Investigation on the crystal structure, magnetic, and electrical transport properties of magnetron sputtered Co2FeGe Heusler alloy films

Crystal structure, magnetic, and electrical resistivity behaviour of Co2FeGe Heusler alloy films deposited at different sputtering parameters have been studied using X-ray diffraction, VSM, and standard four-probe techniques. Though the expected structure was L21, X-ray diffraction studies indicate the A 2-type disordered structure. All films exhibited soft ferromagnetic characteristics having a coercive field of 5-65 Oe and a high ferromagnetic ordering temperature (More than 700 K). The electrical resistivity of the films deposited on the Si substrates was influenced by the substrate temperatures. Out of the different scattering mechanisms present in the low and high-temperature regimes, the two-magnon scattering effect is dominant in all the films. The scattering mechanisms are the same in all films irrespective of the substrate temperature. The optimum sputter deposition parameters that yields good quality Co2FeGe Heusler alloy thin films were found to be 50 W power, 5 mTorr pressure, and 400 °C substrate temperature.


Introduction
The attraction towards Heusler alloys started after the discovery of the ferromagnetic intermetallic compound Cu2MnAl by F. Heusler in 1903 in which no constituent elements are ferromagnetic [1].Heusler alloys are ternary intermetallic compounds having the chemical formula X2YZ for the full Heusler alloy and XYZ for the half Heusler alloy.Here X and Y are mostly transition elements and Z is a sp valent element.These alloys are widely used in spintronic devices due to their high Curie temperature, and high saturation magnetic moment and some of them exhibit a half-metallic nature in which it shows a metallic behavior for either up or down spin carriers and insulating behavior for the other type [2].Half metallic property gives complete spin polarization at the Fermi level which is important for magnetic multilayer structures such as spin valves and magnetic tunnel junctions (MTJ).For these devices, half-metallic Heusler alloy films are used as spin injection electrodes to inject spinpolarized charge carriers into a nonmagnetic layer [3].The efficiency of spin injection will greatly depend on the quality of individual film layers as well as the interface.Hence the optimization of the film deposition parameters such as power, pressure, substrate temperature, and post-annealing conditions are crucial for the device fabrication.Due to high Tc and magnetic moment, Heusler alloys having Co in the X positions are well studied theoretically as well as experimentally [4-7].T. Daibou et al have shown the significant influence of annealing conditions on the tunnel magnetoresistance (TMR) value of Co2MnSi-based MTJ [8].Enhanced TMR value was due to the better crystalline ordering of Co2MnSi at higher annealing temperatures.Zhu Qin et al [9] have reported the influence of substrate temperature on the crystallinity of Co2FeSi thin films and K. Srinivas et al [10] have reported a detailed study on the influence of sputtering conditions on the physical properties of the same alloy.
Researchers have also reported the correlation between sputtering parameters and physical properties of Co2FeAl [11] Heusler alloy films.
Even though the experimental and theoretical study carried out by K. Ramesh Kumar et al. [12] indicated the absence of half metallicity in Co2FeGe Heusler alloy, some of the recent experimental studies and first principle calculations reported in the literature have predicted the possibility of having half metallic characteristics in this system [13,14].As per the report, it has a Curie temperature of 981 K and a saturation magnetic moment of 5.71 μB.Moreover, point contact Andreev reflection measurement shows a spin polarization value of 0.58 which is comparabale to the values reported for the well known Heusler alloys Co2FeAl (0.59) and Co2MnAl (0.6) [15,16].These factors make this alloy one of the attractive candidates for spintronic applications.Even though reports on structural, magnetic [17,18], optical [19], and tunnel magnetoresistance [20] characteristics have been reported on Co2FeGe films, a systematic study on the influence of sputtering parameters such as sputtering pressure and substrate temperature on the physical properties of the films has not been reported so far.Since the alloy possesses an L21 crystal structure, high Curie temperature, and high saturation magnetic moment, we are motivated to do an investigation on the dependence of structural, magnetic, and electrical transport properties on the sputtering parameters.Optimized parameters reported in this paper may be useful for the fabrication of spintronic devices based on this material.

