Manipulation of film quality and magnetic properties of CrO2 (100) films on TiO2 substrates with carrier gas and growth temperature

High-quality CrO2 films were synthesized on TiO2 (100) substrates at different temperatures using the chemical vapor deposition method in argon or nitrogen atmosphere. It was found that the lower limit for the growth temperature of CrO2 films can be reduced to 310 or 300 °C when using Ar or N2 as the carrier gas, respectively. The quality of CrO2 film on TiO2 substrate can thus be improved by optimizing growth temperature in a much larger range (310–400 °C in Ar and 300–430 °C in N2, in contrast with 390–410 °C in O2), which is significant for the practical application of CrO2 films. The best film quality was achieved at 320 °C in either Ar or N2 atmosphere, at which CrO2 film has its narrowest orientation distribution and lowest roughness. Compared to films grown in O2, films grown in Ar were found to have larger saturation magnetizations (Ms) and magnetic anisotropies, possibly due to numerous O vacancies. Films grown in N2 are actually N-doped films, and have lower Ms than those grown in O2. The Curie temperature (Tc) was also tuned by the carrier gas and growth temperature. Films grown in Ar or N2 generally have a higher Tc value than those grown in O2. Furthermore, the thermal stability of the films was found to be remarkably improved when using N2 as the carrier gas.


Introduction
Since chromium dioxide (CrO 2 ) was rst theoretically predicted as a kind of half metal (HM) ferromagnet by Groot in 1983, 1 its half metallicity has been conrmed by different experimental technologies, such as Meservey-Tedrow spin-polarized tunneling 2 and point-contact Andreev reection. 3,4 As a HM material, CrO 2 only has one spin channel involved in electron transport due to its special band structure, which makes the charge carriers 100% spin polarized. [5][6][7] This distinctive property of CrO 2 makes it very promising in the synthesis of high performance spintronic devices, such as giant magnetoresistant (GMR) devices, magnetic tunnel junctions (MTJ), and magnetic random-access memory (MRAM). [8][9][10][11] Although much effort has been devoted in past years, CrO 2 has never been successfully utilized in spintronic devices. One primary obstacle for the practical application of CrO 2 concerns the difficulties for obtaining high lms quality due to other metastability and stringent preparation conditions of CrO 2 lms. Chemical vapor deposition (CVD) in oxygen atmosphere is a commonly used method to synthesize epitaxial CrO 2 lms. [12][13][14] As a metastable state material, CrO 2 easily decomposes into Cr 2 O 3 which is an antiferromagnetic and insulator in room temperature under air atmosphere. 15,16 To prevent the degradation of CrO 2 , isostructural rutile TiO 2 substrates are usually used. However, on TiO 2 substrates, CrO 2 lms can only be synthesized in a narrow temperature range of 390-410 C in an oxygen rich atmosphere. Below 390 C, CrO 2 lms cannot form due to the lack of interfacial energy needed for bonding or nucleating on the surface of TiO 2 . Above 410 C, detectable Cr 2 O 3 phase will appear in lms due to thermal instability. 13 This narrow temperature window greatly limits the qualities of CrO 2 lms for two prominent reasons. Firstly, the growth temperature is so close to the upper limit at which observable lm degradation happens that the existence of tiny Cr 2 O 3 in lms may be unavoidable. Secondly, there is no space for improvements in lm quality through temperature optimization. Considering device fabrication, low temperature growth is desirable to decrease the interface diffusion and simplify the fabrication process. Therefore, for the practical application, it is important to explore a method to expand the temperature window and grow CrO 2 lms at a lower temperature. Recently, Sousa 17 et al. successfully fabricated CrO 2 lms on Al 2 O 3 substrates at a temperature as low as 330 C, broadening the process window by 50 C. However, Al 2 O 3 may be unable to play the same role as TiO 2 in stabilizing CrO 2 lm. Considering that Al 2 O 3 substrates have the same hexagonal structure as Cr 2 O 3 , and the lattice constant differences between them are less than 5%, Cr 2 O 3 may easily form at the interface of CrO 2 /Al 2 O 3 . This kind of interface degradation was observed in experiments. 18 To broaden the temperature window for the growth of CrO 2 and avoid the interface degradation, it is highly signicant to lower the growth temperature of CrO 2 lms on TiO 2 substrates.
