*1.1.3.5 Surface plasmon resonance (SPR)*

Because of its light guidance inside fiber based on the TIR effect, simple and flexible design, compactness, and remote sensing ability, optical fiber has proven to be a very useful instrument in the SPR sensing system. When light travels through the core of the fiber, some of it will pass through the cladding region. The plasmonic metal surface interacts with the evanescently leaked light, which stimulates the surface's free electrons. Surface plasmon is created when the evanescent field and free electron

**Figure 7.** *Fiber optic gas sensing set-up using evanescent wave absorption based on experiment [55].*

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

surface frequencies are in resonance. This generated wave propagates along the metaldielectric interface and exhibits both charge motion in the metal (surface plasmon) and electromagnetic wave in the air or dielectric (polariton) [54].

#### *1.1.3.6 Evanescent wave absorption type (EWA)*

The leakage/loss of electromagnetic energy at the interface of the core and the cladding medium during a total internal reflection (TIR) event determines the evanescent wave absorption based sensing phenomenon. If the diameters of a fiber are big in comparison to the wavelength of light, this method may be applied. When a propagating beam's incident angle (θi) is greater than the critical angle (θc), TIR causes the ray to reflect back from the core-cladding contact. A small fraction of the guided wave's energy enters the cladding medium at each TIR event, generating an electromagnetic field known as a "evanescent wave." The evanescent wave is greatly affected by variations in refractive index at the core-cladding contact, which provides a foundation for sensors based on evanescent wave absorption as shown in **Figure 7** [55, 56].

### **2. Materials used in gas sensing**

### **2.1 Spin frustrated Multiferroics: A novel frustrated ordering studied in gas sensors**

Spin frustration in ME multiferroics have received enormous attention ever since the concept was introduced in 1977 by Gerard Toulouse. It is an important feature in the field of magnetism and multiferroics, as it stems from the relative arrangement of spins. Interestingly, it is not the strength of ME coupling or the high polarization that makes these materials unique; in fact, such systems have weak coupling and low magnitude of polarization compared to other multiferroics [57–60]. Still, the reason for its long sought control of electric properties by magnetic fields, lies in the magnetic origin of their ferroelectricity, which is induced by the presence of complex spin structures, characteristic of frustrated magnets [61]. Basically, in most of the magnetic solids, the magnetic moments form a long range ordered ferromagnetic (**↑↑** moments) or antiferromagnetic (**↑↓** moments) structure when cooled below a certain temperature. However, in "frustrated" magnetic solids, there is no long range ordering at low temperature phase. Instead, they develop high degenerate states with short range spin correlations. Typical examples of geometrically frustrated structures are 2D triangular or tetrahedron non collinear magnetic structures, where the lattice consists of triangular or tetrahedral arrangements of anti-ferromagnetically (AFM) coupled

spins as shown in **Figure 8**. The spin on the third site can be in any one direction, either up (or) down, but then the interaction between this site and the other two sites will be different. As a consequence, the third site will move closer to one site and away from the other, which will break the symmetry and induce ferroelectricity. This kind of lattice geometry creates large degeneracy of ground states within which the system can fluctuate with almost no energy expenditure, even below few milli Kelvin temperatures. A one dimensional anti-ferromagnetic material has ground state in alternating series of spins: up, down, up, down, etc. But, in 2D equilateral triangular AFM lattice, multiple ground states can occur, with three spins, one on each vertex. If each spin can take on only two values (up or down), there are 2<sup>3</sup> = 8 possible states of the system, six of which are ground states. Two situations, which does not favor ground states are when all three spins are up or are all down. However, in other six states, there will be two favorable interactions and one unfavorable one. This illustrates spin frustration: the inability of the magnetic system to find a single ground state [58, 60].

*2.1.1 Factors to consider for sensor interrogation of spin frustrated multiferroic materials*

The main factors that are critical in gas detection, particularly when employing multiferroic materials as sensing materials are

