Inhomogeneous Magnetic State Research Articles

Coexisting magnetic interaction, critical behavior and magnetocaloric effect at high temperature in Fe50Ni50 soft ferromagnetic alloy

Herein, we report the structural, magnetic and magnetocaloric properties of Fe50Ni50 alloy prepared through the planetary ball-mill. Rietveld analysis of the X-ray profiles confirmed the cubic crystallographic phase with the Fm-3 m space group. The magnetic characterization demonstrates that Fe50Ni50 alloy exhibits the coexistence of magnetic interaction within the ferromagnetic phase with Curie temperature 762 K. The Arrott plot results confirmed the second-order nature of the alloy. Critical exponents were also determined using the modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis. Obtained results indicated coexisting magnetic behavior, i.e., long-range ferromagnetism and inhomogeneous magnetic state. This alloy also noted a significant magnetocaloric parameters, i.e., magnetic entropy change (−ΔSM ∼ 2.3 J/kg-K) and relative cooling power (RCP ∼ 230 J/kg) at 30 kOe applied field. Both parameters followed power law dependence of magnetic field, and the exponent value supports the ferromagnetic nature along with the coexisting magnetic state. By normalizing the ΔSM curve with rescaled temperature, a universal master curve is also constructed for this alloy, reiterating the second-order nature of the magnetic transition.

Low field-cooled induced large exchange bias effect and DFT calculations in ferromagnetic Sm2CoMnO6

In the present report, we study the large exchange bias effect in Sm2CoMnO6 (SCMO) polycrystalline samples synthesized with the presence of two crystallographic phases: ordered-phase (monoclinic; P21/n) and disordered-phase (orthorhombic; Pnma). X-ray photoelectron spectroscopy study revealed the presence of mixed valence states for Co (2+ & 3+) and Mn (4+ & 3+). M(T) data exhibits an inhomogeneous magnetic state with the presence of ferromagnetic ordering at TC ∼128 K due to the super-exchange interactions of Co2+-O2--Mn4+ and antiferromagnetic-like spin correlations for T < 50 K, attributed to Co3+-O2--Co3+, and Mn3+-O2--Mn3+ interactions. M(H) loop shift with a conventional exchange bias (EB) effect of 10 kOe for a field-cooled (HFC) of 10 kOe at 2 K was observed. Such a large value of the EB effect for low HFC in SCMO is comparable to that of large EB compounds, such as La1·5Sr0·5CoMnO6 and NiFe2O4/CoO nanocomposites. The zero-field cooled asymmetry in M(H) loop termed as spontaneous exchange bias effect (SEB) observed for T < 20 K. The systematic study of EB effects like HEB and MEB with T(K) and HFC was explained qualitatively by the presence of unidirectional anisotropy formed at the interface of inhomogeneous magnetic phases. Further, density functional theory (DFT) calculations validate the ferromagnetic ground state of SCMO with Co and Mn networks. Moreover, the semiconductor characteristics of SCMO are established with a band gap of 1.3 eV.

Grain size effect on the critical behavior of the ferromagnetic transition in electronically phase separated La0.5Ca0.5MnO3

Mixed valence manganese oxides (manganites) are known to exhibit intrinsic inhomogeneous magnetic states, which are influenced by a great variety of factors, including oxygen stoichiometry, nanostructuration and ceramic grain size. In this context, the prototypical half-doped compound La0.5Ca0.5MnO3was extensively studied. Here we reloaded the phase separated scenario, focusing on the thermodynamical critical behavior of this system at the paramagnetic - ferromagnetic transition and its correlation with the evolution of its ordering capabilities controlled through the ceramic grain size.Our findings establish a clear connection between the correlation length and the grain size, shedding light on the role of this parameter in the control of inhomogeneous magnetic ground states in manganites. Therefore it is possible to tailor fundamental physical properties as the scaling laws of the system just tuning the ceramic grain size.

