annealing assisted substrate coherency and high temperature antiferromagnetic insulating transition in epitaxial la0 67ca0 33mno3 ndgao3 001 films

A typical instance is the La1-xCaxMnO3films grown on SrTiO3001 and LaAlO3001 substrates, in which the large biaxial-strain could induce selective orbital occupancy and phase separation P

insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

L F Wang, X L Tan, P F Chen, B W Zhi, B B Chen, Z Huang, G Y Gao, and W B Wu

Citation: AIP Advances 3, 052106 (2013); doi: 10.1063/1.4804541

View online: http://dx.doi.org/10.1063/1.4804541

View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/3/5?ver=pdfcov

Published by the AIP Publishing

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Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition

in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

L F Wang, X L Tan, P F Chen, B W Zhi, B B Chen, Z Huang,

G Y Gao, and W B Wua

Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, and High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230026, People’s Republic of China

(Received 25 March 2013; accepted 26 April 2013; published online 8 May 2013)

Bulk La0.67Ca0.33MnO3(LCMO) and NdGaO3(NGO) have the same Pbnm

symme-try but different orthorhombic lattice distortions, yielding an anisotropic strain state

in the LCMO epitaxial film grown on the NGO(001) substrate The films are

opti-mally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in

oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below To explain this drastic strain effect, the films

with various thicknesses were ex-situ annealed under various annealing parameters.

We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial

quality, resulting in the films with true substrate coherency and the AFI ground state

And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted

by ex-situ annealing, and moreover, play a key role in controlling the properties of

oxide heterostructures C 2013 Author(s) All article content, except where other-wise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

[http://dx.doi.org/10.1063/1.4804541]

I INTRODUCTION

For the past decades, researches in transition-metal perovskite oxides have flourished The interest in these oxides arises from the fact that they are based on the same simple ABO3 per-ovskite structure and can exhibit a wide diversity of functionalities such as the large and tunable piezoelectricity, ferroelectricity, colossal magnetoresistance, and superconductivity Furthermore, by stacking different perovskites into epitaxial films, multilayers, and superlattices, the reconstructions

of charge, orbital, spin, and lattice degrees of freedom on the nanometer scale may allow not only their properties to be combined but, sometimes, also totally new phenomena to be induced at the heterointerface.1

Specifically, the reconstructions of the flexible corner-sharing BO6networks are always intrigu-ing and crucial in perovskite heterostructures.2On one hand, both the experimental and theoretical results have revealed that the epitaxial strain, generated by the lattice constant mismatch at the heterointerface between two dissimilar perovskites, can directly alter the patterns and magnitudes

of the octahedral rotation and deformation, then giving rise to structures that are not been found

in bulk phase diagram,3 and even structural transitions as a function of the lattice mismatch, sym-metry mismatch, and film thickness.4 7 More importantly, the strong electron-lattice coupling in those correlated perovskites enables the strain-coupled octahedra behaviors to effectively manip-ulate their properties For instance, in the (Nd,Pr)0.5Sr0.5MnO3 films grown on SrTiO3 substrates

a Electronic mail: wuwb@ustc.edu.cn

2158-3226/2013/3(5)/052106/14 3, 052106-1 C Author(s) 2013

with various orientations, the antiferromagnetic charge-orbital-ordering (COO) transition can be either enhanced or suppressed, depending on the strain-stabilized various octahedral deformation modes.8And Zayak et al.9have shown that the strain induced octahedral rotation and deformation could alter the magnetic-nonmagnetic transition in CaRuO3 and SrRuO3 films In addition to the strain-mediated elastic octahedral coupling, recently it has been predicted and demonstrated that the corner-connectivity nature of octahedral networks could enable the octahedral rotation/deformation patterns in the substrate to transfer across the interface and imprint into the films, yielding inter-facial layers with octahedral configurations unstable in bulk.10 – 12 Unlike the epitaxial strain that can be coherently maintained over tens of nanometers, the interfacial octahedral coupling only persists for several unit-cells.2 , 11 Even so, this effect was reported to stabilize novel electronic properties in the interface region Examples include the anomalous dielectric behavior of BiFeO3

near the BiFeO3/La0.7Sr0.3MnO3 heterointerface,12 the improper ferroelectricity in ultrashort pe-riod PbTiO3/SrTiO3superlattice,13and the room-temperature magnetic insulating phase in ultrathin

