Magnetic Transition Temperature Research Articles

Additive manufacturing of Ni-Mn-Sn shape memory Heusler alloy – Microstructure and magnetic properties from powder to printed parts

Ni-Mn-based Heusler alloys such as Ni-Mn-Sn show both elastocaloric and magnetocaloric effects during the magneto-structural phase transition, making these materials interesting for solid-state cooling applications. Material processing by additive manufacturing can overcome difficulties related to the machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitor these properties over the entire processing chain. In the present work, the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Directed energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at.% and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 21 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

The Study of Interplay of Singlet Superconductivity and Ferromagnetism in Superconducting HoMo6Se8

This article reports the effect of magnetic ordering and superconducting order parameter on superconducting and magnetic transition temperatures of interacting singlet superconductivity and ferromagnetism in HoMo6Se8 material. The basic superconducting parameters were calculated starting with the BCS type model Hamiltonian and using the double time temperature dependent Green function formalisms. Results showed that the superconducting critical temperature decreases with enhancement of superconducting order parameter and vice versa. This is perhaps due to the weakening of the binding energy as the temperature approaches its critical value. On the other hand, the superconducting critical temperature demonstrates attenuation with increasing magnetic order parameters . It was also observed that the enhancement of magnetic transition temperature with the increase of the magnetic order parameter demonstrates variation with the alteration of the superconducting order parameter. Moreover, the calculations revealed that there is a temperature region where both superconductivity and magnetic order coexist. Furthermore, the present theoretical analysis is broadly in agreement with existing experimental findings.

Magnetic and magnetocaloric properties of R55Co30Al10Si5 (R = Ho, Dy) metallic glass ribbons

Metallic glass alloys are suitable systems for the development of magnetocaloric refrigerated materials due to the stable physical properties and the adjustable magnetic phase transition temperature. Here, the glass formation ability (GFA), magnetic properties and magnetocaloric effects of R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons prepared by the melt-spinning technique were investigated. The X-ray diffraction and DSC measurement indicated that the metallic glass alloys have a completely amorphous structure and excellent glass formation ability. The amorphous ribbons were demonstrated to undergo a second-order magnetic phase transition (ferromagnetic to paramagnetic) around Tc ∼ 18 K for Ho55Co30Al10Si5, ∼34 K for Dy55Co30Al10Si5, respectively. The maximum magnetic entropy change (-ΔSMmax) and the corresponding refrigerant capacity (RC) values are 12.7 J/kg K, 343 J/kg for Ho55Co30Al10Si5, 9.1 J/kg K, 334 J/kg for Dy55Co30Al10Si5 under a field change ΔH= 5 T. The large -ΔSMmax and high RC suggest that R55Co30Al10Si5 (R = Ho, Dy) amorphous ribbons have potential applications in low temperature magnetic refrigeration.

BaMn2P2 : Highest magnetic ordering temperature 122-pnictide compound

We report the growth of high-quality single crystals of ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type tetragonal ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ and investigation of its structural, electrical transport, thermal, and magnetic properties. Our results of basal plane electrical resistivity and heat capacity measurements show that the compound has an insulating ground state with a small band gap. Anisotropic susceptibility ${\ensuremath{\chi}}_{ab,c}(T)$ data infer a collinear local-moment N\'eel-type antiferromagnetic (AFM) ground state below the ordering temperature ${T}_{\mathrm{N}}=795(15)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is highest among all the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$- and ${\mathrm{CaAl}}_{2}{\mathrm{Si}}_{2}$-type 122-pnictide compounds reported so far suggesting that the strength of magnetic exchange interactions is strongest in this material. The magnetic transition temperatures of ${\mathrm{BaMn}}_{2}P{n}_{2}$ ($Pn=\mathrm{P}$, As, Sb, Bi) compounds exhibit a monotonic decrease with the increase of tetragonal unit cell parameters $a$ and $c$, suggesting a strong dependence of the strength of the decisive magnetic exchange interactions on the separation between the localized spins residing on the Mn ions. The observed monotonic increase of both ${\ensuremath{\chi}}_{ab}$ and ${\ensuremath{\chi}}_{c}$ for $T>{T}_{\mathrm{N}}$ suggests that short-range dynamic quasi-two-dimensional AFM correlations persist above the ${T}_{\mathrm{N}}$ up to the highest temperature of the measurements. The large ${T}_{\mathrm{N}}$ of ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ demands for systematic hole-doping studies on this material as similar investigations on related ${\mathrm{BaMn}}_{2}{\mathrm{As}}_{2}$ with ${T}_{\mathrm{N}}=618\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ have led to the discovery of an outstanding ground state where AFM of localized Mn spins and itinerant half-metallic ferromagnetism with ${T}_{\mathrm{c}}\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ originating from the doped holes coexist together.

