Coexistence Of Different Magnetic Phases Research Articles

Local structure and magnetic properties of a nanocrystalline Mn-rich Cantor alloy thin film down to the atomic scale

The huge atomic heterogeneity of high-entropy materials along with a possibility to unravel the behavior of individual components at the atomic scale suggests a great promise in designing new compositionally complex systems with the desired multi-functionality. Herein, we apply multi-edge X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and X-ray magnetic circular dichroism (XMCD)) to probe the structural, electronic, and magnetic properties of all individual constituents in the single-phase face-centered cubic (fcc)-structured nanocrystalline thin film of Cr20Mn26Fe18Co19Ni17 (at.%) high-entropy alloy on the local scale. The local crystallographic ordering and component-dependent lattice displacements were explored within the reverse Monte Carlo approach applied to EXAFS spectra collected at the K absorption edges of several constituents at room temperature. A homogeneous short-range fcc atomic environment around the absorbers of each type with very similar statistically averaged interatomic distances (2.54–2.55 Å) to their nearest-neighbors and enlarged structural relaxations of Cr atoms were revealed. XANES and XMCD spectra collected at the L2,3 absorption edges of all principal components at low temperature from the oxidized and in situ cleaned surfaces were used to probe the oxidation states, the changes in the electronic structure, and magnetic behavior of all constituents at the surface and in the sub-surface volume of the film. The spin and orbital magnetic moments of Fe, Co, and Ni components were quantitatively evaluated. The presence of magnetic phase transitions and the co-existence of different magnetic phases were uncovered by conventional magnetometry in a broad temperature range.

Influence of Ba–Cr substitution on structural, optical and magnetic properties of nanocrystalline BiFeO3

Bi1−xBaxFe1−yCryO3 (BBFCO: x = 0, y = 0; x = 0.05, y = 0; x = 0, y = 0.05; x = 0.05, y = 0.05; x = 0.1, y = 0; x = 0, y = 0.1; x = 0.1, y = 0.1; x = 0.2, y = 0) nanoparticles were prepared by a simple cost effective ethylene glycol-based sol–gel technique. The calcination temperature was set at 600 °C for 120 min with a heating rate of 10 °C/min. Effects of Ba, Cr dopant on structural, optical and magnetic properties of BiFeO3 (BFO) nanoparticles have been studied. Perovskite structure was confirmed by the metal-oxide absorption bands appeared at 559 cm−1 and 438 cm−1 in Fourier-transform infrared spectroscopy spectra, which are the characteristic bands of FeO6 octahedral groups in perovskite structure. X-ray diffraction patterns confirmed that Barium (Ba) is an effective dopant to suppress the impurity phases in Cr-doped BFO. Phase transitions from rhombohedral to orthorhombic phase in all doped samples were verified by Rietveld refinement and Raman analysis. Rhombohedral, orthorhombic and tetragonal phases coexist in Bi0.9Ba0.1FeO3 and Bi0.8Ba0.2FeO3, which can be ascribed to the serious lattice distortion due to high-level Ba doping. Well-crystalline BBFCO nanoparticles were observed and the particle size distribution of all samples was calculated from transmission electron microscope images. Ba–Cr co-doping resulted in a dramatic reduction in the particle size. Optical absorption spectra indicated that the optical bandgaps of the doped samples are about 2.20 eV. Clear hysteresis loops were observed in all doped BFO, which indicate their ferromagnetic order at room temperature. Gradually enhanced ferromagnetic property was realized in Ba-doped BFO. The magnetization (at 20 kOe) of BFO, BFO-5Ba, BFO-10Ba and BFO-20Ba are 0.24, 0.33, 1.00 and 1.25 emu/g, respectively. The remanent magnetization of them are near zero, 0.08, 0.44 and 0.53 emu/g, respectively. Particular shape of wasp-waist hysteresis loops observed in BFO-5Ba, BFO-5Cr and BFO-5BaCr can be ascribed to the coexistence of different magnetic phases, and the mixture of single domain and superparamagnetic particles.

