## 1 Introduction  
Magnetic polymer composite particles (MPCPs) have garnered widespread attention due to their unique combination of magnetic functionality and polymeric particle structures. This hybrid nature imparts a wide range of capabilities, making them promising candidates for use in areas such as biomedical engineering, bioengineering, magnetic hyperthermia, MRI diagnostics, and enzyme immobilization [1]. Common methods for producing MPCPs generally fall into three categories: physical encapsulation, the in situ formation of magnetic components, and the polymerization of monomers in the presence of magnetic materials. A considerable amount of research has been dedicated to manipulating the shape and structure of micro- and nanoscale particles using a range of emulsion-based methods and polymerization techniques [2]. More recently, there has been growing interest in polymer microspheres exhibiting well-defined and unusual morphologies, driven by their potential in diverse applications such as catalysis [3], photonic crystal engineering [4; 5], self-assembled systems [6], sensing technologies [7], and molecular recognition processes [7]. Despite this progress, the reliable synthesis of such intricately structured microspheres continues to present substantial challenges. From a thermodynamic perspective, smooth-surfaced spherical particles are energetically favorable and thus form more readily. As a result, the development of non-spherical particles has often depended on reshaping or modifying existing spherical templates. Techniques such as seeded polymerization and the self-assembly of complex structures have been employed to produce a variety of particle morphologies--including ellipsoids [8], hollow forms [9], snowman-like geometries [10; 11], and disks [12]. Colloid-based and interfacial lithographic methods [12] have also been investigated for this purpose. Despite these advances, most reported particles still feature smooth surfaces, and generating non-spherical particles with textured or rough surfaces--which are desirable due to their higher surface area--remains a considerable challenge. A significant advancement in this area involved the creation of polymer particles with a walnut-like, multi-hollow architecture [13]. This was accomplished using sulfonated and crosslinked polystyrene spheres as sacrificial templates, prepared via dispersion polymerization followed by sulfonation [13]. Nonetheless, the multistep nature of this fabrication route introduces complexity and limits reproducibility. Magnetic polymer microspheres have emerged as versatile materials for applications in a broad range of fields, including biomedical engineering [14; 15], magnetic hyperthermia [16], MRI diagnostics [17], enzyme immobilization [18], catalysis [19], cellular and genetic separation [20], and environmental cleanup [21]. Combining magnetic functionality with engineered surface textures could unlock additional capabilities in these domains. In previous work [22], we successfully fabricated magnetic polymer composite particles (MPCPs) with encapsulated Fe\({}_{3}\)O\({}_{4}\) via \(\gamma\)-ray induced mini-emulsion polymerization and found that the hydrophobic modification of Fe\({}_{3}\)O\({}_{4}\) enabled high encapsulation, while optimal SDS (surfactant) and CA (co-stabilizer) levels ensured narrow size distribution. In that work, we focused on MPCPs with a smooth sphere shape. This work is the continuation of our previous work but focused on the fabrication of MPCPs with rough surfaces--especially those with a walnut-like shape--by leveraging the knowledge we obtained from previous work. The procedure and formulation to fabricate walnut-like poly (divinyl benzene)/Fe\({}_{3}\)O\({}_{4}\) microspheres were modified from our previous work to make MPCPs [22]. Differing from previous work using cetyl alcohol (CA), here we used hexadecane (HD) as a co-stabilizer to make our mini-emulsion system. This is because HD showed better solubility in our monomer DVB.