Radiofrequency oscillator incorporating: - a magnetoresistive device through which a spin-polarized electric current flows in order to generate an oscillating signal at a frequency of oscillation on an output terminal, this device comprising a stack of at least: • a first magnetic layer, referred to as the "reference layer", capable of spin-polarizing the electric current and the magnetization of which is of fixed direction; • a second magnetic layer, referred to as the "free layer", the magnetization of which may oscillate when the spin-polarized current flows therethrough; and • a non-magnetic layer, referred to as the "spacer", interposed between the two preceding layers in order to form a tunnel junction or spin valve; - a current source (4; 104) for causing a current of electrons to flow through said layers perpendicularly to the latter; and - a terminal (18; 102) for controlling the frequency or the amplitude of the oscillating signal, said control terminal acting on the intensity of the current source or on a characteristic of a generated magnetic field specifically for modifying the frequency of oscillation of the magnetoresistive device, this magnetic field being generated by a generator that is arranged with respect to the device so that the field lines of the magnetic field generated thereby pass through the free layer of the device; and - a feedback loop connected between the output terminal and the control terminal, characterized in that the feedback loop includes an amplifier provided with: - an input that is connected to the output terminal so as to amplify the portion of the oscillating signal detected at the output terminal; and - an output that is connected to the control terminal (18; 102) so as to inject, into this control terminal, the amplified portion of the oscillating signal with a phase shift with respect to the oscillating signal generated on the output terminal that is smaller than π/20 rad. - a magnetoresistive device through which a spin-polarized electric current flows in order to generate an oscillating signal at a frequency of oscillation on an output terminal, this device comprising a stack of at least: • a first magnetic layer, referred to as the "reference layer", capable of spin-polarizing the electric current and the magnetization of which is of fixed direction; • a second magnetic layer, referred to as the "free layer", the magnetization of which may oscillate when the spin-polarized current flows therethrough; and • a non-magnetic layer, referred to as the "spacer", interposed between the two preceding layers in order to form a tunnel junction or spin valve; • a first magnetic layer, referred to as the "reference layer", capable of spin-polarizing the electric current and the magnetization of which is of fixed direction; • a second magnetic layer, referred to as the "free layer", the magnetization of which may oscillate when the spin-polarized current flows therethrough; and • a non-magnetic layer, referred to as the "spacer", interposed between the two preceding layers in order to form a tunnel junction or spin valve; - a current source (4; 104) for causing a current of electrons to flow through said layers perpendicularly to the latter; and - a terminal (18; 102) for controlling the frequency or the amplitude of the oscillating signal, said control terminal acting on the intensity of the current source or on a characteristic of a generated magnetic field specifically for modifying the frequency of oscillation of the magnetoresistive device, this magnetic field being generated by a generator that is arranged with respect to the device so that the field lines of the magnetic field generated thereby pass through the free layer of the device; and - a feedback loop connected between the output terminal and the control terminal, - an input that is connected to the output terminal so as to amplify the portion of the oscillating signal detected at the output terminal; and - an output that is connected to the control terminal (18; 102) so as to inject, into this control terminal, the amplified portion of the oscillating signal with a phase shift with respect to the oscillating signal generated on the output terminal that is smaller than π/20 rad.