Organizing nanometer-scale magnets with bacterial threads

Macroscopic magnetic bacterial threads are formed in which 10 nm Fe₃O₄ particles are intercalated between cell walls. A mutant strain of bacteria is used which forms long filaments of joined cells. Using a drawing technique these strands can be bound together to form a solid thread which can be used as superstructure to template magnetic particles. Dipping a thread into a ferrofluid solution allows the thread to rehydrate and draw the magnetite particles between the filaments. Cross-sectional images of the redrawn threads show the strands of cells form an ordered structure along the length of the fiber and reveal the magnetic particles embedded between the cell walls. The composite bacterial thread is superparamagnetic with a blocking temperature T_B~175 K. Below this temperature the magnetization displays a field-dependent hysteresis indicating anisotropic behavior