Structural, magnetic and electrical properties of double-doped manganites Y0.5+ySr0.5−yMn1−yCryO3 (0≤y≤0.5)

Author's Accepted Manuscript

Structural, magnetic and electrical properties of double-doped manganites Y0.5+ySr0.5-yMn1yCryO3 (0≤y≤0.5) Lei Yang, Qingyong Duanmu, Lin Hao, Xiaoping Wang, Yiyong Wei, Zhongfeng Zhang, Hong Zhu

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S0304-8853(13)00230-8 http://dx.doi.org/10.1016/j.jmmm.2013.04.011 MAGMA58182

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Journal of Magnetism and Magnetic Materials

Received date: 14 January 2013 Revised date: 29 March 2013 Accepted date: 4 April 2013 Cite this article as: Lei Yang, Qingyong Duanmu, Lin Hao, Xiaoping Wang, Yiyong Wei, Zhongfeng Zhang, Hong Zhu, Structural, magnetic and electrical properties of double-doped manganites Y0.5+ySr0.5-yMn1-yCryO3 (0≤y≤0.5), Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j. jmmm.2013.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural, magnetic and electrical properties of double-doped manganites Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5) Lei Yanga, Qingyong Duanmua, Lin Haoa, Xiaoping Wanga, Yiyong Weib, Zhongfeng Zhanga, and Hong Zhua,* a

Department of Physics, University of Science and Technology of China, Hefei, 230026, People’s

Republic of China b

School of Electronic and information Engineering, Hefei Normal University, Hefei, 230601, People’s Republic of China

* Corresponding author. E-mail: [email protected] Keywords: Spin galss; Ferrimagnetism; Cr-doped manganite; X-ray diffraction; Double-exchange interaction.

Abstract We report an investigation on the evolution of structural, magnetic and transport properties in double-doped manganites Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5), remaining eg electron density (x=0.5) unchanged. From a pseudo-cubic perovskite structure for Y0.5Sr0.5MnO3 with y=0, powder X-ray diffraction patterns demonstrate that Cr3+ substitution on the Mn-site leads to a tetragonal distortion, which increases with the doping level y. The temperature dependence of magnetic susceptibility shows that the magnetic state at low temperatures evolves from an antiferromagnetic-spin-glass state for y=0, undergoing a ferromagnetic-spin-glass state, finally to a ferrimagnetic state for y=0.5. The electrical transport properties of all samples show an insulator behavior dominated by the small polaron hopping mechanism and the activation energy increases significantly along with y. Consequently, the enhancement of ferromagnetic component in the Y0.5+ySr0.5-yMn1-yCryO3 series may be attributed to the anisotropic ferromagnetic superexchange interactions of Mn3+ ions collaborating with the ordered

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arrangement of Mn3+/Cr3+ ions, rather than the double exchange interaction via eg-electron hopping process.

1. Introduction Manganese-based perovskite oxides of the generic formula RMnO3 (where R is a rare-earth element) have been intensively studied in recent years due to their fascinating properties and potential applications [1-6]. These rare-earth manganese oxides mainly crystallize in two crystal structures depending on the ionic radius of rare-earth elements. The compounds with relatively large rare-earth elements (R=La-Gd) usually have an orthorhombic perovskite structure with corner-sharing octahedral MnO6 forming a three-dimensional network. Many studies have been done on such orthorhombic RMnO3 doped with alkaline earths because of their unusual properties, such as colossal magnetoresistance and charge ordering phenomena [7-8]. On the other hand, RMnO3 compounds with smaller rare-earth elements (Ho,Y, etc.) favor a hexagonal structure and usually display the coexistence of magnetic and electric orders [9-12]. Moreover, the coupling between magnetic and electric order parameters in such multiferroic RMnO3 is of particular interest, because it means control of magnetization by electric field, and vice verse, becomes physically realizable. However, both the mechanism driving multiferroicity and coupling between order parameters, which are very vital in tailoring the functionality, are not well understood yet [13-14]. We also noticed that most multiferroic manganites display antiferromagnetic orders, which is insensitive to applied magnetic fields. Thus, the enhancement of ferromagnetic component in the compounds is the necessary perquisite to realize the practical applications. As a reminiscence of colossal magnetoresistance materials, the double-exchange (DE) exchange mechanism proposed by C. Zener in 1951 [15] is an important ferromagnetic interaction in doped manganites R1-xAxMnO3 (where A is an alkaline earth element, such as Sr, Ca, Ba, etc.). Partial substitution of divalent alkaline earth ions on the R site effectively introduces Mn4+ ions with eg empty orbital, which in turn leads to the ferromagnetic 2-16

