B. Negulescu

Giant spin-dependent thermoelectric effect in magnetic tunnel junctions

Thermoelectric effects in magnetic nanostructures and the so-called spin caloritronics are attracting much interest. Indeed it provides a new way to control and manipulate spin currents, which are key elements of spin-based electronics. Here we report on a giant magnetothermoelectric effect in a magnetic tunnel junction. The thermovoltage in this geometry can reach 1 mV. Moreover a magnetothermovoltage effect could be measured with ratio similar to the tunnel magnetoresistance ratio. The Seebeck coefficient can then be tuned by changing the relative magnetization orientation of the two magnetic layers in the tunnel junction. Therefore, our experiments extend the range of spintronic devices application to thermoelectricity and provide a crucial piece of information for understanding the physics of thermal spin transport.

Wide range and tunable linear magnetic tunnel junction sensor using two exchange pinned electrodes

A magnetic tunnel junction sensor with both the detection and the reference layers pinned by IrMn is fabricated. Using the differences in the blocking temperatures of the IrMn films with different thicknesses, crossed anisotropies can be induced between the detection and the reference electrodes. The pinning of the sensing electrode ensures a linear and reversible output. It also allows tuning both the sensitivity and the linear range of the sensor. The sensitivity varies linearly with the ferromagnetic thickness of the detection electrode. It is demonstrated that an increased thickness leads to a rise of sensitivity and a reduction of the operating range.

On the control of spin flop in synthetic antiferromagnetic films

The paper presents a systematic study of anneal induced anisotropies in a CoFe/Ru/CoFe synthetic antiferromagnet (SAF) exchange coupled with an IrMn film. When the annealing is done with the SAF in a spin flop state, the magnetic layers can be pinned perpendicular to the annealing field direction. The main parameters controlling this process are identified and analyzed: the value and the direction of the annealing field along with the Ruderman-Kittel-Kasuya-Yosida coupling energy between the two ferromagnetic layers. The induced anisotropy is predicted within a theoretical model taking into account the thermal variation of the coupling constants. Finally, the spin flop annealing is used to orthogonally pin the reference and the detection electrodes in an IrMn/CoFe/Ru/CoFe/Cu/CoFe/IrMn spin valve structure. The magnetoresistance variation in this structure is analyzed as a function of the pinning direction of the SAF acquired during the annealing in the spin flop state. A very good agreement is observed be...

Tuning structure and roughness in exchange biased NiO/permalloy bilayers

Polycrystalline NiO thin films have been grown by pulsed laser deposition on quartz substrates. These films exhibit a strong texture, which can be tuned by changing deposition parameters such as substrate temperature or oxygen partial pressure. By varying the deposition temperature from room temperature up to 900 °C, (220), (111), and (200) textured films are prepared. In the temperature zones separating these orientations, competition between different growth directions leads to smaller crystallites, characterized by broader diffraction lines. Surface roughness measured by atomic force microscopy is strongly correlated with these structural features. Roughness is minimum for highly textured samples (about 7A for 500 A thick films), and it exhibits two peaks in the intermediate zones, with maximum values of about 40 A. In order to correlate exchange bias with these structural features, 100 A thick FeNi layers were deposited by rf sputtering on top of the 500 A thick NiO films. Hysteresis loops were measur...

Interfacial electronic transport phenomena in single crystalline Fe-MgO-Fe thin barrier junctions

Spin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3-8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Delta(1)-like and Delta(5)-like symmetries.

Metastable antiferromagnetic domain configurations in exchange biased bilayers

We report on exchange bias relaxation in NiO/FeNi bilayers. FeNi films have been sputtered on top of NiO films, previously grown on quartz substrates using pulsed laser deposition. As-prepared bilayers show enhanced coercivity but no bias. Their hysteresis loops are essentially isotropic within the sample plane. Exchange bias relaxation is observed after the samples have been magnetized once, and let in their remanent state: exchange bias progressively builds up in zero external magnetic field, over a few days timescale. This behavior may be related to the reorientation of antiferromagnetic domains triggered by the exchange coupling with the ferromagnetic domains throughout the interface. Relaxed samples are exchange biased along the magnetization direction, which becomes an easy axis. Along the perpendicular direction, a two-step magnetization reversal is observed. This two-step process is erased when the samples are field-annealed at elevated temperatures, and it is replaced by the rotation process usua...

Fe/MgO/Fe (100) textured tunnel junctions exhibiting spin polarization features of single crystal junctions

Crystallographic and spin polarized transport properties of (100) textured and (100) epitaxial Fe/MgO/Fe magnetic tunnel junctions are compared. Strong similarities in the transport properties show that structural coherence and magnetic quality at the 25 nm grain scale in textured junctions are sufficient to issue signatures of the spin polarized transport specific to a single crystal junction. This demonstrates that the lateral coherence of the Bloch tunneling wave function is identically limited in both systems. Our analysis leads to model the textured tunnel junction as a juxtaposition of nanometer sized single crystal junctions, placed in parallel.