Intrinsic Tunneling in Phase Separated Manganites

G. Singh-Bhalla, S. Selcuk, T. Dhakal, A. Biswas, and A. F. Hebard*

Department of Physics, University of Florida, Gainesville, Florida 32611, USA(Received 7 May 2008; published 20 February 2009)

We present evidence of direct electron tunneling across intrinsic insulating regions in submicrometer

wide bridges of the phase-separated ferromagnet ðLa; Pr;CaÞMnO3. Upon cooling below the Curie

temperature, a predominantly ferromagnetic supercooled state persists where tunneling across the intrinsic

tunnel barriers (ITBs) results in metastable, temperature-independent, high-resistance plateaus over a

large range of temperatures. Upon application of a magnetic field, our data reveal that the ITBs are

extinguished resulting in sharp, colossal, low-field resistance drops. Our results compare well to

theoretical predictions of magnetic domain walls coinciding with the intrinsic insulating phase.

DOI: 10.1103/PhysRevLett.102.077205 PACS numbers: 75.47.Lx, 71.30.+h, 75.60.Ch

In ferromagnetic oxides [1] such as hole-doped man-ganites, the balancing of electrostatic [2] and elastic [3]energies in addition to competing magnetic interactionsmay lead to coexisting regions of ferromagnetic metallic(FMM) and insulating phases [4–6]. Upon reducing thedimensions of such a system, an increase in the easy-axismagnetic anisotropy and a decrease in electrostatic screen-ing [6] can create conditions which favor phase separationat the ferromagnetic domain boundaries resulting in novelinsulating stripe domain walls which allow direct electrontunneling [6–9]. The manganite ðLa1yPryÞ1xCaxMnO3

provides unique opportunities for exploring the forma-tion of such unique domain walls and intrinsic tunnelbarriers (ITBs) due to its well-documented micrometer-scale phase separation into FMM and insulating regions[3,4]. We present here transport properties of thinðLa0:5Pr0:5Þ0:67Ca0:33MnO3 (LPCMO) films which, whenreduced in dimensions, do indeed exhibit the classic sig-natures of tunneling across ITBs separating adjacent FMMregions. Further, colossal low-field magnetoresistance(MR) measurements suggest that the ITBs coincide withferromagnetic domain walls, implying that the ferromag-netic domain structure in LPCMO is modified.

To clarify the context and implications of our results, wedistinguish between experiments that are sensitive to thepresence of domain walls coincident with grain bounda-ries [10,11] or induced at geometrical constrictions [12–14], and our experiments, where ITBs [6,7] result fromintrinsic phase separation [3]. In the former category,enhanced low-field magnetoresistance observed in poly-crystalline ferromagnetic films of La0:67Ca0:33MnO3

and La0:67Sr0:33MnO3 is attributed to spin-dependentscattering [10] or tunneling [11] across grain bounda-ries. Low-field magnetoresistance has also been attrib-uted to domain walls artificially induced at microcon-strictions in epitaxial La0:67Ca0:33MnO3 films [12] andto domain walls pinned at nanoconstrictions in pat-terned epitaxial La0:67Sr0:33MnO3 films [13,14]. Belowwe show that choosing a system with the appropriatematerial properties and dimensions can give rise to highly

resistive ITB formation in the absence of mechanicaldefects.Calculations incorporating the double exchange model,

which also account for electrostatic interactions andscreening in phase separated ferromagnets show that nar-row stripes of the antiferromagnetic insulating phase (i.e.,ITBs) form at magnetic domain boundaries due to en-hancement of the easy-axis anisotropy in ultrathin (2D)manganite films [6]. This results in an abrupt change inmagnetization between neighboring FMM domains sepa-rated by an ITB, in contrast to classical ferromagnetswhere the direction of magnetization changes over mlength scales near a domain boundary [15]. The calcula-tions can easily be extended to narrow (1D) geometriessuch as ours [6]. We choose LPCMO since near theinsulator-to-metal transition temperature, TIM, the FMMphase coexists with insulating phases: the antiferromag-netic charge-ordered insulating [4] (COI) and the paramag-netic charge-disordered insulating phases [16–18]. BelowTIM LPCMO thin films grown on NdGaO3 are in a pre-dominantly ferromagnetic state [19], making the patternedthin films an ideal system for exploring stripe domain walland ITB formation. A slight change in the chemistry anddoping of the ðLa1yPryÞ0:67Ca0:33MnO3, (y ¼ 0:5) thin

