Magnetoresistive effect

As magnetoresistive effects is any effects that the change in the electrical resistance of a material by applying an external magnetic field described. These include in particular the anisotropic magnetoresistive effect ( AMR effect ), the giant magnetoresistance ( GMR effect ), the CMR effect , the TMR effect and the planar Hall effect .

A distinction is also made between magnetoresistive effects in:

1. non-magnetic materials (Hall effect)
2. magnetic materials (e.g. AMR effect)
3. hybrid components made of non-magnetic and magnetic materials (e.g. GMR, TMR effect).

Lord Kelvin (William Thomson) described the first magnetoresistive effects in 1856. This is an increase in resistance in a conductor when a magnetic field is applied by deflecting the current paths, a galvanomagnetic effect (known as the Thomson effect, less often as the Gauss effect, where the Thomson -Effect should not be confused with the thermoelectric Thomson effect ).

Explanation

The magnetic behavior of solids is determined by the type and strength of the elementary magnets and their interaction with one another, i. H. characterized by cooperative effects and the associated behavior of the moving charges in the solid.

To describe the strength of the respective magnetoresistive effect, one uses the quotient of the change in resistance and resistance without an external field:

${\ displaystyle \ Delta R / R \ [\%] = {\ frac {R (H) -R (0)} {R (0)}} \ cdot 100}$

${\ displaystyle R (H)}$is the resistance as a function of the magnetic field, the resistance without an external field and the characteristic value of the magnetoresistive effect. ${\ displaystyle R (0)}$${\ displaystyle \ Delta R / R}$

In the case of GMR, CMR, TMR, however, the saturation value of the resistance is also often taken as the base value in the standardization.

Magnetoresistance effects change the resistance of magnetic or non-magnetic materials and can be both positive and negative, depending on whether the resistance in the magnetic field increases or decreases.

Magnetoresistive effects in ferromagnetic materials and hybrid structures

Thin layers (in the nanometer range) of ferromagnetic transition metals have an AMR, GMR, CMR or TMR effect of around 3% for AMR, around 50% for GMR (room temperature, magnetic fields up to 2 T), up to 600% for TMR even greater effects in CMR (Colossal Magneto Resistance Effect in Perovskites , change in resistance by a factor of over 1000).

See in particular the articles AMR effect and GMR effect .

With the exception of the TMR effect, they are based on the fact that an external magnetic field influences the direction of magnetization in ferromagnetic materials and the angle between magnetization and current direction has an impact on electrical resistance. With the TMR (tunnel magneto-resistance effect), which was discovered in 1975 and came into the focus of research in the 1990s, the effect is based on spin-dependent tunnel currents of ferromagnetic materials that are separated by insulator layers.

See in particular the article TMR effect .

Planar Hall effect

The planar Hall effect (also pseudo Hall effect or extraordinary Hall effect) has an effect similar to the ordinary Hall effect , but has different causes and is not caused by the magnetic field components perpendicular to the line plane (a geometry similar to caused by the Hall effect), but rather by the magnetic field components in the plane of the conductor. It occurs in ferromagnetic materials and is based on the AMR effect (the anisotropic magnetoresistive effect ): the resistance perpendicular to the direction of the applied magnetic field differs from the resistance parallel to this field. This effect is of secondary importance in applications, e.g. B. Applications in biotechnology were discussed.

history

The influence of an externally applied magnetic field on the electrical behavior of a solid (e.g. its electrical resistance ) varies greatly, depending on which of the above-mentioned effects is at work. Although magnetoresistive effects (especially the AMR effect ) were already known in the middle of the 19th century (discovery of the AMR effect in 1857 by William Thomson, 1st Baron Kelvin ), technical use only became possible towards the end of the sixties, e.g. in the Sensor technology (AMR sensor) only at the beginning of the eighties.

Strength of effects

Comparison of MR effect sizes of the different mechanisms of action.

• AMR :${\ displaystyle \ Delta R / R = 3 \ ldots 4 \%}$
• TMR :${\ displaystyle \ Delta R / R \ leq 600 \%}$
• GMR :${\ displaystyle \ Delta R / R = 6 \ ldots 100 \%}$