A great deal attention has been paid to GMR in granular and muti-layered structure because of their high potential to application and basic physical phenomena.
Thin-film inductive heads can be employed for both recording and reading information, but their use as readback heads is limited to disc drives with high-speed medium motion. So, new types of readout heads are required like GMR head. GMR head, which is a kind of modulating transducer, convert electrical energy(sensing current) to electrical energy(readout voltage) modulated by magnetic field from recording medium. Granular magnetic solids, consisting of single-domain ferromagnetic particles embedded in an immiscible medium have been extensively studied in recent years because of not only underlied basic theories but also their potential use in magnetic recording, optical devices and sensors. Both the high magnetoresistance and the low saturation field are very important GMR materials to be used as the readout head and the low field magnetic field sensor. Also, there are several additional parameters must be considered such as impedance level, current capacity, magnetostriction, temperature coefficient of resistivity and manufactuability. These factors are very closely related to the microstructure of GMR materials.
We have investigated deposition characteristic, structure and GMR of granular $Fe_{20}Ag_{80}$ and ($Ni_{0.8}Fe_{0.2})_{20}Ag_{80}$ thin film prepared by DC magnetron sputtering with mainly XRD, XPS, SEM, TEM and SQUID. Both the deposition rate and the sheet resistivity of $Fe_{20}Ag_{80}$ are decreased. According to the cross-sectional SEM photos of this alloy, growth mode is changed from columnar to equi-axed on increasing nitrogen flow rate. This implies that the preferred orientation is inhibited as the nitrogen partial pressure is increased. The grain size of these films is nearly unchanged till the nitrogen partial pressure is 33.3% but it become smaller above 33.3% of $N_2/ (N_2 + Ar)$. The MR ratio is 38.1% at 4.2K which is maximum value, then $N_2$ flow rate is 20 sccm.
With increasing chamber pressure, deposition rate increase and grain size increase. Phase separation into Ag and (NiFe) is started at $P_{ch}$=5mtorr. When nitrogen flow rate is 5sccm, deposition rate is maximum(7Å/sec) and radically decrease further increasing nitrogen flow rate. At $N_2$ flow rate = 10sccm, nitride is formed, which increase sheet resistivity and degrade Ag (111) texturing. As nitrogen flow rate is raised, electron binding energy(BE) of Ag $3d_{3/2}$ and $3d_{5/2}$ is shifted to high energy side and FWHM is broadend till $N_2$ flow rate = 15sccm. The same as Ag $3d_{3/2}$ and $3d_{5/2}$, BE of Ni $2p_{3/2}$ increase till $N_2$ flow rate = 15sccm and decrease further increasing $N_2$ flow rate. At 861eV in XPS spectrum, satellite peak of Ni $2p_{3/2}$ is shown, which can be assigned to nickel nitride peak. For all as-deposited $Ni_{0.8}Fe_{0.2})_{20}Ag_{80}$ thin film, MR ratio is less than 1% at room temperature but MR ratio is increased to 5.4% for annealed this film at 500℃ for 5min. With decreasing temperature, MR ratio increase and saturation field decreases for all annealed samples. This phenomenon can be considered decrease of thermal vibration energy. Resistivity change is 25% at 1.5K for annealed $Ni_{0.8}Fe_{0.2})_{20}Ag_{80}$ film at 500℃. With increasing annealing temperature, MR ratio shows maximum 5.4% for annealed at 500℃. When nitrogen incorporated in $Ni_{0.8}Fe_{0.2})_{20}Ag_{80}$ thin film about 5at%, both MR ratio and saturation fields are decreased to 2.41% and 1.5T, respectively. This can be tentatively attributed to formation of nitride. The average relative sensitivity is only slightly improved from 1.08% per T to 1.52% per T. If we examine the differential sensitivity(DS) near the zero field, a nitrogenated and subsequently annealed specimen shows a far greater sensitivity. The DS value of a non-nitrogenated specimen is almost zero while that of a nitrogenated specimen is 14.5Ω-cm / Oe.
In $Fe_xAg_{100-x}$ thin film alloys, MR ratio has a maximum value at specific atomic fraction of Fe for both as-deposited and annealed specimens at RT. Also the peak is shown when these samples are measured at 5K. In the early state of the rising composition, MR ratio increase because of increasing the interface area between ferromagnetic particles and matrix which act as effective site for spin-dependent scattering. Then further increasing, the peak appears at 20 % Fe of which value is 3.92 %. But both average and differential sensitivity appear at 25 % Fe. In the case of annealed samples, the same result is obtained.
If the measuring temperature is lowered to 5K, the maximum peak is shifted to 15% Fe and maximum value reach 15%. Lowering temperature has an effect in containing the phonon and magnon scattering, which enhance the effective spin-dependent scattering. Also, the saturation field is reduced to much lower value that is due to decreasing the thermal energy. When the magnetic field is applied to perpendicular to the film surface, the MR ratio lowers to the half of the value of applying field parallel to film surface. It is due to reduction of scattering site and shape anisotropy.
Nitrogen affects the GMR property of $Fe_xAg_{100-x}$. It reduces the MR ratio but doesn't change the saturation field. Also, it increase the Fe atomic fraction because the MN/MFe ＞ MN/MAg. When the PN2 is above 55.5%, the oxide peak start appearing. This oxide may be closely related to the deviation from ordinary GMR behavior of nitrogenated FexAg100-x.
Like single layer $Fe_xAg_{100-x}$, tri-layered the $\fracFe_xAg_{100-x}/Ag/Fe_xAg_{100-x}$ has maximum MR value at specific atomic fraction of Fe. When the $\fracFe_xAg_{100-x}/Ag/Fe_xAg_{100-x}$ is prepared, MR ratio increase to higher value than that of single layer and saturation field is lowered. MR value increase because of enhancing spin-dependent scattering at the interface $Fe_xAg_{100-x}/Ag$ and inside $Fe_xAg_{100-x}$. When the samples are annealed, the magnetic property transforms from ferromagnetic to anti-ferromagnetic. It is due to thinning of Ag layer thickness.
