We report on transport and magnetic measurements of islanded Fe(110) thin films. The electrical resistivity exhibits an anomalous increase at low temperatures, which disappears under the action of a magnetic field. Since such an anomaly completely disappears under the action of a magnetic field, it is inferred that it originates from spin-dependent scattering. We interpret the strong changes in the spin-dependent scattering in our films to be due to a low-temperature spin freezing of the island boundary magnetic regions that impedes ferromagnetic exchange between islands. A consequence of this magnetic behavior is the random arrangement of the individual magnetization, determined by the magnetocrystalline anisotropy of each island, resulting in an increase of the resistivity below the freezing temperature.Z.S. and J.L.M. acknowledge the Comunidad de Madrid for financial support. Work was performed under the financial support of the Comunidad de Madrid and the Spanish Commission of Science and Technology.Peer reviewe

Briones Fernández-Pola, Fernando

Cebollada, Alfonso

Crespo, P.

Hernando, A.

Menéndez, José Luis

Navarro, E.

Sefrioui, Z.

Digital.CSIC

PHYSICAL REVIEW B, VOLUME 64, 224431Correlation between magnetic and transport properties in nanocrystalline Fe thin films:A grain-boundary magnetic disorder effectZ. Sefrioui,1,* J. L. Mene´ndez,1 E. Navarro,1,2 A. Cebollada,1 F. Briones,1 P. Crespo,3 and A. Hernando31Instituto de Microelectro´nica de Madrid-IMM (CNM-CSIC), Isaac Newton 8 (PTM) Tres Cantos, Madrid 28760, Spain2Dept. Fı´sica Aplicada, Facultad de Veterinaria, Universidad Complutense, 28040 Madrid, Spain3Instituto de Magnetismo Aplicado, U.C.M., PO Box 155, 28230 Las Rozas, Madrid, Spain~Received 10 July 2001; published 26 November 2001!We report on transport and magnetic measurements of islanded Fe~110! thin films. The electrical resistivityexhibits an anomalous increase at low temperatures, which disappears under the action of a magnetic field.Since such an anomaly completely disappears under the action of a magnetic field, it is inferred that itoriginates from spin-dependent scattering. We interpret the strong changes in the spin-dependent scattering inour films to be due to a low-temperature spin freezing of the island boundary magnetic regions that impedesferromagnetic exchange between islands. A consequence of this magnetic behavior is the random arrangementof the individual magnetization, determined by the magnetocrystalline anisotropy of each island, resulting in anincrease of the resistivity below the freezing temperature.DOI: 10.1103/PhysRevB.64.224431 PACS number~s!: 75.70.2i, 75.50.BbPhenomena in nanostructured magnetic systems have at-tracted much attention in recent years, especially those inwhich spin-dependent scattering plays an important role. Forexample, the influence of magnetic disorder in spin-dependent scattering shows its clear manifestation in thewell-known giant magnetoresistance effect.1–4 This phenom-enon is due to the change in resistivity of systems in whichthe magnetization directions of individual magnetic entitiescan be aligned on misaligned with respect to each other uponapplication of a magnetic field. These magnetic entities canindeed be of diverse natures: layers separated by nanometer-thick nonferromagnetic spacers in a multilayer or a spinvalve,1,5,6 grains embedded in a matrix to constitute a granu-lar system,7–11 or even magnetic domains separated by do-main walls in a continuous ferromagnetic film.12 In all thesecases, the observed magnetoresistance is due to a transitionfrom a magnetically disordered state ~antiparallel alignmentin a multilayer or spin valve, random orientation in a granu-lar system or multidomain state in a ferromagnetic thin film!to an ordered one ~a parallel configuration or single-domainstate!. In all these cases, a key parameter is the spin diffusionlength, which determines an upper limit for the extension ofthe magnetic disorder to give rise to magnetoresistive effects.It is because of the limited value of this magnitude, in therange of a few nanometers, that a manifestation of thesephenomena is restricted to systems in which at least one ofthe dimensions is structured in the nanometer range. On theother hand, nanostructured materials with characteristicstructural coherence lengths ~such as grain size! in the rangeof a few to a few tens of nanometers are also very interest-ing, because the scale of the microstructure is comparable tothe ferromagnetic exchange length as well as because thelarge relative fraction of atoms at the interfaces. As a conse-quence, dramatic changes in the magnetic behavior aredriven by small variations in both grain size and magneticcorrelation across grain boundaries.13 As recently reported,14nanocrystalline Fe obtained by ball milling exhibits a low-temperature magnetic hardening. Magnetic decoupling dueto a spin freezing of the grain boundaries was invoked as apossible cause of this effect.0163-1829/2001/64~22!/224431~4!/$20.00 64 2244In this paper we show, by means of transport and mag-netic measurements, low-temperature ferromagnetic quench-ing effects in Fe~110! contacting nanometric islands. Theseeffects may be due to either a random spin freezing at theisland boundaries or to a weak antiferromagnetic couplingbetween grains that fluctuations are unable to overcome atlow temperatures. As a result, an anomalous increase of theresistivity as well as a decrease of zero-field-cooled ~ZFC!