The present invention relates to a magnetic nanoparticle having a Curie temperature which is within a biocompatible temperature range, a method for preparing same, and a nanocomposite and a target substance detection composition comprising the magnetic nanoparticle. As the magnetic nanoparticle of the present invention has a Curie temperature within the temperature range of 0 degrees centigrade to 41 degrees centigrade, the ferromagnetic and paramagnetic properties of the magnetic nanoparticle may be controlled within a biocompatible temperature range at a temperature at which a biological control agent is not destroyed, and the temperature of the magnetic nanoparticle is adjusted to control the magnetic properties thereof such that the properties of the magnetic nanoparticle may be used only when ferromagnetic properties are required, such as in the case of signal amplification in detecting, separating, and delivering biological control agents. Accordingly, the magnetic nanoparticle of the present invention can minimize adverse effects of ferromagnetic properties thereof, and can be used in the effective detection and separation of biological control agents.