2.Experimental
Thin films of the Heusler alloy Co2FeGe were deposited on 1cm×1cm Si (100) substrate by using a DC magnetron sputtering system.Before the deposition, substrates were cleaned through ultrasonication by using acetone followed by isopropanol for five minutes.A two-inch Co2FeGe target was used for the deposition.For all depositions, the base pressure achieved was 8×10 -8 Torr.Argon gas was used as the sputtering gas.The sputtering power was maintained at 50 Watt for all deposition.Films were deposited with sputtering pressures 5, 10, and 15 mTorr.For each pressure, the substrate temperature was varied as room temperature (RT), 300 ᴼ C, 400 ᴼ C, and 500 ᴼ C. For all films substrate rotation was set at 10 rpm during the deposition.Films were allowed to cool after the deposition and were cut into pieces having dimensions 0.5cm×1cm for the characterization.For estimating the thickness, a step has been made on the film deposited at 5, 10, and 15 mTorr pressure by keeping the substrate at room temperature.The film thickness was measured using Bruker's contourGT 3D optical microscope.The structural analysis was carried out using a Rigaku smartLab high-resolution X-ray diffractometer with Cu-Kα radiation (λ=1.5418Å) in grazing incident angle mode (Incident angle ω=1 for all measurements).The film stochiometry was identified by using an EDS attachment by Horiba Ltd. with Hitachi S-4800 Type II SEM.Magnetic measurements were carried out by using Lake Shore VSM.Resistivity measurement has been done using a standard four-probe measurement technique using a Cryogenic Probe Station (CRX-4K, Lake Shore Cryotronics).

Crystal Structure
The measured film thicknesses through the optical method are 78 nm, 64 nm, and 44 nm for the films deposited at 5, 10, and 15 mTorr respectively.Figure 1 shows the grazing incident angle (with ω=1) XRD patterns of Co2FeGe films synthesized at various sputtering parameters.Diffraction patterns did not show (111) and (200) superlattice peaks which are the characteristic peaks for the L21 crystal structure.The absence of both peaks indicates the swapping of all the atoms in the unit cell which can be taken as evidence for A2 disorder [21].But the similar atomic scattering factors of the elements Co, Fe, and Ge can also lead to the reduction in super lattice peak intensity.This makes it difficult to figure out the crystal structure of the alloy through X-ray analysis alone.Through EXAFS studies, Benjamin Balke et al have reported that Co2FeGe alloy has the ordered L21 structure even though the alloy appeared to be in A2 disordered structure at first glance [22].
Films deposited at 5 mTorr pressure and substrate temperatures up to 400 0 C have (220), ( 311) and (422) peaks and a low intense (400) peak was observed at 500 ᴼ C (Figure 1a).Also, the intensity of the main peak which is (200) has shown an enhancement at 400 ᴼ C for all sputtering pressures which indicates a better crystallization at this temperature).X-ray spectrums showed a better crystalline order for thin films synthesized at 5 mTorr pressure and higher substrate temperatures (400 ᴼ C and 500 ᴼ C).As per the reported values, the lattice parameter for the Co2FeGe alloy in the bulk form is in the range of 5.70-5.73Å.The lattice parameters for the films investigated here are in the range 5.69-5.73Å which is close to the values reported by other researchers.

Magnetic Properties
Figure 2 a, b, and c shows the magnetic hysteresis curves recorded at room temperature for the samples deposited at various parameters.Except for the film deposited at room temperature, all films exhibited soft ferromagnetic characteristics.Magnetization was saturated within 500 Oe.However, the samples synthesized with RT substrate temperature showed saturation only at higher fields which is around 2000 Oe as shown in figure 2d.This is possibly due to the presence of mixed magnetic interactions such as ferromagnetic and spin/cluster glass type.When the major ferromagnetic phase allows the magnetic domain to align parallel with the field direction the minor spin/cluster glass phase tries to oppose the parallel alignment.The ZFC and FC magnetization measurements showed a maxima around 80 K which is possibly due to the presence of a glassy phase in addition to the dominant ferromagnetic phase This competing interaction results in a lack of magnetic saturation up to 2000 Oe for all the room-temperature deposited films.All films exhibited soft ferromagnetic characteristics having a coercive field of 5-65 Oe.As per the previous reports, Co2FeGe Heusler alloy should possess a magnetic moment value of around 6 μB.Our investigation showed a systematic variation in saturation magnetic moment value from 2.98 μB to 4.95 μB with respect to sputtering parameters.The difference in the magnetic moment values might be because of the atomic disorder and off-stoichiometry in the films.The estimated moment value based on the EDS results and Slater-Pauling rule are 5.08, 5.05, 5.42, and 5.46 μB for the films deposited at 5 mTorr pressure and (RT), 300 ᴼ C, 400 ᴼ C, and 500 ᴼ C substrate temperatures respectively.1d shows the comparison between simulated X-ray spectrum (Bottom) and experimental X-ray spectrum (Top).4

