Cation distribution and magnetic properties of nickel–chromium ferrites NiCr_xFe_2–xO₄ (0 ≤ x ≤ 1.4)

Abstract

Nickel -chromium ferrites NiCr x Fe 2-x O 4 (0 ≤ x ≤ 1.4) were studied using X-ray diffraction, magnetization measurements and Mössbauer spectroscopy. X-ray diffraction patterns show that all samples have cubic spinel structure. The temperature dependent magnetic measurements revealed that a magnetic compensation point appears at chromium concentration x = 0.9, 1.2 and 1.4. Moreover, it was observed that the Curie temperature T C decreases and approaches the compensation temperature T K as Cr 3+ substitution for Fe 3+ increases. The magnetization data at all concentrations are discussed in the light of Néel's molecular field model taking into account the cation distributions obtained using the analysis of Mössbauer spectra.

Cation ordering in NiFe2–xCrx O4‐spinels studied by Mössbauer spectroscopy in external fields

“…Thus it is natural to assign the first one to iron at tetrahedral sites and the second pattern to iron at octahedral sites. In agreement with Rais et al [5] the obtained intensities are Int(tet) ≈55% and Int(oct) ≈45%, while Gismelseed and Yousif [4] got 52% and 48% in a similar fit. Assuming intensities proportional to the amount of iron at the two sites, the result is then incorrect: according to the formula, the amount of iron at the tetrahedral site can not be bigger than the amount of iron at the octahedral site.…”

“…The observed change in lattice constant (a 0 ) with composition (x) in the present study shows partial agreement with results from other studies. Thus for Fe-rich compositions, a decrease in a 0 with increase in Cr-content of about the same magnitude as observed here was also obtained by Lee et al [3], Singhal and Chandra [7], Gismelseed and Yousif [4] and Rais et al [5]. The latter two seem to have used the same samples in their experiments.…”

“…This observation was already made by Kedem and Rothem [14] who therefore used four sextets in fitting their spectrum. To solve the problem using the two sextet model when describing the spectra, Sepelak et al [15] as well as Rais et al [5] used different f-factors, f(oct)/f(tet) = 0.94 at room temperature and f(oct)/f(tet) = 1 at 77 K. Such an assumption is not in full agreement with the Debye model for f-factors, as the ratio then should be 0.98 at liquid nitrogen temperature. Furthermore, the ratio f(oct)/f(tet) = 0.94 at room temperature was firstly given by Sawatsky, van der Woude and Morrish [16] when studying magnetite in the temperature range 300 K-800 K. They achieved f(oct)/f(tet) = 0.99 at 0 K by an extrapolation using the Debye model.…”

“…Partly this may be explained by the fact that neither Lee et al [3] (studied interval x = 0-0.6), nor Singhal and Chandra [7] (studied interval x = 0-1.2) or Gismelseed and Yousif [4] and Rais et al [5] (studied interval x = 0-1.4) synthesized the most Crrich compositions. However, the increase in a 0 with increase in Cr, for the Cr-richest compositions studied in this paper, agrees excellently with recent results by Park and Kim [10], who studied Fe substituted NiCr 2 O 4 in the interval x = 2-1.5 (Fig.…”

“…Chappert and Frankel [2], Lee et al [3], Gismelseed and Yousif [4] and Rais et al [5] all declare that the substituting Cr 3+ ions only populate the octahedral site. In the case for x > 1, iron is according to Rais et al [5] only found in the tetrahedral site while Chappert and Frankel [2] and Gismelseed and Yousif [4] also place iron in the octahedral site. Fayek and Ata-Allah [6] and Singhal and Chandra [7] are however of a contrary opinion.…”