Emerging single-phase state in smallmanganite nanodisksJian Shaoa,b, Hao Liua,b, Kai Zhanga,b, Yang Yua,b, Weichao Yua,b, Hanxuan Lina,b, Jiebin Niua,b, Kai Dua,b,Yunfang Koua,b, Wengang Weia,b, Fanli Lana,b, Yinyan Zhua,b, Wenbin Wanga,b, Jiang Xiaoa,b, Lifeng Yina,b,c,1,E. W. Plummerd,1, and Jian Shena,b,c,1

aState Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China; bDepartment of Physics, Fudan University, Shanghai 200433, China;cCollaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China; and dDepartment of Physics and Astronomy, Louisiana StateUniversity, Baton Rouge, LA 70808

Contributed by E. W. Plummer, June 22, 2016 (sent for review May 6, 2016; reviewed by Zhi-Xun Shen and Jing Shi)

In complex oxides systems such as manganites, electronic phaseseparation (EPS), a consequence of strong electronic correlations,dictates the exotic electrical and magnetic properties of thesematerials. A fundamental yet unresolved issue is how EPS respondsto spatial confinement; will EPS just scale with size of an object, orwill the one of the phases be pinned? Understanding this behavior iscritical for future oxides electronics and spintronics because scalingdown of the system is unavoidable for these applications. In thiswork, we use La0.325Pr0.3Ca0.375MnO3 (LPCMO) single crystalline disksto study the effect of spatial confinement on EPS. The EPS statefeaturing coexistence of ferromagnetic metallic and charge orderinsulating phases appears to be the low-temperature ground statein bulk, thin films, and large disks, a previously unidentified groundstate (i.e., a single ferromagnetic phase state emerges in smallerdisks). The critical size is between 500 nm and 800 nm, which issimilar to the characteristic length scale of EPS in the LPCMO sys-tem. The ability to create a pure ferromagnetic phase in manganitenanodisks is highly desirable for spintronic applications.

manganites | electronic phase separation | magnetization | single phase

Owing to strong coupling between spin, charge, orbital, andlattice (1, 2), different electronic phases often coexist spa-

tially in strongly correlated materials known as electronic phaseseparation (EPS) (3, 4). For colossal magnetoresistance (CMR)manganites, EPS has been observed to have strong influence onthe global magnetic and transport properties (5, 6). Regardingthe physical origin of EPS, it has been shown theoretically thatquenched disorder can lead to inhomogeneous states in manganites(1, 3, 7). Once long-range effects such as coulombic forces (8), co-operative oxygen octahedral distortions (9), or strain effects (10)are included, calculations show infinitesimal disorder (8, 11) oreven no explicit disorder (10) may lead to EPS. Within a phe-nomenological Ginzburg–Landau theory, it has been shown thatEPS is intrinsic in complex systems as a thermodynamic equi-librium state (12).Although the details of the origin of the EPS remain as a

matter of dispute, its very existence as a new form of electronicstate has been well accepted. The length scale of the EPS hasbeen observed to vary widely from nanometers to micrometersdepending on many parameters that can affect the competitionbetween different electronic phases (13–20). It is thus of greatinterest to examine whether the EPS state still exists as the sys-tem is scaled down, especially when the spatial dimension of thesystem is smaller than the length scale of the EPS domains.In this work, we use La0.325Pr0.3Ca0.375MnO3 (LPCMO) as a

prototype system to show a spatial confinement-induced transi-tion from the EPS state to a single ferromagnetic phase state.The LPCMO system is chosen because of its well-known largelength scale of EPS domains (approximately a micrometer) (21),which allows us to conveniently fabricate LPCMO epitaxial thinfilms into disks with diameters that are smaller than the EPSdomain size. In LPCMO bulk (21) and thin films (6, 22), the EPS

state was observed to be the low-temperature ground state.Using magnetic force microscope, we observe that the EPS stateremains to be the ground state in disks with the size of 800 nm indiameter or larger but vanishes in the 500-nm-diameter diskswhose size is distinctly smaller than the characteristic lengthscale of the EPS domains. In the 500-nm disks, only the ferro-magnetic phase can be observed at all temperatures below Curietemperature Tc, indicating that the system is in a single-phasestate rather than a EPS state. Our results further indicate thatthe large length scale EPS in the LPCMO system does not costextra Coulomb energy, which otherwise should lead to a scalingdown of EPS with decreasing size of the LPCMO disks (23, 24).LPCMO films with 60-nm thickness were epitaxially grown on