Structural and magnetic properties of ultra-low density BiFeO3 nanoparticles produced by pulsed laser deposition

Ultra-low-density BiFeO3 nanoparticles have been prepared by pulsed laser deposition and their structure and magnetic properties have been studied. Annealing increases crystallinity and the size of the particles leading to an alteration of magnetic properties, observed from magnetic studies and evaluated using high-resolution transmission electron microscopy, selected area electron diffraction and x-ray diffraction patterns analysis. Transmission electron microscopy results show that the BiFeO3 as-deposited nanoparticles annealed up to 400 °C exhibit a orthorhombic distorted perovskite structure without secondary phase and with diameters varying from 9 nm (as-deposited) to 17 nm (annealed at 400 °C). Magnetic data exhibit exchange bias and magnetic blocking effects at low temperatures and typical superparamagnetic behavior at high temperatures. Meanwhile, the BiFeO3 nanoparticles annealed at 500 °C exhibit a rhombohedrally distorted perovskite structure with typical antiferromagnetic properties and diameter of about 56 nm. The analysis of magnetic relaxation time using the Arrhenius equation suggests a superparamagnetic blocking process of ferromagnetic clusters on the surface of the nanoparticles at low temperature. The magnetic properties are discussed considering the interactions between nanoparticles and the co-existence of different magnetic phases within the nanoparticles: an ordered antiferromagnetic core and ferromagnetic clusters on the surface.

Strong dependence of ferromagnetic phase fraction on divalent doping in (La0.4Pr0.6)1−Ca MnO3 (x = 3/8, 1/3) mixed phase manganites

Abstract We present a comparative study of the effect of divalent doping on structural, magnetic and magneto-transport properties of ( La 0.4 Pr 0.6 ) 1 - x Ca x MnO 3 (x = 3/8, 1/3) mixed phase manganites. Structural analysis, using X-ray and neutron diffraction studies, reveals a similar orthorhombic crystal structure with Pnma space group for both the samples; however, x = 3/8 composition shows higher octahedral distortion. The temperature dependent resistivity shows signature of a first order metal-insulator transition with conduction through percolative paths for both the samples. A broad thermal hysteresis observed in temperature dependent magnetization as well as magneto-transport data indicates coexistence of different magnetic phases like metallic ferromagnet, and insulating charge ordered antiferromagnetic in both the manganites. The variation of magnetization profiles with temperature for these stoichiometrically similar compositions is found to be remarkably different, with the dominance of charge ordered-antiferromagnetic phase in the x = 3/8 composition and ferromagnetism in the x = 1/3 composition. The field dependent magnetization isotherms show significantly higher fraction (≈2 times) of ferromagnetic phase in x = 1/3 composition as compared to x = 3/8 composition. The metal-insulator transition temperature is also found to be higher for composition with x = 1/3 (155 K) than that for x = 3/8 (112 K). In an applied magnetic field of 8 T, the resistivity changes drastically as the charge order melts and gives way to stable ferromagnetic metallic behaviour with higher metal-to-insulator transition temperature. The temperature dependent magnetoresistance shows a significant variation for the samples, with maximum value of around 3 × 10 5 % and 7 × 10 4 % for x = 3/8 and x = 1/3 compositions, respectively. These observations suggest that inspite of a very small variation in divalent doping, the magnetic phase fractions in these two manganites are significantly different.

Spatially limited antiferromagnetic order in a cluster glass compound Tb2Ni0.90Si2.94

In the present study, the synthesis of a new ternary intermetallic compound Tb2Ni0.90Si2.94 has been reported. The detailed studies on structure, static and dynamical magnetic properties of the compound have been investigated by means of powder x-ray diffraction, compositional analysis, dc & ac magnetization, non-equilibrium dynamics, heat capacity and neutron diffraction measurements. The dc & ac magnetic susceptibility reveal that the compound undergoes spin cluster-glass behaviour below 9.9 K. The frequency dependence of the freezing temperature have been analysed on the basis of dynamic scaling laws such as power-law divergence and Vogel-Fulcher law, which further confirm the cluster-glass state formation for the compound. A detailed study on non-equilibrium dynamical behaviour associated with cluster-glass state has been carried out through magnetic relaxation behaviour along with magnetic memory effect in zero-field-cooled (ZFC) as well as field-cooled (FC) conditions and associated aging effect. The zero-field neutron diffraction study reveals the presence of a spatially limited antiferromagnetic phase in addition to the magnetically frustrated cluster-glass state. This result has also been supported through zero-field heat capacity studies. The variation in local electronic environment among the magnetic rare-earth ions caused by the structural disorder associated with Ni/Si ions have been argued to be responsible for the coexistence of different magnetic phases.

