Giant spin-dependent thermoelectric effect in magnetic tunnel junctions

Abstract

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...

Tunneling magnetothermopower in magnetic tunnel junctions

“…Since the system is symmetric, the effective spin polarization [Eq. (17)] is the same in both electrodes, so P eff gl = P eff gr ≡ P . In addition, for the set of parameters discussed throughout this section, the relation P 2 1 is satisfied, hence, Eq.…”

“…(16) [Eq. (17)] k σ 0 and m * l with κ σ 0 and m * r , respectively. In order now to derive analytical expressions for the thermopower and the TMT, we benefit from Mott's relation [33,34], which states that…”

“…In this framework, spin-valves have proved to be excellent systems for probing spin caloritronic phenomena [13][14][15][16][17][18][19][20][21][22][23][24]. An ordinary spin-valve is a heterostructure composed of two ferromagnetic materials separated by a mesoscopic nonmagnetic layer.…”

“…The magnetoSeebeck effect was measured in MgO-based MTJs under heat gradients created optically [14], electrically [15,25], and even without an external heating source by using the heat dissipation of the tunneling current [16]. Giant thermoelectric effects have been observed in Al 2 O 3 -based MTJs [17]. Besides MTJs, all metallic spin-valves have also been probed for spin-dependent thermal properties [18].…”

Electrical contacts to single nanowires: a scalable method allowing multiple devices on a chip. Application to a single nanowire radial p-i-n junction

“…In the last few years, part of the scientific and technological interest in nanowires has been moved to energy harvesting such as photovoltaic [4][5][6][7][8][9][10][11][12][13], thermoelectric devices [14][15][16][17] and batteries [18][19][20][21][22]. Most of these devices are linked to the external world via electrical contacts.…”