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Structural phase transition is studied in high quality CrN thin films grown by molecular beam epitaxy on MgO(001) substrates. Cross-sectional transmission electron microscopy and x-ray diffraction reveal that the epitaxial relationship between CrN film and MgO substrate is [100]_CrN/[100]_MgO, [110]_CrN/[110]_MgO, and [001]_CrN/[001]_MgO. The films show tensile strain/compression at the CrN/MgO(001) interface, which relaxes gradually with the film growth. Temperature dependent x-ray diffraction measurements show a first-order structural phase transition. In addition to the experimental measurements, first-principles theoretical calculations have been carried out for finding a stable model for the CrN/MgO interface. Furthermore, these calculations determine two possible models for themore » interface, where a monolayer of chromium oxide is formed between the CrN and MgO layers.« less

A bilayer of iron on chromium nitride (Fe/CrN) is an interesting system for exchange biasing and sensing applications as the Néel temperature of CrN is 280 K and the Curie temperature of Fe is 1043 K. In this paper, we study the crystal and magnetic structures of the Fe/CrN interface at the atomic level. High quality epitaxial Fe/CrN bilayers prepared by molecular beam epitaxy grow in 001 orientation on MgO(001) substrates with uniform layer thicknesses and sharp interfaces. Our data reveal the epitaxial correlation between Fe and CrN crystals and their magnetic structures at the interface. The magnetic anisotropy directionsmore » of Fe and CrN are found parallel to [110]_MgO. We studied the electronic and magnetic properties of the interface by performing the first-principles total-energy calculations. Here, we present a model that combines the crystal and magnetic structures of the Fe/CrN bilayer and fully explains all results.« less

Here we studied exchange bias and exchange spring effects in magnetic bilayer thin films of Fe and CrN grown by molecular beam epitaxy. First, the relationships between exchange bias, coercivity, and blocking temperature and the Fe and CrN layers thicknesses are studied. Second, the exchange spring effect is observed and studied in all samples. The exchange spring breaks free at critical applied field strengths while creating planar domain walls at lower applied fields. First-principles calculations are performed to understand the magnetic interactions between the Fe and CrN layers at their interfaces. The calculations are key to understanding the experimental observations.

Single phase ε-Mn4N thin and ultrathin films are grown on MgO(001) using molecular beam epitaxy. Reflection high-energy electron diffraction and outof-plane x-ray diffraction measurements are taken for each sample in order to determine the in- and out-of-plane strain for each sample. Vibrating sample magnetometry and superconducting quantum interference device measurements, which are performed on the thin and ultrathin films respectively, are used to plot the magnetization of each sample versus both in- and out-of-plane $$\vec H$$-fields and to determine the magnitude of perpendicular magnetic anisotropy in these films. Three significant components of perpendicular magnetic anisotropy are observed in these filmsmore » and are attributed to sample strain (1 component) and shape (2 components). Among these components, the most significant component (0.8 to 4.9 $$\frac{Merg}{cm^3}$$) is identified as a second term of shape anisotropy, which possesses a negative linear relationship with sample thickness over the range from 9 nm to 310 nm. Atomic (magnetic) force microscopy measurements show the presence of a surface localized magnetic polarization (22% to 82%), which increases with decreasing thickness, when the net magnetization of the films is zero. The second term of shape anisotropy as well as the surface localized polarization, which each depend on sample thickness, are each regarded as a consequence of Ising domains overlapping out-of-plane in these films.« less

