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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Enhanced magnetoresistance in nanocrystalline magnetite

M. Venkatesan, S. Nawka, S. C. Pillai, and J. M. D. Coey

J. Appl. Phys. 93, 8023 (2003); http://dx.doi.org/10.1063/1.1555371 (3 pages) | Cited 19 times

Online Publication Date: 9 May 2003

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The magnetic and magnetotransport properties of nanoparticulate magnetite with different grain sizes are investigated using x-ray diffraction, microscopy, magnetometry, and magnetoresistance measurements. The magnetization varies significantly with grain size and is sensitive to preparation conditions. The reduction in saturation magnetization in coprecipitated particles is probably due to the surface spin disorder. Magnetoresistance of pressed powder compacts is significantly enhanced in material composed of small grain size magnetite particles prepared by coprecipitation. Useful magnetoresistance persists well above room temperature in sintered ceramic material. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Dd Nonmetallic ferromagnetic materials
75.47.De Giant magnetoresistance
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetoresistance of magnetite films prepared by reactive evaporation

Takao Furubayashi

J. Appl. Phys. 93, 8026 (2003); http://dx.doi.org/10.1063/1.1543988 (3 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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It was attempted to fabricate magnetite films by evaporating metallic iron in an oxygen atmosphere onto substrates at room temperature. From the characterization by x-ray diffraction and Mössbauer spectroscopy, the obtained films were found to consist of α-Fe, FeO, and Fe3O4 depending on the oxygen pressure. Pure magnetite was obtained when prepared in 5×10−6 Torr oxygen. The magnetite film exhibited negative magnetoresistance resulting from spin-dependent scattering at the grain boundaries. The result indicates that magnetite films with spin-polarized electrons can be prepared on substrates at room temperature. © 2003 American Institute of Physics.
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75.47.Pq Other materials
72.25.Rb Spin relaxation and scattering
75.50.Gg Ferrimagnetics
75.70.Ak Magnetic properties of monolayers and thin films
68.55.A- Nucleation and growth
61.72.Mm Grain and twin boundaries
76.80.+y Mössbauer effect; other γ-ray spectroscopy

Fabrication and magnetoresistive effect of current perpendicular to plane devices using half-metallic Fe3O4 thin films on metallic films

H. Takahashi, S. Soeya, J. Hayakawa, K. Ito, A. Kida, C. Yamamoto, H. Asano, and M. Matsui

J. Appl. Phys. 93, 8029 (2003); http://dx.doi.org/10.1063/1.1558199 (3 pages) | Cited 17 times

Online Publication Date: 9 May 2003

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The current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices having half-metallic Fe3O4 for their magnetic layers were investigated along with the fabrication of Fe3O4 films on Au layers at low temperature (523 K). The 10–50-nm-thick Fe3O4 films that were grown on a 100 nm Au (111) layer were Fe3O4 (111) oriented. These films showed Verwey transition at ∼120 K. Using these films, the relation between the magnetoresistive (MR) effect of CPP-GMR and the Fe3O4 layer thickness was examined with 2×2 μm2 samples of Ni80Fe20/Au/Fe3O4 trilayers on Au bottom electrode films. At the Fe3O4 layer thickness of 20 nm, the MR ratio was 0.04% and the area magnetoresistance-change product (ΔRA) was 1.5 mΩ μm2. The MR ratio was increased with decrease in the Fe3O4 thickness. The CPP-GMR of Fe3O4/Au/Fe3O4 on the Au layer showed that the MR ratio was 0.04% and the ΔRA was 3.9 mΩ μm2. This MR ratio was four times larger than that of the NiFe-type CPP-GMR for the same Fe3O4 bottom layer thickness. © 2003 American Institute of Physics.
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75.50.Dd Nonmetallic ferromagnetic materials
75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
72.60.+g Mixed conductivity and conductivity transitions
75.70.Ak Magnetic properties of monolayers and thin films
68.55.-a Thin film structure and morphology

Fabrication and magnetoresistance of tunnel junctions using half-metallic Fe3O4

Woochul Kim, Kenji Kawaguchi, Naoto Koshizaki, Mitsugu Sohma, and Tetsuro Matsumoto

J. Appl. Phys. 93, 8032 (2003); http://dx.doi.org/10.1063/1.1557337 (3 pages) | Cited 33 times

Online Publication Date: 9 May 2003

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Magnetite (Fe3O4) is believed to be half metal, providing 100% spin-polarized conduction electrons. The half-metallic nature of magnetic electrodes for tunneling junction devices is expected to induce a large magnetoresistance. We investigated the structural and chemical properties of interfaces in ferromagnet–insulator–ferromagnet (Fe3O4/MgO/Fe) tunnel junctions. Al/Ag/Fe3O4/MgO multilayers for magnetic tunnel junction have been fabricated on α-Al2O3 (001) and MgO (100) substrates by a molecular beam epitaxy system. The Fe3O4 quality was examined by reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), superconducting quantum interference device magnetometry, atomic force microscopy (AFM), and in situ x-ray photoelectron spectroscopy. RHEED and XRD results showed that the epitaxial Fe3O4 layer with a smooth surface was successfully grown on substrates. The stoichiometric Fe3O4 was confirmed by Verway transition in temperature dependence of magnetization. AFM data showed relatively smooth surface for Fe3O4 prepared at Ts=250 °C and P(O2)=3×10−3 Pa. The Fe 2p3/2 and Fe 2p1/2 peak profiles for Fe3O4 layer are little changed by overlaying MgO in the XPS measurements. These results suggest the Al/Ag/Fe3O4/MgO multilayers available for spin-dependent tunnel junction. © 2003 American Institute of Physics.
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75.47.Pq Other materials
72.25.Mk Spin transport through interfaces
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.65.Ac Multilayers

