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Atomic layer deposition of YMnO3 thin films
Journal Pre-proofs Atomic Layer Deposition of YMnO3 thin films Ju H. Choi, Calvin Pham, James Dorman, Taeseung Kim, Jane P. Chang PII: DOI: Reference:
S0304-8853(19)32429-1 https://doi.org/10.1016/j.jmmm.2019.166146 MAGMA 166146
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
16 July 2019 2 October 2019 12 November 2019
Please cite this article as: J.H. Choi, C. Pham, J. Dorman, T. Kim, J.P. Chang, Atomic Layer Deposition of YMnO3 thin films, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm. 2019.166146
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Atomic Layer Deposition of YMnO3 thin films Ju H. Choi,1,2) Calvin Pham,2) James Dorman, 2) Taeseung Kim2) and Jane P. Chang2)* 1) Currently at Korea Photonics Technology Institute, 124 Cheomdanvenchure-ro 108-9, Buk-gu, Gwangju, 61007, Republic of Korea 2) Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095 *
To whom correspondence should be addressed. E-mail: [email protected].
1
Abstract YMnO3 (YMO) thin films were synthesized by radical-enhanced atomic layer deposition (RE-ALD) on silicon (Si) and yttria-stabilized zirconia (YSZ) substrates, to investigate the effect of film composition and substrates on their intrinsic magnetic properties. The crystalline phase of these ultra-thin films depends on both the processing conditions and the substrate lattice parameters. The Mn/Y atomic ratio of the YMO thin films could be controlled near unity by adjusting the Mn:Y precursor pulsing ratio during the RE-ALD processes. The ALD YMO thin film on Si(111) was orthorhombic, regardless of the film thickness with a Néel temperature (TN) between 48 ~ 62 K, as determined through the anomalies observed during DC magnetic susceptibility measurements. However, ultrathin ALD YMO films (~6 nm) on YSZ (111), at a Mn/Y atomic ratio near unity, has both orthorhombic- and hexagonal- phases, yielding two TN anomalies measured at ~48 K and ~85 K. The induction of magnetization of ultra-thin YMO film on Si (111) under an insitu 20 V electric poling indicates that the magnetoelectric coupling was observed below TN, showing that the ALD synthesis could be a promising technique to deposit ultra-thin magnetoelectric films.
Keywords: Multiferroic materials, Yttrium manganate, Radical-enhanced atomic layer deposition, Magnetic properties.
2
Introduction Since multiferroic materials simultaneously exhibit at least two ferroic properties among ferroelectricity, ferroelasticity, and ferromagnetism (or antiferromagnetism), they have considerable promise for applications in information storage, magneto-electric sensors, magneto-capacitive devices, and electrically driven magnetic data storage.[1],[2] Among several potential candidates, multiferroic materials based on rare earth manganites (RMnO3, R=Rare earth ions) have been studied recently due to advances in viable growth techniques alongside theoretical developments regarding the multiferroic properties. The RMnO3 family has two phases based on the crystal structure. One is noncentrosymmetrric hexagonal structure (P63cm)[3] in which geometrically driven effects lead to long-range dipole–dipole interactions and anion rotations to drive the system towards a stable ferroelectric state. [4] The other one is orthorhombically distorted perovskite structure (Pbnm)[5],[6],[7] which also exhibits ferroelectric Curie temperatures (TC ≤ 40K) and antiferromagnetic Néel temperatures (TN = 40-50 K).[8] YMnO3 is particularly interesting because it exists in hexagonal and orthorhombic structures, enabling a direct comparison of the material’s functionality in different lattice symmetries. For example, hexagonal YMnO3 (h-YMO) shows ferroelectricity along the c-axis due to the tilting of MnO5 bipyramids and the subsequent change in distance between O2- and Y3+ ions; additionally, it also exhibits canted antiferromagnetic behavior below the Néel temperature (TN = 65-80 K). In orthorhombic YMnO3 (o-YMO), the ferroelectric displacement is considered to originate from the spin current between noncollinear neighboring spins or the inverse Dzyaloshinskii-Moriya interaction.[9],[10] Although h-YMO is stable in bulk and thin film form, o-YMO can be obtained either by
3
high pressure synthesis[11] or by epitaxial stabilization when an appropriate substrate such as YAlO3Error! Bookmark not defined. and Nb-doped SrTiO3Error! Bookmark not defined. with a small lattice mismatch is chosen. It has also been shown that the magnetic properties of YMnO3 can be manipulated by oxygen vacancies.[12] YMO thin films have been previously fabricated via pulsed-laser depositionError! Bookmark not defined.,Error! Bookmark not defined.,
[13], sol-gel[14], sputtering[15],[16], metal-
organic chemical vapor deposition[17],[18] spark plasma sintering,[19] and molecular beam epitaxial[20] methods. It has been reported that YMO films on Si (111) exhibits both o- and h- phases, depending upon the deposition method used. [21],[22] However, only the h-YMO phase was observed when the substrates have similar lattice parameters, such as YSZ (111), due to the reduced lattice misfit.[23] Atomic layer deposition (ALD), which relies on self-limiting surface reactions during sequential pulsing of precursors,[24] may be a viable alternative deposition method due to its conformal coating and precision atomic control that can also enable complex nanostructured composites in 2D-2D, 3D-1D or 3D-0D configurations for designing engineered multiferroic systems. [25],[26] A thermal ALD processes with ozone as an oxygen source has been shown to be effective in synthesizing h-YMO on Si(100) and o-YMO on LaAlO3 and SrTiO3,Error! Bookmark not defined.,[27]
but the magnetic properties of YMO ultra-thin films were not fully
characterized. As such, this work focuses on synthesizing ultra-thin multiferroic films through the RE-ALD process and assessing the compositional and substrate effects on YMO magnetoelectric properties.
