Device for a rotary haptic interface comprising: - a member for interacting with the user, - an interaction member for interacting with a fluid, the apparent viscosity whereof varies according to a control stimulus, - a shaft suitable for rotating about the axis thereof, the member for interacting with the user and the member for interacting with the fluid being secured to said shaft for rotation therewith, - a fluid, the apparent viscosity whereof varies according to a control stimulus, - a chamber defined by walls and containing said fluid, said interaction element for interacting with the fluid being arranged in said chamber in contact with the fluid, - means for generating a magnetic field in the fluid, - the element for interacting with the fluid comprising at least one movable interaction wall surrounding the shaft, said interaction wall being secured to a first longitudinal end of the shaft opposite a second longitudinal end of the shaft to which the element for interacting with the user is secured, - the shaft passing through a single wall of the chamber, - a sealing means being arranged between the shaft and said wall so as to form a tight passage, - means for guiding the rotation of the shaft arranged about the shaft between the sealing means and the element for interacting with the user, - the means for generating a variable magnetic field comprising a coil arranged in the space defined inside the interaction wall of the interaction element for interacting with the fluid, an inner magnetic core arranged in the coil and an outer magnetic core arranged outside the coil, said inner and outer magnetic cores defining at least in part the chamber, the coil being arranged so as to be facing a part of the interaction wall, the outer magnetic core comprising an axial portion forming an outer peripheral wall of the chamber, characterized in that: - said interaction wall extends from said first end towards the element for interacting with the user, and in that - the outer magnetic core comprising a radial portion forming a transverse wall of the chamber. - a member for interacting with the user, - an interaction member for interacting with a fluid, the apparent viscosity whereof varies according to a control stimulus, - a shaft suitable for rotating about the axis thereof, the member for interacting with the user and the member for interacting with the fluid being secured to said shaft for rotation therewith, - a fluid, the apparent viscosity whereof varies according to a control stimulus, - a chamber defined by walls and containing said fluid, said interaction element for interacting with the fluid being arranged in said chamber in contact with the fluid, - means for generating a magnetic field in the fluid, - the element for interacting with the fluid comprising at least one movable interaction wall surrounding the shaft, said interaction wall being secured to a first longitudinal end of the shaft opposite a second longitudinal end of the shaft to which the element for interacting with the user is secured, - the shaft passing through a single wall of the chamber, - a sealing means being arranged between the shaft and said wall so as to form a tight passage, - means for guiding the rotation of the shaft arranged about the shaft between the sealing means and the element for interacting with the user, - the means for generating a variable magnetic field comprising a coil arranged in the space defined inside the interaction wall of the interaction element for interacting with the fluid, an inner magnetic core arranged in the coil and an outer magnetic core arranged outside the coil, said inner and outer magnetic cores defining at least in part the chamber, the coil being arranged so as to be facing a part of the interaction wall, the outer magnetic core comprising an axial portion forming an outer peripheral wall of the chamber, - said interaction wall extends from said first end towards the element for interacting with the user, and in that - the outer magnetic core comprising a radial portion forming a transverse wall of the chamber.