Origin of high transport spin polarization inLa0.7Sr0.3MnO3

Abstract

Using the point contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, La0.7Sr0.3MnO3 (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a transport half-metal.A half-metallic ferromagnet is a metal that has an energy gap at the Fermi level, E F , in one of the two spin channels. Only the other channel has states available for transport, and thus the electric current is fully spin-polarized. Finding half-metallic or other highly spin-polarized metals would bring about major advances in magnetoelectronics, since device performance improves dramatically as the spin polarization of the metal approaches 100%.1 Although half-metallicity has been predicted in quite a number of materials, the experimental situation is still controversial, especially for the manganese perovskite, La 0.7 Sr 0.3 MnO 3 (LSMO). Theoretical 2 and experimental values 3-6 of the spin polarization of this fascinating material with highly unusual structural, magnetic and electronic properties, obtained by different techniques vary from 35% to 100%. Not surprisingly, when Park et al. concluded from their spin-resolved photoemission spectroscopy measurement that LSMO is completely spin-polarized 3 it attracted immediate attention. This result was important not only from a practical viewpoint, but also as a potential new insight into the microscopic physics of this system, since the values of the spin polarization are extremely sensitive to the band structure of LSMO. Importantly, the measured value of the spin polarization, P n , depends on the experimental technique. It is often possible 10 to define P n in the following form:where N ↑ (E F ), N ↓ (E F ) and v F ↑ , v F ↓ are the majority and minority spin DOS and the Fermi velocities, respectively. This definition allows a direct comparison between different experiments and the theory, since all the quantities in Eq. 1 can be evaluated from the band structure. The spin polarization P 0 (n =0) measured by spinresolved photoemission measurements is determined only by the DOS at the Fermi level.11 Transport experiments measure a different spin polarization, which includes the Fermi velocities (Eq.1). In the ballistic, or Sharvin, limit (mean free path, L, larger than the contact size, d) the DOS is weighted linearly with v F , and P 1 is measured. 12In the diffusive, or Maxwell regime (L < d), as in the classical Bloch-Bolzmann theory of transport in metals, the weighting is quadratic in v F (n=2) and P 2 is measured (a...

Uniaxial pressure-induced half-metallic ferromagnetic phase transition inLaMnO3

“…Hole-doped LaMnO 3 compounds (La 1−x A x MnO 3 , where A = Ca, Sr, Ba) feature the largest reported CMR effect, which is several orders of magnitude larger than that of other systems [18][19][20]. In these materials, CMR can be observed either at the phase transition from a ferromagnetic (FM) half-metallic state to paramagnetic (PM) insulating state or in the melting of the charge-orbital ordered state [21,22].…”

Andreev reflection spectroscopy of niobium point contacts in a magnetic field

“…Earlier experimental works [24][25][26][27][28] on PCAR reported the enhancement of spectral broadening [29] with magnetic field, although without a consistent explanation of the mechanism. The influence of the magnetic field on the tip has not been addressed at all in the context of the spin polarisation measurements previously, despite the significant number of experimental studies on point-contact with ferromagnetic materials [4][5][6][7][8][9][10][11][12][13][14][15][16] and magnetic semiconductors [17,18].…”

“…These results have important implications for interpretation of transport spin polarisation measurements using PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin polarisation on the strength of the interface barrier, and potential misassignment of the inferred spin polarisation.PACS Numbers: 74.45.+c; 75.50.Cc; The recent theoretical [1][2][3] and experimental [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] development of point-contact Andreev reflection (PCAR) spectroscopy is driven by the interest to find materials with high degree of spin polarization P. Such materials can be used for integration in spinbased microelectronics [19][20][21]. The technique may be used to determine the spatial variation of spinpolarised current in semiconductors [22,23].…”

Influence of Barrier Width on Spin-Polarisation Measured by Point Contact Andreev Reflection

“…The fully polarised part is calculated by putting A N (E; d, Z) = 0 in formula (1). In practice, (4) is used to extract the values of spinpolarisation from the measurements of the total conductance by an appropriate fitting procedure [6,7,9,10].…”