4
Experiments and characterization In this work, YMO thin films were deposited by RE-ALD process at low temperatures. Specifically, the metal β-diketonate precursors, tris(2,2,6,6-tetramethyl-3,5heptanedionato) yttrium and tris(2,2,6,6-tetramethyl-3,5-heptanedionato) manganese (Y(TMHD)3 and Mn(TMHD)3 respectively), were used as the precursors for Y and Mn. For the oxidant, oxygen atoms were generated from a coaxial waveguide microwave source, which was operated at 0.6 sccm O2 flow rate and 30-W microwave power.[28] During the RE-ALD process, the metal precursor and oxygen atoms were alternatively pulsed into the reactor at 250
C,
initiating self-limiting half-reactions which
simultaneously deposited a monolayer of thin film and created the reactive surface sites for the next complimentary half-reaction; which were then repeated in a cyclical fashion.[29] In this work, the RE-ALD cycle sequences were composed of a 30-sec Y(TMHD)3/ Y(TMHD)3 pulse followed by a 30-sec purge/pump-down, a 30-sec exposure to oxygen atoms, and then another 30-sec purge/pump-down. The Mn to Y precursor/oxidant pulsing ratio, Mn(TMHD)3/O:Y(TMHD)3/O, defined as Mn:Y, ranged from 1.0-1.8. Both Si (111) and YSZ (111) substrates were used to assess the effect of substrate and the corresponding magnetoelectric characteristics. The growth rate was determined by an ellipsometer (J.A. Woollam Co., M-88) and high resolution SEM cross section analysis (FEI, Nova 600) and ranged from 0.25~0.35 Å/cycle, which was dependent on the choice of substrates and processing conditions such as temperature. The
5
as-deposited amorphous films were annealed by rapid thermal annealing (RTA) process in the range of 800-1000 °C for 2 min to achieve desirable crystallinity. X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD) measurements were used to investigate the initial growth regime and the composition of YMO thin films. All of the spectra were referenced to the C 1s peak at 284 eV. A field emission scanning transmission electron microscope (FEI, Titon S/TEM) was used to image the interface of YMO and the substrate with an accelerating voltage 200 kV. High resolution transmission electron microscope (HR-TEM) images were collected using an ultrascan, 2×2k digital camera (Gatan) with digital micrograph acquisition capabilities. TEM samples were prepared using a SEM equipped with a focused ion beam. The crystal structures of the deposited films were characterized using X-ray diffraction (XRD) (Panalytical X’Pert) equipped with a Cu Kα radiation source (λ = 1.54 Å). The 2θ scan ranging from 20 to 60° was collected with a step size of 0.016°, using a dwell time of 1 sec per point. Crystal structures and atomic environments were modeled using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mn K-edge was measured at Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 4-3. In focus mode, a highly intense beam with ~2×1012 photons/sec could be achieved with an instrument resolution of 0.65 eV. The output monochromatic X-rays were incident on the sample and fluorescence-yield XAFS data for the thin films was acquired with a 13 element Ge detector cooled at 77 K. Magnetic properties were carried out using a superconducting quantum interference device (SQUID, Quantum Design MPMS). The temperature (T) dependence of the DC magnetic susceptibility (χ) (χ vs. T) was measured under zerofield- cooled (ZFC) and field-cooled (FC) conditions with a magnetic field of 100 Oe in
6
the temperature range from 5 K to 298 K. Magnetic field (H) dependence of magnetization (M) (M vs. H) was measured under ZFC and in-plane condition. The crystal phase of the ultra-thin films deposited using ALD depends on processing conditions in addition to the lattice parameters of a substrate. The Mn/Y atomic ratio for YMO thin films could be approached to unity by adjusting the Mn:Y precursor pulsing ratio. Therefore, the effects of substrate and film composition on intrinsic magnetic properties were investigated for ultra-thin YMO films grown on Si (111) and YSZ (111). In order to evaluate magnetoelectric coupling, the induction of magnetization under in-situ 20 V electric poling was analyzed. This analysis indicated that magnetic properties of ultra-thin YMO films were dependent on both the Mn/Y atomic ratio and the substrate, which could be used in the design of composite multiferroic materials. Results and Discussion Figure 1 (a) and (b) show TEM images of ultra-thin YMO films grown on both Si (111) and YSZ (111) substrates. As a reference, the films were annealed in a furnace at 850 C for different time intervals (1 h& 4 h) and it found that a SiO2 layer grown between YMnO3 and Si and YMnO3 layer was divided into regions, i.e., the c-axis oriented and the amorphous YMnO3 layer because it can be assumed that crystallization occurs at the top of the YMnO3 film more easily than at the interface of YMnO3 on Si during annealing process. This indicates that the furnace annealing is not an effective for the crystallization of amorphous YMnO3 film. In this regard, we have used rapid thermal annealing process at different temperatures (900 C and 1000 C) for the grown films and noticed two distinctive layers including a ~6 nm crystalline layer as shown in Figure 1(a). 7
Atomically sharp epitaxial interfaces were not observed but nano polycrystalline domains through the overall thickness were observed by TEM. The arrow in the Figure indicates the nano polycrystalline domains of YMO on Si (111) and YSZ (111) substrates. As shown in Figure 1(b), the YMnO3 films grown on YSZ(111) substrate showed a different aspect. Although, the substrate is well defined and atomically flat, the YMnO3 film reveals that apart from the main orientation, a secondary orientation with a rotation of the c-axis results in different domains. The presence of ferroelectric domains with a polarization oriented differently from the matrix could therefore have a considerable effect on the magnetoelectric properties of films. Figure 2 (a) and (b) show the dependence of the Mn/Y atomic ratio on the Mn:Y precursor pulsing ratio in the YMO films deposited on Si (111) and YSZ (111), respectively. From the XPS survey scan, as shown in the insets, distinctive Mn 2p, Y 3p and O 1s peaks were observed along with the substrate signal (Si 2p and Zr 3d from the two substrates, respectively). The obtained Y 3d5/2 and O 1s binding energies for YMO thin film were 153.6 and 527 eV, respectively, while the Mn 2p3/2 and Mn 2p1/2 spin-orbit doublet components were located at 641 and 652 eV, respectively. The Mn/Y atomic ratio in YMO thin films on Si (111) increased from 0.5 to 1.0 as the Mn:Y ALD precursor pulsing ratio increased from 1.2 to 1.8. On the other hand, the Mn/Y atomic ratio in YMO thin films on YSZ (111) increased from 0.7 to 1.1 as the Mn:Y ALD precursor pulsing ratio increased over the same range. The Mn/Y atomic ratio for YMO thin films on Si (111) and YSZ (111) was successfully tailored by controlling the Mn:Y precursor pulsing ratio, as verified by XPS.
8
The XRD spectra for the YMO thin films grown on Si (111) substrates are shown in Figure 3 (a) to identify the crystal phase obtained as a function of RTA temperature. As shown in Figure 3 (a), the peaks corresponding to o-YMO phase with space group Pbnm (JCPDS: 20-0732) was observed, which were consistent with metastable o-YMO fabricated from a solid state reaction.Error! Bookmark not defined. Apart from the Si (111) peaks, all of the observed peaks are indexed properly to the (202), (113), (004), and (131) planes of o-YMO, confirming a phase-pure o-YMO film with the c-axis normal to the film surface. The half width of the intense peaks ((202), (113), (004), and (131)) of o-YMO film on Si (111) annealed at 900 C and 1000 C were evaluated and were obtained to be 0.40, 0.48, 0.62, 0.81 and 0.39, 0.54, 0.51, 0.93, respectively. The integrated intensities of o-YMO film at both temperatures were found to be 191, 450, 554, 909 and 181, 615, 268, 671 units, respectively. Based on the intensities and half widths, the crystallinity of o-YMO increased as a function of the RTA temperature. The average crystallite size was calculated from the Debye-Scherrer equation and found to be around 17 nm. XRD patterns for YMO thin films on YSZ (111) are shown in Figure 3 (b) for different Mn/Y pulsing ratios. The signal to noise ratio is not as good in these spectra due to the ultra-thin layer of the crystalline film, as shown in Figure 1 (b). It was reported that h-YMO was formed on YSZ (111) because the 2.8% lattice mismatch between the hYMO and the YSZ (111).Error!
Bookmark not defined.