According to binary alloy phase diagrams, 19 researchers have proposed a common theorem that CrO 2 epitaxial lms can only be manufactured under enough high oxygen pressure, where O 2 is usually used as the carrier gas to avoid the formation of Cr 2 O 3 . 20 Most recently, Duarte 21 et al. obtained (110) oriented pure CrO 2 lms on TiO 2 substrates using argon as a carrier gas, suggesting that an oxygen rich atmosphere may not be necessary for the growth of purely-phase CrO 2 lms. However, in the Duarte's study, pure (100) oriented CrO 2 lms were not obtained, and the growth temperature was not decreased.
In the presented work, (100) CrO 2 lms on TiO 2 substrates were fabricated at different temperatures using argon or nitrogen as the carrier gas. It was found that high quality CrO 2 lms can be obtained at a temperature as low as 310 and 300 C using Ar or N 2 as the carrier gas, respectively. And the process the temperature windows are 310-400 C and 300-430 C, respectively, under Ar or N 2 atmosphere. The surface morphology, orientation distribution, and magnetic properties of the lms fabricated at different temperatures and under different atmospheres were investigated and subsequently discussed.

Experimental details
A two zone CVD furnace at atmospheric pressure was utilized to synthesize the CrO 2 lms, since CVD is the best way to manufacture CrO 2 epitaxial lms as suggested by previous studies. 20 CrO 3 powder (purity 99.9%) was placed in low temperature zone as the chromium source and heated to 260 C. 5 Â 5 mm single crystal rutile TiO 2 substrates with the direction of one in-plane crystal axis marked were introduced to the high temperature zone at various temperature. Oxygen (purity 99.99%), argon (purity 99.99%), and nitrogen (purity 99.99%) were used as the carrier gases, and the ow rate was xed to 160 sccm. The quality of the CrO 2 lms was claimed to be critically dependent on the substrate temperature, so to investigate the effects of growth temperature on lm quality, lms were grown at different temperatures in the range of 300-450 C. Before lm deposition, the substrate was pretreated with hydrouoric acid (HF) in an ultrasonic dispersion cleaner for 5 min, and then cleaned with acetone for another 5 min. In order to prevent the deposition of undesirable compounds during the initial stage of the deposition process, the substrate was heated to the deposition temperature before the CrO 3 precursor reached its melting temperature (196 C). The thickness of all the samples used in this study are around 100 nm.
The crystallographic structure and phase of the deposited lms were investigated by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer with Cu K a radiation. The thickness of the lms were evaluated using scanning electron microscopy (SEM) from the cross-section images, while the surface morphology and roughness were characterized using atomic force microscopy (AFM). The elemental compositions of the lms were determined by X-ray photoelectron spectroscopy (XPS). The magnetic properties of the lms were studied using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic eld parallel to c-or b-axis.

