Mn-Mn Interaction Research Articles

Pressure effect on the magnetic transition temperature of Sm 1− xNd xMn 2Ge 2

Magnetic properties of Sm 1− x Nd x Mn 2Ge 2 are reported. The pressure effect on the magnetic phase transition of Sm 0.9Nd 0.1Mn 2Ge 2 has been measured in order to study the atomic distance dependence of the Mn-Mn interaction. The value of d T c/d P, d T K/d P, d T 1/d P, d T 2/d P are found to be -0.8 K/GPa, -4.4 K/Gpa, -220 K/GPa, 176 K/GPa, respectively. The ( P, T) diagram has also been determined. The results indicate that the magnetic phase transition is strongly correlated with the interatomic distances.

Generalization of the pair approximation and its application to (Zn1-xMnx)3As2.

By taking into account the results of recent studies of (${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${)}_{3}$${\mathrm{As}}_{2}$ (ZMA), which is an example of a II-V diluted magnetic semiconductor (DMS) with a very complicated tetragonal crystal structure, we have generalized the pair-approximation model, widely used to analyze the magnetic behavior of II-VI DMS's, for an arbitrary structure. We have subsequently applied our model, which may be called the generalized pair approximation, to reinterpreting the results of magnetic measurements obtained for ZMA. Based on theoretical works that treat the Mn-Mn interaction in DMS's as a sum of superexchange and the Bloembergen-Rowland (BR) mechanism, we have been able to obtain satisfactory agreement between our approach and experiment by introducing only two adjustable parameters, in spite of the complexity of the crystal structure of ZMA. These parameters are the first nearest-neighbor exchange constants for both mechanisms and they have been found to be equal to -53 and -11 K for superexchange and the BR exchange, respectively.

Magnetic properties of rare-earth manganese compounds of the RMn 6Ge 6 type

We determined the magnetic properties of several compounds of the type RMn 6Ge 6 (R is a heavy rare-earth element). Several of these compounds give rise to two ordering temperatures associated with antiferromagnetic ordering of the R and Mn sublattices. For GdMn 6Sn 6 and to some extent also for TbMn 6Sn 6 we observed magnetic behaviour which can be described as “bootstrap ferrimagnetism”: the antiferromagnetic configurations within both the R and Mn sublattices are broken simultaneously by the R-Mn intersublattice interaction, leading to ferrimagnetic alignment of the R and Mn sublattices. This bootstrap mechanism is sufficiently strong only if the intrasublattice Mn-Mn interaction is sufficiently weak. This condition is met only when the unit cell volume is sufficiently large. The interplay between intersublattice coupling and unit cell volume leads to interesting temperature dependences of the magnetization. In the magnetic isotherms the bootstrap mechanism leads to first-order phase transitions.

Preferred Mn spacings in Al-Mn compounds.

A model pair potential is calculated for Mn-Mn pairs in Al using a Green's function method. The pair potential has a preferred spacing of 4.7 A. The energy resulting from these Mn-Mn interactions strongly favors the Al 6 Mn, a-AlMnSi, and icosahedral quasicrystal structures relative to competing simple structures, providing a possible mechanism for stabilizing the complex structures

Superconducting proximity effect in Cu-clad Nb wires doped with Mn, Fe, Co and Ni

The superconducting proximity effect in Cu-clad Nb wires, doped with dilute magnetic impurities, has been studied in relation to the Kondo effect. The spin flip scattering by Mn is found to be rapidly suppressed below 0.5 K, probably due to the Mn-Mn interaction, while that by Fe, Co and Ni decreases linearly with temperature below 1 K.

High-field magnetization of (Cd 1− x− yZn yMn x) 3As 2

We have performed measurements of magnetization for two samples of the tetragonal semimagnetic semiconductor (SMSC) (Cd 1− x− yZn yMn x ) 3As 2 (CZMA) with different compositions, in fields up to 40T. Taking into account the complicated crystal structure of CZMA within the generalized pair approximation and basing on theoretical analyses concerning the Mn-Mn interaction in SMSC which show that the dominant roles in this process are played by superexchange and the Bloembergen-Rowland exchange, we have obtained a very good agreement between our calculated dependences and experimental data.

