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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Universal scaling of magnetoconductance in magnetic nanocontacts (invited)

S. H. Chung, M. Muñoz, N. García, W. F. Egelhoff, and R. D. Gomez

J. Appl. Phys. 93, 7939 (2003); http://dx.doi.org/10.1063/1.1556131 (6 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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We present results of half-metallic ferromagnets formed by atomic nanocontact of CrO2–CrO2 and CrO2–Ni that show as much as 400% magnetoconductance. Analysis of the magnetoconductance versus conductance data for all materials known to exhibit so-called ballistic magnetoresistance strongly suggests that the magnetoconductance of nanocontacts follows universal scaling. If the maximum magnetoconductance is normalized to unity and the conductance is scaled to the resistivity of the material, then all data points fall into a universal curve that is independent of the contact material and the transport mechanism. The analysis was applied to all available magnetoconductance data of magnetic nanocontacts in the literature, and the results agree with theory that takes into account the spin scattering within a magnetic domain wall. © 2003 American Institute of Physics.
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75.75.-c Magnetic properties of nanostructures
73.63.Rt Nanoscale contacts
72.25.Mk Spin transport through interfaces
75.47.Np Metals and alloys
75.47.Pq Other materials
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.60.Ch Domain walls and domain structure
75.50.Cc Other ferromagnetic metals and alloys
75.50.Dd Nonmetallic ferromagnetic materials

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

S. Kämmerer, S. Heitmann, D. Meyners, D. Sudfeld, A. Thomas, A. Hütten, and G. Reiss

J. Appl. Phys. 93, 7945 (2003); http://dx.doi.org/10.1063/1.1556249 (3 pages) | Cited 32 times

Online Publication Date: 9 May 2003

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Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature result in very smooth Co2MnSi Heusler films, enabling the Co2MnSi Heusler alloys to serve as electrodes in tunnel magnetoresistance structures. © 2003 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
68.55.-a Thin film structure and morphology
75.50.Cc Other ferromagnetic metals and alloys
81.15.Cd Deposition by sputtering
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Finite-temperature spin polarization in half-metallic ferromagnets

P. A. Dowben and R. Skomski

J. Appl. Phys. 93, 7948 (2003); http://dx.doi.org/10.1063/1.1556132 (3 pages) | Cited 26 times

Online Publication Date: 9 May 2003

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The temperature dependence of the spin polarization of half-metallic ferromagnets is investigated. A unitary spinor transformation shows that the corresponding spin mixing goes beyond finite-temperature smearing of the Fermi level, leading to a nonzero density of states in the gap of the insulating spin channel. As a consequence, the resistance ratio of the two spin channels changes from infinity to some finite value and, in a strict sense, half-metallic ferromagnetism is limited to zero temperature. Bloch-type spin waves and crystal imperfections contribute to the density of states in the gap but only partly explain the pronounced changes at about 0.2 TC observed in various half-metallic magnets. In the case of NiMnSb, the spin structure depends on a nearly dispersionless transverse optical mode that occurs at about 28 meV. In terms of 3 kBT, this corresponds to 103 K—very close to the temperature at which there is a dramatic loss in the Ni and Mn magnetization in NiMnSb. Similar modes exist in other potential half-metallic systems. © 2003 American Institute of Physics.
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75.30.Ds Spin waves
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
63.20.kk Phonon interactions with other quasiparticles
71.20.-b Electron density of states and band structure of crystalline solids

Extrinsic magnetoresistance in magnetite nanoparticles

Kai Liu, L. Zhao, P. Klavins, Frank E. Osterloh, and H. Hiramatsu

J. Appl. Phys. 93, 7951 (2003); http://dx.doi.org/10.1063/1.1556133 (3 pages) | Cited 26 times

Online Publication Date: 9 May 2003

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Magnetite (Fe3O4) nanoparticles, 8 to 9 nm in size, have been synthesized using an aqueous precipitation technique. X-ray diffraction and chemical titration confirm a single cubic spinel phase with expected stoichiometry. Superparamagnetic behavior has been observed in pressed pellets of the nanoparticles above 200 K. Spin-dependent tunneling through adjacent particles has led to a negative magnetoresistance, −8.6% at 200 K and −4.5% at 300 K in a 70 kOe field. This is caused by the field-induced alignment of the nanoparticle magnetization directions. © 2003 American Institute of Physics.
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75.47.Pq Other materials
72.20.My Galvanomagnetic and other magnetotransport effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.46.-w Structure of nanoscale materials
75.20.-g Diamagnetism, paramagnetism, and superparamagnetism
61.66.Bi Elemental solids
61.66.Dk Alloys

