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Broken inversion symmetry is observed in compounds with a spiral ordering, leading to ferroelectricity has attracted recent attention [1]. CoCr$_2$O$_4$ is a compound with a complex conical-spiral spin ordering of ferrimagnetic nature that has a spontaneous magnetization [2]. This observed spiral ordering has induced ferroelectric polarization [3]. The crystal structure of CoCr$_2$O$_4$ is cubic spinel, where tetrahedral A sites are occupied by Co$^{2+}$ and the octahedral B sites by Cr$^{3+}$ [2, 3]. Isotropic antiferromagnetic A- B and B- B exchange interactions (J$_{AB}$ and J$_{BB}$) among the nearest neighbours with J$_{BB}$/J$_{AB}$ > 2/3, give the solution for the ferrimagnetic spiral ground state having the spins located on the conical surfaces [4, 5]. The basic ordering of spins in the compound is AFM with unequal magnitudes that lead to a net FM order in the case of ferrimagnetic materials [6]. The present work investigates the role of Ce$^{3+}$ substitution at the Cr$^{3+}$ site on spiral ordering and other magnetic transitions in Co(Cr$_{0.95}$Ce$_{0.05}$)$_2$O$_4$ nanoparticles. X-ray diffraction (XRD) studies of the sample calcined at 600 °C revealed phase purity and broadened diffraction peaks, which are signatures of the size effect. The crystallite size (D) estimated from the XRD was 6.3 ± 0.6 nm. The average particle size calculated from the transmission electron microscopy (TEM) data was found to be D$_{TEM}$ = 8.4 ± 0.5 nm, corroborating the XRD results. Electron diffraction patterns confirm the crystalline nature of the nanoparticles having a bi-pyramidal shape. Magnetization as a function applied field shows an increase in coercivity as the temperature was decreased below the Curie temperature, T$_C$. Magnetization measured as a function of temperature indicated the ferrimagnetic behaviour, with T$_C$ = 92.5 ± 0.5 K (using the “knee-point method”). However, the lock-in temperature observed for the Co(Cr$_{0.95}$Ce$_{0.05}$)$_2$O$_4$ nanoparticles, T$_L$ = 15 ± 2 K, is in agreement with that previously reported for pure CoCr$_2$O$_4$. Interestingly the spiral ordering was smeared by substituting Ce$^{3+}$ at the Cr$^{3+}$ site. The present work describes the impact of rare-earth Ce$^{3+}$ ion substitution at the B site that can alter the exchange interaction in such a way that causes suppression of the spin spiral modulation.
References:
[1] D.I. Khomskii, J. Magn. Magn. Mat. 306 (2006) 1.
[2] Y.J. Choi, J. Okamoto, D.J. Huang, K.S. Chao, H.J. Lin, C.T. Chen, M. van Veenendaal, T.A. Kaplan, S-W. Cheong, Phys. Rev. Lett. 102 (2009) 067601.
[3] Y. Yamasaki, S. Miyasaka, Y. Kaneko, J.-P. He,T. Arima, Y. Tokura, Phys. Rev. Lett. 96 (2006) 207204.
[4] D.H. Lyons T. A. Kaplan, K. Dwight, N. Menyuk., Phys. Rev. 126 (1962) 540.
[5] A review: N. Menyuk, in Modern Aspects of Solid State Chemistry, edited by C.N.R. Rao (Plenum, New York,1970), p. 1.
[6] K. Majumdar, S.D. Mahanti, J. Phys.: Condens. Matter 33 (2021) 125801.
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