Magnetic and magnetocaloric results of magnetic field-induced transitions in La1−xCexMn2Si2 (x=0.35 and 0.45) compounds

The La_1−xCe_xMn₂Si₂ compounds (x=0.35 and 0.45) exhibit an antiferromagnetic-ferromagnetic transition caused by the changes in distance between Mn atoms due to temperature changes. A field-induced transition from antiferromagnetic state to ferromagnetic state at a critical field, which decreases with increase in temperature, can also be induced by applying a magnetic field. In this paper our aim is to study the magnetization and magnetocaloric effect, close to transition temperatures. Our subsidiary aim is to examine the temperature dependence of critical field and ferromagnetic fraction of compounds. The variation of magnetocaloric effect with temperature is correlated with the ferromagnetic-antiferromagnetic phase coexistence. Our final aim is to examine the harmony between magnetocaloric effect values calculated both by the Maxwell theory and by the Landau theory.     

Magnetic-field-induced transition from metastable spin glass to possible antiferromagnetic-ferromagnetic phase separation in Cd0.5Cu0.5Cr2O4

Using ac susceptibility, dc magnetization and heat-capacity measurements, we have investigated the magnetic properties of Cd_0.5Cu_0.5Cr₂O₄. Cd_0.5Cu_0.5Cr₂O₄ has an extraordinary magnetic phase including a metastable spin-glass (SG) phase at zero field, a possible phase separation scenario of AFM/FM above ∼0.5 T field, and at intermediate fields, an apparent pseudo reentrant spin-glass (RSG) plateau is observed. These phenomena are closely correlated with the pinning effect of the Cu²⁺ sublattice on the frustrated lattice.     

Spontaneous and field-induced magnetic transitions in YBaCo2O5.5

A detailed study of magnetic properties of cobaltite YBaCo₂O_5.5 has been performed in high (up to 35 T) magnetic fields and under hydrostatic pressure up to 0.8 GPa. The temperatures of paramagnet-ferromagnet (PM-FM) and ferromagnet-antiferromagnet (FM-AF) phase transitions and their pressure derivatives have been determined. It has been revealed that in the compound with yttrium, in contrast to those with magnetic rare earth atoms, the AF-FM field-induced magnetic phase transition is accompanied by a considerable field hysteresis below 240 K, and the magnetic field of 35 T is not sufficient to complete this transition at low temperatures. The hysteresis value depends on the magnetic field sweep rate, which considered as an evidence of magnetic viscosity that is especially strong in the region of coexistence of the FM and AF phases. High values of susceptibility for the field-induced FM phase show that Co spin state in these compounds changes in strong magnetic field.     

Electrical and magnetic properties of TmCoIn5 and YbCoIn5 single crystals

Electrical and magnetic properties of TmCoIn₅ and YbCoIn₅ single crystals were investigated by means of electrical resistivity and magnetization measurements in the temperature range from 300 to 0.5 K under the magnetic field up to 5 T. TmCoIn₅ is an antiferromagnetic metal with a Néel temperature T_N=2.6 K. YbCoIn₅ shows non-magnetic behavior, reflecting of divalent Yb ion.     

Influence of partial rare-earth substitution for Sm in the giant intrinsic magnetic hardness compound SmCo2Ni3

The phenomenon of giant intrinsic magnetic hardness is investigated in compounds R_1−x Sm _x Co₂Ni₃ with R=Y, Pr, Gd, Tb, Er. Partial Er substitution for Sm actually increases magnetic hardness while all other substitutions decrease magnetic hardness. The strength of coercivity is thus dependent on both the sign and magnitude of the crystal field interaction. The temperature dependence of coercivity is complex in the case of Pr substitution as a result of competing effects from thermal activation and a decrease in anisotropy at low temperatures. This study has been supported by a grant from the National Science Foundation.     

Magnetic,dielectric and magnetoelectric properties of new family of orthorhombic multiferroic Eu1−xYxMNO3 manganites

Multiferroic ground states with a spatially modulated antiferromagnetic structure and electric polarization have been revealed in Eu_1−xY_xMnO₃ (0.2?x?0.5$0.2?x?0.5$) single crystals. While the slightly substituted (x?0.1$x?0.1$) compounds exhibited a transition from the incommensurate (IC) to the canted antiferromagnetic (CAF) state at _TCA<_TN$T_{\mathrm{CA}}<T_{\mathrm{N}}$, the transitions from IC to commensurate ferroelectric (C/FE) phase were observed at _Tlock<_TN$T_{\mathrm{lock}}<T_{\mathrm{N}}$ for x>0.2$x>0.2$. Various phase transitions were observed in the magnetic fields up to 250 kOe along a, b, c axes by magnetization, magnetostriction and electric polarization measurements which show an existence of a spontaneous electric polarization below T_lock.     

