Fe Sublattice Research Articles

Element-specific ultrafast lattice dynamics in FePt nanoparticles

Light–matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction (UED) to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

Large magnetocrystalline anisotropic energy and its impact on magnetostriction of L10-FePt

Abstract The origin of magnetocrystalline anisotropic energy (MAE) guided by spin–orbit coupling in the L10-FePt alloy was analyzed. Correlation of MAE with the calculated magnetoelastic constants in the single and polycrystal model was determined by means of first-principles calculations using density functional theory (DFT). More specifically, a systematic analysis of the large MAE and magnetostriction coefficients (λ’s) were done by means of post-processing of eigenvalues (orbital energies) and eigenfunctions (orbital occupancies). Our numerical analysis includes the convolution of the projected wave-function (density of states) of each orbital of the Fe and Pt sub-lattices into their orbital energies, which in principle should be valid for any solid regardless of the strength of the MAE. The determined anisotropic magnetostriction coefficients are of the λ ∼ 10 − 4 –10–3 order for several crystallographic directions; which is in accord to the known experimental data. However, the polycrystalline model based on the uniform stress approximation leads to a decrease of the λ by two orders in the overall magnetostrictive behavior ( λ ∼ 10 − 5 ). Such a large difference is not unexpected as the magnetostriction for polycrystalline FePt thin films were recorded as very low, thus validating the robustness of the current theoretical proposition of analyzing MAE.

Deconvolution of X-ray natural and magnetic circular dichroism in chiral Dy-ferroborate.

Structural chirality and magnetism, when intertwined, can have profound implications on materials properties. Using X-ray imaging and spectroscopic measurements that leverage the natural and magnetic circular dichroic effects present in magnetized chiral crystal structures, we probe the interplay between chirality and magnetism across the field-induced spin-flop transition of Dy ferroborate, ( ) . Deconvolution of natural and magnetic circular dichroic signals at the Fe K and Dy L absorption edges of the non-centrosymmetric structure was enabled by use of tunable temperature and magnetic field, providing access to element-specific magnetic information across the spin-flop transition. The magnetic response of Fe and Dy sublattices was found to be independent of domain chirality. The chiral domains were robust against both the (chirality preserving) R32 to P3 21/P3 21 structural phase transition at 280 K, and application of magnetic field up to 4 Tesla. A third flavor of X-ray dichroism, magneto-chiral dichroism, was not detected within the accuracy of our measurements. The absence of significant Fe magnetization along the screw, c-axis for the magnetic field strength used in this study, together with non-linear coupling of magnetic field to electric polarization across the spin-flop transition, may hinder observation of magneto-chiral dichroic effects in this system.

The B-site ordering in RFe0.5Cr0.5O3 ceramics and its effect on magnetic properties

Abstract To insight into the B-site ordering in RFe0.5Cr0.5O3 ceramics, a series of RFe0.5Cr0.5O3 ceramics (R = La, Y, Lu) were synthesized by the sol–gel method, and the structural and magnetic properties were systemically investigated. By using the Rietveld refinement of all samples, it is found that the structural distortion is increased as the R ionic radius decreases, leading to the weakened interactions between Fe/Cr ions. Moreover, the Fe and Cr are arranged in disorder in LaFe0.5Cr0.5O3, but partially ordered in YFe0.5Cr0.5O3 and LuFe0.5Cr0.5O3, showing an increasing trend of the proportion of ordered domains with the decrease of R ionic radius. Through fitting the temperature-dependent magnetizations, it is identified that the magnetization reversal (MR) in disorder LaFe0.5Cr0.5O3 is resulted from the competition between the moments of Cr and Fe sublattices. In the partially ordered YFe0.5Cr0.5O3 and LuFe0.5Cr0.5O3 ceramics, because of the presence of Fe–O–Cr networks in the ordered domains whose moment is antiparallel to that of Fe–O–Fe and Cr–O–Cr in the disordered domains, the compensation temperature T comp of MR is increased by nearly 50 K. These results suggest that the changing of R-site ions could be used very effectively to modify the Fe–O–Cr ordering, apart from the structural distortion, which has a direct effect on the magnetic exchange interactions in RFe0.5Cr0.5O3 ceramics. Then at values of composition where ordered domains are expected to be larger in number as compared to disordered domains and with a weaker structural distortion, one can expect a higher transition temperature T comp, providing a different view for adjustment of the magnetic properties of RFe0.5Cr0.5O3 ceramics for practical applications.

