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Giant magnetoresistance in insulating granular films and planar tunneling junctions
Materials Science and Engineering A267 (1999) 184 – 192
Giant magnetoresistance in insulating granular films and planar tunneling junctions H. Fujimori *, S. Mitani, K. Takanashi Institute for Materials Research, Tohoku Uni6ersity, 2 -1 -1, Katahira, Aoba-ku, Sendai 980 – 8577, Japan
Abstract The authors’ recent studies of giant magnetoresistance (GMR) in insulating granular films and planar tunneling junctions are reviewed. First, GMR and related properties of sputter-deposited Co – Al – O granular films are described. Electron microscopy observations revealed that Co–Al–O films possess well-defined metal – nonmetal granular structures with Co granules of 2–3 nm in diameter. MR of 10.6% was observed for Co36Al22O42 film at room temperature, which is the largest value of GMR in insulating granular films, and the magnitude of MR is discussed in comparison with those for other granular systems. Anomalous temperature and bias-voltage dependence of MR was found in Co – Al – O granular films, and can be explained by a theory of spin-dependent higher-order tunneling. Improvement of low-field MR response of granular-in-gap (GIG) structures consisting of a Co–Y–O granular film and soft magnetic FeNi films is also shown. Next, GMR and current – voltage characteristics of planar tunneling junctions prepared by an ion beam sputtering technique is shown. MR of 4% was observed for a Fe/Al –O/NiFe/FeMn junction at 77 K. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Giant magnetoresistance; Tunneling; Spin polarization; Coulomb blockade; Insulating granular film; Planar tunneling junctions; Reactive sputtering; Ion beam sputtering
1. Introduction Since the discovery of giant magnetoresistance (GMR) in Fe/Cr superlattices [1], GMR due to spin-dependent scattering [2] and spin-dependent tunneling [3,4] in artificial magnetic nanostructures has attracted growing interest. GMR has opened new perspectives in the physics of magnetotransport, and also provides the possibility for further improvement of MR devices in practical applications. Large MR above 20% at room temperature was recently reported in planar magnetic tunneling junctions [3,5]. These junctions are promising for application to devices such as high-density magnetic recording heads, magnetic random access memories (MRAM) and sensors. However, the following problems remain in planar tunneling junctions: 1. difficulty in preparing a good quality tunneling barrier for reproducible and large MR; * Corresponding author. Tel.: +81-22-215-2223; fax: + 81-22-2152095.
2. decrease in MR with increasing bias voltage [4,6]; and 3. too high resistance for application in devices such as reading heads for high density magnetic recording at present. Insulating granular films consisting of nanometersized magnetic metal granules and an insulating matrix also exhibit GMR [7,8], which is caused by spin-dependent tunneling between magnetic granules. While an insulating granular film can be regarded as an assembly of a large number of small tunnel junctions, it shows characteristic tunneling conductance, which is attributed to the electrical charging effect of nanometersized granules (Coulomb blockade) [9,10]. Of particular interest is the interplay of spin-dependent tunneling and Coulomb blockade. In this paper, the authors’ recent studies of GMR in insulating granular films and planar tunneling junctions are overviewed. First, GMR and related properties of sputter-deposited Co–Al–O granular films are described, with an emphasis on the novel properties due to the interplay of spin-dependent tunneling and Cou-
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lomb blockade [11]. Improvement of the low-field MR response by means of a combination of an insulating granular film and soft magnetic metallic films (granular-in-gap) is also shown [12]. Next, reproducible and large MR and current – voltage (I– V) characteristics of planar tunneling junctions prepared by the ion beam sputtering technique are shown [13]. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%.
2. Insulating granular films
2.1. Sample preparation Insulating granular films are prepared by using sputtering [8,10,14] and evaporation techniques [15]. Fig. 1 shows typical sputtering methods for preparing insulating granular films. A reactive sputtering method (Fig. 1 (a)) is useful for accurate composition control, and Co–Al–O films discussed in this paper were prepared by this method.
Fig. 2. (a) Plan view and (b) cross-sectional TEM micrographs for a Co46Al19O35 film.
2.2. Structure Fig. 2 shows plan view and cross-sectional transmission electron microscopy (TEM) micrographs for a Co46Al19O35 film [16]. It is seen that the Co–Al–O film has isotropic granular structure and consists of Co granules of 2–3 nm in diameter and intergranular Al–oxide of about 1 nm in thickness. In Fig. 3, the high resolution TEM micrograph for a Co52Al20O28 film reveals that crystalline Co granules are surrounded with amorphous Al–oxide [16]. From systematic observations, it is shown that the large MR effect is attributed to the fine granular structures, i.e. the small granules and the thin intergranular insulating barrier.
Fig. 1. Sputtering methods for preparation of insulating granular films: (a) reactive sputtering, (b) sputtering with a composite target, (c) tandem deposition.
Fig. 3. High resolution TEM micrograph for a Co52Al20O28 film.