Phase and structure
To investigate the phase and structure of CrO 2 lms grown at different temperatures in Ar atmosphere, XRD measurements were performed in the scan angle range of 10-90 . XRD results suggest that pure phase (100) CrO 2 lms can be obtained in a growth temperature range of 310-400 C. Fig. 1(a) shows the XRD patterns (for better observation, the value of y-axis is taken logarithm) for (100) oriented CrO 2 lms fabricated at different temperatures when the carrier gas was argon. Here, we only show the spectra in the angle range of 34-44 . It is obvious that only the (200) diffraction peaks of the TiO 2 substrates and CrO 2 lms appear in the spectra, suggesting the lms are pure in phase. The epitaxy of the lms was examined by performing phi-scan aer rotating the horizontal plane from the (100) plane to (110) plane. Here, we use the lm grown at 310 C as an example and show its {110} phi-scan in Fig. 1(b). For comparison, the {110} phi-scan of TiO 2 substrate is also shown. The appearance of (110) and ( 110) peaks suggest that the CrO 2 lm and the TiO 2 substrate have the same two-fold rotational symmetry along the [110] axis. The corresponding peaks of the lm and substrate locate exactly at the same angle, revealing that the CrO 2 lm is epitaxially grown on the TiO 2 substrate. This journal is © The Royal Society of Chemistry 2018 Based on above results, pure CrO 2 (100) lm can be grown at a much lower temperature in Ar atmosphere than lm in O 2 atmosphere. However, it is also noticed that the upper limit of temperature is 400 C, which is 10 C lower than that in O 2 atmosphere. 13 The reduction of growth temperature upper limit indicates that lms fabricated in Ar atmosphere may be less stable than those prepared in O 2 atmosphere.
When nitrogen was used as the carrier gas, purely phased epitaxial CrO 2 lms were also obtained. Fig. 2 shows XRD spectra of CrO 2 lms grown in nitrogen atmosphere at different temperatures. All the spectra only show the (200) peaks of CrO 2 and TiO 2 , while no other phases are detectable in the lms. The epitaxy of the lms was also conrmed by a {110} phi-scan (not showed). Although the lowest temperature shown in Fig. 2 is 310 C, according to our study, purely phased CrO 2 can be obtained at a temperature as low as 300 C when N 2 is used as the carrier gas. Therefore, the temperature window for lm growth is 300 to 430 C in N 2 atmosphere, which is broadened by about 110 C relative to that in oxygen atmosphere. Compared to the temperature window for lm growth in argon atmosphere, CrO 2 lm can be grown at an even lower temperature in nitrogen atmosphere. Another important feature to note is that, purely phased CrO 2 lms can be obtained at a temperature as high as 430 C in N 2 atmosphere-which is 20 and 30 C higher than in O 2 and Ar atmosphere, respectively. The enhancement of upper limit of growth temperature implies that lms prepared under nitrogen atmosphere may have better stability than those grown under O 2 or Ar atmosphere.
In O 2 atmosphere, CrO 2 lms cannot be synthesized below 390 C due to the lack of interfacial energy needed for bonding or nucleating on the surface of TiO 2 . 22 However, in Ar or N 2 atmospheres, CrO 2 lms can be obtained at much lower temperatures, indicating that N 2 and Ar may help to lower the energy barrier for lm bonding or nucleating. However, this kind of role performed by N 2 or Ar may be surface selective. It was found that CrO 2 lms can only be prepared on the TiO 2 (110) substrates at a temperature equal to or higher than 380 C in N 2 or Ar atmosphere.

Film quality
To investigate the effects of growth temperature on lm quality, the surface morphologies and orientation distributions of CrO 2 lms grown at different temperatures were studied.
The orientation distributions of grains in lms were evaluated by analyzing the full width at half maximum (FWHM) of the rocking curve. The FWHMs of lms synthesized at different temperatures using Ar or N 2 as the carrier gas are shown in Fig. 3(a). In oxygen atmosphere, the lowest FWHM of 0.27 is obtained for lms at 390 C, which indicates the harsh synthesis condition needed to synthesize high-quality CrO 2 lms in oxygen atmosphere. According to Fig. 3(a), FWHMs lower than this value can be achieved in a large temperature range in Ar or N 2 atmosphere, where a FWHM lower than 0.27 could be obtained in a temperature range of 310-360 C. A FWHM lower than 0.27 could be obtained in a temperature range of 310-390 C in N 2 atmosphere. Generally speaking, low FWHMs are usually obtained at low temperatures. As the temperature increases, FWHM also increases signicantly. In Ar atmosphere, the lowest FWHM is 0.22 and obtained at 310 C, while in N 2 , the lowest FWHM is 0.19 and obtained at 320 C. Both of these FWHM values are signicantly lower than that obtained in O 2 which is 0.27 . The results suggest that the decrease in growth temperature using Ar or N 2 as the carrier gas is benecial for improving the quality of CrO 2 lms. Comparatively, at most temperatures, lms grown in N 2 atmosphere have lower FWHMs than their counterparts fabricated in Ar atmosphere.
