Thermopower in nano specular spin valves

Andre M. Pereira 2Joao Ventura 2Jose Teixeira 2M. E. Braga 2R P. Pinto 2J. B. Sousa 2Paulo Freitas 1,3A. Veloso 3

1. Instituto Superior Técnico, Lisboa 1049-001, Portugal
2. Departamento de Fisica and IFIMUP, Universidade do Porto (UP), Rua do Campo Alegre, 687, Porto 4169-007, Portugal
3. INESC-MN, Lisbon, Portugal

Abstract

A spin valve (SV) is a magnetic nanostructure constituted by two ferromagnetic (FM) layers separated by a thin non-magnetic (NM) metallic spacer. The magnetization of one of the FM layers is fixed by an underlying antiferromagnetic (AFM) layer, while that of the other FM layer rotates when a small magnetic field is applied. Relatively high magnetoresistance (MR) values can be achieved with such SV design, but new ways to enhance MR are being researched to increase bit density in hard drives. One such way is the fabrication of a nano-oxide layer (NOL) just above and below the free and pinned layers, respectively [1]. Such NOL spin valves can more than double the MR ratio of simpler stacks, because of specular reflection of electrons at the FM/NOL interfaces. However, the microscopic physics of electron specular reflection is still incomplete. We recently showed that the NOL is formed by magnetic oxides with a PM-AFM transition below room temperature that can strongly affect transport properties, particularly MR [2]. To further study the effects of the NOL on the magneto-transport of SVs we measured thermopower (S) and electrical resistance (R) of Ion Beam Deposited MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL specular SVs in the 320-20 K temperature (T) range. We show that at room temperature the oxides constituting the NOL are in the paramagnetic phase and that at TN~150K a magnetic transition to the AFM phase occurs. A linear S(T)-dependence extrapolating to the origin is observed above TN, as characteristic of well behaved metals in the paramagnetic phase. The magnetic transition at TN produces a pronounced change in the S(T) behaviour, leading to a quadratic temperature dependence at low temperatures, here related to the excitation of spin waves and their effect on the Seebeek coefficient. These features will be correlated with the observed peculiar MR(T) behaviour [2]. [1] A.Veloso et al. Appl. Phys. Lett. 77,1020(2000).[2] J.Ventura et al. J. Appl. Phys. 93,7690(2003).

 

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium D, by Joao Ventura
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-18 09:53
Revised:   2009-06-07 00:44