Real-Time Observation of Magnetic Domain Structure Changes with Increasing Temperature for Z-Type Hexagonal Ferrite

Abstract

Z-type hexagonal ferrites have recently received attention for their room-temperature magnetoelectric (ME), which is activated when the temperature at which the transverse-conical spin-state transitions to a ferrimagnetic state is increased. The changes in the magnetic domain structure at the transition have been well-documented; however, they are still not understood in detail. In the present study, Lorentz transmission electron microscopy (TEM) analysis combined with an in situ heating experiment was conducted to demonstrate the shift in magnetic domain structure during the transition from the transverse-conical spin arrangement to a ferrimagnetic spin order. The dynamics of the magnetic domain structure changes with the increasing temperature were acquired in real-time. At 490 K, the magnetization transition from the transverse-conical spin state to the ferromagnetic state was demonstrated. Cross-tie domain walls formed during the magnetic transition process. The increased effect of the demagnetizing field applied to the 180° magnetic domains was caused by a lower magnetocrystalline anisotropy (MCA) at the easy axis of magnetization.

Keywords: Lorentz TEM; hexagonal ferrite; in situ TEM; magnetic moment; magnetization.

Figure 1

(a) Magnetization vs. temperature curves for $\left[11\overline{2}0\right]$ and [0001] directions measured at μ₀H = 50 mT (b) Schematic of the transition of the magnetic spin state (c). XRD pattern of the single phase Z-type hexagonal ferrite.

Figure 2

(a) Schematic of a ray diagram showing the paths of electrons passing through a magnetic specimen (upper), together with the magnetic domain or domain wall contrast for the Fresnel modes (lower). (b) The schematic diagram for imaging the in-plane magnetic configurations using Lorentz TEM. Three images were acquired in the same region under three different defocus conditions. The in-plane magnetization distribution map could be generated from the TIE method. The color wheel represents the magnetization direction at every point.

Figure 3

Sequential BF-TEM images of the Z-type hexagonal ferrite via in situ heating.

Figure 4

Sequential images of the magnetic vector maps of the Z-type hexagonal ferrite by heating from 350 to 500 K. The color wheel represents the magnetization direction at every point.

Figure 5

(a) The cross-tie domain wall evolved after the transition of the magnetic spin axis, (b) schematic diagram of 180° magnetic domain wall structure flux closures within the planes without (left) and with (right) the presence of the cross-tie walls.