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Study on the Properties of Fe-substituted NdMn2O5

The oxide, NdFeMnO5, has been synthesized using a two-step synthesis process. First the precursor oxides are high-energy ball milled and the as-milled powders are then sintered to obtain the NdFeMnO5 phase. X-ray diffraction confirms phase formation and the calculated pattern is obtained by the substitution of Mn3+ sites by Fe3+ sites. Agglomeration of small sized grains is observed from the scanning electron microscopy images. Room temperature magnetization measurement shows a paramagnetic behaviour. A ferrimagnetic ordering is observed to be established below ~110 K from the thermo-magnetization measurements.


Introduction
Materials of the RMn 2 O 5 (R=Rare earths, Bi) family has Mn at two different crystallographic sites [1]. Mn 4+ ions occupy the octahedral pyramidal (4f ) sites where it is coordinated with six oxygen atoms and Mn 3+ ions at tetragonal pyramidal (4h) sites having coordination with five oxygen atoms [1]. They have complex magnetic structures which give rise to magnetodielectric effect around the magnetic transition temperatures [2]. Generally, RMn 2 O 5 systems have the orthorhombic structure with Pbam spacegroup and paramagnetic behaviour at room temperature. They have Neel transition temperature around 40 K below which antiferromagnetic ordering exists.
The magnetic ordering in these systems can be investigated by the substitution of Mn 3+ ions with Fe 3+ ions [3,4]. The magnetic properties of Fe-substituted RMn 2 O 5 in few cases are reported to be ferrimagnetic in nature at low temperatures [5][6][7]. The Fe substitution has its effect on the structural and magnetic properties due to its high spin only effective magnetic moment. We report the synthesis and properties of Fe-substituted NdMn 2 O 5 . It is considered that Fe 3+ occupies the Mn 3+ (4h) sites, and the corresponding change in properties observed is compared with the already reported properties of the parent system.

Experimental
Powders of neodymium oxide, manganese mono-oxide and iron oxide were taken in stoichiometric proportions and milled for a total of 5 h at 400 RPM with a ball to powder ratio of 28:2. The obtained reddish brown powders were then pressed into dense pellets of 12 mm diameter, sintered at 1073 K for 24 h and furnace cooled. Sintered samples were then ground in a mortar to obtain the black powders of NdFeMnO 5 . The X-Ray Diffraction (XRD) data was obtained with Cu-Kα radiation at room temperature. High resolution-scanning electron microscopy (HR-SEM) was done in FEI Quanta FEG 200 HR Microscope. Isothermal magnetization and Zero field cooled (ZFC)-Field Cooled (FC) measurements were carried out in the range 20 K-300 K in a  Lakeshore VSM 7410 instrument. A field of 1000 Oe was applied for the ZFC-FC experiment and for iso-thermal magnetization curves a maximum field of 20 KOe was applied.
3. Results and discussion X-ray diffraction (XRD) pattern is given in fig 1, indicating the formation of stoichiometric NdFeMnO 5 which is iso-structural with NdMn 2 O 5 along with the few secondary phases. The calculated pattern, matching with the experimental pattern, is obtained by using the FULLPROF [8] program by the substitution of Fe 3+ in Mn 3+ (4f ) sites. From the structural point of view, NdFeMnO 5 has the same orthorhombic (Pbam) structure as that of its parent NdMn 2 O 5 . Parent system has the Mn 4+ octahedra network connected through oxygen atoms, extending along c−axis [9], and has Mn 3+ and Nd 3+ layers between these Mn 4+ layers. In the case of NdFeMnO 5 , these Mn 3+ layers are completely replaced by Fe HR-SEM image of NdFeMnO 5 Figure 4. Room temperature magnetization curve measurements confirm the near stoichiometric presence of all elements in NdFeMnO 5 . Room temperature magnetization study, given in fig 4, indicates a paramagnetic behaviour and a weak ferromagnetic property is also observed at initial low fields. The low temperature Zero field cooled-field cooled (ZFC-FC) curves are shown in fig 5, which exhibits a split below ∼110 K and an increase in magnetization values till 20 K. This behaviour is indicative of the ferrimagnetic property below ∼110 K and is supported by the linear dependence of magnetization on the applied magnetic field obtained at 20 K (inset of fig 5). The parent NdMn 2 O 5 , has a Neel transition (T N ) temperature of ∼75K, below which it is reported to have

Conclusion
The compound, NdFeMnO 5 , has been synthesized by initial high-energy ball milling of oxides and then sintering of milled powders. Fe 3+ substitution in the NdMn 2 O 5 at Mn 3+ affects the magnetic ordering at low temperatures due to which a ferrimagnetic ordering occurs below ∼110 K. This is the result of change in interaction in Mn 4+ -O-Mn 4+ chains due to Fe substitution.

Acknowledgments
Authors acknowledge SAIF, IIT Madras for low temperature VSM measurements. KSK thanks DRDO, India for support through fellowship.