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Abstract: Tunability of the Ne ́el temperatures (TN) in case of antiferromagnetic (AFM) materials has significanct importance in exchange bias studies, magnetic and spintronic device applications etc. Interestingly, AFM transition metal oxide thin films demonstrate TN dependance on film thickness and stoichiometry. This article reports the estimation of TN for AFM nickel oxide (NiO) ultrathin films and focuses on its systematic variation with film thickness and Ni:O stoichiometry.
Bulk NiO, with TN=523 K, is a type-II insulating AFM at room temperature. At reduced dimesion, e.g. ultrathin films, TN decreases from its bulk value. The exchange scattered low energy electrons from the (001) AFM NiO surface produce additional p(2x2) fractional order magnetic spots in a Low Energy Electron Diffraction (LEED) pattern owing to the exchange interaction. Intensity of the fractional order spots drops down with raising sample temperature until the magnetic LEED spots vanish at TN.
NiO ultrathin films were deposited on Ag(001) substrate using standard in-situ molecular beam epitaial (MBE) technique [1]. The thicknesses of the films were estimated using a pre-calibrated quartz microbalance. Stoichiometric variations were created at the film surface by different dosages Ar+ ion bombardment. Measurement of film thickness and quantification of relative elemental compositions were characterized by X-ray photoemission spectroscopy (XPS).
Fig. (a) shows the LEED of 15 ML (monolayer) NiO/Ag(001) film at 300 K for the incident beam energy of 30 eV. The fractional ordered (1/2,0) spots of magnetic origin have intensity 2-4% of the integer order spots (beyond the field-of-view of LEED screen at this low beam energy). Fig. (b), on the other hand, shows the LEED pattern of 15 ML NiO film at
T=525 K (>TN), where the fractional order spots disappear. Intensity decrements of these p(2x2) spots against sample temperatures are plotted in figs. (c) and (d) at various film thicknesses. While fig. (c) focuses on the submonolayer thickness range, 0.5 to 2 ML; fig. (d) demonstrates the temperature dependent plots for the cases of thicker films, 2, 6, 10 and 15 ML. The values of TN corresponding to various thicknesses are provided in table 1.
Table:1
Thickness (in ML) 0.5 0.75 1 1.5 2 6 10 15
TN (in K) 260 260 260 360 410 450 475 510
TN decrement as a result of finite size effects could be observed with the lowering of film thickness. Interestingly, at the submonolayer range, no significant change of TN could be noted while the average spot intensities decrease with coverage. This could be attributed to the formation of nano-sized domains with smaller areas at lower coverages.
Fig. (e) shows a systematic variation of TN with the stoichiometric modifications at a 20 ML film surface. Oxygen deficiencies were produced via mild Ar+ ion sputtering (400 eV, 5 min) cycles followed by vacuum annealing at 473 K. Elemental quantifications, estimated using surface sensitive XPS at the grazing emission angle [2], indicate towards weaker surface AFM ordering upon oxygen reduction as shown in table 2.
Table 2:
O: Ni ratio 1 0.94 0.89 0.83 0.78
TN (in K) 510 473 448 418 353
References:
[1] J. Das, K. S.R. Menon, “A revisit to ultrathin NiO(001) film: LEED and valence band photoemission studies,” J. Electron Spectrosc. Relat. Phenom., vol. 203, pp. 71-74, 2015, doi: 10.1016/j.elspec.2015.06.006.
[2] J. Das, K. S.R. Menon, “Effects of surface non-stoichiometry on the electronic structure of ultrathin NiO(001) film,” Appl. Surf. Sci., vol. 359, pp. 61-67, 2015, doi: 10.1016/j.apsusc.2015.09.173.
