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15 Nov 2006

Volume 100, Issue 10, Articles (10xxxx)

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Transformation of electrical transport from variable range hopping to hard gap resistance in Zn1−xFexO1−v magnetic semiconductor films

Y. F. Tian, Shi-shen Yan, Y. P. Zhang, H. Q. Song, G. Ji, G. L. Liu, Y. X. Chen, L. M. Mei, J. P. Liu, B. Altuncevahir, and V. Chakka

J. Appl. Phys. 100, 103901 (2006); http://dx.doi.org/10.1063/1.2386925 (6 pages) | Cited 13 times

Online Publication Date: 16 November 2006

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Transformation of the electrical transport from the Efros and Shklovskii [J. Phys. C 8, L49 (1975)] variable range hopping to the “hard gap” resistance was experimentally observed in a low temperature range as the Fe compositions in Zn1−xFexO1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ρ∝exp[TH/T+(TES/T)1/2], was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.
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75.50.Pp Magnetic semiconductors
73.61.Le Other inorganic semiconductors
72.20.Ee Mobility edges; hopping transport
75.50.Dd Nonmetallic ferromagnetic materials
71.20.Nr Semiconductor compounds
72.25.Dc Spin polarized transport in semiconductors

Magnetotransport properties of Fe3O4 epitaxial thin films: Thickness effects driven by antiphase boundaries

A. V. Ramos, J.-B. Moussy, M.-J. Guittet, A. M. Bataille, M. Gautier-Soyer, M. Viret, C. Gatel, P. Bayle-Guillemaud, and E. Snoeck

J. Appl. Phys. 100, 103902 (2006); http://dx.doi.org/10.1063/1.2386927 (8 pages) | Cited 20 times

Online Publication Date: 16 November 2006

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We present an in-depth study of the magnetotransport properties of epitaxial Fe3O4 films as a function of film thickness. The films, grown on α-Al2O3(0001) single crystals by atomic-oxygen assisted molecular beam epitaxy, exhibit high structural order and abrupt interfaces. These films contain antiphase boundaries (APBs), the density of which is strongly dependent on film thickness. A series of resistivity and magnetoresistance measurements demonstrate a systematic evolution of these properties with decreasing film thickness, revealing the impact of APBs on the transport properties in the films. We present a model based on the spin-polarized transport across an antiferromagnetically coupled APB in order to successfully reproduce our experimental data over a large range of applied magnetic fields. The comparison of this model with experimental results further clarifies the mechanism of the anomalous magnetotransport behavior in Fe3O4.
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75.50.Dd Nonmetallic ferromagnetic materials
72.25.-b Spin polarized transport
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
72.20.My Galvanomagnetic and other magnetotransport effects
75.50.Ee Antiferromagnetics

In-plane magnetotransport properties of Permalloy∕gadolinium∕Permalloy trilayers

R. Ranchal, C. Aroca, and E. López

J. Appl. Phys. 100, 103903 (2006); http://dx.doi.org/10.1063/1.2386931 (5 pages) | Cited 1 time

Online Publication Date: 17 November 2006

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In this work we report on the magnetic and electrical properties of Permalloy∕gadolinium∕Permalloy (Py∕Gd∕Py) trilayers at low temperatures. These results are complemented with a structural study and numerical calculations. The Py-Gd coupling field is derived from magnetoresistance measurements in the transverse configuration of the trilayer with a Permalloy thickness lower than its exchange correlation length. The coupling field obtained was low, likely due to the presence of low Curie temperature Gd1−xNix alloys at the Py∕Gd interfaces.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Np Metals and alloys
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Et Exchange and superexchange interactions
71.70.Gm Exchange interactions
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
68.65.Ac Multilayers
68.35.Ct Interface structure and roughness

Magnetic anisotropies and magnetization reversal of the Co2Cr0.6Fe0.4Al Heusler compound