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tion (M 2) is also shown in Fig. 5 (a). The field dependence of MR is almost the same as that of M 2 [8]. Fig. 6 (a) and (b) show r and M at 4.2 K as a function of H for a Co52Al20O28 film, respectively. MR is enhanced up to 16%. The proportional relationship between MR and M 2 still remains. Since Co–Al–O films have a superparamagnetic nature, MR of the films with low Co content does not saturate in the low magnetic field range. Fig. 7 shows r as a function of H for a Co36Al22O42 film. r becomes almost constant above 70 kOe, resulting in MR of 10.6%. Similar MR and related properties are observed for other insulating granular alloy systems. Figs. 8 and 9 show MR for Fe–Al–O and Co–Si–O films, respectively, as examples. The magnitude of MR reported for insulating granular films to date is summarized in Table 1 [7,8,11,14,15,17 –30]. The values of P 2/(1+P 2) are also shown in Table 1, where P 2/(1+ P 2) is the magnitude of MR expected from the spin polarization of magnetic metal granules [31]. The values of P 2/(1+P 2) for Fe-, Co- and Ni-based systems are calculated using PFe = 0.44, PCo = 0.34 and PNi = 0.2, respectively [32,33], and the value for the Fe-based system is the largest. However, the magnitude of MR observed is not
Fig. 4. Temperature dependence of electrical resistivity (r) for a Co52Al20O28 and Co36Al22O42 film: (a) r vs. T, (b) log r vs. T − 1/2.
2.3. Electrical resisti6ity Fig. 4 (a) shows temperature dependence of electrical resistivity (r) for Co52Al20O28 and Co36Al22O42 films. r depends strongly on temperature, indicating negative temperature coefficients. log r versus T − 1/2 plots for the films are shown in Fig. 4 (b). log r is approximately proportional to T − 1/2 in the wide temperature range. This is the characteristic behavior of r in insulating granular films and is due to charging effect of metallic granules on a nanometer scale [9,10].
2.4. Magnetoresistance Fig. 5 (a) and (b) show r and magnetization (M) as a function of applied magnetic field (H) for a Co52Al20O28 film at room temperature, respectively. MR (Dr/rmax) of 7.8% appears even though r is much larger than those of ordinary ferromagnetic metals, e.g. NiFe alloy. The square of the normalized magnetiza-
Fig. 5. (a) Resistivity r and (b) magnetization M as a function of applied magnetic field (H) for a Co52Al20O28 film at room temperature, respectively. Square of the normalized magnetization is also shown.
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Fig. 8. Magnetoresistance (Dr/rmax) for an Fe – Al – O film at room temperature.
Bias-voltage (V) dependence of MR also shows unusual behavior, as shown in Fig. 11 [11]. r is very sensitive to V, and decreases by three orders of magnitude with increasing V up to 600 mV. In contrast, MR is almost constant, keeping the enhanced value. The temperature and bias-voltage dependence of MR, different from that in macroscopic tunneling junctions, suggests an interplay of spin-dependent tunneling and charging effect of nanometer-sized granules. Fig. 6. (a) Resistivity r and (b) magnetization M as a function of applied magnetic field (H) for a Co52Al20O28 film at 4.2 K, respectively. Square of the normalized magnetization is also shown.
in good accordance with P 2/(1 +P 2). The largest MR is observed for the Co – Al – O system at room temperature. Effective values of the spin polarization factor may be considered for each film. Temperature dependence of MR for a Co36Al22O42 film (closed circles) is shown in Fig. 10 [11,19], as a typical example. The observed MR is anomalously enhanced at low temperatures, while above 100 K, it is nearly constant and is close to P 2/(12 + P 2) for Co ( =10.4%). The experimental results of temperature dependence can not be explained by the previous theories [31,34].
Fig. 7. r as a function of H up to 150 kOe for a Co36Al22O42 film at room temperature.
2.5. Theory In the present theory for tunnel-type GMR, spin-dependent higher-order tunneling processes, which were neglected in the previous theories [11,35], have been taken in account. When usual tunneling processes (sequential tunneling in granular systems) are much suppressed due to the charging effect (Coulomb blockade), it seems impossible to neglect higher-order tunneling probability. Fig. 12 shows a schematic illustration of higher-order tunneling process in granular systems: by
Fig. 9. Magnetoresistance (Dr/rmax) for a Co – Si – O film at room temperature.