Superconductivity and magnetic and transport properties of single-crystalline CaK(Fe1−xCrx)4As4

Members of the CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ series have been synthesized by high-temperature solution growth in single crystalline form and characterized by X-ray diffraction, elemental analysis, magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe$_4$As$_4$ ($T_c$ = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as $x$ is increased from 0 to 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (\textit{T}-\textit{x}) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, $T^*$, appearing near 22~K for $x$ $\sim$ 0.017 and rising slowly up to 60~K for $x$ $\sim$ 0.077. The $T$-$x$ phase diagrams for CaK(Fe$_{1-x}$$T$$_{x}$)$_4$As$_4$ for $T$ = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to $T$ = Co and Ni series for which the $T$-$x$ diagrams scale by a factor of two, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ is also studied as a function of magnetic field. A clear change in $H^\prime_{c2}$($T$)/$T_c$, where $H^\prime_{c2}$($T$) is d$H_{c2}$($T$)/d$T$, at $x$ $\sim$ 0.012 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence length and the London penetration depths are also calculated based on $H_{c1}$ and $H_{c2}$ data. Coherence lengths as the function of $x$ also shows changes near $x$ = 0.012, again consistent with Fermi surfaces changes associated with the magnetic ordering seen for higher $x$-values.

Efficient discovery of room temperature magnetic transition metal monolayers assisted by artificial neural network

The low-dimensional materials have opened a new window to new emerging science and technological applications. Recently the discovery of new two-dimensional (2D) monolayers with long-range magnetic order by theoretical method gets more attention. The key question is how to effectively explore different possible 2D magnetic configurations. However, the computational cost of optimizing and electronically analyzing a a large number of compounds on the ab initio level is very high. A practical method is the combination of density functional theory and machine learning techniques to explore a large number of 2D monolayers. As a proof of principle, the method is implemented for looking at new high transition point 2D magnetic monolayers among transition metals dichalcogenides, dihalides, and chalcogenides/halides TMXY. Our study is intended to provide physical insights into the magnetic properties in two dimensions and introduce new room-temperature magnetic monolayers. For instance, our results show that the effective electronic screening and magnetic transition temperature in 2D monolayers are strongly dependent on the type of non-magnetic ligand atoms. Three different Neural Network (NN), Random Forest (RF), and Supporting Vector Regression (SVR) algorithms are successfully employed for quick and accurate prediction of Hubbard U, cell parameters, and the magnetic exchange parameter for a large number of TMXY (TM: V, Cr, Fe, Co, Mn, Ni, Nb and X, Y: O, S, Se, Te, Cl, Br, I) monolayers. Moreover, the binary classification is employed to determine the magnetic and atomic structure ground state. Finally, the transition temperature of TMXY monolayers is estimated from the predicted value of effective magnetic exchange parameter. The magnetic monolayers with higher transition temperatures are suggested for further theoretical and experimental investigations.

Strain effects on the electronic and magnetic properties of Cr2TaC2 and Cr2TaC2O2 monolayers

Double transition metal MXene materials, such as Cr2TiAlC2, were successfully synthesized recently in addition to the synthesized mono transition metal MXenes. The physical and chemical properties of the MXenes can be tuned by varying the middle metal layer composition by other metal atoms. Thus, the desired mechanical, electronic, thermal, or magnetic properties can be revealed in that MXene material. The other method to tune material properties is by applying external strain on the structure. In this study, we systematically investigate the physical properties, such as dynamical, thermal, mechanical stability, electronic, and magnetic properties of the Cr2TaC2 and the Cr2TaC2O2 MXene monolayers for equilibrium and under biaxial strain. We find that both of MXene monolayers are energetically, thermally, and dynamically stable for certain compressive and tensile strain values. The Cr2TaC2 and the Cr2TaC2O2 MXene monolayers have a ferromagnetic ground state and show a metallic character in an equilibrium state and preserve it up to high tensile strain (ε = 0.10) value. The robust ferromagnetism above room temperature is demonstrated for the Cr2TaC2 with a Curie temperature of 714 K for the strain-free case. This value reduces to 234 K for the Cr2TaC2O2 monolayer. Moreover, our detailed Monte Carlo simulations reveal that the magnetic transition temperature of the system and also hysteresis properties can be controlled by moving from compressive to tensile strain ratios.

