Structure and Magnetization of Thin Film
In this work, we studied the local structure and the magnetization of thin films deposited by a reactive dc magnetron sputtering process. The interstitial incorporation of N atoms in a fcc Co lattice is expected to expand the structure and such expansion yields interesting magnetic properties characterized by a larger than Co magnetic moment and a very high value of spin polarization ratio in . By optimizing the growth conditions, we prepared film having lattice parameter close to its theoretically predicted value. The N concentration was measured using secondary ion mass spectroscopy. Detailed magnetization measurements using bulk magnetization method and polarized neutron reflectivity confirm that the magnetic moment of Co in is higher than that of Co.
keywords:thin films, reactive sputtering, Magnetization
Transition metal nitrides (TMN) are an interesting class of compounds as N incorporation in metals make them less corrosive and results in interesting properties such as superhardness Vepřek (1999); Jhi et al. (2001, 1999); Hao et al. (2006), superconductivity Yamanaka et al. (1998), corrosion and wear resistance Steneteg et al. (2013); Sproul (1996). In particularly, tetra metal nitrides , generally formed only for late 3d TM (e.g. Fe, Co, Ni) Meinert (2016) are somewhat unique in the sense that they share a common fcc structure. Due to a dominant metal-metal interactions in they possess metal like character (unlike metal oxides) Houari et al. (2010). In a metal fcc cage, the incorporation of N atoms at the interstitial positions, results in an expansion of the unit cell, which in turn affects the magnetic integrations Matar et al. (2007); Imai et al. (2010, 2014); Coey and Smith (1999); Mohn and Matar (1999). Generally incorporation of a non-magnetic element in a magnetic one is expected to result in a loss of magnetic moment () explained by the well-known Slater-Pauling curve Houari et al. (2010). However, theoretical calculations predict to be larger for and compounds. This is an interesting proposition as with larger than metal , corrosion resistance and a metal like character can make compounds an alternative their pure metal counterparts. In addition, some recent theoretical predicted a very spin polarization ratio (SPR) for compoundsMatar et al. (2007); Imai et al. (2010). Among them is predicted to have SPR 90 Imai et al. (2010) which is among the highest values for any compound.
However experimental results obtained so far for compounds does not seems to be as exciting as theoretical calculations. This can be understood from the fact that the value of found in most of the experimental works so far is perpetually less than than that of pure Co. A closer look at the thin films reveal that the values of lattice parameter (LP) obtained in most of the experimental works is typically 3.54 Å Sil (2015); Ito et al. (2011, 2014); Silva et al. (2014); Oda et al. (1987); Wang et al. (2009); Jia et al. (2008). This is more close to theoretical values LP for fcc Co at 3.54 Å than that of at 3.72 Å Matar et al. (2007); Imai et al. (2010); Sil (2015). Such a discrepancy in the experimental and theoretical values of LP for requires attention. While investigating the recipes adopted for formation of thin films, we found that films were often deposited at substrate temperatures () similar to those used for thin films. This is convenient approach due to similarity between the and and the absence of a phase diagram for the system Co-N, intuitively makes one to follow paths adopted for preparation of .
However, the energetics of nitride formation for and immediately indicates about the complexity for the later. Theoretical values of enthalpy of formation () for is about -12 kJ mol Tessier et al. (2000), whereas those for are slightly above or below 0 for hcp Co or fcc Co Imai et al. (2010). This also implies that at a higher , system will be less stable as compared to . In a recent work, we studied the phase formation process in the Co-N system at = 300 K Gupta et al. (2015) and 523 K Pandey et al. (2017). We found that at = 523 K, N incorporation in the Co-N system is minimal and the phases formed are similar to a fcc Co having LP3.52Å. On the other hand when is lowered to 300 K Co-N depicts a similar type of phase formation sequence as found for the Fe-N system Gupta and Gupta (2005); Tayal et al. (2015). By optimizing the deposition conditions, film having LP as high as 3.68 Å can be deposited and the value of also supersedes that of Co. Since the films deposited without any intentional heating are expected to have a large fraction of disorder, estimation of LP with x-ray diffraction alone may not be decisive, therefore in the present work, we investigate the local structure and of the thin film deposited at = 300 K using x-ray absorption based techniques. By doing measurements at Co K and L-edges and at N K-edge, we get valuable information about the local structure. In addition, by doing polarizer neutron reflectivity (PNR) measurements at low temperatures (20 K) we determined the value of for thin film. Obtained results are presented and discussed in this work.
