**4. Eddy currents challenge: role of ferrite materials in NFC**

As NFC technology relies on generated mutual inductance between the transmit and receive coil antennas, the amount of magnetic flux between them should be maximized to induce more current, thus increasing data transmission range. However, by placing a NFC tag on a metal surface, the efficiency of data transmission is greatly suppressed due to the generation of eddy currents within the metal surface [39]. To increase the efficiency and range while minimizing the losses due to eddy currents, a soft magnetic ferrite sheet can be inserted between the metal case and the antenna. **Figure 7(a)** illustrates how the magnetic field generates in NFC communication due to nearby conductive surface/plate, and **Figure 7(b)** shows the magnetic field produced when NFC area is shielded by a ferrite sheet from a conductive surface [40]. One can see the shielding effects due to insertion of ferrite sheet [40]. To be amenable to NFC, the ferrite sheets should have high permeability and low magnetic loss to concentrate the magnetic flux generated between the transmit and receive coils (antennas) [39, 41]. Ni-Zn ferrites and Mn-Zn ferrites are the most widely used soft magnetic materials for the preparation of these NFC ferrite sheets at high frequencies. Ni-Zn ferrites have exhibited higher operating frequency range up to 100 MHz as compared to Mn-Zn ferrites (a few MHz), which limits the use of Mn-Zn ferrites in NFC devices. Mn-Zn ferrites have been used in mini dc-dc converters, inductors and power inductors due to their high saturation induction and low losses [42]. For NFC applications, Ni-Zn ferrites offer better suited properties because of its high resistivity, high permeability, low magnetic loss, high operation frequency, and chemical stability. In particular, the high

permeability and low magnetic loss of Ni-Zn ferrite sheets help to concentrate more

To enhance the performance of NFC systems, the employment of high permeability and low loss magnetic sheets is highly desirable. The magnetic permeability and loss properties of Ni-Zn ferrites can be tailored by making strategic changes in crystallography, morphology and microstructure of the material. The relation between the complex relative permeability and frequency is termed as permeability dispersion. The frequency dependent relative permeability is given by Eq. (16) [44],

where *μ<sup>r</sup>* is the ratio of the permeability of the material versus that of the free space (*μ*0). *μ*<sup>0</sup> and *μ*<sup>00</sup> are real and imaginary parts of the relative permeability, respectively. The magnetic loss tangent is the ratio between the real and imaginary parts

*tan <sup>δ</sup><sup>m</sup>* <sup>¼</sup> *<sup>μ</sup>*<sup>00</sup>

**Figure 8** shows a typical relative permeability spectra of flexible ferrite sheet [45]. To achieve high signal transmission efficiency between NFC devices and

than 100 and the loss tangent ( *tan δm*) should be less than 0.05 at the standardized NFC operation frequency of 13.56 MHz [39]. Both *μ*<sup>0</sup> and *μ*<sup>00</sup> of ferrite materials are greatly affected by the composition, microstructure and morphology, which are also quite sensitive to the processing parameters [46]. The key to achieve high performance ferrites for the targeted NFC applications is to tailor and optimize their synthesis process parameters. There are several different ways to synthesize Ni-Zn ferrites such as sol–gel method, citrate precursor method, hydrothermal

NFC devices can communicate in either one of the two modes: active and passive mode. These modes determine how two NFC-enabled devices talk to each

increase the range of transmission, the relative permeability (*μ*<sup>0</sup>

synthesis and solid-state synthesis methods [42].

**5. NFC modes of communication**

**103**

*μ<sup>r</sup>* ¼ *μ*<sup>0</sup> � *jμ*<sup>00</sup> (16)

*<sup>μ</sup>*<sup>0</sup> (17)

) should be greater

magnetic flux and reduce eddy currents via magnetic shielding [42, 43].

*Near-Field Communications (NFC) for Wireless Power Transfer (WPT): An Overview*

*DOI: http://dx.doi.org/10.5772/intechopen.96345*

*A typical representation of relative permeability versus frequency of a ferrite sheet [45].*

given by Eq. (17):

**Figure 8.**

**Figure 7.**

*(a) Eddy currents generated in NFC communication area due to conductive plate in vicinity; (b) magnetic field generated in NFC communication area with an incorporated ferrite sheet shielding [40].*

*Near-Field Communications (NFC) for Wireless Power Transfer (WPT): An Overview DOI: http://dx.doi.org/10.5772/intechopen.96345*

**Figure 8.** *A typical representation of relative permeability versus frequency of a ferrite sheet [45].*

permeability and low magnetic loss of Ni-Zn ferrite sheets help to concentrate more magnetic flux and reduce eddy currents via magnetic shielding [42, 43].