Magneto-electric effect in Y1-xRxFeO3 (R = Sm and Er) ceramics

In the present work, we synthesize a series of Y1-xRxFeO3R=Er,Sm&0.≤x≤1 polycrystalline compound by solid-state reaction method to study the A-site substitution effects on physical properties in YFeO3. The Rietveld refinement of X-ray diffraction analysis indicates that with Er3+ substitution, the lattice parameters have decreased, and more structural distortion has been shown compared to Sm3+ substitution. The obtained magnetic hysteresis loops reveal that Er3+ ions have induced an antiferromagnetic behavior in YFeO3 because of a reduction in the Dzyloshinskii-Moriya interaction. The Y1-xSmxFeO3 family, however, shows a weak ferromagnetic behavior. Furthermore, due to the spin reorientation (SR) transition, a distinct anomaly has been detected in the temperature dependence of Ac susceptibility and dielectric constant curves. With substituting Sm at the Y-site in the YFeO3 compound, SR transition happens at a temperature, TSR, in which the spin structure changes high-temperature Γ4 to Γ2 configuration, and therefore the starting and ending of the SR region depend on Sm concentration. Interestingly, we have observed a gigantic dielectric constant in Y0.7Sm0.3FeO3, Y0.5Er0.5FeO3, and Y0.25Er0.75FeO3. It, however, has demonstrated that Y0.7Sm0.3FeO3 is the only compound that shows a relaxor ferroelectric behavior. The observed peak in the temperature dependence of the dielectric constant has also described a magnetic transition temperature indicating the magneto-dielectric effect in our studied samples.

Electronic signatures of successive itinerant, antiferromagnetic transitions in hexagonal La2Ni7

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic and magnetic properties of La2Ni7, an itinerant magnetic system with a series of three magnetic transition temperatures upon cooling, which end in a weak antiferromagnetic ground state. Our APRES data reveal several electron and hole pockets that have hexagonal symmetry near the Γ point. We observe significant reconstruction of the band structure upon successive magnetic transitions at T 1 ∼ 61 K, T 2 ∼ 57 K and T 3 ∼ 42 K. Several features observed in ARPES data were reasonably well reproduced by DFT calculations, while others were not. In particular, the flat band near E F predicted by DFT in antiferromagnet (AFM) state, was seemingly absent in ARPES data. Our results detail the effects of magnetic ordering on the electronic structure in a Ni-based weak AFM and highlight challenges of current computational methods.

Fe<sub>20</sub>Mo<sub>20</sub>Ni<sub>20</sub>Rh<sub>20</sub>Ru<sub>20</sub> and Fe<sub>14</sub>Mo<sub>35</sub>Ni<sub>15</sub>Rh<sub>15</sub>Ru<sub>21</sub> Ultrahigh-Mixing-Entropy Alloys with Single Hexagonal Close-Packed Structure

Mo35Ni15Rh15Ru35, Fe14Mo35Ni15Rh15Ru21, Mo25Ni25Rh25Ru25, and Fe20Mo20Ni20Rh20Ru20 (at.%) alloys were designed by referring to hexagonal close-packed (hcp) Nb–Mo–Ru–Rh–Pd high-entropy alloys (HEAs) reported by Liu et al., with the help of Pearson’s Crystal Data. X-ray diffraction profiles of the Fe20Mo20Ni20Rh20Ru20 and Fe14Mo35Ni15Rh15Ru21 alloys prepared via the conventional arc-melting and subsequent annealing at 1700 K for 1 h show an hcp structure. Further scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy confirmed the single hcp structure. The Fe20Mo20Ni20Rh20Ru20 HEA annealed at 1700 K for 1 h exhibited a mixing entropy (Smix) normalized by the gas constant (R) of 1.846, 14% higher than the configuration entropy (Sconfig) normalized by R (Sconfig/R = ln5). This study reveals two new ultrahigh-mixing-entropy alloys (UHMixEAs) that satisfy Smix > Sconfig, the Fe20Mo20Ni20Rh20Ru20 and Fe14Mo35Ni15Rh15Ru21 alloys. The evaluation of Smix/Sconfig for the present UHMixEAs and referential Co-containing HEAs from early studies revealed that Smix/Sconfig of the former are constant whereas those of the latter increase at the magnetic transition (Curie) temperature or below.