J. Hamrle, S. Blomeier, O. Gaier, B. Hillebrands, R. Schäfer, and M. Jourdan

J. Appl. Phys. 100, 103904 (2006); http://dx.doi.org/10.1063/1.2387059 (4 pages) | Cited 11 times

Online Publication Date: 20 November 2006

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Magnetic anisotropies and magnetization reversal properties of the epitaxial Heusler compound Co2Cr0.6Fe0.4Al deposited on Fe and Cr buffer layers are studied. Both samples exhibit a growth-induced fourfold anisotropy, and magnetization reversal occurs through the formation of stripy domains or 90° domains. During rotational magnetometric scans the sample deposited on Cr exhibits about 2° sharp peaks in the angular dependence of the coercive field, which are oriented along the hard axis directions. These peaks are a consequence of the specific domain structure appearing in this particular measurement geometry. A corresponding feature in the sample deposited on Fe is not observed.
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75.30.Gw Magnetic anisotropy
75.60.Jk Magnetization reversal mechanisms
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys

Frequency dispersion of hysteresis in uniaxially anisotropic Heisenberg model: Spin-tilting resonance

X. Chen, Y. B. Guo, H. Yu, and J.-M. Liu

J. Appl. Phys. 100, 103905 (2006); http://dx.doi.org/10.1063/1.2387057 (6 pages) | Cited 3 times

Online Publication Date: 21 November 2006

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The frequency dispersion of magnetic hysteresis for uniaxially anisotropic Heisenberg model in two-dimensional square lattice is studied by Monte Carlo simulation and mean-field calculation in order to investigate the high frequency spin-tilting resonance. It is observed that besides the spin reversal resonance which usually occurs under ac magnetic field of low frequency and high amplitude, the hysteresis dispersion may exhibit a second peak which occurs in the regime of high frequency and low amplitude and depends on the uniaxial anisotropy. It is argued that the second peak originates from the spin tilting in resonance with the ac magnetic field, which generates hysteresis far from saturation and is essentially different from the spin reversal resonance peak. The high frequency power-law scaling of the hysteresis is demonstrated for both the spin reversal mode and spin-tilting mode.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.10.Jm Quantized spin models, including quantum spin frustration

Hysteresis of hexagonal arrays of magnetic nanodots

Desheng Xue and Zhongjie Yan

J. Appl. Phys. 100, 103906 (2006); http://dx.doi.org/10.1063/1.2387123 (4 pages) | Cited 2 times

Online Publication Date: 22 November 2006

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The magnetization in an infinite two-dimensional hexagonal array of small magnetic single-domain nanoparticles is studied by theoretical calculations. Various hysteresis loops and analytical expressions of coercivity and saturation field of the system with different particle sizes and array densities are obtained under the assumption of magnetization reversal mechanisms based on the competition between the dipolar interaction energy and the shape anisotropy energy with an external magnetic field perpendicular to the array plane. The hysteresis loop varies from a rectangle to a nonhysteresis straight line through a set of complicated loops composed of two types, which is in accordance with the magnetization reversal process varying from an independent coherent rotation to an “antisymmetric-ferromagnetic chain” mechanism. Our results can give a reasonable reference to the application of perpendicular magnetic recording media.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Ss Magnetic recording materials
75.60.Jk Magnetization reversal mechanisms

Local crystal structure and local electronic structure around Cr in low-temperature-grown GaCrN layers

