B. Viala

Publications (17)18.52 Total impact

• Article: New RF Magnetic Stripe Inductors With Flanges Based on Exchange-Coupled Magnetic Films

  J.P. Michel, Y. Lamy, A.S. Royet, B. Viala
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  ABSTRACT: This work describes magnetically confined inductors using coplanar waveguide (CPW) lines. The fabrication uses antiferromagnetic exchange coupled iron-cobalt films with high ferromagnetic resonance frequency (5 to 7 GHz). A first type of inductors uses sandwiched lines and exhibits a linear density of inductance of 0.9 nHmiddotmm^-1. A second type includes magnetic flanges and indicates 1.15 nHmiddotmm ^-1. The influence of the flanges contributes to 30% increase of the intrinsic coupling with the line. The measured inductance (L) and quality factor (Q) perfectly agree with theoretical permeability considerations. With meanders, we show L of ~3 nH which competes standard spiral inductors. We also demonstrate that Q only depends on the intrinsic values of the reference line (Q₀) and the magnetic material (Q_mat). Therefore, Q-optimization is possible at high frequency. We indicate a typical maximum of ~10 at 5 GHz
  IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
• Article: Fast Analysis of Proximity Effects in Integrated Inductors With High-Permeability Magnetic Material

  B. Orlando, A.-S. Royet, B. Viala
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  ABSTRACT: We used a simple method based on fast magnetostatic computations in order to analyze proximity effects in integrated inductors with high-permeability magnetic material. This method was applied to spiral micro-inductors and showed good agreement with experiment. These results enlighten the importance of the proximity effect contribution on the total losses of integrated magnetic devices and demonstrated the interest of this simple method for quality factor optimization
  IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
• Article: Low-Resistance Integrated Toroidal Inductor for Power Management

  B. Orlando, R. Hida, R. Cuchet, M. Audoin, B. Viala, D. Pellissier-Tanon, X. Gagnard, P. Ancey
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  ABSTRACT: 5.6 mm times 5.6 mm integrated toroidal inductors with reduced thickness down to 200 mum have been realized. A high inductance-to-dc resistance ratio have been achieved using a toroidal geometry. The realization features thick Cu winding and a thick laminated Ni₈₀Fe₂₀ core with crossed anisotropy. Impedance measurements have been performed up to 100 MHz with dc bias current applied. As a typical result, we show an inductor with an inductance of 500 nH up to 10 MHz and a dc resistance of 95 mOmega. To the best of our knowledge, this is the highest inductance-to-dc-resistance ratio demonstrated by a fully integrated inductor. This demonstrates the interest of such integrated inductors to replace discrete components in compact low-power modules
  IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
• Article: NiMn, IrMn, and NiO Exchange Coupled CoFe Multilayers for Microwave Applications

  Y. Lamy, B. Viala
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  ABSTRACT: We compare the static and microwave properties of AF/CoFe/AF multilayers with NiMn, IrMn, and NiO as AF-layers, respectively. These exchange-coupled multilayers combine very high saturation magnetization (4piM_s) with soft rotational behaviors. NiMn has the best exchange coupling and exhibits very high f_FMR (5-10 GHz) and good thermal stability up to 150degC. However, its resistivity is rather low (175 muOmegamiddotcm), and requires very large thickness to be efficient leading to low filling ratio. IrMn exhibits an intermediate exchange coupling leading to f_FMR in the range of 3-6 GHz. It is a good compromise regarding resisitivity (275 muOmegamiddotcm), and the reduced thickness for mu_r~100 at such frequencies. The high resistive NiO exhibits permeability spectra with moderate f_FMR around 3-5 GHz but allows the possibility to stop the eddy currents by lamination. However, NiO and IrMn both exhibit linear decrease of their FMR versus temperature. This work shows that NiMn, IrMn, and NiO multilayers provide suitable magnetic properties for microwave applications, and exhibit complementary properties on selected bandwidths which allow to address the 3-10 GHz range
  IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
• Conference Proceeding: Fast analysis of proximity effects in integrated inductors with high permeability magnetic material

