Structure and magnetic properties of W-type hexaferrites

Abstract

W-type hexaferrites (WHFs) (SrMe 2Fe16O27, Me = Mg, Co, Ni and Zn) are hard magnetic materials with high potential for permanent magnet applications owing to their large crystalline anisotropy and high cation tunability. However, little is known with regards to their complex structural and magnetic characteristics. Here, the substitution of metals (Me = Mg, Co, Ni and Zn) in WHFs is described and their crystal and magnetic structures investigated. From joined refinements of X-ray and neutron powder diffraction data, the atomic positions of the Me atoms were extracted along with the magnetic dipolar moment of the individual sites. The four types of WHFs exhibit ferrimagnetic ordering. For Mg, Ni and Zn the magnetic moments are found to be ordered colinearly and with the magnetic easy axis along the crystallographic c axis. In SrCo2Fe16O27, however, the spontaneous magnetization changes from uniaxial to planar, with the moments aligning in the crystallographic ab plane. Macromagnetic properties were measured using a vibration sample magnetometer. The measured saturation magnetization (M s) between the different samples follows the same trend as the calculated M s extracted from the refined magnetic moments of the neutron powder diffraction data. Given the correlation between the calculated M s and the refined substitution degree of the different Me in specific crystallographic sites, the agreement between the measured and calculated M s values consolidates the robustness of the structural and magnetic Rietveld model.

Texture formation in W-type hexaferrite by cold compaction of non-magnetic interacting anisotropic shaped precursor crystallites

“…The obtained saturation magnetisation values correspond very well with the results reported in the literature, Mørch et al found SrZn 2 Fe 16 O 27 and SrNi 2 Fe 16 O 27 have saturation magnetisations of values 80.4(3) and 69.0(3) Am 2 kg −1 , respectively. 14 Likewise did J.-H. You et al find a M s of 77.3(2) Am 2 kg −1 for SrZn 2 Fe 16 O 27 in the study of the composition range SrZn x Fe 18− x O 27 , while the highest saturation was found for x = 1 with a value of 87.7(5) Am 2 kg −1 . 36 The samples reported by J.-H. You et al also have vanishing coercivity with values between 7.2–9.5 kA m −1 .…”

“…To obtain the intrinsic value of saturation magnetisation for the compounds Sr(Ni 1− x Zn x ) 2 Fe 16 O 27 ( x = 0, 0.5, 1) the corrected saturation magnetization is reported in Table 1 along with the wt% of the phases present. These corrections are described by Mørch et al 14 The weight fractions were extracted from the 2D Rietveld of the refinements of the synchrotron diffraction data, which will be described in detail below.…”

“…13 The specific cation site occupation fraction is crucial as it results in different saturation magnetisation values. 14 This makes the M = Zn 2+ structure interesting, because Zn 2+ is 3d 10 and it occupies the 4e tet site and 4f tet site, where the magnetic ordering is antiparallel. Inserting Zn 2+ into the tetrahedral site leads to an increase in the net magnetic moment due to the fact that Zn 2+ has no unpaired electrons resulting in a zero atomic magnetic dipolar moment.…”

Controlling the magnetic structure in W-type hexaferrites

“…In our previous study we described the uniaxial magnetic ordering in SrZn 2 Fe 16 O 27 with the magnetic space group (MSG) P6 3 /mm 0 c 0 (further details given in Section S2.1), but did not determine the MSG of the planar ordering in SrCo 2 Fe 16 O 27 (Mørch et al, 2019). Fig.…”

“…A system with potential conical spin ordering is the family of W-type hexaferrites, with Co 2+ substituting Fe 2+ as the transition metal. It is known that Co 2+ changes the magnetocrystalline anisotropy axis from uniaxial to planar at room temperature for SrCo 2Àx Zn x Fe 16 O 27 when x goes from 2 to 0 (Mørch et al, 2019). Here, three different magnetic structures of W-type hexaferrites are determined by neutron powder diffraction methods at room temperature.…”