Antiferromagnetic coupling between the Mn and Fe nearest neighbours in ferromagnetic Mn:Pt-10 at.% Fe

The low-temperature nuclear orientation of 54Mn in ferromagnetic Pt-10 at.% Fe has been studied in the temperature range 4-50 mK and in the external magnetic field range 0.5-8.5 T. Considerable noncollinearity of Mn magnetic moments with respect to the bulk magnetization direction has been found in the whole Bext range. A model based on the Heisenberg Hamiltonian in the first-neighbour approximation is proposed, which fully describes the experimental data. The authors found that the first-neighbor Mn-Fe exchange interaction is antiferromagnetic with the value JMn-Fe=-21(1) K, which lies in the range of the nearest-neighbor Mn-Mn or Fe-Fe exchange interaction parameters for Pt-based alloys.

Pressure dependence of the Néel temperature in NdMn 2 and related compounds

Resistivity measurements on Nd(Mn 1 − x Cu x ) 2 compounds in the temperature range from 4.2 to 300 K show that magnetic order disappears for x exceeding 0.07. In order to investigate the role of the lattice parameter in the suppression of magnetic order, the effect of pressure on the Néel temperature has been studied by performing resistivity measurements under pressures up to 5 kbar. A linear decrease with pressure has been found with d T N/d p values of -3.45 K/kbar, -2.76 K/kbar and -1.59 K/kbar for the compounds with x = 0, 0.03 and 0.05, respectively. From a comparison with the effect of chemical pressure observed in (Nd, Lu)Mn 2 compounds, it is concluded that the lattice parameter is not the (only) decisive factor, but that the disturbance of the Mn-Mn interaction (caused by substitution of Mn sites) plays an important role.

Ferromagnetism and critical temperatures in dilute P dMn alloys

Neutron-scattering and magnetization measurements on dilute PdMn alloys have been analyzed using a magnetic environment model of ferromagnetism, atomic short-range order, and Mn-Mn interactions. The analysis allows the calculation of several magnetic properties of PdMn, and the effects of Mn-Mn interactions and atomic short range order on the critical temperatures of ferromagnetism. These temperatures, which are small in dilute PdMn alloys in comparison with other typical giant moment systems such as PdFe, were calculated using the Monte Carlo simulation method, and the results were compared with experimental values, showing good agreement for the concentrations considered.

Pressure effect on the magnetic transition temperature of intermetallic compounds NdMn 2Ge 2 and PrMn 2Ge 2

Pressure effect on the Curie temperature T c of NdMn 2 Ge 2 and PrMn 2 Ge 2 have been measured in order to study the atomic distance dependence of the Mn-Mn interaction for RMn 2 Ge 2 (R: a rare earth metal). The values of d T c /d p are found to be −0.6 K/kbar for NdMn 2 Ge 2 and −0.2 K/kbar for PrMn 2 Ge 2 .

Electronic Band Structures for Non-Magnetic and Ferromagnetic States of Transition-Metal Intercalation Compound Mn1/4TaS2

LAPW band calculations are carried out for the non-magnetic and ferromagnetic states of transition-metal intercalation compound Mn 1/4 TaS 2 . It is clarified that the Mn 3 d states hybridize strongly with the Ta 5 d z 2 state which extends along the c -axis, but the hybridization with the S 3 p states is rather small. These results suggest that the Mn-Mn interaction via Ta atoms is strong along the c -axis. The calculated total magnetic moment per Mn (MnTa 4 S 8 ) is 4.35 µ B , which is in agreement with the observed value 3.9∼4.2 µ B .

