Quantum Interference Devices: SQUID, GMR

QUANTUM INTERFERENCE DEVICES

SQUID

SQUID stands for Superconducting QUantum Interference Device. It is an ultra-sensitive instrument used to measure very weak magnetic field of the order of 10-14 tesla.

Principle

We know that a small change in magnetic field produces variation in the quantum flux.

Description and Working

A SQUID consists of a superconducting ring which can have magnetic fields of quantum values (1, 2, 3...) of flux placed inbetween two Josephson junctions as shown in fig. 2.32.

When the magnetic field is applied perpendicular to the plane of the ring, the current is induced at the two Josephson junctions. The induced current produces the interference pattern and it flows around the ring so that the magnetic flux in the ring can have the quantum value of magnetic field applied.

Application

(i) SQUID can be used to detect the variation of very minute magnetic signals in terms of quantum flux.

(ii) It can also be used as storage device for magnetic flux. 

(iii) SQUID is useful in the study of earthquakes, removing paramagnetic impurities, detection of magnetic signals from the brain, heart etc.

Quantum Interference Transistor (QUIT):

Electrons are made to propagate through two arms of the quantum wire ring as shown in the fig. 2.33.

Suppose an electron wave enters the ring from left to right. The wave entering through "A" gets split up into two partial waves. A constructive interference can be expected to occur at "B" similar to the optical anlogue as they travel through the same distance.

The constructive interference at the output of the device reduces the resistance of the ring. Various methods of introducing a phase difference of л between the two waves have been suggested. This leads to destructive interference which in turn will increase the resistance by reducing the current.

An external voltage can control the nature of interference and the current. This device is expected to act as a high-speed transistor. 

Magneto resistance

Some metallic materials show a large change in resistance on the application of a magnetic field. This effect is called magneto resistance (MR).

Giant Magneto Resistance (GMR)

It is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers.

Definition

The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment.

The overall resistance is resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be be controlled, for example, by applying an external magnetic field.

The effect is based on the dependence of electron scattering on the spin orientation.

The main application of GMR is magnetic field sensors, which are used to read data in data in a hard disk drives, biosensors, microelectromechanical systems (MEMS) and other devices. GMR multilayer structure are also used in magnetoresistive random-access memory (MRAM) as cells that store one bit of information.

Explanation

The GMR is seen in structures which have normal metal and ferromagnetic layers alternatively. The electrical conductivity depends on the relative orientation of magnetization in the successive ferromagnetic layers in the stack.

When the relative magnetizations of the layers are switched from parallel (to the plane of the layers) to antiparallel states, high and low resistivities are obtained in the structure. This corresponds to 0 and 1 states in data storage format.

Two geometries are commonly used in GMR studies and are as shown in fig. 2.34.

(a) Current in Plane (CIP) of layers and

(b) Current Perpendicular to Plane (CPP) of layers. 

Since the layers are only a few nanometers thick, the CIP mode offers high resistance to the small cross sectional area encountered by the electrons.

To alter the resistivity by controlling the spin-dependent scattering, the lateral dimensions of the structure must be small when compared with the electron mean free path.