  B. Orlando, A. Royet, B. Viala
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  ABSTRACT: We used a simple method based on fast magnetostatic computations in order to analyse proximity effects in integrated devices with high permeability magnetic material. This method was applied to various micro-inductors and showed good agreement with experiment. These results enlighten the importance of the proximity effect contribution on the total losses of integrated magnetic devices and demonstrated the interest of this simple method for quality factor optimization.
  Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
• Conference Proceeding: New RF Magnetic Stripe Inductor with Flanges based on Exchange-Coupled Magnetic Films

  J. Michel, Y. Lamy, A. Royet, B. Viala
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  ABSTRACT: A new type of RF-inductor on silicon is shown based on coplanar copper lines covered with a magnetic film. The realization features a new topology where the stripe is fully encapsulated with the film including flanges. It is also compared to three others configurations: top, bottom and sandwich. The realization is based on antiferromagnetic exchange-coupled CoFe films having a high natural ferromagnetic resonance frequency. As a best result, the closed inductor exhibit the highest linear density of inductance with 1.4 nH.mm^-1 and a quality factor of ~6 at 2.2 GHz. Measurements are compared with theoretical and simulation results, and also with more conventional inductors. We show for the first time a simple reversing differential method allowing full permeability measurements up to 20 GHz.
  Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
• Conference Proceeding: Low Resistance Integrated Toroidal Inductors for Power Management

  B. Orlando, R. Hida, R. Cuchet, M. Audoin, B. Viala, D. Pellissier, X. Gagnard, P. Ancey
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  ABSTRACT: 5.6times5.6times0.5 mm³ integrated toroidal inductors with high L/R_DC ratio have been fabricated on Si substrates with microelectronics tools. The realization features thick layer Cu and Ni₈₀Fe₂₀ technologies. Impedance measurements have been performed up to 100 MHz. As a typical result, we show an inductor with L ~ 500 nH up to 10 MHz, R_DC ~ 110 mOmega. To our knowledge, it is the best compromise shown so far. This demonstrates the interest of such integrated inductors to replace discrete ferrite inductors for compact power modules for mobile communication circuits.
  Magnetics Conference, 2006. INTERMAG 2006. IEEE International; 06/2006
• Article: Microfluxgate performance improvement in microtechnology

  H. Joisten, B. Guilhamat, M. Audoin, J.-M. Leger, R. Cuchet, G. Barrois, J.-B. Albertini, P. Gaud, P. Renaux, D. Bloch, B. Viala
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  ABSTRACT: Microfluxgate sensors have been developed to measure very low magnetic fields, of about some nanotesla. This paper describes a recent integrated solution coupled with the basic microfluxgate component. The objective is to decrease the noise level of the sensor down to a few nanotesla per square root of hertz. The technological solution consists of adding coils or single layers, thanks to which the required ac transverse magnetic field can be applied. A large enhancement of the sensor sensitivity is put in evidence on output signal figures with and without stabilization signal. The specifications of the sensor connected with its electronics are also presented.
  IEEE Transactions on Magnetics 12/2005; · 1.36 Impact Factor
• Article: Bidirectional ferromagnetic spiral inductors using single deposition

  B. Viala, S. Couderc, A.S. Royet, P. Ancey, G. Bouche
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  ABSTRACT: Size reduction for spiral inductors is investigated. We reveal bidirectional ferromagnetic inductors with an ∼30% to 135% increase in L over the air-core value. This is advantageously realized by using single deposition. The realization uses above-IC techniques and high-magnetization FeHfN laminated films. We also report a quality factor of ∼10 at 1.5 GHz, which is mitigated while realizing high FMR frequency (up to 6 GHz). This leads to considerations on nonmagnetic excess losses. Finally, a surface reduction of 25% to 50% for conventional spirals is demonstrated.
  IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
• Article: Investigation of anomalous losses in thick Cu ferromagnetic spiral inductors

  B. Viala, A.S. Royet, S. Couderc
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  ABSTRACT: Ferromagnetic spiral inductors were fabricated using thick copper technology. 30% increase in L is shown over the air core value. However, we observe a large restriction in Q when frequency is beyond 1 GHz. This strongly contrasts with the high ferromagnetic resonance (FMR) frequency measured at 6 GHz. Also still increasing the FMR up to 10 GHz using multipatterning do not improve Q. The highlight is on excess losses not originating from magnetic losses (μ'') and the Joule effect. Additionally, when comparing multiturn inductor to single strip inductor, proximity effect or current crowding is shown responsible.
  IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
• Article: Temperature dependence of magnetic properties of AF-biased CoFe films with high FMR

