# Magnetic Vortex and Hyperthermia Suppression in Multigrain Iron Oxide Nanorings
## 1 Introduction  
Spinel ferrite nanoparticles, \(M\)Fe\({}_{2}\)O\({}_{4}\) (\(M=3-d\) transition metal) have attracted considerable attention in the past few decades due to their potential biomedical applications, for example in targeted drug delivery, diagnostics, and magnetic separation [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Spinel ferrite nanoparticles have beenmostly studied because of their unique size- and shape-dependent tunable magnetic properties [14; 15; 16; 17; 18]. For biomedical applications, iron oxide (magnetite or maghemite) has been considered as the most attractive material because of its intrinsic biocompatibility and chemical stability [19; 20; 21]. Large surface to volume ratio, size- and shape-tunable magnetic properties, and high biocompatibility make iron oxide nanoparticles appropriate nanocarriers for magnetic resonance imaging, magnetic hyperthermia, and targeted drug delivery [22; 23; 24; 25; 26; 27; 28; 29; 30]. Magnetic hyperthermia has emerged as a promising alternative to the currently used cancer treatment methods, such as chemotherapy and radiotherapy, which have severe side effects [22; 23; 24; 25; 26; 27; 28; 29; 30]. When iron oxide nanoparticles are exposed to an external alternating current (AC) magnetic field, they convert the electromagnetic energy into thermal energy [22; 23; 24; 25; 26; 27; 28; 29; 30]. Magnetic nanoparticles act as nanometric heating centers that one can use to target a specific tumor area and deliver toxic doses of heat to the tumor area without affecting the neighboring healthy tissues, thus making magnetic hyperthermia a less aggressive and more effective method of targeted cancer treatment. Magnetic hyperthermia using superparamagnetic iron oxide nanoparticles is currently being implemented in combination with chemotherapy or radiotherapy, allowing a reduction of the chemotherapeutic drug dose needed, and yielding several encouraging outcomes in cancer treatment [31; 32; 33]. However, the low heating efficiency of these nanoparticles presents serious concerns about the dose of nanoparticles necessary for an effective cancer treatment [34]. The heating efficiency (estimated by specific absorption rate, SAR) of magnetic nanoparticles depends on their intrinsic properties, including the saturation magnetization and magnetic anisotropy [27; 28; 29; 30]. Size reduction has been reported to alter the saturation magnetization and magnetic anisotropy due to poor crystallinity, which could negatively impact the heating efficiency of the nanoparticles [27; 28; 29; 30]. Additionally, the SAR value of the nanoparticles highly depends on the colloidal stability or inter-particle interaction of the nanoparticles. To overcome these limits, different strategies have been proposed, such as tuning the shape and size of these nanoparticles. In this regard, single-crystalline iron oxide nanorods [27] and nanotubes [28] have recently been synthesized, which have shown excellent magnetic hyperthermia properties. Single-crystalline iron oxide vortex nanorings with high saturation magnetization and negligible remanence and coercivity have also been reported to show excellent magnetic hyperthermia response [35; 36]. However, control over the synthesis of these single-crystalline iron oxide vortex nanorings is a challenging task, raising serious doubt about their practical application [35; 36; 37; 38; 39; 40; 41]. Electron beam lithography and chemical methods have been employed to fabricate single-crystalline vortex nanorings, but a large-scale controlled synthesis of high-quality, defection-free single-crystalline iron oxide nanorings has not been achieved yet [35; 36]. Multigrain iron oxide nanorings are easy to form chemically. A full understanding of the magnetic and hyperthermia responses of these multigrain iron oxide nanorings will provide important insights into the magnetic vortex nature and the large SAR of the single-crystalline vortex iron oxide nanorings.