Bernard Dieny

Exchange-biased spin valves with perpendicular magnetic anisotropy based on (Co/Pt) multilayers

We have prepared spin valves exhibiting perpendicular magnetic anisotropy [perpendicular spin valves (PSVs)] by sputtering. These PSVs associate a “free” (Co/Pt) multilayer with a “pinned” (Co/Pt)/FeMn multilayer separated by various spacer materials (Pt, Cu, Al2O3). We carried out a comprehensive study of the magnetic and magnetotransport properties of the biased multilayers and of the complete spin valves. When the number of repeats in the (Co/Pt) exchange-biased multilayer is larger than 3, the samples present 100% remnant magnetization in the perpendicular configuration. The major hysteresis cycles exhibit two well-separated loops associated with the free and the exchange-biased (Pt/Co) multilayers. When optimized, the exchange-bias field can be larger than the coercivity of the pinned layer. Metallic PSVs with Cu spacers exhibit giant magnetoresistance but the amplitude is only of the order of 1% due to significant current shunting. In contrast, perpendicularly magnetized tunnel junctions are very pr...

Exchange bias in (Pt/Co0.9Fe0.1)n/FeMn multilayers with perpendicular magnetic anisotropy

We report the observation of exchange bias in (Pt/Co0.90Fe0.10)n/FeMn multilayers, with perpendicular magnetic anisotropy. We analyze the behavior of the exchange-bias field Heb as a function of temperature, FeMn thickness, number of (Pt/Co0.90Fe0.10) bilayers, and annealing conditions. Measurements carried out with field applied perpendicular to the plane of the samples indicate that the hysteresis loops present 100% of remanent magnetization, with Heb reaching 200 Oe and a tunable coercive field, depending on Co and Pt thickness ratio and on Pt buffer thickness. Furthermore, spin-valves of the form (Pt/Co0.90Fe0.10)n/NM/(Co0.90Fe0.10/Pt)m/Co0.90Fe0.10/FeMn with NM=Cu or Pt have been prepared. They exhibit two well separated hysteresis loops when the field is applied perpendicular to the plane.

Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation

Pt/Co/M sandwiches (M=Al, Ta, Cr) were prepared by sputtering. The M capping layer was oxidized either by natural oxidation in air or by exposure to an oxygen plasma. By measuring the extraordinary Hall effect in these systems, we observed that the magnetic anisotropy of the Co electrode is extremely sensitive to the degree of oxidation of the capping layer. In particular, when M=Al, the magnetic anisotropy is found in plane when the AlOx layer is under or overoxidized whereas it is out of plane when this oxide layer is stochiometric alumina. The conditions of oxidation which give the perpendicular anisotropy in Pt/Co/AlOx are found to be the same than those which lead to a maximum of specular reflection at Co/AlOx interface in Co/Ru/Co/Cu/Co/AlOx synthetic spin-valves. This crossover of anisotropy therefore provides a very useful way for controlling the oxidation of tunnel barriers. Furthermore, this phenomenon can be used to study the aging of naturally oxidized tunnel junctions exposed to air, and it i...

Spin transfer effect in spin-valve pillars for current-perpendicular-to-plane magnetoresistive heads (invited)

Large spin-transfer effects were observed in spin-valve pillars developed for current-perpendicular-to-plane (CPP) magnetoresistive heads. The investigated samples are complex stacks including laminated free, synthetic/laminated pinned, and nano-oxide layers. The magnetoresistive (MR) transfer curves exhibit current-dependent loop shift and in some particular ranges of current and field, very large noise associated either to stochastic jump of the free layer magnetization or to excitations of steady precessional modes. A very unusual phenomenon of inverted coercivity was observed in the low current range. The spin-transfer effects show an asymmetry as a function of current direction. These experimental results indicate that the spin-transfer effects must be considered in the design of CPP MR heads.

Current driven resistance changes in low resistance x area magnetic tunnel junctions with ultra-thin Al-Ox barriers

Current induced resistance changes were investigated in magnetic tunnel junctions with ultrathin Al-Ox barriers. The nonuniformity of the insulator induced a strong coupling between the two magnetic electrodes and no magnetoresistance. However, the current-voltage (I–V) characteristics at low bias voltages were consistent with a tunnellike behavior. At larger bias voltages, they showed an abrupt change of slope that was reversible for an opposite voltage polarity. The resistance versus current (R–I) curves exhibited reversible resistance changes that reached over 100%. We interpret this as controlled electromigration in local nanoconstrictions of the barrier.

Influence of deposition angle on the properties of NiO spin-valves

Bottom and top spin-valves comprising NiO as pinning layer were prepared by depositing the NiO layer either at normal or oblique incidence onto Si/SiO2 substrates. When the NiO layer is deposited at oblique incidence, a strong uniaxial anisotropy is observed in the hysteresis loop of the Co layer which is exchange coupled to this layer. The easy axis is perpendicular to the plane of incidence. In contrast, when the NiO layer is deposited at normal incidence, the hysteresis loops of the exchange coupled Co layer show an in-plane isotropy. The uniaxial anisotropy observed after oblique incidence deposition is ascribed to growth-induced uniaxial strain and magnetoelastic effects in the NiO antiferromagnetic layer. It is observed that after annealing under a magnetic field, Co/NiO bilayers in which the NiO has been deposited at oblique incidence exhibit symmetric hysteresis loops whereas similar bilayers in which the NiO has been deposited at normal incidence exhibit shifted hysteresis loops. This effect is r...

Spin-torque nano-oscillator based on a synthetic antiferromagnet free layer and perpendicular to plane polarizer

Coupled free layers systems used as active elements in spin-transfer nano-oscillators (STNOs) can have improved microwave performances in terms of linewidth or tunability. Here, we report a numerical study on the magnetization dynamics of a synthetic antiferromagnet (SAF) driven by a perpendicularly spin-polarized current and an in-plane applied magnetic field. Compared to the single free layer STNO, the current-field state diagrams, derived for strong and weak RKKY exchange coupling strength inside the SAF, show a more complex structure, with new static or dynamic states (chaotic dynamics), and also a larger zone of out-of-plane precession state (OPP) oscillations. The OPP frequency behaviour is generally similar to that of the single free layer STNO except for the zone near the chaotic dynamics and for fields larger than the spin-flop field of the SAF, where several frequency jumps have been observed.

Study of spin-transfer-induced dynamics in spin-valves for current-perpendicular-to-plane magnetoresistive heads

We have investigated spin-transfer effects in complex spin-valve pillars developed for current-perpendicular-to-plane (CPP) magnetoresistive heads. The structure of the samples included an exchange-biased synthetic pinned layer; both the reference and the free layer were laminated by inserting ultrathin Cu layers. Despite the low thickness of the polarizer, our results show that the free layer can be switched between the parallel (P) and the antiparallel (AP) states by applying current densities of the order of 107 A cm−2. Due to the use of exchange-biased structures, the stability phase diagrams could be obtained in the four quadrants of the field (H)–current (I) plane, for static as well as for dynamic experiments. Microwave measurements demonstrate the existence of several spin-transfer induced precession regimes, as well as the non-equivalence of the two bias polarities, given the influence of the non-homogeneous fields in the system.