SrTiO3(001) substrates by pulsed-laser deposition. The substrateswere kept at 780 °C in oxygen atmosphere of 5 × 10−3 millibarsduring growth. Unit cell by unit cell growth was achieved as in-dicated by oscillations of intensity of reflection high-energy elec-tron diffraction (RHEED). The films were postannealed to 950 °Cfor 3 h in flowing oxygen to reduce oxygen vacancy and make surethat the films have the same magnetic properties as the bulk. TheLPCMO disks with diameters from 500 nm to 20 μm were fabri-cated from the epitaxial thin films by electron beam lithographywith a negative tone resist (for details, see the sample fabricationmethod and Fig. S1 in the Supporting Information). Magnetic pro-perties of the LPCMO disk arrays were carried out using super-conducting quantum interference device (SQUID) and magneticforce microscope (MFM) measurements.

Significance

Electronic phase separation (EPS) is a common phenomenon incomplex oxides systems. However, little is known regardinghow EPS responds when the size of the system is smaller thanthe characteristic size of EPS. This issue is not only importantfor understanding the physical origin of EPS but also for oxidesdevice applications in which oxides have to be fabricated intosmall-sized structures. Our work on manganites shows a sur-prising transition from the EPS state to a single phase statewhen the spatial size of the system is smaller than the char-acteristic length scale of EPS. This observation paves a way tomanipulate EPS, which is potentially useful for oxides elec-tronic and spintronic device applications.

Author contributions: J. Shao, L.Y., and J. Shen designed research; J. Shao, H. Liu, K.Z., Y.Y.,W.Y., H. Lin, J.N., K.D., Y.K., W. Wei, F.L., Y.Z., and W. Wang performed research; J. Shao, J.X.,L.Y., E.W.P., and J. Shen analyzed data; and J. Shao, E.W.P., and J. Shen wrote the paper.

Reviewers: Z.-X.S., Stanford University; and J.S., University of California, Riverside.

The authors declare no conflict of interest.

Freely available online through the PNAS open access option.1To whom correspondence may be addressed. Email: lifengyin@fudan.edu.cn, wplummer@phys.lsu.edu, or shenj5494@fudan.edu.cn.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1609656113/-/DCSupplemental.

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A distinct signature of the EPS state in the LPCMO system isthe thermal hysteresis for temperature-dependent magnetic andtransport properties. Fig. 1 A–D shows temperature dependentmagnetic properties of LPCMO disks with different diameters.To enhance the measuring signal for SQUID, we fabricate diskarrays for each selected diameter (the optical microscopic imageshown in Fig. 1B, Inset for the 1-μm disk array). Fig. 1 A–D showstemperature-dependent magnetization measured under 1,000 Oein-plane field for 7-μm, 1-μm, 800-nm, and 500-nm disk arrays,respectively. Thermal hysteresis can be observed for disk arrays withsize down to 800 nm, reflecting the fact that ferromagnetic metallic(FMM) and charge order insulating (COI) phases coexist duringthe first-order phase transition (7, 25). For the 500-nm disk array,however, no thermal hysteresis can be observed. This observationimplies that the EPS state may no longer exist in the system (7, 26).The lack of EPS state in the 500-nm disk array is supported by

