GIANT INJECTION MAGNETORESISTANCE IN THE HETEROSTRUCTURE GaAs / GRANULAR FILM WITH COBALT NANOPARTICLES L. V. Lutsev, A. I. Stognij 1, N. N. Novitskii. - презентация

1 GIANT INJECTION MAGNETORESISTANCE IN THE HETEROSTRUCTURE GaAs / GRANULAR FILM WITH COBALT NANOPARTICLES L. V. Lutsev, A. I. Stognij 1, N. N. Novitskii 2 Research Institute 'Ferrite-Domen', St.-Petersburg, Russia 1 Minsk Research Institute of Radiomaterials, Minsk, Belarus 2 Institute of Solid State and Semiconductor Physics, NASB, Minsk, Belarus

2 Preparation of (SiO 2 ) 100-x Co x /GaAs and (TiO 2 ) 100-x Co x /GaAs samples. Experiment. -Current-voltage characteristics. -Suppression of the electron inject current by magnetic field. Magnetoresistance. -Temperature dependencies. -Magnetization curves. -Domain structure. -Spectrum of surface spin waves, propagating in the granular film (SiO 2 ) 100-x Co x.

3 Theory. -Hamiltonian. -Accumulation electron layer at the interface GaAs / granular film. Exchange interaction in the electron layer. -Energy barrier induced by the exchange interaction in the electron layer for injected spin polarized electrons. -Potential jumps and spin-flip scattering of injected spin polarized electrons on the barrier in the magnetic field. Theoretical predictions and experiment. Application. Conclusion. Publication: L. V. Lutsev, A. I. Stognij, N. N. Novitskii, JETP Letters, 2005, 81(10), p.636

4 Preparation of samples Substrate. n-GaAs (100) Carrier concentration cm -3. Prior to the deposition process, substrates were polished by a low- energy oxygen ion beam. The maximum roughness height did not exceed 0.5 nm. Relief cross section of the GaAs substrate.

5 (SiO 2 ) 100-x Co x /GaAs Amorphous silicon dioxide films containing cobalt nanoparticles were grown on GaAs substrates by ion beam co-sputtering of composite quartz and cobalt targets. Co content: at.% Thickness: nm Co particle size: nm (TiO 2 ) 100-x Co x /GaAs Amorphous titanium dioxide films containing layers with cobalt islands were grown layerwise by ion beam sputtering of separated TiO 2 and cobalt targets. Co content: at.% Thickness: nm There were deposited 10 layers of cobalt islands and 10 layers of titanium dioxide: (Co/TiO 2 ) 10.

6 Co content > 50 at.% - ferromagnetic state Co content < 50 at.% - superparamagnetic state

7 Experiment. Current-voltage characteristics TiO 2 (Co)/GaAs with the Co content (1) 34, (2) 55, (3) 76, (4) 100 at.%

8 SiO 2 (Co)/GaAs with the Co content (1) 39, (2) 60, (3) 85, (4) 100 at.%.

9 Influence of the magnetic field on the current-voltage dependence. At U > 20 V the magnetic field suppresses the electron inject current flowing from the granular film SiO 2 (Co) into GaAs. Heterostructure SiO 2 (Co)/GaAs with the Co content x = 60 at.%.

10 Inject current density versus the magnetic field at different applied voltages SiO 2 (Co)/GaAs with x Со = 60 at.%. Magnetic field is parallel to the surface plane.

11 SiO 2 (Co)/GaAs with x Со = 60 at.%. Magnetic field is normal to the surface plane.

12 TiO 2 (Co)/GaAs with x Со = 55 at.%. Magnetic field is parallel to the surface plane.

13 Injection magnetoresistance. R 0, R(H) are the resistances of the structure GaAs / granular film without a field and in the magnetic field H, respectively; j 0 and j(H) are the inject current densities flowing from a granular film into GaAs in the absence of a magnetic field and in the field H. At the voltage U = 50 V for the structure SiO 2 (Co)/GaAs with the Co content x = 60 at.% the value of IMR reaches up to 52 (5200 %) in the saturation limit range at the magnetic field H = 23 kOe.

14 IMR versus the magnetic field H at the applied voltage U = 35 V for the structure SiO 2 (Co)/GaAs with the Co concentrations (1) 39 (2) 60 (3) 85 at.%. Magnetic field is parallel to the film surface.

15 IMR versus the magnetic field H at the applied voltage U = 35 V for the structure TiO 2 (Co)/GaAs with the Co concentrations (1) 34 (2) 55 (3) 76 at.%. Magnetic field is parallel to the film surface.

16 Temperature dependence of the inject current. 1. SiO 2 (Co)/GaAs with the Co content 60 at.%. Activation energies 1 = 0.47 eV, 2 = 0.19 eV. 2. TiO 2 (Co)/GaAs with the Co content 55 at.%. Activation energy = eV.

