Antiferromagnetic Mn Research Articles

Exchange bias and magnetothermal properties in Fe@Mn nanocomposites

We have studied the Exchange Bias (EB) effect in nanocomposite films consisting of Fe nanoparticles (mean size ∼1.9nm) embedded in an antiferromagnetic Mn matrix. They were produced by co-deposition through a gas aggregation cluster source and molecular beam epitaxy and have different Fe volume filling fractions (2.2% and 24.8%). The exchange field, higher in the sample with higher Fe concentration (at T=5K, Hex∼460Oe for 24.8% and ∼310Oe for 2.2% ), in both the samples decreases with increasing T, finally disappearing at T∼40K. The EB properties have been studied in conjunction with results on the thermal dependence of the magnetic coercivity, zero-field-cooled and field-cooled magnetization and thermoremanence. The different Fe content strongly affects the magnetothermal properties, featuring superparamagnetic relaxation in the diluted sample and a reentrant ferromagnet-type transition in the concentrated one. Hence, the EB properties of the two samples have been discussed in consideration of such peculiarities of the magnetic behavior and highlighting the role of the Mn matrix.

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites

Stoichiometric and Cu-poor Cu0.95−xMn0.05InSe2 (x = 0–0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn–Se–Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.

Thermoelectric Properties of Electron-Doped SrMnO3 Single Crystals with Perovskite Structure

Thermoelectric properties have been investigated for single crystals of Sr(Mn1−x Mo x )O3 with the perovskite structure. Similar to (Sr1−x Ce x )MnO3, the Seebeck coefficient for lightly electron-doped compounds (x ≤ 0.01) is enhanced upon G-type antiferromagnetic ordering, while maintaining metallic conduction. This results in enhancement of the figure of merit (ZT). On the other hand, the Seebeck coefficient for the more electron-doped compound (x = 0.025) changes sign from negative to positive within a spin and orbital ordered phase (with C-type antiferromagnetic configuration and Mn 3z 2 − r 2 type orbital order) as the temperature is lowered, whereas the Hall coefficient remains negative in the whole temperature range. The enhancement of the ZT value in the G-type antiferromagnetic phase implies the possibility for improvement of the thermoelectric efficiency by using the coupling between charge, spin, orbital, and lattice degrees of freedom in strongly correlated electron systems.

Magnetovolume effects in substituted Er 1−xGd xMn 6Sn 6 intermetallics

The thermal expansion and spontaneous magnetostriction of polycrystalline samples of Er 1−xGd xMn 6Sn 6 (0 ≤ x ≤ 1) intermetallics with hexagonal HfFe 6Ge 6-type structure are investigated in the temperature range of 77–520 K. The Gd substitution has significant effect on the interatomic distances and especially on inter-sublattice R–Mn couplings. The replacement of Er by Gd causes the lattice constants to increase due to the larger atomic radius of Gd compared with Er. It also results in increasing the ordering temperature as well as the spontaneous magnetostriction values following reinforcement the R–Mn coupling. The examined samples exhibit considerable thermal expansion anomalies at the Néel temperature ( T N = 340 and 335 K for the samples with x = 0 and 0.2, respectively) and also at T M = 309–311 K where the Mn moments experience collapse-like reduction. Whereas, trivial anomalies are revealed at the Curie points ( T C = 77 K for the sample with x = 0, 164 and 383 K for x = 0.2, 419 and 434 K for x = 0.6 and 1, respectively). From the results, it is concluded that the magnetovolume effects in these compounds originate mainly from the antiferromagnetic interlayer Mn–Mn exchange interactions, and the intraplane ferromagnetism does not influence the magnetoelasticity.

Exchange field on the rare earth Sm3+in a single crystal perovskite SmMnO3

Single crystal SmMnO${}_{3}$ has been grown by the floating-zone method. We have measured the magnetization and specific heat in magnetic fields oriented along three principal crystal axes of precisely oriented single crystals. Below ${T}_{N}$ of the Mn${}^{3+}$-ion array, the magnetic moments of the Sm${}^{3+}$ ions are progressively oriented antiparellel to the weak canted-spin ferromagnetic moment of the antiferromagnetic (AF) Mn${}^{3+}$-ion array due to an internal exchange field ${H}_{\mathrm{in}}$ \ensuremath{\parallel} $c$. On cooling through a compensation temperature ${T}_{\mathrm{comp}}$ \ensuremath{\approx} 9 K, the dominant moment parallel to $c$ changes from the canted-spin Mn${}^{3+}$ ions to the Sm${}^{3+}$ moments. A spin reversal in an ${H}_{c}$ \ensuremath{\ge} 1 T changes the magnetic field splitting of the Kramers doublet on the Sm${}^{3+}$ ions from ${H}_{\mathrm{in}}$ \ensuremath{-} ${H}_{c}$ to ${H}_{\mathrm{in}}$ $+$ ${H}_{c}$, where ${H}_{c}$ is a field applied along the $c$ axis. This change, monitored by the Schottky contribution to the specific heat, creates an abrupt change at ${T}_{t}$ $=$ ${T}_{\mathrm{comp}}$ \ifmmode\pm\else\textpm\fi{} \ensuremath{\delta}. We have found no evidence that the transition at ${T}_{t}$ is first-order despite its abrupt nature.

