The influence of Co and Ni ion doping on the crystal structure, phase transitions, and magnetic properties of the Fe7Se8 system has been investigated. The compounds studied are (Fe0.987Ni0.013)7Se8, (Fe0.955Ni0.045)7Se8, (Fe0.915Ni0.085)7Se8, (Fe0.89Ni0.11)7Se8, (Fe0.79Ni0.21)7Se8, (Fe0.975Co0.025)7Se8, (Fe0.951Co0.049)7Se8, and (Fe0.91Co0.09)7Se8 as well as the parent compound Fe7Se8. The crystals are grown using a modified Bridgman method and exist as a hexagonal NiAs-like structure with an ordered distribution of Fe vacancies that determine their magnetic properties. The substitution effect, inducing a systematic decrease in the unit-cell volume due to the ionic radii of doping ions being smaller than the radii of Fe2+ ions, is a source of chemical pressure, acting on the magnetic subsystem and determining the magnetic ordering temperature TC and the spin-reorientation temperature (TSRT). As the most important result of this paper, it was shown that hydrostatic and chemical pressures are strongly correlated in the investigated system. The character of magnetic phase transitions has been designated in the vicinity of TC and TSRT temperatures, and the magnetic entropy change ΔSm and refrigeration potential magnitudes have been determined for this temperature range. These magnetocaloric parameters were shown to depend strongly on chemical pressure. A phenomenological model developed based on the experimental measurements demonstrated that the magnetic properties of the system depend strongly on the crystal field acting on Fe2+ ions and the ratio of the numbers of Fe2+ and Fe3+ ions.

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order". Previously the hidden order in phase II was identified as primary antiferroquadrupolar (AFQ) and field-induced octupolar (AFO) order. Here we present a combined experimental and theoretical investigation of collective excitations in the phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in rotating field is calculated within a localized approach using the pseudo-spin presentation for the Gamma8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of inter-multipolar interactions that stabilize hidden-order phases.

Ferromagnetic semiconductor thin layers of the quaternary (Ga,Mn)(Bi,As) and reference, ternary (Ga,Mn)As compounds, epitaxially grown under either compressive or tensile strain, have been characterized from a perspective of structural and magnetization homogeneity. The quality and composition of the layers have been confirmed by secondary-ion mass spectrometry (SIMS). A thorough evaluation of the magnetic properties as a function of temperature and applied magnetic field has been performed by means of SQUID magnetometry and low-energy muon spin relaxation (µSR) spectroscopy, which enables studying local (on the nanometer scale) magnetic properties of the layers. The results testify that the ferromagnetic order builds up almost homogeneously below the Curie temperature in the full volume fraction of both the (Ga,Mn)As and (Ga,Mn)(Bi,As) layers. Incorporation of a small amount of heavy Bi atoms into (Ga,Mn)As, which distinctly enhances the strength of spin-orbit coupling in the quaternary (Ga,Mn)(Bi,As) layers, does not deteriorate noticeably their magnetic properties.

Series of NiMnSn and NiMnGa Heusler alloys thin films of different compositions were obtained by DC magnetron sputtering in order to achieve strong magnetocaloric effect in the temperature range close to the room temperature. The maximal magnetocaloric effect was found in the temperature range 320 -350 K, with the maximal entropy change - Delta S ranges from 1: 4 x 10(4) erg/gK to 2: 7 x 10(4) erg/gK. Strain modulated ferromagnetic resonance technique was used to determine magnetoelastic properties of the films. Magnetoelastic constants of an order of 10(6) erg/cm(3) were found. (C) 2018 Elsevier B.V. All rights reserved.

