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Thermoelectric effects in magnetic nanostructures and the so-called spin caloritronics are attracting much interests. 1−11,27,28 Indeed it provides a new way to control and manipulate spin currents which are key elements of spin-based electronics.12,13 Here we report on the giant magnetothermoelectric effect in a magnetic tunnel junction. The thermovoltage in this geometry can reach 1 mV. Moreover a magneto-thermovoltage 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.Thermoelectricity has been known since 1821 with T.J. Seebeck. On one hand, the relation between the thermal and the electrical transport is an essential topic for both fundamental physics and for the future of energy-saving technologies.14,15 On the other hand the discovery of the giant magnetoresistance effect (GMR) and the tunnel magnetoresistance effect (TMR) enhanced the interest of the community for spin-dependent conductivity and gave rise to spintronics and multiple applications.12,13 Its interplay with thermal conductivity was introduced to describe the conventional Seebeck effect in ferromagnetic metals. 1−9,16−20 . The magnetothermoelectric effect has then be studied in magnetic systems such as magnetic multilayers and spin valves. 16−20 Moreover the thermoelectric effect has also been observed in non magnetic tunneling devices such as superconductorinsulator-normal metal (or superconductor) tunnel junctions. 21,22 Recently, thermal spin tunnelling effect from ferromagnet to silicon has been reported. Fig. 1a. To generate a temperature difference between the reference layer and the free layer, one electrode lead was heated using the laser beam from a laser diode with a wavelength of 780 nm and a tunable power from 0 to 125 mW. The temperature difference between both sides of the Al 2 O 3 barrier is defined as ∆T whereas the voltage difference is ∆V. In the linear response approximation, the total electric current I in the presence of ∆V and ∆T can be written as 7,16 (1) where G V is the electrical conductance, and G T is the thermoelectric coefficient related to the charge current response to the heat flux.The thermovoltage ∆V can be measured in an opencircuit geometry where I = 0, as shown in Fig. 1b. Considering equation (1) it leads to ∆V = − (G T /G V ) ∆T = − S ∆T, where S = G T /G V is the thermopower (TP) or Seebeck coefficient. ∆V was measured with a nanovoltmeter at room temperature (RT) with a magnetic field H applied along the in-plane easy axis of the free layer. The thermotunnel current was measured by a source-meter connecting the MTJ without any applied voltage, i.e. a closed-circuit, as shown in Fig. 1c. In the closed-circuit geome...

A brief theoretical review points out the specific aspects of electronic transport in single-crystal magnetic tunnel junctions employing bcc(100) Fe electrodes and a MgO(100) insulating barrier. The theoretical predictions are compared to the experimental reality in both equilibrium and out-of-equilibrium regimes. For extremely small MgO thickness, we illustrate that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions. Artificial antiferromagnetic systems based on coupling by spin polarized tunnelling have been elaborated and studied. In the out-of-equilibrium regime and for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we explain the experimental giant tunnel magnetoresistive effects at room temperature, up to 180%, measured in our simple or double barrier tunnel junction systems. Moreover, we illustrate that the magneto-transport properties of the junctions may be skilfully engineered by adjusting the interfacial chemical and electronic structure.

Equipe 101 : Nanomagnétisme et électronique de spinInternational audienceSingle-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJV(-1) m(-1) anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage

The prediction of ultra-low magnetic damping in Co 2 MnZ Heusler half-metal thin-film magnets is explored in this study and the damping response is shown to be linked to the underlying electronic properties. By substituting the Z elements in high crystalline quality films (Co 2 MnZ with Z=Si, Ge, Sn, Al, Ga, Sb), electronic properties such as the minority spin band gap, Fermi energy position in the gap and spin polarization can be tuned and the consequence on magnetization dynamics analyzed. The experimental results allow us to directly explore the interplay of spin polarization, spin gap, Fermi energy position and the magnetic damping obtained in these films, together with ab initio calculation predictions. The ultra-low magnetic damping coefficients measured in the range 4.1 10 -4 -9 10 -4 for Co 2 MnSi, Ge, Sn, Sb are the lowest values obtained on a conductive layer and offers a clear experimental demonstration of theoretical predictions on Half-Metal Magnetic Heusler compounds and a pathway for future materials design.

