Magnetic States Control in Layered Ferromagnetic Materials

Exploring microscopic mechanisms and controlling magnetic states in layered ferromagnetic materials offer opportunities for developing new electronic/spintronic devices such as logic gates and non-volatile memories. Magnetic anisotropy, apart from the Curie temperature and the coercive field, can describe the preferential direction of magnetic moment and is one of the key parameters in spintronic applications of ferromagnetic materials. Essentially, such magnetic anisotropy can be significantly affected by the surrounding environment of magnetic sites, and strongly depending on the interplay between the itinerant electrons and localized magnetic moments. The technical appeal is that the itinerant electron cloud, being surround and screen the local magnetic moment, can serve as a tunable physical parameter of ferromagnetism therein. However, due to the itinerant nature, traditional electric field control techniques are limited by the Thomas-Fermi screening effect therein. Therefore, there is a significant technical challenge for controlling the magnetism of itinerant ferromagnets via electric field. As a result, magnetization switching with a tunable magnetic easy axis can serve as a powerful way to realize practical multifunctionality devices, and thus quantitatively and precisely controlling the easy magnetization axis in ferromagnetic materials remains a challenging scientific problem. However, the precise control over the magnetism of itinerant ferromagnetic materials remains challenging.<br/>In this talk, we demonstrate the control of magnetic states and the emergent hidden Kondo states in layered ferromagnetic materials Fe₅GeTe₂ with the anomalous Hall effect measurements and scanning tunneling microscopy/spectroscopy (STM/STS), as well as quantitative theoretical analysis using the Stoner-Wohlfarth model. First, we achieved continuous and controllable manipulation of the magnetic easy axis through a spin-flop pathway (from out-of-plane to tilted and finally to in-plane direction). Note that the giant magnetic anisotropy energy of Fe₅GeTe₂ can be electrically modulated in an extensive range from −0.38 to 2.11 MJ m^–3, which is almost the largest value among the reported 3<i>d</i> ferromagnetic materials and close to the values of widely used <i>f</i>-orbital permanent magnets such as Nd₂Fe₁₄B, paving a new way for practically tunable spintronic applications. Second, through a diamond anvil cell to apply high pressure to Fe₅GeTe₂, we modulated its magnetic anisotropy from in-plane to out-of-plane and finally to almost isotropic, and further successfully enhancing the Curie temperature to a value as high as 400 K. Such pressure engineering on ferromagnetism provides a novel strategy to explore magnetic exchange interactions and to achieve magnetic materials with <i>T</i>_C far above room temperature. Third and significantly, we observed hidden Kondo states and pronounced enhancement of Kondo screening in itinerant ferromagnetic Fe₅GeTe₂ and we notice that the values of Kondo temperature in such hidden Kondo states can be effectively enhanced with intercalation and Ni-element substitution methods. These research findings provide deeper insights into the microscopic origin of Kondo screening in 3<i>d</i> layered ferromagnetic materials. Additionally, we demonstrating a novel gate-tunable ferromagnetic tunnel junction device with Fe₃GeTe₂/hBN/Fe₃GeTe₂ spin valve geometry. This paves the way for manipulating the magnetism of ferromagnetic materials and exploring spintronics applications.