La0.67Sr0.33MnO3/SrTiO3(110) films.14 For the (Sr,Ca)TiO3/LaAlO3 systems, the interfacial elec-tronic states were also suggested to be affected by the interfacial reconstruction of octahedral patterns.15,16

Accordingly, the strain or interface engineering of octahedral rotation and deformation patterns has provide an opportunity to stabilize familiar perovskites with new functionalities However, synthesis of epitaxial films with substrate coherency, i.e., maintaining pseudomorphic strain state and interfacial octahedral coupling, is always a non-trivial task On one hand, the control of epitaxial strain is challenged by the lattice mismatch between the film and substrate A typical instance is the

La1-xCaxMnO3films grown on SrTiO3(001) and LaAlO3(001) substrates, in which the large biaxial-strain could induce selective orbital occupancy and phase separation (PS) states.17 Nevertheless, since the large lattice mismatch limits the critical film thickness for coherent strain state, the strain-mediated electronic properties in these ultrathin films are unfortunately entangled with many extinct effects, including the grainy surface morphology, finite-size scaling effect, and nonuniform termination of the substrate.18 Further, as the film thickness increases the partially relaxed strain may result in the lattice disorders or defects that are usually unwanted.17,18On the other hand, the oxygen vacancies, interfacial atomic intermixing and cation stoichiometry errors formed during the film growth may also lead to various structure inhomogeneities, which influence the strain state

as well as the octahedral behavior.19–21 In order to improve the substrate coherency, much effort

has been devoted in the in-situ heteroepitaxy processes, such as developing advanced epitaxial

growth techniques,22 sophisticated control of thin film growth modes,23 – 25 and choosing suitable substrates with small lattice and symmetry mismatch.5 , 25 , 26 Moreover, the ex-situ annealing in

oxygen atmosphere has been widely employed to optimize the crystallinity and eliminate oxygen deficiency for complex oxide films.21 , 27 , 28But it was also shown that the high temperature annealing may also facilitate the strain relaxation in large lattice-mismatched systems.17 , 29Therefore, the effect

of ex-situ annealing on substrate coherency should be multiplicate, and surely needs to be further

investigated

In this paper, in order to shed some light on the relationships between ex-situ annealing, substrate

coherency, and electronic properties in perovskite heterostructures, we choose the La0.67Ca0.33MnO3

(LCMO) films grown on NdGaO3(001) [NGO(001)] substrates as a model system In bulk, the NGO

and LCMO show the same orthorhombic Pbnm symmetry with GdFeO3-type octahedral rotation

pattern (tilting system a−a−c+in Glazer notation) but different octahedral rotation angles.30,31And the GaO6octahedra are nearly regular and rigid, whereas the MnO6octahedra are slightly deformed

by the JT distortion Because of the distinct octahedral configurations for these two perovskites, the coherently grown LCMO/NGO(001) film suffers a considerable in-plane anisotropic strain At the same time, the average lattice mismatch between the film and substrate is negligible, which enables the anisotropic strain state to be maintained even in thick films, easily preventing the strain relaxation More strikingly, though the bulk LCMO is optimally doped in the ferromagnetic-metal (FM) ground state, the LCMO/NGO(001) films, after being annealed in oxygen atmosphere, show

an antiferromagnetic insulator phase transition at ∼250 K and the PS with the coexistence of antiferromagnetic-insulating (AFI) and FM phases at the temperature below.32 – 35In the early work

we have demonstrated that the ex-situ annealing process and the NGO(001) substrate stabilized

in-plane anisotropic strain state are both indispensable for the formation of AFI phase.34,35 How-ever, the detailed evolutions of the strain state and octahedral behavior during annealing, which are central for understanding the AFI phase and PS, are still unclear Here we develop a new strat-egy by investigating the surface morphology and magnetotransport property in parallel from the

