Exchange interactions in ɛ-Fe2O3: GGA + U calculations

We have studied the origin of magnetic interaction in ɛ-Fe2O3 by ab-initio electronic structure calculations. The exchange integrals of the Heisenberg Hamiltonian have been calculated using the methods based on the density functional theory (DFT) employing generalized gradient approximation (GGA) with orbital dependent potential extension for 3d electrons of Fe (GGA + U method). The calculations confirm the ground antiferromagnetic (AFM) state with two Fe3+ sublattices oriented up (Fe2 and Fe3) and two Fe3+ sublattices oriented down (Fe1 and Fe4). The calculated exchange integrals, including also the intra-sublattice ones, are all of AFM type. Their strength weighted by the number of neighbors is larger between the Fe sublattices with opposite spins than between the sublattices with equal spin directions. The notable exception is a strong exchange integral between the neighboring tetrahedrally-coordinated sites within the Fe4 sublattice, which effectively decreases the molecular field imposed on Fe4 sites by neighboring sites of other sublattices, namely the antiparallelly oriented Fe2 and Fe3. For this reason, the ordered magnetic moment of Fe4 exhibits the fastest decrease with increasing temperature among the sublattices, leading to an uncompensated AFM arrangement in ɛ-Fe2O3. Considering the competition of the inter- and intra-sublattice exchange integrals and applying symmetry arguments, we infer that the collinear AFM ground state of ɛ-Fe2O3 is prone to an intrinsic canting within the sublattices, retaining at the same time the magnetic group symmetry Pna'21'.

Effect of Tb3+ doping in mixed-valence manganites and cobaltites

The magnetic ordering of four Tb³⁺-doped manganites and cobaltites, La_0.7Tb_0.1Sr_0.2MnO₃, La_0.7Tb_0.1Ca_0.2MnO₃, La_0.7Tb_0.1Sr_0.2CoO₃ and La_0.7Tb_0.1Ca_0.2CoO₃, have been studied by means of neutron diffraction and SQUID magnetometry. All the samples were prepared by sintering of sol-gel precursors and their orthorhombic or rhombohedral perovskite structures at room and low temperatures were refined. A long-range ferromagnetic (FM) order was detected at the Mn and Co sites. In addition, a small but significant ordered moment was observed at A sites of studied cobaltites, which was attributed to local Tb³⁺ moments, aligned by exchange interactions due to FM ordered Co sublattice. No or minor Tb³⁺ contribution was detected in studied manganites.

Charge transport in thin layer Na x CoO2 (x ∼ 0.63) studied by terahertz spectroscopy

Charge transport in Na0.63CoO2 thin film deposited by a spin-coating method was investigated experimentally by time-domain terahertz spectroscopy and theoretically using Monte Carlo calculations of charge response in nano-structured materials. The dominating type of transport mechanism over the entire investigated range of temperatures (20-300 K) is a metallic-like conductivity of charges partly confined in constituting nano-sized grains. Due to the granular character of our thin film, the scattering time at low temperatures is limited by scattering on grain boundaries and the conductivity is strongly suppressed due to capture of a major fraction of charge carriers in deep traps. Nevertheless, our experimental setup and the applied model allowed us to distinguish the parameters related to the grain interior from those influenced by grain boundaries, and to conclude that the metallic type of conductivity is the intrinsic property relevant to single crystal materials.

Effect of Ising-type Tb3+ ions on the low-temperature magnetism of La, Ca cobaltite

Crystal and magnetic structures of the x = 0.2 member of the La0.8-xTbxCa0.2CoO3 perovskite series have been determined from powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below TC = 55 K. The moments first evolve on Co sites, and ordering of Ising-type Tb(3+) moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear Fx for the cobalt subsystem, while it is canted FxCy for terbium ions. The rare-earth moments align along local Ising axes within the ab-plane of the orthorhombic Pbnm structure. The behavior in external fields up to 70-90 kOe has been probed by magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb(3+) Ising character is the observation of a low-temperature region with an anomalously large linear term of heat capacity and its field dependence. Similar behaviors are detected also for other terbium dopings x = 0.1 and 0.3.

