Multiedge refinement of extended x-ray-absorption fine structure of manganese zinc ferrite nanoparticles S. Calvin,* E. E. Carpenter, B. Ravel, and V. G. Harris U.S. Naval Research Laboratory, Washington, D.C. 20375 S. A. Morrison George Washington University, Washington, D.C.20052 Received 12 July 2002; published 6 December 2002 The structure of nanoparticle manganese zinc ferrites synthesized by a reverse micellar method is deter- mined by analysis of the extended x-ray-absorption fine structure in combination with other techniques. Both empirical and theoretical standards are employed; manganese, zinc, and iron edges are refined simultaneously. It is determined that samples synthesized under similar conditions sometimes exhibit a markedly different distribution of cations between the available sites in the spinel structure; this in turn causes significant differ- ences in the magnetic properties of the samples. In addition, it is found that the mean-square displacements for manganese-oxygen bonds are consistently higher than for zinc-oxygen bonds, perhaps due to the presence of manganese ions of more than one valence. DOI: 10.1103/PhysRevB.66.224405 PACS numbers: 75.75.+a, 61.46.+w, 61.10.Ht I. INTRODUCTION For many decades, ferrites have been important magnetic materials for high-frequency industrial applications, due to low conductive losses, high permeabilities, and moderately high saturation magnetizations. 1 These attractive properties, in turn, depend critically on both the magnetic moment of the metal cations and on their distribution between sites with tetrahedral oxygen coordination and those with octahedral coordination. In particular, manganese zinc ferrites MZFO’s have emerged as the leading materials for inductor applications in the MHz f band. In recent years, there has been considerable interest in nanoparticle ferrites, since in many cases they have been found to have technologically desirable magnetic properties relative to ferrites synthesized by traditional routes. 2,3 Some of these properties, such as low coercivity, are attributable directly to the small size of the particles, but others, such as a relatively high Curie tempera- ture, appear to be related to the distribution of cations be- tween sites. 4–8 It is thus important to develop techniques that can identify the cation distribution in these materials. Because of the similar scattering strengths of manganese, zinc, and iron, it is difficult to rely on x-ray diffraction XRD to determine the site occupancy in MZFO’s. This problem is exacerbated in nanoparticles, since the loss of long-range order broadens the diffraction peaks, making full profile reduction considerably more difficult. Mo ¨ ssbauer- effect measurements are effective in determining the symme- try and valence of the iron cations, but provide no direct information on the manganese and zinc cations. For these reasons, extended x-ray-absorption fine-structure EXAFS spectroscopy has been used to determine site occupancy in nanoparticle MZFO’s. Until now, these determinations have examined the manganese, zinc, and iron edges separately, and, although they have been successful in qualitative com- parisons e.g., comparing samples synthesized by different methods, they have generally yielded only limited quantita- tive information see Sec. II B. In this work, we describe a method of analyzing the information from the manganese, zinc, and iron edges simultaneously, along with stoichio- metric information from inductively coupled plasma ICP optical emission spectroscopy, to yield a more precise deter- mination of the site occupancy as well as additional impor- tant structural information. II. BACKGROUND A. Spinel structure MZFO adopts the spinel structure, illustrated in Fig. 1. The oxygens in this structure are based on a face-centered- cubic fcc lattice. One out of eight of the tetrahedral inter- stices A sites and half of the octahedral interstices B sites are occupied by cations, yielding a total of thirty two oxy- gens, eight tetrahedrally coordinated cations, and sixteen oc- tahedrally coordinated cations per unit cell. Charge conser- FIG. 1. Spinel structure. Local environments of both tetrahedral sites A and octahedral sites B are shown. Large circles represent oxygens; small circles metal cations. The large cube is the conven- tional unit cell. For clarity, some of the atoms are not shown. PHYSICAL REVIEW B 66, 224405 2002 0163-1829/2002/6622/22440513/$20.00 ©2002 The American Physical Society 66 224405-1