the CO state in manganites [8]. In the ZP model, each eg electron is considered to be trapped in a Mn–O–Mn trio. The two Mn ions in the trio conserve an intermediate valence state and are ferromagnetically coupled due to the double-exchange interaction induced by the shared eg electrons. The family of bismuth-based manganites Bi1 xAxMnO3 has recently got much interest because of the physics lying behind the unusually high temperature charge ordering phase and the discovery of intrinsic phase separation [9,10]. The properties of bismuth-based oxides depend on the behavior of the highly polarizable 6s2 lone pair electrons of Bi3 þ ions. Bi3 þ ions have been found to push TCO up to 600 K in Bi0.75Sr0.25MnO3. This is the highest TCO reported yet in the manganites [11]. Bi1 xCaxMnO3 is insulating for 0.2oxo1.0, with CO at or above room temperature for 0.4rxr0.6 [9,12]. Its CO temperature is TCO ¼325 K for x¼0.5, it peaks at 335 K for x¼0.6, and drops to 210 K in the Mn4 þ rich region for x¼0.82 [13]. Except for the CO phenomenon, spin clusters and spin-glass behavior are also observed in the Bi1 xCaxMnO3 system with x 0.875 [14].
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In this work, hole- and electron-doped bismuth-based crystals Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 have been synthesized. Detailed structural, magnetic, and transport characterization have been performed on the obtained crystals. The results show that spin-glass behavior is a common feature for these samples.
2. Experimental procedure Single crystals of Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 were grown by the flux melting technique using Bi2O3 as the flux. The precursors Bi2O3, CaCO3, SrCO3, CeO2 and MnO2 were weighed and mixed thoroughly in Bi:(Ce, Ca, Sr):Mn¼0.7:0.3:1 ratio and pre-heated twice at 900 1C for 24 h. The obtained powder with 5 times molar Bi2O3 was ground, pressed and then put into a platinum crucible. The growth was performed in a muffle furnace. The furnace was first heated to 1050 1C and kept at this temperature for 10 h to ensure sufficient melting, mixing and reaction of the raw materials. It was then cooled to 830 1C at a rate of 1 1C/h and finally cooled rapidly down to room temperature at a rate of about 200 1C/h in order to avoid possible twinning. Black tetrahedral-like Bi0.95Ce0.05MnO3 and rectangular-like Bi0.55(Ca,Sr)0.45MnO3 single-crystals could be extracted from the exposed surface and cavities within the solidified flux. The typical size of the crystals is 0.5 mm and 3 mm 2 mm 1.5 mm for Bi0.95Ce0.05MnO3 and Bi0.55(Ca, Sr)0.45MnO3, respectively. The structure and phase purity of the samples was checked by X-ray diffraction (XRD) using Cu Ka radiation at room temperature. The composition of the samples was determined by an energy dispersive spectroscopy (EDS) technique. The resistive measurements were performed with a Quantum Design physical properties measurement system (PPMS) using the Van der Pauw method. DC magnetization measurements were carried out with a vibrating sample magnetometer (VSM) attached to the PPMS system and ac susceptibility measurements were performed with a Quantum Design superconducting quantum interference device (SQUID) MPMS system.
Fig. 1. (a) XRD patterns of powdered single crystals and (b) XRD patterns of the Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 single crystals.
Fig. 2. DC magnetization as a function of temperature with applied magnetic field parallel (triangle) and perpendicular (circle) to the ab plane of the Bi0.55Sr0.45MnO3 single crystal.