DE interaction between Mn3+ and Mn4+ ions. Recently, researchers successfully substituted other transition metal (TM) ions (Cr, Co, Fe, etc.) on the Mn site forming RMn1-xMxO3 compounds [16-21]. Some of them claimed that the ferromagnetic DE exists between the doping TM ions and Mn ions [17, 20, 22]; but some others argued that it is merely a canted antiferromagnetism presented in the Mn-site doped manganites [23]. So the conclusion on the DE interaction between different transition metal ions remains far from unambiguous. To explore the possibility of ferromagnetic interaction in yttrium manganites, in this work we start from Y0.5Sr0.5MnO3 with half-filled eg orbitals, i.e., hole carrier density x=0.5. Because Cr3+ ion has the same electron configuration ([Ar]3d3) as Mn4+ ion, the hole density is also provided by Cr3+ substitution on the Mn site. To remain the hole density unchanged, the same amount of Sr2+ doping level y is decreased as Cr3+

substitution

gradually

increases,

forming

the

double-doped

samples

Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5). Then we investigate the evolution of crystal structure, magnetic and transport properties in the double-doped manganites. Our results show that all the samples adopt an orthorhombic perovskite structure with space group Pnma, and with increasing y a lattice distortion gradually evolves from a pseudo-cubic perovskite for Y0.5Sr0.5MnO3 (y=0) into a pseudo-tetragonal perovskite for YMn0.5Cr0.5O3 (y=0.5), implying a tendency toward B-site cation ordered arrangement of Cr3+/Mn3+ ions with a layered pattern. Detailed analysis of the magnetic properties indicates that the magnetic states of the Y0.5+ySr0.5-yMn1-yCryO3 series start from an antiferromagnetic-dominant spin glass for y=0, through ferromagnetic-like spin glasses for y=0.1~0.3, and finally reach a ferrimagnetic state for y=0.5. The study of electrical transport properties shows that all samples are dominated by the small polaron hopping mechanism and the activation energy increases significantly with y, suggesting that the double exchange interaction mediated by the eg-electron hopping between Cr3+ and Mn3+ ions is impossible. We argue that the enhancement of ferromagnetic component in the Cr3+ samples should be attributed to the anisotropic superexchange interaction of Mn3+ ions in a layered B-site cation ordered structure. 3-16

2. Experimental details Polycrystalline

Y0.5+ySr0.5-yMn1-yCryO3

(0y0.5)

were

synthesized

using

Y2O3(99.99%), SrCO3(99.9%), Cr2O3 (99.9%), and MnO2 as the starting materials by a standard solid-state reaction method. Before weighing, Y2O3 was dehydrated in air at 800°C for 10 h. Stoichiometric quantities of these oxides were then mixed, grinded, and put into a corundum crucible, which was subjected to several heating cycles between 900 and 1200°C in air with several intermediate grindings to obtain a homogeneous mixture. In the final step, the resultant Y0.5+ySr0.5-yMn1-yCryO3 powder was pressed into pellets and annealed at 1400°C in air for 30 h. Room temperature powder x-ray diffraction (XRD) data were measured with a Rigaku TTRAX-III diffractometer employing CuK radiation over the range 20°280°, with a step size of 0.02°. Magnetization (M) as a function of temperature (M-T curve) and field (M-H curve) was measured with a Quantum Design Physical Properties Measurement System (PPMS). M-T curves were recorded in the temperature range of 2–400 K under a magnetic field of H=1 kOe with zero-field-cooled (ZFC) and field-cooled (FC) modes, respectively. M-H curves were obtained up to 50 kOe magnetic fields at 5 K. The dc electrical resistivity was measured by a standard four-probe technique using the same PPMS. 3. Results and discussion 3.1. Structural analysis