SrRuO3/SrTiO3/SrRuO3 heterostructures for magnetic tunnel junctions

G. Herranz, B. Martínez, J. Fontcuberta, F. Sánchez, M. V. García-Cuenca, C. Ferrater, and M. Varela

J. Appl. Phys. 93, 8035 (2003); http://dx.doi.org/10.1063/1.1555372 (3 pages) | Cited 6 times

Online Publication Date: 9 May 2003

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We report on the growth and characterization of SrRuO3 single layers and SrRuO3/SrTiO3/SrRuO3 heterostructures grown on SrTiO3(100) substrates. The thickness dependence of the coercivity was determined for these single layers. Heterostructures with barrier thickness tb=1, 2.5, and 4 nm were fabricated, with electrodes having thickness ranging from 10 to 100 nm. The hysteresis loops of heterostructures with tb=2.5 nm, 4 nm reveal uncoupled magnetic switching of the electrodes. Therefore, these heterostructures can be used for the fabrication of magnetic tunneling junctions. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Tunneling in epitaxial Fe/Si/Fe structures with strong antiferromagnetic interlayer coupling

R. R. Gareev, L. L. Pohlmann, S. Stein, D. E. Bürgler, P. A. Grünberg, and M. Siegel

J. Appl. Phys. 93, 8038 (2003); http://dx.doi.org/10.1063/1.1543989 (3 pages) | Cited 17 times

Online Publication Date: 9 May 2003

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Fe(5 nm)/Si(0.8–2 nm)/Fe(5 nm) structures are grown by molecular-beam epitaxy on Ag(001) buffered GaAs substrates. Ferromagnetic tunneling junctions with crossed electrodes and junction areas ranging from 22 to 225 μm2 are patterned using photolithography. Antiparallel alignment of the magnetizations due to antiferromagnetic interlayer coupling, which is confirmed by longitudinal magneto-optical Kerr effect hysteresis loops, exists for the whole range of spacer thicknesses. Transport properties in current perpendicular to the sample plane geometry are examined by the four-point method in the temperature range from 4 K to room temperature. As a function of spacer thickness, the junctions show a strong increase of the resistance times area product from ≈1 Ω μm2 to more than 10 kΩ μm2. The dI/dV−V curves are parabolic and asymmetric and thus characteristic for trapezoidal tunneling barriers. The mean barrier heights derived from Brinkman fits range from 0.3 to 0.8 eV. The zero-bias resistance of the tunneling junctions moderately decreases with temperature by less than 10% over the whole measured temperature range. All these transport properties fulfill the necessary and sufficient criteria for elastic tunneling. © 2003 American Institute of Physics.
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72.25.Mk Spin transport through interfaces
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
73.40.Sx Metal-semiconductor-metal structures
78.20.Ls Magneto-optical effects
75.50.Ee Antiferromagnetics
75.50.Bb Fe and its alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.66.Bz Metals and metallic alloys

Fe/MgO/FeCo(100) epitaxial magnetic tunnel junctions prepared by using in situ plasma oxidation

S. Mitani, T. Moriyama, and K. Takanashi

J. Appl. Phys. 93, 8041 (2003); http://dx.doi.org/10.1063/1.1557338 (3 pages) | Cited 18 times

Online Publication Date: 9 May 2003

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Fe/MgO/FeCo epitaxial magnetic tunnel junctions (MTJs) were prepared on MgO(100) single crystal substrates by using in situ plasma oxidation for the formation of MgO barriers. The epitaxial relationship of Fe(001)/MgO(001)/FeCo(001) and Fe[100]//MgO[110]//FeCo[100] in the junctions was observed by reflection high-energy electron diffraction. Tunneling transport was clearly observed at low temperatures below about 150 K, and the barrier height of MgO is estimated to be 0.9 eV, which is smaller than the value expected from half of the band gap of bulk MgO. Tunnel magnetoresistance of 23% and 20% was observed at 4.2 and 77 K, respectively. The results suggest that plasma oxidation is useful for fabricating epitaxial magnetic tunnel junctions. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Pq Other materials
81.65.Mq Oxidation
52.77.Dq Plasma-based ion implantation and deposition
72.25.Mk Spin transport through interfaces