M. Hashimoto, S. Emura, H. Tanaka, T. Honma, N. Umesaki, S. Hasegawa, and H. Asahi

J. Appl. Phys. 100, 103907 (2006); http://dx.doi.org/10.1063/1.2386930 (6 pages) | Cited 6 times

Online Publication Date: 27 November 2006

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The local crystal structure and local electronic structure of low-temperature-grown Ga1−xCrxN (0.01<x<0.092) have been studied by x-ray absorption spectroscopy. We found that the majority of Cr atoms are substitutionally incorporated into the GaN matrix, and the solubility limit of Cr in Ga1−xCrxN is around x = 0.04 for the layers grown at 600 °C. Above the solubility limit, the onset of the formation of the CrN-like local structure occurs. The effective valency of Cr in GaCrN was found to be Cr(III) from the x-ray absorption near-edge structure (XANES). The results of XANES also revealed a drastic change in the local electronic structure of Ga1−xCrxN with x between 0.04 and 0.047. This is most likely due to the transformation of the Cr–N atomic configuration from that of substitutionally incorporated GaCrN into a CrN-like local structure. The presence of p-d hybridization between d orbitals of Cr and p orbitals of N has been confirmed in its preedge structure, which could play a role in its ferromagnetism.
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75.50.Pp Magnetic semiconductors
61.66.Fn Inorganic compounds
71.20.Nr Semiconductor compounds
78.70.Dm X-ray absorption spectra
64.75.-g Phase equilibria
75.50.Dd Nonmetallic ferromagnetic materials

Magnetic and electrical transport properties of Ge1−xMnx thin films

Hongliang Li, Yihong Wu, Zaibing Guo, Ping Luo, and Shijie Wang

J. Appl. Phys. 100, 103908 (2006); http://dx.doi.org/10.1063/1.2375015 (9 pages) | Cited 16 times

Online Publication Date: 29 November 2006

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We report on preparation and systematic study of the structural, magnetic, and electrical transport properties of Ge1−xMnx (x = 25%–42%) alloy thin films grown on GaAs (001) substrates by molecular beam epitaxy. Amorphous Ge1−xMnx, and coexistence of amorphous Ge1−xMnx, Ge crystallites, and high TC secondary phases (Mn11Ge8 and Mn5Ge3) are obtained at substrate temperatures of 160 and 200–300 °C, respectively. The amorphous samples are found to consist of a low-temperature highly ordered spin-glass-like phase with an ordering temperature of TC ∼ 20 K (x ∼ 0.39), increasing with the Mn composition, and a high-temperature “clustered dopant” phase with an ordering temperature of TC* ∼ 104 K (x ∼ 0.39), increasing with both the Mn composition and applied external field. The magnetization of the low-temperature phase is found to be coupled antiferromagnetically with that of the high-temperature phase, leading to the appearance of a negative thermal remanent magnetization. Detailed magnetic and electrical measurement revealed that the low-temperature highly ordered spin-glass-like phase consists of both spin-glass-like phase and ferromagnetically ordered region. The amorphous samples also exhibit a negative magnetoresistance and an anomalous Hall effect at low temperature. The good agreement between the values of TC and TC* for amorphous samples and those of the two characteristic temperatures reported in literature for epitaxially grown samples suggests that the ferromagnetic phase of the latter observed in the temperature range of 110–120 K by some reports may not be of intrinsic origin. On the other hand, the samples grown at 300 °C are ferromagnetic up to room temperature and exhibit a positive magnetoresistance.
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75.50.Pp Magnetic semiconductors
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Ee Antiferromagnetics
75.50.Cc Other ferromagnetic metals and alloys
75.70.Ak Magnetic properties of monolayers and thin films
73.61.Jc Amorphous semiconductors; glasses

Magnetostatic waves in layered materials and devices

Pedram Khalili Amiri and Behzad Rejaei

J. Appl. Phys. 100, 103909 (2006); http://dx.doi.org/10.1063/1.2364384 (9 pages) | Cited 1 time

Online Publication Date: 29 November 2006

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Magnetostatic wave propagation in multilayers of ferro-/ferrimagnetic and nonmagnetic, dielectric slabs is investigated using an effective medium theory and the transfer matrix method. The propagation in multilayers with antiparallel directions of magnetization is analyzed, in particular. Antiparallel multilayers support (overall) bulk waves at frequencies much higher than single layers or parallel-magnetization structures. As possible applications of these multilayers, waveguides and resonators are proposed and discussed.
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75.30.Ds Spin waves
85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
84.40.Az Waveguides, transmission lines, striplines

Effects of the substitution of Al for Fe on phase transition, crystal structures, and magnetic properties of Nd3(Fe,Ti)29-type intermetallics

X. F. Liu, J. Y. Lv, Z. B. Hu, S. B. Han, D. F. Chen, Y. J. Xue, J. H. Li, R. Kiyanagi, J. S. Fieramosca, S. Short, and J. Jorgensen