In this work, the data shows that the
synthesized materials have mixed phases: both h-YMO phase with space group P63cm (JCPDS: 25-1079) as well as o-YMn2O5 (112) were observed in samples of Mn/Y=0.7, while more distinctive (0004) and (122) peaks from h-YMO with space group P63cm
9
were obtained in samples of Mn/Y=1.1. This indicates that h-YMO phase with space group P63cm is more dominant as Mn/Y atomic ratio approached toward unity and YMO phase on YSZ (111) is more dependent on atomic composition ratio than on substrate lattice parameters in this system. In order to confirm the Mn3+ crystal structure obtained through the XRD measurements, EXAFS analysis was performed with the YMO films on Si (111) and YSZ (111) substrates. Based on the collected XRD spectra and the processing conditions, it was believed that two types of crystal structures should have existed based on the substrate used. Figure 4 shows the schematic structure of (a) h-YMO with MO5 polyhedron (d(Mn-O1)=1.687Å, d(Mn-O2)=2.043Å, d(Mn-O3)=1.830Å, d(Mn-O4)= 2.090Å) [30] and (b) o-YMO with MO6 polyhedron (d(Mn-O1)=2.209Å, d(Mn-O2)= 1.916Å, d(Mn-O3)= 1.947Å). [31] From the reference o-YMnO3 and h-YMnO3 crystal structures, the theoretical and average lengths in Mn-O bonds are 1.9125 Å for the hexagonal and 2.024 Å for the orthorhombic structure. The Fourier transformed spectra showed two unique R spaces based on the substrate used for YMO deposition. Using the h-YMO and o-YMO crystal structures as a guideline, the peaks were indexed to the respective Mn bonding distances. Additionally, R spaces for the manganate with hexagonal phase[32] and manganate with orthorhombic phase[33] as well as the references are plotted to further identify the crystal structures. The interaction distances of the ALD deposited YMO films were slightly larger than those of the reference but showed similar shape and relative intensities. Based on the comparison, it was confirmed that the YMO on the Si (111) has the orthorhombic phase and the YMO on YSZ (111) has the hexagonal phase. The observed magnetic properties were consistent with the
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observed difference in Mn-O bonding distances between YMO films on YSZ (111) and Si (111), as presented in EXAFS. The DC magnetic susceptibility () as a function of temperature for a ~6nm oYMO thin film on Si (111) were measured after zero-field-cooling (ZFC) and field– cooling (FC) procedures, with a magnetic field of 100 Oe applied in-plane of the film. The ZFC-FC divergence is pronounced in the ALD o-YMO thin films, as shown in Figure 5(a). As can be seen from the figure, a splitting of the ZFC-FC curves, with a maximum in the ZFC curve at TN = 57 K, indicates a spin glass-like behavior. It was also noted that the ZFC-FC curves have not come close above the TN may be due to nano polycrystalline nature of the YMO film confirmed from TEM analysis. The signal from this measurement is weak and shrouded by large paramagnetic contribution from the Si substrate. Nonetheless, o-YMO thin film exhibits a Curie-Weiss like linear temperature dependence at T>100K. It was found that another anomalous enhancement of χ was observed below 28 K. Typical spin–glass behavior was exhibited by a rapid increase in with as the temperature approached 0 K under ZFC condition.Error! Bookmark not defined. This steep anomaly below 28 K indicated the spin reorientation of Mn3+ spin, i.e. spin glasslike. This magnetization behavior was consistent with o-YMO on SrTiO3 (001) with the antiferromagnetic ordering at Neel temperature (TN) of 44 K and the reported spin glass transition at TSR=~28 K. [34] The magnetization measurements of amorphous YMO films also exhibit a spin-glass behavior with a maximum in the ZFC curve between 11 and 19 K. From the d/dT vs. temperature plot (inset of Figure 5(a)), at an external field of 100 Oe, antiferromagnetic ordering was observed at around TN = 57 K, consistent with antiferromagnetic ordering reported for o-YMO. [35],[36],[37] The inverse of as a
11
function of temperature (inset of Figure 5(a)), demonstrated two transitions such as Curie temperature (TC) at ~25 K and Neel temperature (TN) at ~62 K. The TN in thin films is somewhat challenging as the susceptibility of antiferromagnetic materials is small, and substrates and impurities may override film features. In the case of RMnO3 films (R = Rare earth elements) where the R3+ ions are magnetic, the presence of the large paramagnetic moment of the R3+ ions, that largely increases the magnetization upon lowering the temperature, may rend challenging the observation of a cusp of susceptibility associated to any ordering of the much smaller moments of Mn3+ ions. Moreover o-RMnO3 films with R3+ magnetic may display an anisotropic magnetic susceptibility extending far above the TN. This is mainly due to the quadrupolar moment of the R3+ ions that give rise to strong anisotropy of the susceptibility in the paramagnetic state that may mask the features at TN differently depending on the magnetic field direction. Therefore, the observation of features at TN may depend on the anisotropic background provided by the R3+ ions. In the present study, the TN was found to be around 57 K obtained from temperature dependent DC magnetic susceptibility (d/dT) for a 6 nm o-YMO film. The paramagnetic Curie-Weiss temperature (CW), defined as the intercept on the T-axis of the high temperature asymptote to the inverse susceptibility –T curve, was estimated to be -240K, within the range of literature reported values for o-YMnO3,40 suggesting that Mn ions enhanced the ferromagnetic interaction. The effective magnetic moment (eff) per Mn ion was estimated at 4.0B, also consistent with literature reported values.
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The temperature dependence of χ at H= 100 Oe for a ~6 nm h-YMO thin film on YSZ (111) was measured under ZFC and FC condition as shown in Figure 5 (b). In temperature dependence of χ, the ZFC-FC divergences were pronounced in a ~6 nm YMO thin films due to the large paramagnetic contribution from YSZ (111). Below 22 K, the ZFC and FC curves exhibited a rapid increase with decreasing temperature close to zero, reminiscent of typical spin–glass behavior.[38] In order to locate anomalies due to the antiferromagnetic transition, the temperature dependent magnetic susceptibility (d/dT) was plotted for a ~6 nm YMO thin films on YSZ (111) (inset). It clearly showed a distinguished cusp near antiferromagnetic TN ~ 44 K indicative of o-YMO films with Mn/Y=0.7 and 0.8. In the case of YMO thin films with Mn/Y=1.1, a subtle anomaly was observed at 48 K and a distinctive anomaly appeared at 85 K. The anomaly at 85 K was very consistent with the reported TN for h-YMO (TN=82-85 K). [39] These results were also consistent with the XRD results explained previously regarding the transition of the crystalline phase from orthorhombic to hexagonal as the Mn/Y atomic ratio increased towards unity. The plot of -1 vs. temperature for YMO thin films on YSZ (111) with different Mn/Y atomic ratio was used to extract CW and evaluate the antiferromagnetic coupling strength between Mn ions (inset). The extrapolated CW obtained for a ~6 nm YMO thin films with Mn/Y=0.7, 0.80 and 1.1 were -280 K, -440 K, and -580 K respectively. These values are similar to reported experimental values of -417 ~ -550 K for h-YMO,Error! Bookmark not defined.,[40] and the reported theoretical values.Error! Bookmark not defined.
The results suggest that the antiferromagnetic coupling strength between Mn ions
has become enhanced as the Mn/Y ratio approached unity. The enhanced antiferromagnetic interaction, i.e., antiferromagnetic coupling strength, may have driven
13
the systems from a frustrated antiferromagnetic state to a spin glass state at low temperatures.Error! Bookmark not defined. The cw/TN was used to evaluate the geometric frustration of the antiferromagnetic system, known as the frustration parameter (f).Error!
Bookmark not defined.
The range of f for o-YMO reported was 0.7 ~ 1.3, while the range of reported f for hYMO was higher at 5.32 ~ 7.33. Error! Bookmark not defined.,[41],[42],[43],[44] However, in one perovskite RMnO3 manganate system, YbMnO3, the f value for h-YbMnO3 is smaller than that of o-YbMnO3.Error! Bookmark not defined. In this work, f for h-YMO thin films with Mn/Y=1.1 was 6.8 while f for the o-YMO thin film with Mn/Y=0.7 was 6.4. These results suggested that the geometric frustration increased as the Mn/Y ratio in the YMO thin films on YSZ (111) approached unity. In other words, h-YMO was geometrically more frustrated than o-YMO, which was consistent with the trend in cw with YMO system mentioned above. The magnetic field dependence of magnetization, M vs. H was measured at 298 K and 20 K for a ~6 nm YMO thin films on Si (111) as shown in Figure 6. A small hysteresis loop at both 298 K and 20 K were observed. A linear variation in the M vs H curve at room temperature (298 K) indicates a paramagnetic behavior. The o-YMO film on Si (111) of the present study exhibited a small hysteresis loop at low and room temperatures, indicating a ferromagnetic contribution. This feature was not observed for larger crystallite size samples. Bulk YMO is known to be an antiferromagnetic material. However, ferromagnetic behavior in YMO was observed in naoparticleError! Bookmark not defined.
or ultra-thin filmError!
Bookmark not defined.
systems, which were likely to be due to
uncompensated surface spin. As shown in Figure 1, an interrupted lattice line with ~6 nm
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YMO thin films through the overall thickness was observed by TEM, which might result in weak ferromagnetic properties as observed at ultra-thin film system. In addition, the existence of the ferromagnetic behavior in antiferromagnetic material system has been explained by the interaction of canted spins as a result of the Dzyaloshinskii-Moriya interaction,[45],[46] which was considered to be a spin current from ferroelectric displacement between non-collinear neighboring spins in o-YMO thin films.Error! Bookmark not defined.,Error! Bookmark not defined.
The magnetic properties such as remnant magnetization (Mr), saturation magnetizations (Ms), and the coercive magnetic field (HC) were determined from the M vs H analysis curve at 298 K and 20 K. It was found that the Ms for YMO thin films on Si (111) at 298 K and 20 K were about to 3.34 (× 10-5, emu) and 4.24 (× 10-5, emu), respectively while the Mr were almost similar. It is indicative of higher resistance to rotation of spin orientation at low temperature. In order to evaluate the electric field induction of magnetization, the magnetic field dependence of magnetization, M vs. H, was performed under in-situ 20 V electric polling using a SQUID magnetometer. As shown in insets of Figure 6, before electric poling, the splitting of hysteresis loop was not observed at 298 K but it was observed at 20 K for o-YMO films on Si (111). After 20 V poling, the splitting of hysteresis loop was noted at both temperatures (298 K and 20 K). The HC at both temperatures were found to be more or less similar of the order of 100 (Oe) at both temperatures for YMO film on Si (111), indicating a ferromagnetic contribution. However, HC for YMO films on YSZ (111) was nearly unchanged after 20 V poling at 298 K and 20 K (not shown). It might not have been fully poled because of the high dielectric constant (24-29.5) of YSZ substrate.