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Table 1 Material dependence of GMR in insulating granular films at room temperature (RT) and/or low temperatures [7,8,11,14,15,17–30] Insulator
Al oxide Si oxide Mg oxide Hf oxide Rare earth oxide Pb oxide Mg Fluoride P 2/(1+P 2)a
Magnetic material Fe
Co
Ni
3–4% (RT) 25% (4.2 K) 2–3% (RT) – 4% (RT) 14% (4.2 K) 4% (RT) 2% (RT)
6–10% (RT) 16–20% (4.2 K) 2–4% (RT) – – – – 5–6% (RT)
– – 0.6–1% (RT) 3% (20 K) – – – –
10% (RT) 6% (RT) 16%
– – 10%
– – 4%
a P 2/(1+P 2) is the magnitude of MR expected from the spin polarization of magnetic metal granules [31,32].
means of a simultaneous tunneling of three electrons, an electronic charge is transferred from the charged large granule, via the two small ones, to the neutral large one. The theoretical result for r in this model agrees with the experimental value with a fitting parameter of charging energy Ec/kB =110 K [11]. The theoretical
Fig. 10. Temperature dependence of MR for a Co36Al22O42 film (open circles). The dashed line represents P 2Co/(1+P 2Co), which is the magnitude of MR expected from the spin polarization of Co, PCo = 0.34 [31,32], in Co-based insulating granular films. The solid curve represents the theoretical MR given by a spin-dependent higher-order tunneling model with the parameters of the effective spin polarization P = 0.306 and the charging energy of Co granules Ec/kB = 110 K [11]. The inset shows the results for a low temperature range.
Fig. 11. Bias-voltage (V) dependence of (a) r at H =0 Oe and (b) MR for a Co36Al22O42 film at 4.2 K. Open circles represent the experimental results and solid curves represent the theoretical ones by a spin-dependent higher-order tunneling model [11] with the same parameters as those in Fig. 9.
results for temperature and bias-voltage dependence of MR are shown in Figs. 10 and 11, respectively, where the same parameters are used for these calculations. The agreement between the experimental and theoretical results is fairly good, indicating that the enhanced MR and the anomalous temperature and bias-voltage dependence are due to spin-dependent higher-order tunneling. In the higher-order tunneling model, the enhancement of MR originates from the fact that the probability of higher-order tunneling is given by the product of the probability of each tunneling event [11,35]. In addi-
Fig. 12. Schematic illustration of a higher-order tunneling process: by means of a simultaneous tunneling of three electrons, an electronic charge is transferred from the charged large granule (left), via the two small ones, to the neutral large one (right).
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4. Planar tunneling junctions
4.1. Sample preparation
Fig. 13. Schematic illustration of granular-in-gap (GIG) structure consisting of a metal–nonmetal granular film and a couple of soft magnetic metal films [12].
tion, temperature dependence of the enhanced MR is approximately given as Dr/rmax 81 + (C/T)1/2 (C, constant) [11].
3. Granular-in-gap structures From a practical application point of view, magnetic field sensitivity is a serious problem for MR in granular systems, i.e. a high magnetic field is required to magnetize granular magnetic materials. Recently, granular-in-gap (GIG) structure was proposed to reduce saturation field of MR in metal – nonmetal granular systems, in which a high r of metal – nonmetal granular systems is utilized [12]. A schematic illustration of GIG structure is shown in Fig. 13. An insulating granular film is located in the lateral gap of a couple of soft magnetic metal films, and it is expected that a large stray magnetic flux (close to the magnitude of saturation magnetization of the soft magnetic metal film) is generated in the narrow space of the gap when the soft magnetic metal film is magnetized by applying a low external field. Fig. 14 (a) and (b) show the low-field MR response for a GIG structure consisting of Fe66Ni34/Co39Y14O47 granular film/Fe66Ni34 and the magnetization curve of the Fe66Ni34 film, respectively [12]. The MR response appears within a low external field of about 2 Oe. The field dependence of MR is almost the same as that of the magnetization. The MR response is much improved by means of the GIG structure and the field sensitivity at low field is about 250 times larger than that for a single Co39Y14O47 film. It is suggested that GIG structures are applicable to magnetic sensor devices.
In the studies reported to date for planar tunneling junctions, the sample preparation was made by RF or DC sputtering [36,37] or a vacuum evaporation [4] technique. In this study, ion beam sputtering was used for the sample preparation, and insulating barriers were formed by exposing a metallic layer to an oxygen ion beam [13]. Fe/Al–O/NiFe/FeMn and Fe/Al–O/Co/ NiFe/ FeMn junctions were prepared on thermally oxidized Si substrates through metallic masks, to produce the cross stripe configuration as shown in Fig. 15. The junction area was 0.5 ×0.5 mm, and for each change of mask, the vacuum chamber was opened to air. At the first electrode, an Fe 1000 A, layer was deposited at a beam voltage VB of 800 V. An Al metallic layer was deposited on the Fe layer at VB = 800 V, in a circular shape. The insulating barrier was formed by exposing the Al layer, 12 A, in thickness, to Ar+ O2 beam at VB = 100 V, using the assist ion source for 2–30 min. If the oxidization time tox is the same, it is possible to form the insulating layer with a reproducible barrier width and height. At the second electrode, NiFe 150 A, and then FeMn 400 A, were deposited to show a spin valve property. Samples were also prepared with
Fig. 14. (a) Low-field MR response for a GIG structure consisting of Fe66Ni34/Co39Y14O47/Fe66Ni34. (b) Magnetization curve of the Fe66Ni34 film.
