Double Exchange Magnetic Interaction Research Articles

Resistive Switching and Redox Process at the BaTiO3/(La,Sr)MnO3 Multiferroic‐Type Interface

AbstractResistive switching in oxide‐based structures is intensively studied for the development of ultra‐fast non‐volatile memories with low consumption and neuromorphic electronic devices. Here, evidence of interface‐controlled, reversible, bi‐polar resistance switch in prototype multiferroic BaTiO3/La0.75Sr0.25MnO3 (BTO/LSMO) system is given. The analysis of current–voltage hysteresis curves of a thick Au/BTO/LSMO structure reveals the existence of a 2.55 nm‐thick barrier layer at the bottom BTO/LSMO interface, leading to a tunnel electroresistance of ≈1700%. In the native state where BTO polarization points downward, high resolution electron microscopy and electron energy loss spectroscopy show Ba/(La,Sr) and Ti/Mn cationic intermixing, together with a valency reduction of Mn cations and an oxygen deficiency at the interface. This results in a high resistance state, due to partial loss of the metallicity and of the double exchange magnetic interaction in the oxygen‐deficient LSMO. The switch to the low resistance state, achieved by upward electric field, is explained in terms of re‐oxydation of Mn cations at the interface (decrease of the Mn3+/Mn4+ ratio), with oxygen vacancies migration from LSMO to BTO. This work emphasizes the crucial role of ionic exchanges and of redox processes at interfaces, both on ferroelectric bound charge screening and on resistive switching.

Magnetic characteristics of M2FeV3O11 (M = Mg, Zn, Pb, Co, Ni) compounds

The unusual physical characteristics of the multicomponent oxide systems renewed the interest as the potential cathode materials in high-energy cells. Since the earlier magnetic characteristics were not entirely conclusive, we report the results of dc magnetic measurements including higher harmonics of ac magnetic susceptibility of the M2FeV3O11 (M=Mg, Zn, Pb, Co, Ni) compounds. Ferrimagnetic long-range and antiferromagnetic short-range interactions for all compounds under study at low temperatures as well as superparamagnetic-like behavior with the blocking temperature of 29K and the freezing parameter of 0.013 were observed. These effects are discussed within the framework of superexchange and double exchange magnetic interactions as well as the mixed valence band of iron ions.

Characterization of Excited-State Magnetic Exchange in Mn2+-Doped ZnO Quantum Dots Using Time-Dependent Density Functional Theory

Absorption spectra of Mn2+-doped ZnO quantum dots have been studied with the linear-response time-dependent density functional theory. Spectral changes caused by excited state dopant-carrier and dopant–dopant magnetic exchange couplings are investigated. The excitonic transition maximum shifts to higher energy and decreases in intensity with increasing Mn2+ concentration. The lowest excitonic transitions split in the spin-up and spin-down manifolds due to sp–d magnetic exchange between the Mn2+ and ZnO conduction and valence band carriers. Increased Mn2+ concentration leads to a broadening and increase in the intensity of the midgap charge-transfer electronic absorption band. The charge-transfer band broadening results from excited-state splitting arising from double exchange magnetic interactions involving Mn2+ ions and the photogenerated hole. The excited-state double exchange leads to stabilization of the ferromagnetic configuration in the charge-transfer state. The strength of this ferromagnetic double exchange interaction depends on the inter-Mn2+ distance within the quantum dot.

Magnetic Double Exchange Interaction as a Driving Force of the Coexistence of the Negative Giant Magnetoresistance and the Spin Glass State in the Selected Re-Manganites and the Spinels with Chromium

In this article is pointed out the particularly important role of the double exchange magnetic interaction in the appearance of the phenomenon of the coexistence of the negative giant magnetoresistance and the spin glass state in the selected RE-manganites and the spinels with chromium. To explain this phenomenon, its statistical background and the types of magnetic interactions in the compounds under study are taken into account. The particular importance of the double exchange magnetic interaction “cooperating” with the strong external magnetic induction results here from that: i. its coupling constant is several times greater than the superexchange one, ii. the spin orientation of the hopping electrons is conserved (always ferromagnetic coupling!) and the charge transfer makes the p-type metallic conductance.

