Nanostructured ZnFe 2 O 4 ferrites with different grain sizes were prepared by high energy ball milling for various milling times. Both the average grain size and the root mean square strain were estimated from the x-ray diffraction line broadening. The lattice parameter initially decreases slightly with milling and it increases with further milling. The magnetization is found to increase as the grain size decreases and its large value is attributed to the cation inversion associated with grain size reduction. The 57 Fe Mössbauer spectra were recorded at 300 K and 77 K for the samples with grain sizes of 22 and 11 nm. There is no evidence for the presence of the Fe 2+ charge state. At 77 K the Mössbauer spectra consist of a magnetically ordered component along with a doublet due to the superparamagnetic behaviour of small crystalline grains with the superparamagnetic component decreasing with grain size reduction. At 4.2 K the sample with 11 nm grain size displays a magnetically blocked state as revealed by the Mössbauer spectrum. The Mössbauer spectrum of this sample recorded at 10 K in an external magnetic field of 6 T applied parallel to the direction of gamma rays clearly shows ferrimagnetic ordering of the sample. Also, the sample exhibits spin canting with a large canting angle, maybe due to a spin-glass-like surface layer or grain boundary anisotropies in the material.
Nanostructured fluoride powders were prepared by the high-energy ball milling route. A combination of suitable techniques with complementary spatial scales was used for the first time to investigate both structural and microstructural properties: x-ray diffractometry, magnetic measurements and local probes such as 57Fe Mössbauer spectrometry and 19F, 69Ga and 71Ga NMR. The set of data allows us to describe these nanostructured powders on the basis of nanocrystalline grains and grain boundaries. The relevant hyperfine data support then modelling of the microstructure in terms of pseudo-cubic and random packing of corner sharing octahedral units corresponding to nanocrystalline grains (~15 nm diameter) and disordered grain boundaries (a few nanometres thick), respectively. Their different cationic topologies are consistent with the antiferromagnetic and speromagnetic behaviours, respectively, as evidenced by in-field Mössbauer spectrometry. They also support the temperature dependence of the magnetic properties which reveal a progressive magnetic decoupling of grains when the temperature increases, originating a superparamagnetic behaviour above a blocking temperature which is dependent on the thickness of the grain boundaries, i.e. the milling conditions.
Nanostructured iron fluoride powders were prepared using the grinding route for different times and different intensities. Their structural, microstructural and magnetic properties are investigated by means of both transmission Mössbauer spectrometry as a function of temperature and in-field 57 Fe Mössbauer spectrometry. We report a fitting procedure which successfully describes the zero-field Mössbauer spectra recorded at different temperatures. It allows us to describe the powders as crystalline grains and grain boundaries which behave as antiferromagnets and speromagnets, respectively. Such arrangements are confirmed by in-field Mössbauer spectrometry. According to x-ray diffraction data, the size of grains and the thickness of grain boundaries are found to be strongly dependent on the grinding conditions. The occurrence of superparamagnetic effects at high temperature gives clear evidence for the role of grain boundaries in the magnetic coupling of crystalline grains.
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