The roughness of the lms was evaluated using AFM and is depicted in Fig. 3(b). Films grown in Ar atmosphere have roughness around 2 nm. The lowest roughness appears in lm grown at 370 C. For lms grown in N 2 atmosphere, the roughness uctuated from 1.3 to 2.5 nm in temperature range of 320-400 C. As the temperature increased beyond 400 C, or decreased below 310 C, the roughness became larger than 3.5 nm.
Considering both orientation distribution and roughness, the best fabrication temperature was determined to be 320 C for either Ar or N 2 atmosphere.
To investigate the surface morphology, AFM images for lms synthesized at 400, 370, 330, and 310 C are shown in Fig. 4. The surface morphology seems to depend on both the growth temperature and the type of the carrier gas. At 310 C, the lm surface was covered by small grains with spiny shapes for both Ar and N 2 cases. As the growth temperature increased to 330 C, the surface of the lm grown in Ar atmosphere is composed of platelet-like grains with square shape, while the surface of lms grown in N 2 atmosphere exhibited enlarged grains with the Fig. 2 The XRD patterns of films synthesized at different temperatures when nitrogen was used as the carrier gas. length direction along the b-axis. At high temperatures (from 370 to 400 C), the morphology of the lms grown in Ar or N 2 is of nodular type consisting of particles composed of numerous large or small grains. However, the particles in lms grown in different atmospheres possessed different shapes. The particles in the lm grown in Ar are random, while those in the lm grown in N 2 exhibit a rectangular shape with the long sides along the b-axis. The types of surface morphology at different temperatures and under different carrier gases may be the reason for the temperature and carrier gas dependent roughness.

Magnetic properties
To investigate the magnetic properties of CrO 2 lms, hysteresis loops were measured at room temperature (300 K) with an inplane magnetic eld applied along the [010] or [001] direction. Here, we put the hysteresis loops of lms grown at same temperature but in different atmospheres in the same gure to compare their magnetic properties. Fig. 5(a) and (b) show the easy and hard axis loops of samples deposited at 390 C under argon, nitrogen, and oxygen atmosphere, which reveals that all lms have good uniaxial magnetic anisotropy. Despite being grown under different carrier gases, the easy axes of the samples are along the c-axis, while the hard axes are along the b-axis.
Results also indicate that the lm fabricated in Ar has a larger, and that grown in N 2 has a lower saturation magnetization (M s ) than the one deposited in O 2 . According to the hard axis loops shown in Fig. 5(b), lms grown in different atmospheres have different switching elds (H k ), suggesting that their magnetic anisotropies may also vary.
