ИСТИНА

Perovskite manganites RxA1-xMnO3 (R = Rare earth, A = alkaline earth: 0<x<1) are well-known for their correlated charge, spin and lattice degrees of freedom [1]. While studies on hole-doped manganites are almost complete, the physical properties of electron-doped manganites are still being understood. Recent studies on R0.15Ca0.85MnO3 (R = Y, Gd and Dy) revealed interesting metamagnetic transition in the antiferromagnetically ordered state across which the samples show insulator to metal-like change in the electrical resistivity and also switches from an inverse magnetocaloric effect (MCE) to normal MCE [2]. In this work, magnetic and magnetocaloric properties of electron-doped Pr0.15Ca0.85MnO3 are studied in order to understand the effect of light rare earth substitution at the Ca-site of antiferromagnetic CaMnO3. Polycrystalline Pr0.15Ca0.85MnO3 has been prepared by solid state reaction. The X-ray diffraction (XRD) data were collected at several temperatures (T) from 300 K down to 25 K (Cu Ka, Rigaku). It is found that Pr0.15Ca0.85MnO3 crystallizes in orthorhombic structure (space group Pnma, no. 62, oP20) at 300 K and orders antiferromagnetically at 147 K (TN). Paramagnetic susceptibility follows Curie-Weiss law. The effective paramagnetic moment (Meff) and paramagnetic Curie temperature (Op) values are 4.47 mB/f.u and +118 K respectively. The positive Op value indicates the presence of coexisting ferromagnetic interactions in the system. The field dependent magnetization measured at 5 K in fields up to 70 kOe is almost linear suggesting the presence of dominant antiferromagnetic interactions. The magnetization value at 5 K in 70 kOe field is about 0.11 mB/f.u. only. From low temperature XRD, it is observed that the crystal structure changes to monoclinic (space group P21/m, no.11-2, mP20) across the magnetic transition. No metamagnetic transition is evidenced in Pr0.15Ca0.85MnO3 at 5 K in fields up to 70 kOe suggesting robust antiferromagnetic state. Magnetocaloric effect (MCE) has been estimated in terms of isothermal magnetic entropy change (ΔSm) using magnetization vs field (M-H) isotherms measured around TN. Positive MCE (i.e. inverse MCE) is observed below TN [Fig. 1a]. The maximum ΔSm value is about 6.4 Jkg-1K-1 at 137.5 K for 70 kOe field change. The peak in inverse magnetocaloric effect coincides with the magnetostructural transition temperature. The maximum value of isothermal magnetic entropy change, ΔSmmax, at 142.5 K is plotted as a function of magnetic field change (ΔH) and is fitted to a power law, i.e. ΔSmmax  (ΔH)n [Fig. 1b] and the ‘n’ value is found to be 1.75. It is recalled that a value of n = 2 is expected for antiferromagnets [3] and the decrease in the ‘n’ value could be indicative of the coexistence of ferromagnetic interactions in the system. Though Pr0.15Ca0.85MnO3 shows a strong electrical resistivity value in the antiferromagnetically ordered state, the electrical resistivity value decreased by several order below TN. Non adiabatic polaron hopping conduction is observed above TN and polaron activation energy is found to be ~40 meV. Electron spin resonance studies were performed from 300 K to down to 7 K reveal Dysonian broadening as a result of the intrinsic electrical conductivity of the material. The g-value observed at 300 K is less than the free electron g-value and it increases with decreasing temperature down to TN. Thermopower measurements showed strong temperature dependence near TN and a large negative thermopower of ~40 mcV/K was obtained around 100 K. *Corresponding author. Email: nirmala@physics.iitm.ac.in References 1. Y. Tokura, Rep. Prog. Phys. 69 (2006) 797 2. Lakshman Dhal, Eesha Andharia, N. Shukla, T. Geetha Kumary, A. K. Nigam, S. K. Malik, P. N. Santhosh and R. Nirmala, J. Magn. Magn. Mater. 474 (2019) 215 3. A. Biswas, S. Chandra, T. Samanta, M. H. Phan, I. Das and H. Srikanth, J. Appl. Phys. 113 (2013) 17A902