the field-dependent magnetization measurements. Fig. 1 E–Hshows in-plane initial magnetization curves and magnetic hys-teresis loops (M-H loops) for the disk arrays measured at 5 Kafter zero-field cooling. For 800-nm or larger disk arrays, there isa clear difference between the initial magnetization curves andthe corresponding M-H loops due to the coexistence of FMMand COI phases. When the magnetic field is applied from theinitial state, the magnetization of the FMM phase first quicklyaligns along the field direction, leading to the low field fast rise

of the initial magnetization curve. With increasing field, the COIphase is melted and transits into the FMM phase. Once transited,the FMM phase will mostly stay even if the field is reduced, givingrise to the difference between initial magnetization curve and theM-H loop. The difference, however, becomes smaller with de-creasing size. For the 500-nm disk array, the initial magnetizationcurve and the M-H loop virtually superimpose each other, indi-cating no melting of COI phase occurs. Both the temperature- andfield-dependent magnetization measurements show a transitionfrom the EPS state to a single FMM state with decreasing size ofthe disk, and the critical size should be between 500 nm and800 nm.The transition from the EPS state to a single FMM state can be

seen in MFM images shown in Fig. 2 (for MFM imaging details,see micromagnetic mapping method in Supporting Information).Fig. 2A shows morphological appearance of LPCMO disks withdifferent sizes acquired by atomic force microscope (AFM). Fig. 2B–D shows the corresponding MFM images of the LPCMO disksacquired at different temperatures under a perpendicular mag-netic field of 1T. Here, the perpendicular magnetic field is appliedto yield some perpendicular magnetization components for MFMimaging because the easy magnetization axis is in the plane. In thepresent color scale, the contrast below zero (red or black) repre-sents FMM phase, whereas the contrast above zero (green or blue)represents nonferromagnetic phase [i.e., COI phase based onprevious knowledge of the LPCMO system (21, 22)]. Appar-ently, except the 500-nm disk, all other disks show distinctfeatures of the EPS state (i.e., the coexistence of the FMM andCOI phases). Although the portion of FMM phase increasesnoticeably with decreasing temperature, the system stays in theEPS state even at 10 K. The typical length scale of the EPSdomains is around a micrometer, which is consistent with pre-vious reports (21, 27).In stark contrast to the larger disks, the 500-nm disk does not

exhibit any features of EPS in Fig. 2. Instead, the whole disk is ina ferromagnetic phase with a magnetization profile peaking inthe center. To ensure that the EPS state is not diminished by themagnetic field applied during MFM imaging, we took MFMimages of the 500-nm disk at 10 K under different perpendicularmagnetic fields from 0T to 1T, as shown in Fig. 3A. At 0T, signalswith opposite sign can only be seen on two sides of the disk alongthe marked line (MFM images of 4 disks shown in Fig. S3). This

Fig. 1. Temperature dependence of magnetization (black lines for coolingand red lines for warming) under 1,000 Oe (A–D) and initial magnetization(red lines) and hysteresis loop (black lines) (E–H) at 5 K of arrays of LPCMOdisks with sizes of 7 μm (A and E), 1 μm (B and F), 800 nm (C and G), and 500 nm(D and H) in diameter and an area of 3 mm × 3 mm. (B, Inset) The opticalmicroscopic image of d = 1 μm array. (C and D, Insets) Zoomed-in M vs. T looparound the thermal hysteresis region.

Fig. 2. (A) AFM images of LPCMO disks with sizes of 500 nm, 1 μm, 2 μm,3.8 μm, 5 μm, and 7 μm in diameter. (B–D) The MFM images of LPCMO disksunder 1T field (external magnetic field direction is pointing perpendicularlyto the sample surface plane) taken at 10 K (B), 100 K (C), and 180 K (D). Thesizes of disks in MFM images are adjusted and corrected to have same scalesfor each size with the help of scanning electron microscope (SEM) images(shown in Fig. S2) and dash lines show the approximate physical boundary ofdisks. The negative value in MFM image indicates attractive force and pos-itive value indicates repulsive force.