We deposited thin film with a nominal thickness of 120 nm at = 300 K (without intentional heating) using a reactive direct current magnetron sputtering (dcMS) system (Orion-8, AJA Int. Inc.). A one inch diameter and 0.5 mm thick pure Co (purity 99.95) target was sputtered using a gas mixture of Ar and N (both 99.9995% pure) gases. With a base pressure of 110 Torr, the pressure during deposition was about 3 mTorr. More details about deposition process can be found in Gupta et al. (2015). Along with the sample, we also deposited a pure Co thin film under identical conditions as a reference.
To investigate the local and electronic structure, x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure measurements (EXAFS) were performed in the total electron yield (TEY) and florescence mode at BL-01 Phase et al. (2014) and BL-09 beamlines, respectively at the Indus-2 synchrotron radiation source at RRCAT, Indore. The measurements in TEY mode at BL-01 were carried out in a UHV chamber with a base pressure of (210 Torr). To avoid surface contaminations, samples were cleaned using a Ar source kept incident at an angle of 45 . The measurement in florescence mode at BL-09 were carried out at ambient conditions.
The composition of thin film was measured using secondary ion mass spectroscopy (SIMS) depth profiling using a Hiden Analytical SIMS workstation. An oxygen ion beam of energy 4 keV and 200 nA was used as a primary source and the sputtered species were detected using a quadrupole mass analyzer. The SIMS depth profiles were compared with a references sample as described in Gupta et al. (2015). The magnetization measurements were carried out using a Quantum Design SQUID-VSM (S-VSM) magnetometer at room temperature. We did PNR measurements at AMOR reflectometer Gupta et al. (2004) in the time of flight mode at SINQ-PSI Switzerland. To saturate the sample magnetically, a magnetic field of 0.5 T was applied during the PNR measurements. The measurements at low temperature were carried out using a close cycle refrigerator installed inside the electro magnet.
3 Results and Discussion
To quantify the N at.% in our samples, we did SIMS depth profiling measurements. The SIMS depth profiles for the sample are shown in fig. 1 along with a Co reference sample. The depth profiles clearly reveal that N concentration in the sample is more and the Co concentration is less as compared to Co sample. Following a procedure described in ref. Gupta et al. (2015) and measuring a reference sample with known concentration, we found that N at.% comes out to be 18(2)at.% indicating formation of phase.
X-ray diffraction measurements (not shown) carried out in our samples resulted in similar patterns as observed in a earlier work Gupta et al. (2015). Since samples are deposited without any intentional heating, the long range structure is expected to somewhat disordered and more reliable information about the structure can be obtained from a local probe like x-ray absorption spectroscopy. To get complementary information, we did XANES measurements at Co K and L-edges and at the N K-edge. In addition, EXAFS measurements were also carried out at Co K-edge. Fig. 2(a) and (b) shows the normalized XANES and EXAFS patterns for sample together with a Co metal as a reference. It can be clearly seen that the intensity in the white line region is higher in the sample(marked by an arrow in 2(a)), which essentially arises due to hybridization between face centered Co 3d and N 2p bands through anti-bonding and an enhancement in density of states in as compared to Co. Such effects are even more pronounced at Co L-edges as shown in fig. 3(a). Here presence of N in thin film can also be confirmed by comparing the N K-edge spectrum of with that of a Co reference sample as shown in fig. 3(b). The features in N K-edge spectra are labelled as , and . Here the feature is attributed to a dipole transition from N 1s to hybridized states of face centered Co 3d and N 2p orbitals through anti-bonding. Features and represents the dipole transition from N 1s to hybridized states of face centered Co 3d and N 2p orbitals through anti-bonding Ito et al. (2011, 2015). These results obtained from XAS measurements provide a strong evidence for the structure.