Determination of ferromagnetic, superparamagnetic and paramagnetic components of magnetization and the effect of magnesium substitution on structural, magnetic and hyperfine properties of zinc ferrite nanoparticles

Abstract MgxZn1−xFe2O4 nanoparticles (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0) were synthesized by auto combustion method. Structural refinement of the samples using Rietveld method revealed cubic spinel structure and showed migration of Fe3+ ions from octahedral to tetrahedral site with increase in magnesium concentration. TEM analysis showed that Mg doping has no significant influence on average particle size of the samples. The FTIR confirmed spinel structure and shift of ʋ1 and ʋ2 bands with magnesium concentration corroborated cation migration obtained from Rietveld analysis. The simultaneous presence of doublet and collapsed sextet in the Mossbauer spectra is attributed to superparamagnetic or paramagnetic and ferromagnetic particles respectively. The strength of the collapsed sextet increased with the magnesium concentration owing to the enhancement of A-B superexchange interaction. Saturation magnetization is found to be varied with magnesium concentration in accordance with Yafet-Kittel model. The superparamagnetic, ferromagnetic and paramagnetic phases in each sample were resolved by curve fitting the experimental M−H curves which confirm findings of our Mossbauer results. Also the contribution of these phases to saturation magnetization was estimated and the coexistence of different magnetic phases is attributed to distribution of particle size.

Magnetic Phase Transition and Exchange Bias in hbox {Ni}_{45}hbox {Co}_{5}hbox {Mn}_{35.5}hbox {In}_{14.5} Heusler Alloy

Magnetic properties and exchange bias of hbox { Ni}_{45}hbox {Co}_{5}hbox {Mn}_{35.5}hbox {In}_{14.5} ribbons were investigated by vibrating sample magnetometry (VSM) and electron magnetic resonance (EMR). Curie temperatures determined by using VSM and EMR methods are equal to 374 K and 377 K, respectively. Additionally, the EMR measurements revealed the existence of a weak ferromagnetic resonance (FMR) line in the paramagnetic region. The upward deviation of the integral intensity from Curie–Weis low below temperature T^* and above T_mathrm{C} confirms the existence of ferromagnetic phase in this temperature region. Below T_mathrm{C}, the integral intensity does not saturate, which can suggest the coexistence of different magnetic phases at low temperature range. Additional confirmation on the coexistence of magnetic phases in the sample are asymmetric hysteresis loops associated with the spontaneous exchange bias effect. The dependence of EMR signal on cyclic repetition of measurement is also discussed.

Microstructural Investigation, Raman and Magnetic Studies on Chemically Synthesized Nanocrystalline Ni-Doped Gadolinium Oxide (Gd1.90Ni0.10O3−δ)

Nanocrystalline Ni-doped gadolinium oxide (Gd1.90Ni0.10O3−δ, GNO) is synthesized by co-precipitation method. The as-prepared sample is annealed in vacuum at 700°C for 6 h. Analyses of the x-ray diffractogram by Rietveld refinement method, transmission electron microscopy and Raman spectroscopy of GNO recorded at room temperature confirmed the pure crystallographic phase and complete substitution of Ni-ions in Gd2O3 lattice. Magnetization (M) as a function of temperature (T) and magnetic field (H) is measured by a superconducting quantum interference device magnetometer, which suggests the presence of ferromagnetic/antiferromagnetic phases together with a paramagnetic phase. From the M–T curve it can be shown that the ferromagnetic phase dominates over para-/antiferromagnetic phases in the temperature range of 300–100 K, but from 100 K to 50 K, the antiferromagnetic phase dominates over ferro-/paramagnetic phases. Hysteresis loops recorded at different temperatures indicate the presence of weak ferro-/antiferromagnetism, which dominates in the low field region (∼ 4000 Oe), above which magnetization increases linearly. The sharp increase of magnetization in M–T curve observed in the temperature range of 50–5 K confirms the presence of dominating ferromagnetic plus paramagnetic phase over antiferromagnetic part. For the first time a combined formula generated from three-dimensional (3D) spin wave model and Johnston formula is proposed to analyze the coexistence of different magnetic phases in different temperature ranges. Interestingly, the combined formula successfully explains the co-existence of different magnetic phases along with their contribution at different temperatures. The onset of ferromagnetism in Gd1.90Ni0.10O3−δ is explained by oxygen vacancy mediated F-centre exchange (FCE) coupling mechanism.