LCMO/NGO(001) films with various thicknesses after being ex-situ annealed under various an-nealing parameters The surface morphology evolutions demonstrate that the ex-situ anan-nealing can

surprisingly improve the epitaxial quality and thus enhance the substrate coherency And based on observed close relationship between the epitaxial quality and phase evolution, along with the quan-titative analyses of the octahedral response to the anisotropic strain, we suggest that after annealing the enhanced substrate coherency could stabilize the Jahn-Teller distortion in the LCMO films and thus trigger the AFI phase

II EXPERIMENT

LCMO/NGO(001) films of 5-40 nm thick were grown by the pulsed laser deposition method.29,32 The ceramic LCMO target was prepared by standard solid state reactions During deposition, the substrate temperature and O2 pressure were 735◦C and 45 Pa, and the laser energy and repetition rate were set at 2 J/cm2and 5 Hz, resulting in a growth rate of∼5 nm/min After deposition each

film was in-situ annealed for 15 min before being cooled down to room temperature, during this

process the substrate temperature and O2pressure remained at 735◦C and 45 Pa Then the films

were ex-situ annealed in flowing O2of ambient pressure for a fixed duration time and temperature

The magnetization (M) and resistivity ( ρ) were measured on Quantum Design superconducting

quantum interference device (SQUID) magnetometer (MPMS) and physical property measurement system (PPMS) In all theρ measurement the applied current was fixed at 1 μA, and the data beyond

the measurement limit (∼ 10  cm) were not shown The structures of the films were checked by x-ray diffraction (XRD) includingω-2θ linear scan, ω-scan rocking curves (RCs) and reciprocal

space maps (RSMs) using CuKα1radiation (λ = 1.5406 Å, Panalytical X´pert) The film thicknesses

were determined by analyzing the Laue fringes around LCMO(004) diffraction peak, consistent with the evaluated value from growth rate The surface morphologies of the films were measured by the atomic force microscopy (AFM, Vecco, MultiMode V)

III RESULTS AND DISCUSSION

The strain states of the LCMO/NGO(001) films were schematically analyzed in Fig.1(a) In

Pbnm orthorhombic index the bulk NGO (LCMO) has the lattice constants a= 5.4332 Å (5.4717 Å),

b = 5.5034 Å (5.4569 Å), c = 7.7155 Å (7.7112 Å).36According to these data, although the average in-plane lattice mismatch is negligible (0.06%), the coherently grown LCMO/NGO(001) film could take an in-plane anisotropic strain:−0.70% compressive strain along [100] but 0.85% tensile strain

along [010] In Fig.1(b), the AFI phase and PS evolving with film thickness and annealing parameter were briefly depicted by the temperature dependent resistivity (ρ-T) curves The 1 h-annealed film

of 40 nm thick only shows FM transition at 267 K, the same as the bulk counterpart By contrast,

after being ex-situ annealed at 780◦C for 10 h, the film of the same thickness shows not only a

FM transition at 259 K, but also a subsequent AFI phase transition at∼250 K (TAFI, which has been confirmed by previous magnetization measurements), and the subsequent complex hysteresis and large residual resistivity manifest the PS state with competing AFI and FM phases.34,35 With decreasing the thickness to 20 nm, the annealed film shows a higherρ below TAFI, indicating an enhanced AFI phase In fact, by comparing the magnetotransport properties of LCMO films grown

on the NGO substrates with various orientations, we have excluded the interfacial chemical diffusion

and variation of La:Ca doping ratio during ex-situ annealing, and demonstrated that the AFI phase

is elastic-driven.33 , 35In Fig.1(c), theω-2θ linear scans of the 10-h annealed films (20 and 40 nm)

show sharp Laue fringes, further confirming that the LCMO films have high crystal quality and sharp interface with the underlying substrates.4 The strain states of the films were characterized by the RSMs as shown in Fig.1(d) When compared with the (116) reflection from the as-grown film, the

Q[001]of the same reflection from annealed film increases, signifying that the ex-situ annealing can

FIG 1 (a) The schematic top views of LCMO(001) (green) and NGO(001) (wine) unit cells The strain state of the

commensurate LCMO/NGO(001) film was analyzed via the in-plane lattice mismatches in the orthorhombic Pbnm index.

(b)ρ-T curves of 20 and 40 nm films annealed at 780◦C for 10 h The curve of 1 h-annealed 40 nm film was also inserted for comparison The arrows denote the thermal processes (c)ω-2θ scans from the 10 h-annealed films of 20 and 40 nm thick.