The magnetocaloric effect has been studied in high quality single crystals of Fe7Se8 (3c type) grown by using Bridgman’s method. Magnetization and magnetocaloric effect measurements have been carried out in a magnetic field up to 5 T over the temperature range from 2 to 490 K. The spin reorientation transition from the easy c-axis to the easy c-plane, proceeding in an abrupt fashion, as a first-order phase transition, has been observed near the temperature TR ≈ 125 K. The magnetization curves in the vicinity of this transition were shown to have an S-shape with a clear hysteresis. The first order metamagnetic field induced transitions have been identified above and below TR. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above TR, while below TR the inverse magnetocaloric effect was clearly seen. The existence of both kinds of magnetocaloric effect is important from the point of view of large rotating field entropy change in Fe7Se8 single crystals. The refrigeration capacity associated with a second order phase transition from the ferrimagnetic to the paramagnetic state at the Néel temperature TN ≈ 450 K was found to be weaker than that appearing near TR. The giant anisotropy of the magnetocaloric effect was related to the magnetic anisotropy of Fe7Se8 crystals. The one-ion model of the magnetocaloric effect has been developed and its predictions have been compared with experimental data.

Experimental data are discussed showing that in contrast to the paramagnetic phase, in the magnetically ordered state the action of the crystal electric field on the spin dynamics is quantized. In the Curie-Weiss regime of the paramagnetic susceptibility the spin dynamics is determined by local exchange interactions between individual pairs of spins and by single particle anisotropies (crystal field interaction). As we know from Renormalization Group (RG) theory, these local interactions are of no importance on the spin dynamics in the long range ordered state. On the other hand, a sufficiently strong crystal electric field is known to decrease the saturation magnetic moment for T -> 0. In the critical paramagnetic range and for all lower temperatures the spin dynamics is controlled by a field of delocalized bosons instead by exchange interactions between spins. As we could show, the bosons are essentially magnetic dipole radiation emitted by the precessing spins. It is observed that the spontaneous generation of magnetic dipole radiation involves all N = 2S + 1 spin states, and is different in magnets with an integer and a half-integer spin. The dynamics of the boson field therefore is quantized and can be characterized by a limited number of universality classes. The effect of a relevant crystal field interaction is to reduce the number of thermodynamically relevant spin states per magnetic atom by Delta N = 1 or multiples thereof. This happens as discrete crossover events and reduces the saturation magnetic moment for T -> 0 in discrete steps. The dynamics remains quantized. Each reduction by Delta N = 1 changes the universality class. Since a crossover is a threshold induced event, we have to distinguish between a relevant and a non-relevant crystal field interaction. Only a sufficiently strong crystal field interaction can become relevant. The crossover from S to S-eff = S - 1/2 can occur in the critical paramagnetic range, and manifests as a functional change in the temperature dependence of either the longitudinal or the transverse susceptibility. A very particular observation is that in the insulating magnets a relevant crystal field interaction lets the magnetic heat capacity collapse to its absolute minimum. The magnetic entropy saturates at the lowest possible value of R.ln(2), irrespective of the value of S-eff(R = gas constant). This does not mean that a crossover to atomistic Ising behavior has occurred. For the metallic magnets the action of a relevant crystal electric field is also to reduce the number of relevant spin states for T -> 0 but the magnetic entropy saturates only gradually below the expected value of R.ln(2S + 1). Indications are discussed that each reduction of the spin by Delta S = 1/2 generates an additional energy band in the magnon excitation spectrum. In the metals the gap energy of the lowest magnon band is lower than in the insulators. Due to the lower excitation gap, spin dynamics and magnetic heat capacity are less suppressed in the metals compared to the insulators.

The out-of-plane magnetocrystalline anisotropy and magnetoelastic properties of series of epitaxially grown layers of Co(2Fe)0(.4)Mn0.(6S)i (CFMS) and Co2FeGa0.5Ge0.5 (CFGG) Heusler alloys thin films deposited on MgO with chromium buffer layer were investigated by means of the ferromagnetic resonance (FMR), SQUID magnetometer, and by the strain modulated FMR technique. The magnitude of the magnetocrystalline anisotropy constant was found to decrease with increasing layer thickness. The change of the anisotropy is caused mainly by the surface effects. However, for some of the samples series, the change of chemical ordering with the change of the magnetic layer thickness was also observed. An additional silver buffer layer influence on the properties of the magnetic layer was also investigated. For all three of the investigated series of the Heusler alloys, i. e., CFMS without Ag, CFMS with Ag, and CFGG, the experimentally observed magnitude of the magnetoelastic constant increases with the increase of the magnetic layer thickness.

