Mn Exchange Research Articles

Seasonal changes in soil and tissue nutrition in balsam fir and influence on postharvest needle abscission

ABSTRACTPostharvest needle abscission in balsam fir varies based on time of harvest, but little is known about the effect of soil and tree nutrition on postharvest needle abscission. From April to November, 18 balsam fir samples were obtained each month and evaluated for postharvest needle abscission characteristics. Soil samples were collected simultaneously and evaluated for moisture, pH, organic matter, cation exchange capacity, and mineral nutrition. Needle tissue samples were also evaluated for mineral nutrition. Many parameters were found to vary based on month throughout the study, but not all were associated with needle abscission. Soil organic matter, cation exchange capacity, Ca, and Mn all had a significant (P < .01) positive relationship with needle retention; soil Al had a significant (P < .01) negative relationship with needle retention. Needle N, Ca, and Fe were also associated with needle abscission. The strongest positive relationship was found between needle Ca and needle retention (R2 > .50), suggesting that higher concentrations of tissue Ca were linked to superior needle retention in balsam fir. Overall, this study provides evidence that changes to soil management, such as the inclusion of organic matter and certain minerals, may improve needle retention in balsam fir.

Primeira aproximação para solo cultivado com cafeeiro conilon na região atlântica da Bahia

O Sistema Integrado de Diagnose e Recomendação (DRIS), apesar de ser comumente utilizado na interpretação de análises foliares, pode ser empregado em análise química de solo, existindo potencial do uso da metodologia em solos cultivados com cafeeiros. Objetivou-se estabelecer faixas de suficiência e normas DRIS para solos cultivados com cafeeiro conilon na região Atlântica do Estado da Bahia. Coletaram-se 24 amostras de solo em 2012 e 2013, na profundidade de 0-20 cm, sendo quantificados os teores de matéria orgânica, Ca, Mg, K, P, S, Zn, B, Cu, Fe, Mn, saturação de bases e capacidade de troca catiônica a pH 7. Estabeleceram-se normas DRIS e faixas de suficiência para os nutrientes do solo, a partir de lavouras com produtividade, igual e superior a 100 sacas beneficiadas ha-1 de café conilon, sendo aplicáveis na recomendação de adubação da cultura na região. A maioria dos parâmetros estabelecidos apresentou discrepância em relação ao referencial teórico que se tem atualmente para o café conilon. Os resultados sugerem o desenvolvimento de normas regionais e específicas para a aplicação do método DRIS e faixas de suficiência no solo, para lavouras de café conilon.

Mn3 Single-Molecule Magnets and Mn6/Mn9 Clusters from the Use of Methyl 2-Pyridyl Ketone Oxime in Manganese Phosphinate and Phosphonate Chemistry

The syntheses, structures, and magnetochemical properties are reported for five new Mn clusters: [MnIII3O(O2PPh2)3(mpko)3](ClO4) (1), [MnIII3O(O2PPh2)3(ppko)3](ClO4) (2) [MnIII6O2(OMe)4(O2PPh2)4(mpko)4](ClO4)2 (3), [MnIII8MnIIO6(O2CMe)7(O3PPh)2(mpko)3(H2O)] (4), and [MnIII2MnIIO(mpko)3(H2O)4(ClO4)2](ClO4) (5), where mpko- (or ppko-) is the anion of methyl (or phenyl) 2-pyridyl ketone oxime. 1 was obtained by carboxylate substitution on [MnIII3O(O2CMe)3(mpko)3](ClO4) by treatment with diphenylphosphinic acid (Ph2PO2H). The comproportionation reaction between Mn(ClO4)2 and NBun4MnO4 in the presence of Ph2PO2H and ppkoH in EtOH, or mpkoH in MeOH, led to 2 and 3, respectively. 4 was obtained as was 3, but with phenylphosphonic acid (PhPO3H2) instead of Ph2PO2H. 5 was obtained by aerial oxidation of Mn(ClO4)2 in the presence of mpkoH. 1 and 2 contain a triangular Mn3 core, 3 comprises the fusion of two Mn3 units of 1 by MeO- bridges, and 4 has a cagelike structure. 5 is similar to 1 in possessing a triangular core. Variable-temperature, solid-state direct-current (dc) and alternating-current (ac) magnetic data were collected on 1-5: 1 and 2 exhibit ferromagnetic Mn····Mn exchange interactions, S = 6 ground states, and are new single-molecule magnets (SMMs). 3-5 possess S = 4, 5/2, and 5/2 ground states, respectively, from dominant antiferromagnetic interactions. Fits of dc magnetization data in the 1.8-10.0 K and 10-70 kG ranges gave D and g values of: -0.29(2) cm-1 and 1.94(1) for 1, -0.38(2) cm-1 and 1.99(1) for 2, -0.29(2) cm-1 and 1.96(1) for 3, -1.26(4) cm-1 and 1.99(2) for 4, -1.41(4) cm-1 and 1.98(2) for 5, where D is the axial zero-field splitting parameter.

