1. A method for preparing a magnetic iron oxide-graphene composite comprising the steps of:mixing graphite and a halogen salt of iron and heat-treating the same (Step 1);
reacting the product of step 1 with a (C1-20 alkyl) amine (Step 2);
washing the product of step 2 (Step 3);
subjecting the product of step 3 to a high-speed homogenization (Step 4); and
subjecting the product of step 4 to sonication or a high-pressure homogenization (Step 5),
wherein the high-speed homogenization of step 4 is carried out by stirring the product of Step 2 at 5000 to 8000 rpm,
wherein the high-pressure homogenization of step 5 is carried out by passing the product of step 4 through a high-pressure homogenizer including an inlet, an outlet, and a micro-channel that connects between the inlet and the outlet and has a diameter in a micrometer scale,
wherein the product of step 4 is introduced to the inlet of the high-pressure homogenizer under application of a pressure of 500 to 3000 bar and passes through the micro-channel.