Metamagnetism, giant negative magnetoresistance and spin glass state in the Sm1−xSrxMnO3 (x=0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55=Eu0.397Nd0.153, Tb0.25Nd0.30) manganites as well as in the CuCr2X4 (X=S, Se) spinels doped with Sb

Abstract The experimental evidence of metamagnetism, giant negative magnetoresistance and spin glass state is presented for the compounds under study. An attempt is made to explain the jump-like phase transition of the colossal negative magnetoresistance of −4433% at 153.2 K into the colossal positive magnetoresistance of +980% at 154.2 K for the spinel sample with selenium. It was emphasized that in all the phenomena described in this paper the double exchange magnetic interaction plays the main role.

Effect of double exchange on thermoelectric power of Cu xGa yCr zSe 4

A correlation between double exchange magnetic interaction and the thermoelectric power is considered in the ferromagnetically ordered Cu x Ga y Cr z Se 4 nonstoichiometric p-type spinel conductors in the temperature range from 5 to 400 K between y = 0.036 and 0.394. The magnetic and thermopower investigations provide evidence that: (1) the carrier diffusion dominates in the most stoichiometric single crystals and the intensity of diffusion thermopower is five times larger than the intensities of the phonon and magnon excitations, (2) for samples with low gallium content the strong ferromagnetic coupling connected mainly with double exchange mechanism makes easier the spin wave excitations at higher temperatures on one side and the high nonstoichiometry hinders the spin wave excitations on the other, and (3) for samples with higher gallium content, which are more stoichiometric, the ferromagnetic coupling connected here with the weaker double exchange interaction causes a shift of the magnon drag peak into lower temperatures.

Thin films of Co3O4, MnCo2O4 and their solid solution as electrocatalyst: study of their magnetic properties

We present magnetic measurements performed on the MnxCo3–xO4 (0 ≤ x ≤ 1) solid solution, prepared by spray pyrolysis. Thin films of about 20 μm thickness were obtained after depositing aqueous metal nitrates precursors on Ti or conducting glass substrates. Powder material scratched from the substrate's surface was used for physico-chemical characterization. Presence of two oxidation states for each metal ion (Mn3+–Mn4+; Co2+–Co3+) triggers specific conduction mechanisms and double-exchange magnetic interactions. The ferromagnetic components are enhanced when x(Mn) increases, in detriment of the antiferromagnetic interactions. The ordering temperature Tc changes with x (from 22 K up to room temperature, for x = 0 up to x = 1). Based in the paramagnetic moment μeff obtained at T > Tc, we propose a cationic distribution which can be nicely compared to the one obtained from crystallographic analyses. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn1−xCuxCr2Se4 with 0.0≤x≤0.3. It is found that the Zn0.85Cu0.15Cr2Se4 spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T N) and 410 K (Curie temperature; T C) and that the Zn0.70Cu0.30Cr2Se4 spinel has magnetic transitions at 24.5 K (T N) and 415 K (T C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr2Se4 shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn1−x Cu x Cr2Se4; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T N and short-range order of spins (spin glass-like) above T N. The incommensurate antiferromagnetic phase below T N results from a spiral long-range order of the spins of Cr3+. The intermediate ferromagnetic phase between T N and T C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr3+ and Cr4+ mediated by current carriers, positive holes, which is made by the substitution of Zn2+ ions with Cu1+ ions in Zn1−x Cu x Cr2Se4.

ChemInform Abstract: Double Exchange Magnetic Interaction and Giant Negative Magnetoresistivity in the Spin Glass State of the New Compound CuCr1.6Sb0.04S4.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Double exchange magnetic interaction and giant negative magnetoresistivity in the spin glass state of the new compound CuCr 1.6Sb 0.4S 4