3. Results and discussion The nominal composition Bi0.7A0.3MnO3 (A¼Ca, Sr and Ce) and the growth process of the three samples studied in this work are the same. However, the obtained crystals are quite different in their actual composition, crystal shape and size. From the EDS analysis, the actual composition of Ca and Sr in the resulting crystals is about 0.45, whereas that of Ce is about 0.05. The difference may originate from the much smaller ionic size of Ce4 þ ions compared to that of Sr2 þ /Ca2 þ (0.87, 1, and 1.18 for Ce4 þ , Ca2 þ , and Sr2 þ , respectively) in a perovskite structure. Fig. 1(a) shows the room-temperature powder X-ray diffraction (XRD) patterns for the three studied materials. All the reflection peaks in the XRD pattern of Bi0.55Sr0.45MnO3 can be indexed with the tetragonal perovskite structure, whereas the patterns of Bi0.55Ca0.45MnO3 and Bi0.95Ce0.05MnO3 can be indexed with the monoclinic and triclinic cell, respectively. The results are consistent with the earlier reported ones of similar compounds [15,16]. Moreover, y–2y X-ray diffraction was performed on one of the facets of the Ca- and Sr-doped crystals. As shown in Fig. 1(b), only sharp (1 0 0) family peaks are observed in the XRD patterns, indicating the single-crystal structure of the obtained samples. We performed magnetic measurement with applied fields parallel (Mab) and perpendicular (Mc) to the ab plane for both Bi0.55Sr0.45MnO3 and Bi0.55Ca0.45MnO3 samples. As shown in Fig. 2,
Fig. 3. The temperature dependence of DC magnetization for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples in an applied magnetic field of 0.01 T. The inset shows the results for Bi0.95Ce0.05MnO3 in a logarithmic scale.
the temperature dependence of Mab and Mc of Bi0.55Sr0.45MnO3 is almost the same, although Mab is a little bit larger than Mc. In the following, all the magnetic measurements on Bi0.55Sr0.45MnO3 and Bi0.55Ca0.45MnO3 were performed with magnetic field parallel to the ab plane. Fig. 3 shows the temperature dependence of the DC magnetization M(T) for the two samples. The M(T) curve for Bi0.95Ce0.05MnO3 is shown in its inset. The data are collected under both zero-field cooling (ZFC) and field cooling (FC) modes with an applied magnetic field of m0H¼0.01 T. A typical Hopkinson-type maximum exists at Tf in the ZFC curve and the low temperature ZFC and FC curves deviate substantially for all the samples. The phenomenon of the discrepancy between FC and ZFC magnetization
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mB/Mn. However, meff shows much higher value below the transi-
Fig. 4. The temperature dependence of the in-phase component of the ac susceptibility for Bi0.7Ce0.3MnO3 in an ac field of 1 Oe for the frequencies of 1, 10, 100 and 997 Hz.
curves is usually ascribed to the appearance of the spin glass (SG) or cluster glass (CG) state induced by the competition between FM and AFM exchange interactions. The peak temperatures Tf are 37.1 K, 25.2 K and 41.3 K for the Ca-, Sr- and Ce-doped samples, respectively. In order to clarify the characteristics of the glassy state, we also performed ac susceptibility measurements on a single crystal of Bi0.95Ce0.05MnO3. As shown in Fig. 4, the temperature dependence of the in-phase component w0ac ðTÞ of the ac susceptibility is plotted. The susceptibility data were collected in an ac field of 1 Oe at several frequencies of 1, 10, 100, and 997 Hz. w0ac ðTÞ presents a cusp around Tf ¼50 K, which is much higher than the peak temperature obtained from the DC magnetic measurement in an applied magnetic field of 0.01 T as shown in the inset of Fig. 3. With increasing measuring frequency, the cusp shifts toward a higher temperature and the magnitude of the peak magnetic susceptibility decreases, which is the typical behavior of a glassy state. The frequency dependence of the shift for the cusp temperature in w0ac ðTÞ, i.e. p ¼ dT f =ðT f d log10 f Þ 0:0285, is the typical value for the canonical spin glass systems in which p ranges from 0.0045 to 0.28 [17]. Therefore, the result suggests the present system is a SG rather than a CG. With increasing temperature, another clear broad maximum of AFM type at TN in the M vs T curve can be observed for all studied samples. The antiferromagnetic transition temperature is 132.8, 168.7 and 175.5 K for the Ca-, Sr- and Ce-doped samples, respectively. With further increasing temperature, no visible anomaly exists in the M(T) curve of the Ce- and Sr-doped samples. By fitting the magnetic curve above TN with a Curie–Weiss (CW) law w ¼C/(T yc), the effective paramagnetic moment meff and Weiss temperature yc can be obtained as follows: meff ¼ 3.594 mB/ Mn, yc ¼46.1 K, for Bi0.95Ce0.05MnO3 and meff ¼5.757 mB/Mn, yc ¼ 77.5 K for Bi0.55Sr0.45MnO3, respectively. The low effective magnetic moment of Bi0.95Ce0.05MnO3 may be related to the stronger AFM interaction compared to the Bi0.55Sr0.45MnO3 crystal. It should be noted that the obtained yc are positive, signaling the local FM interaction in these samples. Similar