Transport Properties of Rare Earth Nitrides

<p>In this thesis we investigate the transport properties of SmN, NdN and GdN, members of the rare earth nitride series of intrinsic ferromagnetic semiconductors. GdN is the central member of the series with seven occupied majority spin 4f states and seven empty minority spin 4f states. Both the filled and unfilled 4f states are some few eV away from the conduction and valence band extrema, resulting in transport properties which are dominated by the extended Gd 5d band. The half filled 4f shell, with zero net orbital angular momentum, furthermore simplifies calculations and as such GdN is the most studied both experimentally and in theory. As one moves to lighter members, the filled 4f states become unfilled states in the conduction band and the 4f shell now has a net orbital angular momentum. Calculations concerning these members are now significantly more complicated, and as such there exists a wide range of predictions concerning the conduction band minima in the lighter rare earth nitrides. To inform the current theoretical and experimental literature we report on three studies concerning the transport properties of SmN, NdN and GdN. To begin we report on the anomalous Hall effect in SmN, NdN and GdN. Under the symmetry of the rock-salt rare earth nitrides the magnitude of the anomalous Hall effect can imply the wave function of the conduction electron (i.e. d or f band). Measurements of the anomalous Hall effect in moderately doped samples are used to show the conduction channel in SmN and NdN is an f band or hybridised f/d band. Furthermore the sign of the anomalous Hall effect can be used to determine the orientation of the spin magnetic moment of the conduction electrons. Optical measurements of SmN, NdN and GdN films are then reported. Optical measurements provide a probe of the band structure of a material via direct transitions between the valence and conduction bands. Measurements of reflectivity and transmission on undoped SmN and NdN films were used to locate the unfilled majority spin 4f bands which form the conduction band minima in each material. Finally a preliminary study of heavily doped SmN, NdN and GdN is discussed. Structural measurements show a reduced lattice parameter while transport results find a significantly enhanced conductivity in heavily doped films. The Curie temperature is found to be enhanced and optical measurements show an increased absorption and red-shifted optical edge in doped films. The superconducting state of SmN is discussed and it is shown only to be present in moderately doped films, i.e. superconductivity is not present in undoped or degenerately doped SmN, within our measurement limits.</p>

Transport Properties of Rare Earth Nitrides

<p>In this thesis we investigate the transport properties of SmN, NdN and GdN, members of the rare earth nitride series of intrinsic ferromagnetic semiconductors. GdN is the central member of the series with seven occupied majority spin 4f states and seven empty minority spin 4f states. Both the filled and unfilled 4f states are some few eV away from the conduction and valence band extrema, resulting in transport properties which are dominated by the extended Gd 5d band. The half filled 4f shell, with zero net orbital angular momentum, furthermore simplifies calculations and as such GdN is the most studied both experimentally and in theory. As one moves to lighter members, the filled 4f states become unfilled states in the conduction band and the 4f shell now has a net orbital angular momentum. Calculations concerning these members are now significantly more complicated, and as such there exists a wide range of predictions concerning the conduction band minima in the lighter rare earth nitrides. To inform the current theoretical and experimental literature we report on three studies concerning the transport properties of SmN, NdN and GdN. To begin we report on the anomalous Hall effect in SmN, NdN and GdN. Under the symmetry of the rock-salt rare earth nitrides the magnitude of the anomalous Hall effect can imply the wave function of the conduction electron (i.e. d or f band). Measurements of the anomalous Hall effect in moderately doped samples are used to show the conduction channel in SmN and NdN is an f band or hybridised f/d band. Furthermore the sign of the anomalous Hall effect can be used to determine the orientation of the spin magnetic moment of the conduction electrons. Optical measurements of SmN, NdN and GdN films are then reported. Optical measurements provide a probe of the band structure of a material via direct transitions between the valence and conduction bands. Measurements of reflectivity and transmission on undoped SmN and NdN films were used to locate the unfilled majority spin 4f bands which form the conduction band minima in each material. Finally a preliminary study of heavily doped SmN, NdN and GdN is discussed. Structural measurements show a reduced lattice parameter while transport results find a significantly enhanced conductivity in heavily doped films. The Curie temperature is found to be enhanced and optical measurements show an increased absorption and red-shifted optical edge in doped films. The superconducting state of SmN is discussed and it is shown only to be present in moderately doped films, i.e. superconductivity is not present in undoped or degenerately doped SmN, within our measurement limits.</p>

First-Principles Study of Ferromagnetism in Iron Chromite Spinels: FeCr2O4 and CrFe2O4

