Abstract: A metal alloy is provided which includes by weight percentage of the metal alloy, 28 to 55 percent copper, 45 to 63 percent nickel, and 4 to 10 percent iron. The metal alloy may be weight percentage 28 percent copper, 62 percent nickel, and 10 percent iron. The metal alloy may be formed into a foil which may have a thickness of 100 ?m or less or 50 ?m or less. A magnetic field instrument may include a magnetometer core body formed from one or more layers of the foil. The magnetometer core body may be a ring core or a racetrack core. The magnetic field instrument may further include a sense winding and may further include a drive winding. The magnetic field instrument may be a fluxgate magnetometer.