High-quality NiFe thin films on oxide/non-oxide platforms via pulsed laser deposition at room temperature

Soft ferromagnetic NiFe thin films are promising for applications in spintronic devices because of their constituent electrical and magnetic properties. Electron beam evaporation and sputtering techniques have been used to deposit NiFe thin films. For in-situ stacking of NiFe with functional complex oxides, the pulsed laser deposition (PLD) method is highly desirable. However, the growth of high-quality NiFe (and non-oxide thin films in general) by PLD remains a formidable task. Here, we report high-quality NiFe thin films of various thicknesses on oxide/non-oxide substrates with desirable magnetic properties by PLD at room temperature. The magnetic properties are found to be strongly dependent on the laser fluence of the deposition process. The laser fluence of 4 Joule/cm$^2$ produces the highest magnetization of ~547 emu/cc. The small coercivity (few Oersted) and sharp ferromagnetic switching behaviour indicate uniaxial anisotropy with an easy axis along the in-plane direction. In addition, thickness-dependent magnetodynamics characterizations are studied via ferromagnetic resonance. Our findings indicate the ferromagnetic characteristics are sensitive to the quality of the oxide/non-oxide substrate surface. These results offer significant insight into the PLD-based development of thin metal magnetic films.


Introduction
3][4] The essential features of thin permalloy films that anisotropy fields of a few Oe and significant saturation magnetization were obtained. 4Due to the attractive properties of thin NiFe films, it has been used as a constituent of many thin-film devices, such as free layers of giant magnetoresistance, 5 spin injection and accumulation, 6 and ferromagnetic resonance experiment 7 in spintronics devices.
0][11] The anisotropy field can be controlled in various ways, such as deposition in a moderate magnetic field (~100 Oe), 12 substrates, 10 and oblique deposition. 13,14 n the last decades, pulsed laser deposition (PLD) has emerged as one of the most popular and intrinsically simple techniques for depositing a wide range of exciting functional materials.The PLD technique is often used to deposit multi-component oxide films. 15,16 epositions of NiFe by PLD, however, are surprisingly sparsely reported in the literature.Randolph et al. described the PLD growth of NiFe/Ag superlattices. 17However, they showed that the NiFe films change from ferromagnetic to non-ferromagnetic at thicknesses less than ~20 nm.
In this work, we demonstrate the high-quality growth of soft ferromagnetic NiFe thin films on various oxide/non-oxide substrates by PLD at room temperature.Using a stoichiometric target with the chemical formula Ni80Fe20, thin films were deposited in high vacuum at room temperature.Merits of this approach for growing oxide and non-oxide thin films are discussed.Atomic Force Microscopy (AFM) investigations show clean and flat surfaces.The static magnetic measurement indicates the ferromagnetic nature of the thin films.In addition, ferromagnetic resonance (FMR) measurement shows the dynamic properties and determines the damping parameters with respect to the thickness of the thin films and substrates.

Experimental details
High-quality NiFe thin films were grown by PLD at room temperature and at a high vacuum base pressure of ~6×10 -8 mbar to avoid any in-situ oxide formation during the deposition.The NiFe target was ablated by a KrF excimer laser beam (248 nm, 30 ns FWHM) operating at a repetition rate of 5 Hz.The laser fluence (FL) for each deposition was controlled from low power of 1.0 J/cm 2 to a maximum possible high power of 4.0 J/cm 2 .The NiFe thin films with various thicknesses (t) were deposited at pressures <4×10 -7 mbar (high Vacuum) at room temperature on various oxide/non-oxide substrates, namely, HF-treated-SrTiO3 (HF-STO) (001), KTaO3 (KTO) (001), pristine-SrTiO3 (001) (STO), LaAlO3 (LAO) (001), silicon (001) and Quartz.The surface roughness of the former four kinds of oxide substrates is all around 120-300 pm, whereas it is about 800-900 pm for the two kinds of non-oxide substrates.The film thickness was controlled by the number of laser pulses during deposition.The substrates were cleaned with acetone and attached to the holder 7 cm away from the target inside the main vacuum chamber.A 2 nm Al2O3 oxide capping layer was grown in-situ to prevent ex-situ oxidation of the NiFe films.The surface morphology of the films was characterized by Atomic Force Microscopy (AFM).Magnetic characterization was carried out using Magnetic Property Measuring System (MPMS) System fitted with a Superconducting Quantum Interference Device (SQUID) attachment by applying a magnetic field along both in-plane (IP) and out-of-plane (OOP) directions at room temperature.
The ferromagnetic resonance (FMR) measurements were carried out by NanOsc Instruments Cryo ferromagnetic resonance (FMR) in Quantum Design Physical Property Measurement System (PPMS) at 300 K. Frequencies of 2-40 GHz were swept while applying different ranges of magnetic fields.

