Ferromagnetic Mn Research Articles

Growth and characterization of Mn-doped cubic-GaN

We report on the growth and characterization of Mn-doped cubic-GaN films. The n-type carriers, which are resulting from the formation of nitrogen vacancies, were intentionally utilized. For a 3% Mn sample with a high carrier density (∼1×10 20 cm −3), we found substitutional Mn atoms on both the Ga-site and N-site, i.e., (Ga,Mn)N and Ga(N,Mn) exist together. A ferromagnetic behavior was observed in this sample at low temperature (∼5 K), although a 3% Mn semi-insulating sample in which Mn atoms substitute only the Ga-sites in the cubic-GaN lattice showed paramagnetism even at low temperature. The structural and magnetic properties of the relatively high Mn composition (⩾5%) samples were found to be governed by precipitate clusters of antiferromagnetic GaMn 3N and ferromagnetic Mn 4N.

Magnetic phase diagrams of [formula omitted]-MnMoO 4

Field-induced spin-flop transitions in α -MnMoO 4 are summarized in magnetic H – T phase diagrams for different directions of the applied magnetic field up to 12 T. The antiferromagnetic arrangement in the spin-flop phase is preserved at least up to this field for a field parallel to the easy direction. This high transition field is in contrast to the low one of α -NiMoO 4 and favours a model, based on dominant antiferromagnetic supersuperexchange couplings in α -MnMoO 4 over a ferromagnetic Mn 4 “cluster” model. The Néel temperature of 9.8(1) K was determined from the corresponding specific-heat anomaly, measured on a single crystal of α -MnMoO 4.

Structural and magnetic properties of Mn-doped GaAs(1 1 0) surface

We have investigated STM images of the (1 1 0) cross-sectional surface of Mn-doped GaAs using first principles total-energy pseudopotential calculations. We focus on configurations with Mn interstitial in the uppermost surface layers. In particular, we have found that Mn impurities, surrounded by Ga or As atoms, introduce in both cases strong local distortions in the GaAs(1 1 0) surface, with bond length variations up to 8% on surface and non-negligible relaxations effects propagating up to the third subsurface layer. In both cases interstitial Mn induces a spin-polarization on its nearest neighbors, giving rise to a ferromagnetic Mn–As and to antiferromagnetic Mn–Ga configuration.

Valence instability of cerium under pressure in the Kondo-like perovskiteLa0.1Ce0.4Sr0.5MnO3

Effect of hydrostatic pressure and magnetic field on electrical resistance of the Kondo-like perovskite manganese oxide, La$_{0.1}$Ce$_{0.4}$Sr$_{0.5}$MnO$_3$ with a ferrimagnetic ground state, have been investigated up to 2.1 GPa and 9 T. In this compound, the Mn-moments undergo double exchange mediated ferromagnetic ordering at $T_{\rm C}$ $\sim$ 280 K and there is a resistance maximum, $T_{\rm max}$ at about 130 K which is correlated with an antiferromagnetic ordering of {\it cerium} with respect to the Mn-sublattice moments. Under pressure, the $T_{\rm max}$ shifts to lower temperature at a rate of d$T_{max}$/d$P$ = -162 K/GPa and disappears at a critical pressure $P_{\rm c}$ $\sim$ 0.9 GPa. Further, the coefficient, $m$ of $-logT$ term due to Kondo scattering decreases linearly with increase of pressure showing an inflection point in the vicinity of $P_{\rm c}$. These results suggest that {\it cerium} undergoes a transition from Ce$^{3+}$ state to Ce$^{4+}$/Ce$^{3+}$ mixed valence state under pressure. In contrast to pressure effect, the applied magnetic field shifts $T_{\rm max}$ to higher temperature presumably due to enhanced ferromagnetic Mn moments.

Magnetic and theoretical characterization of a ferromagnetic Mn(III) dimer

Reaction of 1,1,1-tris(hydroxymethyl)ethane (H3thme) with the complex [Mn2O2(bpy)4](ClO4)3 produces the dimeric species [Mn2(Hthme)2(bpy)2](ClO4)2 in high yield. Magnetic measurements in the temperature range 1.8–300 K and in fields up to 7 T reveal weak ferromagnetic exchange between the metal centres with J = +2.13 cm−1. A fit of the magnetization data, assuming only the ground state is populated, gives S = 4, g = 1.71 and D = −0.65 cm−1. Low temperature single crystal measurements suggest the co-existence of SMM behaviour and strong intermolecular interactions. Density functional calculations also support a weak exchange interaction between the Mn ions.

