Atomic migration in MgFe2−xCrxO4

Abstract

Mg–Cr ferrite has been studied with Mössbauer spectroscopy, x-ray diffraction, and vibrating sample magnetometer. The crystal structure for this system is spinel structure, and the lattice constant is in accord with Vegard’s law. The Mössbauer spectra consist of two six-line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Curie temperature decreases linearly with Cr concentration, suggesting the superexchange interaction Fe(A)–O–Fe(B) link is stronger than that for the Fe(A)–O–Cr(B) link. Debye temperatures for the A and B sites of MgFe1.9Cr0.1O4 are found to be θA=515±5 K and θB=265±5 K, respectively. Atomic migration of MgFe1.9Cr0.1O4 starts near 350 K and increases rapidly with increasing temperature to such a degree that 50% of the ferric ions at the A sites have moved over to the B sites by 600 K. The temperature dependence of both the magnetic hyperfine field and magnetization of MgFe1.9Cr0.1O4 is explained by the Néel theory of ferrimagnetism using three superexchange integrals: JA–B=−17.9kB, JA–A=1.9kB, and JB–B=7.1kB.

Magnetic and Mössbauer study of Mg0.9Mn0.1Cr x Fe2−x O4 ferrites

“…The XRD data shows that the samples retain the cubic spinel structure as more Cr is added in agreement with the results in [5][6][7][8]. The measured lattice constants decrease with increasing Cr concentration as shown in Figure 1.…”

“…The magnetic moment per formula unit according to Neel model is then 0:5 þ 9hðx; T Þ À 2x. By comparing this relation to the experimental results of m r , it is possible to determine the dependence of h on x and T. In obtaining these relations we assume that h is independent of T for Cr-free systems for T less than about 200°C [5]. The actual value taken for h in this case does not affect the observed trends.…”

“…The magnetization measurements showed that the average magnetic moment and Neel temperature decrease with increasing Cr concentration. Using Mössbauer spectroscopy the contact hyperfine field was also found to decrease with increasing Cr contents [4,5]. In general Cr occupies the B site replacing Fe there; however, both X-ray and Mössbauer studies showed that the concentration of Fe in the A site decreases, while that of Mg increases as more Cr enters the B site.…”

“…In general Cr occupies the B site replacing Fe there; however, both X-ray and Mössbauer studies showed that the concentration of Fe in the A site decreases, while that of Mg increases as more Cr enters the B site. Moreover, the order of inversion h was found to increase with temperature [5].…”

“…On the other hand, in Mn ferrites the Mn 2+ cations were found to reside mostly in the A site with an inversion ratio of about 0.8 [2,3]. The magnesium-chromium ferrites were also intensively studied [4][5][6][7][8]. With inclusion of Mn, the ferrites Mg 0.9 Mn 0.1 Cr x Fe 2−x O 4 were studied against Cr concentration [7,8].…”