Magnetic Phase Stability Research Articles

Disorder-induced time effect in the antiferromagnetic domain state of Fe1+yTe

We report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe1+yTe (y ≈ 0.12). X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the Néel temperature TN = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was achieved by a [100]-oriented biasing field favoring a single-domain state during cooling across TN. Our specific heat and magnetization measurements confirm the bulk character of this AFM phase transition. On longer time scales, however, we observe that the field-biased AFM state is highly unstable even at the lowest temperature of T = 3 K. After switching off the biasing field, the XMLD signal decays exponentially with a time constant τ = 1506 s. The initial XMLD signal is restored only upon repeating a cycle consisting of heating and field-cooling through TN. We explain the time effect by a gradual formation of a multi-domain state with 90° rotated AFM domains, promoted by structural disorder, facilitating the motion of twin-domains. Significant disorder in our Fe1+yTe sample is evident from our X-ray diffraction and specific heat data. The stability of magnetic phases in Fe-chalcogenides is an important material property, since the Fe(Te1-xSex) phase diagram shows magnetism intimately connected with superconductivity.

Ta cap-induced stabilization of interfacial ferromagnetism and enhanced magnetoelectricity in ultrathin FeRh films

We report ab initio electronic structure calculations to study the effect of Ta cap on the magnetic properties and the magnetoelectric response of Ta/FeRh/MgO nanojunctions. The calculations reveal that the Ta cap reverses the magnetic phase stability in ultrathin FeRh films leading to the stabilization of the ferromagnetic phase and interfacial reconstructed ferromagnetism. We demonstrate that the Ta cap induces a large charge transfer which in turn reduces dramatically the magnetic moments of the interfacial Fe atoms regardless the magnetic configuration and enhances the in-plane (out-of-plane) magnetization orientation of the antiferromagnetic (AFM) (ferromagnetic (FM)) phase compared to the uncapped bilayer. The Ta cap modifies substantially and enhances the magnetoelectric response under an external electric field, where the voltage controlled magnetic anisotropy (VCMA) changes from ∨-shape in the AFM to linear behavior in the FM phase with large VCMA efficiency. These findings demonstrate the manipulation of magnetic ordering of FeRh films with heavy metal capping and electric field which can promote the application of FeRh alloy in MeRAM.

A theoretical study of the electronic, magnetic and magnetocaloric properties of the TbMnO3 multiferroic

During the last years, a great attention has been devoted to the discovery of materials that can be used for magnetocaloric applications such as intermetallics and oxides. In this context, the multiferroic TbMnO3 appears as promising magnetocaloric material that can be used for magnetic refrigeration at low temperatures. This multiferroic presents complex electronic and magnetic structures. In this paper, different approaches are used with the aim to get more insight on the driving mechanisms behind the electronic, magnetic and magnetocaloric properties of the TbMnO3 manganite. In this way, a physical model is proposed to describe the magnetic interaction inside this material. Using the density functional theory, the electronic structure, density of states, have been evaluated. The magnetic phase stability and exchange couplings in TbMnO3 have been also investigated in order to understand and to clarify the obscured different magnetic interaction in this compound. Magnetocaloric and magnetic properties have been studied using Monte Carlo Simulation (MCS) within the Ising model. The transition temperatures, the isothermal magnetic entropy change, the adiabatic temperature change and the relative cooling power (RCP) were calculated. The highest obtained isothermal entropy change is found to be −17.8 J/Kg.K for H = 8 T. The RCP maximum value is found to be 507.06 J/Kg and the adiabatic temperature change reaches a maximum value close to 22.5 K in the field change of H = 8 T.

Sustainability of corundum-type Cr2O3 nanoparticles at shock wave loaded conditions

This research work probes into the crystallographic and magnetic phase stability of Corundum-type Cr2O3 nanoparticles (Cr2O3 NPs) at shock wave loaded conditions. The Chromium Oxide nanoparticles were prepared by precipitation method and loaded with shock pulses of counts 50, 100 and 150 having Mach number 2.2. The control and shocked Cr2O3 NPs were analyzed by studying the data of structural, optical and magnetic properties which were observed through analytical techniques such as X-ray diffraction, Diffuse Reflectance Spectroscopy and vibrating sample magnetometer. Based on the analyzed diffraction and spectroscopic measurements, it is clear that the test sample undergoes neither crystallographic nor magnetic phase transitions at shocked conditions and hence it could be considered as a high shock-resistant material and the results of static high-pressure compression also support the consideration. The obtained results reveal that Cr2O3 NPs have outstanding shock resistant behavior as compared to a few other important metal oxides such as TiO2, Co3O4 and α-Fe2O3 NPs. Hence, the title material is suggested for the aerospace device fabrication because of its outstanding structural stability against the impact of shock waves.