The structure and magnetic properties of pure single phase BiFeO3 nanoparticles by microwave-assisted sol-gel method

The pure single phase BiFeO3 nanoparticles have been prepared by microwave-assisted sol-gel process and its structure and magnetic properties have been studied. X-ray diffractometer, infrared and transmission electron microscope results show that the synthesized BiFeO3 nanoparticles at the calcining temperature of 450 °C exhibits a rhombohedrally distorted perovskite structure without secondary phase and the size is about 40 nm. Meanwhile, the pure single phase BiFeO3 particles exhibit typical ferromagnetic properties at room temperature and the obvious large exchange bias phenomenon at 60 K. A exchange bias field (HE) of 302 Oe at 60 K for pure single phase BiFeO3 nanoparticles at calcining temperature 450 °C after field cooling at 20 KOe has been observed. The MT curve at H = 100 Oe shows that the FC and ZFC magnetization curves start to differ in the temperature range from 60 K to 300 K revealing spin glass behavior of BiFeO3 nanoparticles at calcining temperature 450 °C, 500 °C and 550 °C. The ferromagnetic behavior is attributed to size of effects, which partially destroy the long-wavelength cycloid spin structure expected in bulk BiFeO3. The exchange bias effect in single crystalline BiFeO3 nanoparticles arises from co-existence of different magnetic phases of an antiferromagnetic core and a ferromagnetic surface. The ferromagnetic, exchange bias effect and spin glass behavior of BiFeO3 nanoparticles are assigned to the uncompensated or canted spins at the ferromagnetic surface and an antiferromagnetic core of BiFeO3 nanoparticles.

Magnetic properties of sulfur-doped graphene

While studying magnetism of d- and f-electron systems has been consistently an active research area in physics, chemistry, and biology, there is an increasing interest in the novel magnetism of p-electron systems, especially in graphene and graphene-derived nanostructures. Bulk graphite is diamagnetic in nature, however, graphene is known to exhibit either a paramagnetic response or weak ferromagnetic ordering. Although many groups have attributed this magnetism in graphene to defects or unintentional magnetic impurities, there is a lack of compelling evidence to pinpoint its origin. To resolve this issue, we systematically studied the influence of entropically necessary intrinsic defects (e.g., vacancies, edges) and extrinsic dopants (e.g., S-dopants) on the magnetic properties of graphene. We found that the saturation magnetization of graphene decreased upon sulfur doping suggesting that S-dopants demagnetize vacancies and edges. Our density functional theory calculations provide evidence for: (i) intrinsic defect demagnetization by the formation of covalent bonds between S-dopant and edges/vacancies concurring with the experimental results, and (ii) a net magnetization from only zig-zag edges, suggesting that the possible contradictory results on graphene magnetism in the literature could stem from different defect-types. Interestingly, we observed peculiar local maxima in the temperature dependent magnetizations that suggest the coexistence of different magnetic phases within the same graphene samples.

Nitrogen-Induced Change of Magnetic Properties in Antiperovskite-Type Carbide: Mn3InC**Supported by the National Natural Science Foundation of China under Grant Nos 91122026 and 51472017.