  Y. Lamy, B. Viala, I.L. Prejbeanu
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  ABSTRACT: We investigate the temperature dependence of static and dynamic magnetic properties of AF/F/AF multilayers consisting in NiMn as AF-layer and Co₉₀Fe₁₀, Co₅₀Fe₅₀ and Co₃₅Fe₆₅ as F-layer, respectively. The T-dependences of the hysteresis loops have been analyzed in relation with the corresponding microwave permeability spectra. The results indicate a lowering of both the exchange bias (Hex) and the anisotropy field (Hk dc) above 150°C, in agreement with blocking temperature considerations from literature. However, microwave characterizations show better high temperature stability, with a shift of ferromagnetic resonance frequency less than 20% up to 300°C. This reveals a strong discrepancy between the anisotropy field from the static and dynamic measurements including the T-dependence. Finally, this work points out unexpected excellent temperature stability up to 300°C proving that such multilayers are available for future use in RF devices at very high frequencies.
  IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
• Article: FeHfN and FeHfNO soft magnetic films for RF applications

  S. Couderc, B. Viala, P. Ancey, G. Bouche, R. Pantel
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  ABSTRACT: Downsizing RF spiral inductors is thought possible with using ferromagnetic material such as high magnetization FeHfN . However, increasing the natural resistivity of FeHfN would be suitable for a better confinement with cancelling the series capacitance with the inductor. Indeed, the magnetic material can be almost in contact to the metal traces, therefore increasing confinement. In this paper, we first detail optimization of FeHfN films combining microstructural investigations and magnetic characterizations. Then, we reveal the effect O incorporation in FeHfN films. Both processes: nitrogenation and oxydation are rather antagonists but combine advantageously. FeHfNO film consists in nanoscale grains scattered in an oxidized matrix. However, it is still difficult to retain saturation magnetization 4πMs above 10 kG with resistivity higher than 10³μΩ cm. Finally, the dual reactive process is promising but requires further investigations.
  IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
• Article: Combination of ultimate magnetization and ultrahigh uniaxial anisotropy in CoFe exchange-coupled multilayers

  Y. Lamy, B. Viala
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  ABSTRACT: CoFe exchanged-coupled multilayers are studied versus the thickness of the ferromagnetic (F) layer (e_F) and the thickness of the antiferromagnetic (AF) layer (e_AF) . The F layer consists in Co ₉₀ Fe ₁₀ , Co ₅₀ Fe ₅₀ , and Co ₃₅ Fe ₆₅ , respectively. The AF layer is Ni ₅₀ Mn ₅₀ . Very high saturation magnetization (4πM_s) are reported with 18.5, 23, and 24 kG , respectively, combined with excellent soft rotational behaviors. The AF–F–AF multilayers cumulate top and bottom interfacial exchange coupling and show very high effective energy densities (J_ex) of 1.01, 0.81, and 0.98 erg cm ^-2 , respectively. J_ex increases with e_AF up to a maximum for e_AF≥500 Å . This leads to a strong bias field (H_ex) with a classical 1/e_F dependence, which translates into a large uniaxial anisotropy (H_k) at 90° from the pinning direction. Typically, for e_F=150 Å , H_k is 741, 650, and 642 Oe , respectively. However, this translation is rather complex and e_F and 1/e_AF dependences of the ratio H_k/H_ex suggest a contribution of an intrinsic CoFe anisotropy. For all CoFe, it is shown that a ferromagnetic resonance frequency (f_FMR) of 10 GHz is achievable. Co ₃₅ Fe ₆₅ mulilayers exhibit the highest e_F×μ^′ product, μ^′ being very close to the theoretical ultimate permeability. Finally, the soft rotational behavior is shown unchanged over an angular range of ±30° from the pinning direction which is very unusual.
  Journal of Applied Physics 06/2005; · 2.17 Impact Factor
• Article: AF-biased CoFe multilayer films with FMR frequency at 5 GHz and beyond

