L.V. Melo

Publications (12)15.8 Total impact

• Article: Double-barrier magnetic tunnel junctions with GeSbTe thermal barriers for improved thermally assisted magnetoresistive random access memory cells

  S. Cardoso, R. Ferreira, F. Silva, P. P. Freitas, L. V. Melo, R. C. Sousa, O. Redon, M. MacKenzie, J. N. Chapman
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  ABSTRACT: Double-barrier magnetic tunnel junction (MTJ) cells incorporating one thermal barrier (GeSbTe) were fabricated for improved thermally assisted magnetic switching. The MTJ has two Al2O3 barriers with a common weakly pinned structure (storage layer) and two pinned layers (reference). The structural quality of the double junction stack and the roughness at the (buffer/thermal barrier) level were investigated and optimized. To minimize the required heating during writing, the blocking temperature (TB) of the storage layer is reduced to 110 °C by thinning the MnIr layer to 80 Å, while a strong exchange coupling and TB ∼ 300 °C are obtained at the reference layers with a synthetic antiferromagnetically coupled CoFeB/Ru/CoFeB structure pinned to 250-Å-thick MnIr. For the write experiments, the current flowing through the MTJ (patterned down to 2 μm2) increases the temperature above the storage layer TB, under an external field of +/− 80 Oe. Current densities <1 mA/μm2 were enough to write in the MTJs with a thermal barrier (almost half the values needed without thermal barriers, which also showed a stronger dependence of the write power on the junction area). Write power values of the order of 0.3–1.8 mW/μm2 were achieved.
  Journal of Applied Physics. 04/2006; 99(8):08N901-08N901-3.
• Conference Proceeding: Magnetoresistive random access memories: integration issues for novel magnetic devices into memory cells

  H. Boeve, J. Das, G. Borghs, J. De Boeck, R.C. Sousa, L.V. Melo, P.P. Freitas
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  ABSTRACT: We describe a DRAM-like approach towards a nonvolatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. For a good performance in terms of speed and signal, the magnetic storage element itself must be considered in a semiconductor environment which defines the substrate to be used, as well as the read-out electronics for addressing the information. A global assessment for the integration of MRAM cells is addressed.
  Microelectronics, 1999. ICM '99. The Eleventh International Conference on; 12/1999
• Article: Spin valves with synthetic ferrimagnet and antiferromagnet free and pinned layers

  A. Veloso, P.P. Freitas, L.V. Melo
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  ABSTRACT: Spin valve structures with both synthetic ferrimagnet F_a/Re/F_b free layers and synthetic antiferromagnet F₁/Re/F₂/AF pinned layers are demonstrated. Physical thicknesses for the free layer of &ap;50 Å can be maintained while decreasing the effective magnetic thickness (t_eff) to 8-10 Å, without significant signal loss. Inverted structures with the synthetic free layer on top present magnetoresistance (MR) values up to 7% for t_eff~17 Å. The synthetic pinned layer gives high unidirectional exchange H_ex (~1250 Oe) and saturation fields H_s (~3-4 KOe) with no significant change after several hours anneal at 300°C. MR decreases linearly as the temperature increases ((1/MR(RT))x dMR(T)/dT&ap;-0.0017/°C)
  IEEE Transactions on Magnetics 10/1999; · 1.36 Impact Factor
• Article: Technology assessment for the implementation of magnetoresistive elements with semiconductor components in magnetic random access memory (MRAM) architectures

  H. Boeve, C. Bruynseraede, J. Das, K. Dessein, G. Borghs, J. De Boeck, R.C. Sousa, L.V. Melo, P.P. Freitas
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  ABSTRACT: We describe the DRAM-like approach towards a non-volatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. The speed at which the magnetic memory signal can be read depends on many factors. An important factor is the magnetic element itself, the size, magnetic characteristics and absolute resistance. Secondly, the design of the read-out electronics is a key issue. A third determining factor is the technology in which the electronics are fabricated. Some features are indicated that are essential in optimizing MRAM in future
  IEEE Transactions on Magnetics 10/1999; · 1.36 Impact Factor
• Article: Giant magnetoresistive sensors for rotational speed control

  P. P. Freitas, J. L. Costa, N. Almeida, L. V. Melo, F. Silva, J. Bernardo, C. Santos
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  ABSTRACT: This article demonstrates spin valve (sv) sensor applications as rotational speed control devices, in a Wheatstone bridge configuration. The sensor architecture uses two active self-biased sv elements (dimensions 400×6 μm2) in a CoZrNb flux guide. The other two spin valves are inactivated for external fields inferior to 150 Oe. Inactivation is achieved by increasing the ferromagnetic coupling of the inactive spin valves above 200 Oe, by roughening the surface of the underlying oxide layer (12 Å average roughness). When excited by a magnetized wheel with 60 N-S poles rotating at 360 rpm, the sensor yields a 400 mV pp square wave output with rise/fall times corresponding to an angular resolution of 0.17°. Amplitude is independent of speed (0–3000 rpm), and of sensor to wheel separation (0.5 ⩽ d(mm) ⩽ 2.0). Bridge output decreases 26% when temperature is increased to 150 °C. Bridge offset of 350 mV is caused by the resistance differences between the active and inactive sv elements. © 1999 American Institute of Physics.
  Journal of Applied Physics 04/1999; 85(8):5459-5461. · 2.17 Impact Factor
• Conference Proceeding: Giant magnetoresistive sensors for rotational speed control and current monitoring applications