Spin switching and magnetocaloric effect of high-entropy orthoferrite single crystal

The concept of high-entropy oxides/alloys was introduced into the rare-earth orthoferrite (RFeO3, R represents a rare-earth ion) system. Doping five different rare-earth ions with equal molar ratios at the R-site, a single crystal of high-entropy orthoferrite (Y0.2Sm0.2Eu0.2Gd0.2Er0.2)FeO3 (HERFO) was synthesized using optical floating zone method. At high temperatures, the weak ferromagnetic (wFM) moments induced by the distortion of the Fe sublattices align along the c axis, with a spin configuration of Γ4. As the temperature decreases, a spin reorientation transition (SRT) occurs from Γ4 to Γ2 due to the Fe3+-R3+ super-exchange interactions, with an immediate state Γ24. The wFM moments gradually rotate from the c axis to the a axis within the ac plane. In the zero-field-cooling (ZFC) mode, both the type-I and type-II spin switching (SSW) are observable along the a axis. Under low magnetic fields, the type-II SSW is magnetically controlled. The trigger temperature and variation in net moment of the type-II SSW exhibit systematic changes concerning the applied magnetic field. The rich magnetic phase transitions in HERFO make it a promising candidate for spintronic applications aimed at spin manipulation. Additionally, HERFO single crystal exhibits a large magnetocaloric effect (MCE) and magnetic entropy anisotropy, with the negative magnetic entropy change (‐ΔSm) along the c axis (14.45 J/(kg·K)) being greater than that along the a and b axes (9.57 J/(kg·K) and 10.32 J/(kg·K)). Consequently, HERFO holds great promise as a novel material for magnetic refrigeration applications aimed at achieving rotational magnetic cooling.

Optical-Helicity-Dependent Orbital and Spin Dynamics in Two-Dimensional Ferromagnets.

Disentangling orbital (OAM) and spin (SAM) angular momenta in the ultrafast spin dynamics of two-dimensional (2D) ferromagnets on subfemtoseconds is a challenge in the field of ultrafast magnetism. Herein, we employed a non-collinear spin version of real-time time-dependent density functional theory to investigate the orbital and spin dynamics of the 2D ferromagnets Fe3GeTe2 (FGT) induced by circularly polarized light. Our results show that the demagnetization of the Fe sublattice in FGT is accompanied by helicity-dependent precession of the OAM and SAM excited by circularly polarized lasers. We further identify that precession of the OAM and SAM in FGT is faster than demagnetization within a few femtoseconds. Remarkably, circularly polarized lasers can significantly induce a periodic transverse linear response of the OAM and SAM on very ultrafast time scales of ∼600 attoseconds. Our finding suggests a powerful new route for attosecond regimes of the angular momentum manipulation to coherently control helicity-dependent orbital and spin dynamics in 2D ferromagnets.

Thermal stability of refined Al6(Fe, Mn) phase formed in laser powder bed fusion process

The addition of Mn in Al–Fe alloys stabilizes the Al6Fe metastable phase (orthorhombic structure, oC28) owing to the partial occupation of Fe sublattice sites by Mn atoms (formation of Al6(Fe, Mn) phase), thereby achieving superior high-temperature strength of Al–Fe–Mn ternary alloys that are manufactured via laser powder bed fusion (L-PBF) process. This study aimed to fundamentally understand the thermal stability of the refined Al6(Fe, Mn) phase formed in the L-PBF fabricated Al–2.5%Fe–2%Mn alloy in terms of a thermodynamic approach and the kinetics of morphological changes in the Al6(Fe, Mn) phase at elevated temperatures ranging from 200 to 500 ℃. The L-PBF processed samples showed a high hardness of approximately 125 HV due to the formation of numerous dispersoids of the Al6(Fe, Mn) phase with sizes of several tens of nanometers in the α-Al matrix containing concentrated solute elements of Fe and Mn. The hardness and microstructure were almost unchanged even after exposure to a high temperature of 200 ℃ for an extended period of 1000 h. Upon exposure to 300 ℃, nucleation and growth of the Al6(Fe, Mn) phase occurred locally, particularly at grain boundaries in the α-Al matrix. Such a local growth contributed to a reduction in hardness upon thermal exposure. Simultaneously, numerous nanoscale precipitates were formed in the α-Al matrix, resulting in a suppressed reduction in hardness. Contrary to the result of the thermodynamic calculations, the θ-Al13Fe4 phase was scarcely found even after long-term exposure to 500 ℃. The θ-phase formation was accompanied by the dissolution of the refined Al6Fe metastable phase for strengthening, which significantly reduced the hardness during thermal exposure. The suppressed θ-phase formation could be associated with the thermodynamically stable Al6(Fe, Mn) phase developed during the L-PBF process, which predominantly contributed to the high microstructural stability of the L-PBF fabricated Al–Fe–Mn alloy at elevated temperatures.

