Magnetic Transition Temperature Tc Research Articles

Understanding theμSR spectra of MnSi without magnetic polarons

Transverse-field muon-spin rotation (μSR) experiments were performed on a single crystal sample of the noncentrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature TC at the 4a muon site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field μSR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse field μSR. In view of the presented evidence we conclude that the μSR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state.5 MoreReceived 2 May 2014DOI:https://doi.org/10.1103/PhysRevB.89.184425©2014 American Physical Society

A comparative study of magnetic behaviors in TbNi2, TbMn2 and TbNi2Mn

All TbNi2, TbMn2, and TbNi2Mn compounds exhibit the cubic Laves phase with AB2-type structure in spite of the fact that the ratio of the Tb to transition-metal components in TbNi2Mn is 1:3. Rietveld refinement indicates that in TbNi2Mn the Mn atoms are distributed on both the A (8a) and B (16d) sites. The values of the lattice constants were measured to be a = 14.348 Å (space group F-43 m), 7.618 Å, and 7.158 Å (space group Fd-3 m) for TbNi2, TbMn2, and TbNi2Mn, respectively. The magnetic transition temperatures TC were found to be TC = 38 K and TC = 148 K for TbNi2 and TbNi2Mn, respectively, while two magnetic phase transitions are detected for TbMn2 at T1 = 20 K and T2 = 49 K. Clear magnetic history effects in a low magnetic field are observed in TbMn2 and TbNi2Mn. The magnetic entropy changes have been obtained.

Magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2as revealed by single-crystal neutron diffraction

The magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu2+ moments along the c-direction is revealed below the magnetic phase transition temperature Tc = 17 K. In addition, the antiferromagnetism of the Fe2+ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe-spin density wave (SDW) order and the structural phase transition are significantly suppressed to Tn = 70 K and Ts = 90 K, respectively, compared to the parent compound EuFe2As2.We present the microscopic evidences for the coexistence of the Eu-ferromagnetism (FM) and the Fe-SDW in the superconducting crystal. The superconductivity (SC) competes with the Fe-SDW in Eu(Fe0.82Co0.18)2As2.Moreover, the comparison between Eu(Fe1-xCox)2As2 and Ba(Fe1-xCox)2As2 indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.

Irreversibility, remanence, and Griffiths phase in Sm0.1Ca0.9MnO3 nanoparticles

Magnetic properties of compacted Sm0.1Ca0.9MnO3 particles with 25 and 60 nm average sizes have been investigated. Particular attention has been paid to Griffiths-like features at temperatures above magnetic transition temperature TC and to the system glassiness at low temperatures. Griffiths-like features in inverse magnetic susceptibility of Sm0.1Ca0.9MnO3 nanoparticles have been linked to the presence of short range ferromagnetically correlated spin clusters above TC. Glassy behavior has been revealed in temperature and frequency dependence of ac-susceptibility, temperature and field dependence of thermoremanent and isothermoremanent magnetization, and time decay of the remanent magnetization. Experiments revealed the major impact of the glassy component on magnetic properties of investigated nanoparticles. The magnetic relaxation associated with glassy features was found to be much more pronounced in smaller particles, where a formation of collective state in an ensemble of phase separated nanoparticles may take place. Thermomagnetic irreversibility found in Sm0.1Ca0.9MnO3 nanoparticles has been linked to martensitic strain effects.

Synthesis and Characterization of Cobalt doped Manganese Oxide Nanoparticles by Chemical Route

Abstract Cobalt doped α-Mn2O3 nanoparticles have been synthesized by chemical route. The structural properties were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The SEM image shows that the nanoclusters having spherical geometry. The optical properties were analyzed by ultraviolet - visible (UV-Vis) and Fourier transform infrared (FTIR) spectroscopy. UV-Vis spectra illustrates that Co doped Mn2O3 nanosystem acquire blue shift from bulk value (2.5 eV). The chemical composition and purity of the samples were examined using energy dispersive analysis of X-ray spectroscopy (EDAX). Magnetic properties were studied using electron paramagnetic resonance (EPR) and Guoy's method. The high magnetic moment of 3.5 μB shows the enhancement in magnetic transition temperature (Tc) of Mn2O3 nanosystem due to the incorporation of cobalt ions.