Four representive XRD patterns of the Y0.5+ySr0.5-yMn1-yCryO3 series with y varying from 0 to 0.5 are displayed in Fig. 1. With increasing Cr content y, it is clear that the diffraction peaks split gradually, indicating lowering of the crystal symmetry. All of the XRD peaks can be indexed with a single phase of orthorhombic structure in the Pnma space group, without any impurity phase. Figure 2 summaries the lattice parameters, which are divided by 2 or 2 for

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comparing with the pseudo-cubic structure of doped manganites, and lattice volume of Y0.5+ySr0.5-yMn1-yCryO3 series. The decrease in lattice volume with increasing Cr3+ content, as shown in Fig. 2(a), should be mainly attributed to the simultaneous decrease of larger Sr2+ (1.31 Å) ion on the A-site. According to Ref. [24], the ionic size of Sr2+ is much larger than that of Y3+ (1.075 Å), while Cr3+ ion (0.615 Å) is merely a little bit larger than Mn4+ ion (0.53 Å). Therefore, the lattice volume decreases finally with decreasing Sr2+ content, while the increase of Cr3+ doping just plays a minor opposite role on it. In Fig. 2(b), one can see that the lattice of samples with y0.1 is close to a pseudo-cubic structure with a / 2 | b / 2 | c / 2 . As further increasing Cr3+ content to y0.2, b / 2 | c / 2 decreases while a / 2 increases continuously, indicating a remarkable tetragonal distortion increases regularly. As we know, the lattice symmetry of perovskite structure is governed by the tolerance factor t

( rA !   rO !) / 2( rB !   rO !) , where rA, rB, and rO are the ionic radii of the

ions A, B, and O in ABO3, respectively. So we calculated the tolerance factor t for the Y0.5+ySr0.5-yMn1-yCryO3 series. As shown in Fig. 2(c), t gradually decreases from 0.923 for Y0.5Sr0.5MnO3 to 0.862 for YMn0.5Cr0.5O3, which is coincide with the evolution of lattice distortion in the Y0.5+ySr0.5-yMn1-yCryO3 samples, i.e., larger t corresponds to higher lattice symmetry, and vice verse. We like to point out that the pseudo-cubic structure of Y0.5Sr0.5MnO3 and the tetragonal distortion of YMn0.5Cr0.5O3 suggest in some extent the B-site cation ordering in a rock salt/disorder pattern and a columnar/layered pattern, respectively [25]. S.V. Trukhanov et al. have reported a structural evolution in (Pr,Ba)MnO3 perovskite manganites from an orthorhombic structure for the A-site disordered Pr0.7Ba0.3MnO3 to a tetragonal structure for the A-site ordered PrBaMn2O6 [26]. And they found that the cation odering remarkably influences the magnetic properties of the samples, e.g., the Curie temperature increases from ~173 K for the former to ~313 K for the latter. Here for the double-doped manganites Y0.5+ySr0.5-yMn1-yCryO3, it is also possible to form an A/B site ordered pattern accompanying with the structure evolution from orthorhombic (pseudo-cubic) 5-16

structure to tetragonal-like perovskite tructure, especially for the y=0.5 sample. In general, for cation ordered structures, there should be a superlattice peak appearing at 2T lower than 20° in XRD patterns. For the YMn0.5Cr0.5O3 compound, the superlattice peak should be localized at 2T ~19.7° marked as a black line in Fig. 1. In the experimental XRD patterns, however, the superlattice peak corresponding to the cation ordering is almost invisible. The reason may be that the Mn3+ and Cr3+ ions have similar atomic scattering factors, and X-ray diffraction is not sensitive enough to exactly distinguish Mn3+ and Cr3+ ions. So to clarify this point, future investigation with the help of high quality samples and advanced methods, such as single crystal and neutron diffraction, is still needed. Referring to the reported studies, at present we speculate that there is a tendency

to

B-site-ordered

double

perovskite

structure

for

the

Y0.5+ySr0.5-yMn1-yCryO3 compounds with increasing y. Considering Mn3+ content (x=0.5) remains unchanged in this series, the change in their arrangement may be responsible to the evolution of magnetism discussed below, because Mn3+ ions have both of antiferromagnetic covalent bond and ferromagnetic semicovalent bond depending on the bonding direction [27]. 3.2. Magnetic properties