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Fig. 15. Schematic illustrations of the fabrication process for planar tunneling junctions. The left and right figures show cross sections and top views, respectively.
a Co 30 A, layer was inserted between the insulating barrier and the NiFe layer.
4.2. Magnetoresistance and junction resistance Resistance (R) as a function of applied magnetic field, i.e. MR, at 77 K is shown for Fe/Al– O/NiFe/ FeMn and Fe/Al– O/Co/NiFe/FeMn (tox =5 min) in Fig. 16 (a) and (b), respectively. Interestingly, inserting a Co layer between the insulating layer and the NiFe layer enhances the magnitude of MR from 4.0 to 18%. The authors consider that one possible reason for the enhanced MR is the high spin polarization of conduc-
Fig. 17. (a) Junction resistance (RJ) and (b) magnetoresistance (DR/ R) for Fe(1000 A, )/Al(12 A, ) – O/Co(30 A, )/NiFe(150 A, )/FeMn(400 A, ) as a function of the oxidization time (tox). Solid lines (77 K) and dashed lines (290 K) are guides for the eyes.
tion electrons of Co compared to that of NiFe [37]. The junction resistance RJ and the MR DR/R are shown as a function of tox in Fig. 17 (a) and (b), respectively. RJ increases with tox at 77 K and at room temperature. On the other hand, the MR shows no strong dependence on tox. The maximum MR is obtained at tox =15 min, and being 28% at 77 K and 16% at room temperature. MR at room temperature increases as tox decreases, and at tox = 2 min, it is larger than that at 77 K. The authors consider this to be caused by the geometrical enhancement [38] due to very low RJ. Fig. 18 shows a typical I–V curve for Fe/Al–O/ Co/NiFe/FeMn (tox = 15 min), which shows non-linear characteristics indicating tunneling transport. The current density is given theoretically by the Simmons’ formula [39], I=a(V+ gV 3),
(1)
where a= 3.16× 1010 × (f 1/2/L)exp{− 1.025Lf 1/2} (2) and g= 0.0115×L /f. 2
Fig. 16. Magnetoresistance of (a) Fe(1000 A, )/Al(12 A, )–O/NiFe(150 A, )/FeMn(400 A, ) and (b) Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/ NiFe(150 A, )/FeMn(400 A, ) at 77 K (tox = 5 min).
L and f mean the barrier width (A, ) and the barrier height (eV), respectively. Fitting the I–V curve in Fig. 4 to Eq. (1) gives L= 25 A, and f=0.40 eV. Fig. 19 shows the barrier height f and width L as a function of tox. f increases with tox, although these values of barrier
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5. Concluding remarks
Fig. 18. I – V characteristics for Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/ NiFe(150 A, )/FeMn (400 A, ) at tox = 15 min. The solid line is a best fit to the Simmons’ formula.
height are smaller than those generally known for Al2O3 [40] On the other hand, L shows no large tox dependence. For tox shorter than 5 min, L is somewhat larger. The authors consider this to be because f is so small that the Simmons’ expansion to V 3 in Eq. (1) is insufficient for fitting the experimental I – V curve.
This paper has reviewed our recent studies of GMR in insulating granular films and planar tunneling junctions. MR of 10.6% was observed for Co36Al22O42 film at room temperature, which is the largest value of GMR in insulating granular films. Anomalous temperature and bias-voltage dependence of MR was found in Co–Al–O granular films and was explained by a theory of spin-dependent higher-order tunneling. The granular-in-gap (GIG) structures improve the low-field MR response dramatically. GMR and I–V characteristics of planar tunneling junctions prepared by the ion beam sputtering technique were also shown. MR of 4% was observed for a Fe/Al–O/NiFe/FeMn junction at 77 K. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%. From the large MR in the Co-based granular films and the effect of Co layer insertion, it is suggested that Co possesses high spin polarization in the magnetic nanostructures and is suitable for electrode materials of tunnel-type GMR devices.
Acknowledgements The studies discussed in this paper have been performed through several joint research works. We would like to thank Dr S. Ohnuma and Mr N. Kobayashi of The Research Institute for Electric and Magnetic Materials, and Mr K. Yakushiji, Mr H. Yamanaka and Mr K. Saito of IMR, Tohoku University, for their work and experimental cooperation. We would like to thank Dr S. Takahashi and Professor S. Maekawa of IMR, Tohoku University for their theoretical cooperation and valuable discussions. We are grateful to Dr M. Ohnuma and Dr K. Hono of the National Research Institute for Metals for TEM observations and analyses. Our special thanks are also due to Dr S. Nagata, Dr K. Takahiro, Professor S. Yamaguchi of IMR, Tohoku University, for RBS analysis. A part of this work was performed at the Laboratory for Developmental Research of Advanced Materials, IMR, Tohoku University. This work was supported by a grant from the JSPS, Research for the Future Program.
References
Fig. 19. Barrier height and width calculated from I– V characteristics of Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/NiFe(150 A, )/FeMn(400 A, ) as a function of tox. Dashed lines are guides for the eyes.