  P.P. Freitas, F.B. Silva, L.V. Melo, J.L. Costa, N. Almeida, J. Bernardo
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  ABSTRACT: Giant magnetoresistance sensors for rotational speed control (0-3200 rpm, anti-blocking system) and for current monitoring (0-2000 A) were fabricated and tested. A bridge design is used, where the individual sensors are spin valves (sv) with dimensions 200×3 μm². The ABS sensor uses flux guides for two of the sv elements, and has the other two sv's inactivated, leading to a square wave output response to a magnetized wheel with n N-S poles (±0.2 V, rise time <30 μs at 320 rpm). The analog current monitoring system consists of 4 unshielded sv elements where 2 are inactivated. The sensor was tested for high current monitoring (0-2000 A AC, 50 Hz). It shows less than ±1.5% linearity deviation in this current range and a resolution of 35 μV rms/A for a sense current of 14 mA
  Electronics, Circuits and Systems, 1998 IEEE International Conference on; 02/1998
• Article: Novel spin-valve memory architecture

  L.V. Melo, L.M. Rodrigues, P.P. Freitas
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  ABSTRACT: Giant magnetoresistance (GMR) materials are used for random access, non-volatile memories. Different memory architectures have been proposed, both using GMR multilayers and spin-valve (SV) sandwich structures. In most of these approaches extra word lines are needed for writing purposes in addition to read contacts. This makes three interconnect levels. We show results on a simpler SV memory architecture, where writing is achieved using the read current contacts. This is relevant in terms of fabrication, as only two interconnecting layers are necessary to address a matrix of bits. A 400 bit memory matrix was fabricated and tested. A 2 mV signal between “0” and “1” states was measured for a bit inside this matrix
  IEEE Transactions on Magnetics 10/1997; · 1.36 Impact Factor
• Article: Spin‐valve sensors exchange‐biased by ultrathin TbCo films

  P. P. Freitas, J. L. Leal, L. V. Melo, N. J. Oliveira, L. Rodrigues, A. T. Sousa
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  ABSTRACT: Ultrathin TbCo films are used to exchange‐bias spin‐valve sensors as an alternative to the commonly used FeMn films. Magnetoresistance values of 7.8% and sensitivities of 2.5%/Oe are demonstrated for spin‐valve stripes where the pinned NiFe layer is exchange biased by a 120‐Å thick TbCo film. The exchange field can be fine tuned by changing the TbCo layer thickness. A well‐biased and linear spin‐valve sensor was fabricated, with a height of 2 μm and a trackwidth of 25 μm, where the free layer is biased by the sense current.
  Applied Physics Letters 08/1994; · 3.84 Impact Factor
• Article: Spin‐valve structures exchange biased with a‐Tb0.23Co0.77 layers

  P. P. Freitas, J. L. Leal, T. S. Plaskett, L. V. Melo, J. C. Soares
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  ABSTRACT: Spin‐valve structures, glass/NiFeI/Cu/NiFeII/a‐TbCo/Cu, were prepared where the pinned Permalloy layer is exchanged biased by a 200 to 400 Å thick a‐Tb 0.23 Co 0.77 layer. Exchange fields between 50 and 250 Oe were achieved with TbCo thicknesses below 400 Å, for a pinned Permalloy layer 150 Å thick. The exchange fields are strongly dependent on substrate bias. The magnetoresistance of these structures reaches 4.5% when thin Co layers are added at the NiFe/Cu interfaces.
  Journal of Applied Physics 06/1994; · 2.17 Impact Factor
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  Article: Magnetic and transport properties of sputtered Gd‐Y multilayers

  P. P. Freitas, M. From, L. V. Melo, T. S. Plaskett
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  ABSTRACT: Gd‐Y‐Gd multilayers were prepared that show a magnetoresistance enhancement for an Y layer separation of 30 Å. This magnetoresistance enhancement is an interface effect and occurs in samples where some degree of antiferromagnetic coupling is present.
  Journal of Applied Physics 05/1991; · 2.17 Impact Factor
• Article: Magnetoresistance anisotropy and magnetization reversal processes in two different kinds of amorphous ferromagnets: a-Tb20(Fe 4Co1)80 and a-U45Sb45 Mn10

  P.P. Freitas, L.V. Melo, T.S. Plaskett
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  ABSTRACT: New amorphous U-Sb-Mn ferromagnets with magnetoresistance anisotropy reaching 9% at low temperature have been recently discovered. The magnetoresistance hysteresis cycles for these systems are compared with those obtained in amorphous Tb-Fe-Co films. Results are discussed in terms of excess resistivity caused by domain walls at ± H _c and resistivity anisotropy at saturation. Both effects are important in a-Tb-Fe-Co and U-Sb-Mn. The excess resistivity at ± H _c scales with |β'|² where β' is the tangent of the Hall angle
  IEEE Transactions on Magnetics 10/1990; · 1.36 Impact Factor
• Conference Proceeding: Magnetoresistance anistrophy and magnetization reversal processes in two different kinds of amorphous random ferromagnets: a-Tb₂₀(Fe₄Co₁)₈₀ and a-U₄₅ Sb₅₅

  P.P. Freitas, L.V. Melo, A.A. Gomes, T.S. Plaskett
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  ABSTRACT: Not Available
  Magnetics Conference, 1990. 1990 Digests of INTERMAG '90. International; 05/1990