The sharp peaks at 2 θ = 47.074◦can be indexed as NGO(004), and the broad humps at the higher angle side stands for the LCMO(004) reflections (d) RSMs measured from the as-grown and 10 h-annealed films (40 nm) around (116) reflection.

The open and solid arrows denote the reflections from the films and the substrates, respectively.

cause oxygen incorporation and then the contraction of the out-of-plane film lattice.28On the other hand, the reflections from the as-grown and annealed films are both sharp and concentrated, and have exactly the same in-plane lattice spaces as those of the substrates, suggesting that LCMO/NGO(001)

films are coherently strained even after being ex-situ annealed.17 , 26 , 29 , 35 That means the strain relaxation during annealing may not be the driven force for the AFI phase

For these LCMO/NGO(001) thin films with negligible lattice mismatch, the local structure changes during annealing could be averaged out by the X-ray diffraction, and the AFM used for characterizing the surface morphology might be a more effective approach Based on this consideration, as presented in Fig 2, the AFM images along with zero-field ρ-T curves were

correspondingly measured from a set of as-grown and annealed films with unequal thicknesses For the as-grown films, as observed in Fig 2(a)–2(e), although all the surfaces are very smooth with the root-mean-square roughness less than 1 nm, the randomly distributed grainy structures indicate

that a high density of defects accumulated during the in-situ growth, which may be attributed to

3-dimentional island film growth mode in our deposition condition.24By contrast, after being ex-situ

annealed at 780 ◦C for 5 h, the films of 8, 12 and 16 nm thick [Fig 2(f)–2(h)] exhibit single-unit-cell stepped terraces with sharp edges and uniform widths As the thickness increases to 24 and 40 nm [Fig 2(i)and 2(j)], the atomic terraces in the annealed film surfaces turn out to be distorted and discrete Aside from this drastic morphology change, as shown in Fig.2(k)and2(l), the close correspondences between the surface morphologies and electronic transport properties are rather interesting The as-grown films with grainy surfaces only show bulk-like FM transition with

a gradually reduced TCas the thickness decreases In contrast, for the annealed films of 8, 12 and 16

nm thick, accompanying with the improvement of surface morphology, the FM transition disappears, instead theρ-T curves show the AFI phase transition followed by insulating behavior, signifying an

AFI ground state And for those 24 and 40 nm films that display distorted terraces, theρ-T curves

show a typical PS behavior as characterized by the thermal hysteresis and multiple metal-insulator transitions

FIG 2 AFM images (1× 1 μm2 ) scanned from (a)-(e) as-grown and (f)-(j) 5 h-annealed films with different thicknesses as denoted The as-grown film morphologies for all the thickness show grainy surface, whereas the annealed film morphologies show single-unit-cell stepped terraces evolving with film thickness (k) [(l)] The ZFC-ZFWρ-T curves measured from the

as-grown (annealed) films.

FIG 3 (a) ZFC-FW and (c) FC-ZFWρ-T curves measured from the LCMO/NGO(001) films with various thickness, as

denoted The solid arrows denote the cooling or warming The ZFC-ZFWρ-T curves (gray, dashed line) were also inserted

in (a) for comparison In (c) the AFI reentrance temperatures TR were marked by the solid triangles (b) [(d)] The isothermal

ρ-H curves measured after ZFC the 16, 24 and 40 nm films to 5 K (130 K) The open arrows denote the cycling of H (0→

4→ 0 T), and the AFI phase melting (reentry) fields HM(HR ) were marked by the solid arrows In all the measurements the

magnetic fields were applied along the c axis.

