Magnetic Transitions in Chemically Synthesized Nanoparticles of CoCr2_{2} O4_{4}

Bulk CoCr2O4 undergoes a transition from paramagnetic to long-range ferrimagnetic phase at Ts (94 K) to a long-range and/or short-range spiral order at Ts (~24 K), and finally shows a lock-in-transition below 15 K. The spiral component induces an electric polarization and also a spontaneous magnetization for which it is said to be multiferroic. Reducing the size of a CoCr2O4 multiferroic material to ~50 nm by a coprecipitation method, we obtain a pure cubic phase with space group, Fd3m and lattice parameter (8.334 ± 0.003°A). A rich sequence of magnetic transitions are examined by measuring temperature and field-dependent magnetization and diffused neutron scattering (DNS) using polarized neutron at different temperatures. While paramagnetic to ferrimagnetic transition is enhanced from 97 K in bulk to 99 K at 0.5 kOe field, followed by a decrease in lock-in-transition (TL) from 15 K in bulk to 8 K, spiral ordering temperature does not show a significant change. A strong disagreement between paramagnetic moment obtained from the fitting of χ-1 = (T/C) + (1/χo) - (b/T - θ) and ferrimagnetic moment obtained from the M versus H loop taken at 2 K, nonsaturated magnetization at 50-100 kOe field, two order of magnitude higher coercivity (Hc), and splitting of ac susceptibly confirm the core-shell structure of the particles. Furthermore, a magnetic scattering analysis clearly shows that while the paramagnetic to ferrimagnetic transition is continuous, the spiral ordering is sharp, short range, and commensurate in contrast to incommensurate spiral order observed single crystal of CoCr2O4

Size-dependent magnetic transitions in CoFe0.1_0.1Cr1.9_1.9O4_4 nanoparticles studied by magnetic and neutron-polarization analysis

Multiferroic, CoCr2O4 bulk material undergoes successive magnetic transitions such as a paramagnetic to collinear and non-collinear ferrimagnetic state at the Curie temperature (T C) and spiral ordering temperature (T S) respectively and finally to a lock-in-transition temperature (T l). In this paper, the rich sequence of magnetic transitions in CoCr2O4 after mixing the octahedral site with 10% of iron are investigated by varying the size of the particle from 10 to 50 nm. With the increasing size, while the T C increases from 110 to 119 K which is higher than the T C (95 K) of pure CoCr2O4, the T S remains unaffected. In addition, a compensation of magnetization at 34 K and a lock-in transition at 10 K are also monitored in 50 nm particles. Further, we have examined the magnetic-ordering temperatures through neutron scattering using a polarized neutron beam along three orthogonal directions after separating the magnetic scattering from nuclear-coherent and spin-incoherent contributions. While a sharp long-range ferrimagnetic ordering down to 110 K and a short-range spiral ordering down to 50 K are obtained in 50 nm particles, in 10 nm particles, the para to ferrimagnetic transition is found to be continuous and spiral ordering is diffused in nature. Frequency-dependent ac susceptibility (χ) data fitted with different phenomenological models such as the Neel–Arrhenius, Vogel–Fulcher and power law, while ruling out the canonical spin-glass, cluster-glass and interacting superparamagnetism, reveal that both particles show spin-glass behavior with a higher relaxation time in 10 nm particles than in 50 nm. The smaller spin flip time in 50 nm particles confirms that spin dynamics does not slow down on approaching the glass transition temperature (T g)

Effect of size reduction on cation distribution and magnetic transitions in CoCr2_{2} O4_{4} multiferroic: EXAFS, magnetic and diffused neutron scattering measurements

A rich sequence of magnetic transitions such as para to long-range ferrimagnetic transition at Curie temperature, TC, to a short range non-collinear spiral ordering at spiral ordering temperature, TS, and finally to a lock in transition, TL, are examined in ∼10 and ∼50 nm samples of CoCr2O4 multiferroic through dc, ac magnetic measurements and diffused neutron scattering using polarized neutrons. Analysis of extended X-ray absorption fine structure (EXAFS) spectra and Fourier transforms of Co and Cr edges in real (r) and momentum (k) space show no change in cation distribution among A and B sites even after reducing the size to the nanometer range due to the high crystal field stabilisation energy of Cr3+ towards the B site. While TS remains independent, TC increases from 97.2 to 100.1 K and TL increases from 5 to 8.5 K with varying size from 10 to 50 nm. Temperature dependent ac susceptibility (χ) measurements demonstrate that χ′ and χ′′ do not show any dispersion behaviour in the 10 nm sample. However, we observe splitting of χ′′ into two peaks and one of them shows dispersion behaviour in the 50 nm sample, indicating a core–shell structure. Magnetization vs. magnetic field measurement show hysteresis behaviour with unsaturated magnetization at high magnetic field indicating a ferrimagnetic core surrounded by disordered surface spins. Fitting of χ′′ with an empirical relation Φ = ΔTB/TBΔlog10 ω, and memory effect measurement, further confirm the spin-glass behaviour of the shell in the 50 nm sample. Magnetic ordering temperatures are examined through neutron scattering using a polarized neutron beam, and reveal that while the para- to ferrimagnetic transition, TC, is continuous and long-range in both nanocrystalline particles, TS is found to be sharp, short-range, and commensurate in 50 nm sampl