Ab-initio study of magnetic and electronic properties of the perovskites RFeO3: 4f-R valence electrons effects

We involve ab-initio calculations within the generalized gradient approximation (GGA + U) based on the density functional theory (DFT) and Heisenberg Hamiltonian model to study the magnetic phase stability, exchange interactions, electronic and magnetic properties of rare-earth orthoferrites RFeO3 (R = Dy, Er, Tm). We strongly believe that this model can provide an accurate description of the magnetism in this series of compounds. The 4f-electrons of rare earth elements are treated as valence electrons. In all selected perovskites, 3d shells hybridize with oxygen to form Jahn-Teller distortion, which guarantee superexchange interaction. GGA + U results show that insulating G-AFM configuration is the most appropriate to the spin arrangement of R/Fe ions and agree well with the experimental data. This ground state is typified by small superexchange coupling parameters JFe-Fe (3.30–3.38 meV) and Néel temperature values (669.93–685.50 K). The volume and the Jahn-Teller distortion (Q3) are responsible to decrease of JFe-Fe and Néel temperature values when changing R ionic radius. Our results reveal that the 4f valence states are critical factors to properly reproduce the ground state of these compounds.

Assessment of crystallographic and magnetic phase stabilities of cubic copper ferrite at shocked conditions

In recent years, dynamic shock wave-driven investigation carried out on the crystallographic phase stabilities of nano-materials has led to the accumulation of massive explosion of innovations which materialize in identifying the efficient materials so that such kinds of assessments are highly required before putting them into practical applications. Surprisingly, at shocked conditions, most of the materials are found to have undergone phase transition or a variety of changes have been observed in their stability as well as efficiency. Hence, device engineers are highly focused on the search of high shock-resistant materials for applications point of view especially for aerospace, defense, and military applications. In the present context, we have chosen one of the most familiar divalent metal ferrites of cubic copper ferrite nanocrystalline material (CuFe2O4 NPs) for the analysis of structural stability and the results have been screened by X-ray diffraction (XRD) as well as ultra-violet diffuse reflectance spectroscopic (UV-DRS) techniques. Magnetic phase stability has been evaluated by vibrating sample magnetometer (VSM). Interestingly, the title ferrite does not experience any crystallographic and magnetic phase transition even though it has polymorphic nature and variety of magnetic states. Therefore, it could be confirmed that CuFe2O4 NPs have considerable shock-resistant behavior for both crystallographic and magnetic phases. The results are discussed in the upcoming sections.

Intrinsic hard magnetic properties of Sm(Fe,Co)12−xTix compound with ThMn12 structure

Sm1.1(Fe0.8Co0.2)12-xTix thin films were grown epitaxially with various Ti contents (x = 0, 0.4, 0.6, 0.9, and 1) to investigate the effects of Ti on the intrinsic hard magnetic properties and phase stability. A single ThMn12-type structure with the (001) texture was obtained for x up to 0.9. A further increase in the Ti content led to a multiphase structure in the Sm1.1(Fe0.8Co0.2)11Ti1 film; this was in contrast to the behavior in the bulk, where x = 1 is necessary to form a single ThMn12-type structure. Meanwhile, increasing the Ti content from x = 0–0.6 in the Sm1.1(Fe0.8Co0.2)12-xTix thin films decreased the saturation magnetization, anisotropy field, and Curie temperature from 1.84 T to 1.55 T, 12–8.7 T, and 800–730 K, respectively. This thin film model shows that the Sm(Fe0.8Co0.2)12-xTix compounds can compete with Nd2Fe14B at 300 K when phase stabilization is achieved at reduced Ti content, for x ≤ 0.5.