Large low-field reversible magnetocaloric effect in K3Gd(PO4)2 at sub-Kelvin temperature

We synthesized a potassium gadolinium phosphate K3Gd(PO4)2 with monoclinic structure (P21/m) by the high-temperature solid–state reaction method. Its lattice constants obtained by Rietveld refinements of XRD curve are a = 0.7411 nm, b = 0.5624 nm, and c = 0.9433 nm. Magnetic and magnetocaloric properties of K3Gd(PO4)2 were investigated in detail. The magnetic phase transition temperature of K3Gd(PO4)2 was determined by ultra-low temperature testing to be 0.58 K. K3Gd(PO4)2 exhibits large magnetic entropy changes (–ΔSM). The maximum –ΔSM are 20.2 and 29.4 J/(kg·K) in the magnetic field variation (ΔH) of 1 and 2 T, respectively, which are larger than that of the commercial Gd3Ga5O12 (GGG). The maximum adiabatic temperature change (ΔTad) reaches 5.91 K under a magnetic field change (ΔH) of 1 T. K3Gd(PO4)2 is a potential magnetic refrigeration material for obtaining sub-Kelvin temperatures.

Enhancement magnetic properties of GdMn1−xHfxO3 nanoceramics by regulation of vacancy defects and Mn valence state

GdMn1−xHfxO3 nanoceramics were synthesized by sol–gel method to investigate the regulation of the vacancy defects and Mn valence state on the magnetic properties of GdMn1−xHfxO3 nanoceramics. XRD and Raman analysis indicated that GdMn1−xHfxO3 nanoceramics with single-phase structure were synthesized by sol–gel method; Hf4+ ions had replaced the Mn ions in GdMnO3 and resulted in lattice deformation. SEM measurements verified that all GdMn1−xHfxO3 nanoceramics had nanoscale particle sizes. XPS analysis indicated that Hf4+ substitution could tune the Mn valence state and oxygen vacancy concentration. The results of positron annihilation lifetime spectroscopy revealed that cation vacancies concentration in the interface region could be effectively tailored by Hf4+ substitution. The magnetic measurements indicated that appropriate Hf4+ substitution could enhance the magnetic transition temperatures and magnetization of GdMnO3 samples, which could be mainly attributed to the cation vacancies concentration in the interface region and Mn valence state.

Combinatorial Synthesis of Cation-Disordered Manganese Tin Nitride MnSnN2 Thin Films with Magnetic and Semiconducting Properties

Magnetic semiconductors may soon improve the energy efficiency of microelectronics, but materials exhibiting these dual properties remain underexplored. Here, we report the computational prediction and realization of a new magnetic and semiconducting material, MnSnN2, via combinatorial sputtering of thin films. Grazing incidence wide-angle X-ray scattering and laboratory X-ray diffraction studies show MnSnN2 exhibits a wurtzite-like crystal structure with cation disorder. This new material has a wide composition tolerance, with a single-phase region ranging from 20% < Mn/(Mn + Sn) < 65%. Spectroscopic ellipsometry identifies an optical absorption onset of 1 eV, consistent with the computationally predicted 1.2 eV bandgap. Resistivity measurements as a function of temperature support the semiconducting nature of MnSnN2. Hall effect measurements show carrier density has a weak inverse correlation with temperature, indicating that the charge transport mechanisms are more complex than in a pristine semiconductor. Magnetic susceptibility measurements reveal a low-temperature magnetic ordering transition (≈10 K) for MnSnN2 and strong antiferromagnetic correlations. This finding contrasts with bulk, cation-ordered MnSiN2 and MnGeN2, which exhibited antiferromagnetic ordering above 400 K in previous studies. To probe the origin of this difference, we perform Monte Carlo simulations of cation-ordered and cation-disordered MnSnN2. They reveal that cation disorder lowers the magnetic transition temperature relative to the ordered phase. In addition to discovering a new compound, this work shows that future efforts could use cation (dis)order to tune magnetic transitions in semiconducting materials for precise control of properties in microelectronics.