phenomenon is also observed in Bi0.75Sr0.25MnO3 and Bi0.5Ca0.5MnO3 compounds [11,18]. The experimental value of meff for Bi0.55Sr0.45MnO3 is larger than the theoretically expected value mtheo ef f ¼4.467 mB/Mn, which is calculated for a mixture of Mn3 þ and Mn4 þ (in a ratio of 0.55:0.45). For the Bi0.55Ca0.45MnO3 sample, in the M(T) curve above TN a weak maximum exists at about 270 K. Charge ordering was suggested to be the physical origin for this transition. By fitting CW laws above and below this CO transition we have found a change in the effective paramagnetic moment and Weiss temperature from meff ¼4.451 mB/Mn and yc ¼ 66.4 K to meff ¼6.213 mB/Mn and yc ¼57.6 K. The value of meff above the transition is consistent with the theoretical value mtheo ef f ¼4.467
tion temperature. An increment of the effective moment in the low temperature phase is a characteristic of manganites exhibiting charge and orbital-ordering. A proper interpretation has been given in the framework of the ZP model [8]. The ZP model predicts a strong ferromagnetic coupling of Mn-pairs induced by the shared eg electron, which to align the magnetic moments of the Mn ions ferromagnetically in the paramagnetic region below TCO, leading to large fluctuating magnetic moments. Similar changes in the effective magnetic moment have also been reported in Bi0.75Sr0.25MnO3 manganite, where the transition temperature is as high as 600 K [11,19]. The Bi0.55Sr0.45MnO3 sample in present study may also be in a CO state above TN with a transition temperature well above 350 K, which induces the unexpected value of meff found in the CW fitting as aforementioned. The temperature dependence of the magnetization is also measured for all samples in fields of 0.01 T, 0.1 T, 0.5 T, 1 T and 5 T. The results for the Bi0.55Ca0.45MnO3 sample are shown in Fig. 5 as an example. The spin-glass transition temperature Tf decreases with increasing applied magnetic field. For the Ca- and Ce-doped samples, the FC curve almost superposes on the ZFC curve in applied fields above 1 T, indicating that a 1 T field is sufficient to melt the glass state in the two samples. However, the deviation between the FC and ZFC curves of Bi0.55Sr0.45MnO3 is still obvious even in a field of 3 T, which indicates that the glass state in the Sr-doped sample is much stronger than that in the Caand Ce-doped samples. The AFM transition in all samples and the CO transition in Bi0.55Ca0.45MnO3 exist for all the applied fields. TN of the Ce- and Sr-doped samples is almost not affected by the applied field. However, both TN and TCO in the Ca-doped sample increase monotonously with increasing field as shown in Table 1, implying that the applied magnetic field can stabilize the charge-
Fig. 5. The temperature dependence of the magnetization for Bi0.55Ca0.05MnO3 in magnetic fields of 0.01 T (a), 0.05 and 0.1 T (b), 0.5 and 1 T (c), and 5 T (d).
Table 1 Magnetic and transport properties of the studied crystals. The units of m0H and r are T and O cm, respectively.
m0H/r
0.01 0.05 0.1 0.5 1 5
r300 K r150 K
Bi0.95Ce0.05MnO3
Bi0.95Ca0.45MnO3
Bi0.95Sr0.45MnO3
Tf (K)
TN (K)
Tf (K)
TN (K)
TCO (K)
Tf (K)
TN (K)
41.3 32.1 17.9 – – – – –
175.5 175.5 175.4 176.1 176.1 176.1
37.1 33.1 27.2 11.2 7.2 – 2.82 3202.2
132.8 134.8 136.9 138.8 142.7 142.8
264.4 266.4 266.5 268.3 274.3 274.3
25.2 – 11.3 – 7.2 – 3.02 780.6
168.7 – 168.6 – 168.6 –
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ordering state in the present sample. The enhancement of TCO under applied magnetic field may be attributed to the highly polarizable 6s2 character of Bi3 þ ions [20]. The applied field can put the orientation of the 6s2 lone pair toward a surrounding anion (O2 ), which can produce a local distortion or even hybridization between Bi-6s orbits and O-2p orbits [21]. This hybridization would avoid the movement of eg electrons from Mn to Mn through the Mn–O–Mn bridges, thus localizing these electrons and favoring charge ordering. The supposition above can be used to explain the positive magnetoresistance observed in the sample as discussed below. The isothermal M vs H curves obtained at 5 K after a zero-field cooling process are shown in Figs. 6 and 7. Almost linear M–H evolution of Bi0.55Sr0.45MnO3 in the figure can be observed below 3 T, which is considered to originate from a progressive spin canting in the AFM structure. Interestingly, a weak field-induced transition starts at about 3.5 T. A similar transition is also observed in Bi0.5Sr0.5MnO3 and Bi0.75Sr0.25MnO3, but the required field is as high as 37 T. In contrast with the linear M–H evolution of Bi0.55Sr0.45MnO3, the isothermal magnetization curves for Bi0.55Ca0.45MnO3 show a nonlinear increase at low fields. The residual magnetization and coercive field for the latter sample are Mr ¼0.0088 mB/Mn and HC ¼ 453 Oe, respectively. The marked
Fig. 6. Isothermal magnetization hysteresis loops at 5 K for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples. The inset shows the magnified M(H) curve for Bi0.55Sr0.45MnO3.