  B. Viala, G. Visentin, P. Gaud
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  ABSTRACT: Antiferromagnetic/ferromagnetic/antiferromagnetic (AF/F/AF) multilayers are investigated. The AF layer is 500-Å-thick Ni₅₀Mn₅₀. Films are studied as a function of the thickness of the ferromagnetic layer (e_F). Films with Ni₈₀Fe₂₀, Co₅₀Fe₅₀, and Co₉₀Fe₁₀ exhibit interfacial exchange coupling energy densities (J_ex) of 0.22, 0.82, and 0.97 erg.cm^-2 respectively. High J_ex ensures strong exchange coupling (H_ex) and large uniaxial anisotropy fields (H_ua) at 90° from the pinning direction. This promotes soft magnetism in CoFe films while they are not soft naturally. With e_F=150 Å,H_ua is of 570 and 755 Oe for Co₅₀Fe₅₀ and Co₉₀Fe₁₀, respectively. Thicknesses of ∼0.3 μm are also shown possible for practical use by means of multiple alternations (n ≥ 20). Microwave characterizations confirm coherent reversible magnetization processes. Co₅₀Fe₅₀ multilayers are the best candidates with the highest μ' and the lowest μ'' at high frequency because an exceptional combination of a high 4πM_s of 21 kG and a small damping factor of 0.015. ferromagnetic resonance frequencies of ∼5 GHz are reported, and values of ∼10 GHz are shown realistic, for the first time.
  IEEE Transactions on Magnetics 08/2004; · 1.36 Impact Factor
• Article: RF planar ferromagnetic inductors on silicon

  B. Viala, A.S. Royet, R. Cuchet, M. Aid, P. Gaud, O. Valls, M. Ledieu, O. Acher
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  ABSTRACT: Two-port coplanar ferromagnetic inductors using solid magnetic planes were fabricated on silicon. L, R, and Q measurements are presented. Cutoff frequencies exceed 10 GHz, indicating no capacitance limitation. The increase in L over the air-core value is ∼15%, and the quality factor Q is of ∼10 up to 1.5 GHz. The inductance scales with HA-alignment and spiral excitation ratio. Agreement with High-Frequency Structure Simulator simulations including ferromagnetic resonance led to a better understanding in terms of L and Q. Current increase in L is limited by the film thickness (0.2 μm). Thicker films (0.5-1 μm) would lead to larger gain over L (23% to 31%) with similar Q (∼10). The restriction in Q from the air-core inductor consists in magnetic losses but not only. Another contribution would come from anomalous ohmic losses from the spiral itself enhanced by the proximity of the magnetic planes.
  IEEE Transactions on Magnetics 08/2004; · 1.36 Impact Factor
• Article: GMR head for helical-scan recording with a 5000-h head life

  K. Hallamasek, M. Boots, F. Souchon, R. Hida, M. Panabiere, J.-B. Albertini, B. Viala
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  ABSTRACT: We describe a giant magnetoresistance (GMR) head for helical-scan recording that overcomes the problems of wear, corrosion, and thermal spikes. A flux guide GMR head is built on a very thin silicon structure that is protected by low-wear bearing materials. We describe both the design of the flux guide microstructure that provides a high-bandwidth path for the flux to the GMR spin-valve sensor and the investigation of bearing materials that give low wear rate with acceptable pole tip recession. Avoiding large closure domains gives the flux guide a permeability greater than 1000 for a bandwidth better than 100 MHz. A bearing structure of Al₂O₃SiC gives a head life in excess of 5000 h. Tests with a 19-mm data recorder using conventional dual-layer metal particle tape suggest that this GMR head will support a threefold increase over current recording densities and still allow the reproduction of previous generation recordings.
  IEEE Transactions on Magnetics 10/2003; · 1.36 Impact Factor
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  Available from: arxiv.org

  Article: The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

  A Masseboeuf, C Gatel, P Bayle-Guillemaud, Y Lamy, B Viala
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  ABSTRACT: Lorentz Transmission Electron Microscopy (LTEM) combined with in situ magnetizing experiments is a powerful tool for the investigation of the magnetization reversal process at the micron scale. We have implemented this tool on a conventional Transmission Electron Microscope (TEM) to study the exchange anisotropy of a polycrystalline Co 35 Fe 65 /NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the Differential Phase Contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.