Site Occupancy Preference and Magnetic Properties in Nd2(Fe,Co)14B

Partial replacement of Fe by Co is an effective method to increase Curie temperature (TC), which improves the thermal stability of magnetic properties in Nd2Fe14B-based permanent magnets. The correlation between Fe substitution and magnetic properties has been studied in Nd2(Fe,Co)14B via a first-principles calculation. The calculated Fe substitution energies indicate that the Co atoms avoid the 8j2 site, which agrees with the experiments. The Co atoms are ferromagnetically coupled with Fe sublattice and show magnetic moments of about 1.2 to 1.7 μB at different crystallographic sites, less than that of Fe (2.1–2.7 μB), resulting in the decrease in total magnetization at ground state (0 K) with increasing Co content. The effective exchange interaction parameter, derived from the energy difference between varied magnetic structures, increases from 7.8 meV to 17.0 meV with increasing Co content from x = 0 to x = 14 in Nd2Fe14−xCoxB. This change in the effective exchange interaction parameter is responsible for the enhancement of TC in Nd2(Fe,Co)14B. The total magnetization at 300 K, derived from mean-field theory, shows a peak maximum value at x = 1 in Nd2Fe14−xCoxB. The phenomenon results from the interplay between the reduction of the magnetic moment in the Fe(Co) sublattice and the enhancement of TC with increasing Co content.

Structural distortion and dynamical electron correlation driven enhanced ferromagnetism in Ni-doped two-dimensional Fe5GeTe2 beyond room temperature

Achieving beyond room-temperature ferromagnetism in two-dimensional (2D) magnets is immensely desirable for spintronic applications. Fe5GeTe2 is an exceptional van der Waals metallic ferromagnet due to its tunable physical properties and relatively higher Curie temperature ( TC ) than other 2D magnets. Using density functional theory combined with dynamical electron correlation and Monte Carlo simulations, we find the TC of (Fe 1−δ Ni δ)5 GeTe2 monolayer can increase up to ∼400 K at δ∼0.20 (δ: fractional occupation). Two specific Fe sublattices are identified to be the most energetically preferred sites to host Ni. Exchange interactions between particular Fe pairs play a dominating role in controlling TC , influenced by the dopant-induced structural distortions. Dynamical electron correlation induces site- and orbital-specific quasi-particle mass of Fe-d states with varying Ni concentrations. This work provides fundamental insights into 2D magnetism as an interplay of structural and electronic aspects and would guide to tailoring exciting magnetic phenomena in similar systems.

Transient Non-Collinear Magnetic State for All-Optical Magnetization Switching.

Resonant absorption of a photon by bound electrons in a solid can promote an electron to another orbital state or transfer it to a neighboring atomic site. Such a transition in a magnetically ordered material could affect the magnetic order. While this process is an obvious road map for optical control of magnetization, experimental demonstration of such a process remains challenging. Exciting a significant fraction of magnetic ions requires a very intense incoming light beam, as orbital resonances are often weak compared to above-band-gap excitations. In the latter case, a sizeable reduction of the magnetization occurs as the absorbed energy increases the spin temperature, masking the non-thermal optical effects. Here, using ultrafast X-ray spectroscopy, this work is able to resolve changes in the magnetization state induced by resonant absorption of infrared photons in Co-doped yttrium iron garnet, with negligible thermal effects. This work finds that the optical excitation of the Co ions affects the two distinct magnetic Fe sublattices differently, resulting in a transient non-collinear magnetic state. The present results indicate that the all-optical magnetization switching (AOS) most likely occurs due to the creation of a transient, non-collinear magnetic state followed by coherent spin rotations of the Fe moments.