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Summary Ultra-thin ALD YMO films on Si (111) and YSZ (111) were synthesized using the REALD method. The atomic composition, crystal phase and Mn3+ local bonding environment in YMO thin films were investigated as well as magnetic properties. Anomalies in the temperature dependent DC magnetic susceptibility (d/dT) at TN= ~57 K for YMO thin films on Si (111) indicated the o-YMO crystal phase. The ALD YMO thin films on YSZ (111) showing two anomalies at ~48 K and at ~85 K when Mn/Y ratio approached unity in the temperature dependent DC magnetic susceptibility, where XRD showed the peaks corresponding to dominant h-YMO phase and minor o-YMO phase. The YMO phase on YSZ (111) was more dependent on atomic composition ratio than on substrate lattice parameters. It was also confirmed that YMO on Si (111) has the orthorhombic phase and YMO on YSZ (111) has the hexagonal phase based on EXAFS analysis. In order to evaluate the electric field induction of magnetization, the magnetic field dependence of magnetization, M vs. H, was performed under in-situ 20 V electric poling. The HC for YMO on Si (111) at 298 K and 20 K was obtained to be 100 Oe, which indicated magneto-electric coupling below TN. The RE-ALD was shown as a viable technique with attractive features to fabricate the ultra-thin films in engineered multiferroic systems. The detailed analysis of magnetic properties of ultra-thin ALD YMO films in terms of the Mn/Y atomic ratio and type of substrate could be used for the design of composite multiferroic systems with 3D-3D or 3D-1D geometries.
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Acknowledgements This work was supported in part by the FAME Center, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA, as well as Functional Engineered Nano Architectonics (FENA), an SRC FCRP center. Part of this work was carried out in part at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The authors would also like to thank Dr. Ignacio Martini in the UCLA Department of Chemistry for his assistance in performing SQUID magnetic measurements.
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Figure captions Figure 1. Bright field TEM images of ~6 nm YMO ultra thin films on (a) Si (111) and (b) YSZ (111) Figure 2. (a) Composition ratio of ~6 nm YMO ultra thin films on Si (111) with different Mn:Y precursor pulsing ratio. Inset shows survey XPS spectra for YMO thin film on Si (111) with Mn/Y=0.95, (b) Composition ratio of ~6 nm YMO ultra thin films on YSZ (111) with different Mn:Y precursor pulsing cycles. Inset shows survey XPS spectra for YMO thin film on YSZ (111) with Mn/Y=1.07. Figure 3. (a) The XRD of YMO thin films on Si (111) as function of RTA condition. The crystalline peaks according to RTA condition at 900oC and 1000oC are compared to the peaks from bare Si (111) substrate. (b) The XRD of YMO thin films on YSZ (111) as function of Mn/Y ratio after RTA 900oC. The crystalline peaks according to Mn/Y ratio are compared to the peaks from bare YSZ (111) substrate. Figure 4. Schematic structure of (a) h-YMO with MO5 polyhedron (d(Mn-O1)=1.687Å, d(Mn-O2)=2.043Å, d(Mn-O3)=1.830Å, d(Mn-O4)=2.090Å)32 and (b) o-YMO with MO6 polyhedron (d(Mn-O1)=2.209Å, d(Mn-O2)=1.916Å, d(Mn-O3)=1.947Å)33. (c) The Fourier transformed EXAFS R spectra for 80 nm YMO thin films on Si (111) and 80 nm YMO thin films YSZ (111). The spheres represent the O atoms, the and represent the Y and Mn atoms, respectively. Figure 5. (a) DC magnetic susceptibility () for the ~6 nm YMO thin films on Si (111) under 100 Oe. Solid symbols represent ZFC condition and open symbols represent FC condition. The two insets shows d/dT and -1, respectively. (b) DC magnetic susceptibility () for ~6 nm YMO thin films on YSZ (111) with different Mn/Y atomic ratios under 100 Oe. The two insets shows d/dT and -1, respectively. The two insets shows d/dT and -1, respectively. Figure 6. Magnetic hysteresis loop (M vs. H) measured at 20 K and 298 K for a ~6 nm YMO ultra-thin films on Si (111) before and after 20 V electric polling. The inset shows the zoom-in view of the coercive field measurements.
18
Figure 1
19
1000
Si: 2p 3/2
0.4
YMO/Si (111) Mn/Y=0.53 Mn/Y=0.67 Mn/Y=0.95
Si: 2S
Y: 3P 1/2
Y: 3P 3/2
Mn: 2P 3/2
Mn: 2P 1/2
5
Composition ratio (%)
(a) 0.6
Intensity (x10 Arb. Unit)
Figure 2
800
600
400
Binding energy (eV)
200
0
0.2
0.0 Mn
Y
Si
C
1000
Zr: 3d 5/2
YMO/YSZ (111) Mn/Y=0.69 Mn/Y=0.80 Mn/Y=1.07
Zr: 3d 3/2
0.4
Y: 3P 1/2 Y: 3P 3/2
0.6
Intensity (x10 5 Arb. Unit)
Composition ratio (%)
(b)
Mn: 2P 1/2 Mn: 2P 3/2
Composition elements
800
600
400
200
Binding energy (eV)
0
0.2
0.0
Mn
Y
Zr
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20
C
Figure 3
(a)
Intensity (arb. unit)
YMO on Si (111) o-YMO: Pbnm JCPDS: 20-0732
Si (111) o
1000 C o
30
35
40 45 2 (degree)
(b)
(131)
(004)
(113)
(202)
(211)
(120)
(020)
900 C
50
55
YMO on YSZ (111) h-YMO: P63cm
Intensity (arb. unit)
JCPDS: 25-1079
30
35
(0006)
(112)
Mn/Y=1.07
O-YMn2O5
(0004) (1122)
YSZ (111)
40 45 2 (degree)
21
Mn/Y=0.69
50
55
Figure 4
5 YMO /Si (111)
4
Intensity (arb. unit)
Mn-O2
Mn-O1
Mn-Y
3 2
YMO /YSZ (111)
Mn-O2
1 0 -1
0
1
2
3
R+R (Å)
22
4
5
Figure 5
(a)
0.010 6.0
(emu/Oe cm3)
ddT
0.008
~80 nm YMO YMO/Si (111)
0.006
20
40
60
(1/emu) 10
5
6nm YMO
TC~25K
TN~62K
5.8 5.5 5.3 0
80
Temperature (K)
30 60 90 120 Temperature (k)
FC
0.004
0.002
0.000
ZFC 0
50
100
~6nm YMO/Si (111) 150
200
250
300
Temperature (K)
Solid: ZFC Open: FC , : Mn/Y=0.69 , : Mn/Y=0.80 , : Mn/Y=1.1
0.10
0.0003
20
40
60
Temperature (k)
80
3
(emu/Oe cm )
3
(cm /emu )
0.11 d /dT
(b) 0.0004
0.09 Mn/Y=1.07
0.08 -600
0.0002
Mn/Y=0.80 Mn/Y=0.69
-450
-300
-150
0
Temperature (K)
150
300
0.0001
0
50
100
150
200
Temperature (K)
23
250
300
Figure 6
-5
4x10
-5 -5
2x10
YMnO3 on Si (111) 298K 20K
0.005 20K 298K
0.004 0.003
Before
0.002
-5
M (emu)
1x10
0.001 0.000
-0.001 -0.002
0
-0.003 -0.004 -0.005
-400
-200
0
200
400
H (Oe)
-5
-1x10
0.005 0.004 0.003
20K
After
298K
0.002
-5
-2x10
M (emu)
Magnetization, M (emu)
3x10
0.001 0.000
-0.001 -0.002
-5
-3x10
-0.003 -0.004 -0.005 -500
-5
-4x10
-250
0
250
500
H (Oe)
-2000 -1000
0
1000
2000
Magnetic Field, H (Oe)
24
3000
References
References
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27
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S0304-8853(19)32429-1 https://doi.org/10.1016/j.jmmm.2019.166146 MAGMA 166146
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
16 July 2019 2 October 2019 12 November 2019
Please cite this article as: J.H. Choi, C. Pham, J. Dorman, T. Kim, J.P. Chang, Atomic Layer Deposition of YMnO3 thin films, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm. 2019.166146
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© 2019 Published by Elsevier B.V.