Picosecond laser ultrasonic techniques for acoustic wave generation and detection have been employed to probe shear acoustic waves in liquid glycerol at gigahertz frequencies. The experimental approach uses a unique laser pulse shaping technique and a crystallographically canted metal layer to generate frequency-tunable transverse acoustic waves, and uses time-domain coherent Brillouin scattering to detect the waves after they propagate through a liquid layer and into a solid substrate. A linear frequency dependence is found for both the shear speed and attenuation rate in glycerol. DOI: 10.1103/PhysRevLett.102.107402 PACS numbers: 78.20.Hp, 43.35.+d, 78.40.Dw, 78.47.JÀ Fast structural relaxation dynamics in liquids continue to pose major fundamental challenges [1], in large measure because direct experimental access to key relaxing degrees of freedom over the time or frequency ranges of interest remains elusive. Both density and shear relaxation play central roles in the complex structural responses of viscoelastic materials. On slow time scales, dynamic mechanical analysis and sonic or related measurement methods can be used, while faster responses require measurements of longitudinal and shear acoustic waves in the megahertz and gigahertz frequency ranges. Much of the MHz range is now accessible to ultrasonics and impulsive stimulated thermal or Brillouin scattering (impulsive stimulated thermal or Brillouin scattering [2-5]), and (usually isolated) frequencies in the low GHz range may be accessed through spontaneous Brillouin scattering [6]. Recent work in x-ray Brillouin scattering has accessed THz longitudinal acoustic frequency ranges [7], but frequencies in the tens to hundreds of GHz range, where fast relaxation features occur, have remained difficult to access. Deep-UV Brillouin scattering from longitudinal acoustic waves in this range has been demonstrated [4,7], but its utility is limited by strong absorption in most materials. Picosecond ultrasonics [8], in which a short optical pulse generates a single-cycle acoustic pulse that is observed after propagation through a sample, has provided tabletop access to much of the GHz-frequency range for longitudinal acoustic waves. Adaptations of the method to enable GHz shear wave generation [9][10][11][12] have been developed. However, shear waves in liquids have remained elusive, to the extent that the challenges in ''seeking shear waves in liquids with picoseconds ultrasonics'' [13] have been elaborated explicitly. The use of multiple optical pulses to generate frequency-tunable, multiple-cycle longitudinal waves [14] has been demonstrated to improve acoustic spectral brightness for characterization of frequency-dependent material responses. Here we demonstrate this approach for generation of frequency-tunable shear as well as longitudinal acoustic waves in the GHz-frequency range. We further demonstrate a sample and optical configuration that permit the measurements to be conducted in viscoelastic liquids, whose GHz-frequency acoustic responses are of...

In combining spin- and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe(1-x)Co(x)(001)/MgO/Fe(1-x)Co(x)(001) magnetic tunnel junctions. Contrary to previous theoretical predictions, we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic% as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Δ(1) symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe(1-x)Co(x)(001) free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system.

Half Metal Magnets are of great interest in the field of spintronics because of their potential full spin-polarization at the Fermi level and low magnetization damping. The high Curie temperature and predicted 0.7eV minority spin gap make the Heusler alloy Co 2 MnSi very promising for applications.We investigated the half-metallic magnetic character of this alloy using spin-resolved photoemission, ab initio calculation and ferromagnetic resonance. At the surface of Co 2 MnSi, a gap in the minority spin channel is observed, leading to 100%spin polarization. However, this gap is 0.3 eV below the Fermi level and a minority spin state is observed at the Fermi level. We showthat a minority spin gap at the Fermi energy can neverthelessbe recovered either by changing the stoichiometry of the alloy or by covering the surface byMn, MnSi or MgO. This results in extremely small damping coefficients reachingvalues as low as7x 10 -4 .