Nanocrystals of CuCr₂S_4−xSe_x chalcospinels with tunable magnetic properties

Abstract

Ferromagnetic materials exhibiting large spin polarization at room temperature have been actively pursued in recent years for the development of next-generation spintronic devices. Chromium-based chalcospinels are the only ternary chalcogenide-containing magnetic materials with Curie temperatures above room temperature. However, the magnetic and electronic properties of chromium-based chalcospinels at the nanoscale level are not well understood. We have developed a facile colloidal method for the synthesis of CuCr 2 S 4Àx Se x (0 r x r 4) nanocrystals over the entire composition range.Systematic changes in the lattice parameter and elemental composition confirm formation of CuCr 2 S 4Àx Se x (0 r x r 4) nanocrystals. The dimensions of the nanocrystals, as determined from TEM images, vary from 12 AE 1.4 nm to 21 AE 1.4 nm. The Curie temperature (T C ) shows a systematic increase with increasing selenium content. Saturation magnetization and coercivity values of CuCr 2 S 4Àx Se x (0 r x r 4) nanocrystals at 5 K are found to steadily increase up to x = 3. Electronic structure calculations as a function of composition and size using density functional theory suggest ferromagnetic ordering over the entire composition range with partial spin polarization for the bulk materials. Furthermore, the calculations predict complete opening-up of a gap at the Fermi level in the minority spin channel at reduced dimensions to render them completely spin polarized, i.e., display half-metallic characteristics.

Magnetic resonance studies of mixed chalcospinel CuCr2SxSe4−x (x = 0; 2) and CoxCu1−xCr2S4 (x = 0.1; 0.2) nanocrystals with strong interparticle interactions

“…The parameters of fitting providing the best description of the blocking temperatures are summarized in Table 1. CuCr 2 Se 4 6.410 5 12.9 8.110 5 11.6 26.3 1.610 -9 The particle sizes determined from these fits are close to the average sizes determined by TEM [25]. It is interesting, that the correlation length, defining the effective volume of the clus- [29] and magnetite [31].…”

“…On the other hand, considering the real temperature dependence of the anisotropy constant [27], the estimation of the blocking temperature for the non-interacting CuCr 2 Se 4 particles of the size <d> = 12 nm gives T B = 120 K, which is lower than the value of T B = 385 K obtained in [25]. Apparently, the same situation with the description of the blocking temperatures is implemented in other studied samples in which the sizes of the nanoparticles are similar to that of CuCr 2 Se 4 and the blocking temperatures are also high and close to the corresponding Curie points.…”

“…Using Eq. (2) with K eff = 0.910 5 erg/cm 3 at T=295 K determined for the bulk compound CuCr 2 Se 4 [27] and T B =385 K determined for the nanoparticles [25], the particle size is estimated as d 26 nm which is more than twice larger than the experimental value <d> = 12 nm obtained by TEM [25].…”

“…The shift is caused by the appearance of the average magnetic anisotropy fields (1) whose magnitudes measured at T = 4.2 K for all the studied nanoparticles are shown in Table 2. [24,25] are also characterized by strong interparticle interactions. These interactions are responsible for the high values of the blocking temperatures in the nanoparticles, found both in our resonance studies and in the magnetic measurements [24,25].…”

“…x Se x (x = 0 and 2) chalcospinel nanocrystals were carried out following previously reported procedures [24,25]. Briefly, Co x Cu 1−x Cr 2 S 4 nanocrystals were synthesized by reacting chloride salts of copper, cobalt, and chromium with 1-dodecanethiol in octadecylamine at 360°C.…”