films used in this experiment can drastically alter this phasecomposition [4,20].To fabricate our samples, we epitaxially deposited single

crystalline, 30 nm thick LPCMO films on heated (820 C)NdGaO3 (110) substrates using pulsed laser deposition[19]. We first measured (Fig. 1) the temperature-dependentresistance RðTÞ of several unpatterned LPCMO films be-fore standard photolithography and a manganite wet-etchwere used to pattern the films into 2:5 8 m four-terminal bridge structures (see Fig. 1). Measurementswere performed on these bridges before they were reducedin width using a focused ion beam as shown in the SEMimage in Fig. 1. Care was taken to avoid gallium ioncontamination by either directly depositing a polymerfollowed by a metal (70 nm) or just a metal layer on ourstructure. Four-terminal resistance measurements were

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made by sourcing a 1 nA dc current and measuring theresulting voltages, except for the current-dependent datashown in Figs. 2 and 3.

Figure 1 shows RðTÞ data for a film patterned into abridge geometry of 2:5 m width, which is on the orderof individual domain length scales [4,5]. In this case,RðTÞ emulates unpatterned thin-film behavior (grey) withthe exception of small steplike features [21] below theinsulator-to-metal percolation transition temperature,TIM ¼ 100 K [22]. However, when the bridge width isreduced to 0:6 m, transport is clearly dominated by afew metallic and insulating regions. We attribute the pro-nounced reduction of TIM to 64 K for this narrow bridge todimensionally-limited percolation (from 2D to nearly 1D).Multiple steplike drops in R occur for T < TIM due todiscrete numbers of insulating regions converting toFMM phase. Recent observations of discrete resistivitysteps on narrow bridges of other mixed phase manganitesalso provide evidence of single FMM and insulating re-gions spanning the full width of the structure [21,23,24].

Below T 50 K, the steps in RðTÞ of the bridge shownin Fig. 1 cease and a nearly temperature-independentresistance in a supercooled state dominates. Thoughmagnetization measurements on unpatterned epitaxialLPCMO thin films confirm a fully ferromagnetic metallic(3:8B=Mn) state below 50 K [19], it is possible that thenarrow geometry of the bridge favors the formation of aninsulating state. However, the temperature range of ap-proximately 50 K over which this high-resistance plateauoccurs cannot be explained by such a scenario since any

hopping transport associated with the insulating COI phasein LPCMO [20,25] would show a pronounced resistanceincrease with decreasing temperature. Additionally, theresistance, R 5 108 , of the zero-field-cooled(ZFC) temperature-independent plateau is 5 orders of mag-nitude larger than the quantum of resistance h=2e2 ¼12:9 k. By the scaling theory of localization [26] thelarge resistance value implies that for all dimensions, theT ¼ 0 state must be an insulator with infinite resistance,contrary to observation (down to 2 K). We therefore con-clude that transport across the 0:6 m wide bridge istemperature-independent direct tunneling through ITBscomprising atomically thin insulating regions.Also, unique to the 0:6 m wide bridge is an abrupt

colossal (1000-fold) drop in resistance near 40 K fromthe low-temperature field-cooled (Hz ¼ 5 kOe) high-resistance plateau upon field warming (FW) (cf. Fig. 1).Similar but relatively small and smooth drops in R areobserved in bulk and thin-film LPCMO samples, thoughthe mechanism is unclear [5]. To understand this drop inresistance upon FW, we recall that near TIM in LPCMO theinsulating and metallic regions are not pinned but evolve inshape and size with changing temperature [5,20]. However,below TB 40 K, the blocking [20] or the supercoolingglass transition [27,28] temperature, the phase separatedregions are ‘‘frozen’’ in place [20,27,28]. Thus, uponwarming up again into the dynamic state above TB, thephase separated regions and thus the metastable ITBs areno longer frozen in space, possibly giving way field-enhanced FMM conversion of ITBs resulting in a colossalresistance drop.To confirm that the temperature-independent resistance