Magnetocaloric properties of single crystalline YbTiO3 with second order phase transition

Magnetic entropy change and refrigerant capacity in applied magnetic fields up to 5 T have been investigated for the single crystalline YbTiO3. The maximal magnetic entropy changes at the second order magnetic phase transition temperature TC (42 K) are about 2.47 and 5.25 J kg−1 K−1 under 1.5 and 5 T, respectively. The magnetic entropy change is attributed to the sharp magnetization jump, related with anomalies of the lattice parameters just at the Curie temperature. The magnetic entropy change for YbTiO3 can be well described by a phenomenological universal curve behavior. The field dependence of the magnetic entropy change and the relative cooling power was also studied. From the analysis of the relationship between the local exponent n and the critical exponents, we conclude that the critical behavior of YbTiO3 belongs to the three-dimensional Heisenberg class, which was also confirmed by the heat capacity measurement.

Pressure effect on transport properties of NdCo2

Electrical resistivity ρ and thermopower S of the Laves phase compound of NdCo2 has been investigated at temperatures from 2 K to 300 K. ρ has been measured under pressures up to 8 GPa and S has been measured under pressures up to 3 GPa. The magnetic transition temperature TC obtained by ρ measurement decreases with increasing pressure. The temperature Tmim where the thermopower S takes minimum at high temperature region increases linearly with increasing pressure. The high-temperature minimum of S is associated with a sharp peak in density of states related mainly to the Co 3d-electron density. Since the width of an itinerant electronic band depends on the extent of the corresponding overlapping of 3d orbitals, the pressure variation of Tmin can be attributed to the broadening of the peak width of 3d electron density of states.

Structure, Magnetism and Magnetoresistance Effect of ${\rm Cd}_{1-{\rm x}}{\rm Cu}_{\rm x}{\rm Cr}_{2}{\rm S}_{4}$ ($x=0.01$, 0.04, 0.1, 0.2)

A series of Cd1-xCuxCr2S4 (x=0.01, 0.04, 0.1, and 0.2) ceramics have been prepared by a solid state reaction method. The X-ray diffraction patterns suggest that the samples have a cubic spinel phase with a space group Fd3m and a tiny amount of Cr2O3 impurity phase. X-ray photoemission spectroscopy reveals a bivalence of Cu and trivalence of Cr in Cd0.8Cu0.2Cr2S4. The replacement of Cd by Cu leads to a slight shift of magnetic transition temperature TC. The saturation magnetic moment per formula unit at 2 K and effective moments in paramagnetic region decrease with Cu content, which is attributed to the coexistence of low spin state Cr3+ with S=1/2 and high spin state Cr3+ with S=3/2 as deduced from the M-H and M-T measurements. The transport measurements suggest that the Cu2+ doping enhances the carrier density. In contrast, a significant decrease in magnetoresistance is observed upon Cu doping. The magnetoresistance effect is discussed based on the magnetic polaron theory.

Reversible solid-state hydrogen-pump driven by magnetostructural transformation in the prototype system La(Fe,Si)13Hy

In magnetocaloric La(Fe,Si)13 alloys, it is known that hydrogen addition shifts the magnetic transition temperature TC near room temperature. By partial hydrogen desorption, TC can be adjusted precisely towards the working temperature of a magnetic cooling device. In this work, we studied the stability of partially desorbed LaFe11.6Si1.4Hy and show that the large volume difference of ferro- and para-magnetic phases drives the system from a single phase to a stable two-phase configuration. The hydrogen concentration separates on a macroscopic scale. The phase-coexistence is visualized by Kerr Microscopy. Differential scanning calorimetry measurements show that hydrogen can be reversibly recombined and separated again. We explain the separation by the coexistence of a high-volume ferromagnetic and low-volume paramagnetic phase that can be induced either by temperature or other external stimuli. This phenomenon can be applied to material systems that show a coupling of physical and structural transitions.