To comprehensively investigate the effects of growth temperature and atmosphere, the magnetic properties of lms synthesized at different temperatures and in different atmospheres are summarized and listed in Table 1. Compared with M s value of 460 emu cm À3 (for the lm grown in oxygen atmosphere), M s value of the lms grown in argon atmosphere are much higher, while M s values of those grown in N 2 are signicantly lower. In addition to the dependence on growth atmosphere, M s also shows dependence on growth temperature. As the growth temperature changes, M s also varies to some extent. Considering that Ar is an inert gas, it will not take part in chemical reactions for lm formation when being used as a carrier gas. However, the existence of a large amount of Ar around the substrate may lead to a large number of oxygen vacancies in the obtained lms due to O deciency. To conrm this, the elemental compositions of the surfaces of the lm grown in Ar were analyzed by XPS (results not shown) and compared with those synthesized in O 2 . It was found that the   ion. The two le 3d electrons will be lled in majority t 2g states. Therefore, each Cr ion possesses around 2 m B moment. When one O vacancy appears around a Cr ion, the Cr ion will lose fewer electrons. Although appearance of the O vacancy will lead to small changes in the energies of 3d states of the Cr due to the variation of crystal eld, majority t 2g states still have lower energies than minority ones. All 3d ion electrons of the Cr will be lled in majority states. As a result, the moment of the Cr ion will increase 23 . When a Cr ion loses more than one neighbored O atoms, its minority 3d states may have lower energy than some majority ones due to large crystal eld splitting and be partially lled, which may lead to a decrease of the moment of the Cr ion. However, in this case, the moments of Cr ions in neighbored octahedrons may increase signicantly since each O vacancy is shared by three Cr ions (read reference [23] for detail). As a result, the total moment still increases. In the discussion above, we suppose the density of O vacancy in CrO 2 is not that high and there no vacancy aggregation. This supposition may be reasonable because very high density and aggregation of O vacancy may lead to the appearance of Cr 2 O 3 phase. In our lms, no Cr 2 O 3 was detected. Based on the discuss above, the larger M s of lms fabricated in Ar atmosphere may be due to existence of a larger number of O vacancies compared to the lms grown in O 2 . The variation in the quantity of O vacancies in lms grown at different temperatures may the reason for the growth temperature dependence of M s . The analysis of the elemental compositions found that a signicant number of N ions exist in the lms grown in N 2 ; suggesting that they are actually N-doped CrO 2 lms. This nding indicates that N is involved in the chemical reaction for lm formation due to its relatively high chemical reactivity. In their rst principle study, 24 Y. Xie et al. found N doping reduces the magnetization of CrO 2 . The comparatively lower M s of the lms grown in N 2 as shown in Table 1 may be attributed to the substitutions of N atoms for some of the O atoms in the lms. To investigate the reasons for the reduction of M s , density of states of N-doped CrO 2 was studied using rst principle method (results are not shown). It was found, even though the introduction of a N atom slightly changes the energies of 3d states of neighbored Cr ions, the majority t 2g states still have lower energy than minority ones. So, the 3d electrons are lled in spin up states. Therefore, the half metallicity of CrO 2 maintains. However, the crystal eld splitting led by N dopants makes the 3d electrons of Cr ions more delocalized, leading to a decrease of the moments of those Cr ions. Moreover, due to their incompletely occupied 2p states, N ions possess larger negative moments than O ions. The reduction of moments of Cr ions and fairly large negative moments of N are the reasons for the decrease of M s in N-doped CrO 2 lms. Based on XPS, N concentrations in lms grown at 320, 350, 370 and 390 C were determined to be 2.07%, 2.48%, 3.91%, and 2.39%, respectively. These results show some extent of temperature dependence, which may help to explain the observed growth temperature dependence of M s . In coherent switching model, the effective magnetic anisotropy (K) can be calculated using following equation: where M s and H k are the saturation magnetization and the hard axis switching eld, respectively. 25 The anisotropy constants shown in Table 1 indicate that lms grown in Ar or N 2 have higher magnetic anisotropies than those grown in O 2 . The rst principle study also suggests that N doping could increase the magnetic crystalline anisotropy of CrO 2 (results not shown). Therefore, higher anisotropies obtained for lms grown in Ar or N 2 may be attributed to the increase in the quantity of O vacancies or the substitution of N atoms for some of the O atoms in the lms. Materials with strong magnetic anisotropy are desirable for the improvement of thermal stability of spintronic devices. 26 CrO 2 lms with enhanced magnetic anisotropy may have important applications in devices. The temperature dependence of magnetizations for the lms grown at different temperatures and in different atmospheres was measured in a temperature range of 300-500 K with a 500 Oe magnetic eld applied along the c-axis. The Curie temperatures (T c ) were obtained from the M s vs. T curves, which are shown in Table 1. Generally speaking, the lms grown in Ar and N 2 have higher T c value than those grown in O 2 , suggesting that ferromagnetic (FM) phase of CrO 2 has better stability when synthesized in Ar or N 2 .

