## 1 Introduction
The micropolar fluids have many engineering applications, such as the polymer fluids extrusion, animal bloods, exotic lubricants, colloidal and suspension solutions and liquid crystals solidification. A few investigators have studied micropolar fluid natural convection in enclosures ([PERSON] and [PERSON] [20], [PERSON] et al. [21], [PERSON] et al. [22]). [PERSON] et al. [23] has recently investigated transient micropolar double diffusive natural convection for buoyancy-opposing ratio and noticed that a monotonic decrease in the flow strength and heat and mass transfer ratesoccurs due to the vortex viscosity parameter. It should be noted that major researches in literature for the three-dimensional flow of micropolar fluids concerned numerical and/or analytical study of a flow due to stagnation point or stretching flat surface ([PERSON] and et al. [24], [PERSON] and al. [25], [PERSON] and et al. [26]). With the wide use of a non-Newtonian fluids model with suspended particles in modern technology and industries, the extension to micropolar nanofluid model is desirable. In this context, [PERSON] and [PERSON] [27] introduced a micropolar nanofluid model to carry out theoretical investigation of natural convection in nanofluids and compared the nanofluidic suspensions with the micropolar theory. It is observed that the average Nusselt number increases with increase Rayleigh number when the microrotation number is kept constant. [PERSON] et al. [28] investigated natural convection in Cu-water micropolar nanofluid with a heat-generated source in a porous medium. They found that as the material parameter increased, rotational strength of vortices and the particles micro-rotation increase and decrease, respectively. [PERSON] et al. [29] explored the transient natural convection in micropolar nanofluid on a vertical plate using five types of oxide nanoparticles. They established that the temperature of nanofluid based on graphene oxide is much higher than the other nanofluids. Magneto-convection research has been of extensive interest due to its vast engineering applications such as microelectronic mechanical devices, electronic packaging, and crystal growth in liquids [30]. Most of the researchers considered an electrically conducting fluid with low thermal conductivity which, in turn, restricts the heat-transfer enhancement in the cavity, and the use of nanofluid is a better option to improve the mechanism of heat transfer. [PERSON] et al. [31] considered the magnetic field impact on an Al\({}_{2}\)O\({}_{3}\) water-nanofluid natural convection flow in square cavity and found that the heat transfer deteriorated with increased Hartmann number. [PERSON] and [PERSON] [32] investigated, numerically, nanofluid natural convection flow in a square enclosure in the presence of uniform generation and absorption of heat and magnetic field. They concluded that heat transfer is enhanced with solid nanoparticles' volume fraction but decreased when the Hartmann number increased. [PERSON] et al. [33] examined the natural convection in a two-dimensional triangular enclosure filled with nanofluid under the influence of magnetic field and noticed that the nanoparticles are more efficient once the Raleigh number is as high as \(10^{4}\). [PERSON] and [PERSON] [34] conducted numerical investigation of natural convection in CuO-water nanofluid to understand the impact of hydromagnetic heat transfer on. They analyzed the combined impact of both high magnetic field and nanofluids in heat transfer enhancement. [PERSON] et al. [35] studied the magnetic field impact on natural convection in Cu-water nanofluid and claimed that increase in nanoparticle volume fraction and Rayleigh number increases the average Nusselt number; however, it diminishes as the Hartmann number increases. Similar finding are reported by [PERSON] and [PERSON] [36]. [PERSON] et al. [37] carried out the lattice Boltzmann method based numerical investigation of natural convection in nanofluid with nanoparticle's Brownian motion in open cavity under the influence of magnetic field.