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Giant magnetoresistance in insulating granular films and planar tunneling junctions H. Fujimori *, S. Mitani, K. Takanashi Institute for Materials Research, Tohoku Uni6ersity, 2 -1 -1, Katahira, Aoba-ku, Sendai 980 – 8577, Japan
Abstract The authors’ recent studies of giant magnetoresistance (GMR) in insulating granular films and planar tunneling junctions are reviewed. First, GMR and related properties of sputter-deposited Co – Al – O granular films are described. Electron microscopy observations revealed that Co–Al–O films possess well-defined metal – nonmetal granular structures with Co granules of 2–3 nm in diameter. MR of 10.6% was observed for Co36Al22O42 film at room temperature, which is the largest value of GMR in insulating granular films, and the magnitude of MR is discussed in comparison with those for other granular systems. Anomalous temperature and bias-voltage dependence of MR was found in Co – Al – O granular films, and can be explained by a theory of spin-dependent higher-order tunneling. Improvement of low-field MR response of granular-in-gap (GIG) structures consisting of a Co–Y–O granular film and soft magnetic FeNi films is also shown. Next, GMR and current – voltage characteristics of planar tunneling junctions prepared by an ion beam sputtering technique is shown. MR of 4% was observed for a Fe/Al –O/NiFe/FeMn junction at 77 K. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Giant magnetoresistance; Tunneling; Spin polarization; Coulomb blockade; Insulating granular film; Planar tunneling junctions; Reactive sputtering; Ion beam sputtering
1. Introduction Since the discovery of giant magnetoresistance (GMR) in Fe/Cr superlattices [1], GMR due to spin-dependent scattering [2] and spin-dependent tunneling [3,4] in artificial magnetic nanostructures has attracted growing interest. GMR has opened new perspectives in the physics of magnetotransport, and also provides the possibility for further improvement of MR devices in practical applications. Large MR above 20% at room temperature was recently reported in planar magnetic tunneling junctions [3,5]. These junctions are promising for application to devices such as high-density magnetic recording heads, magnetic random access memories (MRAM) and sensors. However, the following problems remain in planar tunneling junctions: 1. difficulty in preparing a good quality tunneling barrier for reproducible and large MR; * Corresponding author. Tel.: +81-22-215-2223; fax: + 81-22-2152095.
2. decrease in MR with increasing bias voltage [4,6]; and 3. too high resistance for application in devices such as reading heads for high density magnetic recording at present. Insulating granular films consisting of nanometersized magnetic metal granules and an insulating matrix also exhibit GMR [7,8], which is caused by spin-dependent tunneling between magnetic granules. While an insulating granular film can be regarded as an assembly of a large number of small tunnel junctions, it shows characteristic tunneling conductance, which is attributed to the electrical charging effect of nanometersized granules (Coulomb blockade) [9,10]. Of particular interest is the interplay of spin-dependent tunneling and Coulomb blockade. In this paper, the authors’ recent studies of GMR in insulating granular films and planar tunneling junctions are overviewed. First, GMR and related properties of sputter-deposited Co–Al–O granular films are described, with an emphasis on the novel properties due to the interplay of spin-dependent tunneling and Cou-
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lomb blockade [11]. Improvement of the low-field MR response by means of a combination of an insulating granular film and soft magnetic metallic films (granular-in-gap) is also shown [12]. Next, reproducible and large MR and current – voltage (I– V) characteristics of planar tunneling junctions prepared by the ion beam sputtering technique are shown [13]. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%.
2. Insulating granular films
2.1. Sample preparation Insulating granular films are prepared by using sputtering [8,10,14] and evaporation techniques [15]. Fig. 1 shows typical sputtering methods for preparing insulating granular films. A reactive sputtering method (Fig. 1 (a)) is useful for accurate composition control, and Co–Al–O films discussed in this paper were prepared by this method.
Fig. 2. (a) Plan view and (b) cross-sectional TEM micrographs for a Co46Al19O35 film.
2.2. Structure Fig. 2 shows plan view and cross-sectional transmission electron microscopy (TEM) micrographs for a Co46Al19O35 film [16]. It is seen that the Co–Al–O film has isotropic granular structure and consists of Co granules of 2–3 nm in diameter and intergranular Al–oxide of about 1 nm in thickness. In Fig. 3, the high resolution TEM micrograph for a Co52Al20O28 film reveals that crystalline Co granules are surrounded with amorphous Al–oxide [16]. From systematic observations, it is shown that the large MR effect is attributed to the fine granular structures, i.e. the small granules and the thin intergranular insulating barrier.
Fig. 1. Sputtering methods for preparation of insulating granular films: (a) reactive sputtering, (b) sputtering with a composite target, (c) tandem deposition.
Fig. 3. High resolution TEM micrograph for a Co52Al20O28 film.
