" This dependency can be described by the relative change in resistivity, DR/R being proportionalto cos2u. This means that when the current and magnetic moment, M, are parallel, the resistanceof the strip is greatest; when they are at a 90° angle to each other, it is smallest. Disregarding theinfluence of film thickness, the maximum relative change in resistance is a material constant. InPermalloy it is typically 2%–3%.As can be deduced from Figure 6, the characteristics of asimple strip make it less than desirable for measurementapplications. The reasons are that it exhibits a vanishingsensitivity at low fields; that the direction of the external fieldcannot be determined; and exFigure 7. To detect the directionof an external magnetic field, thecept for the region around 45°, there is nearly no signalPermalloy strip is augmented withlinearity. To make a usable device, a trick shown in Figure 7small structures made of a highlyis applied.conductive material to create aAdding small structures made of a highly conductive materialbarber pole configuration.such as aluminum to the strip creates a barber poleconfiguration. In the simplest case, barber poles are just tinyblocks of material sitting on top of the Permalloy. Their typical linear dimension in all directionsis a couple of microns, and they are arranged at an angle of <45° to the long axis of the strip.22Serving as local shorts for the Permalloy, they force the current to flow through it at that angle.Thus, an angle of 45° between current flow and magnetization, M, results when no external fieldis applied. This gives the barber pole device the desired maximum sensitivity at zero field and acorresponding linearity around the working point. Working at roughly an average value betweenmaximum and minimum resistance of a simple strip, the barber pole device reacts in anunequivocal way to changes in the external field direction.In the majority of practical applications AMR sensor chips are composed of barber pole deviceswired as Wheatstone bridges. AMR chips have significant advantages over other technologiessuch as Hall devices:They can be used at high frequencies (some MHz are no problem) and high temperatures such asthose in automotive environments (continuous service at 150°C, peak values at 190°C), andexhibit low and stable offset values. Due to their high sensitivity (~10 3 that of Hall devices) theycan be used to detect very weak fields (the present limit is ~10 nT).The anisotropic magnetoresistive effect (AMR effect) is known to be present in a whole family offerromagnetic alloys. Most of these alloys are composed of iron (Fe), nickel (Ni), and chromium(Cr), and may be binary (two components) or ternary (three components). They have in common amore or less strong anisotropy in their magnetic properties. Whenever these materials are exposedto a magnetic field during crystal formation, a preferred orientation in magnetization will result.The same happens when the materials are forced into shape, i.e., a mechanical anisotropy isimposed.Long before chip or even thin film technologies were developed, simple wires drawn of NiFe23were known to possess an orientation of their magnetization, a magnetic moment, along theirlinear axes. Interestingly, it was found that changing the orientation of the magnetic moment inthe wire caused a current passing through it to change correspondingly. The orientation could bechanged by applying an external magnetic field, and generally an increase in current (i.e., adecrease in resistance) was observed. This phenomenon was called anisotropic magnetoresistiveeffect.Today, the ferromagnetic materials can be deposited as thin films and structured into small stripsthat are typically 40 nm thick, 10 mm wide, and 100 mm long. A magnetic field is applied duringthe process. In modern device fabrication an alloy commonly called Permalloy (81% Ni, 19% Fe)turns out to be the best compromise in terms of device sensitivity, longevity, and reproducibility.Figure 6 shows the position of a Permalloy strip. As does the wire, the strip has an orientation ofthe inner magnetic moment, M0, parallel to its long axis. When an external field is applied, thetotal magnetization, M, of the strip is turned at an angle, f.In the most general case, the electrical resistance of AMR material depends on the angle, u,between the direction of the magnetization, M, and the direction of the current going through it.This dependency can be described by the relative change in resistivity, DR/R being proportionalto cos2u. This means that when the current and magnetic moment, M, are parallel, the resistanceof the strip is greatest; when they are at a 90° angle to each other, it is smallest. Disregarding theinfluence of film thickness, the maximum relative change in resistance is a material constant. InPermalloy it is typically 2%–3%.As can be deduced from Figure 6, the characteristics of a simple strip make it less than desirable24 "