Pressure effects on magnetism in Ca2Mn2O5 -type ferrites and manganites

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (with a ${d}^{5}$ electronic configuration) and ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$ configuration), exhibiting the ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (${d}^{5}$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\ensuremath{\approx}24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ and shifts it to higher pressures.

Crystallographic and magnetic phase stabilities of NiFe2O4 nanoparticles at shocked conditions

In the present article, we have reported the crystallographic and magnetic phase stabilities of nickel ferrite nano particles (NiFe2O4 NPs) at different number of shock wave-loaded conditions. Series of shock pulses have been loaded on the test samples such as 50,100 and 150 shock pulses with Mach number 2.2. Powder X-ray diffractometer (XRD), diffused reflectance spectroscopy (DRS) and vibrating-sample magnetometer (VSM) are utilized to examine the crystallographic, electronic and magnetic phase stabilities. The observed XRD, DRS and VSM results indicated that no crystallographic, electronic and magnetic phase transitions occurred by the impact of shock waves. Fascinatingly, SEM images show the shock wave assisted shape modification at 150 shocks. The accomplished magnetic phase stability results have compared with our previous report on Fe2O3 NPs for better understanding the structure-property stabilities of title ferrite against the impact of shock waves. Based on the observed results, we wish to suggest that the title material is a suitable candidate for high pressure, high temperature applications and for aerospace applications due to the outstanding shock resistance properties.

Mn-Induced Thermal Stability of L10 Phase in Fept Magnetic Nanoscale Ribbons.

Magnetic nanoscale materials exhibiting the L10 tetragonal phase such as FePt or ternary alloys derived from FePt show most promising magnetic properties as a novel class of rare earth free permanent magnets with high operating temperature. A granular alloy derived from binary FePt with low Pt content and the addition of Mn with the nominal composition Fe57Mn8Pt35 has been synthesized in the shape of melt-spun ribbons and subsequently annealed at 600 °C and 700 °C for promoting the formation of single phase, L10 tetragonal, hard magnetic phase. Proton-induced X-ray emission spectroscopy PIXE has been utilized for checking the compositional effect of Mn addition. Structural properties were analyzed using X-ray diffraction and diffractograms were analyzed using full profile Rietveld-type analysis with MAUD (Materials Analysis Using Diffraction) software. By using temperature-dependent synchrotron X-ray diffraction, the disorder–order phase transformation and the stability of the hard magnetic L10 phase were monitored over a large temperature range (50–800 °C). A large interval of structural stability of the L10 phase was observed and this stability was interpreted in terms of higher ordering of the L10 phase promoted by the Mn addition. It was moreover found that both crystal growth and unit cell expansion are inhibited, up to the highest temperature investigated (800 °C), proving thus that the Mn addition stabilizes the formed L10 structure further. Magnetic hysteresis loops confirmed structural data, revealing a strong coercive field for a sample wherein single phase, hard, magnetic tetragonal L10 exists. These findings open good perspectives for use as nanocomposite, rare earth free magnets, working in extreme operation conditions.

Assessment of crystallographic and magnetic phase stabilities on MnFe2O4 nano crystalline materials at shocked conditions

Today's advanced science and technology has witnessed extensive efforts put forth by researchers whose persistent work has been continuing quite consistently to find high shock resistant and technologically important device materials. Ferrites are right at the top in terms of contribution for the making of modern electronic and spintronics devices which are predominantly used in magnetic sensors as well as data storage applications. On the other hand, several ferrite materials do not fulfill the real practical requirements of the device engineers even though they have the required functional properties for device fabrications. Hence, the journey of finding ideal materials for device fabrication is still an ongoing arduous process. On this context, we have made systematic investigation of unearthing and understanding the properties and especially the stability of MnFe2O4 NPs at shock wave loaded conditions. Powder X-ray diffractometer (PXRD), Ultra-Violet diffuse reflectance spectroscopy (UV-DRS) and vibrating sample magnetometer (VSM) techniques have been undertaken so as to find out the crystallographic, electronic and magnetic phase stabilities at shocked conditions. The observed XRD, UV-DRS and VSM studies reveal that the title ferrite NPs has high shock resistant behavior so that it is suggested for the application of device fabrication.