Thermal stability
To evaluate the stability of lms fabricated at low temperatures, lms grown in Ar or N 2 at 320 C were annealed at different temperatures in air for 60 min. The phase and crystal structures of each annealed lms were characterized using XRD, and the highest annealing temperature the lm can withstand was obtained. The XRD spectra of the lms annealed at different temperatures are shown in Fig. 6. Aer annealing at 410 C, only diffraction peaks of Cr 2 O 3 are observable, revealing that CrO 2 completely degraded into Cr 2 O 3 in the lm grown in Ar. The XRD spectrum without detectable Cr 2 O 3 peaks is only obtained Fig. 6 The XRD pattern of CrO 2 films synthesized (red) in argon atmosphere annealed at 410 C, (blue) in oxygen atmosphere annealed at 420 C, and (green) in nitrogen atmosphere annealed at 450 C.
when the annealing temperature is lower than 400 C. For lms grown in O 2 , signicant amounts of Cr 2 O 3 appears when the annealing temperature reaches 420 C. Although the main phase is still CrO 2 , the highest temperature it can withstand is 410 C. For the lm grown in N 2 atmosphere, even aer being annealed at 450 C, no Cr 2 O 3 peak is detectable in the XRD spectrum. Cr 2 O 3 phase appears in lms grown in N 2 when the annealing temperature is higher than 470 C. According to the different tolerances to annealing exhibited by the lms, lms grown in N 2 possessed the best thermal stability, while those grown in Ar are least stable. As discussed above, more O vacancies may form in lms when Ar is used as the carrier gas in instead of O 2 due to O deciency. According to our theoretical study, 23 once an O vacancy exits in an octahedral of CrO 2 crystal, new O vacancy tends to form in the same octahedral. As a result, Cr ion in the center of the octahedral will be reduced to Cr 3+ . Therefore, the existence of a larger number of O vacancies in the lms grown in Ar make them less stability compared to those grown in O 2 because Cr 2 O 3 more easily forms. In the lm growns in N 2 , a small number of N atoms substituted for O atoms. Since N is in 2p 3 valence state and the most stable ionic form of N is N 3À , a N atom can accept one more electron than an O atom. When N ions exist in a lm, they can weaken the effects of O vacancies around them. Therefore, the existence of N ions may retard the reduction of neighbored Cr ions, leading to an enhancement of thermal stability of the lm. It is pertinent to mention that N doping does not affect the half metallicity of CrO 2 according to the results of our rst principle study (results are not shown and will be published elsewhere).

Conclusion
Using Ar or N 2 as the carrier gas, epitaxial CrO 2 (100) oriented lms were synthesized on TiO 2 at different growth temperatures. The quality, magnetic properties, and thermal stability of the lms were evaluated. The conclusions are summarized as follows: (1) High quality, pure rutile phased CrO 2 (100) oriented lm can be grown at a temperature as low as 310 C on TiO 2 substrate using Ar or N 2 as the carrier gas.
(2) In Ar and N 2 atmospheres, the temperature windows for lm fabrication were 310-400 C and 300-430 C, respectively, which was more greatly broadened that the window in O 2 . This makes it possible to improve lm quality by growth temperature optimization.
(3) In Ar and N 2 atmospheres, lms with the best quality were obtained at 320 C, which both had a narrow orientation distribution and low roughness.
(4) The saturation magnetization, anisotropic energy, and Curie temperature of CrO 2 lms can be manipulated by adjusting the growth temperature and changing the carrier gas.
(5) The thermal stability of CrO 2 lm can be enhanced by using N 2 as the carrier gas, which may be of great signicance for practical applications.

Conflicts of interest
There are no conicts of interest to declare.