A magneto-structural phase transition is investigated in single crystal CrN thin films grown by rf plasma molecular beam epitaxy on MgO(001) substrates. While still within the vacuum environment following MBE growth, in-situ low-temperature scanning tunneling microscopy, and in-situ variable low-temperature reflection high energy electron diffraction are applied, revealing an atomically smooth and metallic CrN(001) surface, and an in-plane structural transition from 1×1 (primitive CrN unit cell) to $$\sqrt{2}$$ × $$\sqrt{2}$$ -R45° with a transition temperature of 278 ± 3 K, respectively. Ex-situ temperature dependent measurements using neutron diffraction are also per formed, looking at the structural peaks and likewise revealingmore » a first-order structural transition along the [111] out-of-plane direction, with a transition temperatures of 268 ± 3 K. Turning to the magnetic peaks, neutron diffraction confirms a clear magnetic transition from paramagnetic at room temperature to antiferromagnetic at low temperatures with a sharp, first-order phase transition and a Néel temperature of 270 ± 2 K or 280 ± 2 K for two different films. In addition to the experimental measurements of structural and magnetic ordering, we also discuss results from first-principles theoretical calculations which explore various possible magneto-structural models.« less

Single phase ε-Mn₄N and ζ-Mn₁₀N thin films are grown on MgO(001) using molecular beam epitaxy. The films are identified and characterized using reflection high-energy electron diffraction, x-ray diffraction, back scattered electron scanning electron microscopy, atomic/magnetic force microscopy and Rutherford backscattering spectrometry. These films are found to be highly smooth with root-mean-squared roughnesses 3.39 nm and below. The quality of ε-Mn₄N grown is strongly dependent on substrate temperature during growth. Epitaxial growth of substantial grains composed of the antiferromagnetic η-phase Mn₃N₂ side by side with ferrimagnetic ε-phase grains is observed when growth temperature is below 480 °C. Ising domains isolated withinmore » areas roughly 0.5 μm across are observed in the ferrimagnetic ε-phase grains of samples consisting of a mix of η- and ε-phase grains. Magnetic domains following semi-continuous paths, which are 0.7–7.2 μm across, are observed in single phase ε-Mn4N. Measurements of the ζ-phase detail the structure and magnetism of the material as high Mn content γ-type ζ-phase with a regular surface corrugation along the [100]-direction and antiferromagnetic.« less

Based on the interest in, as well as exciting outlook for, nitride semiconductor based structures with regard to electronic, optoelectronic, and spintronic applications, it is compelling to investigate these systems using the powerful technique of spin-polarized scanning tunneling microscopy, a technique capable of achieving magnetic resolution down to the atomic scale. However, the delicate surfaces of these materials are easily corrupted by in-air transfers, making it unfeasible to study them in stand-alone ultra-high vacuum STM facilities. Therefore, we have carried out the development of a hybrid system including a nitrogen plasma assisted molecular beam epitaxy/pulsed laser epitaxy facility for samplemore » growth combined with a low-temperature, spin-polarized scanning tunneling microscope system. The custom-designed molecular beam epitaxy growth system supports up to eight sources, including up to seven effusion cells plus a radio frequency nitrogen plasma source, for epitaxially growing a variety of materials, such as nitride semiconductors, magnetic materials, and their hetero-structures, and also incorporating in-situ reflection high energy electron diffraction. The growth system also enables integration of pulsed laser epitaxy. The STM unit has a modular design, consisting of an upper body and a lower body. The upper body contains the coarse approach mechanism and the scanner unit, while the lower body accepts molecular beam epitaxy grown samples using compression springs and sample skis. The design of the system employs two stages of vibration isolation as well as a layer of acoustic noise isolation in order to reduce noise during STM measurements. This isolation allows the system to effectively acquire STM data in a typical lab space, which during its construction had no special and highly costly elements included (such as isolated slabs) which would lower the environmental noise. The design further enables tip exchange and tip coating without breaking vacuum, and convenient visual access to the sample and tip inside a superconducting magnet cryostat. A sample/tip handling system is optimized for both the molecular beam epitaxy growth system and the scanning tunneling microscope system. The sample/tip handing system enables in-situ STM studies on epitaxially grown samples, and tip exchange in the superconducting magnet cryostat. Finally, the hybrid molecular beam epitaxy and low temperature scanning tunneling microscopy system is capable of growing semiconductor-based hetero-structures with controlled accuracy down to a single atomic-layer and imaging them down to atomic resolution.« less