A modular designed ultra-high-vacuum spin-polarized scanning tunneling microscope with controllable magnetic fields for investigating epitaxial thin films

A room-temperature ultra-high-vacuum scanning tunneling microscope for in situ scanning freshly grown epitaxial films has been developed. The core unit of the microscope, which consists of critical components including scanner and approach motors, is modular designed. This enables easy adaptation of the same microscope units to new growth systems with different sample-transfer geometries. Furthermore the core unit is designed to be fully compatible with cryogenic temperatures and high magnetic field operations. A double-stage spring suspension system with eddy current damping has been implemented to achieve ≤5 pm z stability in a noisy environment and in the presence of an interconnected growth chamber. Both tips and samples can be quickly exchanged in situ; also a tunable external magnetic field can be introduced using a transferable permanent magnet shuttle. This allows spin-polarized tunneling with magnetically coated tips. The performance of this microscope is demonstrated by atomic-resolution imaging of surface reconstructions on wide band-gap GaN surfaces and spin-resolved experiments on antiferromagnetic Mn(3)N(2)(010) surfaces.

The low temperature resistivity of thermally evaporated antiferromagnetic Mn_{100-X} Ru_x thin films

Electrical resistivity measurements on thermally evaporated Mn_{100-x} Ru_x thin films (with x = 0.05 at.% Ru and x = 2.5 at.% Ru) have been carried out over the temperature range from 300 to 1.4 K. A behaviour typical of the bulk \alpha-Mn has been observed on the film with x = 0.05 at.% Ru. The Néel point of this specimen is established at 90 \pm 1 K which is typical of the bulk. This Néel point is raised to 100 \pm 1 K with x = 2.5 at.% Ru in \alpha-Mn. A relatively long range magnetic ordering was observed with x = 0.05 at.% Ru in \alpha-Mn indicating that the concentration of Ru has an adverse effect on antiferromagnetism in \alpha-Mn. The low temperature resistivity of the 2.50 at.% Ru in \alpha-Mn can be explained in terms of Kondo scattering.

Magnetic properties of full-Heusler alloy Co2Fe1−xMnxAl films grown by molecular-beam epitaxy

We have investigated magnetic properties of single-crystalline full-Heusler alloy Co2Fe1−xMnxAl films (0<x<1) grown on GaAs (001) by molecular-beam epitaxy at different temperatures. It is found that as x increases, the films grown at 160 °C have complex magnetic phases, but show pure ferromagnetic properties when grown at 280 °C. We attribute this magnetic behavior to the competition between ferromagnetic and antiferromagnetic Mn–Mn interactions, which are proved by x-ray magnetic circular dichroism measurements. Moreover, we have investigated in-plane magnetic anisotropies of Co2Fe1−xMnxAl films with pure ferromagnetic phases and deduced the corresponding anisotropy constants which also remarkably depend on x.

Magnetic properties of Bi2FeMnO6: A multiferroic material with double-perovskite structure

Single phase Bi2FeMnO6 was synthesized on Si substrates by an electrospray method. Three peaks were observed in the temperature dependence of magnetization curve, which is attributed to the inhomogeneous distribution of Fe3+ and Mn3+. The observed magnetic peaks at 150 K, 260 K, and 440 K correspond to orderings of the ferrimagnetic Fe–O–Mn, and antiferromagnetic Mn–O–Mn and Fe–O–Fe, respectively. Heat capacity measurements were carried out to confirm these magnetic transitions. The Debye temperature of Bi2FeMnO6 is 339 K, calculated from Debye–Einstein fitting.

Absence of magnetic ordering in low dimensional mixed magnetic Mn 1− xNi xCl 2·H 2O

A new mixed magnet, Mn 1− x Ni x Cl 2·H 2O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17 K (Mn) and 5.6 5 K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl 2MCl 2M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ M= C/( T− θ). C( x) is linear but θ( x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6 K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T max almost 20% lower than in the pure Mn component but T c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range.