Dynamics of Exciton–Mn Energy Transfer in Mn-Doped CsPbCl3 Perovskite Nanocrystals

Recent success in Mn2+ ion doping in cesium lead halide (CsPbX3) nanocrystals opened the door to exploring new optical, magnetic and charge carrier transport properties mediated via exciton–dopant exchange coupling in this new family of semiconductor nanocrystals. Here, we studied the dynamics of energy transfer from exciton to Mn2+ ions in Mn-doped CsPbCl3 nanocrystals to gain an insight into the relative strength of exciton–Mn exchange coupling compared to more extensively studied Mn-doped II–VI quantum dots. The comparison of exciton–Mn energy transfer times in CsPbCl3 nanocrystals and CdS/ZnS core/shell quantum dots suggests that exciton–Mn exchange coupling in CsPbX3 is not far behind that of CdS/ZnS despite the lack of quantum confinement. With further progress in the synthesis of Mn-doped CsPbX3 nanocrystals, such as imposing quantum confinement and expanding the range of host chemical composition, one could fully benefit from many properties of CsPbX3 superior to those of other semiconductor nanoc...

Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La0.3Ln0.3Sr0.3Mn1.1O3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, 55Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La3+, Pr3+, Nd3+, Sm3+, or Eu3+ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T ms) and ferromagnet–paramagnet (T C) phase transitions and the increase in electrical resistivity ρ and activation energy E a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d 4(Mn3+)–2p 6(O2–)–3d 3(Mn4+)–V (a)–3d 4(Mn3+). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric 55Mn NMR spectra support the high-frequency electronic double exchange Mn3+(3d 4) ↔ O2–(2p 6) ↔ Mn4+(3d 3) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.

Structure defects, phase transitions, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ ceramics

Structure and its defects, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R3¯c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal–semiconductor Tms and ferromagnetic–paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ O2– ↔ Mn4+. The crystal structure of the compositions x = 0 and 0.1 contains nanostructured planar clusters, causing anomalous magnetic hysteresis at T = 77 K. Broad asymmetric 55Mn NMR spectra confirm high-frequency electron double exchange Mn3+(3d4) ↔ O2–(2p6) ↔ Mn4+(3d3) and indicate inhomogeneity of the manganese environment due to the surrounding ions and vacancies. The effective local fields of the hyperfine interaction HHF at 55Mn nuclei have been calculated by decomposing asymmetric NMR spectra into three Gaussian components. The constructed experimental phase diagram of the magnetic and conducting states of the La0.6–xEuxSr0.3Mn1.1O3–δ ceramics revealed strong correlation between the composition, structural imperfection, phase state, and magnetotransport properties of rare-earth manganites.

54Mn Radiotracers Demonstrate Continuous Dissolution and Reprecipitation of Vernadite (δ-MnO2) during Interaction with Aqueous Mn(II)