Atomic Layer Deposition of YMnO3 thin films Ju H. Choi,1,2) Calvin Pham,2) James Dorman, 2) Taeseung Kim2) and Jane P. Chang2)* 1) Currently at Korea Photonics Technology Institute, 124 Cheomdanvenchure-ro 108-9, Buk-gu, Gwangju, 61007, Republic of Korea 2) Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095 *
To whom correspondence should be addressed. E-mail: [email protected].
1
Abstract YMnO3 (YMO) thin films were synthesized by radical-enhanced atomic layer deposition (RE-ALD) on silicon (Si) and yttria-stabilized zirconia (YSZ) substrates, to investigate the effect of film composition and substrates on their intrinsic magnetic properties. The crystalline phase of these ultra-thin films depends on both the processing conditions and the substrate lattice parameters. The Mn/Y atomic ratio of the YMO thin films could be controlled near unity by adjusting the Mn:Y precursor pulsing ratio during the RE-ALD processes. The ALD YMO thin film on Si(111) was orthorhombic, regardless of the film thickness with a Néel temperature (TN) between 48 ~ 62 K, as determined through the anomalies observed during DC magnetic susceptibility measurements. However, ultrathin ALD YMO films (~6 nm) on YSZ (111), at a Mn/Y atomic ratio near unity, has both orthorhombic- and hexagonal- phases, yielding two TN anomalies measured at ~48 K and ~85 K. The induction of magnetization of ultra-thin YMO film on Si (111) under an insitu 20 V electric poling indicates that the magnetoelectric coupling was observed below TN, showing that the ALD synthesis could be a promising technique to deposit ultra-thin magnetoelectric films.
Keywords: Multiferroic materials, Yttrium manganate, Radical-enhanced atomic layer deposition, Magnetic properties.
2
Introduction Since multiferroic materials simultaneously exhibit at least two ferroic properties among ferroelectricity, ferroelasticity, and ferromagnetism (or antiferromagnetism), they have considerable promise for applications in information storage, magneto-electric sensors, magneto-capacitive devices, and electrically driven magnetic data storage.[1],[2] Among several potential candidates, multiferroic materials based on rare earth manganites (RMnO3, R=Rare earth ions) have been studied recently due to advances in viable growth techniques alongside theoretical developments regarding the multiferroic properties. The RMnO3 family has two phases based on the crystal structure. One is noncentrosymmetrric hexagonal structure (P63cm)[3] in which geometrically driven effects lead to long-range dipole–dipole interactions and anion rotations to drive the system towards a stable ferroelectric state. [4] The other one is orthorhombically distorted perovskite structure (Pbnm)[5],[6],[7] which also exhibits ferroelectric Curie temperatures (TC ≤ 40K) and antiferromagnetic Néel temperatures (TN = 40-50 K).[8] YMnO3 is particularly interesting because it exists in hexagonal and orthorhombic structures, enabling a direct comparison of the material’s functionality in different lattice symmetries. For example, hexagonal YMnO3 (h-YMO) shows ferroelectricity along the c-axis due to the tilting of MnO5 bipyramids and the subsequent change in distance between O2- and Y3+ ions; additionally, it also exhibits canted antiferromagnetic behavior below the Néel temperature (TN = 65-80 K). In orthorhombic YMnO3 (o-YMO), the ferroelectric displacement is considered to originate from the spin current between noncollinear neighboring spins or the inverse Dzyaloshinskii-Moriya interaction.[9],[10] Although h-YMO is stable in bulk and thin film form, o-YMO can be obtained either by
3
high pressure synthesis[11] or by epitaxial stabilization when an appropriate substrate such as YAlO3Error! Bookmark not defined. and Nb-doped SrTiO3Error! Bookmark not defined. with a small lattice mismatch is chosen. It has also been shown that the magnetic properties of YMnO3 can be manipulated by oxygen vacancies.[12] YMO thin films have been previously fabricated via pulsed-laser depositionError! Bookmark not defined.,Error! Bookmark not defined.,
[13], sol-gel[14], sputtering[15],[16], metal-
organic chemical vapor deposition[17],[18] spark plasma sintering,[19] and molecular beam epitaxial[20] methods. It has been reported that YMO films on Si (111) exhibits both o- and h- phases, depending upon the deposition method used. [21],[22] However, only the h-YMO phase was observed when the substrates have similar lattice parameters, such as YSZ (111), due to the reduced lattice misfit.[23] Atomic layer deposition (ALD), which relies on self-limiting surface reactions during sequential pulsing of precursors,[24] may be a viable alternative deposition method due to its conformal coating and precision atomic control that can also enable complex nanostructured composites in 2D-2D, 3D-1D or 3D-0D configurations for designing engineered multiferroic systems. [25],[26] A thermal ALD processes with ozone as an oxygen source has been shown to be effective in synthesizing h-YMO on Si(100) and o-YMO on LaAlO3 and SrTiO3,Error! Bookmark not defined.,[27]
but the magnetic properties of YMO ultra-thin films were not fully
characterized. As such, this work focuses on synthesizing ultra-thin multiferroic films through the RE-ALD process and assessing the compositional and substrate effects on YMO magnetoelectric properties.
4
Experiments and characterization In this work, YMO thin films were deposited by RE-ALD process at low temperatures. Specifically, the metal β-diketonate precursors, tris(2,2,6,6-tetramethyl-3,5heptanedionato) yttrium and tris(2,2,6,6-tetramethyl-3,5-heptanedionato) manganese (Y(TMHD)3 and Mn(TMHD)3 respectively), were used as the precursors for Y and Mn. For the oxidant, oxygen atoms were generated from a coaxial waveguide microwave source, which was operated at 0.6 sccm O2 flow rate and 30-W microwave power.[28] During the RE-ALD process, the metal precursor and oxygen atoms were alternatively pulsed into the reactor at 250
C,
initiating self-limiting half-reactions which
simultaneously deposited a monolayer of thin film and created the reactive surface sites for the next complimentary half-reaction; which were then repeated in a cyclical fashion.[29] In this work, the RE-ALD cycle sequences were composed of a 30-sec Y(TMHD)3/ Y(TMHD)3 pulse followed by a 30-sec purge/pump-down, a 30-sec exposure to oxygen atoms, and then another 30-sec purge/pump-down. The Mn to Y precursor/oxidant pulsing ratio, Mn(TMHD)3/O:Y(TMHD)3/O, defined as Mn:Y, ranged from 1.0-1.8. Both Si (111) and YSZ (111) substrates were used to assess the effect of substrate and the corresponding magnetoelectric characteristics. The growth rate was determined by an ellipsometer (J.A. Woollam Co., M-88) and high resolution SEM cross section analysis (FEI, Nova 600) and ranged from 0.25~0.35 Å/cycle, which was dependent on the choice of substrates and processing conditions such as temperature. The
5
as-deposited amorphous films were annealed by rapid thermal annealing (RTA) process in the range of 800-1000 °C for 2 min to achieve desirable crystallinity. X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD) measurements were used to investigate the initial growth regime and the composition of YMO thin films. All of the spectra were referenced to the C 1s peak at 284 eV. A field emission scanning transmission electron microscope (FEI, Titon S/TEM) was used to image the interface of YMO and the substrate with an accelerating voltage 200 kV. High resolution transmission electron microscope (HR-TEM) images were collected using an ultrascan, 2×2k digital camera (Gatan) with digital micrograph acquisition capabilities. TEM samples were prepared using a SEM equipped with a focused ion beam. The crystal structures of the deposited films were characterized using X-ray diffraction (XRD) (Panalytical X’Pert) equipped with a Cu Kα radiation source (λ = 1.54 Å). The 2θ scan ranging from 20 to 60° was collected with a step size of 0.016°, using a dwell time of 1 sec per point. Crystal structures and atomic environments were modeled using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mn K-edge was measured at Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 4-3. In focus mode, a highly intense beam with ~2×1012 photons/sec could be achieved with an instrument resolution of 0.65 eV. The output monochromatic X-rays were incident on the sample and fluorescence-yield XAFS data for the thin films was acquired with a 13 element Ge detector cooled at 77 K. Magnetic properties were carried out using a superconducting quantum interference device (SQUID, Quantum Design MPMS). The temperature (T) dependence of the DC magnetic susceptibility (χ) (χ vs. T) was measured under zerofield- cooled (ZFC) and field-cooled (FC) conditions with a magnetic field of 100 Oe in
