Electric field controlled magnetization and terahertz spin current pulse in artificial multiferroic systems

Nanomagnets are the stable component of non-volatile magnetic memories, where the binary bit is stored in different magnetization state to store the information. In the past, many different approaches have been adopted for altering the magnetization state of a nanomagnet to enable the writing of a binary bit. Strain-controlled magnetic storage logic devices are widely considered as a promising route to store the information in a binary bit with low energy consumption. One of the most essential problems with earlier/existing current-driven memory devices is the conversion of the current into heat leading to significant energy loss, making them energy inefficient. In such a scenario, to reduce energy consumption per write operation the magnetization switching by electric field induced strain in two-phase multiferroics is very promising. Theoretically, it is calculated that ferromagnetic and ferroelectric based multiferroic can form the basis of ultra-low-power computing and signal processing. The deterministic realization of bi-stable magnetic switching using electric field induced strain is essential for information storage which forms the motivation of this Ph.D. thesis using voltage-controlled (or electric field controlled) magnetization switching in ferromagnetic/ferroelectric (FM/FE) heterointerfaces. In our first work reported in this thesis, we have presented a synthesis of a flexible artificial multiferroic composite consisting of nickel ferrite (NFO) nanoparticles (NPs) in a polyvinylidene fluoride (PVDF) matrix. The fabrication of PVDF based flexible free standing artificial multiferroic composite is highly desirable to achieve an energy-efficient magnetoelectric device. In this direction, the efforts have been made to design the flexible artificial multiferroic by solution casting method to achieve reliable room temperature magnetoelectric coupling. The research work was extended to demonstrate the electric field control of magnetic order by synthesizing the artificial multiferroic nanofiber consisting of similar composition. The advantage of the decrease in dimension of the composite was significant enhancement in the magnetoelectric response and the control of the magnetic order by just applying the electric field in the range of few kV cm-1. Secondly, we have designed a thin film based artificial multiferroic based terahertz (THz) spintronic device to demonstrate the electric field induced strain control of effective magnetization of a ferromagnet and it was probed through the change in magnitude as well as phase of the THz spin current pulse upon photoexcitation by a femtosecond laser pulse. Finally, we have performed the Micromagnetic simulation studies to understand the role of defects in a geometrically frustrated square artificial spin ice (ASI) system comprising of peanut shaped nanomagnets. Further, we combine the ASI system with ferroelectric to form the artificial multiferroic system. In this study, we have used a close square ASI consisting of peanut shaped nanomagnets to demonstrate the pulsed electric field-controlled magnetization switching. The magnetization switching is obtained by applying the electric field pulse at few specific angles to the system. Additionally, the new scheme has been proposed to switch the magnetization at lower magnitude of applied electric field in materials with smaller magnetostriction coefficient.