Fig. 7. Isothermal magnetization hysteresis loops of Bi0.95Ce0.05MnO3 at 5 K (a), 15 K (b), 40 K (c), 60 and 80 K.
curvature in the M(H) isotherms indicates the induction of ferromagnetism and the ferromagnetic component increases with increasing magnetic field. Compared to the Ca- and Sr-doped samples, the 5 K hysteresis loop of Bi0.95Ce0.05MnO3 displays a large coercivity of about 0.6 T and a much larger magnetic moment of about 20 emu/g under 4 T magnetic field, which is about twice compared to that of the Caand Sr-doped samples. In order to understand the magnetic properties deeply, we also measured the field dependence of the magnetization behavior at 15, 40, 60, and 80 K (as shown in Fig. 7). Before each hysteresis loop measurement, the sample was first warmed to 300 K and then cooled to the settled temperature at zero magnetic field. From Fig. 7(b), we can see that the hysteresis loop at 15 K looks like a dumbbell and the coercive field is about 0.26 T. The result shows that the AFM interaction in the crystal plays a more important role as temperature increases. The hysteresis loop at 40 K is similar to that of 15 K, while its coercive field decreases considerably to about 0.11 T. The variation of the magnetic phenomena above indicates that the crystal system changes gradually from the FM state to the AFM state with increasing temperature. As temperature increases to 60 K, the M(H) curve shows no hysteresis and nonlinear behavior. In the temperature range above 80 K, all M(H) curves obey stringent linear behavior, indicating the whole system are in the PM state. One possible explanation for the observed M(H) behavior is as follows. In BiMnO3 system, all Mn ions are in the form of Mn3 þ and the interaction between adjacent Mn ions is FM at low temperatures [21]. A slight of Ce is doped into the system only replace the Mn3 þ ions in a very small regions and should not change the FM arrangement in the parent compound BiMnO3. In general, the valence of the doped Ce in manganite is considered to þ4 and bring Mn2 þ ions in the sample as predicted in previous works [22,23]. Accordingly, a mixture of Mn2 þ and Mn3 þ ions are present in the region around the doped Ce4 þ ions and the interaction between Mn2 þ and Mn3 þ is FM due to the doubleexchange mechanism. That is to say, there are two magnetic domains of the Mn2 þ /Mn3 þ and Mn3 þ /Mn3 þ coexist around the doped Ce ions and they are both FM, but they are antiparallel with each other. Similar two antiparallel magnetic domains is also observed in the slight Ce-doping NdMnO3 samples [24]. The two magnetic domains will be locked randomly in the ZFC process. When the magnetic field is applied, the two magnetic domains will rotate subsequently and then result into the dumbbell-like hysteresis loop as aforementioned. We tried to measure the electrical resistivity of all studied samples but the resistivity of Bi0.95Ce0.05MnO3 is too high to measure. The temperature dependence of the electrical resistivity r(T) is shown for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 in Figs. 8 and 9, respectively. Both samples show semiconducting transport behavior throughout the range of our investigation ( 120–380 K). For Bi0.55Ca0.45MnO3, the onset of the chargeordered phase is characterized by a clear increase in the resistivity at about 280 K. This is consistent with the suppression of the carrier hopping associated with the charge-ordering phase. No clear resistivity anomalies are detected at the Ne´el temperature in the r vs T curves for both samples. It should be noted that the resistivity of Bi0.55Sr0.45MnO3 is larger than that of Bi0.55Ca0.45MnO3 at 300 K, whereas it has a smaller value at 150 K. For comparison, the resistivities at 300 K and 150 K are listed in Table 1. The ionic radius and the charge-ordering state may contribute to the variation of the resistivity of the two samples. In the left inset of Fig. 8 we compare the logarithmic variations of the resistivity for Bi0.55Ca0.45MnO3 vs (1/T)1/4 and (1/T). The resistivity above TCO of Bi0.55Ca0.45MnO3 can be well approximated by a r ¼ r0 expðE0 =kB TÞ form, indicating an activated variation nature of the carriers in the crystal [25]. Similar to most