plateau below TB ¼ 40 K (see Fig. 1, main text) is due toITBs, we measured current-voltage (I-V) curves at 5, 10,and 15 K as shown in Fig. 2. By numerically differentiatingthe I-V curve at 15 K, the differential conductance(dI=dV-V) curve shown in the inset of Fig. 2 is obtained.Assuming one ITB in the bridge, the solid red curve wasfitted to the data using the equation, dI=dV ¼ þ 3 V2

giving ¼ 9:8ð1Þ 109 S and ¼ 1:0ð1Þ 106 S=V2. Using Simmons’ model [29] and the valuesfor and , we calculate the average barrier height to be0.47 eV and the barrier thickness (t) to be 67 A.Interestingly, our value for is also typical for polycrys-talline manganites where tunneling occurs across a singlegrain boundary (GB) [30,31]. Unlike GBs however, ITBsare metastable and upon application of a field, bulk resis-tivity values are recovered in the bridge, confirming theabsence of GBs in our structure.The nonlinearity of the I-V curves in Fig. 2 could also be

due to current-induced Joule heating. Evidence supportingthe absence of Joule heating is shown in Fig. 2, bottominset, where RðTÞ is measured at the indicated currents.Here, TIM increases with increasing applied current, incontrast to the behavior found by Sacanell et al. where adecrease in TIM with increasing applied current is attrib-uted to Joule heating [32].

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FIG. 1 (color online). R vs T upon zero-field cooling andwarming (ZF-C/W) of bridges patterned from the sameLPCMO film are labeled for the unpatterned film (gray), the2:5 m 8 m bridge (black) and the 0:6 m 8 m bridge(green), respectively. A FC curve for the 0:6 m 8 m bridge(blue) is also shown. Insulator-metal transitions for the twobridges are indicated by the vertical color-coded dashed lines.Lower inset: schematic of the four-terminal configuration alongwith the applied field directions. Upper inset: scanning electronmicrograph of the 0:6 m wide bridge.

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Next, we explore the magnetic properties of ITBs. If theformation of ITBs is linked to the ferromagnetic domainstructure of LPCMO, the magnetic field required for thecollapse of the ITB will couple to the intrinsic magneticanisotropy of the thin film. Sensitivity of ITBs to magneticfield direction is verified in Fig. 3(a) with a subset of themeasured field-cooled RðTÞ traces for the three field ori-entations (Hx, Hy, and Hz) illustrated schematically in

Fig. 1. The inset highlights the cooling field directionaldependence at 5 K. Here, after a resistance of 107 isreached, cooling in a slightly higher field results in anabrupt hundred fold drop in resistance, suggesting a rapidand sudden disappearance of the ITB. Clearly, the in-planefields Hx and Hy are more effective than Hz in coupling to

the magnetization to reach the low resistance FMM state(R< 105). These observations of anisotropic field-inducedITB extinction show a coupling of the ITBs with the mag-netic easy axis which lies within the plane of the film formanganites deposited on (110) NdGaO3 [1]. The magneticfield values required to reach the low resistance state are onthe order of 1 kOe, which though much greater than themeasured coercive field in unpatterned films, are not atypi-cal for narrow ferromagnetic wires [33]. The anisotropicMR associated with the ITBs thus suggests that they mayindeed coincide with the FMM domain boundaries forminginsulating stripe domain walls. The necessity of intrinsicphase separation for stripe domain wall formation is ap-parent when considering recent measurements of notchedbridges on nonphase separated La0:67Sr0:33MnO3 which didnot show the presence of highly resistive ITBs [14].