Fe and Cr co-intercalation in 2H-NbS2 single crystals for realization of perpendicular magnetic anisotropy and large anomalous Hall effect

Recently, many interesting magnetic and electrical properties have been demonstrated in transition-metal intercalated $2\text{H-Nb}{\mathrm{S}}_{2}/\mathrm{Ta}{\mathrm{S}}_{2}$ layered single crystals. In this work, single crystals of Cr and Fe co-intercalated $2\text{H-Nb}{\mathrm{S}}_{2}$, with the compositions of ${({\mathrm{Cr}}_{1--x}{\mathrm{Fe}}_{x})}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2} (x=0.0--1.0)$, are successfully grown and their magnetic and transport properties are systematically studied. With increasing Fe content, the magnetic transition temperature continually decreases from 117 to 37 K, and the magnetic coupling transforms from ferromagnetic to antiferromagnetic. Importantly, a rotation of the magnetic easy axis occurs in this process, with perpendicular magnetocrystalline anisotropy and considerable coercive field achieved in the $x=0.13--0.66$ compounds. Both $x=0.33$ and $x=0.50$ exhibit relatively low carrier concentration, stable anomalous Hall effect, and butterfly-shape magnetoresistance. Notably, large anomalous Hall conductivity is identified in $x=0.50$, whose origin is discussed from the calculated electronic structures. The realization of perpendicular anisotropy and large anomalous Hall conductivity in this two-dimensional ferromagnetic system can facilitate their future application in spintronic devices.

Bottom-up building of two-dimensional magnetic materials with self-assembly of superatom TM@Sn12 (TM = Sc, Ti, V, Cr, Mn, Fe) clusters

The miniaturization of electronic devices is increasingly requiring some low-dimensional magnetic materials with excellent properties, so ultra-thin two-dimensional magnetic materials have attracted extensive attention. However, most two-dimensional materials exfoliated from bulk either lack intrinsic magnetism or have low magnetic transition temperatures, which greatly limits their practical applications. Here, using magnetic superatom TM@Sn12 (TM = Sc, Ti, V, Cr, Mn, Fe) clusters as building blocks, a series of two-dimensional materials are designed and the underlying mechanism for magnetic order and stability are explained by direct exchange of outer superatom orbitals (1G, 2P and 2D). The honeycomb lattice of TM@Sn12 (TM = V, Cr, Fe) and the square lattice of Ti@Sn12 are ferromagnetic. The Cr@Sn12 honeycomb lattice has a large out-of-plane magnetic anisotropic energy of 2.21 meV and its Curie temperature reaches 162 K, while the Fe@Sn12 honeycomb lattice has a large in-plane magnetic anisotropic energy of 3.58 meV. This research provides a new avenue for developing novel magnetic materials with excellent properties.

Spin reorientation in easy-plane kagome ferromagnet Li9Cr3(P2O7)3(PO4)2

We report the successful growth and characterization of Li9Cr3(P2O7)3(PO4)2 single crystal, and investigate its magnetic properties under external magnetic fields via magnetization and heat capacity measurements. Our study reveals that Li9Cr3(P2O7)3(PO4)2 is an easy-plane kagome ferromagnet with S = 3/2, as evidenced by the Curie–Weiss temperature of 6 K which implies a ferromagnetic exchange coupling in the material. Under zero magnetic field, Li9Cr3(P2O7)3(PO4)2 undergoes a magnetic transition at T C = 2.7 K from a paramagnetic state to a ferromagnetically ordered state with the magnetic moment lying in the kagome plane. By applying a c-axis directional magnetic field to rotate the spin alignment from the kagome plane to the c-axis, we observe a reduction in the magnetic transition temperature as the field is increased. We construct a magnetic phase diagram as a function of temperature and magnetic field applied parallel to the c-axis of Li9Cr3(P2O7)3(PO4)2 and find that the phase boundary is linear over a certain temperature range. Regarding that theoretically, the field-induced phase transition of the spin reorientation in the easy-plane ferromagnet can be viewed as the ferromagnetic magnon Bose–Einstein condensation (BEC), the phase boundary scaling of field-induced (B ∥ c) magnetic transition in Li9Cr3(P2O7)3(PO4)2 can be described as the quasi-2D magnon BEC, which has been observed in other ferromagnetic materials such as K2CuF4.