The half-metallic Co 2 Fe 0.4 Mn 0.6 Si (CFMS) and Co 2 FeGa 0.5 Ge 0.5 (CFGG) Heusler alloys are representative materials showing high spin-polarization of conduction electrons. The main purpose of this work is to shed light on the mechanism of perpendicular anisotropy related to the internal stresses of the film through the magnetoelastic coupling. Magnetoelastic coupling and magnetostriction of these films have not been studied yet. The out-of-plane magnetocrystalline anisotropy and magnetoelastic properties of series of epitaxially grown layers of CFMS and CFGG Heusler alloys have been investigated by means of the strain modulated ferromagnetic resonance (SMFMR), the ferromagnetic resonance technique and by SQUID magnetometer. We study the perpendicular magnetocrystalline anisotropy as well as the magnetoelastic properties of a series of the CFMS and CFGG Heusler alloys of the thicknesses of tens of nanometers.

The magnetocaloric effect in the vicinity of the martensitic transformation for a single crystalline alloy with a composition close to the stoichiometric Ni2MnGa has been determined indirectly by M(T,H) magnetization measurements. It has an inverse character. The magnetocaloric parameters, i.e., the magnetic entropy change, refrigeration capacity and various hysteretic effects have been calculated from the M(T,H) dependences. Besides the martensitic transition a weak entirely separated intermartensitic transition was observed. These two successive magneto-structural transformations give contributions to the observed magnetocaloric effect. Unusual dependence of entropy change as a function of magnetic field has been explained as arising because of two different mechanisms. Additionally, to confirm that studied martensitic transformation is a first order phase transition electrical resistivity and thermoelectric power measurements have been performed.

Thin epitaxial film of La0.8Sr0.2MnO3 has been deposited on yz-cut LiNbO3 by pulsed laser ablation. The film was characterized by X-ray diffraction and magnetic measurements as a single phase. The magnetic properties and the magnetocaloric effect have been measured as a function of magnetic field and temperature. A reasonably high magnetocaloric effect indicates that this film might provide an efficient material for micro magnetic refrigerators.

Thin epitaxial lm of La0.8Sr0.2MnO3 has been deposited on yz-cut LiNbO3 by pulsed laser ablation. The lm was characterized by X-ray di raction and magnetic measurements as a single phase. The magnetic properties and the magnetocaloric e ect have been measured as a function of magnetic eld and temperature. A reasonably high magnetocaloric e ect indicates that this lm might provide an e cient material for micro magnetic refrigerators.

Recently, room temperature magnetocaloric materials increasingly attracted attention in the development of magnetic refrigerators. In this paper, an effect of P/Ge substitution on the magnetic phase transition in the series of Mn1.15Fe0.85P1-xGex (0.25 < x < 0.32) magnetocaloric compounds prepared by the arc melting technique and subsequent homogenization process has been studied. Calorimetric and magnetization results show that the temperature of structural phase transition coincide with the Curie temperature and fall within the temperature range 270-355 K. The magnetic entropy change reaches the maximum value for the compound with x = 0.28 and equals to 32 J/(kg K) for the magnetic field change of 5 T. The adiabatic temperature change for the same sample, measured using magnetocalorimeter, is equal to 1.2 K for the magnetic field change of 1.7 T. It was found that the increase of Ge content in the sample causes weakening of first order magnetic transition, which is manifested by the lowering difference in transition temperature measured in two zero-field-cooling and field-cooled-cooling regimes.

Structural properties, magnetization and magnetocaloric effect measurements in the compounds based on lanthanum-strontium-bismuth manganites La0.7Sr0.3-xBixMnO3 (LSBiMO) with 0.05 <= x <= 0.30 have been performed. The valence states of elements entering into the composition of investigated LSBiMO samples were analyzed for x = 0.15 and 0.3 using the HR XPS spectra of Bi 4f, La 3d, Mn 3s and Sr 3d. According to X-ray powder diffraction data the crystalline structure of the samples is perovskite-like with a small rhombohedral distortion. The a and c lattice parameters and unit cell volume V are established to increase with increasing Bi content. All the manganites show ferromagnetic-like ordering with second order phase transition to paramagnetic state. The Curie temperature and magnetization decrease at substitution of Sr2+ for Bi3+ ions. The magnetic entropy changes were calculated using the field dependence of isothermal magnetization in the terms of the thermodynamic Maxwell relation. The maximum magnetic entropy change value was shown to be 0.94 J kg(-1) K-1 for x = 0.05 for a field change of 1 T. (C) 2015 Elsevier B.V. All rights reserved.

