Resistance minimum in ultrathin ferromagnetic films of Fe on MgO(100)

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Journal of Magnetism and Magnetic Malerials I i I , I9921 L225-L230 North-itollatad

Letter to the Editor

Resistance minimum in ultrathin ferromagnetic films of Fe on MgO(100) C. Liu, Y. Park ~ and S.D. B a d e r Materials Science Dicision, Argonne National Laborato~', Ar,,onne, IL 60430, USA

Received 2 April 1992

Ultrathin films of Fe grown on MgO(100) in ultrahigh vacuum were studied in situ as a function of temperature and thickness by means of sheet-resistance R u , Kerr-effect, electron-diffraction and Auger measurements. Ferromagnetic hysteresis appears at room t e m p e r a t u r e for Fe dosages > 4 monolayers ( ~- 5.7 ,~). For films -" 11 to 14 .~, thick R [] has a minimum at the relatively high temperature of = 260 to 140 K, respectively, attributed to a crossover from 2 dimensional disordered to and 3D metallic system.

For over a decade the electrical-transport properties of disordered two-dimensional (2D) films have attracted attention associated with weak-localization and interaction effccts [1,2]. P h e n o m e n a of interest include metal-insulator transitions [3], superconductor-insulator transitions [4], high-T~ superconductivity [5], as well as Kondo [6], spin-glass [7] and ferromagneti: [8] behavior. The effccts of interlayer coupling in magnetic [9] and non-magnetic [10] multiiayer systems, and ultrathin metallic films [11,12] can also gi,~e rise to unusual transport properties. In metals, for film thickness less than a characteristic length, e.g., z Thouless length LTh, the resistance can i ":tease on cooling, typically with a In T dependence at low T, and exhibit a minim u m at a higher T. Morphological effects ;uch as due to island growth, can also lead to an increasing resistance at low T, and thermallyactivated hopping could yield a resistance minimum as well. It is of interest to measure a well-

Correspondence to: Dr. C. Liu, Materials Science Division, Bldg. 223, Argonne National Laboratory, Argonne, Ik 60439, USA. Tel.: + 1-708-252-5019: telefax: + 1-708-252-7777. t Also at the Dept. of Physics, Northwestern University, Evanston, IL 602118~ USA.

characterized, clean metal film on a flat, insuk ing substrate. The system Fe on MgOtl00) is lattice matched to within 4%, and is a good candidate to study 2D-transport properties of relatively ordered metal films on an insulating substrate. In this Letter we present results of low-energy electron-diffraction (LEED) and Auger studies of the film growth, surface magneto-optic Kerr-effect ( S M O K E ) ~tudie~ of the magx:cti~. properties, and dc electrical resistance measurements for ultrathin F e / M g O ( 1 0 0 ) films. A resistance minimum is observed at a temperature 7 that decreases from = 260 to -- 140 K for Fe dosages corresponding to --- 8 - I 0 monolayers (or 11-14 A), respectively. These films possess ordered L E E D patterns and long-range ferromagnetic order with full remanence. First we v~ill describe the experimental conditions and the magnetic and structural analyses, and then we present the tran~;port results and discuss their significance. The films were gro~:,n in l0 ~ Tort ultrahigh vacuum ( U H V ) from a resistive cx~porator, w'hich was made by winding a pure iron wire on a tungsten sphal = 8 mm in diameter. The epitaxially polished MgO(100) sug~trate was mounted on a manipulator with the [! i0] aligned horizono

0304-8853/92/$05.00 ~:' 1992 - Elsevier Science Publishers B.V. All rights reserved

LETTER ,

TO

THE

EDITOR

h

C. Liu et ai. ,/Resistance minimum in uhrathin fihns o f Fe on MgO( lO0)

L226

tally. The MgO exhibited a p(1 x 1) LEED pattern characteristic of the bulk-terminated structure even before any cleaning treatment in vacuum. Carbon and/or CO on the surface, as indicated by monolayer-level carbon Auger signals, were removed by Ar + sputtering, followed by an anneal at =1000 K in 2>:10 -~' Torr O~. Our LEED observations indicate that Fe grows epitaxiaily on clean MgO(100) at substrate temperatures from 100 to 450 K, in agreement with ref. [13]. 100 K growth gave broader LEED spots, as expected. It was reported previously [13] that Fe films < 10 A average thickness have a bodycentered-tetragonal structure and that the bcc structure develops with increasing thickness. Measurements of the Auger intensity vs. deposition time were carried out to study the growth mode of Fe/MgO(100). Fig. 1 shows a typical result for room-temperature growth. The data correspond quite well to exponential growth and decay curves, but no distinct break points were observed for any growth temperature in our study. Similar observations were reported by Urano and Kanaji [14], but were interpreted as layer-by-layer growth. We interpret our observations to indicate a simultaneous-multilayer growth mode, by which the film grows with a series of terraces of different heights, and multilayers are built up simultaneously with the growth of the first layer. To calibrate the evaporation rate, the Auger measurements were repeated for Fe/Ru(0001),

