Magnetic properties of PrCo2 and its ternary hydride PrCo2H4

Magnetization and susceptibility data on PrCo₂ and PrCo₂H₄ are presented. The ac susceptibility of PrCo₂ measured in zero dc field displays a sharp and high peak at T_c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T_c, indicate that PrCo₂ orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ_B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T.Similar experiments on PrCo₂H₄ provide evidence that the introduction of hydrogen in PrCo₂ not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo₂H₄ is found to have decomposed into PrH₂ and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Néel model for superparamagnetic particles with randomly oriented local anisotropy axes.     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Observation of mixed-phase behavior in the Mn-doped cobaltite La0.7Sr0.3Co1−xMnxO3 (x=0-0.5)

Studies on La_0.7Sr_0.3Co_1−xMn_xO₃ (x=0-0.5) compounds evidence that the interaction between Mn and Co ions in this system is antiferromagnetic super-exchange and not ferromagnetic (FM) double-exchange (DE). As a result, antiferromagnetism and magnetic glassiness develop steadily with increasing Mn content and the system becomes a spin glass at x∼0.1. Analyses of high-field magnetization data indicate that the system consists of two major phases: a metallic FM phase which magnetically saturates in rather low field, and an insulating non-FM phase which has a linear dependence of magnetization on magnetic field. In the low doping regime, the fraction of the non-FM component expands with temperature at the expense of the FM phase and becomes maximal at T_C. Ferromagnetism reappears in highly doped (x≥0.2) compounds due to the presence of DE interaction between the Mn ions. The small volume fraction of the FM phase derived from the M(H) data in high-field region supports the coexistence of insulating and FM behaviors in the highly doped samples.     

Uncompensated magnetization and exchange-bias field in La0.7Sr0.3MnO3/YMnO3 bilayers: The influence of the ferromagnetic layer

We studied the magnetic behavior of bilayers of multiferroic and nominally antiferromagnetic o-YMnO₃ (375 nm thick) and ferromagnetic La_0.7Sr_0.3MnO₃ and La_0.67Ca_0.33MnO₃ (8…225 nm), in particular the vertical magnetization shift M_E and exchange-bias field H_E for different thickness and magnetic dilutions of the ferromagnetic layer at different temperatures and cooling fields. We have found very large M_E shifts equivalent to up to 100% of the saturation value of the o-YMO layer alone. The overall behavior, including XMCD magnetization shift measured at the Mn-L edge of the LSMO layer only, indicates that the properties of the ferromagnetic layer contribute substantially to the M_E shift and that this does not correlate straightforwardly with the measured exchange-bias field H_E.     

Structural first-order transformation in La2/3Ba1/3MnO3: ESR study

We have studied by the electron-spin resonance (ESR) and static magnetic field techniques, the La_2/3Ba_1/3MnO₃ perovskite, which was previously shown to exhibit a martensitic phase transformation in the vicinity of T_s∼200 K [Physical Review B 68, 054109 (2003)], leading to its structural phase-segregated state. Resonant absorptions reveal that in the temperature interval from 100 K to 340 K the compound represents a mixture of two ferromagnetic phases possessing different magnetizations, in varying proportions depending on the temperature, and a small amount of a paramagnetic phase. The results agree well with the previous neutron diffraction study. Applied in the ESR experiments, magnetic fields (2–6 kOe) strongly affect the magnetization curves: even magnetic field as high as 700 Oe modifies the anomaly in the phase transformation region and removes the difference between the zero-field cooled and field-cooled magnetization curves, which implies that the difference in the magnetic susceptibility of the coexisting phases is small and the magnetic domain configuration can be easily changed.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Magnetic dynamics of charge ordered Nd0.80Na0.20MnO3 compound

The magnetic dynamics of charge ordered Nd_0.8Na_0.2MnO₃ compound was studied by measuring the temperature variation of magnetization for different magnetic fields up to 7 T and, the field variation of magnetization at different temperatures down to 5 K. This sample exhibits a charge-ordering transition at 180 K, followed by a weak ferromagnetic (FM) transition at around 100 K and a spin glass like transition below 40 K. Suppression of charge-ordering and spin glass like transition and increase in FM T_C were observed with an increase in magnetic field. A reversible metamagnetic transition above a threshold field (H_f) of 4.5 T was observed at 130 K, followed by a saturation magnetization of 3.2 μ_B/f.u. However at 5 K, an irreversible field induced first order phase transition from charge ordered state to FM state was observed at H_f=5 T. For comparison, the temperature and field variations of magnetization were studied on a FM compound from the same series with the composition Nd_0.90Na_0.10MnO₃. A clear FM transition with a T_C of 113 K and a saturation magnetization of 4.3 μ_B/f.u was observed.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Ferromagnetism and spin reorientation in Sm12Fe14Al5