6
the temperature range from 5 K to 298 K. Magnetic field (H) dependence of magnetization (M) (M vs. H) was measured under ZFC and in-plane condition. The crystal phase of the ultra-thin films deposited using ALD depends on processing conditions in addition to the lattice parameters of a substrate. The Mn/Y atomic ratio for YMO thin films could be approached to unity by adjusting the Mn:Y precursor pulsing ratio. Therefore, the effects of substrate and film composition on intrinsic magnetic properties were investigated for ultra-thin YMO films grown on Si (111) and YSZ (111). In order to evaluate magnetoelectric coupling, the induction of magnetization under in-situ 20 V electric poling was analyzed. This analysis indicated that magnetic properties of ultra-thin YMO films were dependent on both the Mn/Y atomic ratio and the substrate, which could be used in the design of composite multiferroic materials. Results and Discussion Figure 1 (a) and (b) show TEM images of ultra-thin YMO films grown on both Si (111) and YSZ (111) substrates. As a reference, the films were annealed in a furnace at 850 C for different time intervals (1 h& 4 h) and it found that a SiO2 layer grown between YMnO3 and Si and YMnO3 layer was divided into regions, i.e., the c-axis oriented and the amorphous YMnO3 layer because it can be assumed that crystallization occurs at the top of the YMnO3 film more easily than at the interface of YMnO3 on Si during annealing process. This indicates that the furnace annealing is not an effective for the crystallization of amorphous YMnO3 film. In this regard, we have used rapid thermal annealing process at different temperatures (900 C and 1000 C) for the grown films and noticed two distinctive layers including a ~6 nm crystalline layer as shown in Figure 1(a). 7
Atomically sharp epitaxial interfaces were not observed but nano polycrystalline domains through the overall thickness were observed by TEM. The arrow in the Figure indicates the nano polycrystalline domains of YMO on Si (111) and YSZ (111) substrates. As shown in Figure 1(b), the YMnO3 films grown on YSZ(111) substrate showed a different aspect. Although, the substrate is well defined and atomically flat, the YMnO3 film reveals that apart from the main orientation, a secondary orientation with a rotation of the c-axis results in different domains. The presence of ferroelectric domains with a polarization oriented differently from the matrix could therefore have a considerable effect on the magnetoelectric properties of films. Figure 2 (a) and (b) show the dependence of the Mn/Y atomic ratio on the Mn:Y precursor pulsing ratio in the YMO films deposited on Si (111) and YSZ (111), respectively. From the XPS survey scan, as shown in the insets, distinctive Mn 2p, Y 3p and O 1s peaks were observed along with the substrate signal (Si 2p and Zr 3d from the two substrates, respectively). The obtained Y 3d5/2 and O 1s binding energies for YMO thin film were 153.6 and 527 eV, respectively, while the Mn 2p3/2 and Mn 2p1/2 spin-orbit doublet components were located at 641 and 652 eV, respectively. The Mn/Y atomic ratio in YMO thin films on Si (111) increased from 0.5 to 1.0 as the Mn:Y ALD precursor pulsing ratio increased from 1.2 to 1.8. On the other hand, the Mn/Y atomic ratio in YMO thin films on YSZ (111) increased from 0.7 to 1.1 as the Mn:Y ALD precursor pulsing ratio increased over the same range. The Mn/Y atomic ratio for YMO thin films on Si (111) and YSZ (111) was successfully tailored by controlling the Mn:Y precursor pulsing ratio, as verified by XPS.
8
The XRD spectra for the YMO thin films grown on Si (111) substrates are shown in Figure 3 (a) to identify the crystal phase obtained as a function of RTA temperature. As shown in Figure 3 (a), the peaks corresponding to o-YMO phase with space group Pbnm (JCPDS: 20-0732) was observed, which were consistent with metastable o-YMO fabricated from a solid state reaction.Error! Bookmark not defined. Apart from the Si (111) peaks, all of the observed peaks are indexed properly to the (202), (113), (004), and (131) planes of o-YMO, confirming a phase-pure o-YMO film with the c-axis normal to the film surface. The half width of the intense peaks ((202), (113), (004), and (131)) of o-YMO film on Si (111) annealed at 900 C and 1000 C were evaluated and were obtained to be 0.40, 0.48, 0.62, 0.81 and 0.39, 0.54, 0.51, 0.93, respectively. The integrated intensities of o-YMO film at both temperatures were found to be 191, 450, 554, 909 and 181, 615, 268, 671 units, respectively. Based on the intensities and half widths, the crystallinity of o-YMO increased as a function of the RTA temperature. The average crystallite size was calculated from the Debye-Scherrer equation and found to be around 17 nm. XRD patterns for YMO thin films on YSZ (111) are shown in Figure 3 (b) for different Mn/Y pulsing ratios. The signal to noise ratio is not as good in these spectra due to the ultra-thin layer of the crystalline film, as shown in Figure 1 (b). It was reported that h-YMO was formed on YSZ (111) because the 2.8% lattice mismatch between the hYMO and the YSZ (111).Error!
Bookmark not defined.
In this work, the data shows that the
synthesized materials have mixed phases: both h-YMO phase with space group P63cm (JCPDS: 25-1079) as well as o-YMn2O5 (112) were observed in samples of Mn/Y=0.7, while more distinctive (0004) and (122) peaks from h-YMO with space group P63cm
9
were obtained in samples of Mn/Y=1.1. This indicates that h-YMO phase with space group P63cm is more dominant as Mn/Y atomic ratio approached toward unity and YMO phase on YSZ (111) is more dependent on atomic composition ratio than on substrate lattice parameters in this system. In order to confirm the Mn3+ crystal structure obtained through the XRD measurements, EXAFS analysis was performed with the YMO films on Si (111) and YSZ (111) substrates. Based on the collected XRD spectra and the processing conditions, it was believed that two types of crystal structures should have existed based on the substrate used. Figure 4 shows the schematic structure of (a) h-YMO with MO5 polyhedron (d(Mn-O1)=1.687Å, d(Mn-O2)=2.043Å, d(Mn-O3)=1.830Å, d(Mn-O4)= 2.090Å) [30] and (b) o-YMO with MO6 polyhedron (d(Mn-O1)=2.209Å, d(Mn-O2)= 1.916Å, d(Mn-O3)= 1.947Å). [31] From the reference o-YMnO3 and h-YMnO3 crystal structures, the theoretical and average lengths in Mn-O bonds are 1.9125 Å for the hexagonal and 2.024 Å for the orthorhombic structure. The Fourier transformed spectra showed two unique R spaces based on the substrate used for YMO deposition. Using the h-YMO and o-YMO crystal structures as a guideline, the peaks were indexed to the respective Mn bonding distances. Additionally, R spaces for the manganate with hexagonal phase[32] and manganate with orthorhombic phase[33] as well as the references are plotted to further identify the crystal structures. The interaction distances of the ALD deposited YMO films were slightly larger than those of the reference but showed similar shape and relative intensities. Based on the comparison, it was confirmed that the YMO on the Si (111) has the orthorhombic phase and the YMO on YSZ (111) has the hexagonal phase. The observed magnetic properties were consistent with the
10