Crystal structure, piezoelectric and magnetic properties of solid solutions BiMn1-xFexO3 (x ≤ 0.4)

Сrystal structure, piezoelectric and magnetic properties of solid solutions BiMn1-xFexO3 (x ≤ 0.4) prepared by solid-phase reactions from a stoichiometric mixture of simple oxides at high pressures and temperatures have been studied. The structure of the compounds is characterized by the concentration driven phase transition from the monoclinic structure to the orthorhombic structure at x ≈ 0.2; wherein the ordering dz2 of the orbitals of Mn3+ ions is destroyed, and the inhomogeneous magnetic state is stabilized. Solid solutions with 0.2 ≤ x ≤ 0.4 are characterized by a nonzero piezoelectric response, wherein both ferroelectric and magnetic domain structures exist, the ferroelectric switching voltage decreases with an increase of iron ions concentration, while the residual magnetization value decreases. The maximum value of the piezoresponse signal is observed in the compound BiMn0.7Fe0.3O3. The work clarifies the relationship between the chemical composition, the crystal structure, piezoelectric and magnetic properties of solid solutions BiMn1-xFexO3. The presence of both magnetic and electric dipole ordering indicates the perspectives for the practical usage of such materials.

Oxygen deficiency in Sr2FeO4-x: electrochemical control and impact on magnetic properties.

The oxygen-deficient system Sr2FeO4-x was explored by heating the stoichiometric Fe4+ oxide Sr2FeO4 in well-defined oxygen partial pressures which were controlled electrochemically by solid-state electrolyte coulometry. Samples with x up to about 0.2 were obtained by this route. X-ray diffraction analysis reveals that the K2NiF4-type crystal structure (space group I4/mmm) of the parent compound is retained. The lattice parameter a slightly decreases while the c-parameter increases with increasing x, which is in contrast to the Ruddlesden-Popper system Sr3Fe2O7-x and suggests removal of oxygen atoms from FeO2 lattice planes. The magnetic properties were studied by magnetization, 57Fe Mössbauer, and powder neutron diffraction experiments. The results suggest that extraction of oxygen atoms from the lattice progressively changes the elliptical spiral spin ordering of the parent compound to an inhomogeneous magnetic state with coexistence of long-range ordered regions adopting a circular spin spiral and smaller magnetic clusters.

Magnetism and temperature dependence of nano-TiO2: Fe EPR spectra

Nanocrystalline TiO2 samples of various compositions (up to 5 at.% Fe) with anatase structure and an average particle size of about 20 nm were synthesized using sol-gel technology. The magnetic properties of nano-TiO2: Fe was studied by electron paramagnetic resonance (EPR) and Faraday balance. The existence of an inhomogeneous magnetic state in TiO2: Fe samples of different compositions were revealed by EPR spectroscopy in a wide temperature range. The analysis of the EPR spectra in the L -, X - and Q-bands allowed us to calculate the quadratic fine structure (D, E) parameters, “axial” and “rhombic” ones, respectively. The value of D turned out to be quite small, which indicates an insignificant anisotropy, which can be ignored describing the magnetic properties of TiO2:Fe. It was shown that the temperature behavior of different separate components of the integral EPR spectra can be qualitatively interpreted in the model of coexistence in the TiO2: Fe system, mainly, dimers with a strong negative exchange interaction and isolated paramagnetic centers. No ferromagnetic state in TiO2: Fe-based samples after etching of aE-prepared state were detected.

Inhomogeneous Magnetic State of YFeO3 Thin Films According to NMR Spectroscopy Data

Inhomogeneous Magnetic State of YFeO3 Thin Films According to NMR Spectroscopy Data

Magnetic properties and spin glass behavior in an antiferromagnet KFeO2

We report the inhomogeneous magnetic state with the characteristics of spin-glass like behavior in KFeO2 from the low temperature magnetization and magnetization relaxation of zero field cooled measurements. The results reveal a possible spin glass state below the typical glass transition temperature (Tg). The low temperature isothermal hysteresis loops reveal a spin configuration consisting of a small ferromagnetic component within an antiferromagnetic ground state. The observation of spin-glass state in KFeO2 may be attribute to the competition between ferromagnetic and antiferromagnetic component. And the ferromagnetic component might origin from the frustration effect or nonstoichiometric caused mixed valence state of Fe ions in KFeO2. Hence, we expect that a similar phenomenon would exist in related AFeO2 (A: alkali metal) family with strong super-exchange interactions.