Stability and magnetic properties of Nd[formula omitted]Fe[formula omitted]X (X = Al, Si, Cu, Zn, and Ga)

The stability and magnetic properties of the RE6Fe13X compounds, as well as their preferential localization in Nd-Fe-B magnets, have been investigated by first-principles calculations. We find that the RE6Fe13X (X = Al, Si, Zn, and Ga) have negative formation energies and are more likely to form in the triple junctions, while Nd6Fe13Cu with the formation energy of 0.73 eV tends to localize in the thin intergranular phase. These compounds contribute to the exchange decoupling of Nd2Fe14B grains and thus enhance the coercivity of Nd-Fe-B magnets. Such decoupling can be more pronounced in the cases of Nd6Fe13Cu and Nd6Fe13Zn, which exhibit antiferromagnetic ordering in the Fe sublattice according to our calculations. We propose that the partial substitution of Nd atoms with Pr can gradually reduce the formation energy of these phases and maintain their original antiferromagnetic states.

Interplay of Magnetic Interaction and Electronic Structure in New Structure RE-12442 Type Hybrid Fe-Based Superconductors

We present detailed first-principles density functional theory-based studies on RbRE2Fe4As4O2 (RE = Sm, Tb, Dy, Ho) hybrid 12442-type iron-based superconducting compounds with particular emphasis on competing magnetic interactions and their effect on possible magneto-structural coupling and electronic structure. The stripe antiferromagnetic (sAFM) pattern across the xy plane emerges as the most favorable spin configuration for all the four compounds, with close competition among the different magnetic orders along the z-axis. The structural parameters, including arsenic heights, Fe-As-Fe angle, and other relevant factors that influence superconducting Tc and properties, closely match the experimental values in stripe antiferromagnetic arrangement of Fe spins. Geometry optimization with inclusion of explicit magnetic ordering predicts a spin–lattice coupling for all the four compounds, where a weak magneto–structural transition, a tetragonal-to-orthorhombic structural transition, takes place in the relaxed stripe antiferromagnetic spin configuration. Absence of any experimental evidence of such structural transition is possibly an indication of nematic transition in RE-12442 compounds. As a result of structural distortion, the lattice contracts (expands) along the direction with parallel (anti-parallel) alignment of Fe spins. Introduction of stripe antiferromagnetic order in Fe sub-lattice reconstructs the low-energy band structure, which results in significantly reduced number of bands crossing the Fermi level. Moreover, the dispersion of bands and their orbital characteristics also are severely modified in the stripe antiferromagnetic phase similar to BaFe2As2. Calculations of exchange parameters were performed for all the four compounds. Exchange coupling along the anti-parallel alignment of Fe spins J1a is larger than that for the parallel aligned spins J1b. A crossover between the super-exchange-driven in-plane next-nearest-neighbor exchange coupling J2 and in-plane exchange coupling J1a due to lanthanide substitution was found. A large super-exchange-driven next-nearest-neighbor exchange interaction is justified using the construction of 32 maximally localized Wannier functions, where the nearest-neighbor Fe-As hopping amplitudes were found to be larger than the nearest- and the next-nearest-neighbor Fe-Fe hopping amplitudes. We compare the hopping parameters in the stripe antiferromagnetic pattern with non-magnetic configuration, and increased hopping amplitude was found along the anti-parallel spin alignment with more majority-spin electrons in Fe dxz and dxy but not in Fe dyz. On the other hand, the hopping amplitudes are increased in stripe antiferromagnetic phase along the parallel spin alignment with more majority-spin electrons in only Fe dyz. This difference in hopping amplitudes in the stripe antiferromagnetic order enables more isotropic hopping.

Multiple spin switching and magnetocaloric effects in orthoferrite Yb0.25Tb0.75FeO3 single crystal

YbFeO3 exhibits spin switching (SSW) and spin reorientation transition (SRT) at low temperatures, but its mediocre magnetocaloric properties limit its further applications. As for TbFeO3 showing a large magnetocaloric effect, its SSW is missing. Motivated by this, we chemically put Yb and Tb with a ratio of 1:3 on the rare-earth site. Potential applications of magnetic and magnetocaloric properties are both preserved in one single phase Yb0.25Tb0.75FeO3, which is exceptional among the rare-earth orthoferrites. Spin switching temperature (TSSW) in the field-cooled-cooling mode increases to around 100 K compared with YbFeO3. A novel multiple SSW emerges in the field-cooled-warming mode originating from the complex competing magnetic interactions between Yb/Tb and Fe sublattices. We also addressed the mechanism of spin modification of magnetic ions with temperatures. Arrott plots created from the isothermal magnetization curves demonstrate that SRT is a second-order phase transition. The negative entropy change (refrigeration capacity) vs. temperatures [−∆Sm(RC) vs. T] diagrams indicate that the −∆Sm(RC) are effectively increased compared with YbFeO3 by rare-earth doping. The pronounced magnetic refrigeration performance and SSW effect with a low-field dependence make it promising to be employed in both magnetic sensing and refrigeration devices.

