Origin of the constricted hysteresis loop in cobalt ferrites revisited

Correlating Size and Composition-Dependent Effects with Magnetic, Mössbauer, and Pair Distribution Function Measurements in a Family of Catalytically Active Ferrite Nanoparticles

“…Further evidence for a more magnetically anisotropic system is displayed in the higher remanence ratios ( M R / M S ) of the cobalt ferrite nanoparticles (Table ). Alternatively, this can also be ascribed to competition between different spin ordering structures, such as spin canting effects and/or a random distribution of cations between the (A) and [B] subsites, as aging experiments on CoFe 2 O 4 have shown …”

“…As the coexistence of doublets and sextets in the observed Mössbauer spectra extends over a large temperature range, the blocking temperature is determined where the area of the doublets and sextets are equal; this designates the average blocking temperature for the ensemble of the nanoparticles in the sample. Information about the average blocking temperature and average nanoparticle size has been used in eq to obtain K = 2.5 × 10 4 J/m 3 and τ 0 = 0.5 × 10 –9 s. Bulk CoFe 2 O 4 , as with all spinels, possesses cubic magnetic anisotropy, − with its first coefficient of magnetocrystalline anisotropy, K 1 , in the range of (1.8–3.0) × 10 5 J/m 3 , while τ 0 is of the order of a time scale of 10 –9 s, usually assumed for superparamagnetic nanoparticles. We note that the value of K obtained is an order of magnitude smaller than that of the magnetocrystalline anisotropy of the bulk, in contradiction to prior reports of increased anisotropy of up to 2 orders of magnitude for nanoparticles, due to shape, surface, and strain anisotropy contributions. , Moreover, the K value is consistent with use in the PDF studies presented in Section .…”

“…Alternatively, this can also be ascribed to competition between different spin ordering structures, 62 such as spin canting effects 59 and/or a random distribution of cations between the (A) and [B] subsites, as aging experiments on CoFe 2 O 4 have shown. 63 Nanostructures are known to trap non-equilibrium states; a lack of cation order within the lattice will fail to establish a clearly preferred anisotropy axis. In this case, under a weak applied field, intrinsic random neighbor spin−spin interactions could trigger the production of a spin disordered state with a sharply reduced magnetization and a constricted hysteresis loop.…”

“…As the coexistence of doublets and sextets in the observed Mössbauer spectra extends over a large temperature range, the blocking temperature is determined where the area of the doublets and sextets are equal; this designates the average blocking temperature for the ensemble of the nanoparticles in the sample. Information about the average blocking temperature and average nanoparticle size has been used in Equation (1) to obtain K = 2.5•10 4 J/m 3 and 0  = 0.5•10 -9 s. Bulk CoFe 2 O 4 , aswith all spinels, possesses cubic magnetic anisotropy,[61][62][63] with its first coefficient of magnetocrystalline anisotropy, K 1 , in the range of (1.8-3.0)•10 5 J/m 3 , while 0…”

Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3Me)2MnII5(CN)12 and (NEt2Me2)2MnII5(CN)12: A Cation‐Adaptive Structure

“…Preliminary data also reveals another discontinuity at 65 kOe suggestive of a spin‐flop transition at higher field. The M ZFC ( T ) follows a similar curvature to the other corresponding quadrants of the hysteresis curve, and this unusual curvature is due to a constriction of the hysteresis loop, rather than a spin‐flop like transition [20, 21, 23, 24] …”