Spin polarization and electronic structure of ferromagnetic Mn5Ge3 epilayers

AbstractGermanium‐based alloys hold great promise for future spintronics applications, due to their potential for integration with conventional Si‐based electronics. High‐quality single phase Mn5Ge3(0001) films, grown by solid‐phase epitaxy on Ge(111) and GaAs(111), exhibit strong ferromagnetism up to the Curie temperature TC ∼ 296 K. Point Contact Andreev Reflection (PCAR) measurements on Mn5Ge3 epilayers reveal a spin‐polarization P = 42 ± 5% for both substrates. We also calculate the spin polarization of bulk Mn5Ge3 in the diffusive and ballistic regimes using density‐functional theory (DFT). The measured spin polarization exceeds the theoretical estimates of PDFT = 35 ± 5% and 10 ± 5% for the diffusive and ballistic limits, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic properties of Cu substituted NdMn2Si2 intermetallics

AbstractThe structure and magnetic properties of NdMn2–xCuxSi2 (0.2 ≤ x ≤ 1) were studied by X‐ray powder diffraction and magnetization measurements. In this study, we investigate the variations in the magnetic properties of NdMn2–xCuxSi2 as a function of Cu concentration by examing the evolution of the features in the temperature dependence of the magnetization. Earlier neutron diffraction experiments showed that the ferromagnetic Mn planes are ordered antiparallel along the c‐axis below 380 K and the Nd sublattice orders at 33 K in NdMn2Si2. The ordering of the Nd sublattice reconfigures the ordering in Mn sublattice and leads to ferromagnetic ordering. With increasing amount of Cu, the Curie temperature has a maximum value of 120 K at x = 0.7 and decreases for the samples with x ≥ 0.8. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of magnesium substitution on the magnetic properties of Nd 0.7Sr 0.3MnO 3

Effects of magnesium substitution on the magnetic properties of Nd 0.7Sr 0.3MnO 3 have been investigated by neutron powder diffraction and magnetization measurements on polycrystalline samples of composition Nd 0.7Sr 0.3MnO 3, Nd 0.6Mg 0.1Sr 0.3MnO 3, Nd 0.6Mg 0.1Sr 0.3Mn 0.9Mg 0.1O 3, and Nd 0.6Mg 0.1Sr 0.3Mn 0.8Mg 0.2O 3. The pristine compound Nd 0.7Sr 0.3MnO 3 is ferromagnetic with a transition temperature occurring at about 210 K. Increasing the Mg-substitution causes weakened ferromagnetic interaction and a great reduction in the magnetic moment of Mn. The Rietveld analyses of the neutron powder diffraction (NPD) data at 1.5 K for the samples with Mg concentration, y = 0.0 and 0.1, show ferromagnetic Mn moments of 3.44(4) and 3.14(4) μ B, respectively, which order along the [001] direction. Below 20 K the Mn moments of these samples become canted giving an antiferromagnetic component along the [010] direction of about 0.4 μ B at 1.5 K. The analyses also show ferromagnetic polarization along [001] of the Nd moments below 50 K, with a magnitude of almost 1 μ B at 1.5 K for both samples. In the samples with Mg substitution of 0.2 and 0.3 only short range magnetic order occurs and the magnitude of the ferromagnetic Mn moments is about 1.6 μ B at 1.5 K for both samples. Furthermore, the low-temperature NPD patterns show an additional very broad and diffuse feature resulting from short range antiferromagnetic ordering of the Nd moments.

Identification of extrinsic Mn contributions in Ga1−xMnxAs by field-dependent magnetic circular X-ray dichroism

We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field-dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1−xMnxAs and observe different Mn constituents: ferromagnetic Mn with an nd>5 lineshape and paramagnetic Mn with distinct nd=5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency.