Figure 3(a) shows the temperature dependence of magnetization with field-cooled (FC) and zero-field-cooled (ZFC) modes at magnetic field H=1 kOe for the Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5) series. For the Y0.5Sr0.5MnO3 sample (y=0), it can be seen that the magnetization increases gradually as paramagnetic behavior with decreasing temperature till ~23 K, and then MZFC decreases at lower temperatures while MFC changes little. The significant difference between ZFC and FC curves implies the magnetic inhomogeneity or frustration, such as spin glass behavior, occurs at low temperatures. Usually, two parameters are defined to describe the frustrated interactions, i.e., magnetic ordering temperature TMO corresponding to the minimum value of dMFC/dT and freezing temperature TF corresponding to 6-16

the peak of ZFC curve [28]. For the Y0.5Sr0.5MnO3 sample, TMO and TF are 42 K and 23 K, respectively. For the Cr-doped samples (y>0), the M-T curves show notable difference at two points compared with that of the Cr-undoped sample. The first is that with decreasing temperature the magnetization begins to increase sharply at ~75 K, corresponding to a ferromagnetic-like transition. The defined ordering temperature TMO increases to 47 K and 51 K for y=0.1 and 0.3 samples, respectively. And the freezing temperature TF also increases to 25 K and 29 K, implying an increase in the size of ferromagnetic clusters [28].The second is that the low-temperature M is one order of magnitude larger than that of y=0 sample. These results indicate that Cr-substitution on the Mn-site is in favor of ferromagnetic component in the Y0.5+ySr0.5-yMn1-yCryO3 series. It has been reported that external pressure enhances the ferromagnetic component and suppresses spin glass behavior [29]. In our case, while Mn3+ and Cr3+ cations on the B sites have similar radiuses, the Y3+ is much smaller than Sr2+ as mentioned above. Then the average radius of A-site cations substantially decreases with increasing y, which in turn results in an enhancement of the internal chemical pressure. Considering the similarity between internal and external pressures, so it is reasonable for the increase of ferromagnetic component in the Y0.5+ySr0.5-yMn1-yCryO3 compounds with increasing y. While the Cr-doped samples with y<0.5 remain a spin-glass-like behavior with the difference between MZFC and MFC, it is also noteworthy that disappearance of the low-temperature suppression of MZFC for YMn0.5Cr0.5O3 compound with y=0.5, suggesting the latter enters a more stable magnetic state at low temperatures. To further investigate the magnetic behavior, we plot 1 versus T for four of the Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, 0.5) samples in Fig. 3(b). For the samples with yd0.3, it can be seen that the curves are linear over the whole paramagnetic temperature range, revealing that the Curie-Weiss law can account for the behavior of 1 versus T. While for y=0.5, the 1-T curve is not linear but hyperbola over the paramagnetic temperature range, suggesting a ferrimagnetic nature of this sample. We 7-16

fitted the 1-T curves at higher temperatures by using the Curie–Weiss law F 1

(T - T CW ) / C

for

y0.3

and

Néel

molecular

field

theory

of

ferrimagnetism F 1 T / C  F o 1  U / (T  T CW ) for y=0.5 [30], respectively. Shown as the solid lines in Fig. 3(b), the fitting results are in good agreement with the experimental data. Two principal fitting parameters, i.e. the Curie–Weiss temperature CW and the Curie constant C, are presented in Table 1. The parent compound Y0.5Sr0.5MnO3 has a negative Curie–Weiss temperature CW = 12 K, clearly indicating the antiferromagnetic feature. When Cr is introduced into Y0.5Sr0.5MnO3, CW increases and becomes positive, indicating the enhancement of ferromagnetic interaction in the Cr-doped samples. As considering for the Curie constant C, the fitting value C=3.19×10-5 K·m3/mol of the YMn0.5Cr0.5O3 sample (y=0.5) is consistent with the theoretical value Ctheo=3.07×10-5 K·m3/mol, which is calculated by using

Ctheo

1 N o P o g 2 P B 2 [ S Mn ( S Mn  1)  S Cr ( S Cr  1)] 2

(1)