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Thickness (MLE) Fig. 2. Thickness d e p e n d e n c e of the longitudinal Kerr intensity for F e / M g O ( 1 0 0 ) films grown and m e a s u r e d at 1(10 I~ and straight-line extrapolation to zero film thickness. The upper inset shows typical longitudinal (a) and polar (b) Kerr-eifect response for an = 3 M L E Fe film and lower inset shows the longitudinal K e r r loop for an = 8 M L E film.

which has been studied previously [15]. The deposition time needed to complete one monolayer of Fe on Ru(0001) was then used to determine the thickness of Fe/MgO(100) in monolayer equivalents (MLE) according to the planar lattice densities. Periodically repeated Auger measurements confirmed that the evaporation rate remained constant within 5% during the time period of the experiments. In-situ SMOKE measurements were used to probe both in-plane and perpendicular magnetization directions as the films were growing [16]. Fig. 2 shows the Kerr intensity (hysteresis loop height) vs. Fe thickness for films grown and measured at 100 K. Hysteretic behavior was not detected for films thinner than = 4 MLE, where only an unsaturated magnetic response was present. An example of this type of response is shown in the insets (a) and (b) of fig. 2 for a 3 M: E film, which suggests that while the film is magnetizable along the surface normal, the easy axis of magnetization is neither in-plane nor perpendicular to the film. Such films are either superparamagnetic or have an equilibrium domain structure [17] that eliminates any remanence. Longitudinal Kerr loops started to develop at := 5 MLE and extended to thicker films. A hys-

('. l.iu et al. / Resistance m i n i m u m in ultrathin films o t Ire on ~Ig()( iOO) •

teresis loop is shown in the inset (c) of fig. 2 for an = 8 MLE film. Straight-line extrapolation of the thicker-film data in fig. 2 to zero film thickness indicates that the magnetism is expected to persist throughout the ultrathin regime, and that each Fe layer appears to contribute similarly to the total magnetization. This extrapolation behavior is similar to that observed for Fe/Pd(100), but different from that of F e / R u ( 0 0 0 1 ) w h e r e the first 2 MLE are not ferromagnetically aligned [18]. Recent polarized neutron reflection measurements for 3-12 M L E of Fe on MgO(100) overcoated with gold indicated that all films studied are ferromagnetic at 40 K, with a magnetic moment similar to that of bulk Fe [19]. Li and Freeman [20] recently proposed that F e / M g O ( 1 0 0 ) is a nearly ideal 2D magnetic system, and predicted a giant enhancement of the magnetic moment for the first two monolayers of Fe on MgO with an in-plane magnetization. O u r observations pertaining to the first few monolayers of F e / M g O ( 1 0 0 ) indicate an absence of hysteresis., a tendency for the magnetization to cant out of plane, and a zero-intercept extrapolation of the Kerr-intensity vs. thickness plot which are contrary to the theuretical prediction.,, of giant, in-plane ferromagnetic moments for 1-2 ME of Fe on MgO(100). The theory assumes perfect epitaxy, but experimentally we obviously have no! achieved that goal. We have tried different growth conditions and in-situ cleaving of the MgO, but all of our efforts led to the same conclusions as reported above. r o make in-situ sheet resistance measurements, - 1 0 0 0 A thick Au pads were pre-c cposited onto an MgO(100) substrate for the fourprobe electrical leads and the chromel-alumel thermocouple contact. The substrate was sputtercleaned and annealed at = 800 K to ensure that ao ( < 1%) impurities were present. The dimensions between the two voltage probes were 2 x 3 mm 2. The dc electrical resistance was measured using a Keithley digital multimeter (Model 1901 with a low current (typically 1).5-51) g A ) which gave an ohmic response. For films thinner than = 8 MLE (or = l l A), a R ~ / a r < 0 [see fig. 3(a)]. As the film became thicker, a transition to metallic behavior, signified o

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by aRD/OT > 0 with a resistance minimum Rmin, was observed between = 11 and = 14 ,~ as shown in fig. 4. The location of Rmm gradually shifted to lower T as the film thickness was increased. The temperature at which Rmin occurs depends sensitively on the surface condition of the substrate. For a clean surface, R,,,, moves from = 26() to 130 K in the narrow thickness range of = 11-14 ,k for which long-rang,', ferromagnetic order persists (see fig. 4). Observations of resistance minima have been reported previously for uitrathin Pt films sputtered on Si [12] and |or Fe on GaAs [21] and on giass [22]. The fact that "oR ~/i~T ,:.. tl in fig. 31a) and in the low T part of fig. 4 can have either of two different origins. Firstly, it is know that for variable range hopping the conductivity ~r can

LETTER TO THE EDITOR , ,

"]