Critical size limits for collinear and spin-spiral magnetism in CoCr2O4

The multiferroic behavior of CoCr$_2$O$_4$ results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size dependent behavior, which collapses when reaching a critical particle size. Here, the magnetic phase stability of CoCr$_2$O$_4$ in the size range of 3.6 - 14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr$_2$O$_4$.

Magnetic phase stability of transition metals doped (4,4) AlN nanosheet

Magnetic phase stability in transition metals doped (4,4) armchair AlN nanosheet were studied using first principle calculations for three different configurations and two magnetic states. Energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) state suggest that the preferable magnetic state for the AlN nanosheet doped with a pair of Fe and Co atoms independent from doping site is AFM, while it is FM for Ni and Mn impurities. For the rest transition metals (Ti, V, Cr, and Cu) doped AlN nanosheet, magnetic state alters as the transition metals configuration varies. In order to identify the most stable configuration, the formation energy is calculated for all compounds. The results obtained from formation energy calculations revealed that the most stable configuration for a pair of (V, Mn, Fe, Co, and Cu) atoms doped AlN nanosheet is the configuration of I, while for Cr and Ni is the configuration of III. AlN nanosheets doped with (Ti, V, Fe, and Ni) atoms are more stable than nanosheets doped with Cr, Mn and Cu.

A Review of Ab Initio Calculation on Lattice Distortion in High-Entropy Alloys

Lattice distortion is one of the four core effects responsible for the unprecedented mechanical behaviors of high-entropy alloys (HEAs). Lattice distortion in HEAs has been investigated in experiments and estimated in theory. In this review, we focus on the lattice distortion based on available ab initio calculations. The degree of lattice distortion and its influence on the ground-state property, local magnetic moments, phase stability, and mechanical performance are reviewed in single-phase high-entropy alloys.

First-Principle Simulation of Ferromagnetism in Gd-Doped Mg2X (X=Si, Ge and Sn)

Magnetic phase stability and electronic properties of Gd-doped Mg2X ([Formula: see text], Ge and Sn) were investigated by first-principle calculations based on density functional theory. The present calculations were performed using the Generalized Gradient Approximation (PBE-GGA) and the modified Becke–Johnson (mBJ-GGA). The 4[Formula: see text] electrons’ behavior has been investigated as a function of the Coulomb repulsion [Formula: see text] by varying it from 0[Formula: see text]eV to 8[Formula: see text]eV. The ground state properties for the pure compounds are consistent with experimental values and other theoretical data. All doped compounds are metallic and ferromagnetic with large Curie temperature values. It was found that both exchange potentials ([Formula: see text] and [Formula: see text]) provide a better description of the electronic structures of pure and doped systems than regular GGA and mBJ.

Theoretical Study of the Electronic and Magnetic Properties and Phase Stability of the Full Heusler Compound Pd2CoAl

Based on first principles calculation, a systematical investigation has been performed to study the electronic, magnetic, dynamic, and mechanical properties of the full Heusler compound Pd2CoAl. It is found that the L21-type structure is energetically more stable than the XA-type due to the lower total energy. The obtained lattice constant in cubic ground state is 6.057 Å, which matches well with previous study. The calculated electronic band structure reveals the metallic nature of Pd2CoAl and its total magnetic moment of 1.78 μB is mainly contributed by Co atom from strong spin splitting effect, as indicated with the distinctive distributions of the density of states in two spin directions. Under uniform strains from −5% to +5%, the variation of total magnetic moment has been obtained and it is still caused by the much larger change from Co atom, compared with Pd and Al atoms. The tetragonal structure has further been analyzed and we found that there is possible martensitic phase transformation because the total energy can be further reduced when the cubic structure is varied into the tetragonal one. The large energy difference of 0.165 eV between the tetragonal and cubic phases is found at the c/a ratio of 1.30. The total density of states has been compared between the cubic and tetragonal phases for Pd2CoAl and results show tetragonal phase transformation could reduce the states at the Fermi energy level in both directions. In addition, the dynamic and mechanical stabilities have also been evaluated for Pd2CoAl in both cubic and tetragonal structures and results confirm that the tetragonal phase shows good stability against the cubic phase, which further verifies that the tetragonal phase transformation is highly expected. In the end, the strong elastic anisotropy in the tetragonal structure has been clearly shown with the calculated directional dependence of the Young’s modulus and shear modulus.