To enhance the performance of NFC systems, the employment of high permeability and low loss magnetic sheets is highly desirable. The magnetic permeability and loss properties of Ni-Zn ferrites can be tailored by making strategic changes in crystallography, morphology and microstructure of the material. The relation between the complex relative permeability and frequency is termed as permeability dispersion. The frequency dependent relative permeability is given by Eq. (16) [44],

$$
\mu\_r = \mu' - j\mu'' \tag{16}
$$

where *μ<sup>r</sup>* is the ratio of the permeability of the material versus that of the free space (*μ*0). *μ*<sup>0</sup> and *μ*<sup>00</sup> are real and imaginary parts of the relative permeability, respectively.

The magnetic loss tangent is the ratio between the real and imaginary parts given by Eq. (17):

$$
\tan \delta\_m = \frac{\mu''}{\mu'} \tag{17}
$$

**Figure 8** shows a typical relative permeability spectra of flexible ferrite sheet [45]. To achieve high signal transmission efficiency between NFC devices and increase the range of transmission, the relative permeability (*μ*<sup>0</sup> ) should be greater than 100 and the loss tangent ( *tan δm*) should be less than 0.05 at the standardized NFC operation frequency of 13.56 MHz [39]. Both *μ*<sup>0</sup> and *μ*<sup>00</sup> of ferrite materials are greatly affected by the composition, microstructure and morphology, which are also quite sensitive to the processing parameters [46]. The key to achieve high performance ferrites for the targeted NFC applications is to tailor and optimize their synthesis process parameters. There are several different ways to synthesize Ni-Zn ferrites such as sol–gel method, citrate precursor method, hydrothermal synthesis and solid-state synthesis methods [42].

## **5. NFC modes of communication**

NFC devices can communicate in either one of the two modes: active and passive mode. These modes determine how two NFC-enabled devices talk to each other. The distinction between modes depends on whether, a device generates its own RF field or used power from another device. In communication, initiator is the device that starts the communication, and target is the device that receives the signal from initiator. The main differences between main properties of passive technologies (NFC, Chipless RFID and UHF RFID) and active technologies (Bluetooth and Zigbee) are summarized in **Table 2** [47–49].

is sent between two devices using amplitude shift keying (ASK) i.e., the base RF field signal (13.56 MHz) is modulated with data using coding schemes (Miller and Manchester Coding). Data transfer rates are higher in this mode and it can work

*Near-Field Communications (NFC) for Wireless Power Transfer (WPT): An Overview*

In passive mode, the initiator sends the RF field to power the target. In turn, target used the RF field and sends back the stored data via a process called load modulation (Manchester coding) [52]. It is the most common mode for NFC, as it

Three different combinations of communications are possible when two NFC device communicates with each other wirelessly, active-active, active-passive and

While working in active and passive modes, the NFC devices perform different operation during communication. This means NFC device 1(initiator) must send signal first to NFC device 2 (target) to get the response back from device 2 (target).

It is not possible for NFC device 2 (Target) to send data to device 1 without receiving any initial signal. All the possible interaction styles of NFC devices are

These are three main modes, under which a NFC device can operate [14]:

Active Active The RF field is generated by both devices Active Passive The RF field is generated by device 1 only Passive Active The RF field is generated by device 2 only

**Initiator device Target device** NFC mobile NFC tag NFC mobile NFC mobile NFC reader NFC mobile

**Device 1 Device 2 RF field generation**

*Various possible communication arrangements between two NFC devices [53].*

*Various possible interaction styles of NFC devices [14].*

According to the NFC forum's device requirement, a device must have the functionality i.e., device needs to operate in reader/writer mode and in peer mode in order to be NFC-compliant [54]. i.e., a device must behave as an initiator during passive communication and an initiator or target during active communication. Initially, the NFC operating frequency of 13.56 MHz was unregulated. In 2004, NFC forum was established to standardize the tags and their operating protocols. There were three tasks standardized by the NFC Forum: including transferring power from a NFC device to a NFC tag, sending information from a NFC device to a NFC tag via signal modulation, and sensing the modulation by the load created on the NFC tag while performing load modulation to receive information from a NFC device. These three operation modes were designated by the NFC forum as reader/ writer, peer-to-peer, and card emulation communications, as depicted in **Figure 9**.

well at longer distances [50, 51].

*DOI: http://dx.doi.org/10.5772/intechopen.96345*

requires no battery and it is less expensive [53].

passive-active. These are listed in **Table 3** [53].

**5.2 Passive mode**

listed in **Table 4** [14].

**Table 3.**

**Table 4.**

**105**