(54)Mn radiotracers were used to assess Mn atom exchange between aqueous Mn(II) and vernadite (δ-MnO2) at pH 5.0. Continuous solid-liquid redistribution of (54)Mn atoms occurred, and systems are near isotopic equilibrium after reaction for 3 months. Despite this extensive exchange, X-ray diffraction and X-ray absorption spectroscopy data showed no major changes in vernadite bulk mineralogy. These results demonstrate that the vernadite-Mn(II) interface is dynamic, with the substrate undergoing continuous dissolution and reprecipitation mediated by aqueous Mn(II) without observable impacts on its mineralogy. Interfacial redox reactions between adsorbed Mn(II) and solid-phase Mn(IV) and Mn(III) are proposed as the main drivers of this process. Interaction between aqueous Mn(II) and structural Mn(III) likely involves interfacial electron transfer coupled with Mn atom exchange. The exchange of aqueous Mn(II) and solid-phase Mn(IV) is more complex and is proposed to result from coupled interfacial comproportionation-disproportionation reactions, where electron transfer from adsorbed Mn(II) to lattice Mn(IV) produces transient Mn(III) species that disproportionate to regenerate aqueous Mn(II) and structural Mn(IV). These findings provide further evidence of the importance of Mn(II)(aq)-MnO2(s) interactions and the attendant production of transient Mn(III) intermediates to the geochemical functioning of phyllomanganates in environments undergoing Mn redox cycling.

Electric Field Modulation on Special Interfacial Magnetic States in Tetragonal La2/3Sr1/3MnO3/BiFeO3 Heterostructures

By first-principles calculations on tetragonal La2/3Sr1/3MnO3(LSMO)/BiFeO3(BFO) heterostructures, we demonstrate two interfacial couplings, direct and indirect, for achieving the net Fe moment in the first layer of interfacial regions. The direct Fe–-Mn (indirect Fe+-OB-MnO) exchange couplings induce a net Fe moment of 0.55 μB (−1.07 μB) in MnO2/Fe–O2 (MnO2/O2–Fe) model, which is parallel(antiparallel) to adjoining Mn around interfaces. Especially, the interfacial net Fe moment in MnO2/Fe–O2 model with 100% spin polarization can be reversed into the opposite direction by an electric field. In the spin-valve device, such controllable interfaces will develop a specific magnetic field region, where the external electric field can modulate the high and low magnetoresistances. These results lay foundations for developing energy-efficient spintronics and multifunctional devices.

Mn3O4@CoMn2O4-CoxOy Nanoparticles: Partial Cation Exchange Synthesis and Electrocatalytic Properties toward the Oxygen Reduction and Evolution Reactions.

Mn3O4@CoMn2O4 nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed Mn3O4 NPs. Selecting the proper cobalt precursor, the nucleation of CoxOy crystallites at the Mn3O4@CoMn2O4 surface could be simultaneously promoted to form Mn3O4@CoMn2O4-CoxOy NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. Mn3O4@CoMn2O4-CoxOy NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 V at -3 mA·cm(-2) and a small Tafel slope of 52 mV·dec(-1) for ORR, and overpotentials of 0.31 V at 10 mA·cm(-2) and a Tafel slope of 81 mV·dec(-1) for OER, thus outperforming commercial Pt-, IrO2-based and previously reported transition metal oxides. This cation-exchange-based synthesis protocol opens up a new approach to design novel heterostructured NPs as efficient nonprecious metal bifunctional oxygen catalysts.

Nanometer-scale exchange interactions between spin centers in diamond

Exchange interactions between isolated pairs of spin centers in diamond have been calculated, based on an accurate atomistic electronic structure for diamond and any impurity atoms, for spin-center separations up to 2~nm. The exchange interactions exceed dipolar interactions for spin center separations less than 3~nm. NV$^-$ spin centers, which are extended defects, interact very differently depending on the relative orientations of the symmetry axis of the spin center and the radius vector connecting the pair. Exchange interactions between transition-metal dopants behave similarly to those of NV$^-$ centers. The Mn\---Mn exchange interaction decays with a much longer length scale than the Cr\---Cr and Ni\---Ni exchange interactions, exceeding dipolar interactions for Mn\---Mn separations less than 5~nm. Calculations of these highly anisotropic and spin-center-dependent interactions provide the potential for design of the spin-spin interactions for novel nanomagnetic structures.

Manganese and iron release from mangrove porewaters: A significant component of oceanic budgets?