Results and discussion
Figure 1(a) shows the degree of unevenness of the film surface (morphology) in a scan area of 2×2 μm 2 of the NiFe films grown on silicon at FL=4.0 J/cm 2 with t=12 nm.The roughness of the film surface is ~0.263 nm. Figure 1(b) shows the room-temperature resistivity ρ of the obtained structures as a function of the FL used for the NiFe growth.For samples grown at low FL (< 3 J/cm 2 ), the resistivity ρ is beyond the measurement limit of our PPMS transport measurement tool.
Whereas the thin films are observed to be conducting above a certain high FL fluence threshold (≥3 J/cm 2 ).Laser fluence (FL) plays a critical role in the growth of high-quality NiFe films.The resistivity ρ of thin films on silicon as a function of thickness for FL = 3 and 4 J/cm 2 is shown in Fig. 1(c).It is noteworthy that below 10 nm of film's thickness (t), the resistivity increases by nearly two orders of magnitude while the behaviour of the resistivity remains almost constant (around 10 μΩcm) for thicker films.In addition, the resistivity ρ of NiFe is inversely related to the Fluence FL.This suggests that high FL is essential for the growth of high-quality NiFe.Moreover, we deposited NiFe films on various substrates at room temperature, and the trend of the resistivity depends on the quality of the substrate surface (Fig. 1(d)).As can be shown, NiFe films grown on HF-STO and KTO exhibit the lowest resistivities, suggesting that the substrate surface influences the NiFe film growth.The magnetization hysteresis loops (M-H) along IP and OOP at room temperature of 20 nm NiFe films grown on silicon at a laser fluence FL of 4 J/cm 2 .We observed a sharp magnetic reversal with a strong magnetization (M) and small coercivity, as expected in all NiFe thin films except below 6 nm.NiFe films with t < 6 nm show weak hysteresis loops (inset of Fig. 2  For NiFe films grown at FL = 4 J/cm 2 , the Ms value was gradually varied from 9.6 emu/cc to 547.5 emu/cc with an increasing t ranging from 3 to 20 nm.For NiFe films grown at FL = 3 J/cm 2 , the Ms value was gradually varied from 8.9 emu/cc to 270.1 emu/cc with an increasing t ranging from 2.4 to 12 nm.9][20][21] The magnetic dead layer, which can be deduced by extrapolating Ms-t relation to Ms → 0, is ~2 nm and ~3 nm for FL=3 J/cm 2 and 4 J/cm 2 , respectively.In addition, substrate-dependent Ms of 20 nm thin NiFe films is also observed, as shown in Fig. 2(d).The Ms values vary minorly with different substrates, which indicates the growth is precise.The NiFe films on the LAO substrate exhibit the weakest magnetism with Ms of ~450.5 emu/cc.The difference between each NiFe thin film on various substrates could be due to the different surface roughness.Room temperature Ms values for 20 nm NiFe films have been reported in literature to be about 800 emu/cm 3 and 546 emu/cm 3 in single and bilayer structures, respectively, deposited by sputtering. 12,18,21,22The reduced magnetization value may be due to the oxidation and magnetic dead layer effects of the NiFe films, and this fact almost exists in many soft and hard magnetic materials. 23Our NiFe films exhibit almost similar magnetization with low coercivities compared to those grown by other deposition techniques.The small coercivity and sharp switching behaviour indicate uniaxial anisotropy with an easy axis along the IP direction.Moreover, the NiFe films grown at higher FL show higher magnetization.Their Ms values are higher and Hc values are lower comparing to NiFe films grown at lower FL with same t.To find out the dynamic and damping characteristics at room temperature in the NiFe films, magnetodynamics measurements are performed using the FMR technique.FMR experimental data for NiFe thin film deposited at FL = 4 J/cm 2 with t = 20 nm is presented as an example in Figure 3. Figure 3(a) displays the FMR spectra in the wide frequency (f) range from 2 to 40 GHz as a function of the external magnetic field (H).The circle symbols give the experimental data, while the solid lines show the fits to the derivatives of a Lorentzian function. 24From the spectra, the resonance field (Hres) and the linewidth (∆H) can be extracted by determining the peak-to-peak (inset of Fig. 3(a)). 25The extracted ∆H as a function of f is summarized in Figure 3(b).The Gilbert damping could be obtained from the linearly fitted curves (red lines) based on the following equation: (c) The resonance frequency vs the resonance magnetic field (H res ) at 300 K.The red line is the fitting result using the Kittel formula (eq.2).
in which γ is the geomagnetic ratio, and ΔH0 is related to the inhomogeneous properties of the NiFe films.The Gilbert damping () at 300 K is estimated to be 0.01059.We plot the field dependence of the extracted resonance frequency in Fig. 3(c).It is notable that the Hres shifts toward higher fields as f increases, which is consistent with the Kittel model, 26 where Meff is the effective magnetization which contains the saturation magnetization and other anisotropy contributions.As the fitted line shown in Fig. 3(c), the 4πMeff for 20 nm NiFe films is obtained to be ~7.30kG at 300 K.The extracted Gilbert damping α as a function of the NiFe thicknesses (t) is summarized in Fig. 4(a).As t increases, the Gilbert damping α decreases as expected for magnetic metallic samples, which indicates a surface/interface-enhanced damping for thin NiFe films. 27As reported, analyzing the damping as a function of 1/t can separate the damping contributions due to the bulk and the surface/interface, as shown in the inset of Fig. 4(a). 28As predicted, it follows the equation Here, the b α and s α represent the bulk and surface damping, respectively.The best-fitted parameters for b α and s α are 0.0099 ± 0.0004 and 0.0409 ± 0.0035 nm.The α of NiFe film grown at 4 J/cm 2 is higher than that of at 3 J/cm 2 .For example, it is 0.01494 and 0.01299 for t = 12 nm NiFe film grown at 3 J/cm 2 and 4 J/cm 2 , respectively.The thickness dependences of the 4πMeff for NiFe films are shown in Fig. 4(b).The 4πMeff are obtained to be ~ 5.14 kG to 8.34 kG for t from 5.5 nm to 30 nm.In addition, substrate-dependent Gilbert damping α (left axis) and effective magnetization 4πMeff (right axis) of thin NiFe films are depicted in Fig. 4(c).The NiFe film on LAO presents the largest  and lowest 4πMeff values.