The effects of N substitution on the magnetic properties of Mn3InC1−xNx (0.0 ≤ x ≤ 0.7) are investigated systematically. Partial substitution of N for C leads to the monotonic reduction in both the Curie temperature TC and saturated magnetization MS. The final results obtained from thermo-magnetic curves demonstrate that Mn3InC1−xNx samples show a magnetic phase transition from a paramagnetic (PM) state to a ferrimagnetic (FIM) state consisting of ferromagnetic (FM) and antiferromagnetic (AFM) components. In addition, there is a competition between the AFM component and the FM component in the FIM state with the change of the N-doped content. Magnetic measurements of Mn3InC at 100 Oe and 5000 Oe indicate the metastability and the coexistence of different magnetic phases at lower temperature. The spans of FIM phase broaden gradually with further N doping. The mechanism for the induction of the complicated magnetic state is still in controversy. However, the results clearly show that the doping at the X site in antiperovskite Mn3AX materials is as useful as that of the A and Mn sites.

The effect of Ca-substitution in La-site on the magnetic properties of La2CoMnO6

Double perovskite La2-xCaxCoMnO6 (x = 0, 0.1, 0.2, 0.5) ceramics samples were synthesized by the standard solid-state reaction method. The crystal structures have been confirmed to be monoclinic perovskite structure with space group of P21/n at room temperature by the powder x-ray diffraction (Cu-Kα). The grains of the samples are relatively dense and distribute in random directions from the scanning electron microscope patterns. Holes are doped into La2CoMnO6 with the substitution of the Ca2+ ions in La-site, and further lead to the valences of Mn ions and Co ions transforming from Mn4+ and Co2+ to Mn3+ and Co3+, respectively. Ferromagnetic coupling of Mn4+-O2–-Co2+ and antiferromagnetic coupling of Mn3+-O2–-Co3+ are coexistent in La2-xCaxCoMnO6 system, which correspond to two transitions existed in field-cooling magnetization versus temperature curves. Curie temperature of La2-xCaxCoMnO6 system decreases with the increasing of Ca-substitution amount. The coexistence of different magnetic phases leads to the exchange bias effect.

BaTiO3–ferrite composites with magnetocapacitance and hard/soft magnetic properties

BaTiO3–ferrite multiphase composites were prepared starting from di-phase mixtures of α-Fe2O3 and BaTiO3 powders. During the sintering step, the formation of small amounts of secondary phases with multifunctional character as BaFe12O19 or Ba12Ti28Fe15O84 was promoted. The resulting multiphase ceramic compounds show interesting dielectric, magnetic and small magnetocapacitance effect at low temperature. The coexistence of different magnetic phases with contrasting coercivities (hard/soft) was detected by the presence of ‘wasp-waisted’ M(H) hysteresis loops and first-order reversal curve analysis. The present approach demonstrates that active materials can be realised by controlling in situ reactions at the interfaces in ferroelectric–magnetic oxide composites.

Magnetic and transport properties in thin film of Fe2CrAl

Preliminary results on magnetic and transport properties of a thin film of disordered Fe2CrAl deposited on (111) Si by pulsed laser deposition is reported in this study. While the bulk arc melted ingot used as target for the deposition is purely ferromagnetic, the film shows the co-existence of different magnetic phases, which arises due to the granular nature of the film. This is because, although the average composition pertains to Fe2CrAl, local disorder results in the formation of grains with different magnetic states that may be ferromagnetic or may consist of magnetic clusters. The resulting magnetic inhomogeneity of the thin film leads to large values of negative temperature coefficient of resistance of ∼22%/K at 260 K.

Magnetic Nonuniformity and Thermal Hysteresis of Magnetism in a Manganite Thin Film

We measured the chemical and magnetic depth profiles of a single crystalline (La(1-x)Pr(x))(1-y)Ca(y)MnO(3-δ) (x=0.52±0.05, y=0.23±0.04, δ=0.14±0.10) film grown on a NdGaO(3) substrate using x-ray reflectometry, electron microscopy, electron energy-loss spectroscopy, and polarized neutron reflectometry. Our data indicate that the film exhibits coexistence of different magnetic phases as a function of depth. The magnetic depth profile is correlated with a variation of chemical composition with depth. The thermal hysteresis of ferromagnetic order in the film suggests a first-order ferromagnetic transition at low temperatures.