Crystal field and magnetism of Pr³⁺ and Nd³⁺ ions in orthorhombic perovskites

Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO3 perovskites (R=Pr, Nd, M=Ga, Co) are calculated using a first-principles electronic structure and the Wannier projection. The method contains a single adjustable parameter that characterizes the hybridization of R(4f) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of the trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the 4f electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within a few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO3 the theory is also compared with the magnetic measurements on a single crystal presented by Luis et al (1998 Phys. Rev. B 58 798). Good agreement between the experiment and theory is found.

Ground-state properties of the mixed-valence cobaltites Nd0.7Sr0.3CoO3, Nd0.7Ca0.3CoO3 and Pr0.7Ca0.3CoO3

The electric, magnetic, and thermal properties of three perovskite cobaltites with the same 30% hole doping and ferromagnetic ground state were investigated down to very low temperatures. With decreasing size of large cations, the ferromagnetic Curie temperature and spontaneous moments of cobalt are gradually suppressed: TC = 130 K, 55 K and 25 K and m = 0.68 μB, 0.34 μB and 0.23 μB for Nd0.7Sr0.3CoO3, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3, respectively. The moment reduction with respect to the moment of the conventional ferromagnet La0.7Sr0.3CoO3 (T(C) = 230 K, m = 1.71 μB) in the so-called low spin/intermediate spin (IS/LS) state for Co(3+)/Co(4+) was originally interpreted using a phase-separation scenario. Based on the present results, mainly the analysis of the Schottky peak originating from Zeeman splitting of the ground-state Kramers doublet of Nd(3+), we find, however, that the ferromagnetic phase in Nd0.7Ca0.3CoO3 and likely also in Pr0.7Ca0.3CoO3 is uniformly distributed over the whole sample volume, despite the severe drop of moments. The ground state of these compounds is identified with the LS/LS-related phase derived theoretically by Sboychakov et al (2009 Phys. Rev. B 80 024423). The ground state of Nd0.7Sr0.3CoO3 with an intermediate cobalt moment is inhomogeneous due to competition between the LS/LS and IS/LS phases. In the theoretical part of the study, the crystal field split levels for 4f(3) (Nd(3+)), 4f(2) (Pr(3+)) and 4f(1) (Ce(3+) or Pr(4+)) are calculated and their magnetic characteristics are presented.

Distribution of cations in nanosize and bulk Co-Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.

The assessment proposal for long-term and short-term tolerable hygrothermal microclimatic conditions

A group of four efficient mine rescuers 25 to 35 years old were exposed to a load of a cyclo-ergometer (stages A and B) and a hand ergometer (stage E) in a climate chamber. The total 120 min period of work was divided into four work intervals, 30 min each. There were 5-min breaks between the individual intervals. The load on the ergometer was selected in the range of 25 to 150 W, Tg=20 to 40 degrees C, rh=40 to 80% and v(a)=0.2 to 1.5 m x s(-1). The thermal resistance of the working suit was 0.65 clo at stage A, 1.07 clo at stage B and 0.81 clo at stage E. A total of 200 experiments with 50 combinations of the work and climate loads were made. The heart rate, oxygen consumption, carbon dioxide production, body temperature, skin temperature, water loss by sweating and perspiration, dry and wet bulb air temperature, air velocity and globe temperature were measured during the experiments. The expected production of sweat (SR) and the amount of accumulated heat in the body (Qmax) were calculated for each combination of the work-climate conditions by a computing program ISO 7933:1989 as well as by our own program. Good agreement was reached between the measured and predicted SR values, calculated by the ISO program (r=0.871) as well as between the values calculated by the two programs, respectively (r=0.985). The experimental results have shown good agreement between the predicted and actually measured values of temperature of the body core as an index of short-term tolerable climate load. The values of short-term tolerable time of work calculated at the level of accumulated heat in the body of 50 W x h x m(-2) resulted in an increase of body core temperature by 0.8 to 1.0 K. The values of heart rate did not mostly exceed 140 beats/min, reaching in exceptional (three) cases values above 150 beats/min. The authors recommend to limit the long-term work-heat (climatic) load during a higher metabolic rate (M >80 W x m(-2) including the basal metabolic rate) of acclimatized males and females at a sweat rate SR=270 g x h(-1) x m(-2), of non-acclimatized persons at SR=206 g x h(-1) x m(-2). The limit for low metabolic rates (M < or = 80 W x m(-2)) for non-acclimatized and acclimatized persons is proposed for long-term tolerable loads of SR=147 g x h(-1) x m(-2). The short-term tolerable load by heat storage within the organism for all categories is proposed as Qmax=50 W x h x m(-2).