First-principles study of exchange interactions and Curie temperatures of half-metallic ferrimagnetic fullHeusler alloys Mn2V Z (Z = Al,Ge)

We report the parameter-free, density functional theory calculations of interatomic exchangeinteractions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloysMn2V Z (Z = Al,Ge). To calculate the interatomic exchange interactions we employ the frozen-magnonapproach. The Curie temperatures are calculated within the mean-fieldapproximation to the classical Heisenberg Hamiltonian by solving a matrixequation for a multi-sublattice system. Our calculations show that, although alarge magnetic moment is carried by Mn atoms, competing ferromagnetic(inter-sublattice) and antiferromagnetic (intra-sublattice) Mn–Mn interactions inMn2V Al almost cancel each other in the mean-field experienced by the Mn atoms. InMn2V Ge the leading Mn–Mn exchange interaction is antiferromagnetic. In bothcompounds the ferromagnetism of the Mn subsystem is favoured by strongantiferromagnetic Mn–V interactions. The obtained value of the Curie temperature ofMn2V Al is in good agreementwith experiment. For Mn2V Ge there is no experimental information available and our calculation is a prediction.

Magnetic ordering of the Pr sublattice with the substitution of Cu for Mn inPrMn2Si2 intermetallics

The magnetic properties of PrMn2−xCuxSi2 () were studied by field-cooled and zero-field-cooled magnetization measurements in thetemperature range 5 K K in low external fields (5 mT) and by magnetic-field-dependent magnetization measurementsin fields up to 5.5 T. Substitution of Cu for Mn leads to a linear decrease in the lattice constantc and the unitcell volume V and a linear increase in the lattice constanta. Earlier neutron diffraction experiments showed that Pr does not order down to 1.6 K inPrMn2Si2 while the ferromagnetic Mn planes are ordered antiparallel along thec axis. With the increasing Cu content, the magnetization increases rapidly at lowtemperatures for the samples with . Cu substitution strongly changes the magnetic properties and leads to the magneticordering of the Pr sublattice. This is mainly deduced from the discussion of the valuesof the magnetic moments at low temperatures. Below the Curie temperaturesTC, the spins in the Mn sublattice are arranged parallel to the Pr sublattice. With increasing Cu,TC(x) has a maximumvalue of 155 K at x = 0.6 and decreases for samples with .

Te-co-doping experiments in ferromagnetic Mn(Ga)As/GaAs-cluster hybrid layers by MOVPE

Hybrid structures consisting of ferromagnetic Mn(Ga)As clusters, which are embedded defect-free in a p-GaAs:Mn matrix, are realized by epitaxial growth using metalorganic vapour-phase epitaxy (MOVPE). The successful Te-co-doping of the GaAs:Mn matrix leads to a change from p- to n-type carrier transport. This behaviour enables the growth of structures for studying electron spin-injection effects. First laser devices including a co-doped hybrid structure in the n-type region of the contact of the device are realized. Investigations by SQUID magnetometer show that the ferromagnetic properties of the Mn(Ga)As clusters are not influenced by the Te co-doping of the surrounding GaAs matrix.

A cyano-bridged molecule-based magnet containing manganese(III) Schiff base and octacyanotungstate(V) building blocks

A hetero-bimetallic complex K[Mn(acacen)] 2[W(CN) 8] · 2H 2O (acacen 2− = dianion of N, N′-ethylenebis(acetylacetonylideneaminato)) consists of two-dimensional gridlike {[Mn(acacen)] 2[W(CN) 8]} − layers that are linked by K + ions giving rise to a three-dimensional network. Ferromagnetic Mn(III)–W(V) coupling and magnetic ordering at 18.0 K ( T c) have been observed in the complex.

A dinuclear Mn(II) chloro-bridged compound with a weak ferromagnetic Mn–Mn interaction: Synthesis, structure, EPR and magnetism of [Mn(μ-Cl)(2,2′-bi-imidazoline) 2] 2Cl 2

In this study an unusual dinuclear bis-chloride bridged Mn(II) compound, [Mn(μ-Cl)(biz) 2] 2Cl 2 (in which biz = 2,2′-bi-imidazoline), has been described and characterised. Also the magnetic susceptibility and EPR investigations are made. The distorted octahedral geometry around the Mn(II) ion is formed by four nitrogen atoms of two dichelating biz ligands and two bridging chloride atoms. The lattice is further stabilised by hydrogen bonds between uncoordinating N atoms of the ligand and the coordinating and uncoordinating chloride atoms. Magnetic susceptibility measurements reveal a weak ferromagnetic interaction between the two high-spin Mn(II) ions of J = 0.33 cm −1.