observed difference in Mn-O bonding distances between YMO films on YSZ (111) and Si (111), as presented in EXAFS. The DC magnetic susceptibility () as a function of temperature for a ~6nm oYMO thin film on Si (111) were measured after zero-field-cooling (ZFC) and field– cooling (FC) procedures, with a magnetic field of 100 Oe applied in-plane of the film. The ZFC-FC divergence is pronounced in the ALD o-YMO thin films, as shown in Figure 5(a). As can be seen from the figure, a splitting of the ZFC-FC curves, with a maximum in the ZFC curve at TN = 57 K, indicates a spin glass-like behavior. It was also noted that the ZFC-FC curves have not come close above the TN may be due to nano polycrystalline nature of the YMO film confirmed from TEM analysis. The signal from this measurement is weak and shrouded by large paramagnetic contribution from the Si substrate. Nonetheless, o-YMO thin film exhibits a Curie-Weiss like linear temperature dependence at T>100K. It was found that another anomalous enhancement of χ was observed below 28 K. Typical spin–glass behavior was exhibited by a rapid increase in with as the temperature approached 0 K under ZFC condition.Error! Bookmark not defined. This steep anomaly below 28 K indicated the spin reorientation of Mn3+ spin, i.e. spin glasslike. This magnetization behavior was consistent with o-YMO on SrTiO3 (001) with the antiferromagnetic ordering at Neel temperature (TN) of 44 K and the reported spin glass transition at TSR=~28 K. [34] The magnetization measurements of amorphous YMO films also exhibit a spin-glass behavior with a maximum in the ZFC curve between 11 and 19 K. From the d/dT vs. temperature plot (inset of Figure 5(a)), at an external field of 100 Oe, antiferromagnetic ordering was observed at around TN = 57 K, consistent with antiferromagnetic ordering reported for o-YMO. [35],[36],[37] The inverse of as a
11
function of temperature (inset of Figure 5(a)), demonstrated two transitions such as Curie temperature (TC) at ~25 K and Neel temperature (TN) at ~62 K. The TN in thin films is somewhat challenging as the susceptibility of antiferromagnetic materials is small, and substrates and impurities may override film features. In the case of RMnO3 films (R = Rare earth elements) where the R3+ ions are magnetic, the presence of the large paramagnetic moment of the R3+ ions, that largely increases the magnetization upon lowering the temperature, may rend challenging the observation of a cusp of susceptibility associated to any ordering of the much smaller moments of Mn3+ ions. Moreover o-RMnO3 films with R3+ magnetic may display an anisotropic magnetic susceptibility extending far above the TN. This is mainly due to the quadrupolar moment of the R3+ ions that give rise to strong anisotropy of the susceptibility in the paramagnetic state that may mask the features at TN differently depending on the magnetic field direction. Therefore, the observation of features at TN may depend on the anisotropic background provided by the R3+ ions. In the present study, the TN was found to be around 57 K obtained from temperature dependent DC magnetic susceptibility (d/dT) for a 6 nm o-YMO film. The paramagnetic Curie-Weiss temperature (CW), defined as the intercept on the T-axis of the high temperature asymptote to the inverse susceptibility –T curve, was estimated to be -240K, within the range of literature reported values for o-YMnO3,40 suggesting that Mn ions enhanced the ferromagnetic interaction. The effective magnetic moment (eff) per Mn ion was estimated at 4.0B, also consistent with literature reported values.
12
The temperature dependence of χ at H= 100 Oe for a ~6 nm h-YMO thin film on YSZ (111) was measured under ZFC and FC condition as shown in Figure 5 (b). In temperature dependence of χ, the ZFC-FC divergences were pronounced in a ~6 nm YMO thin films due to the large paramagnetic contribution from YSZ (111). Below 22 K, the ZFC and FC curves exhibited a rapid increase with decreasing temperature close to zero, reminiscent of typical spin–glass behavior.[38] In order to locate anomalies due to the antiferromagnetic transition, the temperature dependent magnetic susceptibility (d/dT) was plotted for a ~6 nm YMO thin films on YSZ (111) (inset). It clearly showed a distinguished cusp near antiferromagnetic TN ~ 44 K indicative of o-YMO films with Mn/Y=0.7 and 0.8. In the case of YMO thin films with Mn/Y=1.1, a subtle anomaly was observed at 48 K and a distinctive anomaly appeared at 85 K. The anomaly at 85 K was very consistent with the reported TN for h-YMO (TN=82-85 K). [39] These results were also consistent with the XRD results explained previously regarding the transition of the crystalline phase from orthorhombic to hexagonal as the Mn/Y atomic ratio increased towards unity. The plot of -1 vs. temperature for YMO thin films on YSZ (111) with different Mn/Y atomic ratio was used to extract CW and evaluate the antiferromagnetic coupling strength between Mn ions (inset). The extrapolated CW obtained for a ~6 nm YMO thin films with Mn/Y=0.7, 0.80 and 1.1 were -280 K, -440 K, and -580 K respectively. These values are similar to reported experimental values of -417 ~ -550 K for h-YMO,Error! Bookmark not defined.,[40] and the reported theoretical values.Error! Bookmark not defined.
The results suggest that the antiferromagnetic coupling strength between Mn ions
has become enhanced as the Mn/Y ratio approached unity. The enhanced antiferromagnetic interaction, i.e., antiferromagnetic coupling strength, may have driven
13
the systems from a frustrated antiferromagnetic state to a spin glass state at low temperatures.Error! Bookmark not defined. The cw/TN was used to evaluate the geometric frustration of the antiferromagnetic system, known as the frustration parameter (f).Error!
Bookmark not defined.
The range of f for o-YMO reported was 0.7 ~ 1.3, while the range of reported f for hYMO was higher at 5.32 ~ 7.33. Error! Bookmark not defined.,[41],[42],[43],[44] However, in one perovskite RMnO3 manganate system, YbMnO3, the f value for h-YbMnO3 is smaller than that of o-YbMnO3.Error! Bookmark not defined. In this work, f for h-YMO thin films with Mn/Y=1.1 was 6.8 while f for the o-YMO thin film with Mn/Y=0.7 was 6.4. These results suggested that the geometric frustration increased as the Mn/Y ratio in the YMO thin films on YSZ (111) approached unity. In other words, h-YMO was geometrically more frustrated than o-YMO, which was consistent with the trend in cw with YMO system mentioned above. The magnetic field dependence of magnetization, M vs. H was measured at 298 K and 20 K for a ~6 nm YMO thin films on Si (111) as shown in Figure 6. A small hysteresis loop at both 298 K and 20 K were observed. A linear variation in the M vs H curve at room temperature (298 K) indicates a paramagnetic behavior. The o-YMO film on Si (111) of the present study exhibited a small hysteresis loop at low and room temperatures, indicating a ferromagnetic contribution. This feature was not observed for larger crystallite size samples. Bulk YMO is known to be an antiferromagnetic material. However, ferromagnetic behavior in YMO was observed in naoparticleError! Bookmark not defined.
or ultra-thin filmError!
Bookmark not defined.
systems, which were likely to be due to
uncompensated surface spin. As shown in Figure 1, an interrupted lattice line with ~6 nm
14
YMO thin films through the overall thickness was observed by TEM, which might result in weak ferromagnetic properties as observed at ultra-thin film system. In addition, the existence of the ferromagnetic behavior in antiferromagnetic material system has been explained by the interaction of canted spins as a result of the Dzyaloshinskii-Moriya interaction,[45],[46] which was considered to be a spin current from ferroelectric displacement between non-collinear neighboring spins in o-YMO thin films.Error! Bookmark not defined.,Error! Bookmark not defined.
The magnetic properties such as remnant magnetization (Mr), saturation magnetizations (Ms), and the coercive magnetic field (HC) were determined from the M vs H analysis curve at 298 K and 20 K. It was found that the Ms for YMO thin films on Si (111) at 298 K and 20 K were about to 3.34 (× 10-5, emu) and 4.24 (× 10-5, emu), respectively while the Mr were almost similar. It is indicative of higher resistance to rotation of spin orientation at low temperature. In order to evaluate the electric field induction of magnetization, the magnetic field dependence of magnetization, M vs. H, was performed under in-situ 20 V electric polling using a SQUID magnetometer. As shown in insets of Figure 6, before electric poling, the splitting of hysteresis loop was not observed at 298 K but it was observed at 20 K for o-YMO films on Si (111). After 20 V poling, the splitting of hysteresis loop was noted at both temperatures (298 K and 20 K). The HC at both temperatures were found to be more or less similar of the order of 100 (Oe) at both temperatures for YMO film on Si (111), indicating a ferromagnetic contribution. However, HC for YMO films on YSZ (111) was nearly unchanged after 20 V poling at 298 K and 20 K (not shown). It might not have been fully poled because of the high dielectric constant (24-29.5) of YSZ substrate.
