J. Gosk

Paramagnetism and antiferromagnetic d–d coupling in GaMnN magnetic semiconductor

The magnetization of Ga1−xMnxN (x<0.1) crystals was measured as a function of the magnetic field and temperature. Paramagnetic behavior typical of spin S=5/2 expected for Mn2+ (d5) magnetic centers was observed in the temperature range of 2 K<T<300 K. On the other hand, antiferromagnetic coupling between Mn ions was clearly visible. The nearest neighbor (NN) coupling constant JNN/kB=−1.9 K was estimated from the temperature dependence of the magnetization.

Possible origin of ferromagnetism in (Ga,Mn)N

Ferromagnetic behavior of GaN doped with Mn (Ga1−zMnzN) grown by the ammonothermal and chemical transport methods is discussed in terms of a second phase (ferromagnetic one) produced during the growth process. The reference manganese nitride samples grown by the same method as (Ga,Mn)N reveal room-temperature ferromagnetic behavior, depending on the growth details. Different MnxNy phases are suggested to be responsible for ferromagnetic behavior of (Ga,Mn)N.

Structure and magnetism of MnAs nanocrystals embedded in GaAs as a function of post-growth annealing temperature

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

Structural and magnetic properties of GaN/Mn nanopowders prepared by an anaerobic synthesis route†

A new oxygen-free molecular precursor system based on (i) ammonolysis in refluxing/liquid NH3 of selected mixtures of gallium tris(dimethyl)amide Ga(NMe2)3 and manganese bis(trimethylsilyl)amide Mn[N(SiMe3)2]2 (Me = CH3, initial Mn-contents = 0.1, 5, 20, 50 at.%) followed by (ii) pyrolysis under flowing ammonia gas at 500, 700, and 900 °C afforded a range of nanocrystalline powders in the GaN/Mn system. The nanopowders were characterized mainly by powder XRD diffraction, FT-IR spectroscopy, Raman spectroscopy, SEM/EDX morphology examination, and XRF elemental analysis. Magnetization measurements as a function of magnetic field and temperature were carried out with a SQUID magnetometer. Structurally, the materials were shown to be single-phases based on the gallium nitride lattice. The presence of small quantities of residual amorphous Mn/N/Si/C species due to an incomplete transamination/removal of the trimethylsilylamide groups during ammonolysis was deduced from the XRF, FT-IR, Raman, and magnetization data. Magnetic properties for all nanopowders consistently pointed to a paramagnetic GaMnN phase with antiferromagnetic interactions among Mn-centers that under favorable circumstances reached the level of 3.8 at.% Mn. The paramagnetic phase was accompanied by a residual antiferromagnetic phase due to a facile oxidation in air of excessive Mn-containing by-products.

Magnetic and optical properties of (GaMn)N nanocrystalline powders prepared by the aerosol-assisted vapour phase synthesis and anaerobic imide route methods

Herein, we report a study on magnetic and optical properties of hexagonal (GaMn)N nanocrystalline powders obtained by two methods: aerosol-assisted vapour phase synthesis and anaerobic imide route. Measurements of the magnetization as a function of temperature and magnetic field for the powders show a typical paramagnetic behaviour. In addition, antiferromagnetic contribution originating from a residual MnO by-product and a small ferromagnetic contribution are observed. Magnetization measured as the function of magnetic field shows a smaller saturation effect than expected for non-interacting Mn-ions. Electron paramagnetic resonance (EPR) measurements reveal a single, structureless resonance line with a g-factor equal to 2.008±0.003 indicating a presence of Mn 2+ -centres in the samples. Both magnetic and EPR measurements suggest weak AF interactions between Mn-ions incorporated in (GaMn)N.

Magnetic Properties of (Ga,Fe)N

Wurtzite (Ga,Fe)N bulk crystals were, for the first time, successfully grown by AMMONO and chemical transport methods. The magnetization measurements of (Ga,Fe)N crystals revealed the coexistence of paramagnetic and ferromagnetic contributions. The paramagnetic component was shown to be growth condition dependent. The Brillouin-type behavior was observed in the samples obtained by both methods. Some (Ga,Fe)N samples (especially those codoped with Si) grown by the chemical transport method show a Van Vleck–type paramagnetic behavior.

Ferromagnetic spins interaction in alternating branched polyarylamines

The chemical oxidation of alternating branched polyarylamines leads to the formation of radical cations, the presence of which was manifested by the appearance of new bands in the UV–visible–near infrared spectra. Spins of radical cations can be magnetically coupled. The nutation pulsed-EPR technique was used to determine the multiplicity of spin systems for different oxidant:polymer unit ratios. It was found that spins of polymers oxidized to a radical cation in each conjugated amine segment interact magnetically to form dominant quartet state (S = 3/2). Magnetization measurements confirmed the ferromagnetic interaction of spins, returning an exchange integral J = +0.75 meV.

The nature of Cr center in GaN: Magnetic anisotropy of GaN:Cr single crystals

Magnetization measurements of the strain-free bulk GaN:Cr single crystals of wurtzite structure are reported. Strong magnetic anisotropy at low temperatures (2–10 K) was observed. The data were analyzed assuming Cr2+(d4) configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static Jahn-Teller distortion, and the spin-orbit interaction provide a good description of the experimental magnetization data.

Magnetic properties of MnSb inclusions formed in GaSb matrix directly during molecular beam epitaxial growth

Despite of intensive search for the proper semiconductor base materials for spintronic devices working at room temperature no appropriate material based on ferromagnetic semiconductors has been found so far. We demonstrate that the phase segregated system with MnSb hexagonal inclusions inside the GaSb matrix, formed directly during the molecular beam epitaxial growth reveals the ferromagnetic properties at room temperature and is a good candidate for exploitation in spintronics. Furthermore, the MnSb inclusions with only one crystalline structure were identified in this GaMn:MnSb granular material. The SQUID magnetometry confirmed that this material exhibits ferromagnetic like behavior starting from helium up to room temperature. Moreover, the magnetic anisotropy was found which was present also at room temperature, and it was proved that by choosing a proper substrate it is possible to control the direction of easy axis of inclusions’ magnetization moment between in-plane and out-of-plane; the latter is ...

Analysis of one-dimensional disordered structures by simulation technique

A study on one-dimensional disordered structures (ODDS) in close packed (cp) crystals by Monte Carlo computer simulation is reported. Calculations of diffraction intensity distributions along the 10.L reciprocal lattice row from: 5T, 8H(44) and 12R(31)3 structures with deformation stacking faults (SFs) are presented. In particular, using the simple frequency function of fault-to-fault distances, both random and non-random distributions of SFs are considered. Distinctive features of the diffraction patterns corresponding to the chosen examples of transformations from the parent structures 5T, 8H(44) and 12R(31)3, into other polytypes are discussed in detail.