Reentrant spin glass behavior in polycrystalline La0.7Sr0.3Mn1-X FeX O 3

The magnetic and transport properties of the compound La0.7Sr0.3Mn1-xFe xO 3 (0.1 < x <0.4) have been studied by means of electrical resistivity, AC magnetic susceptibility, and DC magnetization. At low concentrations (x <0.1), the system displays essentially para-to-ferromagnetic transitions as the temperature is decreased, although a decrease in the magnetic moment has been observed in previous studies at temperatures a little below T C. This ferromagnetism is explained by double exchange theory in terms of the formation of Mn+3/Mn+4 ions pairs in the system. At concentrations in the range 0.1 < x <0.4 the system is more complex. Increased Fe doping not only weakens the ferromagnetic (FM) order and augments the resistivity of the samples, but also induces the appearance of a reentrant spin glass phase at low temperatures (T < 60 K). Irreversibility of the magnetization measured with zero field cooling and with field cooling has been observed. In addition, the AC susceptibility peak position varies with frequency. All these effects are characteristic of spin glass behavior. The results have been interpreted based in an increase of frustration due to increasing competition between FM Mn+3/Mn+4 interactions and antiferromagnetic interactions between ions at the boundaries of Fe clusters.

Determination of hole-induced ferromagnetic Mn–Mn exchange in p-type Zn 1− xMn xTe by inelastic neutron scattering

Direct determination of the hole-induced ferromagnetic contribution to the nearest-neighbor (NN) Mn–Mn exchange interaction in Zn 1− x Mn x Te is reported. The difference in the NN exchange constants for insulating and strongly p-type specimens was found by comparing the spectra of inelastic magnetic neutron scattering from isolated NN spin pairs in the samples. Our results are in good agreement with the predictions of a Zener/RKKY model, and thus indicate that the ferromagnetic exchange is mediated by weakly localized holes.

Interacting networks and spin reversal in (RE, Ca)MnO 3

Magnetic properties of (RE, Ca)MnO 3 perovskites (RE=Gd, Dy) have been studied in the ordered state. The remanent magnetization M rem reverses its magnetic moment, as if the external field was applied opposite to the initial direction. Results are interpreted in terms of two interacting networks, with negative exchange interaction between the RE and the ferromagnetic Mn lattices.

Atomistic modeling of polar LaMnO 3 surfaces

The results of the atomic and electronic structure calculations, with a focus on the surface relaxation and polarization, are presented for the LaMnO 3 (1 1 0) O-terminated polar surface. We compare results of the classical shell model (SM) and ab initio Hartree–Fock calculations for three possible surface models, including (1×2) (1 1 0) surface reconstruction, and demonstrate that the latter has the lowest surface energy. The surface energy is saturated only when six to eight near-surface atomic planes are relaxed which is accompanied by considerable macroscopic surface polarization. Properties of stoichiometric surfaces with oxygen vacancies and non-stoichiometric defect-free surfaces are compared, along with ferromagnetic and antiferromagnetic (AFM) Mn spin orderings in slabs.

Mn implanted GaAs by low energy ion beam deposition

High dose Mn was implanted into semi-insulating GaAs substrate to fabricate embedded ferromagnetic Mn–Ga binary particles by mass-analyzed dual ion beam deposit system at room temperature. The properties of as-implanted and annealed samples were measured with X-ray diffraction, high-resolution X-ray diffraction to characterize the structural changes. New phase formed after high temperature annealing. Sample surface image was observed with atomic force microscopy. All the samples showed ferromagnetic behavior at room temperature. There were some differences between the hysteresis loops of as-implanted and annealed samples as well as the cluster size of the latter was much larger than that of the former through the surface morphology.

Effect of oxygen on the Mn–Co ferromagnetic coupling

The paper describes the study of the effect of oxidation on the magnetic coupling between Mn and Co atoms in ultrathin Mn films deposited on fcc Co(0 0 1). The magnetic map has been investigated through tight-binding linear-muffin-tin scheme within generalized gradient approximation of the density functional. The calculations indicate that oxygen tends to destabilize the ferromagnetic Mn–Co coupling in favour of an antiferromagnetic coupling found by experiment when oxygen is added. Nevertheless, the (1×1) structures observed do not seem to be the most stable ones. We speculate that a disordered oxidized Mn-rich MnCo surface alloy might explain the experimental findings.