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1 Sep 2005

Volume 98, Issue 5, Articles (05xxxx)

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Spin collinearity and thermal disorder in ε-Fe2O3

E. Tronc, C. Chanéac, J. P. Jolivet, and J. M. Grenèche

J. Appl. Phys. 98, 053901 (2005); http://dx.doi.org/10.1063/1.2034653 (4 pages) | Cited 10 times

Online Publication Date: 2 September 2005

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Studies of magnetoelectric ε-Fe2O3 nanoparticles (30 nm) in silica by math Mössbauer spectrometry (4.2–600 K,0–6 T) are reported. They indicate a magnetic/structural transition at ∼ 130 K, a high-temperature collinear ferrimagnetic spin structure similar to FeGaO3 and FeAlO3, and the Curie temperature TC at 585 K. Reduced hyperfine field versus T/TC is (S = 5/2) Brillouin-like or Langevin-like according to the structural site. The magnetization at 300 K is estimated to 0.25μB/Fe in accord with published data.
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75.50.Gg Ferrimagnetics
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
64.70.K- Solid-solid transitions
75.80.+q Magnetomechanical effects, magnetostriction
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.80.+y Mössbauer effect; other γ-ray spectroscopy
71.70.Jp Nuclear states and interactions

Spin filtering and spin-polarization reversal in multilayered ferromagnetic metal/semiconductor heterostructures

Yong Guo, Xiao-Wei Yu, and Yu-Xian Li

J. Appl. Phys. 98, 053902 (2005); http://dx.doi.org/10.1063/1.2030410 (7 pages) | Cited 7 times

Online Publication Date: 2 September 2005

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We investigate spin-filtering effect in multilayered ferromagnetic (F)/semiconductor (S) heterostructures within the Landauer framework of ballistic transport. Spin-dependent transmission and polarization are calculated and analyzed for different magnetizations of three ferromagnetic layers in a F/S/F/F structure proposed in this work. The results indicate that in such a multilayered configuration and when the magnetizations of the middle and the right ferromagnetic layers are antiparallel, the transmission for spin-up and spin-down electrons can be separated, which is quite different from the transport properties in the F/S/F structure, where electrons of different spin orientations have exactly the same contributions to transmission if the magnetic moments of the two ferromagnetic layers are antiparallel. It is also shown that the F/S/F/F structure can have big values of the polarization than the F/S/F structure. The quantum size effect of the length of the middle ferromagnetic layer and that of the semiconductor layer are discussed. Moreover, the polarization can be reversed when we switch the magnetizations of the middle and the right ferromagnetic layers.
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72.25.Mk Spin transport through interfaces
73.23.Ad Ballistic transport
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.30.Cr Saturation moments and magnetic susceptibilities
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Large transport eddy-current loss in quasisaturated CoFeSiB amorphous wire

D.-X. Chen and Y.-F. Li

J. Appl. Phys. 98, 053903 (2005); http://dx.doi.org/10.1063/1.2009812 (3 pages) | Cited 1 time

Online Publication Date: 7 September 2005

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The transport eddy-current loss of a nearly nonmagnetostrictive CoFeSiB amorphous wire under a large longitudinal bias field can be several times greater than the value commonly expected by the classical model. Related to the overlarge circular susceptibility under the same condition, this is attributed to the formation of disklike soft-magnetic microstructures around hard magnetic inclusions randomly embedded in a soft magnetic matrix.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Cc Other ferromagnetic metals and alloys
61.43.Dq Amorphous semiconductors, metals, and alloys

Temperature study of the spin-transfer switching speed from dc to 100 ps

T. Devolder, A. Tulapurkar, Y. Suzuki, C. Chappert, P. Crozat, and K. Yagami

J. Appl. Phys. 98, 053904 (2005); http://dx.doi.org/10.1063/1.2012512 (8 pages) | Cited 18 times

Online Publication Date: 7 September 2005

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We study the speed of the magnetization switching resulting from spin transfer in pillar-shaped CoFe/Cu/CoFe spin valves and the temperature dependence thereof. The switching speed was investigated with current pulses of durations from 100 ps to dc while the temperature was varied from 50 to 300 K. Quasistatic loops indicate that the reversal events imply transition states with reduced remanences. Their interval of occurrence shrinks gradually to almost null when the temperature is raised to 300 K. The curvature of resistance versus current hysteresis loops is different in the antiparallel and parallel branches, which evidences the influence of the Ampere field on the quasistatic micromagnetic configuration. In the dynamical regime, the pulse-induced parallel to antiparallel transition speed is not much temperature dependent from 50 to 300 K. In contrast, the pulse-induced antiparallel to parallel transition is thermally disfavored and much faster at 150 K than at 300 K. We model the experimental behavior by a competition between thermal fluctuations and the Ampere-field-related C-like bending of the magnetization in the free layer. The contribution of the Ampere field dominates in most cases. These contributions are amplified or damped together by the spin-transfer torque, but since the C-like bending is a response to a magnetic field, it sets in at a gradual pace ruled by the classical Gilbert relaxation. Most of the difference between quasistatic switching and pulse-induced switching results from this complete or incomplete alignment with the total effective field. Our demonstration of 100-ps switching validates spin-transfer switching for fast memory applications.
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75.50.Bb Fe and its alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Jk Magnetization reversal mechanisms
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields

Variation of domain-wall structures and magnetization ripple spectra in permalloy films with controlled uniaxial anisotropy

A. Gentils, J. N. Chapman, G. Xiong, and R. P. Cowburn

J. Appl. Phys. 98, 053905 (2005); http://dx.doi.org/10.1063/1.2033152 (7 pages) | Cited 5 times

Online Publication Date: 7 September 2005

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We have studied domain-wall structures and magnetization ripple in permalloy films with widely varying values of anisotropy field and thickness. Both the Fresnel and differential phase contrast imaging modes of Lorentz microscopy have been used, the latter proving particularly valuable for providing reliable quantitative data. Cross-tie walls were frequently observed but the density of the cross ties increased markedly with anisotropy field while the cross-tie length decreased. In accord with expectation, the mean ripple wavelength and magnitude of the ripple decreased with increasing anisotropy field, albeit the variation was more rapid than could be accounted for using simple theories. Generally the root-mean-square dispersion angle was found to be greater than that in earlier experimental work, a factor that was ascribed to the use of an incomplete analysis procedure in the past.
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75.50.Bb Fe and its alloys
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure

Effect of domain and grain shapes on the dynamical behavior of polycrystalline ferrites: Application to the initial permeability

Philippe Gelin, Patrick Quéffélec, and François Le Pennec

J. Appl. Phys. 98, 053906 (2005); http://dx.doi.org/10.1063/1.2039269 (6 pages) | Cited 10 times

Online Publication Date: 8 September 2005

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This paper describes a theoretical approach to calculate the permeability tensor of polycrystalline ferrites in various magnetization states, from the completely demagnetized state to saturation. Demagnetizing dynamic fields related to the magnetic domain and grain shapes are introduced in the initial equations to describe the interactions between domains in a grain and between grains. In this paper, we focus on the calculation of the initial permeability of polycrystalline ferrites. Results obtained from the model are compared in a completely demagnetized state to those given by the well-known Schloemann model and to experiments. In all the models given in the literature, an unusually high value of the damping factor must be introduced into the calculations to correctly describe the low-field loss region. Values of the damping factor comparable to those calculated from the measured resonance linewidth of ferrites permit the prediction of the spreading of the magnetic losses as a function of frequency experimentally observed at low bias fields and especially in the demagnetized state.
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75.60.Ch Domain walls and domain structure
75.50.Gg Ferrimagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets in contact with selected nitrides MN (M = Al, B, Nb, Ta, Ti, and V)

Ying Yang, Y. Austin Chang, Jianhua Yang, Chengxiang Ji, Peter F. Ladwig, Feng Liu, Bharat B. Pant, and Allan E. Schultz

J. Appl. Phys. 98, 053907 (2005); http://dx.doi.org/10.1063/1.2040002 (5 pages) | Cited 1 time

Online Publication Date: 8 September 2005

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Nitride tunnel barriers have potential applications in magnetic tunnel junctions (MTJs). Thermal stability of the interfaces between Co-, Ni-, and Fe-based ferromagnets and these nitride tunnel barriers is critical to device performance. With guidance from low-temperature ternary isothermal phase diagrams of the Co–M–N, Ni–M–N, and Fe–M–N systems (M = Al, B, Nb, Ta, Ti, and V), the interfaces in Co/MN, Ni/MN, and Fe/MN structures were evaluated in terms of two criterions: the phases in contact must (1) be in equilibrium with each other (i.e., connected by a stable tie line) and (2) have negligible mutual solubility in the phase diagram at the temperatures of interest. Of the investigated interfaces, Co/AlN, Co/BN, Co/NbN, Co/TaN, Co/TiN, Ni/BN, Ni/TaN, Fe/BN, Fe/NbN, Fe/TaN, and Fe/TiN were found to be thermodynamically stable. However, in light of some simplifications made in this analysis, the current evaluation of interfacial stability serves as a useful step in preselecting candidate nitride-based MTJ tunnel barrier materials for detailed study.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetic and transport properties of (Mn, Co)-codoped ZnO films prepared by radio-frequency magnetron cosputtering

Zheng-Bin Gu, Chang-Sheng Yuan, Ming-Hui Lu, Jing Wang, Di Wu, Shan-Tao Zhang, Shi-Ning Zhu, Yong-Yuan Zhu, and Yan-Feng Chen