The Magnetic Domain Structure Properties in Diluted Magnetic Semiconductors

We present a comprehensive analysis of the domain structure formation in the ferromagneticphase of diluted magnetic semiconductors (DMS) of the p-type. Our analysis is carried outon the base of the effective magnetic free energy of DMS calculated by us earlier. This freeenergy, substituting DMS (a disordered magnet) by an effective ordered substance, permits usto apply the standard phenomenological approach to the domain structure calculation. Usingthe coupled system of Maxwell equations with those obtained by the minimization of the freeenergy functional, we show the existence of the critical ratio vcr of concentration of chargecarriers and magnetic ions such that the sample critical thickness Lcr (such that the sampleis monodomain at L &lt; Lcr) diverges as v → vcr. At v &gt; vcr, the sample is monodomain. Thisfeature makes DMS different from conventional ordered magnets, as it gives a possibility tocontrol the sample critical thickness and the emerging domain structure period by a variationof v. As the concentration of magnetic impurities grows, vcr → ∞, restoring a conventionalbehavior of ordered magnets. Above facts have been revealed by the examination of the tem-perature of the transition to an inhomogeneous magnetic state (stripe domain structure) inthe slab of a p-type DMS with finite thickness L. Our theory can be easily generalized for anarbitrary disordered magnet.

Magnetic resonance force spectroscopy of multilayer films Co/Pt with perpendicular magnetic anisotropy

The magnetic resonance force microscopy (MRFM) in combination with magneto-optical Kerr effect spectroscopy and magnetic force microscopy is used to study ferromagnetic resonance in multilayer thin-film structures [Co(0.5 nm)/Pt(1 nm)]5 with perpendicular magnetic anisotropy. Both the uniformly magnetized state and nonhomogeneous domain structures with different spatial scales are studied. It is shown that labyrinth domain structure manifests itself in the MRFM spectra as specific resonances that are split in an external magnetic field. In zero magnetic field the resonant frequencies depend on the size of oppositely magnetized domains due to internal demagnetizing fields and magnetostatic interaction through overlapping strain fields. For composite sample [Co(0.5 nm)/Pt(1 nm)]5Co(1.3 nm) the decrease in the spatial scale of domain structure and decrease in resonance frequencies are observed, which are due to the effect of interface exchange field. The obtained results demonstrate the ability of MRFM to study the resonance properties of highly inhomogeneous magnetic states of ferromagnetic structures, especially in zero magnetic fields.

Studying the Phase Transformation Kinetics of the U–6Nb Alloy Using NMR Methods

The inhomogeneous magnetic state arising from isothermal aging of the rapidly quenched U–6Nb alloy (6.3 wt % or 14 at % niobium) was studied for the first time using nuclear magnetic resonance of the 93Nb nucleus. In the process of phase transformation during isothermal annealing at Тan = 500°C, the fraction of niobium atoms in the bulk of the alloy increases in regions with a magnetic susceptibility corresponding to the alloys U1–хNbх (х > 0.14). It is shown that the process of isothermal transformation, accompanied by the formation of niobium-enriched structural precipitates, is fully completed after 60 hours.

Structural, Magnetic, and Magneto-Caloric Properties of Cu-Substituted Nd0.67Ba0.33MnO3 Manganites

The crystal structure and magnetic properties of polycrystalline Nd0.67Ba0.33Mn1 – xCuxO3 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) manganites are discussed. All the samples are crystallized into an orthorhombic crystal structure with Imma symmetry space group and the obtained lattice parameters confirm the presence of a co-operative Jahn–Teller effect for all the studied compounds. X-ray photoelectron spectra suggest the ratio of Mn3+/Mn4+ ions are observed to be decreasing systematically with the increase of Cu-content substitution and affects the Cu-ions oxidations states. The ferromagnetic (FM) to paramagnetic (PM) transition and magnetic moment is found to decrease systematically with the increase of Cu-content substitution leading to the suppression of double-exchange interactions. The theoretical and experimentally observed values of effective PM moment and saturation magnetic moment confirm the presence of inhomogeneous magnetic states containing FM and antiferromagnetic phases in all the samples. A magnetic phase change, i.e., from second-order transition to first-order transition is witnessed in between the 4 to 6 at wt % of the Cu substitution in Nd0.67Ba0.33MnO3 mixed-valence manganite.