Conclusion
In

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 1 .
Figure 1.Growth and characterization of NiFe thin films.(a) Surface morphology investigations of the NiFe film grown on silicon at a laser fluence (F L ) 4 J/cm 2 with t = 12 nm (scan scale of 2 × 2 µm 2 ).(b) Room temperature resistivity ρ of 20 nm NiFe films grown on silicon with different F L .(c) Experimental thicknessdependent resistivity of thin NiFe films grown on silicon.(d) Experimental substrate-dependent resistivity of 20 nm thin NiFe films.
(b)), indicating the dead layer of the films.For the t > 6 nm sample, the saturation magnetization Ms is reached in only a few Oe. Figure 2(b) shows the thickness-dependent behaviors of the coercive field (Hc) along the IP direction.The Hc decreases from 129.8 Oe to 3.7 Oe in FL = 3 J/cm 2 samples and decreases from 87.5 Oe to 1.8 Oe in FL = 4 J/cm 2 samples (seen in Fig. 2(b)).The IP saturation magnetization (Ms) values as a function of the thickness of the NiFe films are shown in Fig. 2(c).

Figure 2 .
Figure 2. Magnetic properties of the NiFe films.(a) In-plane (black square) and out-of-plane (red circle) magnetic hysteresis loops at 300 K for 20 nm NiFe films grown on silicon at a laser fluence of 4 J/cm 2 .Evolution of (b) the coercive field (H c ) values and (c) the saturation magnetization (M s ) as a function of the NiFe film thickness grown on silicon.The inset in (b) shows the in-plane magnetic hysteresis loop for 4 nm NiFe film.(d) The substrate-dependent saturation magnetization (M s ) of thin NiFe films.

Figure 3 .
Figure 3. FMR data of a 20 nm NiFe thin film grow on silicon measured at 300 K with the magnetic field applied in the plane of the NiFe thin film.(a) The measured FMR signals at different frequencies.The solid lines are the fittings.Inset shows a single FMR spectrum measured at 20 GHz.(b) FMR linewidth as a function of frequency.The effective damping was determined to be 0.01059 from a linear fit based on eq. 1 (red curve).(c)The resonance frequency vs the resonance magnetic field (H res ) at 300 K.The red line is the fitting result using the Kittel formula (eq.2).

Figure 4 .
Figure 4. Gilbert damping and effective magnetization of NiFe thin films.(a) The Gilbert damping as a function of the NiFe thickness, t, measured at 300 K grown on silicon.Inset shows the linear fitting corresponds to eq.3 of Gilbert damping as a function of 1/t measured at 300 K.The slope and the intercept are related to the surface contribution and bulk contribution to the total Gilbert damping.(b) Thickness dependence of the effective magnetization M eff .(c) Substrate-dependent Gilbert damping (left axis) and effective magnetization (right axis).
conclusion, we report the successful growth of high-quality soft ferromagnetic NiFe thin films on oxide/non-oxide substrates at room temperature through PLD employing a KrF pulsed laser source.The fluence of the pulsed laser during deposition affects the magnetization of NiFe thin films.The thin films deposited at high laser fluence exhibits smooth flat surface and excellent ferromagnetic behavior.A few Oe of coercive fields and strong saturation magnetizations were obtained for film thickness above 6 nm.It is found that the magnetization and the damping parameters depend substantially on the film thickness.A noteworthy magnetization value of 547.5 emu/cc is attained at 20 nm of thickness.In addition, results indicate that the ferromagnetic characteristics do depend on the quality of the oxide/non-oxide substrate surface.Our experimental results offer new platforms for research in ferromagnetic thin films and spintronic devices.