Fe3O4 and its magnetic tunneling junctions grown by ion beam deposition

Ken-ichi Aoshima and Shan X. Wang

J. Appl. Phys. 93, 7954 (2003); http://dx.doi.org/10.1063/1.1558633 (3 pages) | Cited 25 times

Online Publication Date: 9 May 2003

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Magnetic properties of Fe3O4 and magnetic tunnel junctions with Fe3O4 bottom electrode have been investigated. Highly conductive V/Ru layers were used as an underlayer of the Fe3O4 films. The V/Ru/Fe3O4 on [110] out-of-plane oriented MgO single crystal substrate show an anisotropy and high squareness along [1math0] direction, while the Fe3O4 films with an underlayer of just Ru show isotropic behavior and low squareness. X-ray diffraction shows tensile stress on Fe3O4 for V/Ru/Fe3O4 samples. The anisotropy was shown to be induced by the stress. Finally, magnetic tunnel junction stacks of MgO/V/Ru/Fe3O4/AlO/CoFe/NiFe/Ru were deposited and the magnetic tunnel junctions with a junction size ranging from 2×2 μm2 to 9×9 μm2 were fabricated by optical lithography. The junctions show magnetoresistance ratios of ∼14% and no geometrical effect due to the junction size. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Pq Other materials
75.30.Gw Magnetic anisotropy
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Fe3O4 thin films sputter deposited from iron oxide targets

Yingguo Peng, Chandro Park, and David E. Laughlin

J. Appl. Phys. 93, 7957 (2003); http://dx.doi.org/10.1063/1.1556252 (3 pages) | Cited 21 times

Online Publication Date: 9 May 2003

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Fe3O4 thin films have been directly sputter deposited from a target consisting of a mixture of Fe3O4 and Fe2O3 onto Si and glass substrates. The magnetic properties and microstructures of the films have been characterized and correlated. The columnar growth of the Fe3O4 grains was found to be initialized from the substrate surface without any critical thickness. Substrate bias was found to be a very effective means of improving the crystal quality and magnetic properties of the thin films. The crystallographic defects revealed by high resolution transmission electron microscopy seem to be a characteristic of the films prepared by this method. © 2003 American Institute of Physics.  
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81.15.Cd Deposition by sputtering
75.70.Ak Magnetic properties of monolayers and thin films
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.A- Nucleation and growth
61.72.-y Defects and impurities in crystals; microstructure
68.37.Lp Transmission electron microscopy (TEM)
68.55.-a Thin film structure and morphology

Magnetic moment in an ultrathin magnetite film

P. Morrall, F. Schedin, S. Langridge, J. Bland, M. F. Thomas, and G. Thornton

J. Appl. Phys. 93, 7960 (2003); http://dx.doi.org/10.1063/1.1556251 (3 pages) | Cited 9 times

Online Publication Date: 9 May 2003

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We have investigated the magnetic properties of a Cu capped thin film of magnetite (Fe3O4) grown epitaxially on Pt(111). Conversion electron Mössbauer spectroscopy data show good agreement with those from bulk Fe3O4, evidencing a good degree of structural order. The data point to in-plane ferrimagnetic alignment of the magnetic moment in the Fe3O4 layer. Polarized neutron reflectivity (PNR) data determines the layer thinknesses to be 53±6 Å for the magnetite film and 106±5 Å for the Cu capping layer. The average magnetic moment determined by PNR for the Fe3O4 layer is 2.8±0.3 μB, smaller than the value of 4.1 μB for bulk Fe3O4. It is suggested that the reduced moment is in part a result of a reduced ordering temperature in the ultrathin film. © 2003 American Institute of Physics.
Show PACS
75.70.Ak Magnetic properties of monolayers and thin films
76.80.+y Mössbauer effect; other γ-ray spectroscopy
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Gg Ferrimagnetics
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