L228

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C Liu et al. / Resistance minimum in uhrathin fihns oJ" Fe on MgO( lO0)

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T dependence ~r(T)= ~r{}cxp[-(Eh//kl~T)"], where Ce varies from 0.25 to 0.6 [23] depending on the dimcnsionality and the functional form of the density of states. But fitting of our data in fig. 3(a) gives unrealistically small (submillivolts) values of the hopping energy Eh. The other mechanism to understand our resuits involves the consideration of disordered 2D conductors [2], for which

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¢r(T) = o'(T0) +/3~ I n ( T / T o ) ,

where ~ = e"/2~r~h, and /3 is expected to be of order of unity. We find a /3 value of = (1.5 by fitting the data in fig. 3(a). In 3D metals the resistance can be expressed in the simplified form:

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(2)

where R, is the residual resistance and y is determined by the dominant inelastic scattering

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mechanism. Fitting the data of thick films [e.g., fig. 3(c)], gives 31 values of = 1.7-2.2, consistent with the ;esults for bulk Fe [24] in this T interval. The transport properties in 2D are controlled by weak localization and interaction effects if LTh Ot T -I'/2 > d, where d is the film thickness and p is the temperature index of inelastic scattering rate. The condition for two dimensionality in the regime dominated by electron interaction effects is ( D / k B T ) ~ / 2 > d, where D is the diffusion constant. We can observe the 2D and 3D regimes in figs. 3(a) and (c), respectively. Since LTh and ( D / k l 3 T ) I/2 ale T dependent, they can go from tile value larger to smaller than d with increasing T. The dimensional crossover [25] is observed in fig. 4. We fitted the data in fig. 4 with the expression

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¢r(T) = ¢r(T,,) + ,Bs~ 1,'1(T/To) + B( T ~ - T,~'). (3)

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Temperature (K) Fig. 4. R vs. T for a series of four films: (a) = 11 ~, (b) = 12 A, (c) = 13 /~ and (d) = 14 7\ to demonstrate the shift in R m i rl •

Thc fitting paramctcrs/3 and y for four diffcrcnt films are shown in table 1. We find reasonable agreement with the many reported values of /3 for various materials [26], although /3 can vary quite widely depending on the size of the electron-electron interaction effect; and on the number of subbands at the Fermi surface if the quantum size effect plats a role [27]. However the important fact is that our Fe films are ferromagnetic with strong remanence. Singh and Fradkin [28] argue that the magnetization due to spin-split bands in itinerant fe~romagnet~; dees not affect weak localization effects in bulk systems. But it is not clcar if thi,, also is applicable to 2D systems where the demagnetization may destroy the phase coherence of the electron wavefunction that is essential to weak localization. 2D interaction and quantum size effects, however, are not affected to

C. Liu et ai. / Resistance minimum in uhrathin [ihns o[ Fe on M~O( IOO) •

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magnetic films show thc crossover from disordered 2D to a 3D metallic regimc with increasing tcmperaturc and thc prcscncc of resistance minima. The thickness dependence of the conductivity shows behavior related to the crossover.

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lowest order by magnetic fields. Since the T range we studied is in the vicinity of dimensional crossover, the condition is not ideal for determining a precise value of 3'. However, the trend observed in table 1 of 3' increasing with thickness is approaching the expectation. In addition, we also studied the thickness dependence of the resistivity and observed a slope change as shown in fig. 5 for an as-dcpositcd film. To understand this behavior we note that recent evidence [29] of the inadequacy of the classical theory [30] has motivated quantum mechanical treatments of the size effects [31,32! which predict p = p o d -'s, where 6---6 for a single-band semiconductor, and 8 = 2.3 for multi-band metallic film [32]. Our effective exponents shown in fig. 5 are surprisingly similar to these values and the slope change occurs at - 8 ~,, near to where the dimensional crossover discussed above occurs. In summary, we report that SMOKE measurements of Fe films grown on MgO(100) do not show ferromagnetic hysteresis up to ---4 MLE. We also conclude that Fe grows in a simultaneous-multilayec mode in which weakly linked multi!ayers may produce randomly canted easy axes of magnetization. The easy axes of magnetizatio~ reorient in-plane for films thicker than = 4 MLE. No evidence was found for the enhanced magnetic moments, which is contrary to the theoretical prediction that assumes perfect layer-by-layer growth. Rcsistivity measurements in the ferro-

This work was supported by the US Department of Energy,, Office of Basic Energy Sciences, Division of Materials Sciences, under contract no. W-31-109-ENG-38. We thank Tao Pang, N. Rivier, I.K. Schuller and N. Trivedi for helpful discussions.

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L E T T E R TO THE EDITOR

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C Liu et al. / Resistance mhzimum in ultrathin fihns of Fe on MgO( lO0)

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[24] [25] [26] [27] [28] [29] [30] [31] [32]

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