Curie Temperature of Diluted Magnetic Semiconductors: the Influence of the Antiferromagnetic Exchange Interaction

The coherent potential approximation and mean field approximation are used to calculate the free energy of the coupled carrier – localized spin system in III-V diluted magnetic semiconductors. Thus the magnetic transition temperature Tc can be determined and its dependence on important model parameters. We show that the strong antiferromagnetic superexchange interaction between nearest neighbour sites considerably reduces the Curie temperature.

Structural and Magnetic Properties of Ln2CoMnO6(Ln=Dy and La) Produced by Combustion Synthesis

In this article, the structural and magnetic properties of the Ln2MnCoO6 (Ln=La and Dy) produced by the combustion method are reported. The samples were characterized by X-ray diffraction (XRD) with Rietveld refinement, and their magnetic properties by studying the dependence of the magnetization with the magnetic field and temperature. XRD analysis reveals that the La2MnCoO6 (LCM) consists of a mixture of crystalline orthorhombic, monoclinic, and rhombohedral phases. This mixture might be originated by the mixing of valence states of the Co+2/Mn+4 and Co+3/Mn+3. Dy2CoMnO6 (DCM) presents a mixed composition of hexagonal and orthorhombic structures with the transition metal atoms in mixed valences. Magnetic measurements show that the magnetic transition temperature TC decreases from 150 K to 85 K when the La ions are replaced by Dy. We also observe that the magnetic order changes from a ferromagnetic to a ferrimagnetic when La is replaced by Dy.

Coupled phonons, magnetic excitations, and ferroelectricity in AlFeO3: Raman and first-principles studies

We determine the nature of coupled phonons and magnetic excitations in AlFeO3 using inelastic light scattering from 5 K to 315 K covering a spectral range from 100-2200 cm-1 and complementary first-principles density functional theory-based calculations. A strong spin-phonon coupling and magnetic ordering induced phonon renormalization are evident in (a) anomalous temperature dependence of many modes with frequencies below 850 cm-1, particularly near the magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band positions of high frequency Raman bands between 1100-1800 cm-1, in particular a broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies at ~ 100 K, due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of phonons spectrum and their coupling with spins.

β-Detected NMR Search for Magnetic Phase Separation in Epitaxial GaAs:Mn

Abstract To test for the microscopic magnetic phase separation in the dilute magnetic semiconductor Ga1− x Mn x As sug-gested by low energy muon spin rotation measurements [1] , we present a detailed analysis of the amplitudes of the 8 Li β-detected nuclear magnetic resonance in an epitaxially grown thin film of x = 5.4% Mn doped GaAs on a semi-insulating GaAs substrate with magnetic transition temperature TC =72 K. The spectrum at 100 K corresponds to 73% of the full room temperature amplitude, and at 60 K to about 62%. The 11% loss of signal through the magnetic tran-sition is much smaller than that ∼ 50% found by low energy μ SR [1] , and may be entirely due to an amplitude change intrinsic to GaAs. This lack of evidence for phase separation is, however, consistent with the full volume fraction magnetism found by a second low energy μSR measurement on a different sample using weak transverse field [2] .

Structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3(x = 0.0, 0.1 and 0.2)

We investigate the effect of Erbium doping on the structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3 (x = 0.0, 0.1 and 0.2) powder samples. Our polycrystalline samples were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted orthorhombic system with Pnma space group. All our synthesized samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The magnetic transition temperature TC is found to decrease continuously with increasing Er concentration. It was found that the maximum value of the magnetic entropy change for Pr0.4Er0.2Sr0.4MnO3 sample reached max M

Physical and structural studies of chemical Vapor deposited lacunar neodymium manganite thin films

Lacunar Nd1-xMnO3-δ thin films were successfully deposited by a liquid source metal-organic MOCVD technique on (001) SrTiO3 (STO), LaAlO3 (LAO) and Si substrates. Optimal growth conditions are reported. TEM and X-ray diffraction characterisations reveal that the thin films grown on STO and LAO substrates are single crystalline layers epitaxially grown on the top of the substrates. The relationship between the crystallographic orientation of the films and those of the substrates were determined. Magnetic transition temperatures (Tc) of the as-grown STO films, deduced from Squid Magnetometer measurements, are compared to the corresponding bulk values (typically 100K). Our magnetic measurements also suggest a complicated magnetic behavior close to the one observed in bulk samples: magnetization curves obtained under different applied magnetic fields indicate a possible reverse of the magnetization sign at low temperatures.

Pressure effect on transport and magnetic properties of Nd1-xTbxCo2

The electrical resistivity and magnetization of Laves phase Nd1−xTbxCo2 compounds have been investigated at temperatures from 2 K to 300 K in magnetic fields up to 10 T under pressures up to 8 GPa. The magnetic transition temperature TC increases linearly with increasing Tb concentration x, which indicates that the 4f-3d exchange interaction energy increases linearly with increasing x. The spontaneous magnetization MS at small x decrease linearly, vanishes around x ≈ 0.35 and then increases linearly with increasing x. This behavior suggests that the Tb magnetic moment is antiparallel to that of Nd, while Co moment is almost independent of x.

PHOTOINDUCED MAGNETISM IN THE FERROMAGNETIC SEMICONDUCTORS

We study the enhancement of the magnetic transition temperature Tc due to incident light in ferromagnetic semiconductors such as EuS. The photoexcited carriers mediate an extra ferromagnetic interaction due to the coupling with the localized magnetic moments increasing the Tc. The Hamiltonian consists of a Heisenberg model for the localized moments and an interaction term between the photoexcited carriers and the localized moments. The model predicts a small enhancement of the transition temperature, about a fraction of a degree K, in semi-quantitative agreement with the experiments.

Electrical resistivity and thermopower of Nd1−xTbxCo2 compounds

Electrical resistivity ρ and thermopower S of Nd1−xTbxCo2 Laves phase quasibinary alloys (0 ≤ x ≤ 1) are investigated at temperatures from 2 K to 300 K. The magnetic transition temperature TC, determined from resistivity magnetic anomaly, increases linearly with increasing x. The low temperature thermopower changes its sign from negative to positive at the critical composition where the relative orientation of total magnetization and cobalt 3d moment is changed. We propose that this change is related to the dependence of s-d scattering rate on relative polarization of conduction electrons and cobalt 3d band.

CURIE TEMPERATURE OF THE TWO-BAND MODEL FOR DILUTED MAGNETIC SEMICONDUCTORS IN COHERENT POTENTIAL APPROXIMATION

Coherent potential approximation (CPA) is extended to a two-band model for diluted magnetic semiconductors (DMS). The magnetic transition temperature Tc as a function of magnetic coupling constants, hopping parameters and carrier densities is calculated by minimizing the free energy of the coupled carrier-localized spin system. It is shown that Tc can be substantially raised by considering multiband systems.

Instability of the Fe23B6 phase during thermomechanical treatment

Experimental results that indicate an unusual state detected in the fcc structure of the metastable Fe23B6 phase upon compacting and additional annealing are presented. This state manifests itself macroscopically in the form of the processes of α-Fe dissolution and precipitation without changing the magnetic-transition temperature TC of the mechanically synthesized alloy. The Mossbauer data of the mechanically synthesized Fe23B6 alloy suggest that the nature of this phenomenon is determined by changes in the parameters of the local surrounding of iron atoms in crystallographic position Fe(4a) in the fcc unit cell.