Further information about the LP can be obtained from EXAFS measurements. Fig. 2(b) shows the modulli of k weighted EXAFS data for and with a Co reference. For structure we consider space group and LP = 3.74 Å. Among other parameters the fitting of EXAFS data also yields the distance between corner Co atoms which is equal to the LP. We found that the LP comes out to be 3.92 Å with ( = 0.0145 Å); these values for the Co reference sample are 3.47 Å with ( = 0.0093 Å). The value of LP obtained in our case is considerably larger than that of Silva Sil (2015) in a recent work for their thin film. They found LP = 3.53 Å with ( = 0.005 Å). Considering even large inaccuracy of EXAFS technique in determination of LP, our values are considerably larger and clearly indicate an expansion in the Co lattice by incorporation of N. It may be noted that Silva Sil (2015) deposited their samples at a = 523 K and at this temperature it may happen that most of N atoms diffuse out of the system leaving behind a dominant fcc Co as found in our recent work Pandey et al. (2017).
It may be noted that in most of experimental work carried out in the thin films always a high was used and the measured values of LP were closer to fcc Co rather than that of . Since LP and the are correlated and an expansion in the LP due to interstitial incorporation of N atoms is responsible for enhanced Imai et al. (2010), the values of obtained in most of the works were closer to pure Co. To measure in our samples we did bulk magnetization and PNR measurements. Inset of fig. 4 shows M-H loop measured using S-VSM at room temperature for a thin film. We find that = 1195 emu/cc which approximately corresponds 1.6 per Co atom, which is slightly lower than the theoretically predicted value of for . Exact determination of from bulk magnetization measurements in a thin film sample require precise values of sample volume and density. While the former can be measured with a great accuracy, estimation of later is not easy due to relatively large fraction of defects etc. in thin films. In addition diamagnetism of substrate always interferes with sample magnetization.
It is well-known that in a PNR measurement, absolute value of can be measured as in PNR technique, measurement of is not influenced by sample volume and substrate magnetism. On the other hand density of the film is inherently measured in PNR. In this context it is surprising to note that PNR has not been used to measure in thin films. In most of the works available so far, was measured using bulk magnetization methods and there seems to be a large variations in estimation of for thin films ranging from 1.3 to 1.6/atom Ito et al. (2014); Oda et al. (1987); Wang et al. (2009); Jia et al. (2008); Ito et al. (2011).
We performed PNR measurement at room temperature and at 17 K (to minimize thermal fluctuations) under an applied magnetic field of 0.5 T, which is sufficient to saturate sample magnetically (see inset of 4). The spin-up down reflectivities clearly show a separation typically expected for a ferromagnetic sample. The fitting of PNR patterns was carried out using SimulReflec programme Ott (2011) and the obtained values of are 1.73(0.05) /atom at 300 K and 1.75(0.05)/atom at 17 K. This clearly shows that is larger than Co in thin film. Though such enhancement in was theoretically predicted, it has been unambiguously demonstrated in this work.
In conclusion, by measuring the local structure and a we found that LP of thin films far exceeds that of previous works and is more closer to its theoretical value. For thin films an enhancement in was expected. By by doing precise magnetization measurements using PNR we found enhancement in the Co magnetic moment. In addition, our SIMS depth profile measurements cleanly reveal the formation of phase and N K-edge measurements further confirms the formation of phase.
A part of this work was performed at AMOR, Swiss Spallation
Neutron Source, Paul Scherrer Institute, Villigen, Switzerland. We
acknowledge Department of Science and Technology, New Delhi for
providing financial support to carry out PNR experiments. We are
thankful to Layanata Behera for the help provided in sample
preparation and various measurements. We acknowledge Ram Janay
Choudhary and Malvika Tripathi for S-VSN measurements. Thanks are
due to D. M. Phase and Rakesh Sah for support in XAS beamline. We
are thankful to V. Ganesan and A. K. Sinha for support and
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