The single crystal of the new ternary compound Sm₁₂Fe₁₄Al₅ was grown and its crystallographic and magnetic properties were investigated. Sm₁₂Fe₁₄Al₅ has a hexagonal structure of the space group p-3m1 and shows ferromagnetism with a Curie temperature of 245 K. The easy direction of magnetization is parallel to the c-axis at temperatures between 245 and 85 K; however, it changes to the c-plane below 85 K through a first-order-like phase transition. No saturation is observed in the magnetization curve even under the applied field of 55 kOe at 5 K. Sm₁₂Fe₁₄Al₅ seems to have a large coercive field at very low temperatures. The anisotropy field was estimated at 5 and 120 K and the saturation magnetization of low temperature phase is explained assuming a ferromagnetic coupling between Fe and Sm sublattices.     

The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy

The magnetic properties of the Co₃₈Ni₃₄Al₂₈ alloy have been studied. The alloy exhibits a first order austenite-martensite phase transition in the temperature region between 155 and 247 K. A strain of 0.07% is produced across this phase transition. The Arrott plots obtained from the isothermal magnetic field dependence of magnetization indicate the presence of spontaneous magnetization both in the austenite and martensite phases, confirming the ferromagnetic character of the alloy up to room temperature. The temperature dependence of the high field magnetization indicates the presence of spin wave excitations, spin wave excitation gap and spin wave-spin wave interactions in the martensite phase. The magnetic anisotropy energy constant for the Co₃₈Ni₃₄Al₂₈ alloy is estimated both with the help of the standard law of approach to saturation of magnetization, and also from the field dependence of magnetization using the field for technical saturation of magnetization. The temperature dependences of these energy terms are compared. The estimated values of the magnetic anisotropy constant seem to be in agreement with the magnitude of the spin wave excitation gap estimated from the temperature dependence of high field magnetization.     

Low temperature study of micrometric powder of melted Fe50Mn10Al40 alloy

Melted Fe₅₀Mn₁₀Al₄₀ alloy powder with particle size less than 40 μm was characterized at room temperature by XRD, SEM and XPS; and at low temperatures by Mössbauer spectrometry, ac susceptibility, and magnetization analysis. The results show that the sample is BCC ferromagnetic but with a big contribution of paramagnetic sites, and presents super-paramagnetic and re-entrant spin-glass phases with critical temperatures of 265 and 35 K, respectively. The presence of the different phases detected is due to the disordered character of the sample and the competitive magnetic interactions. The obtained values of the saturation magnetization and the coercive field as a function of temperature present a behavior which indicates a ferromagnetic phase. However, the behavior of the FC curve and that of the coercive field as a function of temperature suggest that the dipolar magnetic interaction between particles contributes to the internal magnetic field in the same way as was reported for nanoparticulate powders.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.     

Effect of thermal treatment on the magnetization processes of nanocrystalline Fe80Ge3Nb10B7 alloys

The dynamic magnetization processes of nanocrystalline Fe₈₀Ge₃Nb₁₀B₇ alloys after annealing at different temperatures are studied through the permeability spectroscopy. Three steps of crystallization are found when amorphous Fe₈₀Ge₃Nb₁₀B₇ alloys are heated from 300to 1200 K. The dominant magnetization process varies with different annealing temperatures. Domain wall bulging is the main magnetization mechanism under weak applied field. When the applied field exceeds pinning field H_p, the depinning-involved domain wall displacement occurs. Different annealing temperature results in different H_p. The lower value of μ′ and high relaxation frequency after heating at 923 and 973 K are due to the strengthened domain wall pinning and the increase of magnetocrystalline anisotropy.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

Magnetic characterization of Mn5SiB2 and Mn5Si3 phases

In this work the Mn₅Si₃ and Mn₅SiB₂ phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn₅Si₃ and near single-phase Mn₅SiB₂ microstructures. The magnetic behavior of the Mn₅Si₃ phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn₅SiB₂ phase shows a ferromagnetic behavior presenting a saturation magnetization M_s of about 5.35×10⁵ A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments.     