(M ZFC) magnetization are observed. Ferromagnetic ex-change between contacting nanocrystals depends on the ca-pability of the interface to transmit exchange. Therefore, it isinferred that grain boundaries of Fe that transmit ferromag-netic exchange at high temperatures become ferromagneticexchange insulators at low temperatures.Granular ferromagnetic thin films formed of nanometer-sized islands in physical contact are very adequate systems tostudy both temperature-dependent magnetic interactions aswell as transport properties, since a careful control of thegrowth conditions allows a fine design of the nanostructure.In the present work, Fe~110! islands were deposited onAl2O3(0001) substrates in an ultrahigh-vacuum triode sput-tering system at 700 °C,15 and a 2.5-nm Pt capping layer wasgrown at room temperature to protect from oxidation. Itshould be noted here that in the case of ordered Fe-Pt alloys,which are usually annealed at high temperature ~450–500 °C!, Pt can give rise to perpendicular anisotropy.16,17However, we did not observe the presence of perpendicularanisotropy in our samples due to the low temperature of thePt growth necessary to avoid alloying and ordering at theinterface Fe-Pt. Island sizes were easily varied simply bychanging the deposition time. Three in-plane equivalent azi-muthal orientations,1–10 Fei(11– 20)Al2O3, as expected fromthe symmetry of the substrate, are concluded from TEMcharacterization,15 while the out-of-plane orientation for allthe islands is ~110!. Therefore, three equivalent in-plane easyaxes separated by 60° are also expected. If we assume a bulkstructure, the easy axis would be Fe@001#; however due tosurface anisotropy and/or strain, often present in thin films,the easy axis might be different, although always keeping a©2001 The American Physical Society31-1Z. SEFRIOUI et al. PHYSICAL REVIEW B 64 224431sixfold symmetry due to the crystalline symmetry. In order totest this assertion, magneto-optical Kerr effect loops wereperformed along different in-plane directions, showing noessential differences in the hysteresis loops, consistent with ahighly symmetric sixfold structure. This therefore indicatesthe presence of a low magnetic anisotropy in this system.Due to the reduced island height for all the fabricated struc-tures ~limited to less than 4 nm! and the elevated depositiontemperature, it is very reasonable to assume that every indi-vidual island is a single crystal. An atomic force microscopy~AFM! image for a sample with 20-nm in-plane island size isshown in Fig. 1 ~a!. The system under study in this paperconsists of individual single-crystalline Fe~110! islands ofnanometric dimensions with a discrete in-plane magneto-crystalline anisotropy distribution. In order to confirm thatislands are magnetically connected through their bases by anFe boundary region, Kerr hysteresis loops at room tempera-ture were performed, showing a square shape. This indicatesthe absence of superparamagnetism, despite the small size ofthe islands ~the maximum volume for the onset of superpara-magnetism in bcc Fe at room temperature is around103 nm3!. The sharpness of the loops is inconsistent with aFIG. 1. ~a! AFM image of a selected Fe~110! islanded samplewith a 20-nm typical in plane island size. ~b! Sketch of the islandedstructure showing three in-contact islands with the three equivalentin-plane Fe crystallographic orientations. The shaded area corre-sponds to the islands boundary regions.22443system being a mixture of superparamagnetic and ferromag-netic regions. Superparamagnetic regions acting indepen-dently should give rather smooth loops due to the distribu-tion of local anisotropies, which would result in adistribution of coercive fields. This observation serves as aconfirmation that islands observed in the AFM image mustbe in physical contact, leading to an exchange interactionthrough the in-contact islands boundaries. In Fig. 1~b! weshow a sketch of the fabricated structure illustrating this pic-ture.The temperature dependence of the electrical resistivityfor this system was measured for samples with different is-land sizes, under applied magnetic fields in the range 0–0.5T, using a dc four-terminal method. While r(T) presents atypical metallic behavior in the whole temperature range forsamples with in-plane island diameters d, greater than 30 nm,we observe a nonmetallic behavior below a specific tempera-ture (T f) ranging from 50 to 150 K for samples with grainsizes varying from d526– 16.5 nm, respectively, giving riseto a minimum in the resistivity around T f . Figure 2~a! showsthe temperature dependence of the electrical resistivity withand without magnetic field for a sample with the smallestgrain size. It is clear that the upturn of the resistivity disap-pears under the action of a magnetic field of 0.5 T. Therefore,below 150 K the system exhibits a decrease of resistivitywith further cooling down, up to a value of 5.5% at 10 K.The results of measurements performed with the same ap-plied magnetic field for a sample with a 26-nm grain sizeindicate a smaller ~;0.4%! magnetoresistance. Since such ananomaly completely disappears under the action of a mag-netic field, it is inferred that the low-temperature electricaltransport is dominated by spin-dependent scattering. In orderto confirm the magnetic origin of this phenomenon, we haveperformed magnetic measurements using a superconductingquantum interference device magnetometer. Figure 2~b! dis-plays the temperature dependence of zero-field-cooled(M ZFC) and field-cooled (M FC) magnetization under a mag-netic field of 0.1 T for a sample with a 26-nm in-plane islandsize. While M FC increases below 50 K, we observe a de-crease of M ZFC below 50 K. A similar dependence of zero-field cooling was observed for a sample with a 16.5-nm in-plane island size @see the inset of Fig. 2~b!