## 1 Introduction  
Cancer treatment through magnetic hyperthermia relies on delivering magnetic nanoparticles (MNPs) to a tumor area in order to deactivate cancer cells by locally raising the temperature while avoiding collateral damage to healthy tissues [1; 2; 3; 4]. Since it was proposed by [PERSON] et al. in 1957 [5], the technique has progressively advanced and nowadays there are already a few clinical trials being carried out in different hospitals around the world [3; 6]. In addition, in the last years, different strategies have been devised to improve the theranostic efficiency of these nano-agents such as combining magnetic hyperthermia and photothermia in magneto-plasmonic nanostructures or using magneto-mechanical actuation for further disrupting the cancer cells [7; 8; 9; 10; 11; 12; 13; 14; 15]. Despite a large number of advancements in the field, there are still several issues that warrant further studies such as improving the delivery efficiency of the MNPs to the tumor or optimizing the heating efficiency (i.e., the specific absorption rate, SAR) of the MNPs. Concerning this last issue, one of the most complex problems that arises in the effort of increasing the SAR is the problem of magnetic interactions [16; 17]. Interparticle magnetic interactions appear when the separation between the individual MNPs is small enough. These types of interactions tends to give rise to an undesired agglomeration of the MNPs inside the human body, which can be challenging [18; 19]. The presence of these interactions has been reported to reduce the heating efficiency of the MNPs. The degradation of the SAR with increasing interparticle interactions can be, in principle, associated with the magnetic disorder and additional anisotropies introduced by magnetic interactions [20; 21; 22; 23; 24; 25]. The hysteresis losses are often reduced in the agglomerated MNPs during the application of the AC field, resulting in lower SAR values in comparison with the case of the well-separated MNPs [26]. There are, nevertheless, exceptions to this rule: for example, when the interparticle magnetic interactions can be controlled so that they give rise to specific arrangements of the MNPs such as chains, it has been demonstrated that their heating efficiency can increase in comparison with the non-interacting MNPs [27; 28]. However, it is not straightforward to obtain MNPs forming such arrangements. Apart from interparticle magnetic interactions, one must also consider the presence of intraparticle magnetic interactions as has been reported in several experimental and theoretical works [29; 30; 31; 32; 33]. For example, when the MNPs are multigranular or multidomain, short-range dipolar interactions between nanograins forming part of a nanostructure can also impact their magnetic response and therefore their heating efficiency [34]. To this respect, in the last years several new magnetic nanostructures for magnetic hyperthermia have been brought forward as an alternative to the typically employed spherical single domain MNPs. Among these, we have synthesized magnetic nanodiscs, nanoflowers and core/shell MNPs, for example [35; 36; 37; 38]. By changing the morphology of these nanostructures, we can improve their heating efficiency while exploiting new biomedical capabilities. Recent SANS experiments in multigranular nanoflowers with strong interactions between the nanograins have revealed that a comparatively large heating efficiency can be obtained in these nanostructures due to their intrinsic enhanced spin disorder [38]. Other good examples of these kinds of novel MNPs are iron oxide nanotubes and nanorings [39; 40]. These nanostructures have recently been proposed as promising candidates for combined drug delivery and magnetic hyperthermia therapy: by making the particles hollow, the surface area available for attaching drug molecules to the MNPs is increased [41; 42]. In addition, the tunable aspect ratio and morphology of these nanostructures provide us with an additional degree of control over their magnetic response.