EPR and magnetic properties of MgO-CaO-SiO2-P2O5-CaF2-Fe2O3 glass-ceramics

Glass-ceramics have been derived from 4.5MgO(45−x)CaO34SiO₂16P₂O₅0.5CaF₂xFe₂O₃ (x=5, 10, 15, 20 wt%) glasses by heat treatment. Room temperature electron paramagnetic resonance (EPR) spectra and temperature-dependent magnetic susceptibility (χ) of the glass-ceramics have been obtained. The EPR absorption line centered at g≈4.3 disappeared at higher concentrations of iron oxide. The intensity and line width of the EPR absorption line centered at g≈2.1 increased as the iron oxide concentration was increased. Temperature-dependent magnetization of samples with low iron oxide content revealed ferrimagnetic as well as paramagnetic contributions. Information about the structural changes involving iron ions, their valence state and the type of magnetic interactions between the Fe ions as a function of composition was obtained using EPR and χ studies.     

Magnetic behavior of nanocrystalline LaMn2Ge2

The compound, LaMn₂Ge₂, crystallizing in ThCr₂Si₂-type tetragonal crystal structure, has been known to undergo ferromagnetic order below (T_C=) 326 K. In this article, we report the magnetic behavior of nanocrystalline form of this compound, obtained by high-energy ball milling. T_C of this compound is reduced marginally for the nanoform, whereas there is a significant reduction of the magnitude of the saturation magnetic moment with increasing milling time. The coercive field however increases with decreasing particle size. Thus, this work provides a route to tune these parameters by reducing the particle size in this ternary family.     

Magnetic transitions in layered triangular antiferromagnets

Magnetic states and phase transitions of the layered triangular antiferromagnets in an applied field are studied. It is shown that in compounds like VBr₂ and VCl₂ quantum effects change the ground-state structure and cause successive phase transitions as the magnetic field increases. Coplanar structures of different spin configuration are realized far from the saturation field and a noncoplanar structure of umbrella-type configuration is realized near this field. The ground-state phase diagram is constructed, and a finite region of fields where the collinear phase is also possible is indicated.     

Anisotropic Applied Field Dependency of Two Successive Magnetic Transitions in LiCu2O2

The anisotropy of two successive transitions of the spin-ladder compound LiCu2O2 is studied by the specific heat （ C） under magnetic fields with H / / c and H / / ab （written as H^c and H^ab in the following） up to 14 T. The peak of specific heat at 24.5 K in zero field shifts to lower temperature when the field is increased and the magnitude of the peak is suppressed by the field. On the contrary, the peak of 22.5 K shifts to higher temperature, especially at 14 T. Its magnitude increases in the field of H^c, whereas it decreases in the field of H^ab. We calculate the entropy change between 21 K and 63 K. The different influence of the spin ordering by fields of different direction is obtained. Our experimental results suggest a mixed state between the long range incommensurate helimagnetic ground state and the higher-T short-range dimer liquid state. The temperature range of mixed state is shrunk with the increasing field. Possible mechanism is discussed.     

Effects of sintering temperature and atmosphere on magnetism in Mn-doped TiO2 bulk samples

Magnetic properties have been investigated on Mn doped TiO₂(Ti_0.98Mn_0.02O₂) bulk samples prepared by solid state reaction, which were sintered at different temperature ranging from 450 ^°C to 900 ^°C in air and argon atmosphere, respectively. The results show that the magnetic properties were strongly dependent on the sintering temperature and atmosphere. For samples sintered in air, the magnetization initially increase with the increase of sintering temperature up to 600 ^°C and thereafter it decrease. While the magnetization of samples sintered in argon atmosphere decreases monotonically with the increase of sintering temperature. Furthermore, for samples sintered at 600 ^°C in air, the magnetic susceptibility exhibits a dominant Curie-Weiss behaviour and no magnetic transition is observed over the temperature range from 10 to 300 K. In contrast, for samples sintered in argon atmosphere, besides the magnetic transition near 45 K perhaps caused by Mn₃O₄, another magnetic transition appears near room temperature.     

