The device uses a permanent magnet to produce a static magnetic field (B0) and includes a device for creating a radio frequency (RF) magnetic field (B1) in two half- spaces which each have a common zone with the permanent magnet. The force lines of the RF magnetic field in the two common zones are in opposite directions The static magnetic field is cylindrical with a plane of symmetry (60) and the two half-spaces are bounded by the plane of symmetry. The static magnetic field is North-South (22) in a direction perpendicular to the axis of the cylinder (23) and the RF magnetic field force lines are in opposite directions perpendicular to the North-South direction of the permanent magnet. An antenna (24) has two connection terminals suitable for one to connect to a generator producing controlled radio-frequency electrical current and the other to a receiver to analyze the electrical signals delivered by the antenna to the terminals. This antenna is made of at least one closed loop in a plane parallel to the North-South direction and surrounding the permanent magnet. The terminals are formed of two points (28, 29) in the loop and cut the loop into two equal parts. The permanent magnet is cylindrical and the two points in the loop cut the axis of revolution of the permanent magnet. The two branches of the closed loop defined by the terminals have a variable electrical impedance. The RF magnetic field can be provided by a magnetic coil made of a number of parallel closed loops as described, with the first and second terminals of each loop electrically connected to a first and a second assembly respectively. Alternatively, the RF magnetic field can be provided by a number of half-loops as described.
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