The spatial distribution of magnetisation density in Mn5Ge3

The spatial distribution of magnetisation density in the intermetallic compound Mn5Ge3 has been determined from a polarised neutron diffraction single-crystal study at 4.2 K. The positional and thermal parameters of the structure were derived from an unpolarised neutron study at 60 K, which also served to characterise the extinction in the material. The observed magnetic structure factors were fitted by a multipole expansion based on free-atom form factors for each of the two crystallographically inequivalent manganese atoms in the unit cell. The magnetic moment of the Mn1 atom with site symmetry 32 is 1.96(3) mu B and that of Mn2 with site symmetry mm is 3.23(2) mu B. The radial form factor of the Mn1 atom is found to be significantly contracted relative to that of a free Mn2+ ion. Evidence is presented that Mn-Mn interactions are the major factor leading to the difference in the moments on the two sites.

Preparation, structure, and magnetochemistry of hexanuclear manganese oxide complexes: chemically and thermally induced aggregation of aquahexakis(benzoato)oxobis(pyridine)trimanganese forming products containing the [Mn6O2]10+ core

A variety of synthetic procedures are described that convert the trinuclear complex Mn30(02CPh)6(py)2(H20) (1) into the hexanuclear complexes [Mn602(02CPh)lo(py)2(MecN)2].2MeCN (2) and [Mn602(02CPh)io(py)4].Et20 (3). With two exceptions, the procedures involve treatment of 1 with phenolic molecules (phenol, p-cresol, tyrosine, 2,2'-biphenol, 8-hydroxyquinoline) or the mononuclear Mn' complexes [Mn(bi~hen)~(biphenH)]~and [Mn(Br4biphen)2(02CPh)]2in MeCN and lead to high (50-75%) yields of complex 2. It is proposed that the mechanism involves reduction of the [Mn30l6+ unit of 1 to a [Mn30I5+ species, which spontaneously aggregates to 2 containing the [Mn602]10t core. This proposal is supported by the formation of 2 when 1 is reduced with the outer-sphere reductant sodium acenaphthylenide. Complex 3 is obtained in 34% yield when a PhCN solution of 1 is refluxed for 10 min. Complex 2 crystallizes in triclinic space group PT with, at -140 OC, a = 15.389 (4) A, b = 19.513 (6) A, c = 14.433 (4) A, a = 91.84 (1)O, (3 = 94.08 ( 1 ) O , y = 87.85 (l)', and Z = 2. The structure was solved and refined by using 5999 unique reflections with F > 3.0u(F). Complex 3 crystallizes in triclinic space group P i with, at -160 C, a = 24.394 (16) A, (3 = 19.876 (11) A, c = 19.245 (12) A, CY = 89.71 (3)O, (3 = 105.43 (3)O, y = 79.89 (3)O, and Z = 2. The structure was solved and refined by using 6000 unique reflections with F > 3.0u(F). Final values of discrepancy indices R (R , ) for 2 and 3 are 7.82 (7.17) and 7.51% (7.63%), respectively. Complex 2 contains a [Mn602],io+ core that can be conveniently described as two edge-sharing Mn4 tetrahedra at the center of each of which is a p4-02ion. Peripheral ligation to the octahedrally coordinated Mn centers is provided by ten bridging benzoate, two terminal py, and two terminal MeCN groups. The complex is mixed valence (Mn4Mn11'2), and the Mn' centers are assigned as the two central metal ions bridged by two 02ions. Complex 3 contains two independent [M~I ,O , ] '~ complexes in the asymmetric unit, each of which is essentially identical with that in complex 2, except that the terminal ligands are all py. The results of solid-state magnetic susceptibility studies on complex 2 in the temperature range 2.95-300 K are described. With use of the idealized symmetry of two edge-sharing tetrahedra (D2h), the derivation by the Kambe vector-coupling method of a theoretical model to account for the intramolecular exchange interactions is described. Least-squares fitting of the susceptibility versus temperature data to the model yields the parameters J, = -42.0 cm-I, J2 = -0.8 cm-I, J3 = -2.4 cm-l, and g = 1.90, where the J values refer to the Mn'-Mn', Mn-Mn', and Mn-Mn interactions, respectively. These exchange parameters give an ST = 0 ground state with two degenerate ST = 1 states at 4 cm-I and degenerate ST = 0, 1, 2 states at 9 cm-' higher in energy. The magnitudes and signs of the exchange parameters are compared with those reported for other oxo-bridged Mn complexes. Complexes 2 and 3 join a small but growing number of high-nuclearity Mn aggregates, and the prospective and potential procedures that could be employed for the synthesis of higher nuclearity Mn aggregates that might display molecular ferromagnetism are discussed.

