Modification of low temperature magnetic interactions in Dy1−xEuxMnO3

Solid solutions of rare earth ion (Eu3+) substituted DyMnO3, Dy1−xEuxMnO3 (x = 0.0–1.0) have been synthesized by ceramic method. Powder X-ray diffraction revealed single phase nature of the compounds with orthorhombic structure. Contributions from the atomic vibrations to the observation of Raman bands have been established and assigned to symmetry stretching and anti symmetry stretching, bending and tilting modes. Raman band frequencies of tilting, asymmetric stretching and bending modes were found to decrease with increasing europium concentration showing softening. Transport studies revealed that all the compounds show semiconducting nature. While the end compounds display hopping process for electrical conduction, all the substituted compounds showed activated type of conduction, and activated energy was found to reduce with increase in x. Molar susceptibility of the substituted compounds for x = 0.1, 0.3 and 0.5 revealed an antiferromagnetic transition corresponding to Mn ions. The fitted Curie–Weiss temperatures also suggested the existence of antiferromagnetic interactions in all the materials. The magnetic field dependent magnetization at various temperatures revealed paramagnetic nature down to 8 K below which hysteresis loops are observed. The presence of strong ferromagnetic correlations between Dy and Mn spins through apical oxygen ions results in the large coercive fields. For temperatures above the antiferromagnetic temperature of manganese ions (39 K) M–H curves show almost straight lines implying the absence of ferromagnetic interactions in the compounds. Different magnetic transitions: from high temperature paramagnetic state to intermediate temperature antiferromagnetic state to low temperature ferromagnetic states are observed in the M–H data.


Introduction
Perovskite oxides, ABO 3 (A: alkali/rare earth ion and B: transition metal ion), are an important group of functional materials due to their diverse physical properties over a wide range of temperatures. Superconductivity, colossal magneto resistance, piezo/pyro/ferroelectricity and structural, magnetic and transport properties are some of the intriguing properties exhibited by these materials 1,2 that attracted researchers attention from theoretical, experimental and application point of view. Among ABO 3 family of oxides, rare earth (R) manganites, RMnO 3 display spin dependent transport and magnetic properties [3][4][5] thus making them good candidates for memory devices and spintronics applications. However, magnetic and transport properties are largely inuenced by the ionic size of the R ion (r R ). Kimura et al. 6 reported a magnetic phase diagram as a function of r R . RMnO 3 compounds adopt GdFeO 3 perovskite structure where R and Mn atoms are deviated from the ideal positions in the cubic unit cell resulting in distortions of MnO 6 octahedra with Mn-O-Mn bond angle less than 180 degrees. As a consequence of these changes, a series of magnetic transitions from high temperature paramagnetic to low temperature antiferromagnetic state with several spin arrangements 7,8 have been observed. For example, in the case of orthorhombic DyMnO 3 (DMO), high temperature paramagnetic phase changed to incommensurate sinusoidal antiferromagnetic (AFM) order ($40 K) which in turn changed to commensurate magnetic structure (18 K) with the lowering of temperature. 7,9 Whereas another member of the series, HoMnO 3 with a smaller ionic radius at R site, displayed a commensurate magnetic structure that can be identied with "up-up-down-down" spin structure or E-type anti ferromagnetic structure. 10 The modication of magnetic structures is understood to be due to the decrease of bond angle Mn-O-Mn in the crystal structure as a consequence of GdFeO 3type distortions in addition to Jahn-Teller (JT) distortions. Yet in another member, YMnO 3 with orthorhombic structure, frustrated antiferromagnetic ground state is observed. 11 In particular, TbMnO 3 and DyMnO 3 have shown ferroelectric properties at low temperatures induced by spiral antiferromagnetic spin ordering of Mn ions from a high temperature sinusoidal antiferromagnetic incommensurate magnetic structure. 7 Ferroelectricity is understood to be due to unconventional magnetic transition exhibited by DyMnO 3 and TbMnO 3 compounds. On other hand, EuMnO 3 (EMO) with the same orthorhombic structure is reported to have antiferromagnetic ground state but without any magnetic induced ferroelectricity. 12 Thus RMnO 3 compounds show complex magnetic ground states and the magnetic interactions have been understood to be due to two types of interactions: super exchange interactions between nearest neighbor (NN) and next nearest neighbor (NNN) Mn ions. [13][14][15] In view of the different properties shown by RMnO 3

Experimental details
