YbN: An intrinsic semiconductor with antiferromagnetic exchange

Warring, H.; Ruck, B. J.; McNulty, James; Anton, Eva-Maria; Granville, S.; Koo, Annette; Cowie, Bruce C. C.; Trodahl, H. J.

doi:10.1103/physrevb.90.245206

Cited by 16 publications

(11 citation statements)

References 32 publications

(55 reference statements)

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“…This will in-turn alter the nature of the temperature dependence of the resistivity with degenerately doped films taking a positive temperature coefficient of resistance. Measurements of the resistivity as a function of temperature showing a similar non-metallic behaviour as seen for GdN in Figure 2.3 have been seen in DyN [56], NdN [43], YbN [57] and SmN [58].…”

Section: Electrical Transport and Optical Measurementssupporting

confidence: 71%

“…In addition to the redshift of the absorption onset the development of an additional contribution to the absorption at ∼ 1.7 eV in low-temperature measurements signals transitions between the minority spin bands at the X point [61]. Optical measurements of DyN [62], NdN [22,43], YbN [57], EuN [27] and SmN [21,63] have all found bandgaps near 1 eV and at most weak indications of free carriers in moderately doped samples.…”

Section: Electrical Transport and Optical Measurementsmentioning

confidence: 99%

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Transport Properties of Rare Earth Nitrides

Holmes-Hewett¹

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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.

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Section: Electrical Transport and Optical Measurementssupporting

confidence: 71%

Section: Electrical Transport and Optical Measurementsmentioning

confidence: 99%

Transport Properties of Rare Earth Nitrides

Holmes-Hewett¹

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“…The results were supported by a thorough study of the electronic structure of bulk YbN samples in the 1990's where optical spectroscopy, specifically infrared absorption, gave further evidence for a semi-metallic band structure [52]. However these older studies are again, like many other rare-earth nitrides, at odds with both recent experimental and theoretical considerations which suggest that YbN may be a semiconductor [1,51].…”

Section: Conduction Bandmentioning

confidence: 80%

“…However, in contrast to Eu it is only the Yb 3+ ions that carry a magnetic moment, and the 4-f magnetic moment of Yb 3+ is smaller than that of Eu 2+ . This divalent/trivalent competition also means that in thin films Yb metal will only react with ionised nitrogen to form a nitride [51]. Early (1960s) electrical transport studies of YbN yielded a positive temperature coefficient of resistance (TCR), suggesting metallic behaviour [22].…”

Section: Experimental Picturementioning

confidence: 99%

Device Applications of Rare-Earth Nitrides

Warring¹

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In this thesis the properties of thin film spintronic devices are investigated. These devices incorporate rare-earth nitrides as the active elements in a geometry with vertical transport perpendicular to the layers. Many rare-earth nitrides are ferromagnetic semiconductors with a rich range of magnetic properties arising from their 4-f magnetic moments. These magnetic moments contain both spin and orbital contributions, in contrast to the quenched, spin-based magnetism frequently exploited in spintronic devices based on transition metals. Magnetic tunnel junctions are demonstrated with the ferromagnetic electrodes made from the intrinsic ferromagnetic semiconductor GdN surrounding GaN and AlN barriers. Fitting of the current-voltage characteristics of a GdN/GaN/GdN device determines a barrier height of 1.5 eV at room temperature. This puts the GdN Fermi level close to the GaN mid-gap, consistent with recent theoretical predictions of the band alignment at the GdN/GaN interface [Kagawa et al., Phys. Rev. Applied 2, 054009 (2014)]. This barrier height is found to scale with the band gap of the group-III nitride barrier, being approximately twice as large for AlN barriers. It was observed that the barrier height reduces as the AlN barrier thickness increases, signalling the formation of Schottky barriers at the interface. These polycrystalline junctions exhibit a tunnel magnetoresistance of a few percent but do not show clear signs of homogeneous switching. The transport properties of the GdN/GaN/GdN junctions are heavily influenced by the electronic structure of the semiconducting GdN layers, making junctions based on rare-earth nitrides promising candidates for further investigation. A fully semiconductor-based magnetic tunnel junction that uses spin-orbit coupled materials made of intrinsic ferromagnetic semiconductors is then presented. Unlike more common approaches, one of the electrodes consists of a near-zero magnetic moment ferromagnetic semiconductor, samarium nitride, with the other electrode comprised of the more conventional ferromagnetic semiconductor gadolinium nitride. Fabricated tunnel junctions exhibit magnetoresistances as high as 200%, implying strong spin polarisation in both electrodes. In contrast to conventional tunnel junctions, the resistance is largest at high fields, a direct result of the orbital-dominant magnetisation in samarium nitride that requires the spin in this electrode aligns opposite to that in the gadolinium nitride when the magnetisation is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetisation phase in the samarium nitride, a direct result of the orbital-dominant ferromagnetism. Thus, new functionality can be brought to magnetic tunnel junctions by use of novel electrode materials, in contrast to the usual focus on tuning the barrier properties. Finally, highly resistive GdN films intentionally doped with Mg are demonstrated. These films are found to have increased resistivities and decreased carrier concentrations, with no observed degradation in crystal quality as compared with undoped films. An increase of the Curie temperature in conductive films is observed which is consistent with the existence of magnetic polarons centred on nitrogen vacancies. The prospect of doping rare-earth nitride films in this manner promises greater control of the material properties and future device applications.