Theory of bound magnetic polarons in diluted magnetic semiconductors

A theory of bound magnetic polarons (BMPs) in dilute magnetic semiconductors is formulated by applying the functional integral method (FIM). The effects of the antiferromagnetic Mn-Mn interaction on BMPs in dilute systems such as Cd1-xMnxSe, Cd1-xMnxTe, and Cd1-xMnxS are included in terms of clusters. Within the framework of the static approximation to the FIM, the spin-flip Raman scattering spectrum and the binding energy of BMPs are calculated. The theoretical results are compared with available experimental data.

Energetics of acceptor-bound magnetic polarons in diluted magnetic semiconductors.

We present a theory of the bound magnetic polaron (BMP) in diluted magnetic semiconductors, valid for large ${\mathrm{Mn}}^{2+}$ magnetic ion concentrations x, and investigate the formation of acceptor-BMP in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te as a function of x and of the temperature T. Experimentally determined magnetization up to 40 T is used to derive a self-consistent potential expressed in terms of the magnetization in order to investigate the BMP binding energy for x up to 0.5. The nonlinear Schr\"odinger equation obeyed by the BMP is solved numerically to obtain the BMP wave function. This permits the separate estimation of the binding due to the Coulomb potential and that due to the self-consistent exchange potential, including fluctuation effects. The antiferromagnetic Mn-Mn interaction leads to a lowering of the effective value of x. With increasing x, the BMP binding energy saturates. It is proposed that stannite structures containing ${\mathrm{Mn}}^{2+}$, with their absence of nearest-neighbor Mn-Mn interactions, be investigated in order to observe larger BMP binding energy due to an increase in the effective value of x to 0.25. Theoretical estimates for the binding energy of BMP in such structures, due to the exchange interaction alone are found to be about 5 times that in ${\mathrm{Cd}}_{0.75}$${\mathrm{Mn}}_{0.25}$Te. The formation of free polarons, formed by free carriers lowering their energy and localizing in the exchange potential, is also investigated. In stannite structures, with their large effective Mn concentration, it is found that free-hole--polaron formation is favored at temperatures below 15 K.

Antiferromagnetism of the hexagonal Laves phase ThMn2

Magnetization, thermal expansions and neutron diffraction measurements have been performed on ThMn2. Below TN, one Mn atom (2a) in four remains nonmagnetic whilst the others order in a triangular configuration. The large anistropic thermal expansions effects minimize the frustration of the Mn-Mn interactions.

Magnetic properties of Pb1-xEuxTe grown by molecular-beam epitaxy.