Thermodynamics evaluation of half-metallic ferromagnetism in N-doped XTiO3 (X = Ca, Sr, and Ba) systems: DFT calculations

Half-metallic (HM) materials with high magnetic transition temperature (TC) combined with a wide energy gap (Eg) in insulating spin channel are desired for the realization of modern spin based devices due to their stable ferromagnetic (FM) ordering, spin-filtering, and high mean free path. Herein, the formation energetics, electronic, and magnetic characteristics of N-doped XTiO3 (XTO) (X = Ca, Sr, and Ba) perovskite oxides (PO) were inspected by first-principles calculations, where one of the oxygen (O) atom is replaced by nitrogen (N) atom. The calculated formation energy and elastic constants values confirm the thermodynamical as well as elastic stability of all doped systems. Our calculations show that all undoped PO systems display a non-magnetic insulating behavior. On the other hand, all doped motifs show magnetic character with an integral induced magnetic moment of 1.00 μB. Further, it has also been observed that magnetism in all these doped systems mainly originates from p orbitals of the dopant N. Interestingly, all doped systems have 100% spin splitting at Fermi level depicting a HM FM behavior with large Eg of 1.61, 1.65, and 1.59 eV in spin minority channels for XTO (X = Ca, Sr, and Ba) systems, respectively. Further, a long range stable FM ordering with TC > RT is predicted in these doped materials by computing the magnetic energy solutions for FM and anti-ferromagnetic (AFM) ordering at various distances between two N atoms. The current study suggests that doping strategy with non-magnetic dopant in different PO materials could be a useful approach to modulate the physical features and make them effective for emerging spintronic devices operating above RT.

Size Effects of Magneto-Structural Correlation in Mn3O4 Nanoparticles

We synthesized spinel manganese oxide Mn3O4 nanoparticles with sizes ranging from 7 to 25 nm and investigated their crystal structure and magnetic properties through X-ray diffraction, magnetization, and electron spin resonance (ESR) measurements. As the particle size decreased, the lattice constant a increased monotonically, while the lattice constant c and tetragonal distortion of the unit cell reached a minimum at 11 nm and then increased rapidly below 9 nm. The ESR spectra of the nanoparticles differed from those of the bulk crystal, and the nanoparticles exhibited a hyperfine structure of Mn2+ ions at the nanoparticle surface. The size dependence of the line width of the ESR spectrum of the nanoparticles was similar to that of the tetragonal distortion. The magnetic transition temperatures and coercive field of the nanoparticles decreased rapidly below 10 nm. The Mn3O4 nanoparticles maintained three magnetic transition temperatures similar to those of bulk crystals above 9 nm. However, the nanoparticles exhibited only two distinct transition temperatures below 9 nm. Distortion of the crystal structure leads to a change in the magnetic interactions and magnetic state. It is suggested that the change of a phase boundary appears to exist at approximately 9 nm in the phase diagram of the magnetic properties as a function of the particle size.

Anomalous low temperature magnetic behaviour of Ca2Fe2(1-x)Cr2xO5 (x = 0, 0.1 and 0.2)

In the present study, systematic investigations on structural, magnetic, and thermal properties of a series of polycrystalline samples Ca2Fe2(1-x)Cr2xO5 (x = 0, 0.1, and 0.2) are reported. These compounds have been successfully synthesised by solid-state reaction method and are found to crystallise in brownmillerite orthorhombic structures with space group Pnma. In the case of the parent compound Ca2Fe2O5, a negative magnetisation (sign reversal of magnetisation) is observed in temperature-dependent magnetic study, which is anticipated to arise from multiple sublattice interactions. On the other hand, isothermal magnetisation studies exhibit the presence of weak ferromagnetic coupling in Ca2Fe2O5. From Magnetocaloric calculations, it is evident that antiferromagnetic interactions are predominant at the low-temperature regime (below 150 K). The partial substitution of Cr in place of Fe leads to a remarkable change in the magnetic behaviour of the compound along with suppression of negative magnetisations and ferromagnetic interactions. Additionally, the Differential Scanning Calorimetry study indicates a gradual shift in magnetic transition temperatures with an increase in Cr substitution.

Doping induced tailoring of properties in polycrystalline spinel vanadate – MnV2O4

We investigate the structural, electronic and magnetic properties of Mn1-xCuxV2O4 (0 ≤ x ≤ 0.1) to study the role of Cu in spinel vanadate MnV2O4. X-ray diffraction studies show the synthesized materials are single phase. However, X-ray photoelectron spectroscopy reveals the presence of cations in more than one valence state. The higher valence states of cations together with the desired ones confirm the presence of excess oxygen in a formula unit and non-stoichiometry increases with increasing Cu content. Further, with increasing Cu content, lattice parameters, unit cell volumes as well as different atomic separations shrink, whereas magnetic transition temperature gradually decreases and the structural transition disappears for x = 0.1. Moreover, magnetic hysteresis loops at 5 K remain unsaturated up to 6 Tesla applied field, whereas coercivity gradually increases and remanent magnetization decreases with increasing Cu content, in tandem with the increasing trend of grain size.