17 Temperature dependence of the injection magnetoresistance. SiO 2 (Co)/GaAs с with the Co content 60 at.%. U = 35 V H = 2.6 kOe

18 Difference between the magnetodiode and IMR effects

19 Magnetization curves. SiO 2 (Co)/GaAs with x Со = 85 at.%, room temperature (A. Stashkevich)

20 SiO 2 (Co)/GaAs with x Со = 85 at.%

21 Temperature dependence of magnetization curves. (A. Stashkevich) SiO 2 (Co)/GaAs x Со = 85 at.% H = 200 Oe. fc - field-cooling zfc - zero-field- cooling

22 Domain structure. SiO 2 (Co)/GaAs with x Со = 85 at.%, room temperature.

23 Surface spin-wave spectrum in the granular film SiO 2 (Co)/GaAs with x Со = 85 at.% measured by Brillouin Light Scattering technique. (A. Stashkevich) BLS intensity spectrum for the angle of incidence of 40° Ar + laser power = 350 mW wavelength = 514 nm H e = 5000 Oe

24 Spin-wave spectrum measured by BLS technique. Dots are measured data. The solid curve represents numerical simulations of the Damon-Eshbach mode.

25 Theoretical model.

26 Accumulation electron layer at the interface GaAs / granular film. Difference of chemical potentials at the interface 1 is the difference of chemical potentials between GaAs and dielectric (is high). 2 is the difference of chemical potentials between GaAs and Co (is low). p 1, p 2 are concentrations at the interface. Dielectric forms accumulation electron layer in GaAs. Co polarizes electrons in the accumulation layer.

27 Energy barrier induced by the exchange interaction in the electron layer for injected spin polarized electrons. W is the barrier induced by the exchange interaction between d-electrons of Co and electrons in GaAs

28 Calculation of the height of the exchange barrier. Hamiltonian.

29 Hamiltonian of electrons in GaAs

30 Exchange interaction between d-electrons and electrons in GaAs Electrical field

31 Electron Green function in the semiconductor

32 Equation for electrical field

33 Effective exchange interaction in the one-loop approximation.

34

35 Calculations have been done under the following conditions: J 0 (r -R) = J 0 exp(- r -R ) n = 0, R = 0, = 10 nm -1, J 0 = 2 eV S(R) 0 = 1/2 (r) 0 = 1/2 (r) 2, (r) 0 is parallel to the interface plane a = 0.23 nm 4 M = 4 g B S 0 /a 3 = 10 kOe for the granular structure.

36 Effective exchange interaction = 150 meV n 0 = cm -3 T = 300 K n = cm -3 at the interface

37 Energy barrier induced by the exchange interaction in the electron layer for injected spin polarized electrons. z (inj) (r) = 1/2 (r-r o )

38 Energy barrier height versus the difference of chemical potentials at the interface T = 300 K Electron concentration in the semiconductor at a great distance from the interface n 0 = cm -3

39 Energy barrier height versus the temperature Electron concentration in the semiconductor at a great distance from the interface n 0 = cm -3 (at T = 300 K)

40 Energy barrier height versus the electron concentration in the semiconductor T = 300 K = 100 meV

41 Trajectories of injected electrons without a change of spin polarizations in the absence of magnetic field. In the magnetic field in the absence of domain structure injected electrons must jump over the potential barrier W. max IMR = exp(W/kT)

42 Application 1. Spin valve 2.

43 Conclusion. IMR reaches up to 5200 % at room temperature on the structure SiO 2 (Co)/GaAs with the Co content x = 60 at.%. IMR is observed in a narrow d-metal concentration range.

44 Сравнение с экспериментом Различие IMR в касательном и перпендикулярном к гетероструктуре магнитных полях объясняется размагничивающим фактором H dem = -p4 M co (p - объемная доля частиц). Температурная зависимость IMR для SiO 2 (Co)/GaAs с x Co = 60 at.% имеет максимум при T = 280 K. Эта зависимость соответствует гетероструктуре с разностью химических потенциалов = 0.23 eV. Максимум IMR при комнатной Т достигается для SiO 2 (Co)/GaAs с x Co = 60 at.%. Меньшие значения IMR для гетероструктур с другими x Co объясняются зависимостью W RKKY от.

45 Выводы. В структуре арсенид галлия / гранулированная пленка с наночастицами кобальта обнаружено новое явление - инжекционное магнитосопротивление (IMR). Значения коэффициента IMR достигают 5200 % при комнатной температуре. Эти значения на два порядка выше, чем аналогичные значения TMR и GMR в магнитных туннельных структурах и металлических мульти слоях.

46 IMR объясняется рассеянием спин-ориентированных инжектированных электронов на RKKY-барьере обогащенного электронного слоя вблизи интерфейса. В отсутствии магнитного поля существуют траектории инжектированных электронов, которые проходят обогащенный слой без изменения спиновой поляризации. В магнитном поле при отсутствии доменной структуры инжектированный электрон должен преодолеть RKKY-барьер. Локализованные электронные состояния в обогащенном слое вносят главный вклад в высоту RKKY-барьера. Наличие локализованных состояний определяет максимумы на температурной зависимости RKKY- барьера и на зависимости от.

47 Движение поляризованного электрона по междоменным каналам (1) в полупроводнике без магнитного поля.

48 Движение поляризованного электрона в полупроводнике в магнитном поле.