The effect of the Sr doping on electronic structure in single crystals of (La(1-x)Sr(x))(Ga(1-y)Mn(y))O3 solid solutions (LSGM) is investigated by means of electron magnetic resonance (EMR). The EMR results are supplemented by magnetic susceptibility and optical spectroscopy measurements. The compositions with small concentration of Mn doping (y<1%) and overdoped content of Sr (the ratio x(Sr)/y(Mn) up to 8) are used to maximally enhance the role of divalent doping. The experimental results provide evidence of the holes delocalization in the overdoped compound (x(Sr)/y(Mn)>1). This delocalization is accompanied by appearance of the new charge transfer transitions in the optical spectrum and dynamical valence change of manganese atoms. Additionally we observe the thermally activated narrowing of resonance EMR lines due to the internal motion, which is characterized by the energy barrier depending strongly on the ratio x(Sr)/y(Mn). The energy barrier is found to be associated with the charge carrier (hole) self-trapped energy. Fitting the EMR spectra in three orthogonal planes to an orthorhombic spin Hamiltonian enables extracting the zero-field splitting (ZFS) parameters and the Zeeman g-factors for Mn(4+) (S=3/2) ions in LSGM. The experimental ZFS parameters are modeled using superposition model analysis based on an orthorhombic symmetry approximation.

Magnetic and Structural Study of Mn1.15Fe0.85P1−xGex (0.25 < x < 0.32) Magnetocaloric Compounds Prepared by Arc Melting L. Hawelek, P. Wlodarczyk, P. Zackiewicz, M. Polak, M. Kaminska, R. Pu1niak, I. Radelytskyi and A. Kolano-Burian Institute of Non-Ferrous Metals, J. Sowi«skiego 5, 44-100 Gliwice, Poland Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warszawa, Poland

The frequency, temperature, and the dc and ac dependences of the magnetic susceptibility of the YbCoGaO4 and LuCoGaO4 single crystals are investigated. The YbCoGaO4 behaves like an Ising spin glass with a strong uniaxial anisotropy. In contrast LuCoGaO4 is Heisenberg-like spin glass. Dynamical scaling reveals a three dimensional phase transition near Tg and yields critical exponent values between those of Heisenberg- and Ising-like systems.

The magnetic properties and magnetocaloric effect of single crystals RCoGaO4 (R = Lu, Yb) layered cobaltites were investigated. The strong anisotropy of magnetic susceptibility for YbCoGaO4 crystal confirms its Ising spin glass behavior while in LuCoGaO4 crystal Heisenberg-like spin glass phase is observed.

The Mn2-xFexP0.5As0.5 compounds (x = 0.7 and 1.0) studied exhibit the magnetic phase transitions, which are accompanied by a magnetic entropy change. For x = 1 the PM FM transition is of the first order one with a weak (2-3 K) thermal hysteresis in the vicinity of T-C = 275 K. The Mn1.3Fe0.7P0.5As0.5 compound possesses two magnetic transitions: the second-order PM FM transition at T-C = 190 K, followed by the FM-AFM transition at T-N = 90 K, leading to normal and inverse magnetocaloric effects, respectively. The maximum values of magnetic entropy change are equal to 17 J kg(-1) K-1 in MnFeP0.5As0.5 and 5 J kg(-1) K-1 in Mn1.3Fe0.7P0.5As0.5 for a field change of 5 T. The magnetic entropy changes were calculated using both the isofield magnetization curves versus temperature and the isothermal magnetization curves versus applied magnetic field. The magnetocaloric effect in MnFeAs0.5P0.5 is discussed in the terms of both the thermodynamic Maxwell relation and the Clausius-Clapeyron equation. (C) 2014 Elsevier Masson SAS. All rights reserved.