Local symmetry breaking and magnetic properties of acceptor-donor codoped ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ hybrid improper ferroelectrics

Layered hybrid improper ferroelectrics are very important for multiferroic devices; however, it has been very challenging to obtain multiferroic properties at room temperature. In this study we report that acceptor-donor codoping can be used to realize ferromagnetism in ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ improper ferroelectric ceramics at room temperature. The transmission electron microscope observation and first-principles calculation revealed that the obtained ferromagnetic properties result from the ${\mathrm{MnO}}_6$ octahedron tilt caused by the large lattice distortion near the acceptor-donor pair. At the same time, an obvious magnetoimpedance effect was observed.

Figure 1

XRD and TEM characterizations of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ceramics. (a) The phase structure of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005,0.015,0.02\right)$ ceramics. (b) High-resolution phase of ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ ceramic taken at room temperature. (c) High-resolution phase of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_{0.005}{\mathrm{Ca}}_{2.995}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ceramic taken at room temperature. (d–e) Geometric phase analysis results of ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ ceramic. (f–g) Geometric phase analysis results of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_{0.005}{\mathrm{Ca}}_{2.995}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ceramic.

Figure 2

Calculation results of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ based on first principles. (a) Schematic diagram of ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ crystal structure. (b) ${\mathrm{MnO}}_6$ octahedron tilt deviates from the iron pair direction. (c) Density of states calculation result of ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ after doping.

Figure 3

Magnetic performance characterization of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_{0.005}{\mathrm{Ca}}_{2.995}{\mathrm{Mn}}_2{\mathrm{O}}_7$. (a, b) Zero-field-cooled and field-cooled curves of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005\right)$ ceramics under 100 Oe. (c, d) Hysteresis loop of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005,0.015,0.02\right)$ ceramics at room temperature and local amplification of hysteresis loop under low magnetic field of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005\right)$.

Figure 4

Variation rules of magnetoresistance change rate with frequency under different magnetic field strengths. (a–d) Magnetoresistance behaviors change with frequency of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005,0.015,0.02\right)$. (e) Magnetoresistance change rate of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7 \left(x=0,0.005,0.015,0.02\right)$ under 3 kOe, 6 kOe, and 9 kOe. (f) Magnetoresistance change rate of ${\left({\mathrm{Li}}_{0.5}{\mathrm{Al}}_{0.5}\right)}_x{\mathrm{Ca}}_{3-x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ change with doping content under 9 kOe at1 kHz.