3k B

with the effective moment of Mn3+ (3d4, SMn=2) and Cr3+ (3d3, SCr=3/2) ions, confirming the ferrimagnetic phase in this sample. The result also supports the expectation of Cr substitution on the Mn site with valence state Cr3+ in the samples. In the Y0.5+ySr0.5-yMn1-yCryO3 series, the contents of magnetic 3d4 (Mn3+) and 3d3 (Mn4++Cr3+) ions consistently maintain as hole carrier density x=0.5, and then the Curie constants for these sample should remain unchanged because Ctheo is independent of long-range magnetic orders, such as (anti)ferromagnetism or ferrimagnetism. As shown in Table 1, however, the Curie constants for the samples with lower Cr content are actually larger than Ctheo, especially for Y0.5Sr0.5MnO3 (y=0). We noted that one of the features for spin glasses is the enlargement of Curie constants due to the short-range ferromagnetic fluctuation in systems [31]. Therefore, the larger Curie constants are consistent with the spin-glass behavior in the y<0.5 samples, as shown in Fig. 3(a). With increasing y from 0 to 0.5, the Curie constant decreases gradually to Ctheo, meaning that the system approaches to a more stable 8-16

long-range order, i.e. ferrimagnetic state. Figure 4 displays the field dependences of magnetization (M-H curves) at 5 K and a corresponding paramagnetic (PM) curve (dotted line), which is calculated by using the Brillouin function, for comparation. It can be seen that the parent compound Y0.5Sr0.5MnO3 shows an AFM behavior with a linear M vs. H curve much lower than the PM curve in the whole magnetic field range. Combining with the results mentioned

above,

we

conclude

that

the

Y0.5Sr0.5MnO3

sample

is

an

antiferromagnetic-dominant spin glass at low temperatures. As increasing Cr-doping content y to 0.1 and 0.3, the magnetization at lower magnetic fields is higher than the PM curve, corresponding to a ferromagnetic behavior; while at higher fields the magnetization is lower than the PM curve and shows a continuous increase with magnetic field, corresponding to a spin glass behavior [32]. Therefore, it is evidenced again that the Cr-doped samples are ferromagnetic-dominant spin glasses at low temperatures. As for the YMn0.5Cr0.5O3 sample (y=0.5), the magnetization shows small negative values at magnetic fields lower than ~8 kOe. As reported by A. Sundaresan et al [33], it is a common artifact under ZFC condition due to the trapped field in superconducting magnet, particularly for samples with large coercive field. Eliminating this artifact, one can see that the magnetization increases significantly at lower fields, and then it is gradually “saturated” to MS~0.5 B/f.u., which is in good agreement with a ferrimagnetic system consisting of antiferromagntic-coupled Mn3+ (3d4) and Cr3+ (3d3) ferromagnetic sublattices. So the YMn0.5Cr0.5O3 sample finally enters a ferrimagntic phase at low temperatures. 3.3. Resistivity

The electrical resistivity measurements as a function of temperature are shown in Fig. 5(a) for Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5) samples. The overall behavior of the  (T) data is essentially the same for all samples and the data exhibit semiconducting nature for the studied temperature range (50-300 K), i.e., the resistivity increases with decreasing temperature. However, a close correlation between Cr content and

9-16

transport behavior does exist. It can be seen that the temperature coefficient increases sharply with increasing Cr-doping. The electrical resistivity data in the paramagnetic phase of manganites have been usually fitted by Mott and Davis’s adiabatic small polaron hopping model due to polaron transport picture in this system [34]. The electrical resistivity in this model can be written as U

UoT exp( Ea / kBT )

(2)

