Controlling the Ba/Sr Ratio in U-type Ba_xSr_4-xCo₂Fe₃₆O₆₀ Enables the Stabilization of the Spin Cone Symmetry Achieving the Magnetoelectric Coupling Effects at Room Temperature

Multiferroic materials, characterized by the coexistence of ferroelectricity and ferromagnetism, exhibit remarkable magnetoelectric coupling effects. The unique property endows them with broad application prospects in multi-state memory devices, highly sensitive magnetoelectric sensors, and multifunctional devices. Currently, the majority of single-phase multiferroic materials exhibit limited magnetoelectric coupling performance and the operational temperature can hardly achieve room-temperature conditions, which constrains the practical application value. Therefore, there is an imperative to synthesize single-phase multiferroics with room-temperature magnetoelectric coupling performance. This study synthesized a ferroelectric U-type hexaferrites ceramic Ba_xSr_4-xCo₂Fe₃₆O₆₀ (0.6 ≤ x ≤ 1.4) by the solid-state sintering method. The dynamic magnetoelectric coupling properties of this materials were investigated for the first time, and the phenomenon of polarization reversal with low magnetic field was discovered. The magnetoelectric coupling in Ba_xSr_4-xCo₂Fe₃₆O₆₀ demonstrates progressive enhancement with increasing Ba content. The first-order dynamic magnetoelectric coupling coefficients rises from 0.33 mV/(cm · Oe) at x = 0.6 to 0.54 mV/(cm · Oe) at x = 1.2 under ambient conditionswhile the second-order dynamic magnetoelectric coupling coefficients reaches 2.1 µV/(cm · Oe²). In addition, the magnetic structure and magnetic properties of Ba_xSr_4-xCo₂Fe₃₆O₆₀ were also investigated. Both magnetodielectric curves and magnetic hysteresis loop demonstrate that adjusting the ratio of Ba/Sr can make the transverse conical magnetic structure more stable. The U-type hexaferrites ceramic Ba_xSr_4-xCo₂Fe₃₆O₆₀ (0.6 ≤ x ≤ 1.2) exhibit magnetoelectric coupling properties throughout the temperature range from low temperature to room temperature, reaching its maximum performance in the 300-320 K range. The Ba/Sr ratio regulation strategy effectively tunes the magnetic phase transition temperature of Ba_xSr_4-xCo₂Fe₃₆O₆₀, expanding the U-type hexagonal hexaferrites magnetoelectric coupling operating temperature breaks through room temperature, and providing a new strategy for the development of multistate memory.