Within the context of stripe domain walls, the MRanisotropy arises when spins in neighboring domains par-tially align with the field, resulting in reduced spin-dependent scattering at the ITB. The large drop in resis-

tance signifies a conversion of the ITBs to FMM, analo-gous to field-induced extinction of Bloch or Neel domainwalls. This notion is confirmed in Fig. 3(b) where themagnetic field dependence of the I-V curves obtained at15 K is shown. As Hz increases, the curvature of the I-Vcurves changes [Fig. 3(c)]. For fields sufficiently largeenough to drive the bridge into a low resistance state(9 kOe in Fig. 3(a), inset), the I-V curves become linear[Fig. 3(c)] suggesting ITB extinction.The isothermal magnetoresistance curve measured at

57 K in Fig. 4(a) follows a similar trend with small in-cremental decreases in resistance followed by a large (100-fold), irreversible, steplike drop to a low resistance state.Each resistance drop can be explained by either the ex-tinction of an ITB spanning the bridge width, or the in-cremental conversion of single ITBs to a FMM state.Qualitatively similar R vs H curves were measured forall T < TIM ¼ 64 K. For temperatures 100 T TIM ¼64 K we observe [Fig. 4(b)] colossal (100-fold) field-induced resistance changes which unlike for T < TIM oc-cur at well-defined anisotropic switching fields (i.e., theresistance can be switched back to the zero-field state uponfield removal). At the switching field, there is a reduction in

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FIG. 3 (color online). (a) ZFC and FC resistance transitions arelabeled by the field directions defined with respect to bridgeorientation in Fig. 1. Inset: R measured at 5 K when the bridge iscooled in separate runs at the indicated fields. (b) I-V character-istics of the 0:6 m wide bridge zero-field-cooled to 15 K,measured at the indicated values of Hz and (c) normalized tothe voltage, Vmax, measured at maximum applied current.

FIG. 2 (color online). I-V characteristics for the 0:6 m widebridge at the three indicated temperatures all measured duringone cooling cycle. Top inset: dI=dV-V curve at 15 K with a fit(red curve) to the Simmons’ model. Bottom inset: RðTÞ curvesobtained at the indicated currents.

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the free energy of the FMM phase and the dominant insu-lating phase undergoes a first-order phase transition and aconcomitant colossal resistance drop [1,8,16,20]. Becausethe changes in the bridge occur more readily for in-plane(Hy, blue curves) easy-axis fields, we speculate that exter-

nal field-induced spin alignment between neighboringFMM domains enhances the first-order insulator-to-metaltransition, just as ITBs are suppressed below TB whenneighboring domain spins are aligned (Fig. 2). Below50 K, the inferred resistance-area product [15] (from Rvs H measurements and the zero-field resistance plateaushown in Fig. 1) of a single ITB is 105 m2 [1] ormore conservatively, for ten ITBs within the bridge,106 m2 [1]. These values are 100 000 times greaterthan the highest value (1011 m2) reported for mangan-ites [12–14].

In summary, we have shown that phase-separation inmanganites is strongly modified in confined geometriesand leads to the formation of insulating regions thinenough to allow direct electron tunneling, which mayalso coincide with domain walls separating adjacentFMM domains [6]. Magnetotransport studies of LPCMObridge structures with submicron widths less than theaverage size of an FMM region enable us to observetunneling across these metastable ITBs with record-high-

resistance-area products. The high resistance of the ITBs,is extremely sensitive to temperature and the magnitudeand direction of applied magnetic fields, giving rise tocolossal low-field magnetoresistance. In addition to offer-ing rich physical insights into the formation of ferromag-netic domains in phase separated systems, the presence ofITBs introduces new opportunities for manipulating high-resistance barriers on the nanometer length scale.We thank Dmitri Maslov, Pradeep Kumar, and Denis

Golosov for discussions. This research was supported bythe U.S. National Science Foundation under GrantNo. DMR-0704240 (A. F. H.) and No. DMR 0804452(A. B.).

*Corresponding author.afh@phys.ufl.edu

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FIG. 4 (color online). Field sweeps (H) between 20 kOe attemperatures (a) T ¼ 57 K (below TIM) and (b) T ¼ 70 K(above TIM) for both increasing (bold curve) and decreasing(thin curve) fields: Hy (black) and Hz (blue). Inset: schematic

showing the coalescence of FMM (white) regions at the expenseof insulating (black) regions with applied fields or lowering tem-peratures. The rectangular overlay depicts the 0:6 m bridge.

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