Properties of Ilmenite-Hematite Ferromagnetic Semiconductors

Transcription

1 Properties of Ilmenite-Hematite Ferromagnetic Semiconductors D. Allen, P. Padmini, H. Alouach, R. Schad, W. Butler and R.K. Pandey

2 Ilmenite - FeTiO 3 Oxide wide bandgap semiconductor; p-type, E g = 2.58 ev. Antiferromagnetic, T N = 57 K Ferroelectric, Tc 6 K Excellent radiation resistant material A dominant mineral on lunar surface

3 Ilmenite - Hematite (I-H) solid solutions (FeTiO 3 ) (1-x). (Fe 2 O 3 ) x show, depending on concentration x: Semiconducting properties (p or n type) Ferrimagnetic or antiferromagnetic properties - T c or T N > room temperature Radiation resistant system (Neutrons)

4 Ceramic Processing (1-x)FeTiO 3.xFe 2 O 3 FeTiO 3 +Fe 2 O 3 X = to.67 Ball Milling Isostatic 4 psi 12h in Ar Targets of (1-x) FeTiO 3.xFe 2 O 3 ceramics

5 Pulsed Laser Deposition Excimer Laser of wavelength 248 nm Target : (1-x)FeTiO 3.xFe 2 O 3, x = -.67 Substrate-Target distance : 3-5 in Laser energy : 3-8 mj Substrate temperature : C Base Pressure : 1x1-6 Torr Gas : Ar/O 2

6 BULK PROPERTIES OF IH

7 Resistivity Measurement Resistivity as a function of temperature for (1-x)FeTiO 3.xFe 2 O 3 R.T as a function of composition 8 4 log resistivity (Ohm-cm) x= x=.1 x=.2 x=.33 x=.45 x=.67 Resistivity@RT (Ohm-cm) Unradiated Radiated T(K) Comp- x-fe 2 O 3

8 Seebeck Measurement S(uV/K) Seebeck coefficient as a function of composition and temperature p n Composition, xfe 2 O 3 X <.2 p type; and x >.2 n type S(uV/K) p -1 n T (K) S(x=) S(x=.1) S(x=.2) S(x=.27) S(x=.33) S(x=.45) S(x=.67)

9 Magnetic Measurements Magnetic moment and transition temperature for different compositions of Ilmenite-Hematite 3K and ->K,1kOe (emu/cm 3 ) AF 2 FM AF composition x open symbols: Ishikawa, Akimoto J.Phys.Soc.Jap.12, 183 (1957) Tc and T N (K)

10 Magnetic Measurements Magnetic moment before and after neutron irradiation (emu/g) composition: % 2% 33% 45% radiation: Dose: 4.4 x 1 1 n/cm 2 Energy: 73MeV (ave) empty: before filled: after temperature (K)

11 THIN FILMS OF IH

12 Structural Characterization X-ray diffraction of IH film on Sapphire (c-cut) and MgO tw 2 35 Al2O3(6) intensity (cps) IH(6) intensity (cps) MgO MgO θ (deg) θ (deg)

13 Optical Characteristics Transparent Ilmentite/hematite films on MgO 19nm (FeTiO 3 ).67 (Fe 2 O 3 ).33 33nm (FeTiO 3 ).8 (Fe 2 O 3 ).2

14 Electrical Characterization Resistivity of x=.45 film on MgO at two different deposition temperature Seebeck Coefficient of x =.45 film on MgO log resistivity (ohm-cm) C 85C T(K) S(uV/K) T(K)

15 Magnetic Characterization Magnetic properties of x =.33 film on MgO Magnetic moment as a function of composition 2.x1-4 c33emu1a 8 magnetic moment (emu) 1.5x1-4 1.x1-4 5.x x x1-4 c33emu1b c33emu2b c33emu29a c33emu29b 3K (emu/cc) Film Bulk -1.5x x magnetic field (Oe) Comp - xfe 2 O 3

16 Conclusions (1-x)FeTiO 3.xFe 2 O 3 ; x <.2 - p-type and x >.2 - n-type x <.1 - antiferromagnetic;.1 < x <.67 ferrimagnetic Resistivity decreased after neutron irradiation Magnetic moments enhanced by neutron irradiation. Highly textured and transparent films on MgO and sapphire

17 Future Work Optimization of the deposition parameters to get a good epitaxial film. Bandgap measurements Polycrystalline films to evaluate the varistor property. pn junction devices Characterize films for Ferroelectric and Magnetic properties.