Metastability across the antiferromagnetic–ferromagnetic intermediate phase transition and enhanced giant magnetoresistance in Zn-doped antiperovskite compounds Ga1−xZnxCMn3

We report the detailed investigations of the magnetic properties, electronic transport, and specific heat in Zn-doped antiperovskite compounds Ga1−xZnxCMn3. Magnetic measurements indicate the metastability of the antiferromagnetic (AFM)–ferromagnetic intermediate (FI) phase transition and the coexistence of different magnetic phases at lower temperature. The enhanced giant magnetoresistance (GMR) is observed with a maximum value of 70% spanning a temperature range of 100 K at 50 kOe. The analysis of heat capacity reveals that the metastability of the AFM–FI transition and the enhanced GMR observed in Ga1−xZnxCMn3 may originate from the reconstruction of Fermi surface accompanying AFM–FI transition.

Electric and magnetic modulation of fully strained dead layers inLa0.67Sr0.33MnO3films

We report the electrical transport and magnetic properties of the dead layer of La(0.67)Sr(0.33)MnO(3) grown on LaAlO(3) substrate under the influence of magnetic field and electric voltage. The electrical resistance of the dead layer shows exponential decrease with both magnetic field and electric voltage, leading to colossal magnetoresistance and electroresistance, respectively. However, the sample cannot be driven into a metallic state with the available magnetic field and electric voltage. At low temperatures, the magnetic-field dependence of both magnetization and resistance show remarkable hysteresis. Exchange bias effect was observed in the magnetization vs magnetic-field curves. Magnetic force microscope measurement reveals the coexistence of different magnetic phases in the dead layer. The results were discussed in terms of phase separation in the dead layer. This work demonstrates the presence of phase separation in the manganite dead layer and its tunability by magnetic field and electric voltage.

EMR study of magnetic inhomogeneities in crystalline bulk and nanometer-sized La 0.7Ca 0.3MnO 3

Electron magnetic resonance (EMR) was studied on crystalline bulk and nanometer-sized samples of the same La 0.7Ca 0.3MnO 3 manganite compound aiming to compare their magnetic homogeneities. The results obtained show that single-crystalline bulk sample is less homogeneous than the nanocrystalline one. Except for higher homogeneity, the nanometer-sized sample also demonstrates a weaker magnetic anisotropy. As a result, well-pronounced coexistence of different magnetic phases (coexistence of ferromagnetic and paramagnetic resonance signals) is observed in the bulk sample, while it is absent in the nanocrystalline one. It is suggested that such strong difference is directly connected to the difference in structural state of the samples.

Magnetic inhomogeneities in crystalline bulk and nanometer sized La0.7Ca0.3MnO3: ESR probing

AbstractElectron spin resonance was studied on three (two bulk and one nanometer sized) samples of the same La0.7Ca0.3MnO3 manganite compound aiming to compare their magnetic homogeneities. The results obtained show that both ceramic and single crystalline bulk samples are less homogeneous than nano‐crystalline one. Except for higher homogeneity, the nano sample demonstrates also a weaker magnetic anisotropy. As a result, well pronounced coexistence of different magnetic phases (coexistence of ferromagnetic and paramagnetic resonance signals) is observed in bulk samples, while it is absent in nano. It is suggested that such strong difference is directly connected to the difference in structural state of the samples. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inhomogeneous magnetic states in the Nd(Mn1−x Crx)O3 system

The crystal structure and magnetic properties of the Nd(Mn−xCrx)O3 system (x≤0.85) have been studied. Substitution of chromium for manganese was shown to induce a transition from the antiferromagnetic to ferromagnetic state (x≈0.2) and a decrease in the critical temperature followed, conversely, by an increase in the Neel temperature and decay of spontaneous magnetization. At low temperatures, the magnetization was found to behave anomalously as a result of magnetic interaction between the ferromagnetic and antiferromagnetic phases. The formation of the ferromagnetic phase is attributed to destruction of cooperative static orbital ordering, while the coexistence of different magnetic phases is most probably due to internal chemical inhomogeneity of the solid solutions.