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tion (M 2) is also shown in Fig. 5 (a). The field dependence of MR is almost the same as that of M 2 [8]. Fig. 6 (a) and (b) show r and M at 4.2 K as a function of H for a Co52Al20O28 film, respectively. MR is enhanced up to 16%. The proportional relationship between MR and M 2 still remains. Since Co–Al–O films have a superparamagnetic nature, MR of the films with low Co content does not saturate in the low magnetic field range. Fig. 7 shows r as a function of H for a Co36Al22O42 film. r becomes almost constant above 70 kOe, resulting in MR of 10.6%. Similar MR and related properties are observed for other insulating granular alloy systems. Figs. 8 and 9 show MR for Fe–Al–O and Co–Si–O films, respectively, as examples. The magnitude of MR reported for insulating granular films to date is summarized in Table 1 [7,8,11,14,15,17 –30]. The values of P 2/(1+P 2) are also shown in Table 1, where P 2/(1+ P 2) is the magnitude of MR expected from the spin polarization of magnetic metal granules [31]. The values of P 2/(1+P 2) for Fe-, Co- and Ni-based systems are calculated using PFe = 0.44, PCo = 0.34 and PNi = 0.2, respectively [32,33], and the value for the Fe-based system is the largest. However, the magnitude of MR observed is not
Fig. 4. Temperature dependence of electrical resistivity (r) for a Co52Al20O28 and Co36Al22O42 film: (a) r vs. T, (b) log r vs. T − 1/2.
2.3. Electrical resisti6ity Fig. 4 (a) shows temperature dependence of electrical resistivity (r) for Co52Al20O28 and Co36Al22O42 films. r depends strongly on temperature, indicating negative temperature coefficients. log r versus T − 1/2 plots for the films are shown in Fig. 4 (b). log r is approximately proportional to T − 1/2 in the wide temperature range. This is the characteristic behavior of r in insulating granular films and is due to charging effect of metallic granules on a nanometer scale [9,10].
2.4. Magnetoresistance Fig. 5 (a) and (b) show r and magnetization (M) as a function of applied magnetic field (H) for a Co52Al20O28 film at room temperature, respectively. MR (Dr/rmax) of 7.8% appears even though r is much larger than those of ordinary ferromagnetic metals, e.g. NiFe alloy. The square of the normalized magnetiza-
Fig. 5. (a) Resistivity r and (b) magnetization M as a function of applied magnetic field (H) for a Co52Al20O28 film at room temperature, respectively. Square of the normalized magnetization is also shown.
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Fig. 8. Magnetoresistance (Dr/rmax) for an Fe – Al – O film at room temperature.
Bias-voltage (V) dependence of MR also shows unusual behavior, as shown in Fig. 11 [11]. r is very sensitive to V, and decreases by three orders of magnitude with increasing V up to 600 mV. In contrast, MR is almost constant, keeping the enhanced value. The temperature and bias-voltage dependence of MR, different from that in macroscopic tunneling junctions, suggests an interplay of spin-dependent tunneling and charging effect of nanometer-sized granules. Fig. 6. (a) Resistivity r and (b) magnetization M as a function of applied magnetic field (H) for a Co52Al20O28 film at 4.2 K, respectively. Square of the normalized magnetization is also shown.
in good accordance with P 2/(1 +P 2). The largest MR is observed for the Co – Al – O system at room temperature. Effective values of the spin polarization factor may be considered for each film. Temperature dependence of MR for a Co36Al22O42 film (closed circles) is shown in Fig. 10 [11,19], as a typical example. The observed MR is anomalously enhanced at low temperatures, while above 100 K, it is nearly constant and is close to P 2/(12 + P 2) for Co ( =10.4%). The experimental results of temperature dependence can not be explained by the previous theories [31,34].
Fig. 7. r as a function of H up to 150 kOe for a Co36Al22O42 film at room temperature.
2.5. Theory In the present theory for tunnel-type GMR, spin-dependent higher-order tunneling processes, which were neglected in the previous theories [11,35], have been taken in account. When usual tunneling processes (sequential tunneling in granular systems) are much suppressed due to the charging effect (Coulomb blockade), it seems impossible to neglect higher-order tunneling probability. Fig. 12 shows a schematic illustration of higher-order tunneling process in granular systems: by
Fig. 9. Magnetoresistance (Dr/rmax) for a Co – Si – O film at room temperature.
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Table 1 Material dependence of GMR in insulating granular films at room temperature (RT) and/or low temperatures [7,8,11,14,15,17–30] Insulator
Al oxide Si oxide Mg oxide Hf oxide Rare earth oxide Pb oxide Mg Fluoride P 2/(1+P 2)a
Magnetic material Fe
Co
Ni
3–4% (RT) 25% (4.2 K) 2–3% (RT) – 4% (RT) 14% (4.2 K) 4% (RT) 2% (RT)
6–10% (RT) 16–20% (4.2 K) 2–4% (RT) – – – – 5–6% (RT)
– – 0.6–1% (RT) 3% (20 K) – – – –
10% (RT) 6% (RT) 16%
– – 10%
– – 4%
a P 2/(1+P 2) is the magnitude of MR expected from the spin polarization of magnetic metal granules [31,32].
