Y.Q. Ma

Separated CoFe2O4/CoFe nanoparticles by the SiOx matrix: revealing the intrinsic origin for the small remanence magnetization

In order to clarify the intrinsic reason for the smaller remanence (Mr)-to-saturation (Ms) magnetization ratio Mr/Ms than that expected by the Stoner–Wohlfarth model in CoFe2O4/CoFe2 nanoparticles in the previous report, we first prepared well-dispersed CoFe2O4 nanoparticles, and then they were diluted in the SiO2 matrix followed by reduction in H2 as far as possible to exclude or reduce disadvantageous variables (such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing) affecting magnetic properties. Such an idea has not been taken into account before to our knowledge. The analyses on the magnetic results indicate that the CoFe2O4/CoFe2 nanoparticles herein reported are a pure dipolar system, in which the coercivity (Hc) and Mr/Ms ratio are very sensitive to the anisotropy and the strength of dipolar interaction. These results signify that it is important to maintain the CoFe2O4/CoFe2 nanoparticles with higher anisotropy and weaker dipolar interaction for improving Mr/Ms and Hc. This suggestion was further confirmed by our another result wherein an Mr/Ms value of 0.64 was obtained even though no exchange coupling was observed in the CoFe2O4/CoFe2 nanoparticles, and further work is in process.Graphical abstractNumerous efforts have devoted to improve the values of Ms and Mr/Ms by compositing hard CoFe2O4 (CFO) ferrite with soft CoFe2 (CF) alloy, which unfortunately give the low Mr/Ms value (<0.5) even in presence of the exchange coupling. Key issues involve the preparation of CFO/CF composite. Previously the preparation of CFO/CF undergoes the synthesis of CFO and the subsequent reducing in the H2 ambient, as shown in Figure (a), while in this work well dispersed CFO nanoparticles were first prepared , and then diluted in the SiO2 matrix followed by reducing in H2 to exclude or reduce disadvantageous variables, such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing, as shown in Figure (b). Our results suggest that higher anisotropy and weaker dipolar interaction favor the larger Mr/Ms value, as shown in Figure (c).

The magnetization reversal in CoFe2O4/CoFe2 granular systems

AbstractThe temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., MFC and MZFC, measured under different magnetic fields from 500xa0Oe to 20xa0kOe have been investigated on two exchange–spring CoFe2O4/CoFe2 composites with different relative content of CoFe2. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature Tirr. For the sample with less CoFe2, the curves of −d(MFCxa0−xa0MZFC)/dT versus temperature T exhibit a broad peak at an intermediate temperature T2 below Tirr, and the moments are suggested not to fully freeze till the lowest measuring temperature 10xa0K. However, for the −d(MFCxa0−xa0MZFC)/dT curves of the sample with more CoFe2, besides a broad peat at an intermediate temperature T2, a rapid rise around the low temperature T1~15xa0K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2xa0kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.n Graphical abstractCoFe2O4/CoFe2 composites with different relative content of CoFe2 were prepared by reducing CoFe2O4 in H2 for 4xa0h (S4H) and 8xa0h (S8H). The temperature-dependent FC and ZFC magnetizations, i.e., MFC and MZFC, under different magnetic fields from 500xa0Oe to 20xa0kOe have been investigated. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at field-dependent irreversible temperature Tirr. For the S4H sample, the curves of −d(MFCxa0−xa0MZFC)/dT versus temperature T exhibit a broad and field-dependent relaxing peak at T2 below Tirr (figure a), and the moments were suggested not to fully freeze till the lowest measuring temperature 10xa0K. However, for the S8H sample, it exhibits the reentrant spin-glass state around 50xa0K, as evidenced by a peak in the MFC curve (inset in figure b) and as a result of the cooperative effects of the random anisotropy of CoFe2O4, exchange–spring occurring at the interface of CoFe2O4 and CoFe2 together with the inter-particle dipolar interaction (figure c); in −d(MFCxa0−xa0MZFC)/dT curves, besides a broad relaxing peat at T2, a rapid rise around the low-temperature T1~15xa0K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2xa0kOe leads to the shift of T2 and Tirr towards a lower temperature, and the shift of T2 is attributable to the moment reversal of CoFe2O4.

Improvement of magnetic properties of Cr3+ substituted SrFe12O19

Abstract M type strontium hexaferrites SrFe12u2009−u2009xCrxO19 (xu2009=u20090.0–0.6) were prepared by the ceramic process. The structure, morphology and magnetic properties of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and B–H hysteresis curve measurements. The XRD patterns show the formation of the M-type hexaferrite phase in the pre-sintered samples with non-magnetic phase, α-Fe2O3. The SEM results indicate that the Cr substitution does not obviously affect the structure of strontium ferrite. The results of magnetic mensuration, especially for the SrFe11.8Cr0.2O19 samples, reveal that rather high magnetic properties can be obtained in the sintering temperature range of 1135–1165°C.