Unveiling a hidden multiferroic state under magnetic fields in BaHoFeO4

In this paper, we report field-induced ferroelectricity in ${\mathrm{BaHoFeO}}_{4}$ in a narrow range of applied magnetic fields ($1.5\phantom{\rule{4.pt}{0ex}}\text{T}\ensuremath{\le}H\ensuremath{\le}5\phantom{\rule{4.pt}{0ex}}\text{T}$). Magnetization measurements reveal three magnetic anomalies, the first one at ${T}_{N}^{\text{Fe}}\ensuremath{\sim}52.8$ K, the second one at ${T}_{{N}_{2}}\ensuremath{\sim}36.4$ K, and the third one at ${T}_{N}^{\text{Ho}}\ensuremath{\sim}6.8$ K, corresponding to the antiferromagnetic ordering of ${\mathrm{Fe}}^{3+}$, a possible spin structure change of ${\mathrm{Fe}}^{3+}$, and the antiferromagnetic ordering of ${\mathrm{Ho}}^{3+}$ sublattices, respectively. Surprisingly, the heat capacity data did not show any anomaly at ${T}_{N}^{\text{Fe}}$ but exhibited a broad maximum at ${T}_{N}^{\text{Ho}}$. Below 40 K, isothermal magnetization measurements indicate a metamagnetic transition at 1.5 T. The temperature-dependent dielectric measurements performed below 1 T reveal two anomalies, around 6 and 36 K, corresponding to ${\mathrm{Ho}}^{3+}$ ordering and a possible change in the magnetic phase of the Fe sublattice. These anomalies disappear at higher magnetic fields, and a different dielectric anomaly appears above the metamagnetic transition with a broad asymmetric peak around 13 K (${T}_{P}$), which shifts to higher temperatures, reaches a maximum under 2 T, and then is suppressed further on increasing the magnetic field up to 9 T. A switchable electric polarization appears below 25 K and above the metamagnetic transition field ($H\ensuremath{\ge}1.5$ T). Intriguingly, the polarization disappears above 5 T. The presence of electric polarization in intermediate magnetic fields and its strong magnetic field dependence are interesting and make this material unique.

Influence of high pressure on the remarkable itinerant electron behavior in Y0.7Er0.3Fe2D4.2 compound

A monoclinic Y0.7Er0.3Fe2D4.2 compound exhibits unusual magnetic properties with different field induced magnetic transitions. The deuteride is ferrimagnetic at low temperature, and the Er and Fe sublattices present magnetic transitions at different temperatures. The Er moments are ordered below TEr = 55 K, whereas the Fe moments remain ferromagnetically coupled up to TM0 = 66 K. At TM0, the Fe moments display a sharp ferromagnetic–antiferromagnetic transition (FM–AFM) through itinerant electron metamagnetic behavior very sensitive to any volume change. Y0.7Er0.3Fe2D4.2 becomes paramagnetic above TN = 125 K. The pressure dependence of TEr and TM0 has been extracted from magnetic measurements under hydrostatic pressure up to 0.49 GPa. Both temperatures decrease linearly upon applied pressure with dTEr/dP = −126 and dTM0/dP = −140 K GPa−1 for a field of B = 0.03 T. Both magnetic Er and ferromagnetic Fe orders disappear at P = 0.44(4) GPa. However, under a larger applied field B = 5 T, dTM0/dP = −156 K GPa−1, whereas dTEr/dP = −134 K GPa−1 showing weaker sensitivity to pressure and magnetic field. At 2 K, the decrease of the saturation magnetization under pressure can be attributed to a reduction of the mean Er moment due to canting and/or a crystal field effect. Above TM0, the magnetization curves display metamagnetic behavior from an AFM to FM state, which is also very sensitive to the applied pressure. The transition field Btrans, which increases linearly upon heating, is shifted to a lower temperature upon applied pressure with ΔT = −17 K between 0 and 0.11 GPa. These results show strong decoupling of the Er and Fe magnetic sublattices vs temperature, applied field, and pressure.