Intermixing of aluminum-magnetic transition-metal bilayers

J. D. R. Buchanan, T. P. A. Hase, B. K. Tanner, P. J. Chen, L. Gan, C. J. Powell, and W. F. Egelhoff

J. Appl. Phys. 93, 8044 (2003); http://dx.doi.org/10.1063/1.1544094 (3 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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Grazing incidence x-ray scattering has been used to study interfacial intermixing in thin films of aluminum/transition metal bilayers grown by dc magnetron sputter deposition at room temperature. As with all transition metals, the ferromagnets Fe, Co and Ni have dramatically different interface widths between X/Al and Al/X (X=Fe,Co,Ni). Intermixing lengths are larger for X on Al than for Al on X. © 2003 American Institute of Physics.
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68.35.Fx Diffusion; interface formation
78.70.Ck X-ray scattering
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Cd Deposition by sputtering

High bias voltage dependence in tunneling magnetoresistance of a ramp-type junction

Sang-Suk Lee, Young-Il Kim, Do-Guwn Hwang, Kungwon Rhie, Sun-Wook Kim, and Jang-Roh Rhee

J. Appl. Phys. 93, 8047 (2003); http://dx.doi.org/10.1063/1.1557339 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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A ramp-type tunneling magnetoresistance (TMR) junction having structure NiO(60 nm)/pinned Co(10 nm)/NiO(60 nm)/barrier Si3N4(2–6 nm)/free NiFe(10 nm) with the 15° slope was investigated. We obtained nonlinear I–V characteristics for ramp-type tunneling junctions that are distinctively different with and without an applied magnetic field. In the barrier with a Si3N4 thickness of 4 nm, the bias voltage dependence of TMR was stable up to 10 V with a negative TMR ratio of about −10%. The negative TMR is very peculiar for an asymmetric tunneling process between a wedge Co pinned layer and a free NiFe layer. © 2003 American Institute of Physics.
Show PACS
75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
75.50.Bb Fe and its alloys
75.47.Np Metals and alloys
75.50.Cc Other ferromagnetic metals and alloys

Thermal dependence of magnetotransport in nanogranular magnetic media

M. B. A. Jalil

J. Appl. Phys. 93, 8050 (2003); http://dx.doi.org/10.1063/1.1544111 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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Magnetotransport simulation is performed on granular nanomagnets (Co) in insulator, of average radius of 2.5 nm, over a temperature range (5<T<1000 K) which straddles the single-domain and superparamagnetic regimes. The M-H hysteresis is calculated based on a two-state model, which is solved analytically using the Master Equation. The two-state model is then refined to account for fluctuations into states in the vicinity of the two minima. The occupation probability of these states is determined by a birth–death chain analysis. The resulting M-H hystereses show decreasing coercivity with T. At higher T>200 K, the M-H curve approaches the Langevin function, but with a small discrepancy, due to the intrinsic anisotropy of Co. The magnetization results are then combined with a stochastic Monte Carlo transport model which combines the effects of stochastic spin-polarized tunneling, Coulomb blockade, and the magneto- and electrostatic influence of the contacts. The tunneling magnetoresistance shows a complex thermal dependence, with distinct behavior for different types of contact electrodes used. © 2003 American Institute of Physics.
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75.47.Np Metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy

Conductance anomaly and density of state effects in magnetic tunnel junctions

X. H. Xiang, T. Zhu, J. Du, and John Q. Xiao

J. Appl. Phys. 93, 8053 (2003); http://dx.doi.org/10.1063/1.1544112 (3 pages)

Online Publication Date: 9 May 2003

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We have investigated the bias dependence of conductance in magnetic tunnel junctions and found that the conductance minima can have a large shift from zero bias and the shifts are spin dependent. These anomalous behaviors have not been observed in tunnel junctions based on nonmagnetic electrodes. With a modified Brinkman’s model, by incorporating the voltage dependent density of states of the ferromagnetic electrodes, these conductance anomalous behaviors can be explained. The proposed model can also explain the bias dependence of magnetoresistance and resistance over a large bias range. All these results demonstrate that the density of states of the ferromagnetic electrodes play an important role in defining the bias dependence behaviors in magnetic tunnel junctions. © 2003 American Institute of Physics.
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72.25.Mk Spin transport through interfaces
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
73.20.At Surface states, band structure, electron density of states

Two-band model of spin-polarized tunneling incorporating discrete charging energy

X. Wang and M. B. A. Jalil

J. Appl. Phys. 93, 8056 (2003); http://dx.doi.org/10.1063/1.1555373 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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Tunneling transport across a double-junction system, consisting of a small magnetic metallic island, coupled to ferromagnetic contacts by tunnel barriers, is studied by incorporating the effects of source–drain Va and gate Vg voltages, and the island charging energy into the model Hamiltonian. The transmission coefficients and current across the double barrier are evaluated using quantum mechanical transfer matrix method. The tunneling J–Va characteristic exhibits a staircase pattern, while the tunneling current oscillates with the gate voltage. The device also exhibits a bias-dependent tunneling magnetoresistance with a peak value exceeding 35%. We attribute these behaviors to the combined effect of spin-polarized tunneling and discrete charging of the island. © 2003 American Institute of Physics.
Show PACS
72.25.Mk Spin transport through interfaces
75.47.Np Metals and alloys
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
85.75.Mm Spin polarized resonant tunnel junctions
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