where kB is the Boltzmann constant, Ea is activation energy and T is absolute temperature. In order to investigate in more detail the effect of Cr-doping on the electrical transport behavior, we replotted the resistivity curves as ln(/T) vs. 1000/T, and fitted resistivity data using the small polaron hopping model [35]. Figure 5(b) shows the experimental data (scatter symbols) and fitting results (solid lines). These plots reveal that the fitting results are in good agreement with the data for the whole temperature range of measurement, indicating the conduction being governed by the small polaron hopping mechanism. The fitting parameters Ea and Uo for these samples are listed in Table 2. It is clear from Table 2 that the activation energy Ea increases fast with the Cr substitution from 0.11 eV for Y0.5Sr0.5MnO3 (y=0) to 0.36 eV for YMn0.5Cr0.5O3 (y=0.5), which means that Cr substitution on the Mn-site strongly restrains the hopping probability of eg electrons/polarons᧨especially at higher Cr-doping levels. At first glance, it seems natural because a lattice distortion gradually evolves with increasing Cr content as shown in Fig. 2(b), which in turn suppresses hopping of eg electrons from Mn3+ to Mn4+/Cr3+. But we note that a heady growth of the activation energy Ea by more than doubles from Y0.5Sr0.5MnO3 to YMn0.5Cr0.5O3 suggests that something else must play a more important role. As we know, double exchange of 3d electrons usually occurs between different ions of same element, such as eg electrons of Mn3+/Mn4+ ions in doped manganites [15, 36-37] and t2g electrons of Fe2+/Fe3+ in Fe3O4 [38]. In the present case, however, it is apparent that the configurations Mn3+-O2--Cr3+ and Mn4+-O2--Cr2+ are totally nondegenerate. Consequently, double 10-16

exchange between Mn3+ and Cr3+ is impossible, and thus the activation energy Ea increases greatly with increasing Cr content. By now we have seen that Y0.5Sr0.5MnO3 presents an antiferromagnetic-dominant spin-glass-like behavior with a medium hopping activation energy, which is consistent with the results reported by Chatterjee et al [39]. Such a spin-glass-like behavior can be attributed the competition between the charge-ordering related antiferromangetic superexchange and ferromagnetic double-exchange governed by itinerant eg electrons. Charge-ordering (CO) in the A-site half doped manganites is strongly affected by the average radius of the A-site cation, , which is directly related to the one-electron eg bandwidth, and thus double-exchange interaction. With a larger =1.24Å, for example, Nd0.5Sr0.5MnO3 is a ferromagnetic metal with a TC of ~250 K and transforms to an antiferromagntic CO state around 150 K [40]. Whereas with a smaller =1.13Å, Y0.5Ca0.5MnO3 shows only a charge-ordering transition at 260 K, without ferromagnetic transition [41]. For the Y0.5Sr0.5MnO3 sample, we like to point out that its =1.20 Å falls just in between above two values, which implies that the magnitudes of AFM superexchange and FM double-exchange must be comparable with each other in this system. Therefore, it is reasonable that such comparable contrarious interactions result in a spin-glass-like phase. With increasing Cr doping, we have found that ferromangtic component increases in

the

Y0.5+ySr0.5-yMn1-yCryO3 serious.

Some

authors

attributed

it

to

the

double-exchange interaction between Cr3+ (3d3) and Mn3+ (3d4) via oxygen ions. In this work, however, we do not think this reason is realistic. As mentioned above, the tolerance factor and lattice distortion increase with increasing y, meaning that the delocalization of eg electrons must be reduced. Actually, the transport measurement results give a significant increase in the hopping activation energy with y, which excludes the possibility of double-exchange between Cr3+ and Mn3+ ions. So the ferromagnetic enhancement in the Cr-doped samples must result from other mechanisms. At present, we consider that the anisotropic superexchange interactions and B-site 11-16

ordered arrangement of Mn3+ ions may play an important role in this phenomenon [28]. As well known, the superexchange interactions are anisotropic depending on the

3d-orbital occupation. For example, in LaMnO3, which is a typical A-type antiferromagnestim, the Mn3+-O2--Mn3+ superexchange interaction within the a-b plane is ferromagnetic through a filled and an empty hybrid orbital, but antiferromagnetic through two occupied orbitals between the a-b planes. Whereas in G-type antiferromagnetic CaMnO3, both the in-plane and out-of-plane superexchange interactions are antiferromagntic due to the identical occupation of 3d orbitals along 3 axes. In our case, Mn3+ (3d4) and Mn4+/Cr3+ (3d3) are half doped with each other. When 3d4+ and 3d3+ ions are arranged at each B site randomly or in the rock-salt structure, the system enters an antiferromagnetic-dominant spin glass state, as Y0.5Sr0.5MnO3 presented. When Cr3+ (3d3) and Mn3+ (3d4) ions are gradually separated into ordered layers, the ferromagnetic component due to anisotropic superexchange from Mn3+ ions must be increased. Actually, we can get some sort of clue on the B-site ordering from the structural evolution with Cr-doping, i.e., the structure of Cr-doped samples displays a clear tetragonal-distortion with a/

2 !b/2|c/

2 as

shown in Fig. 2(b). Finally, when y reaches 0.5, YMn0.5Cr0.5O3

becomes a planar-type ferrimagnetism with ferromagnetic ordered Mn3+ and Cr3+ layers and antiferromagnetic coupling between each other. Detailed studies on the ferrimagnetism in YMn0.5Cr0.5O3, including first- principle calculations, are still in progress.