The Ir4+ substitution dependence of electric polarization as a probe of magnetic phase stability in multiferroic MnWO4

Hubnerite MnWO4 is a highly frustrated magnetic compound that has been known for its multiferroic properties. The intrinsic connection of ferroelectric polarization and magnetically frustrated structure allows an opportunity to probe the stability of magnetic structures against perturbations by means of measuring ferroelectric polarization. In this work, we investigate the ferroelectric polarization of Mn1 − 2xIrxWO4 to probe the stability of the low-temperature (T) collinear antiferromagnetic (AF1) phase against the Ir substitution, considering the strong spin-orbital coupling of Ir4+ that would enhance the single-ion anisotropy, on the one hand, and would favor the noncollinear spin alignment, on the other hand. Different from Mn1 − xRux/2WO4, it is suggested that the AF1 phase is only partially suppressed by the Ir substitution, allowing the emergence of the noncollinear antiferromagnetic (AF2) phase in coexistence with the collinear AF1 phase. Proper Ir substitution may promote both the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction, thus making the modulation of the magnetic structure more complicated.

Effect of Dopant Concentrations on Magnetic States of Ti, Mn, And Ni Doped Iii-V Zincblende Compounds

The effect of Ti, Mn, and Ni dopants concentration on various magnetic states of III-V zincblende compounds (Al1-xTx)M is presented by calculation of multiple scattering theory based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), where T denotes Ti, Mn, and Ni atoms and M represents As and P atoms. The calculations reveal the phase changes in various magnetic states, which are solely controlled by dopant concentrations. The turning points of these behaviors are reported, which are responsible for the change of stable ferromagnetic (FM) state to unstable disordered local moment (DLM) state. The Curie temperature, magnetic moments, and density of states at different concentrations of the dopants are calculated and explain the concurrency with magnetic phase stability.
 Journal of Bangladesh Academy of Sciences, Vol. 42, No. 2, 177-182, 2018

Cr and Fe Substitution Effects on Magnetic Properties and Phase Stabilities in Tetragonal Mn3(Ga,Ge)

We have investigated Cr and Fe substitution effects on magnetic properties and phase stabilities in D022-type Mn3Ga0.5Ge0.5. The single phase of the D022 structure was obtained within the small amount of substitution. The structural parameters in the Cr substitution system show opposite composition dependencies to those in the Fe substitution system. This result implies that the structural properties of the D022 structure depend on the number of the valence electron. In contrast to this, the thermal stability of D022 phase is influenced by the Fe substitution, whereas the Cr substitution has an insignificant effect on the thermal stability. Magnetization increases with the increasing Cr content but decreases with increasing Fe content. In addition, Mossbauer spectra reveal that the Fe atom occupies at 4d site. Not only the number of the valence electron, but also the site preference of the Cr and Fe atoms plays an important role for the magnetic properties.

Half-Metallic Ferromagnetism in V-Doped FmP Binary Monopnictide Compounds: an Ab Initio Calculations

The ab initio calculations of the structural phase stability, magnetic phase stability, and electronic and magnetic properties of the FmP binary monopnictides and the half-metallic ferromagnetic properties of their Fm1−xVxP (for x = 0.25, 0.50, 0.75, and 1)-doped alloys have been assured through the spin-polarized density functional theory (DFT), and they are performed by employing the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW + lo) method that is implemented in the WIEN2k package. The exchange and correlation potential is parameterized by the generated gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) scheme. According to the studies of the magnetic phase stability, the fermium monopnictides (FmP) have a paramagnetic regime, while the ferromagnetic behavior appears when we dope this compound with vanadium. Through the ferromagnetic character of all Fm1−xVxP alloys, the spin-polarized band structures and densities of states exhibit a nearly half-metallic character of Fm0.75V0.25P alloy, whereas Fm0.50V0.50P, Fm0.25V0.75P, and VP compounds showed metallic character. Moreover, the total magnetic moment of the three ternary alloys is mainly contributed by Fm and V elements, where the p-d hybridization reduces the atomic magnetic moment of V element from its free space charge and produces feeble magnetic moments on the nonmagnetic P sites.