Bulk materials of the solid solution series of Dy 1Àx Eu x MnO 3 (x ¼ 0.0, 0.1, 0.3, 0.5, 0.8 and 1.0) were synthesized by the conventional ceramic method from the raw oxides of Dy 2 O 3 , Eu 2 O 3 and MnO 2 as per the stoichiometry of the nominal compositions. The mixture was ground thoroughly and heated around 1000 C for 12 hours with intermediate grinding. Subsequently, mixture was once again heated at 1300 C for 12 hours. The mixture was then compressed into circular pieces which were sintered at 1300 C for 24 hours to obtain dense pellets for resistivity measurements. X-ray diffraction (XRD) technique using STOE diffractometer (Germany) with Cu k a radiation (1.5406Å) was used to characterize the compounds as well as to determine their crystal structure. To determine the stoichiometry in the compounds, chemical analysis was carried out by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) technique. In order to avoid spectral interference from the rare earth ions in ICP-OES technique, Atomic Absorption Spectroscopy was used to analyze Mn. X-ray photoelectron spectroscopy technique was employed to determine the oxidation states of ions using SPECS Surface Nano Analysis GmbH (Germany) spectrometer equipped with a monochromatic Al k a X-ray source (hn ¼ 1486.7 eV) operating at 15 kV and 9.0 mA. Electron detection was done with a 9 channel train detector. A pressure of 1.2 Â 10 À9 mbar was maintained in the vacuum chamber during the measurement and the takeoff angle of the photoelectron was 57.4 . Before recording the XPS spectra, to remove adsorbed surface impurities, sample surfaces were cleaned by Ar ion sputtering with 1 keV energy. Raman spectra of the materials were recorded at room temperature with 514.5 nm Ar ion laser using InVia micro-Raman spectrometer (Renishaw, UK) in backscattering conguration. The effects of spin disorders induced by europium substitution on the transport properties are studied by measuring electrical resistivity as a function of temperature using four-probe method from room temperature (300 K) down to a low temperature depending on the resistance of the compounds. Magnetic properties were investigated from the magnetization measurements. The magnetization was measured both as a function of temperature and eld using MPMS 3 Ever cool SQUID-VSM (Quantum Design) magnetometer. The temperature dependent dc magnetization data (M-T) were collected in both Zero Field Cooled (ZFC) and Field Cooled (FC) modes at selected magnetic elds in the range of 50 Oe to 10 kOe in the temperature range of 2-400 K. Magnetization with respect to magnetic eld (M-H) was also recorded at different temperatures in the range between AE7 kOe.

Results and discussion
Phase formation of the solid solution has been examined by Xray powder diffraction technique. Fig. 1 shows XRD patterns of  Table 1. Dy 0.5 Eu 0.5 MnO 3 could not be dissolved by acid digestion method. As seen from the table, there is a good agreement between the chemical analysis results and calculated results by weight percentage as per the nominal composition.
Raman spectra for all the compounds are illustrated in Fig. 2 (main panel). It is seen from the Fig. 2 that broad Raman bands observed between 200 and 1000 cm À1 are all internal modes and those below 200 cm À1 are associated with the lattice modes corresponding to heavy rare earth ions. Room temperature as well as temperature variation of Raman scattering spectra of manganites were reported by several authors [17][18][19][20][21] where individual phonon modes have been correlated with structural distortions including Jahn-Teller ion (Mn 3+ ) distortions. Raman spectra were tted using Peakt to obtain Raman mode frequencies. The mode frequencies agree with the reported values for similar manganites. 22 Raman band frequencies of the JT/asymmetric stretching (ASS), tilting (T) and bending (B) modes decreased with increase in Eu concentration whereas that of symmetric stretching (SS) modes showed only a marginal decrease with increase in x. Inset of Fig. 2 shows the experimental ndings of variation of Raman mode frequencies with europium concentration. Fig. 3(a) shows temperature dependence of electrical resistivity, r(T) of Dy 1Àx Eu x MnO 3 from 150 K to 300 K. It is observed that the semiconducting behavior persists in the whole temperature range of measurement. In order to understand the charge transport mechanism for the observed semiconducting behavior, various models such as thermal activation process involving nearest neighbor hopping 23,24 (Arrhenius law), hopping of small polarons (SPH) in the adiabatic approximation [25][26][27] and variable range hopping 28 (VRH) conduction process were used to t the r(T) data. The equation for thermal activation process for nearest neighbor hopping conduction with activation energy (E a ) is given by Arrhenius equation: The equations for SPH and VRH conductions are: Fitting of the r(T) data to all the three models was carried out and the best ts showed that the nature of the electronic transport varied between the two end compounds and from the intermediate compounds as well. It was found that the conduction mechanism for the substituted compounds (x ¼ 0.1 to 0.8) is dominated by the activated type of conduction in the whole temperature range whereas the end compounds DyMnO 3 and EuMnO 3 followed SPH and Mott type VRH mechanisms respectively. Linear ts of ln(r/T) with 1/T, ln r with 1/T and ln r with 1/T 1/4 are shown in Fig. 3(b)-(d) respectively. The activation energies for the nearest neighbor hopping calculated from the t parameters are: 110.0, 168.8, 104.4 and 94.4 meV for x ¼ 0.1, 0.3, 0.5 and 0.8 respectively. Effect of europium substitution at Dy site on the activation energies for charge carriers can be presumably explained by changes in bond distances between Mn ions. Hopping amplitude of carriers between two Mn ions also decreases when the angle between the two Mn ions becomes smaller than 180 degrees. Thus the decrease in the activation energies can be interpreted in terms of the changes in the hopping distances. Moreover, carriers cannot nd continuous paths between ions due to bending and tilting of MnO 6 octahedra. In the composition regime 0.1 to 0.8, the hopping paths may be disturbed due to the probable disruption of the three dimensional network. This may be one of the reasons responsible for decreasing the conductivity in this composition range.