4. Conclusions In this work, we have synthesized Sr/Cr double-doped Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5) polycrystalline samples successfully and investigated their structures, magnetic and transport properties. Powder XRD patterns demonstrate that the substitution of Cr for Mn leads to a tetragonal distortion from a pseudo-cubic perovskite structure for the parent compound Y0.5Sr0.5MnO3. The temperature dependence of magnetic susceptibility shows that the magnetic states evolve from an

12-16

antiferromagnetic-spin-glass state for y=0, undergoing ferromagnetic-spin-glass states for 0
Acknowledgments

We would like to thank Prof. Li Pi, from High Magnetic Field Laboratory of CAS, for stimulating discussion and technical help during the experiment. This work was supported by the National Nature Science Foundation of China (Grant No. 11174261).

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Table 1. The Curie–Weiss temperature CW and Curie constant C for Y0.5+ySr0.5-yMn1-yCryO3 (0y0.5). CW and C are obtained by fitting the inverse susceptibility to F 1

(T - T CW ) / C for

(0y0.4) and F 1 T / C  F o 1  U / (T  T CW ) (y=0.5), respectively. y

0

0.1

0.2

0.3

0.4

0.5

CW (K)

-12

63

72

68

27

67.76

C (K·m3/mol)

6.4×10-5

4.55×10-5

3.65×10-5

3.10×10-5

2.91×10-5

3.19×10-5

Table 2. The values of o and activation energy Ea for different Cr-doping concentration. y

0.0

0.1

0.2

0.3

0.4

0.5

Ea (eV)

0.11

0.13

0.14

0.18

0.24

0.36

o (·cm)

3.68×10-4

2.70×10-5

1.15×10-4

1.26×10-4

5.04×10-4

2.10×10-3

Figure captions Fig. 1 X-ray powder-diffraction patterns for Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, and 0.5) at room 15-16

temperature, one of them is indexed on an orthorhombic lattice with space group Pnma. The black line shows the the theoretical position of a superlattice peak for B-site-ordered structure. Fig. 2. Variations of (a) lattice volume V (Å3), (b) lattice parameters a,b,c (Å), and (c) tolerance factor t with y for Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, and 0.5). Fig. 3. (a) (Color online) Temperature dependence of the field-cooled (FC) and zero-field-cooled (ZFC) dc molar magnetization of Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3 and 0.5) measured at magnetic field of 1000 Oe. The inset demonstrates the temperature dependence of the derivative of the FC curves. (b) (Color online) Temperature dependence of the inverse molar magnetic susceptibility (data points) and the Curie–Weiss fitting of the paramagnetic behavior (solid line) for Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, and 0.5). Fig. 4. (Color online) Magnetization curve of Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3 and 0.5) and the paramagnetic magnetization (dashed line) simulated by using the Brillouin function at 5 K. Fig. 5. (Color online) (a) Temperature dependence of the electrical resistivity for Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, and 0.5). (b) The fitting results of the adiabatic small polaron hopping model for the experimental data of Y0.5+ySr0.5-yMn1-yCryO3 (y=0, 0.1, 0.3, and 0.5). The solid lines show the best-fitting curves of this model by formula U

U oT exp( Ea / kBT ) .

Highlights

z z z z

The substitution of Cr for Mn leads to a tetragonal distortion from a pseudo-cubic perovskite structure. Magnetic measurements reveal an evolution from a spin-glass state to a ferrimagnetic state. Electrical transport properties are dominated by the small polaron hopping mechanism. The double-exchange is unreasonable in the Cr-doped manganites.

16-16

Figure

Fig.2.tif

Figure

Fig.4.tif

Fig.5.tif