A Chinese research team from High Magnetic Field Laboratory, Chinese Academy of Sciences demonstrated the induced formation of a new type of Domain walls (DWs): the structural DW, and how it shows up in variable processes with surprisingly completely reproduced patterns and confined the phase dynamics in strained manganite thin films.

DWs have been a central research topic in the field of magnetic materials during recent decades. Generally speaking, DWs are the boundaries between adjacent phases or domains with different ordering properties.

They are at the transition regions where the order parameters change in a material having broken symmetry. They are important because they often exhibit exotic physical properties that do not exist in their surrounding domains.

Two striking examples are the possible nucleation of superconductivity at the charge-ordering DWs and the discovery of enhanced conductivity at DWs in ferroelectric oxides.

So, the discovery or design of exceptional DWs in these strongly correlated electron systems, and the phenomena that are unique to the DW regime, merit in-depth exploration.

However, to date, very few studies have focused on designing new types of DWs, or investigating their roles in phase dynamics.

Recently, Chinese scientists Prof. LU Qingyou and Prof. WU Wenbin induced a new type of DW: the structural DW, and demonstrated how it shows up in variable processes with surprisingly completely reproduced patterns as well as confined the phase dynamics in strained manganite thin films.

This was in essence different from the previously observed various phase domains in phase-separated manganites.

Using a home-built low-temperature and high-magnetic-field magnetic force microscopy (or MFM), they systematically explored the physical properties of the DWs and their critical roles in the phase separation states microscopically.

Specifically, the DWs form a large diversity of closed loops with, in some cases preferred orientations and exhibit little dependence on temperature or magnetic field, showing the structural origins of the domains.

This type of immobile DW, which has rarely been reported before, is a new addition to the family of nano scale DWs.

Through direct imaging with their MFM, they also confirmed that the DWs are ferromagnetic and metallic at low temperatures, while their surrounding domains are not.

They found that the DWs can act as immobile spatial restrictions that confine the growth and emergence of the phase-separated domains, implying the strong interactions of electronic phases with the lattice.

Each domain exhibits binary occupation by a single pure phase, resulting in a quasi-periodic phase separation. Hence, the DWs could play a deterministic role in phase competition and the evolution of phase-separated manganite films.

The universal behaviors of the multiple engineered films illuminate the possibility of controlling the formation of DWs and tuning phase dynamics through DW design.

Magnetic images during magnetic field sweeping (Image by FENG Qiyuan)