Abstract: A magnetic field sensor in the form of a multi-material optical fiber is described. The magnetic sensing optical fiber of the present disclosure can leverage optics and magnetostriction to sense an external magnetic field adjacent to the fiber. The magnetic sensing optical fiber can be customized to achieve various desired sensing sensitivities for various applications, including measuring while drilling and unconventional oil and gas applications. In one example, an optical fiber can include a cladding that can extend from a first end to a second end of the optical fiber. The optical fiber can further include an optical core within the cladding. The optical core can extend along the optical fiber between the first end and the second end. The optical fiber can also include a magnetostrictive element within the cladding. The magnetostrictive element can extend along the optical fiber between the first end and the second end.