The structural, electronic and magnetic properties of FeCr2O4 and CrFe2O4 spinels have been investigated by the first-principles approach based on density functional theory (DFT) and the full-potential linearized augmented plane-wave method, within the generalized gradient approximation (GGA-PBE) and GGA-[Formula: see text] scheme. The stability of these spinels in the normal and inverse phases is evaluated. The spin-polarized electronic band structures and density of states of FeCr2O4 calculated by GGA-PBE and GGA-[Formula: see text] show that the minority spin channel has metallic nature and the majority spin channel has a half-metallic (HM) gap of 0.25[Formula: see text]eV and 1.30 eV, respectively. CrFe2O4 shows that both minority and majority spin channels have metallic nature when using GGA-PBE and half-semiconducting behavior with half-semiconductor gap of 0.71[Formula: see text]eV when using GGA-[Formula: see text], with magnetic moment of 2[Formula: see text][Formula: see text] per formula unit. Analysis of density of states of these compounds indicates that the magnetic moment mainly originates from the strong spin-polarization of 3[Formula: see text] states of Fe and Cr atoms. Presence of HMF in FeCr2O4 and CrFe2O4 spinels makes these compounds promising compounds for spintronic applications.

Strain Induced Modifications of Valence Band Features in Fe3O4 Epitaxial Heterostructures

We report systematic investigations of lattice mismatch strain and the strain relaxation induced modifications on the valence band electronic structure of the epitaxial Fe3O4/Si (100) and Fe3O4/MgO (100) heterostructures. The Fe3O4 films on Si (100) and MgO (100) substrates were investigated though the angle-integrated photoemission spectroscopy (AIPES) at room temperature in the energy range 45-65 eV. Depending on the strain state of the films, the Raman modes and the Verwey transition temperature show deviations from the corresponding bulk values. The valence band feature at 2.7 eV (3.5 eV) shifts towards (away) the Fermi level with an increase (decrease) in strain resulting in decrease (increase) of density of states (DOS) of the minority spin Fe(A) 3d-eg band (majority spin Fe(B) 3d-t2g band). The effect is more pronounced in the case of films on MgO (100) substrate in comparison to the films on Si (100) substrate.

Interface Characterization of Current-Perpendicular-to-Plane Spin Valves Based on Spin Gapless Semiconductor Mn2CoAl

Employing the first-principles calculations within density functional theory (DFT) combined with the nonequilibrium Green’s function, we investigated the interfacial electronic, magnetic, and spin transport properties of Mn2CoAl/Ag/Mn2CoAl current-perpendicular-to-plane spin valves (CPP-SV). Due to the interface rehybridization, the magnetic moment of the interface atom gets enhanced. Further analysis on electronic structures reveals that owing to the interface states, the interface spin polarization is decreased. The largest interface spin polarization (ISP) of 78% belongs to the MnCoT-terminated interface, and the ISP of the MnMnT1-terminated interface is also as high as 45%. The transmission curves of Mn2CoAl/Ag/Mn2CoAl reveal that the transmission coefficient at the Fermi level in the majority spin channel is much higher than that in the minority spin channel. Furthermore, the calculated magnetoresistance (MR) ratio of the MnCoT-terminated interface reaches up to 2886%, while that of the MnMnT1-terminated interface is only 330%. Therefore, Mn2CoAl/Ag/Mn2CoAl CPP-SV with an MnCo-terminated interface structure has a better application in a spintronics device.

EFFECTS OF THE MIXED Cu/O LAYER ON THE TRANSPORT PROPERTIES OF Cu/EuO-BASED TUNNEL JUNCTIONS

The spin-dependent transport properties of Cu/EuO-based tunnel junctions are investigated by means of the first-principle calculations combined with the non-equilibrium Green’s function (NEGF) method. It is found that the Cu/EuO-based junctions exhibit excellent spin-filtering effect. Furthermore, the mixed Cu/O layer enhances the tunneling of the majority spin through the EuO barrier for the junctions with Cu/O layers due to the fact that the valance-band maximum of the Eu-4[Formula: see text] states shifts to high energies with respect to the Fermi level for these junctions. These results permit the existence of the mixed Cu/O layer in Cu/EuO-based tunnel junctions and promote future applications of these tunnel junctions in spintronic devices.

Spin-dependent electron transport through a Mn-phthalocyanine molecule — A steady-state density functional theory (SS-DFT) study

We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin-dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allowing spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnel through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of the spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is presented.

Strong spin-filtering and spin-valve effects in a molecular V–C60–V contact

Motivated by the recent achievements in the manipulation of C60 molecules in STM experiments, we study theoretically the structure and electronic properties of a C60 molecule in an STM tunneljunction with a magnetic tip and magnetic adatom on a Cu(111) surface using first-principles calculations. For the case of a vanadium tip/adatom, we demonstrate how spin coupling between the magnetic V atoms, mediated by the C60, can be observed in the electronic transport, which display a strong spin-filtering effect, allowing mainly majority-spin electrons to pass (>95%). Moreover, we find a significant change in the conductance between parallel and anti-parallel spin polarizations in the junction (86%) which suggests that STM experiments should be able to characterize the magnetism and spin coupling for these systems.