means of a simultaneous tunneling of three electrons, an electronic charge is transferred from the charged large granule, via the two small ones, to the neutral large one. The theoretical result for r in this model agrees with the experimental value with a fitting parameter of charging energy Ec/kB =110 K [11]. The theoretical
Fig. 10. Temperature dependence of MR for a Co36Al22O42 film (open circles). The dashed line represents P 2Co/(1+P 2Co), which is the magnitude of MR expected from the spin polarization of Co, PCo = 0.34 [31,32], in Co-based insulating granular films. The solid curve represents the theoretical MR given by a spin-dependent higher-order tunneling model with the parameters of the effective spin polarization P = 0.306 and the charging energy of Co granules Ec/kB = 110 K [11]. The inset shows the results for a low temperature range.
Fig. 11. Bias-voltage (V) dependence of (a) r at H =0 Oe and (b) MR for a Co36Al22O42 film at 4.2 K. Open circles represent the experimental results and solid curves represent the theoretical ones by a spin-dependent higher-order tunneling model [11] with the same parameters as those in Fig. 9.
results for temperature and bias-voltage dependence of MR are shown in Figs. 10 and 11, respectively, where the same parameters are used for these calculations. The agreement between the experimental and theoretical results is fairly good, indicating that the enhanced MR and the anomalous temperature and bias-voltage dependence are due to spin-dependent higher-order tunneling. In the higher-order tunneling model, the enhancement of MR originates from the fact that the probability of higher-order tunneling is given by the product of the probability of each tunneling event [11,35]. In addi-
Fig. 12. Schematic illustration of a higher-order tunneling process: by means of a simultaneous tunneling of three electrons, an electronic charge is transferred from the charged large granule (left), via the two small ones, to the neutral large one (right).
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4. Planar tunneling junctions
4.1. Sample preparation
Fig. 13. Schematic illustration of granular-in-gap (GIG) structure consisting of a metal–nonmetal granular film and a couple of soft magnetic metal films [12].
tion, temperature dependence of the enhanced MR is approximately given as Dr/rmax 81 + (C/T)1/2 (C, constant) [11].
3. Granular-in-gap structures From a practical application point of view, magnetic field sensitivity is a serious problem for MR in granular systems, i.e. a high magnetic field is required to magnetize granular magnetic materials. Recently, granular-in-gap (GIG) structure was proposed to reduce saturation field of MR in metal – nonmetal granular systems, in which a high r of metal – nonmetal granular systems is utilized [12]. A schematic illustration of GIG structure is shown in Fig. 13. An insulating granular film is located in the lateral gap of a couple of soft magnetic metal films, and it is expected that a large stray magnetic flux (close to the magnitude of saturation magnetization of the soft magnetic metal film) is generated in the narrow space of the gap when the soft magnetic metal film is magnetized by applying a low external field. Fig. 14 (a) and (b) show the low-field MR response for a GIG structure consisting of Fe66Ni34/Co39Y14O47 granular film/Fe66Ni34 and the magnetization curve of the Fe66Ni34 film, respectively [12]. The MR response appears within a low external field of about 2 Oe. The field dependence of MR is almost the same as that of the magnetization. The MR response is much improved by means of the GIG structure and the field sensitivity at low field is about 250 times larger than that for a single Co39Y14O47 film. It is suggested that GIG structures are applicable to magnetic sensor devices.
In the studies reported to date for planar tunneling junctions, the sample preparation was made by RF or DC sputtering [36,37] or a vacuum evaporation [4] technique. In this study, ion beam sputtering was used for the sample preparation, and insulating barriers were formed by exposing a metallic layer to an oxygen ion beam [13]. Fe/Al–O/NiFe/FeMn and Fe/Al–O/Co/ NiFe/ FeMn junctions were prepared on thermally oxidized Si substrates through metallic masks, to produce the cross stripe configuration as shown in Fig. 15. The junction area was 0.5 ×0.5 mm, and for each change of mask, the vacuum chamber was opened to air. At the first electrode, an Fe 1000 A, layer was deposited at a beam voltage VB of 800 V. An Al metallic layer was deposited on the Fe layer at VB = 800 V, in a circular shape. The insulating barrier was formed by exposing the Al layer, 12 A, in thickness, to Ar+ O2 beam at VB = 100 V, using the assist ion source for 2–30 min. If the oxidization time tox is the same, it is possible to form the insulating layer with a reproducible barrier width and height. At the second electrode, NiFe 150 A, and then FeMn 400 A, were deposited to show a spin valve property. Samples were also prepared with
Fig. 14. (a) Low-field MR response for a GIG structure consisting of Fe66Ni34/Co39Y14O47/Fe66Ni34. (b) Magnetization curve of the Fe66Ni34 film.
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Fig. 15. Schematic illustrations of the fabrication process for planar tunneling junctions. The left and right figures show cross sections and top views, respectively.
a Co 30 A, layer was inserted between the insulating barrier and the NiFe layer.