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where r0 is the resistivity at zero field and rH is the resistivity at m0H¼5 T. It is interesting to note that the applied magnetic field suppresses the conductivity considerably and induces a positive MR as large as 70.7% at applying a 5 T field near TCO in Bi0.55Ca0.45MnO3. The enhancement of the charge-ordered state in a magnetic field as aforementioned may attribute to the observed large positive MR. MR decreases with decreasing temperature from TCO while a weak negative MR appears in the temperature range below 127 K. Similarly low temperature negative MR is also observed in the r(T) curve of Bi0.55Sr0.45MnO3. However, no obvious MR effect is detected in the high temperature region.
4. Conclusion
Fig. 8. The temperature dependence of the resistivity for Bi0.55Ca0.45MnO3 at 0 T. The right inset shows the MR vs T plot at an applied field of 5 T. The left inset shows ln r vs T 1/4 and ln r vs T 1 dependences.
Structural, magnetic and transport measurements of the bismuth-based manganites Bi0.95Ce0.05MnO3, Bi0.55Ca0.45MnO3, and Bi0.55Sr0.45MnO3 single crystals have been compared studied. DC magnetization and ac susceptibility measurements evidence spinglass behavior in the low-temperature region for all studied samples. With increasing temperature, a AFM transition at TA and a charge-order transition at TCO are observed in Bi0.55Ca0.45MnO3, whereas only a AFM transition exists in Bi0.95Ce0.05MnO3 and Bi0.55Sr0.45MnO3 crystals in the studied temperature range. Both TN and TCO increase monotonically with increasing magnetic field. The temperature dependence of the resistivity r(T) for Bi0.55(Ca,Sr)0.45MnO3 displays semiconducting behavior over the whole measured temperature range. The transport mechanism in Bi0.55Ca0.45MnO3 changes from the high-temperature thermal-active model to low-temperature variable-range-hopping (VRH) model at TCO, whereas the r(T) curve of Bi0.55Sr0.45MnO3 fits the VRH model in the whole studied temperature range. In addition, positive magnetoresistance as large as 70.7% near TCO is observed in the Bi0.55Ca0.45MnO3 crystal. The results may be caused by the rotation of the polarized Bi-6s2 lone pair in a magnetic field.
Acknowledgment Fig. 9. Resistivity as a function of temperature for a Bi0.55Sr0.45MnO3 single crystal. The inset shows the fitting results according to the VRH model.
oxide semiconductors, low temperature resistivity data below TCO in Bi0.55Ca0.45MnO3 can be well fitted with Mott’s variable range hopping (VRH) model, r ¼ r0 expðT 0 =TÞ1=4 , where T0 is characteristic temperature related to the localization length x and the density of state N (EF) in the vicinity of the Fermi energy level as kBTE21/[x3N(EF)]. It is suggested that the onset of the chargeordered phase plays a main role in determining the evolution of the transport mechanism in the crystal. As for the sample of Bi0.55Sr0.45MnO3, the r(T) curve can be well fitted according to the VRH model for T4230 K, with (T0)1 ¼1.43 108 K, and for To230 K, with (T0)2 ¼2.52 108 K, as shown in the inset of Fig. 9. It is found that T0 increases obviously with decreasing temperature, implying that the decrease of the localization length and the carrier mobility is intimately related to the localization of the carriers. The resistivity data obtained in an applied magnetic field of 5 T for Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 single crystals are also recorded. The temperature dependence of MR for Bi0.55Ca0.45MnO3 is shown in the right inset of Fig. 8. Here MR is defined as
Dr=r0 ¼
rH r0 100% r0
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