Role of intersublattice exchange interaction in ultrafast longitudinal and transverse magnetization dynamics in permalloy

We report on element-specific measurements of ultrafast demagnetization and magnetization precession damping in permalloy thin films. Magnetization dynamics induced by an optical pump at 1.5 eV is probed simultaneously at the ${M}_{2,3}$ edges of Ni and Fe with high-order harmonics for moderate demagnetization rates (less than 50%). The role of the intersublattice exchange interaction in both longitudinal and transverse dynamics is analyzed with a Landau-Lifshitz-Bloch description of ferromagnetically coupled Fe and Ni sublattices. It is shown that the intersublattice exchange interaction governs the dissipation during demagnetization as well as precession damping of the magnetization vector.

Canted ferromagnetic order in nonsuperconducting Eu(Fe1−xNix)2As2

The magnetic order in a series of nonsuperconducting $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for $\mathit{x}\ensuremath{\le}0.02$, in which the spins lie in the $\mathit{ab}$ plane, to a pure canted ferromagnetic order for $0.04\ensuremath{\le}\mathit{x}\ensuremath{\le}0.20$, in which the spins are canted with a small angle off the $\mathit{c}$ axis. By comparison with the $\mathit{c}$-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the ${\mathrm{Eu}}^{2+}$ moments in an intermediate temperature range is observed for $\mathit{x}=0.20$, in which some in-plane spin fluctuations persists above the long-range ordering temperature ${T}_{\mathrm{Eu}}$.

Simulations of Temperature-Dependent Magnetization in FexGd100−x (20 ≤ x ≤ 80) Alloys

Theoretical calculations of the temperature-dependent magnetization in FeGd alloys were done with the use of Heisenberg-type atomistic spin Hamiltonian and Monte Carlo algorithms. The random allocation of atoms in the desired crystal structure was used for simulations of magnetically amorphous alloys. Performed calculations for the two different crystal structures have shown an important role of coordination number on the observed critical temperature and compensation point. Moreover, the value of the exchange interaction between Fe and Gd sublattices plays a key role in the simulations—an increase in the Fe–Gd exchange constant provides an increase in critical temperature for each concentration of elements, which explains the higher temperature stabilization of Gd moments. It was shown that obtained temperature-dependent magnetization behavior is consistent with experimental observations, which confirms the applicability of the atomic model used to study FeGd or other magnetic alloy structures.

Crucial role of Fe in determining the hard magnetic properties of Nd2Fe14B

${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$'s unsurpassed, hard magnetic properties for a wide range of temperatures result from a combination of a large volume magnetization from Fe and a strong single-ion anisotropy from Nd. Here, using finite temperature first-principles calculations, we focus on the other crucial roles played by the Fe atoms in maintaining the magnetic order on the Nd sublattices, and hence the large magnetic anisotropy, and directly generating significant uniaxial anisotropy at high temperatures. We identify effective spins for atomistic modeling from the material's interacting electrons and quantify pairwise and higher order, nonpairwise magnetic interactions among them. We find the Nd spins couple most strongly to spins on sites belonging to two specific Fe sublattices: $8{j}_{1}$ and $8{j}_{2}$. Moreover, the Fe $8{j}_{1}$ sublattice also provides the electronic origin of the unusual, nonmonotonic temperature dependence of the anisotropy of ${\mathrm{Y}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$. Our work provides atomic-level resolution of the properties of this fascinating magnetic material.

Role of Ni doping in magnetic dilution of Fe sublattice and in tailoring optical properties of CoFe2O4

The magnetic measurements of the prepared samples of CoFe2-xNixO4 were investigated by SQUID-VSM. We observed a decrease in the coercive field on increasing the doping content, attributed to lower magneto-crystalline anisotropy of dopant Ni2+ than that of the host Fe3+. Also, the merging of FC-ZFC curves approximately at 300 K for all the three samples predicts that the samples are ferrimagnetic up to 300 K. Two sextets can be seen in the Mossbauer spectroscopy, which also indicates that the samples are ferrimagnetic in nature. The bandgap energy calculated from Tauc-plots for pristine and doped samples was found to vary between 2.16 and 2.30 eV. The present bandgap values show the semiconducting behavior of pristine and Ni-doped cobalt spinel ferrite. Therefore, a variety of semiconductor device applications can make use of this material.