Mangrove porewater can be highly enriched in dissolved manganese (Mn), iron (Fe), and other trace metals. As a result, porewater exchange may release dissolved metals to surface waters. This study assessed dissolved Mn exchange with the coastal ocean in four mangroves ecosystems, and whether porewater exchange represents a major driver of the oceanic exchange along a latitudinal gradient in Australia (from 28° S to 12° S). Dissolved Fe was also determined but concentrations were below detection in most surface water samples, preventing any flux estimates. Average concentrations of Mn in porewater were approximately an order of magnitude greater than surface waters at all sites, resulting in average porewater-derived Mn fluxes of 441kmolkm−2year−1 at the four sites. Time series surface water observations indicate that average Mn concentrations decrease at lower latitudes. The average dissolved Mn export rate from the four mangrove systems to the coastal ocean was 88kmolkm−2year−1. Porewater-derived Mn inputs were greater than surface water exports, which may be explained by dissolved Mn precipitation, oxidation or flocculation at the sediment water interface. While the removal of Mn at the sediment-water interface brings about uncertainties in the estimated porewater fluxes, it has no impact on estimated surface water exports to the coastal ocean. If our surface water export estimates are representative of the global mangrove area (140,000km2), mangroves may deliver 12Gmolyear−1 of dissolved Mn to the coastal ocean. These fluxes are greater than the estimated flux from global riverine (5.4Gmolyear−1) and atmospheric (11Gmolyear−1) sources, demonstrating that mangroves may be a major player in the oceanic cycle of Mn.

ChemInform Abstract: A Novel Dimeric Polyanion Based on Mono‐Manganese Substituted Keggin‐Type Phosphotungstate: (MnPW11O39)210‐.

AbstractThe new polyoxotungstate (III) crystallizes in the monoclinic space group P21 with Z = 2 (single crystal XRD, detailed description).

Solid-State Redox Switching of Magnetic Exchange and Electronic Conductivity in a Benzoquinoid-Bridged Mn(II) Chain Compound.

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) with Mn(III) affords the chain compound Mn((N,O)L)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn(II) centers bridged by (N,O)L(2-) ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn((N,O)L)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn((N,O)L)](-), as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm(-1), and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10(-12) S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10(-8) S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe](+) regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.

The role of structural and magnetic inhomogeneities in the formation of magneto-transport properties of the La0.6−xSmxSr0.3Mn1.1O3−δ ceramics

X-ray diffraction, magnetic, transport and SEM measurements were used to investigate the role of structural and magnetic inhomogeneities in the formation of magneto-transport properties of the La0.6−xSmxSr0.3Mn1.1O3−δ (x=0–0.6) ceramics. Reduction of the a parameter of perovskite structure and change of its symmetry are shown to be due to both replacement of La3+ by Sm3+ and an increase concentration of anion and cation vacancies. Broad asymmetric 55Mn NMR spectra are indicative of the high frequency electron double exchange Mn3+↔Mn4+ and demonstrate the heterogeneity of magnetic and valence Mn-states. An excess manganese is dissolved in perovskite structure statistically without formation of planar antiferromagnetic clusters of Mn2+ ions in the deformed A positions. Constructing phase diagram of “composition–structure–properties” describes a strong correlation between structural, resistive and magnetic properties as well as reflects a special role of structural defects in the formation of magneto-transport properties of the rare-earth manganites.

A novel dimeric polyanion based on mono-manganese substituted Keggin-type phosphotungstate: (MnPW11O39)210 −

The novel dimeric polyoxometalate, (MnPW11O39)210− (1a), has been synthesized by reacting [B-α-PW9O34]9− with Mn2+ ions in weak acid aqueous solution (pH=5.3) under hydrothermal condition and isolated in the form of (C14H14N4)5H10(MnPW11O39)2·2H2O (1), which was characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, and magnetic measurements. The polyanion represents the first example of dimeric polyanion based on mono-transition metal (TM) substituted Keggin-type polyanions linked by two TM-μ2-O-TM bridges. Magnetic measurements show that the Mn┄Mn exchange interactions are weakly antiferromagnetic (J=−0.53cm−1).

Structural and magnetic properties of He+ irradiated Co2MnSi Heusler alloys

We have investigated the atomic disorder induced by a 150 keV He+ ion irradiation in a 40 nm thick Co2MnSi Heusler alloy. Disorder parameters on each atomic site are deduced from normal and anomalous x-ray diffraction measurements with Co and Cu Kα sources. While the film grows mainly in the L21 phase with inclusion of B2 grains, we observe an increase of both the Mn–Si and Co–Mn exchanges with the ion fluence. HAADF-STEM analysis demonstrates that the increase in Mn–Si disorder corresponds to a growing size of the B2 grains while the Co–Mn exchange is accounted for a D03 disorder type in the L21 matrix. These structural modifications are shown to decrease the average magnetization of the alloy, which is due to D03 disorder and local defects induced by irradiation.

Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear Mn(II)2Gd(III)2 and Co(II)2Eu(III)2 Complexes.

In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes Mn(II)2Gd(III)2 and Co(II)2Eu(III)2. The energy pattern of the Mn(II)2Gd(III)2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the χT product observed for the Mn(II)2Gd(III)2 complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger (J(Mn, Mn) = -1.6 cm(-1), Js(Mn, Gd) = 0.8 cm(-1), g = 1.97). The model for the Co(II)2Eu(III)2 complex includes uniaxial anisotropy of the seven-coordinate Co(II) ions and an isotropic exchange interaction in the Co(II)2 pair, while the Eu(III) ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co(II) ions) dominates (weak-exchange limit) in the Co(II)2Eu(III)2 complex (D = 20.5 cm(-1), J = -0.4 cm(-1), gCo = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co(II) pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-(1)/2 Hamiltonian that is deduced for the Co(II)2 pair in the weak-exchange limit.

Pressure-induced ferroelectricity and enhancement of Mn-Mn exchange striction in GdMn2O5

We report the effects of hydrostatic pressure up to p ∼ 1.73 GPa on the multiferroic and magnetoelectric properties of GdMn2O5 single crystals. The ferroelectric (FE) polarization can be enhanced at low pressures, while it decreased after reaching a maximum at p ∼ 1.30 GPa. This pressure induced variation of polarization can be ascribed to the combined results of Mn–Mn and Gd–Mn exchange striction under pressure. Our results show that the polarization induced by Mn–Mn interaction increases monotonously with increasing pressure and finally saturates above p ≳ 1.30 GPa. Interestingly, we find that a new FE phase appears in the temperature range of 31 K–38 K with pressure p ≳ 0.88 GPa. This new FE phase can be suppressed by magnetic field, but stabilized by pressure. The pressure-temperature (p–T) phase diagram of GdMn2O5 was obtained. Our analyses suggest that the pressure-induced new FE phase could arise from a non-collinear, incommensurate magnetic structure.

Manipulating coupling state and magnetism of Mn-doped ZnO nanocrystals by changing the coordination environment of Mn via hydrogen annealing**Project supported by the National Basic Research Program of China (Grant No. 2013CB934001) and the National Natural Science Foundation of China (Grant Nos. 51072012 and 51272015).

Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO. Mn ions initially enter into interstitial sites and a Mn3+O6 octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn3+ and O2 ions could form a Mn3+–O2−–Mn3+ complex. After H2 annealing, interstitial Mn ions can substitute for Zn to generate the Mn2+O4 tetrahedral coordination in the nanocrystals, in which neighboring Mn2+ ions and H atoms could form a Mn2+–O2−–Mn2+ complex and Mn–H–Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn3+–O2−–Mn3+ complex, while the annealed samples exhibit their ferromagnetism, which originates from the Mn–H–Mn bridge structure and the Mn–Mn exchange interaction in the Mn2+–O2−–Mn2+ complex.

First-principles investigation of the magnetic structures and pressure-induced magnetic phase transition in magnetocaloric MnRhAs

Density functional theory was applied to investigate the magnetic orderings and pressure-induced magnetic phase transition of hexagonal, Fe2P-type MnRhAs. Effective exchange parameters were evaluated to measure the strength of pair-wise metal–metal magnetic couplings. Results indicate that indirect Mn⋅⋅⋅Mn exchange couplings dominate in MnRhAs. Weak direct exchange couplings for near-neighbor Rh–Rh and Rh–Mn contacts are caused by small moments on the Rh atoms. In addition, RKKY and superexchange mechanisms were used to analyze indirect Mn⋅⋅⋅Mn exchange couplings in MnRhAs. The relationship between inter-layer Mn–Mn magnetic couplings and volume changes in MnRhAs was examined to understand the pressure-induced magnetic phase transition.