Lithium Ferrite Nanocrystals Embedded in a non-Magnetic Glass Matrix

Nanocrystalline LiFe,O, particles embedded in an amorphous matrix were obtained by heat treatments of the nonmagnetic oxide glass 32Li20-8Fe0,-60B203, between 200 and 770'~. The X-ray spectra reveal the formation of the spinel phase of Li-ferrite in the initially amorphous materials only for annealing temperatures higher than 440°C and the hybrid structure becomes more evident with the increase of temperature. The occurrence of the magnetic spinel phase (LiFe,O,) was proved by magnetizat~on and Curie temperature measurements too. The evolution of the magnetic properties of these glass-ceramic compounds in terms of annealing temperature was investigated. The magnetic properties (o,, H,) are cot~elated with the average size of the LiFeSOs nanoparticles dispersed in the glass matrix. The X-ray diffraction data indicated an evident increase in the average crystal block size from 3.5 to 50 nm with the increase of annealing temperature from 440' to 770'C. In an earlier paper [ I ] we shown that by thermal annealing of the metastable oxide glasses 32 Li20-8 Fe203 -60B20, occurs the crystallization of Li-ferrite (LiFes08) and the resulting material is a glass-ceramic named ferriglass. Crystals of ferrite, randomly dispersed in the amorphous matrix, of nanometer sizes (3.5 nm to 50 nm) depending on the annealing temperature, were obtained. The irreversible changes in the structure of the amorphous matrix was investigated by X-ray analysis and magnetic measurements. In this work we extend previous report dealing with the relationship among thermal annealings, crystal size and magnetic properties of the glass-ceramic containing lithium ferrite nanoparticles. The amorphous ribbons were fabricated by rapid quenching of the liquid state between two cooper rollers. The resulting ribbons were 3-5 mm wide and 10-20 pm thick. The details of the sample procedure are described in [I]. The amorphous structure of the ribbons was confirmed by X-ray diffraction. The specimens were successively annealed at various temperatures between 200-770'~. The mean grain size of the Li-ferrite crystallites was determined using Schemer's formula. The magnetic measurements were carried out with a vibrating sample magnetometer. The occurrence of LiFe,O, magnetic phase in the amorphous nonmagnetic compound Li20-Fe203-B2O3 was revealed by magnetization measurement during the rapid heating of the amorphous ribbons between 20-700'~ . Fig 1 displays the variation of the specific magnetization o, measured at 8.8 kOe versus temperature and shows several interesting facts: i)up to about 5 0 0 ' ~ there is no appreciable magnetization; ii)the descendent curve between 500-600'~ shows the same shape with a weak ferrimagnetic material; iii)the Curie point (660+10'~) is very close to that measured on the LiFe50S single crystals grown In our laboratory by the flux method [2]. This later concordance attests the crystallization of the Li ferrite. To investigate the thermal behaviour of the amorphous compound Li20-Fe203-B203, the same specimen was subjected to successive thermal annealings, between 200 and 7 7 0 ' ~ for a long time (2 h for each annealing). The X-ray analysis revealed the start of the spinel phase nanocrystallization after annealing at about 440 'c. The occurrence of the magnetic phase was proved by magnetization and Curie temperature measurements. No evidence of magnetic ordering has been detected up to 440'~. The obtained results for annealed samples are compiled in Table 1. With increasing To, o, increases up to a value of -12 emu g-l. An abruptly increase in (J, can be noticed for samples annealed in the temperature range 440-530°C, due to the Liferrite crystallization. The saturation like behaviour of (J, for high temperatures is the result of the crystallization of the whole amount of Li ferrite within the glass host. Of course, due to the dispersion of the ferrite particles in a non-magnetic residual phase, a, value of this ferriglass material is much lower than that reported for bulk lithium ferrite (62.5 emug-').Thus the 6 values may give information on the percentage of LiFeSOs crystallized in the amorphous network. We estimated it after each thermal annealing.(Table 1). One can observe an increase of the percentage from 1 to 19% when To increases from 440 to 770 '~ . At high temperatures a slow increase of this percentage was evaluated, that can be explaked by finishing of the crystallization process of the Li ferrite. From Table 1 one remarks an increase of the mean grain size D from 3.5 nm to 50 nm with increasing of To. A pronounced increase of the crystallites was noticed for annealing above 5 8 0 ' ~ . Two simultaneous process seem to take place at higher temperatures: a normal grain increase due to the progressive crystallization and, on the other hand, a formation of agglomerated clusters, in which the larger grains grow at the expense of the smaller by a "penetrating" process . A special attention is given to the relationship between the grain size of LiFe50s nanoparticles and the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19971228 JOURNAL DE PHYSIQUE IV magnetic properties (a, HJ of the vitroceramic material. Fig 2 reveals a, and H, as a function of mean grain size 5. The sharp increase of a, for small grain sizes (at low annealing temperatures) may be explained by the multiplication of the nucleation centres that leads to a rapid increase of the crystallization percentage of the Li ferrite.. At higher temperatures (above 580'~) the increase of the grains by penetrating process dominates and determines a slight increase of the magneJizatio>. The relationship between a, and D is found to be: a,D3, for D < D , and a, D 0*08 for 5 > fi ;. From the fig 2 one observes that H, increases with the increasing grains, and then, for a further increase of the grains, it decreases. This decrease is attributed to the transition from single to multidomain magnetization mechanism (it is easier to displace a domain wall than to rotate the individual atomic spins). In this connection we estimate the critical size for a single domain particle with formula[3]: ~,,,=(9q,/2tr~;), where q,,=(2kbT. /Ki //a)'" is the wall Table 1 : Material parameters vs. annealing density energy, /K1 /is the anisotropy constant, T, the Curie point, M, the temperature saturation magnetization and a the lattice constant. Using /Kl /=8.104 erg/cm3 [4], M.=3 10G, a=8.37.10-~cm, Tc=933K, it was obtained D ,,,=75 nm. For the mean sizes 5 >75nm, the particle is already multidomain. Below this size, the particle is a uniformly magnetized single-domain. However, there is a lower limit, D, , i.e. a particle size below which a super-paramagnetic state appears, estimated as 5,=6nm according to [5] (fig. 2). For 5 < fii no measurable coercivity is found. Therefore, the main size of the single domain crystallites should be in the range of 30-40 nm, in good agreement with our results.