4.2. Magnetoresistance and junction resistance Resistance (R) as a function of applied magnetic field, i.e. MR, at 77 K is shown for Fe/Al– O/NiFe/ FeMn and Fe/Al– O/Co/NiFe/FeMn (tox =5 min) in Fig. 16 (a) and (b), respectively. Interestingly, inserting a Co layer between the insulating layer and the NiFe layer enhances the magnitude of MR from 4.0 to 18%. The authors consider that one possible reason for the enhanced MR is the high spin polarization of conduc-
Fig. 17. (a) Junction resistance (RJ) and (b) magnetoresistance (DR/ R) for Fe(1000 A, )/Al(12 A, ) – O/Co(30 A, )/NiFe(150 A, )/FeMn(400 A, ) as a function of the oxidization time (tox). Solid lines (77 K) and dashed lines (290 K) are guides for the eyes.
tion electrons of Co compared to that of NiFe [37]. The junction resistance RJ and the MR DR/R are shown as a function of tox in Fig. 17 (a) and (b), respectively. RJ increases with tox at 77 K and at room temperature. On the other hand, the MR shows no strong dependence on tox. The maximum MR is obtained at tox =15 min, and being 28% at 77 K and 16% at room temperature. MR at room temperature increases as tox decreases, and at tox = 2 min, it is larger than that at 77 K. The authors consider this to be caused by the geometrical enhancement [38] due to very low RJ. Fig. 18 shows a typical I–V curve for Fe/Al–O/ Co/NiFe/FeMn (tox = 15 min), which shows non-linear characteristics indicating tunneling transport. The current density is given theoretically by the Simmons’ formula [39], I=a(V+ gV 3),
(1)
where a= 3.16× 1010 × (f 1/2/L)exp{− 1.025Lf 1/2} (2) and g= 0.0115×L /f. 2
Fig. 16. Magnetoresistance of (a) Fe(1000 A, )/Al(12 A, )–O/NiFe(150 A, )/FeMn(400 A, ) and (b) Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/ NiFe(150 A, )/FeMn(400 A, ) at 77 K (tox = 5 min).
L and f mean the barrier width (A, ) and the barrier height (eV), respectively. Fitting the I–V curve in Fig. 4 to Eq. (1) gives L= 25 A, and f=0.40 eV. Fig. 19 shows the barrier height f and width L as a function of tox. f increases with tox, although these values of barrier
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5. Concluding remarks
Fig. 18. I – V characteristics for Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/ NiFe(150 A, )/FeMn (400 A, ) at tox = 15 min. The solid line is a best fit to the Simmons’ formula.
height are smaller than those generally known for Al2O3 [40] On the other hand, L shows no large tox dependence. For tox shorter than 5 min, L is somewhat larger. The authors consider this to be because f is so small that the Simmons’ expansion to V 3 in Eq. (1) is insufficient for fitting the experimental I – V curve.
This paper has reviewed our recent studies of GMR in insulating granular films and planar tunneling junctions. MR of 10.6% was observed for Co36Al22O42 film at room temperature, which is the largest value of GMR in insulating granular films. Anomalous temperature and bias-voltage dependence of MR was found in Co–Al–O granular films and was explained by a theory of spin-dependent higher-order tunneling. The granular-in-gap (GIG) structures improve the low-field MR response dramatically. GMR and I–V characteristics of planar tunneling junctions prepared by the ion beam sputtering technique were also shown. MR of 4% was observed for a Fe/Al–O/NiFe/FeMn junction at 77 K. Inserting a thin Co layer between the insulating barrier and the NiFe layer improved the MR up to 18%. From the large MR in the Co-based granular films and the effect of Co layer insertion, it is suggested that Co possesses high spin polarization in the magnetic nanostructures and is suitable for electrode materials of tunnel-type GMR devices.
Acknowledgements The studies discussed in this paper have been performed through several joint research works. We would like to thank Dr S. Ohnuma and Mr N. Kobayashi of The Research Institute for Electric and Magnetic Materials, and Mr K. Yakushiji, Mr H. Yamanaka and Mr K. Saito of IMR, Tohoku University, for their work and experimental cooperation. We would like to thank Dr S. Takahashi and Professor S. Maekawa of IMR, Tohoku University for their theoretical cooperation and valuable discussions. We are grateful to Dr M. Ohnuma and Dr K. Hono of the National Research Institute for Metals for TEM observations and analyses. Our special thanks are also due to Dr S. Nagata, Dr K. Takahiro, Professor S. Yamaguchi of IMR, Tohoku University, for RBS analysis. A part of this work was performed at the Laboratory for Developmental Research of Advanced Materials, IMR, Tohoku University. This work was supported by a grant from the JSPS, Research for the Future Program.
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Fig. 19. Barrier height and width calculated from I– V characteristics of Fe(1000 A, )/Al(12 A, )–O/Co(30 A, )/NiFe(150 A, )/FeMn(400 A, ) as a function of tox. Dashed lines are guides for the eyes.
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