Nanopatterned hard/soft bilayer magnetic antidot arrays with long-range periodicity

A top-down approach using focused ion beam has been employed to fabricate Co/Permalloy hard-soft bilayer magnetic antidot arrays. These nanopatterned films are studied with particular emphasis on magnetic coercivity. The antidots have a diameter of 40 nm and the studied antidot symmetries are square and hexagonal. A dependence of magnetic coercivity on the relative thicknesses of the magnetically hard (Co) and soft (Permalloy) layers is found; increasing Permalloy thickness results in lower magnetic coercivity. Furthermore, the long-range periodicity of top-down nanopatterned antidots results in higher magnetic coercivity and a stronger magnetic domain-wall pinning, compared to identical hard/soft bilayers of short-range order deposited on porous anodic alumina. The combination of antidot symmetry and hard/soft thickness, allow for efficient tailoring of the magnetic properties of nanopatterned thin films. Finally, magnetic force microscopy imaging of the antidot array magnetic configuration shows striking qualitative differences between the two array symmetries: square symmetry arrays have inhomogeneous magnetic state and a high density of immobile super-domain walls, whereas hexagonal symmetry arrays show a homogeneous magnetic configuration.

Antiferromagnetic states of BiFeO3 single crystal multiferroic in external magnetic field

Room-temperature multiferroics take special attention due to possibilities for applications in technology, but they are also interesting from fundamental point of view. Bismuth ferrite is the most studied multiferroic demonstrating high ordering temperatures and fascinating properties in thin films and could be used as a functional element in magnetoelectric memory. For better utilization of bismuth ferrite in spintronic devices the detailed understanding of its properties is of great importance. In this work we apply the developed thermodynamic model for description of magnetic states and magnetic field driven transformation of magnetic structure of single crystal BiFeO3. Presence of magnetic field may lead to a situation, when the transition from inhomogeneous magnetic state having cycloidal structure into homogeneous state proceeds through emergence of intermediate magnetic state with conical-like structure. Magnetic field transitions emerging under influence of magnetic field should somehow lead to alternation of magnetization. For revealing this influence the magnetization curves were calculated for three directions of magnetic field and the comparison with experimental results was carried out. Good agreement with experimental curves allows us to say, that peculiarities of magnetization curves may be caused by the emergence of intermediate conical state.

Structural, magnetic and electronic properties of Ti-doped BaFeO3-δ exhibiting colossal dielectric permittivity

We report a systematical investigation of the structural, magnetic, electronic, and dielectric properties of BaFe1-xTixO3-δ (x = 0.05, 0.10, 0.15 and 0.20) samples synthesized via the solid-state reaction method. Rietveld refinement using a combination of room-temperature X-ray (XRD) and neutron powder diffraction (NPD) data confirms single phases with a 6H-type structure for all the samples. X-ray absorption spectroscopy (XAS) study reveals a coexistence of Fe3+ and Fe4+, whose ratio remains almost unchanged with increasing Ti concentration. Magnetization measurements reveal a low-temperature inhomogeneous magnetic state with coexisting ferromagnetic (FM), AFM clusters and a paramagnetic phase. Moreover, magnetization of x = 0.10 shows a pinched hysteresis loop behavior, suggesting the coexistence of hard and soft FM phases in samples with high Ti concentrations. Low-temperature NPD measurements also confirm the absence of any long-range magnetic order in the samples. A colossal dielectric permittivity and two relaxation processes are observed for most of the studied samples. There is a change in electric conduction mechanism from the small polaron hopping to the Mott's variable-range hopping with increasing Ti concentration.

High-pressure magnetism of the double perovskite Sr2FeOsO6 studied by synchrotron Fe57 Mössbauer spectroscopy