Magnetic properties of the novel nanocomposite (Zn0.15Ni0.85Fe2O4)0.15/(SiO2)0.85 at room temperature

The value of the effective magnetic anisotropy constant of the ferrimagnetic nanoparticles Zn_0.15Ni_0.85Fe₂O₄ embedded in a SiO₂ silica matrix, determined through ferromagnetic resonance (FMR), is much higher than the magnetocrystalline anisotropy constant. The higher value of the anisotropy constant is due to the existence of surface anisotropy. However, even if the magnetic anisotropy is high, the ferrimagnetic nanoparticles with a 15% concentration, which are isolated in a SiO₂ matrix, display a superparamagnetic (SPM) behavior at room temperature and at a frequency of the magnetization field equal to 50 Hz. The FMR spectrum of the novel nanocomposite (Zn_0.15Ni_0.85Fe₂O₄)_0.15/(SiO₂)_0.85, recorded at room temperature and a frequency of 9.060 GHz, is observed at a resonance field (B_0r) of 0.2285 T, which is substantially lower than the field corresponding to free electron resonance (ESR) (0.3236 T). Apart from the line corresponding to the resonance of the nanoparticle system, the spectrum also contains an additional weaker line, identified for a resonance field of ∼0.12 T, which is appreciably lower than B_0r. This line was attributed to magnetic ions complex that is in a disordered structure in the layer that has an average thickness of 1.4 nm, this layer being situated on the surface of the Zn_0.15Ni_0.85Fe₂O₄ nanoparticles that have a mean magnetic diameter of 8.9 nm.     

Vacancy-induced spin-glass behavior of Prussian blue analogue Fe1.1Crx[Cr (CN)6]0.6−x·nH2O nanowires

Prussian blue analogue Fe^II_1.1Cr^II_x[Cr^III(CN)₆]_0.6−x·nH₂O nanowires were synthesized by electrodeposition. The magnetic properties investigation indicates that the nanowires exhibit cluster spin-glass behavior, which undergoes a magnetic transition to a frozen state below about 62 K. Spin disorder arising from reduced coordination and broken exchange bonds between spin centers due to the structural defects may be the reason that causes the spin-glass freezing behavior. The negative magnetization observed at temperature lower than the compensation temperature (T_comp∼43 K) at a field of 10 Oe may be due to the different temperature dependences of the ferromagnetic site Fe-Cr and antiferromagnetic site Cr-Cr.     

Phase coexistence in NaZn13-type LaFe11.4Al1.6C0.02 compound

In NaZn₁₃-type LaFe_11.4Al_1.6C_0.02 compound, a signature of weak ferromagnetism is observed at ∼100 K under a low field by ac magnetic-susceptibility and electrical-resistivity measurements, implying the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases. The hysteresis in isofield magnetization curves and large magnetic relaxation demonstrate the metastability of the magnetic state in the AFM-FM transition region. The variations of magnetization with temperature, time and field show distinct step-like behaviors, which is probably attributed to the discontinuous growth of ferromagnetic cluster in antiferromagnetic matrix.     

Influence of sample geometry on determination of magnetocaloric effect for Gd60Co30Al10 glassy ribbons using direct and indirect methods

Rare-earth based metallic glasses with high saturation magnetization show a sizeable magnetocaloric effect (MCE) and are subject of extensive research concerning magnetic refrigeration materials. In this work, the magnetic phase transition from paramagnetic to ferromagnetic of Gd₆₀Co₃₀Al₁₀ metallic glass has been characterized and three different methods were applied for the determination of its magnetocaloric specific parameters: (a) direct measurement of the adiabatic temperature change by exposing the material to an adiabatically applied magnetic field; (b) determination of the magnetization M(H,T) and calculation of the temperature dependent magnetic field induced entropy change ΔS_m by application of the Maxwell relation and (c) measuring the total heat capacity C_p(H,T) in zero and non-zero magnetic field. Gd₆₀Co₃₀Al₁₀ glassy ribbons were prepared by melt spinning, a technique that offers very high cooling rates due to the low dimensionality of the sample. Depending on the particular method of measurement, pieces of these glassy ribbons form samples with different appropriate total volume and dimensions. We show that the combination of the pronounced two-dimensionality of the ribbon pieces (aspect ratio ∼100) together with the very high magnetic permeability principally can cause strong internal demagnetizing fields that cannot be neglected when evaluating the intrinsic MCE parameters obtained from different methods.     

Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.