J. Appl. Phys. 100, 103910 (2006); http://dx.doi.org/10.1063/1.2386945 (5 pages)

Online Publication Date: 29 November 2006

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Effects of the substitution of nontransition metal Al on phase transition, crystal structures, and magnetic properties of Nd3(Fe,Ti)29-type intermetallics have been systematically investigated by means of x-ray diffractions, time-of-flight powder neutron diffraction, and magnetic measurements. Rietveld analyses of x-ray diffraction patterns indicate that Nd3Fe27.5−xTi1.5Alx compounds mainly crystallize in Nd3(Fe,Ti)29-type structure (A2/m space group) when x ⩽ 1.5, but the main phase was replaced by Th2Zn17-type structure (Rmathm space group) when x>1.5. The lattice parameters a, b, c, and unit cell volume V of 3:29 phase in Nd3Fe27.5−xTi1.5Alx increase linearly with the substitution of Al. The site distributions of Ti and Al atoms were determined by refining the powder neutron diffraction data and it was found that Ti atoms prefer to occupy 4i1, 4i2, and 4g sites with the largest number of Fe neighbors while Al atoms prefer to take 4i4 and 8j4 sites with the largest number of rare earth neighbors. The Curie temperature of Nd3Fe27.5−xTi1.5Alx increases monotonously while the saturated magnetization decreases almost linearly with increasing Al content (x ⩽ 1.5).
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64.70.K- Solid-solid transitions
61.66.Dk Alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetization processes in hard Co-rich Co–Pt films with perpendicular anisotropy

M. Ghidini, G. Zangari, I. L. Prejbeanu, G. Pattanaik, L. D. Buda-Prejbeanu, G. Asti, C. Pernechele, and M. Solzi

J. Appl. Phys. 100, 103911 (2006); http://dx.doi.org/10.1063/1.2357869 (10 pages) | Cited 11 times

Online Publication Date: 30 November 2006

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We present a study of the magnetic properties and magnetization processes in hard Co-Pt (Pt ∼ 20 at. %) films. Co-rich Co-Pt films, with thickness t ranging from 5 nm up to 2 μm, were prepared by electrodeposition on (0001)-oriented Ru underlayers. All samples displayed strong perpendicular magnetic anisotropy and high coercivity. Virgin magnetic domain structures for varying thickness were investigated by magnetic force microscopy (MFM). The observed increase of domain width with film thickness is well understood by full two-dimensional micromagnetic computations with no adjustable parameters. The easy-axis magnetization process, as observed by measuring virgin curves by magnetometry and imaging the corresponding magnetization configurations by MFM in variable field, consists of two stages separated by a well-defined critical field, marking the onset of domain wall propagation. A thorough analysis of the out-of-plane angular dependence of the switching field points out that unpinning of domain walls is the dominant magnetization process.
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75.50.Cc Other ferromagnetic metals and alloys
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Ch Domain walls and domain structure

Transport properties of metallic La1−xSrxCoO3 (0.30 ≤ x ≤ 0.50) ferromagnet

P. Mandal, A. Hassen, and P. Choudhury

J. Appl. Phys. 100, 103912 (2006); http://dx.doi.org/10.1063/1.2363236 (5 pages) | Cited 7 times

Online Publication Date: 30 November 2006

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To elucidate the nature of transport mechanism below Curie temperature TC, we have analyzed the temperature dependence of resistivity (ρ) and thermopower (S) of metallic La1−xSrxCoO3 (0.30 ≤ x ≤ 0.50). At low temperature well below TC both ρ and S exhibit T2 behavior whereas over a temperature interval just below TC, they show a T4 dependence due to the scattering of charge carriers by magnetic fluctuations. These fluctuations can be suppressed by a magnetic field, leading to a negative bulk magnetoresistance in cobaltates and in other itinerant ferromagnets such as manganites and CrO2.
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72.20.My Galvanomagnetic and other magnetotransport effects
75.47.Pq Other materials
72.20.Pa Thermoelectric and thermomagnetic effects
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.50.Dd Nonmetallic ferromagnetic materials
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