X. Obradors

Cation distribution and intrinsic magnetic properties of Co‐Ti‐doped M‐type barium ferrite

The structural and magnetic properties of BaFe12−2xCoxTixO19 (0≤x≤1.0) M‐type barium ferrite have been investigated by means of neutron powder diffraction and high‐field magnetization measurements. The cationic distribution and magnetic moments of the five different metallic sublattices are determined and compared to the experimental saturation magnetization. It is found that about 50% of Co ions occupy tetrahedral sites, thus being ineffective in the reduction of the uniaxial magnetic anisotropy, while Ti ions prefer the 4fVI octahedral sites. All in all, a clear hierarchy of sublattice preferences is defined. Although the collinearity of the magnetic structure is progressively broken, mainly above x≊0.7, the overall behavior remains ferrimagnetic. The strong local spin canting is tentatively attributed to the localization of diamagnetic Ti ions in the 4fVI octahedral sites.

Magnetic frustration and lattice dimensionality in SrCr8Ga4O19

Abstract DC magnetic susceptibility measurements of the magnetoplumbite-like compound SrCr 8 Ga 4 O 19 show that no magnetic phase transition occurs up to T=4.2K in spite of very high antiferromagnetic interactions making apparent a very high degree of magnetic frustration (O/Tg 117). The determination of a preferencial siblattice cation distribution by neutron diffraction allows us to interpret this effect as associated with an effective decrease of the lattice dimensionality.

Crystal structure of strontium hexaferrite SrFe12O19

The room temperature structure of SrFe/sub 12/O/sub 19/ hexagonal ferrite has been refined from X-ray single-crystal data. Structural isomorphism of the title compound with the hexagonal ferrite BaFe/sub 12/O/sub 19/ has been demonstrated. In particular, it is corroborated that the bipyramidal Fe ions have a fast diffusional motion within a quasiharmonic double-well potential as in BaFe/sub 12/O/sub 19/. Some structural differences within the R-block are found among Ba and Sr magnetoplumbites which may account for the different dynamic, magnetic, and crystallochemical behavior of both compounds.

Synthesis and characterization of new substituted barium-ferrite particles for magnetic recording

The synthesis and the structural and magnetic characterization of new BaFe/sub 12-2x/Co/sub x/Sn/sub x/O/sub 19/ barium ferrite small particles is reported. The synthesis of the crystallites was performed by the liquid mix technique and TEM (transmission electron microscopy) photographs show a well-developed hexagon-like morphology with a high diameter-to-thickness ratio and a narrow size distribution. X-ray diffraction line broadening has been used to follow the evolution of the crystal size with composition. Hysteresis loop measurements prove that a coercivity suitable for magnetic recording may be obtained in this Co-Sn series with a smaller rate of Fe substitution than in the Co-Ti series. The different magnetic behavior of both series is tentatively attributed to a different sublattice cation distribution. >

Spin glass behaviour in an antiferromagnetic non-frustrated lattice: Sr2FeNbO6 perovskite

From magnetic susceptibility and hysteresis loop measurements on the cubic perovskite Sr2FeNbO6, it is shown that a spin glass transition does exist in this compound which has a non-frustrated lattice, and only antiferromagnetic interactions. This experimental results allows the authors to conclude that lattice frustration is not an essential feature for the spin glass transition in antiferromagnetic insulators. Moreover, it is argued that short-range atomic correlations, which modify the magnetic phase diagrams of the diluted compounds, make this spin glass transition possible.

Bandwidth narrowing in bulk magnetoresistive oxides

The key structural factor governing the one-electron bandwidth (W) in (A = Ca, Sr) magnetoresistive oxides has been identified by a neutron study. Reducing the size of the lanthanide, the Mn - O bondlength in octahedra does not change () and hence does not contribute to the narrowing of the conduction bandwidth. In contrast, the Mn - O - Mn angle bends at a rate . The calculation of the bandwidth dependence on has been performed and it has been found to be proportional to the measured Curie temperature, both having almost a dependence.

Exchange interactions in BaFe12O19

In the scope of the mean field approximation the nine nearest neighbour exchange interactions existing among the ferric ions in BaFe12O19 hexagonal ferrite have been determined from the experimental sublattice magnetizations. It is shown that these exchange interactions are well explained by means of the superexchange theory. It is also concluded that some direct exchange interaction does exist among the octahedral 4fVI ions. The calculated magnetization, paramagnetic susceptibility, and Curie temperature are in good agreement with the corresponding experimental values. The stability of the collinear Gorter-type magnetic structure of BaFe12O19 is discussed as well.

Hexagonal ferrite particles for perpendicular recording prepared by the precursor method

Single domain MFe 12 O 19 (M=Ba Sr) hexagonal ferrite particles have been prepared by the precursor or liquid mix technique. It is shown that single phase samples with different mean diameter and diameter to thickness ratio may be prepared by modifications of thermal treatments. It is shown as well that surface spin canting has been formed and that the magnetocrystalline anisotropy is reduced by the platelet-like shape anisotropy.

Cation distribution and high field magnetization studies on SrFe 12-x Cr x O 19

With the aim of a better understanding of both cationic distribution and magnetic properties of the uniaxial SrFe 12-x Cr x O 19 hexagonal ferrites, Mossbauer spectroscopy, neutron diffraction and high field magnetization measurements have been carried out. The Cr3+ions occupy the octahedral sites of the M structure with a preference hierarchy within them. The magnetic measurements, together with the deduced cationic distribution, indicate that some sublattices have a random spin canting around the c-axis.

Neutron diffraction study of the crystallographic and magnetic structures of the BaFe12−xMnxO19 m-type hexagonal ferrites

Abstract The cationic distributions, ionic valencies and magnetic structures of the magnetoplumbite-like BaFe 12- x Mn x O 19 hexagonal ferrites have been studied from powder diffraction data in the paramagnetic and ferrimagnetic phases. The paramagnetic diagrams show that the Mn cations enter all the sublattices of the M-type structure except the pseudo-tetrahedral one within the R-block. It is also concluded that, in the spinel block, the tetrahedral site is occupied by Fe 3+ and Mn 2+ ions whereas in the remaining octahedral sites are located Fe 3+ , Mn 3+ , Mn 4+ cations in a selective way and with a hierarchy of preferences. From the magnetic diagrams it is concluded that even if the long range magnetic ordering has a collinear ferrimagnetic character, the 12k and 2a octahedral sublattices present a random moment canting which decreases further the saturation magnetization.