Review of Giant Magnetoresistive (GMR) Materials

Abstract

Chapter 7 reviews giant magnetoresistive (GMR) materials. Magnetoresistance is defined as the property of a material to change its electrical conductivity or its inverse electrical resistance when an external magnetic field is applied to it. In 1851, William Thomson (Lord Kelvin), discovered that when pieces of iron or nickel are placed within an external magnetic field the electrical resistance increases when the current is in the same direction as the magnetic force which is aligned with the magnetic N–S vector and decreases when the current is perpendicular to the direction of the magnetic force. He noted that the magnitude of changes in conductivity or resistivity was greater in nickel than iron. This magnetoresistance effect is referred to as anisotropic magnetoresistance (AMR). Lord Kelvin was unable to reduce the electrical resistance of any metal by more than about 5%. This effect is commonly called the ordinary magnetoresistance (OMR) effect to differentiate it from the more recent discoveries of giant magnetoresistance (GMR), colossal magnetoresistance (CMR), tunnel magnetoresistance (TMR) and extraordinary magnetoresistance (EMR). Giant magnetoresistive materials generally possess alternating layers of ferromagnetic and non-magnetic but conductive layers such as iron–chromium and cobalt–copper.