## 1 Introduction  
Magnetic gears (MGs) have emerged as a robust alternative to conventional mechanical systems due to their contactless torque transmission, low maintenance requirements,and silent operation advantages [1; 2]. Their high efficiency and structural durability in key parameters, such as power, torque, and speed transmission, make them particularly attractive for compact design applications [3]. However, the saturation limits of the magnetic materials used impose physical constraints on the torque density achievable in such systems. The development of neodymium iron boron (NdFeB) magnets in the 1980s marked a significant milestone in the evolution of magnetic gear technology, enabling solutions that are not only highly energy-dense but also more cost-effective [4]. Studies in the literature have shown that increasing the number of pole pairs positively affects torque transmission capacity [3; 5; 6].  
Magnetic gears are increasingly applied in advanced engineering fields, such as aerospace, renewable energy systems, electric transportation, and space technologies [7; 8; 9; 10; 11; 12; 13; 14]. Nonetheless, their relatively low torque density and certain geometric limitations compared to classical mechanical gears remain key barriers to broader industrial adoption. In this context, recent research efforts have increasingly focused on novel design strategies and material-oriented optimizations aimed at enhancing torque performance [15; 16].  
In magnetic gears, torque is transmitted without physical contact through the modulation of the magnetic fields generated by permanent magnets. This principle not only eliminates mechanical wear but also enhances reliability in dynamic applications [17; 18]. Critical parameters such as the air gap between rotors, the number of poles, and pole alignment directly influence overall system performance. Particularly in flux-modulated structures, ferromagnetic elements placed between rotors guide magnetic flux paths, thereby enabling effective torque transmission [19; 20].  
This study aims to provide a comprehensive review of the recent advances in magnetic gear systems, evaluating their topological diversity, torque transmission principles, material selection, and performance outcomes. While some research focuses directly on improving key performance indicators such as torque density, losses, and efficiency, others explore the design of new topologies tailored to specific applications, material optimization, and thermal analysis.  
The primary objective of this work is to examine the structural and magnetic parameters that influence the engineering design of magnetic gear systems from a holistic perspective. The study also compares various methodologies adopted in the literature and identifies prominent research gaps driving the field forward. Furthermore, the magnetic, thermal, and mechanical properties of core materials and their impacts on system performance are addressed in detail.  
In the following sections, magnetic gear classification and operating principles, along with the performance characteristics of modulated and non-modulated topologies, design parameters, thermal behavior, core material manufacturability, and their comparative impacts on system performance are addressed. Finally, current application potentials and future research and design strategies are discussed.