Noncollinear magnetism in manganese nanostructures

We present ab initio spin-density-functional calculations of the magnetic properties of Mn nanostructures with a geometry varying between a straight linear wire and a three-dimensional nanorod, including collinear and noncollinear, commensurate and incommensurate magnetic configurations. With decreasing tension along the axis of the nanostructure we find a series of transitions first from a straight to a zigzag wire, then to planar triangular or hexagonal stripes and further to a nanorod consisting of a periodic stacking of distorted octahedra. At local equilibrium all nanostructures are in a high-moment state, with absolute values of the local magnetic moments per atom varying between $3.79{\ensuremath{\mu}}_{B}$ for a straight noncollinear antiferromagnetic Mn monowire, $3.54{\ensuremath{\mu}}_{B}$ for a triangular collinear antiferromagnetic stripe, $3.40{\ensuremath{\mu}}_{B}$ for a hexagonal collinear ferrimagnetic stripe, and $2.96{\ensuremath{\mu}}_{B}$ for an octahedral noncollinear ferrimagnetic nanorod. For all low-dimensional nanostructures except the monowire we find collinear and noncollinear magnetic structures to be energetically nearly degenerate, if the geometric and magnetic degrees of freedom are relaxed simultaneously. The energetic consequences of a modest change in the interatomic distances are comparable to those of a large canting of the magnetic moments. Compression of the nanostructures leads to a decrease in the magnetic moments.

LARGE COERCIVITY IN ANTIFERROMAGNETIC Mn2O3/ Mn5O8 AND MnO/Mn NANOPARTICLES

Mn 2 O 3/ Mn 5 O 8 and MnO/Mn nanoparticles were prepared by annealing manganese nanoparticles in air and hydrogen, respectively. The antiferromagnetic Mn 2 O 3/ Mn 5 O 8 and MnO/Mn nanoparticles exhibit anomalous magnetic properties, such as large moments, and large coercivity of up to 169 kA/m and 105 kA/m, respectively. This ferromagnetic-like behavior is attributed to the finite size effect of antiferromagnetic nanoparticles and the presence of ferrimagnetic Mn 3 O 4 phases.

Antiferromagnetic Mn 50Fe 50 wire with large magnetostriction

This work presents a study on the relation between the fiber texture and the magnetostrictive performance in an antiferromagnetic Mn 50Fe 50 alloy wire, which was prepared through the combining process of hot rolling and cold drawing. The face-centered cubic (fcc) crystal structure can be retained during the plastic deformation process. Mixed fiber textures consisting of both 〈1 1 0〉 and 〈1 0 0〉 components were formed along the drawing direction (DD) in the wire. A large magnetostriction of 750 ppm was obtained along DD under 1.2 T, which can be ascribed to the single γ phase and the formation of preferred crystal orientation.

Magnetic and electronic structure calculations of antiferromagnetic Mn 2As

Magnetic and electronic structure calculations are performed for Mn 2As with antiferromagnetic (AFM), ferromagnetic (FM), and ferrimagnetic (FIM) spin ordering, using the full-potential linearized augmented plane-wave (FLAPW) method based on the generalized gradient approximation (GGA). It is shown that AFM is the magnetic ground state of Mn 2As, which is in agreement with the experimental observations. At a low temperature (0 K), AFM–FIM transition is also predicted which is consistent with the previous predictions. The ground state stability of the magnetic structure of Mn 2As is attributed to the nearest Mn (I) and Mn (II) antiferromagnetic interaction. The calculated magnetic moment of Mn (II) 3.44 μ B is found to be in good agreement with the neutron diffraction experiment 3.50 μ B while there is a disagreement for the magnetic moment of Mn (I). The different magnetic moments are reflected in the electronic structures of Mn 2As and the exchange splitting between Mn atoms is shown to be an intra-atomic effect.

Correlating exchange bias with magnetic anisotropy in ion-beam bombarded NiFe/Mn-oxide bilayers

The exchange bias field dependence on the Mn-oxide and its microstructure in NiFe/Mn-oxide bilayers was investigated. Transmission electron microscopy results have shown that the bilayer bottom consisted of either α-Mn, rocksalt MnO, or a composite of tetragonal Mn3O4+MnO, depending on the ratio of O2/Ar used during dual ion-beam deposition. Magnetometry results at 5 K indicate that the exchange bias field (Hex∼−300 Oe) is largest in a NiFe/Mn (0%O2/Ar) bilayer. The MnO formation by in situ Mn oxidation results in a decrease in Hex in a NiFe/Mn-oxide (21%O2/Ar) bilayer. In contrast, a further increase in the O2/Ar ratio during deposition results in larger Hex and Hc. This is attributed to the oxidation of MnO into a harder ferrimagnet, Mn3O4. Our results indicate that the antiferromagnetic Mn enabled stronger coupling with NiFe than MnO. In addition, we find that the MnO–Mn3O4 coupling dominates the exchange bias effects at high oxygen concentrations.