It may be noted that different transport mechanisms operate in the solid solutions Dy 1Àx Eu x MnO 3 . SPH was found in the pristine compound, DMO and then there is a crossover from SPH to activated behavior for the substituted compounds again  to VRH for EuMnO 3 . Various parameters such as average ionic size, grain morphology, charge and structural disorders can directly affect the transport properties. Furthermore, increase in the resistivity with decrease in the temperature observed for all the compounds in the entire temperature range indicates a robust insulating behavior.
X-ray photoelectron spectra of Dy 1Àx Eu x MnO 3 (x ¼ 0.1-1.0) were recorded in the energy range 0-1000 eV. Representative survey scans corresponding to x ¼ 0.0, 0.5 and 1.0 are depicted in Fig. 4(a). It is seen from the wide spectrum that all the elements are present in their respective binding energy scale. XPS spectra corresponding to manganese are shown in Fig. 4(b). Fig. 4(c) and (d) display a typical deconvoluted spectrum of Mn element for x ¼ 0.5 and normalized spectra for DEMO for Eu 3d respectively. The binding energies determined from the tting the spectra are tabulated in Table 2. Binding energies of Eu and Mn calculated from the tting are found to be in agreement with the respective ions in the trivalent states. 29  lock-in transition of Mn 3+ ions (T N2 -12 K) and antiferromagnetic ordering of Dy 3+ ions (T N -3.62 K) respectively. Molar susceptibility (c) of Dy 1Àx Eu x MnO 3 (x ¼ 0.1-1.0) as a function of temperature at different applied elds is displayed in Fig. 5. It is seen from the Fig. 5 that the c ZFC and c FC curves measured under various strengths of the magnetic eld overlap whose magnitude increased with decreasing the temperature in the entire temperature range for compounds with x ¼ 0.1, 0.3 and 0.5 respectively. However, there appear broad peaks in both the c ZFC and c FC curves at all the elds whose peak temperature decreases slightly with eld strength indicating the existence of a magnetic transition. Moreover, the temperature at which c ZFC and c FC curves bifurcate decreased when the external magnetic eld increased. Cwik 31 investigated magnetic and magnetoelectric properties of bulk compounds of (Dy 0.9 Er 0.1 ) 1Àx Gd x Co 2 (0.0 # x # 0.25). The irreversibility of the susceptibility between the ZFC and FC processes observed in our compounds is similar to that reported by Cwik. Further, the magnetic transition temperature is reduced with increasing the europium concentration, x up to x ¼ 0.5. In view of the broader peaks, transition temperatures were identied from the minimum observed in the rst derivative of the susceptibility curves, dc FC /dT versus temperature. Thus the estimated transition temperatures are $12 K, $10 K and $5 K for x ¼ 0.1, 0.3 and 0.5 respectively. Fig. 6 reports typical dc FC /dT -T plots for selected compounds.