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“…The optical band gap has been measured for polycrystalline GdN to be 1.31 eV [27], and indirect and direct transitions found to be 0.95 eV and 1.18 eV respectively in epitaxial GdN [28]. The optical band gap has also been measured in DyN to be 1.2 eV [29], EuN with a gap of 0.9 eV [30], NdN with a gap of 0.9 eV [8] and YbN with a gap of 1.5 eV [31].…”

Section: Electronic Transportmentioning

confidence: 92%

Growth and Structural Properties of Rare Earth Nitride Thin Films

Chan¹

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In this thesis aspects of the growth of rare earth nitride thin films and characterisation of the resulting structural, electronic and magnetic properties of the film are investigated. The rare earth nitrides are a class of materials which combine interesting electronic and magnetic properties with potential applications in novel spintronic device structures. We study the formation of a preferential orientation in polycrystalline thin films of GdN deposited by electron-beam physical vapour deposition. X-ray diffraction is used to characterise the crystalline structure of films of varying thickness to identify a preference to grow along the [111] crystal direction, understood in terms of an evolutionary selection process. Variations in the film microstructure as a result of growth parameter variation are also correlated to electronic and magnetic properties. Investigation of the epitaxial growth of SmN on AlN surfaces revealed a novel growth orientation transition, controllable only via the growth temperature. Epitaxial integration of rare earth nitrides with III-nitride surfaces has previously only resulted in (111)-oriented growth on the (0001) surface as is expected from matching of the close-packed planes in the different crystal structures. High growth temperatures (≥800 ℃) induce (001)-oriented growth of SmN on the same (0001) AlN surface. This unexpected cube-on-hexagon geometry is confirmed through ex situ x-ray and in situ electron diffraction, the latter for which a computational simulation tool was developed to model and understand. The viability of using Sm as a temporary capping layer for rare earth nitride thin film samples is investigated. Capping layers are required to passivate samples due to their high reactivity, limiting the range of ex situ characterisation techniques that can be performed on them. Elemental Sm is relatively volatile raising the possibility of removing a Sm cap in situ using moderate annealing temperatures (400 °C to 600 ℃). The ability to remove the capping layer would allow in situ characterisation techniques to be performed in ultra high vacuum systems not directly connected to the growth system. In situ electron diffraction is used to characterise the growth and thermal annealing of Sm grown on top of epitaxial GdN layers, and the effects of the thermal removal process on the structural, electronic and magnetic properties of the GdN layer are investigated.

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YbN: An intrinsic semiconductor with antiferromagnetic exchange

Cited by 16 publications

References 32 publications

Transport Properties of Rare Earth Nitrides

Transport Properties of Rare Earth Nitrides

Device Applications of Rare-Earth Nitrides

Growth and Structural Properties of Rare Earth Nitride Thin Films

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