Electrical Transport Properties
The contribution to the electrical resistivity of a material mainly arises due to electron scattering by defects and impurities, electron-electron interaction (T 2 dependence), electron-phonon interaction (T), and electron-magnon interaction (In the case of magnetic material only).The magnon contribution could be either due to one magnon scattering (T 2 ) or due to two magnon scattering (T 9/2 at lower temperature and T 7/2 at higher temperature).In addition to the above-mentioned contributions, spin wave excitations (T 2 at lower temperature and T at higher temperature) and Stoner fluctuations (T 2 and T 5/3 ) can also contribute to electronic resistivity [23].Figure 3(a)-(d) shows the experimental (in black) and fitted (in red and green) ρ-T graphs for the films deposited on Si substrate at 5 mTorr pressure.The surface roughness of the films obtained through AFM measurements were in the range of 0.4 to 4.1 nm.The RT films deposited film has shown a semiconducting behavior as the temperature decreases and is followed by an almost temperature-independent resistivity.This unusual behavior has restricted us from fitting the curves.Another interesting observation is the presence of a minimum around 240 K for the film deposited at 300 ᴼ C.This minimum in the resistivity curve has become a point of inflection for the samples synthesized at 400 ᴼ C and 500 ᴼ C. For fitting, curves were divided into two regions where the scattering contributions are different.Different combinations of scattering terms were considered for fitting and the least χ2 fit was chosen for the discussion.The respective equations and temperature ranges used for the fitting are ρ=ρo+AT 2 +BT 9/2 for 7 K ≤ T ≤ 60 K and ρ=ρo+CT+DT 2 +ET 7/2 for T > 60 K Here A, B, C, D, and E are the respective coefficients for various scattering mechanisms which are obtained as fit parameters.It was evident that the prominent contribution to the resistivity at low temperatures arises from two magnon scattering (T 9/5 ).Since electron-electron, one magnon scattering (absence of half metallicity), and magnetic excitations such as spin wave and Stoner fluctuations have the same temperature dependence (T 2 ) at low temperatures, it is difficult to separate the individual scattering mechanisms.The second region of the resistivity curve is contributed by three terms having T, T 2, and T 7/5 dependence.The first term could be due to the electron scattering with phonon and spin wave excitations and the second term could be due to one magnon scattering.Since the phonon energy is directly dependent on the temperature, the emergence of T dependent term at high temperature regime is understandable.The third term shows that there is a presence of strong two magnon scattering in the film samples.

Conclusion
In this article, the effect of magnetron sputtering parameters on the physical properties of Co2FeGe Heusler alloy has been investigated.X-ray analysis revealed a disordered A2 crystal structure for all the deposited films.Better crystalline order and desired stoichiometry were observed at 400 ᴼ C for all sputtering pressures.DC magnetization measurements revealed a soft ferromagnetic nature for all films.The optimum sputtering parameters for for obtaining films with low coercivity and high saturation magnetization values are 50 W power, 5 mTorr pressure, and 400 ᴼC substrate temperature.Anomalous electrical transport behavior was observed for the films deposited at lower substrate temperatures.

Acknowledgement
One of the authors, P.V.Midhunlal thanks IIT-Madras and DMRL-Hyderabad, for their help in the fabrication and characterization of the films.Midhunlal P V also thank Dr R.B. Gangineni for the AFM characterization.

Figure 1 :
Figure 1: Grazing incident angle x-ray spectrum recorded using Cu-Kα radiation for Co2FeGe films synthesized at (a) 5mTorr (b) 10 mTorr (c) 15 mTorr pressures and different substrate temperatures.Figure1dshows the comparison between simulated X-ray spectrum (Bottom) and experimental X-ray spectrum (Top).

Figure
Figure 1: Grazing incident angle x-ray spectrum recorded using Cu-Kα radiation for Co2FeGe films synthesized at (a) 5mTorr (b) 10 mTorr (c) 15 mTorr pressures and different substrate temperatures.Figure1dshows the comparison between simulated X-ray spectrum (Bottom) and experimental X-ray spectrum (Top).

Figure 2 :
Figure 2: Isothermal magnetization curves for Co2FeGe films deposited at 50 W power and (a) 5 (b) 10 and (c) 15 mTorr pressures.Figure5dshows the saturation at higher fields for the samples synthesized at room temperature and at different sputtering pressures.

Figure 3 :
Figure 3: Temperature variation of electrical resistivity of thin films of Co2FeGe synthesized at different substrate temperatures Casper F, Winterlik J, Balke B, Fecher G H and Felser C 2009 Crystal structure of new heusler compounds Z Anorg Allg Chem 635 976-81 [2] Kawasaki J K, Chatterjee S and Canfield P C 2022 Full and half-Heusler compounds MRS Bull 47 555-8 [3] Akmal A, Arshad F, Shakeel R, Shabir T, Riaz S and Naseem S 2023 Study of electronic, structural and magnetic properties of electrodeposited Co2MnSn Heusler alloy thin films Journal of Materials Research and Technology 22 1-16