In the case of x ¼ 0.8, (Fig. 5(d)) the c ZFC and c FC curves merge down to 50 K below which they deviate with magnitude of c FC greater than c ZFC . However, the susceptibility data measured at 1 tesla eld does not show any deviation between c ZFC and c FC . Irrespective of the applied eld strength, both c ZFC and c FC continue to increase with decrease in the temperature without any anomalies. In the absence of signatures for a typical antiferromagnet whose magnetization decreases as temperature decreases, the magnetic response may be ascribed to random arrangement of magnetic spins. These experimental ndings point to the paramagnetic nature of this compound in the entire temperature range. For EuMnO 3 , the c ZFC and c FC curves merge with each other down to $50 K, below which c ZFC displayed a peak while c FC increased. A large bifurcation in the curves was observed at low temperatures and low elds. dc FC /dT curve revealed two types of transitions as marked by arrows in the Fig. 6 around 50 K and 35 K. By comparing with literature, a small broad anomaly at <50 K may be ascribed to the onset of incommensurate antiferromagnetic ordering of Mn ions (T N1 ) and the low temperature anomaly around 35 K may be associated with the collinear arrangement of Mn 3+ ions in the antiferromagnetic state (T N2 ). Nevertheless, magnetization at low temperatures tends to increase/saturate with lowering temperature resembling that of a ferromagnet. This may imply the existence of ferromagnetic correlations inside the antiferromagnetic state. Hence the low temperature magnetic ground state may be A-type AFM. This is corroborated with the observation of large coercive elds from M-H curves whose results will be discussed in the ensuing sections. c(T) data of all the compounds follow Curie-Weiss behavior in the paramagnetic state. A representative CW t to eld cooled c(T) data of EMO collected at 10 kOe is shown in Fig. 5(e). The CW t parameters of the compounds obtained by tting the c FC data measured at various elds were used to determine the values of effective magnetic moment, m eff. . Variation of m eff. with external magnetic eld is displayed in Fig. 7.
The effective magnetic moment is calculated from the atomic moments of the magnetic ions as per the chemical formula of the compounds from the relation: (cal.) thus calculated are found to reduce with increase in the europium concentration, x in DEMO. It may be remarked here that m eff. (expt.) obtained from 1 tesla data is close to the calculated value (Table 3).
As discussed in the earlier sections, the origin for a series of magnetic transitions observed in DMO was attributed to antiferromagnetic coupling between Mn and Dy ions. Comparison of the experimental ndings of the substituted compounds with those of the end compounds, DMO and EMO, the following conclusions can be made: (a) The x ¼ 0.1, 0.3 and 0.5 compounds with a single anomaly in the c(T) data exhibit a magnetic transition from paramagnetic to antiferromagnetic structure.
(b) x ¼ 0.8 compound does not undergo any type of magnetic ordering.
(c). EMO indicates the onset of IC-AFM for T # 50 K and C-AFM around 35 K which agrees with the neutron diffraction measurements carried out by Ferreira et al. 32 revealing A-AFM.
Further, to clarify the observations of the temperature dependence of magnetization data and to identify the magnetic nature of these materials, isothermal magnetization measurements at different xed temperatures were performed. M-H curves for all the compounds at selected temperatures are presented in Fig. 8. A linear relationship between the magnetic moment and magnetic eld was observed at high temperatures (300 K and 100 K) for all the compounds. It is seen from the Fig. 8, that the isothermal curves measured at 2 K for the compounds with x ¼ 0.1 to 0.8 show S shape wherein the moment increases gradually in the low magnetic eld region without hysteresis followed by a slope change which again increases with increase in the eld strength. As temperature of the measurement increases, M-H curves show linear behavior. These S type curves have a very low remnant magnetization and do not exhibit coercivity. On the other hand, EMO has a large coercive eld. Hysteresis curves of DMO and EMO with appreciable coercive elds show no sign of saturation up to 7 T. However, the remnant magnetization is small, 0.174 m B f.u. À1 and 0.214 m B f.u. À1 for DMO and EMO respectively. These   features indicate the short range ferromagnetic correlations within the antiferromagnetic state (Fig. 9).

Conclusions
Polycrystalline Dy 1Àx Eu x MnO 3 (x ¼ 0.0-1.0) compounds were prepared by solid state reaction. All the compounds crystallized in the orthorhombic structure. Contributions from the atomic vibrations to the observation of Raman bands have been established and assigned to symmetry stretching and anti symmetry stretching, bending and tilting modes. Raman band frequencies of the tilting and bending modes in DEMO decreased with increase in europium content showing soening. The transport results revealed that all the compounds show semiconducting nature. All the substituted compounds showed activated type of conduction. EMO showed VRH type conduction. Hopping energy (E hop. ) is calculated and it is observed that the activated energy was found to reduce. In contrast to the parent compound, DMO, all DEMO compounds showed only one type of magnetic transition i.e.,

Conflicts of interest
There are no conicts to declare.