The ρ-T curves during various thermal processes and isothermal ρ-H curves were measured

from the annealed films in order to characterize the phase evolution and instability in detail As shown in Fig 3(a), after zero-field-cooling (ZFC) the 40, 24, and 16 nm films to 5 K and then

during field warming (FW) at H = 2 T, the ρ-T curves show steep drops in resistivity, which can

be explained as the phase transition from ‘frozen PS’ to ‘dynamic PS’.34 , 35 , 37 It is clear that this phase transition shifts to higher temperatures as the film thickness decreases, indicating a more stable frozen state in a thinner film In Fig.3(b), theρ-H curves measured after ZFC the films to

5 K show irreversible AFI phase melting behavior, consisting with the dynamic T-H phase diagram

as previously constructed.34For the films of 16, 24 and 40 nm thick, the AFI phase can be completely

melted at 5.1, 4.5 and 3.9 T (HM), respectively The increased HMfurther confirms that the frozen

state becomes more robust against H as the thickness decreases In Fig.3(c), after field-cooling at

4 T (6 T for the 8 nm film) to 10 K then during zero-field warming (FC-ZFW), the films can keep

the FM phase till TD followed by a sharp increase inρ (nearly five orders for the 16 nm film),

signifying the AFI phase reentrant behavior during the phase transition from FM-dominated PS to AFI-dominated PS.34 , 38For the 8 and 16 nm films, the sharp AFI reentrance is stabilized at∼94 K,

while for the 24 and 40 nm films the TDincreases rapidly to 121 and 140 K, respectively, and the resistivity jump becomes sluggishly That means the FM phase could become energetically preferred

as the film thickness increases In Fig.3(d), theρ-H curves at 130 K show not only the AFI phase

melting behavior but also the thickness dependent AFI phase reentrance.38 In the 16 nm (24 nm)

film the AFI phase can reenter as H is decreased to 2.15 T (1.06 T), while no reentrant AFI phase was observed in the 40 nm film, because the TD of this film is well above 130 K Based on the

FIG 4 AFM images (1× 1 μm2 ) from (a)-(d) 14 nm and (e)-(h) 28 nm films during the accumulative annealing processes The surface terraces are gradually improved as the annealing temperature and duration time increases (i) [(j)] The corresponding

ρ-T curves of 14 (28) nm films.

morphology data as well as theρ-T and ρ-H curves presented in Fig.2and Fig.3, we may conclude

that in the LCMO/NGO(001) epitaxial system the ex-situ annealing improved surface morphologies

are closely related to the electronic phase instability: the annealed thin films that display uniform terraced surfaces tend to show stable AFI ground state; and for those thicker ones, in accord with the imperfect surface morphologies, the AFI phase is weakened and the FM phase is enhanced, thus leading to the unstable PS state

In order to further elucidate the ex-situ annealing effect on the improvement of surface

mor-phologies and the formation of the AFI phase, as shown in Fig.4, the AFM images and zero-field

ρ-T curves were correspondingly measured from a set of the films after being ex-situ annealed under

various annealing parameters For the films deposited and ex-situ annealed in the same run, we

have confirmed the good reproducibility ofρ-T data, but the surface morphology changes could be

affected by not only the annealing parameters but also other extinct factors such as the variation of miscut angle for different substrates.39Hence, a special procedure, named accumulative annealing process, was used for the three kinds of measurements to obtain the most reliable data In detail, five commensurate films of 14 nm (28 nm) thick were deposited and then annealed accumulatively side by side for four times with progressively increased annealing temperature (duration time) At the end of each annealing procedure, the AFM images were always measured from one specified sample, while the Pt electrodes were prepared on one of the rest samples for the correspondingρ-T

measurements After these steps, the morphology and transport data of the 14 nm (28 nm) films evolving with gradually increased annealing temperature (duration time) were obtained, as presented

in Fig.4(a)–4(d)[Fig.4(e)–4(h)] and Fig.4(i)[Fig.4(j)], respectively For the 14 nm film, after being annealed at 600◦C for 5 h, the grainy surface is similar to that of as-grown film, and theρ-T curve

only shows the bulk-like FM transition As the annealing temperature rises to 660◦C and 720◦C, the surface terraces and unit-cell-height 2D islands appear, and the coalescence of these islands and steps leads to the meandering of the terrace edges Further being annealed at 780◦C, the 2D islands disappear and the uniform terraces were observed At the same time, for the samples annealed at

660, 720 and 780◦C, all theρ-T curves exhibit an evident AFI transition followed by the insulating

behavior, further confirming the close linkage between the AFI phase ground state and the terraced surface morphology A similar correspondence was also observed in the 28 nm films annealed at