Magnetic anisotropy-induced spin-reorientation in spinel FeCr2S4

In this paper, the low-field magnetic behavior of polycrystalline FeCr₂S₄ was investigated by thermal circling the sample from 5 to 200 K at a constant magnetic field. We show that, upon cooling the sample at 50 Oe to a temperature below the Curie temperature then warming back, the magnetization displays irreversibility between cooling and warming sequence. The irreversible behavior was suppressed gradually with increasing magnetic field. By considering spin-reorientation due to the increase of magnetic anisotropy upon cooling, the irreversible behavior has been explained qualitatively.     

Structural and magnetic properties of SmCo5.6Ti0.4 alloy

Three series of SmCo_5.6Ti_0.4 samples were prepared by quenching, melt spinning, and ball milling, respectively. Annealing at different temperatures was carried out for the three series. The influence of the processing routes on the structural and magnetic properties was systematically investigated for this alloy. The as-quenched bulk sample consisted of three phases with a rather coarse grain microstructure. Low intrinsic coercivity (_iH_c) of 0.12 T was obtained in this sample. While the as-spun ribbons and as-milled/annealed powders showed the CaCu₅-type phase (1:5) plus Th₂Zn₁₇-type phase (2:17), and the 1:5 phase plus TbCu₇-type phase (1:7), respectively, with nanograin microstructure. The _iH_c of as-spun ribbons and as-milled/annealed (700 °C for 2 h) powders was found to be 0.59 and 2.23 T, respectively. Coercivity mechanism of these as-spun ribbons is mainly of nucleation type. In the as-milled/annealed powders, the network of the nanograin boundaries is believed to provide strong pinning sites for the domain wall movement.     

Metastable states in phase separated electron doped Sm0.15Ca0.85MnO3

In order to study the mechanism behind the phase separation scenario in the Sm_0.15Ca_0.85MnO₃ compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K<T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM (P2₁/m) phase resulting in field-induced insulator-metal transition, which is irreversible below T₁=75 K. In zero field the content of a G-type phase in the mixed C-G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T₁ depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.     

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Inhomogeneous ferrimagnetic-like behavior in Gd2/3Ca1/3MnO3 single crystals

We present a study of the magnetic properties of Gd_2/3Ca_1/3MnO₃ single crystals at low temperatures, showing that this material behaves as an inhomogeneous ferrimagnet. In addition to small saturation magnetization at 5 K, we have found history dependent effects in the magnetization and the presence of exchange bias. These features are compatible with microscopic phase separation in the clean Gd_2/3Ca_1/3MnO₃ system studied.     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.     

Structural characterization and magnetization of Mg0.7Zn0.3SmxFe2−xO4 ferrites

Mg_0.7Zn_0.3Sm_xFe_2−xO₄ ferrites were prepared by the solid-state reaction method and were characterized by X-ray diffraction and magnetization measurements. A single spinel phase was obtained in the range 0.00?x?0.03$0.00?x?0.03$. The lattice parameter was found to increase at x=0.01$x=0.01$ and then decreases up to x=0.03$x=0.03$, which may indicate a distortion in the spinel lattice. The saturation magnetization was found to decrease with the increase in x up to 0.04, due to the replacement of the Fe³⁺ ions by the Sm³⁺ ions.     

Yafet-Kittel-type magnetic ordering in Ni0.35Zn0.65Fe2O4 ferrite detected by magnetosensitive microwave absorption measurements

Magnetosensitive microwave absorption measurements of polycrystalline ferrite Ni_0.35Zn_0.65Fe₂O₄ was carried out at 9.4 GHz (X-band) as a function of temperature. Temperature dependence of the total linewidth (ΔH_pp) deduced from the resonance spectra showed the passage through the Curie point (T_c~430 K). Additionally, the plot ΔH_pp vs. T also indicated the existence of another magnetic phase transition at ~240 K, which can be associated with a Yafet-Kittel-type canting of the magnetic moments. Low-field microwave absorption (LFMA) and the magnetically modulated microwave absorption spectroscopy (MAMMAS) were used to give a further knowledge on this material. For low temperature, these techniques give evidence of a Yafet-Kittel-type canting of the magnetic moments.     

Magnetic and structural study of Cu-doped TiO2 thin films

Transparent pure and Cu-doped (2.5, 5 and 10 at.%) anatase TiO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. The samples were structurally characterized by X-ray absorption spectroscopy and X-ray diffraction. The magnetic properties were measured using a SQUID. All films have a FM-like behaviour. In the case of the Cu-doped samples, the magnetic cycles are almost independent of the Cu concentration. Cu atoms are forming CuO and/or substituting Ti in TiO₂. The thermal treatment in air promotes the CuO segregation. Since CuO is antiferromagnetic, the magnetic signals present in the films could be assigned to Cu substitutionally replacing cations in TiO₂.