J. Appl. Phys. 98, 053908 (2005); http://dx.doi.org/10.1063/1.2039279 (4 pages) | Cited 28 times

Online Publication Date: 9 September 2005

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(Mn, Co)-codoped ZnO films have been synthesized on c-sapphire (0001) by a radio-frequency magnetron sputtering system in which two targets were sputtered together. X-ray-diffraction measurements indicate that the films are highly c-axis oriented. X-ray photon spectra show that the doped Mn and Co ions in (Mn, Co) ZnO films are both in the divalent states. The films show ferromagnetic behavior with a coercivity of about 90 Oe and a saturation moment of 0.11μB/(0.3Mn2++0.7Co2+) at 300 K. In the lower temperatures between 5 and 20 K, a relatively large positive magnetoresistance over 10% was observed for (Mn0.03, Co0.07)Zn0.90O film. The number of carrier concentration is experimentally established to be 1.5613×1018 cm−3 and the mobility to be 2.815 cm2 V−1 s−1 for (Mn0.03, Co0.07)Zn0.90O film by Hall measurements at 300 K. The origins of the room-temperature magnetism and the large positive magnetoresistance are also discussed.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
81.05.Hd Other semiconductors
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.47.Pq Other materials
75.30.Cr Saturation moments and magnetic susceptibilities
73.61.Le Other inorganic semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.20.My Galvanomagnetic and other magnetotransport effects
78.66.Li Other semiconductors
78.70.-g Interactions of particles and radiation with matter
81.15.Cd Deposition by sputtering
68.55.A- Nucleation and growth

Remanent magnetization states and interactions in square arrays of 100-nm cobalt dots measured using transmission electron microscopy

T. J. Bromwich, A. Kohn, A. K. Petford-Long, T. Kasama, R. E. Dunin-Borkowski, S. B. Newcomb, and C. A. Ross

J. Appl. Phys. 98, 053909 (2005); http://dx.doi.org/10.1063/1.2011780 (8 pages) | Cited 3 times

Online Publication Date: 13 September 2005

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Large area square arrays of circular cobalt dots, nominally 100 nm in diameter and 20 nm in thickness, were patterned using interference lithography. Magnetic remanent states were measured using off-axis electron holography in the transmission electron microscope (TEM). The results show that the dots are mostly single domain, although vortex states and multidomain configurations are occasionally observed. Significant magnetic interactions between adjacent dots result in variations in their magnetization direction away from the direction of the applied field. The suitability of such dots for data storage applications is discussed. Quantitative magnetic phase measurements were also obtained by applying the transport of intensity equation to images acquired using the Fresnel mode of Lorentz microscopy in the TEM. The consistency between the electron holography and transport of intensity equation (TIE) results is assessed.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.65.Hb Quantum dots (patterned in quantum wells)
75.50.Ss Magnetic recording materials
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
75.60.Ch Domain walls and domain structure

Electron paramagnetic resonance and luminescent properties of Mn2+:MgGa2O4 phosphor

B. Yasoda, R. P. Sreekanth Chakradhar, J. Lakshmana Rao, N. O. Gopal, and C. N. Xu

J. Appl. Phys. 98, 053910 (2005); http://dx.doi.org/10.1063/1.2037210 (5 pages) | Cited 9 times

Online Publication Date: 14 September 2005

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Electron-paramagnetic-resonance (EPR) and photoluminescence (PL) studies on magnesium gallate (MgGa2O4) phosphor activated with divalent manganese have been carried out. The EPR spectrum exhibits a six-line hyperfine structure centered at g = 1.995, which is a characteristic of a Mn2+ ion with a nuclear spin I = 5/2. The spin-Hamiltonian parameters have been evaluated. The concentration and temperature (123–433 K) dependences of EPR signals have been studied. The number of spins participating in resonance (N) with temperature obeys the Boltzman law and from the graph of log N versus 1/T the activation energy has been evaluated. The paramagnetic susceptibilities (χ) have been calculated from the EPR data at different temperatures and it is interesting to note that it obeys the Curie-Weiss behavior. The Curie constant has been evaluated from the 1/χ versus T graph. A bright green photoluminescence according to the transition math→math whose emission peak is located at 502 nm is observed from the phosphor under UV excitation when excited by 267-nm light. Such a stable luminescence performance is promising for use in the field of flat panel light-emitting devices. The mechanism involved in the generation of green emission has been explained in detail.
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78.55.Hx Other solid inorganic materials
75.30.Cr Saturation moments and magnetic susceptibilities
76.30.-v Electron paramagnetic resonance and relaxation
75.10.Dg Crystal-field theory and spin Hamiltonians
75.20.Ck Nonmetals
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