Layered antiferromagnetism with high Neel temperature in the intermetallic compound Mn2Au

On the basis of earlier experimental studies the intermetallic compound Mn2Au has been characterized as a nonmagnetically ordered material. Here we report the results of first-principles calculations based on local spin-density approximation that describes Mn2Au to have a narrow band ground state with rigid local moments on the Mn sites. Calculations of the interatomic exchange constants based on the magnetic force theorem and a Monte Carlo modeling of the resulting Heisenberg-like Hamiltonian predict a high Neel temperature of ∼1600 K. This temperature is considerably higher than for the other known high-temperature antiferromagnetic L10-type Mn based binary alloys used in magnetic storage applications.

Spin Wave Dispersion on the Nanometer Scale

Hot electrons injected into antiferromagnetic Mn layers from the tip of a low temperature scanning tunneling microscope have been used to determine the energies, lifetimes, and momenta of antiferromagnetic spin waves on the nanometer scale. The spin waves show a linear dispersion with a velocity of 160+/-10 meV A and lifetimes that scale linearly with energy in agreement with neutron scattering and theory. It is shown that the method is sensitive enough to detect the influence of surface anisotropies on the spin wave dispersion.

Testing the High Spin MnII9WV6 Cluster as Building Block for Three-Dimensional Coordination Networks

Pentadecanuclear high spin cyano-bridged {M' II 9 M v 6 (CN)48(solv)24) (M' = 3d metal ions; M = Mo, W; solv = H 2 O, alcohol) clusters are considered as building blocks for three-dimensional open networks. Self-assembly of Mn II 9 W v 6 clusters (S = 39/2) with 4,4'-bipyridine leads to their organization into an organic-inorganic hybrid coordination network {Mn II 9 (4,4'-bpy) 4 [W v (CN) 8 ] 6 (EtOH) 12 (H 2 O) 4 } ·10EtOH 1 (System, triclinic; space group: P1, a = 17.1569(2) A, b = 17.3835(2) A, c = 19.2530(2) A, a = 103.5520(10)°, P = 115.3950(10)°, y = 98.3060(10)°, Z = 1). The clusters are connected through 4,4'-bpy spacers into a two-dimensional square-grid coordination framework extended in the ab crystallographic plane with the smallest intercluster separation along the a (Mn···W = 7.12 A) and b (Mn···W = 7.15 A) directions significantly lower than that along the c crystallographic direction (Mn···W = 8.64 A). Direct current magnetic measurements suggest strong dominance of intracluster antiferromagnetic Mn II -W v coupling over a very weak intercluster antiferromagnetic interaction.

Grafting Derivatives of Mn6 Single-Molecule Magnets with High Anisotropy Energy Barrier on Au(111) Surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.

A Mössbauer spectroscopy investigation ofh-YbMnO3

170Yb and57Fe Mössbauer spectra are reported for hexagonal phaseYbMnO3 and Yb(57FexMn1−x)O3 (x = 0.005, 0.01), respectively. The dilute concentrations of57Fe are demonstrated to provide a reliable, non-perturbing Mössbauer spectroscopyprobe of the Mn sub-lattice magnetization. Substitution of up to 1 at.%57Fe exerts negligible influence on the Néel temperature(TN1≈88–89 K) and point charge model estimates of the electric field gradient agree well with experimental57Fe Mössbauer results in terms of both sign and magnitude. The170Yb Mössbauer spectrum recorded at 4.5 K provides support for strong crystal fieldquenching with isolated Kramers doublet ground states at both Yb sites.A ‘static’, five-line sub-spectrum is tentatively assigned to the 4b site, forwhich the slowed fluctuation of the Kramers doublet is attributed to non-zeromagnetic exchange with the antiferromagnetic Mn sub-lattice. This is not the casefor the 2a site whose sub-spectrum is a motionally narrowed single line. On thebasis of this work, the saturation magnetization for the ordered Yb sub-lattice(TN2≤3 K) isestimated at ≈1.8 µB per formula unit, in close agreement with single-crystal magnetization data reportedelsewhere.