The pressure dependence of the magnetism of the double perovskite ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ was investigated by temperature- and magnetic-field-dependent synchrotron $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy in the energy domain up to $\ensuremath{\sim}50$ GPa. ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ is known to feature antiferromagnetic ordering below ${T}_{\mathrm{N}}\ensuremath{\sim}140$ K and a change in spin structure from AF1 to AF2 near 70 K at ambient pressure. Previous Os ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) spectra [Veiga et al., Phys. Rev. B 91, 235135 (2015)] indicated a pressure-driven change from the antiferromagnetic to the ferrimagnetic state. Raman spectra of ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ collected at room temperature and up to 34 GPa are in agreement with previous x-ray-diffraction studies which verified that the tetragonal ambient pressure crystal structure is retained at high pressure. The M\ossbauer investigations show that the Fe ions remain in the +3 high-spin $({{t}_{2g}}^{3}{e}_{g}^{2})$ state over the whole pressure range. The magnetic ordering is strongly stabilized by high pressure and for $pg30$ GPa the ordering temperature is increased to above room temperature. Broad magnetic hyperfine patterns as well as coexistence of magnetic and paramagnetic signals at high pressures indicate the formation of inhomogeneous magnetic states. The shapes of applied-field M\ossbauer spectra at low temperature are similar at 2 and 33 GPa and do not allow resolution of distinct antiferromagnetic and ferrimagnetic components. Pressure-induced spin canting in a basically antiferromagnetic spin structure or magnetic phase separation involving a pressure-induced minority ferrimagnetic phase may be the origin for the ferromagnetic XMCD signal.

Small‐Angle Neutron Scattering and Magnetically Heterogeneous State in Sr2FeMoO6–δ

Single‐phase strontium ferromolybdate (Sr2FeMoO6–δ) samples with different degrees of the superstructural ordering of the Fe/Mo cations (P) are obtained from partially reduced SrFeO3–х, SrMoO4 precursors by the solid‐state technology. The study of the temperature dependences of the magnetization measured in the field‐cooling and zero‐field‐cooling modes indicated an inhomogeneous magnetic state of the samples. The presence of magnetic regions of different nature has also been revealed by the small‐angle neutron scattering. For the Sr2FeMoO6–δ samples with different P values and for all values of the magnetic field induction up to 1.5 T and of the scattering vector in the interval 0.1 &gt; q &gt; 0.002 Å−1, the analytical dependence I ∼ q–α obeys the Porod law (α ≈ 4), which corresponds to an object with a smooth and well‐marked surface and polydisperse grain size. Deviations from the Porod law in the q &gt; 0.1 Å−1 region and a weakening of the neutron scattering in applied magnetic fields may be ascribed to magnetic inhomogeneities with diameters D &lt; 6 nm, which are partially destroyed/oriented by magnetic fields В ≥ 1.5 T. It is established that the magnetic homogeneity of the Sr2FeMoO6–δ compound is enhanced with increasing superstructural ordering of the Fe/Mo cations.

Inhomogeneous Magnetic State of Се2Fe17 Evidenced by Mössbauer Spectroscopy

A comparative analysis of the magnetic properties and Mossbauer spectra of two Ce2Fe17 samples prepared by different methods and demonstrating different magnetic behavior is carried out. A model for processing the spectra is proposed, which gives a good description of the Mossbauer spectra of samples in different magnetic states by a superposition of eight subspectra. It is shown that in the samples in the ferromagnetic state there are regions with antiferromagnetic ordering at the local level. The relative fractions of iron atoms in these regions differs for different samples and increases when approaching the temperature of the transition from the ferromagnetic to the antiferromagnetic state.

Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analogue of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including $^{63,\, 65}$Cu nuclear quadrupole resonance, $^{13}$C nuclear magnetic resonance and muon spin rotation to show that the magnetic ground state of the Cu$^{2+}$ ($S=1/2$) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish an onset of intrinsically inhomogeneous magnetic state at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexist with the remnant high-temperature dynamical component down to $T_l = 20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components we conclude that the two components coexist on a microscopic level.

Magnetic Phases and Inhomogeneous Micromagnetic Structures in a Ferrite Garnet Film with Orientation (210)

Possible magnetic states of a cubic ferromagnet with a uniaxial anisotropy induced along the [210] direction are theoretically investigated. It is demonstrated that the orientation phase diagram of a magnet is nontrivial and admits the existence of three types of magnetic phases that differ in transformational properties and the presence of quintuple points, isostructural phase transitions, etc. It is found that magnetic inhomogeneities, regardless of the values of parameters of the matter, have a common structure: they correspond to 180-degree domain boundaries with a noncircular trajectory of the magnetization vector. The features of homogeneous and inhomogeneous magnetic states, found in these materials, make it possible to explain the manifestation of a flexomagnetoelectric effect in them.