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pattern is a typical MFM image for an in-plane ferromagneticsingle domain, because only the two ends of an in-plane mag-netic dipole yield perpendicular field gradient (with oppositesigns) for the MFM tip to detect. Once a perpendicular fieldof 0.15T is applied, the in-plane magnetization is driven out ofplane, leading to a center peaked MFM contour. The MFMsignal increases with increasing field, as shown in Fig. 3B by themarked line profiles extracted from Fig. 3A.The field-dependent behavior of the MFM contrast of the

500-nm disk is in qualitative agreement with micromagneticsimulations. Based on the MFM observation, the 500-nm disk isin an in-plane, single-domain state. Using this model as input, weperformed micromagnetic simulation and obtained the Z-compo-nent of magnetic stray field distribution at 100 nm above samplesurface (Fig. 3D; for details, see the micromagnetic simulationmethod in the Supporting Information), which is virtually the signaldetected by MFM tip. The corresponding magnetic structuresunder different magnetic fields are shown in Fig. 3E. The markedline profiles extracted from simulation (Fig. 3D) are shown in Fig.3C alongside with the experimental MFM line profiles (Fig. 3B).The subtle differences between Fig. 3B and Fig. 3C are likely

caused by the fact that experimental MFM images are convolutedfrom signals of both the LPCMO disks and the MFM tips (∼100nm in size). The consistency of MFM images and simulationconfirms that the 500-nm disk is in a ferromagnetic single-domainstate with an in-plane easy magnetization axis.Finally, we show that the 500-nm disk is in a single FMM state at

all temperatures. Fig. 4 shows MFM images of the 500-nm diskacquired every 20 K, from 20 K to 200 K under 1,000 Oe. Otherthan the center-peaked FMM phase, no traces of COI phase canbe observed. The MFM signal decreases with increasing temper-ature, which is consistent with the behavior of the temperature-dependent magnetization shown in Fig. 1. Considering the fact thatwe have never observed pure COI phase in the 500-nm disks, webelieve this phenomenon may be caused by the existence of theferromagnetic metallic edge state in the LPCMO system (22),which assists the 500-nm disk to be in pure ferromagnetic statewhen a single state is energetically preferred in the 500-nm disk.In summary, we discovered a spatial confinement-induced

transition from a EPS state featuring coexistence of FMM and COIphases to a single FMM state in the LPCMO system. The criticalsize for the transition is between 500 nm and 800 nm, which issimilar to the characteristic length scale of the EPS state in theLPCMO system. Combining the MFM data and the micromagneticsimulation, we conclude that the 500-nm LPCMO disk is in asingle-domain ferromagnetic state at all temperatures below Tc. Asimilar conclusion can be reached for 300-nm LPCMO disks(shown in Fig. S4), although it needs to be studied further whethera new state would emerge if the disk size becomes a few tens ofnanometers or smaller. Our work opens a way to control EPSwithout external field or introducing strain and disorder, which ispotentially useful to design electronic and spintronic devices incomplex oxides systems.

ACKNOWLEDGMENTS. J. Shen, L.Y., J.X., and W. Wang were supported byNational Key Research Program of China (2016YFA0300702). J. Shao, H. Liu,K.Z., Y.Y., H. Lin, J.N., K.D., Y.K., W. Wei, F.L., Y.Z., and J. Shen weresupported by National Basic Research Program of China (973 Program) Grant2014CB921104, National Natural Science Foundation of China Grant 91121002,and Shanghai Municipal Natural Science Foundation Grant 14JC1400500. L.Y.was supported by National Basic Research Program of China (973 Program)Grant 2013CB932901 and National Natural Science Foundation of ChinaGrants 91121002 and 11274071. W.Y. and J.X. were supported by NationalNatural Science Foundation of China Grant 91121002. W. Wang was supportedby National Natural Science Foundation of China Grant 11504053. E.W.P. wassupported by US Department of Energy (DOE) Grant DE-SC0002136.

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