Nearly total spin polarization in La2/3Sr1/3MnO3 from tunneling experiments

Abstract

We have performed magnetotransport measurements on La 2/3 Sr 1/3 MnO3 / SrTiO3 / La 2/3 Sr 1/3 MnO3 magnetic tunnel junctions. A magnetoresistance ratio of more than 1800 % is obtained at 4K, from which we infer an electrode spin polarization of at least 95 %. This result strongly underscores the half-metallic nature of mixed-valence manganites and demonstrates its capability as a spin analyzer. The magnetoresistance extends up to temperatures of more than 270K. We argue that these improvements over most previous works may result from optimizing the patterning process for oxide heterostructures.PACS numbers: 73.40Rw, 71.20.Eh Magnetic tunnel junctions (MTJ) have been studied actively from the mid 90's [1] due to both the underlying physics and their potential applications as magnetic memories (MRAMs) or sensors. These structures consist of two ferromagnetic metallic electrodes (FM) sandwiching a thin insulating barrier (I). When a bias voltage V DC is applied, electrons near the FM/I interface tunnel through the barrier and, since they are spin-polarized, the resistance depends on the relative orientation of the electrodes' magnetization. The tunneling magnetoresistance (TMR) ratio is defined aswhere R AP and R P are the resistances of the junction in the antiparallel and parallel configurations, respectively. In Julliere's model [2], the TMR ratio is related to the spin polarizations P 1 and P 2 of the two ferromagnetic electrodes as:Within this simple model, large TMR ratios result from electrodes, or from electrode-barrier interfaces [3], with large effective spin polarization values. Junctions which integrate amorphous barriers such as Al 2 O 3 and transition ferromagnets, for which the spin polarization does not exceed around 50 % [1, 4], do not show TMR ratios larger than 60 %. Preliminary work has been reported[5] on obtaining large interfacial spin polarizations owing to band structure effects, but the simplest route to achieving large TMR ratios relies on the use of so-called "half-metals" with a nearly total intrinsic spin polarization.A regarding their half-metallicity. Indeed, whereas spinpolarized photoemission spectroscopy experiments [10] have confirmed the half-metallic character of LSMO, the maximum spin polarization as inferred from tunneling experiments does not exceed 86% in LCMO [11] and 83% in LSMO [12].In this letter, we report a TMR ratio of more than 1800 % at T=4.2K and V DC =1mV in La 2/3 Sr 1/3 MnO 3 / SrTiO 3 / La 2/3 Sr 1/3 MnO 3 fully epitaxial MTJs, from which we deduce a spin polarization of at least 95 % for LSMO. This result confirms for the first time the transport half-metallic nature [13] of this material, which can therefore be used as a spin analyzer in tunneling experiments [3]. We argue that this large TMR value arises both from preserving the quality of the LSMO / STO (STO : SrTiO 3 ) interfaces during our upgraded patterning process, and from designing junctions of small size. The TMR extends to temperatures of about 280K, an improvement compared to previous resul...

Interface-induced room-temperature multiferroicity in BaTiO3

“…The TMR is negative, which (within Jullière's model 25 and in view of the positive spin polarization of LSMO; ref. 26) signals a negative spin-polarization for the Fe/BTO interface, in agreement with first-principles predictions 16,17 . As expected when the tunnel barrier is ferroelectric, the junctions also exhibit tunnel electroresistance (TER; refs 20,27,28), that is, a variation of their resistance with polarization direction at fixed magnetic field.…”

Spin filtering through ferromagneticBiMnO3tunnel barriers

“…One also knows that the half-metallic character of LSMO is conserved at the interface with STO. 11,22 Atomic force microscopy ͑AFM͒ images of this structure show a very smooth surface suitable for patterning the sample into tunnel junctions with the following structure: Au/BMO/STO/LSMO.…”

Magnetic tunnel junctions with MgO-EuO composite tunnel barriers

“…Various approaches have been explored in order to enhance the TMR value, for example, incorporate half-metallic electrodes for their near ideal spin polarizations [3,4,5], to construct epitaxial MgO barriers coupled with bcc-ferromagnets for the establishment of coherent tunneling [6,7,8,9], or to take advantage of magnetic insulators for their spin-filter capabilities [ 10 , 11 , 12,13 ]. Tremendous progress has been made with these approaches for the purpose of generating and enhancing TMR [14].…”