780◦C for different duration time For the films annealed for 2 and 7 h, the surfaces exhibit a high density of 2D islands, and theρ-T curves show a characterized PS behavior When the annealing

duration goes up to 15 and 20 h, the films turn out to display sharp surface terraces as well as the insulatingρ-T behavior in the whole temperature range These results clearly show that both the high

annealing temperature and long duration are quite necessary for inducing the AFI ground state and the terraced surface structure Especially for the 28 nm films, the required annealing duration time

is much longer than that for oxygen incorporation and strain relaxation,28 , 29indicating an unusual mechanism lies behind this annealing effect

In Fig.5, the annealing induced structure changes were examined by the XRDω-2θ linear scans

and RCs around the LCMO(004) reflections, from the 14 and 28 nm films during the aforementioned accumulative annealing processes During the entire annealing procedures, the film thickness, as calculated by the Laue fringes, remains unchanged And the appearance of the sharp fringes and the narrow full width at half maximum of the RCs exclude any crystal degeneration after annealing.4 , 26

As the annealing temperature or the duration time increases, accompanying with the gradually improved surface morphology and enhanced AFI phase, the LCMO(004) reflections first shift to

a higher Bragg angle side because of the oxygen uptake [Fig 5(b) and5(g)], and then barely move after the annealing temperature (duration time) reaches 660 ◦C (7 h) [Fig 5(c)–4(e) and Fig.5(h)–5(j)], reflecting an already saturated oxygen content.28 Accordingly, it may be concluded that the improvement of film morphology and the oxygen incorporation are two intrinsically different

physical processes that occur during the ex-situ annealing In addition, after annealing, the Laue

fringes at the higher angle side of the LCMO(004) reflection become depressed, while the ones at the lower angle side remain sharp This asymmetric change is still an open question, and we suggest that it may be related to the anisotropic strain enhanced orthorhombic distortion in the annealed films

Based on the results presented above, here we can discuss the role of ex-situ annealing in

im-proving the surface morphology and triggering the AFI phase Previously, the relationship between

surface morphology and in-situ film growth mode has been widely reported Specifically, the irregular

grainy surface structures manifest the 2-dementional/3-dimentional island growth mode, leading to

a high density of defects and strain relaxation.18 , 24 , 25On the contrary, the terraced surface structures are commonly observed in the epitaxial films grown in the well-controlled step-flow or layer-by-layer mode, which usually signifies an defect-free structure and coherent strain state.4 , 16 , 24 , 40Furthermore, the symmetry mismatch, which is originated from the differences in octahedral configurations be-tween the film and substrate, has been reported to increase the surface roughness.5These facts clearly reveal that the surface morphology could intrinsically manifest the structure coherency between film and substrate Along this line, for the LCMO/NGO(001) films, the presence of atomically flat surface

FIG 5. ω-2θ scans and RCs measure from the (a)-(e) 14 nm films and (f)-(j) 28 nm films during the accumulative annealing

processes The dashed lines are guidance for eyes for the shift of LCMO(004) peaks In (b)-(e) and (g)-(j) theω-2θ scans of

the as-grown films were inserted for comparison.

terraces suggests that the ex-situ annealing with high temperature and long duration time may not

only supply the oxygen uptake but also lead to a recrystallization which assists the true substrate coherency To support this point, we compared the surface morphologies of the LCMO film and the underlying substrate after being cut and annealed separately In detail, one as-received NGO substrate was cut into two pieces and the LCMO film (14 nm) was deposited on one of them Prior to annealing, as shown in Fig.6(a), the atomic terraces on the surface of the as-received NGO substrate are indistinct due to the mixed A-site and B-site surface termination, and the film displays the typical grainy surface (not shown) After being simultaneously annealed side by side at 780 ◦C for 10 h, however, the substrate [Fig.6(b)] and film [Fig.6(c)] both display atomically flat surfaces with sharp terraces.41 Moreover, the terraces in the film share almost the same width and pattern as those in the annealed substrate, which strongly suggests that the recrystallization during annealing is not just limited in the surface but related to structure coherency of the entire film with the substrate If we take the anisotropic strain as the driving force of the AFI phase, the picture over annealing-assisted substrate coherency can harmoniously explain the correspondence between the surface morphology