#. The observationof an upturn in the electrical resistivity, together with a sup-pression of the magnetization, allow us to conclude that be-low T f the magnetization becomes inhomogeneous with de-creasing temperature. Moreover, the combination ofmagnetic and transport measurements indicates that the softferromagnetic system evolves toward a noncollinear mag-netic structure, as the temperature drops below T f . This ob-servation also shows the importance of the intergrain ex-change in the overall magnetic behavior. For temperaturesabove T f the spins of the boundary are magnetized along thedirection of the adjacent grain by contact to with their mo-lecular field. Thus, the magnetization being homogeneous inthe intergrain regions, the whole system is ferromagnetic.Below T f the boundary spins start to freeze toward theground state reducing the ferromagnetic exchange betweenislands. This gives rise to a progressive increase of the elec-trical resistivity due to the enhanced magnetic scattering of1-2CORRELATION BETWEEN MAGNETIC AND TRANSPORT . . . PHYSICAL REVIEW B 64 224431the carriers at the grain boundaries. As a consequence of thelocal anisotropy a random distribution at grains and bound-aries appears parallel to the spin freezing, and the total mag-netization drops. Further support for a magnetic hardeningdue to a boundary spin freezing in our nanocrystalline Fethin films is based on an analysis of the low-temperaturehysteresis loops. In Fig. 3 we show the hysteresis loop at 40K for a sample with a 16.5-nm in-plane island size. First ofall, let us note that the hysteresis loop is squared at hightemperatures, and that the magnetization saturates abruptly ata low magnetic field, indicating a uniform reversal of themagnetization. At low temperature ~40 K!, however, the loopis smooth ~see Fig. 3!, indicating that the boundary graduallyreverses its magnetization, and that the boundary spins rota-tion is not coherent as at high temperature. This is indicatedby the fact that the coercive field at 40 K is one order ofmagnitude higher than at high temperature. Moreover, theFIG. 2. ~a! Temperature dependence of the electrical resistivityat zero magnetic field and in a magnetic field of 0.5 T for a samplewith the smallest obtained grain size ~16.5 nm!. Inset: resistancecurve at zero magnetic field for an islanded Fe~110! thin film withan island size of 26 nm. ~b! Magnetization ~M! vs temperature,measured after zero-field cooling ~ZFC! and field cooling ~FC! in amagnetic field of 0.1 T for a sample with an island size of 26 nm.~b! Temperature dependence of the zero-field-cooled (M ZFC) mag-netization ~0.1 T! for a sample with an island size of 16.5 nm.22443saturation field, Hs , increases at low temperature@Hs(200 K)’30 Oe and Hs(40 K)’175 Oe# reducing thesquareness of the loop, since this quantity is mainly associ-ated with the progressive alignment of the boundary spinstoward the field direction. As a result of the above consider-ations, our data strongly indicate that this soft ferromagneticsystem evolves toward a magnetic structure not completelycollinear at low temperature.The key argument about spin freezing was obtained froma generalization of the random-anisotropy model ~RAM!,13,18for which the self-consistent energy includes reduced inter-grain exchange. The basic idea is the competition betweenthree energy scales: ~a! the anisotropy energy within a finitevolume island, ~b! the exchange energy for adjusting themagnetization between such island, and ~c! the exchange en-ergy across grain boundaries. According to a two-phase gen-eralized RAM,13,18 the ferromagnetic exchange between twoadjacent islands is given by gA , where A is the exchangecoupling coefficient for bcc Fe, typically A510211 J m21,and g is a factor, ranging between 0 and 1, which accountsfor the possible modification of the interactions introducedby the interface structure. The exchange correlation lengththrough the intergrain boundary, L* is of the order of(gA/k)1/2, where k is the experimental anisotropy constantof bcc Fe, k54.8104 J m23 at room temperature.19 If L*exceeds the maximum grain dimension d, the magnetizationdoes not follow the easy direction of each individual grain,but tends to align itself along a common easy axis. Note thatL*.d implies that g.d2K/A , and therefore there exists aferromagnetic exchange between islands, g;1, and thewhole system is ferromagnetic and soft. Since experimen-tally we observe that L* decreases with cooling, there couldbe a temperature for which L* scales d, thus giving rise to aprogressive inhomogeneous magnetization for further de-creasing temperature. According to this argument, the low-temperature progressive inhomogeneous magnetization ofthe phases must be due to a decrease of g.In summary, we have investigated a magnetic thin-filmFIG. 3. Hysteresis loop at 40 K for a samples with 16.5-nmin-plane island sizes.1-3Z. SEFRIOUI et al. PHYSICAL REVIEW B 64 224431granular system that exhibits magnetic and transport proper-ties that strongly depend on the intergrain exchange. 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Correlation between magnetic and transport properties in nanocrystalline Fe thin films: A grain-boundary magnetic disorder effect

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