We have measured the magnetization and magnetic susceptibility of molecular-beam-epitaxy grown ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Eu}}_{\mathrm{x}}$Te with x=0.042, 0.096, 0.316, and 1.00, in the temperature range 2 K<T<300 K and for fields 0<H<4 T. The magnetic susceptibility follows a Curie-Weiss behavior: \ensuremath{\chi}\ensuremath{\sim}${N}_{\mathrm{Eu}/(\mathrm{T}\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\Theta}}}$). For all the concentrations studied, \ensuremath{\Theta} is negative indicating that the Eu-Eu interaction is predominantly antiferromagnetic. However, the absolute values of \ensuremath{\Theta} are 1 or 2 orders of magnitude smaller than those observed in (II,${M}_{n}$)-VI diluted magnetic semiconductors. The magnetization can be described by a modified Brillouin function: M\ensuremath{\sim}${N}_{\mathrm{Eu}{}^{\mathrm{*}}\mathrm{B}[(\mathrm{T}\mathrm{\ensuremath{-}}{T}_{0}{)}^{\mathrm{\ensuremath{-}}1}}$] in which, for each sample, the number of Eu atoms ${N}_{\mathrm{Eu}{}^{\mathrm{*}}}$ and the parameter ${T}_{0}$ are nearly the same as ${N}_{\mathrm{Eu}}$ and \ensuremath{\Theta} respectively, obtained from the susceptibility measurement. The results of the present study indicate that Eu-Eu exchange effects in ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Eu}}_{\mathrm{x}}$Te are much weaker than Mn-Mn interactions in (II,${M}_{n}$)-VI compounds.

Bound magnetic polaron in semimagnetic semiconductors : Effects of Mn-Mn exchange interaction

We study the effects of the antiferromagnetic Mn-Mn exchange interaction on a bound magnetic polaron (BMP) in dilute systems like Cd 1−xMn xTe. The density matrix expansion method is used to show that at high T the magnetic binding energy is proportional to 1 (T + τ) , where τ is smaller than the Curie-Weiss temperature. We discuss the relevance of this result to the usually employed “antiferromagnetic temperature” T 0 and experimental data indicating that (T 0) BMP < (T 0) magnetization. We next consider the low temperature limit and examine the approximation of using a reduced effective concentration ( x ). Finally, a quantum mechanical cluster model is used for studying the effects of increasing average Mn-Mn interaction on the binding energy versus temperature curve of an acceptor BMP. As the magnitude of the interaction increases, the low temperature binding energy decreases and the BMP effect gradually disappears in favour of a magnetic fluctuation-dominated behaviour.

Electronic structure of MnSb.

Self-consistent, spin-polarized energy-band calculations have been performed for MnSb, for both ferromagnetic and antiferromagnetic spin alignments. For ferromagnetic MnSb we find a 3d5.5 configuration on the Mn atom, with a magnetic moment of 3.3μB on the Mn sites, and –0.06μB on the Sb sites. The charge transfer of Mn to Sb is very small. A detailed analysis is made of the strong covalent Mn-Mn and Mn-Sb interactions. The coupling between the moments is described in terms of covalent interactions of unoccupied states of the Sb 5p band with the Mn 3d bands. These interactions stabilize the ferromagnetic alignment. The calculated spin density, which agrees very well with experimental data, can be easily understood in terms of a model of covalent bonding. We compare the calculated Fermi surface, total density of states, and joint density of states. with results of transport measurements, x-ray photoelectron spectroscopy, and optical measurements, respectively.

Determination of exchange interactions of solid solutions of Mn2+in Pd and Pd hydride through ESR experiments

The authors present ESR data concerning manganese impurities dissolved in pure palladium and in the beta phase of palladium hydride. In the pure Pd matrix, the behaviour of the resonance of the Mn impurities in low-concentration alloys shows that: (i) the resonance is bottlenecked, (ii) the g factors of the conduction electrons and Mn2+ ions are different, (iii) the conduction electron-manganese coupling is strongly positive with a Korringa shift triangle gK/gi approximately=0.5 which agrees with the presence of a giant moment on the manganese ions. In the hydrides they observe a smaller but negative g shift, in agreement with the familiar covalent mixing interaction and some indication of a bottlenecked resonance. The spatial dependence of the conduction electron-manganese interaction is analysed using the ESR data and the residual resistivity. The influence of indirect and direct Mn-Mn interactions on the ESR is observed and discussed for the more concentrated alloys.