Abstarct: The research on ferrofluids as a form of nanofluid has been developed rapidly since ten years ago because it can exhibit different behavior patterns compared to other fluids under the influence of magnetic and electric fields and in some cases, it can improve thermal behavior. Although many studies have been done, there is still no complete understanding of the mechanisms and formulation of heat transfer by ferrofluids under external magnetic field. In this article, at the beginning and after presenting the basic concepts, an overview of ferrofluid production methods, materials, stability, surfactant, its electrical and magnetic properties, types of induction of external magnetic field on ferrofluid, and its tools to improve heat transfer and test conditions. Then, baxsed on the initial design, the purpose of which is to investigate the heat transfer of the free movement of the water/Fe3O4 ferrofluid in slow flow in a square microchannel with dimensions of 0.01×0.8×0.01×m, under the influence of zero/constant/alternating magnetic field and affected by various heat fluxes. and the fact that the purpose of which is to increase the heat transfer, the explanation of the appropriate laboratory system, and the theory of the tests are discussed. The current research has been carried out on the topics and with the summary of the following results; Investigating the Berger method in the production of 0.5% and 1% ferrofluid by studying the thermophysical characteristics of ferrofluid such as zeta potential whose value is 62.1 mV and magnetic saturation whose values were obtained as 0.6 emu/g and 2 emu/g respectively. The rate of sedimentation of sub-particles over time was also investigated. Investigating the many experimental relationships that have been proposed in previous research regarding the physical characteristics of this type of fluid and sometimes do not entirely match the results of the experimental tests of the fluid by comparing and evaluating the thermo-physical properties of the manufactured ferrofluid with the existing experimental relationships and finding the closest experimental relationships with the results of the viscosity tests. , the specific heat coefficient and density for the water/Fe3O4 ferrofluid, the research was conducted and validated using pure water, and the maximum percentage error was calculated, which is 0.2%. In the design of the magnetic field, wire tube and Helmholtz coil were used with the intensity of the produced fields of 1464 Gauss. In the investigation of the local Nusselt number by ferrofluid in different concentrations under the influence of the external local magnetic field with the intensity of zero/constant/alternating fields under several uniform and different heat fluxes applied to all faces of the channel, it was determined that the highest local Nusselt number is related to the alternating magnetic field and in At the end of the channel, 5.22 and 4.98 correspond to the heat flux of 380, respectively, for 1% and 0.5% ferrofluid. Among other cases, the effect of changing the volume percentage of zero/half percent/one percent under a zero/constant/alternating external applied magnetic field under several uniform and different heat fluxes applied to all sides of the channel is on the local Nusselt number of the ferrofluid. The effect of uniform heat flux applied to all sides of the channel on the local Nusselt number of the ferrofluid under the influence of zero/constant/alternating magnetic field was also investigated. It should be mentioned that the geometry of the present problem, which is one of the innovations of this research, is used in heat exchangers. Another innovation of this research is the use of the Helmholtz coil to create a uniform alternating field. The results show that the local Nusselt number increases with the help of a magnetic field. This method may pave the way for the next generation of high-efficiency heat transfer engineering. In all these cases, the heat flux is constant and perpendicular to the fluid flow by the heater, which is applied uniformly around the channel to create uniformity.