# Unraveling Dissipation-Related Features in Magnetic Imaging by Bimodal Magnetic Force Microscopy
## 1 Introduction  
In 1981, 40 years ago, science began a new revolution with the development of the Scanning Tunneling Microscope [1]. This is the starting point of the family of Scanning Probe Microscopes (SPM) and all its alternatives. Among them, the branch of Scanning Force Microscopy (SFM) [2] has become a very powerful tool for nanomaterials characterization such as nanoparticles, nanowires and nanostructures or low dimensional systems. These techniques can be used not only to characterize the topography but also to detect a variety of interactions such as magnetic, chemical, mechanical, electrical properties, surface potential or even thermal gradients with extraordinary sensitivity and resolution. In the field of materials for magnetic applications, the development of the Magnetic Force Microscopy (MFM) has been crucial in the characterization of those materials at the nanoscale [3, 4]. Advantages of MFM include relatively high spatial resolution, simplicity in operation as well as sample preparation. MFM is a very active topic of investigation and developments [5]. One of the most relevant achievements has been the capability to study magnetization processes by applying in situ magnetic fields [6, 7, 8, 9]. Moreover, the use of the MFM was extended to investigate biological systems thanks to the possibility of operating in different environments as liquid media with improved signal/noise ratio [10]. Other recent development is related to the evaluation of the dissipation of energy during the MFM operation by measuring variations in the cantilever oscillation, with the so-called Magnetic Dissipation Force Microscopy (MDFM) mode. The dissipative maps in MDFM have been used to distinguish between Neel and Bloch domain walls [11], to detect magnetic nanoparticles [12] or to obtain a 3D map of the sample stray field [13] Despite the great advances in MFM, there are still some challenges as to obtain quantitative information and to increase both the spatial resolution and the sensitivity. Different alternatives have been proposed to achieve quantitative MFM characterization [14; 15; 16; 17]. Apart from the well stablished methods based on the tip function transfer evaluation, bimodal MFM has recently been used for quantitative imaging with high-spatial resolution [18]. Bimodal SFM is based on the simultaneous excitation and detection of two cantilever vibration modes, usually, the 1 st and the 2 nd modes. The multifrequency concept in SFM and its applications has been developed in the last ten years [19] for the characterization at the nanoscale of mechanical [20] or electrical properties in samples with heterogeneous behavior [21]. Moreover, different groups have also explored the opportunities of the magnetic force microscope operating in a bimodal configuration to separate short and long-range (topographic and magnetic, respectively) forces in ferromagnetic samples [22] to detect ferritin molecules in liquid media by measuring a nanomechanical effect [23] or to operate in high vacuum environment [24]. Regarding the resolution and sensitivity of the MFM, different routes can be followed to enhance them. The first approach is the development of high-performance MFM probes by using different methods to control the tip stray field [25; 26]. On the other hand, the dissipation maps obtained by using the MDFM mode, is an alternative way to improve the resolution of the magnetic imaging. In a previous work [27], we demonstrated that the MDFM can be used to detect the magnetization switching of a relatively small number of spins (with dissipated energies in the range of few fW). The corresponding MDFM images present a spatial resolution better than 8 nm, much higher than the actual MFM resolution limit. In this attempt to increase the sensitivity of magnetic images, the presence of artifacts when measuring non-conventional magnetic samples must be considered [28; 29]. Those artifacts can have undesirable effects on magnetic images leading to misinterpretations of the signal [30].