15
Summary Ultra-thin ALD YMO films on Si (111) and YSZ (111) were synthesized using the REALD method. The atomic composition, crystal phase and Mn3+ local bonding environment in YMO thin films were investigated as well as magnetic properties. Anomalies in the temperature dependent DC magnetic susceptibility (d/dT) at TN= ~57 K for YMO thin films on Si (111) indicated the o-YMO crystal phase. The ALD YMO thin films on YSZ (111) showing two anomalies at ~48 K and at ~85 K when Mn/Y ratio approached unity in the temperature dependent DC magnetic susceptibility, where XRD showed the peaks corresponding to dominant h-YMO phase and minor o-YMO phase. The YMO phase on YSZ (111) was more dependent on atomic composition ratio than on substrate lattice parameters. It was also confirmed that YMO on Si (111) has the orthorhombic phase and YMO on YSZ (111) has the hexagonal phase based on EXAFS analysis. In order to evaluate the electric field induction of magnetization, the magnetic field dependence of magnetization, M vs. H, was performed under in-situ 20 V electric poling. The HC for YMO on Si (111) at 298 K and 20 K was obtained to be 100 Oe, which indicated magneto-electric coupling below TN. The RE-ALD was shown as a viable technique with attractive features to fabricate the ultra-thin films in engineered multiferroic systems. The detailed analysis of magnetic properties of ultra-thin ALD YMO films in terms of the Mn/Y atomic ratio and type of substrate could be used for the design of composite multiferroic systems with 3D-3D or 3D-1D geometries.
16
Acknowledgements This work was supported in part by the FAME Center, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA, as well as Functional Engineered Nano Architectonics (FENA), an SRC FCRP center. Part of this work was carried out in part at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The authors would also like to thank Dr. Ignacio Martini in the UCLA Department of Chemistry for his assistance in performing SQUID magnetic measurements.
17
Figure captions Figure 1. Bright field TEM images of ~6 nm YMO ultra thin films on (a) Si (111) and (b) YSZ (111) Figure 2. (a) Composition ratio of ~6 nm YMO ultra thin films on Si (111) with different Mn:Y precursor pulsing ratio. Inset shows survey XPS spectra for YMO thin film on Si (111) with Mn/Y=0.95, (b) Composition ratio of ~6 nm YMO ultra thin films on YSZ (111) with different Mn:Y precursor pulsing cycles. Inset shows survey XPS spectra for YMO thin film on YSZ (111) with Mn/Y=1.07. Figure 3. (a) The XRD of YMO thin films on Si (111) as function of RTA condition. The crystalline peaks according to RTA condition at 900oC and 1000oC are compared to the peaks from bare Si (111) substrate. (b) The XRD of YMO thin films on YSZ (111) as function of Mn/Y ratio after RTA 900oC. The crystalline peaks according to Mn/Y ratio are compared to the peaks from bare YSZ (111) substrate. Figure 4. Schematic structure of (a) h-YMO with MO5 polyhedron (d(Mn-O1)=1.687Å, d(Mn-O2)=2.043Å, d(Mn-O3)=1.830Å, d(Mn-O4)=2.090Å)32 and (b) o-YMO with MO6 polyhedron (d(Mn-O1)=2.209Å, d(Mn-O2)=1.916Å, d(Mn-O3)=1.947Å)33. (c) The Fourier transformed EXAFS R spectra for 80 nm YMO thin films on Si (111) and 80 nm YMO thin films YSZ (111). The spheres represent the O atoms, the and represent the Y and Mn atoms, respectively. Figure 5. (a) DC magnetic susceptibility () for the ~6 nm YMO thin films on Si (111) under 100 Oe. Solid symbols represent ZFC condition and open symbols represent FC condition. The two insets shows d/dT and -1, respectively. (b) DC magnetic susceptibility () for ~6 nm YMO thin films on YSZ (111) with different Mn/Y atomic ratios under 100 Oe. The two insets shows d/dT and -1, respectively. The two insets shows d/dT and -1, respectively. Figure 6. Magnetic hysteresis loop (M vs. H) measured at 20 K and 298 K for a ~6 nm YMO ultra-thin films on Si (111) before and after 20 V electric polling. The inset shows the zoom-in view of the coercive field measurements.
18
Figure 1
19
1000
Si: 2p 3/2
0.4
YMO/Si (111) Mn/Y=0.53 Mn/Y=0.67 Mn/Y=0.95
Si: 2S
Y: 3P 1/2
Y: 3P 3/2
Mn: 2P 3/2
Mn: 2P 1/2
5
Composition ratio (%)
(a) 0.6
Intensity (x10 Arb. Unit)
Figure 2
800
600
400
Binding energy (eV)
200
0
0.2
0.0 Mn
Y
Si
C
1000
Zr: 3d 5/2
YMO/YSZ (111) Mn/Y=0.69 Mn/Y=0.80 Mn/Y=1.07
Zr: 3d 3/2
0.4
Y: 3P 1/2 Y: 3P 3/2
0.6
Intensity (x10 5 Arb. Unit)
Composition ratio (%)
(b)
Mn: 2P 1/2 Mn: 2P 3/2
Composition elements
800
600
400
200
Binding energy (eV)
0
0.2
0.0
Mn
Y
Zr
Composition elements
20
C
Figure 3
(a)
Intensity (arb. unit)
YMO on Si (111) o-YMO: Pbnm JCPDS: 20-0732
Si (111) o
1000 C o
30
35
40 45 2 (degree)
(b)
(131)
(004)
(113)
(202)
(211)
(120)
(020)
900 C
50
55
YMO on YSZ (111) h-YMO: P63cm
Intensity (arb. unit)
JCPDS: 25-1079
30
35
(0006)
(112)
Mn/Y=1.07
O-YMn2O5
(0004) (1122)
YSZ (111)
40 45 2 (degree)
21
Mn/Y=0.69
50
55
Figure 4
5 YMO /Si (111)
4
Intensity (arb. unit)
Mn-O2
Mn-O1
Mn-Y
3 2
YMO /YSZ (111)
Mn-O2
1 0 -1
0
1
2
3
R+R (Å)
22
4
5
Figure 5
(a)
0.010 6.0
(emu/Oe cm3)
ddT
0.008
~80 nm YMO YMO/Si (111)
0.006
20
40
60
(1/emu) 10
5
6nm YMO
TC~25K
TN~62K
5.8 5.5 5.3 0
80
Temperature (K)
30 60 90 120 Temperature (k)
FC
0.004
0.002
0.000
ZFC 0
50
100
~6nm YMO/Si (111) 150
200
250
300
Temperature (K)
Solid: ZFC Open: FC , : Mn/Y=0.69 , : Mn/Y=0.80 , : Mn/Y=1.1
0.10
0.0003
20
40
60
Temperature (k)
80
3
(emu/Oe cm )
3
(cm /emu )
0.11 d /dT
(b) 0.0004
0.09 Mn/Y=1.07
0.08 -600
0.0002
Mn/Y=0.80 Mn/Y=0.69
-450
-300
-150
0
Temperature (K)
150
300
0.0001
0
50
100
150
200
Temperature (K)
23
250
300
Figure 6
-5
4x10
-5 -5
2x10
YMnO3 on Si (111) 298K 20K
0.005 20K 298K
0.004 0.003
Before
0.002
-5
M (emu)
1x10
0.001 0.000
-0.001 -0.002
0
-0.003 -0.004 -0.005
-400
-200
0
200
400
H (Oe)
-5
-1x10
0.005 0.004 0.003
20K
After
298K
0.002
-5
-2x10
M (emu)
Magnetization, M (emu)
3x10
0.001 0.000
-0.001 -0.002
-5
-3x10
-0.003 -0.004 -0.005 -500
-5
-4x10
-250
0
250
500
H (Oe)
-2000 -1000
0
1000
2000
Magnetic Field, H (Oe)
24
3000
References
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