CuCr 1.6Sb 0.4S 4 crystallizes in the spinel structure with the lattice parameter a=9.9680(1) Å and the anion parameter u=0.3820(1). The very hard magnetization of the sample, the shape of the magnetization isotherms as well as the lack of magnetic saturation even in a field of B=14 T indicate that the antiferromagnetic superexchange interaction dominates the long-range magnetic interactions between the magnetic moments localized on the chromium ions. A high positive value of the paramagnetic Curie–Weiss temperature Θ C–W (equal to 318 K) testifies that the short-range interaction is ferromagnetic and strong which means that in the corresponding microregions the double exchange interaction is present and predominates. A suggestion is made that these microregions behave like superparamagnetic clusters. As the result of the competition of the interactions mentioned above the sample under study reveals a spin glass state at low temperatures, which is confirmed by the temperature dependence of the magnetic susceptibility curves. The spin-freezing temperature is lower than 50 K. CuCr 1.6Sb 0.4S 4 is a semiconductor with a giant negative magnetoresistivity equal to 74% at 38 T and 3 K. The driving force of this effect is the double exchange magnetic interaction. Three values for the activation energy were observed: E 1=0.118 eV, E 2=0.004 eV and E 3=0.033 eV. These are most probably connected with the energy levels generated by the crystal field.

Colossal Magnetoresistance Without Double Exchange Mechanism in Spinel FeCr2S4

The crystal structure of spinel FeCr2S4, which shows a semiconductor-metal transition and paramagnetic-ferrimagnetic transition at Tc = 170 K, was determined by x-ray powder diffraction. It is found that the spinel FeCr2S4 belongs to a normal spinel structure [Fe2+]t[Cr23+]oS4. The result clearly shows that the spinel FeCr2S4 has neither mixed valence of Fe3+ and Fe2+ ions or Cr3+ and Cr2+ ions for a double-exchange magnetic interaction, nor a Jahn-Teller cation such as Cr3+. The spinels represent a distinct class of colossal magnetoresistance materials, with magnetic ordering driven by superexchange rather than double exchange.

Large Low-Field Magnetoresistance in Perovskite-typeCaCu3Mn4O12without Double Exchange

We report the observation of large low-field magnetoresistance (MR) behavior in ${\mathrm{CaCu}}_{3}{\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$ with a perovskite-related $({\mathrm{AA}}^{\ensuremath{'}}_{3}B_{4}{\mathrm{O}}_{12})$ structure. ${\mathrm{Ca}}^{2+}{\mathrm{Cu}}_{3}^{2+}{\mathrm{Mn}}_{4}^{4+}{\mathrm{O}}_{12}$ is semiconducting and orders ferromagnetically at 355 K. However, it has neither mixed valency of Mn for double-exchange magnetic interactions, nor Jahn-Teller ${\mathrm{Mn}}^{3+}$ ions, nor a metal-insulator transition. The MR smoothly increases with decreasing temperature and is $\ensuremath{-}40%$ at 20 K. The low-field MR response does not show the strong temperature-dependent decay characteristic of the ${\mathrm{LnMnO}}_{3}$ perovskite-based systems and appears consistent with an intergrain-interdomain tunneling origin.

Structure, Magnetism and Colossal Magnetoresistance Behavior in A2Mn2O7 Pyrochlores (A = Dy-Lu, Y, Sc, In or Tl)

This paper presents a bird's eye view of the structural, electrical and magnetic properties of A 2 Mn 2 O 7 (A = rare earth, Y, Sc, In, Tl) phases crystallizing in pyrochlore-related structure. Electrical measurements indicate the compounds containing rare earth, Y, Sc or In are insulators whereas Tl 2 Mn 2 O 7 is highly conducting. All the compounds show spontaneous magnetization at low temperatures indicating ferromagnetic transitions. Detailed ac and dc susceptibility as well as neutron diffraction studies do not provide any evidence for long-range order in A 2 Mn 2 O 7 phases where A is a rare earth, Y or Sc, but suggest a spin-glass like behavior. However, powder neutron diffraction and small-angle neutron scattering investigations substantiate the long-range magnetic ordering in Tl 2 Mn 2 O 7 , which is also now known to exhibit colossal magnetoresistance (CMR) behavior. Structural analysis of this phase shows no deviations from ideal stoichiometry and gives an Mn-O distance of ∼1.90 A, significantly shorter than the Mn-O distances in the range 1.94 to 2.00 A observed in phases based on LaMnO 3 perovskites which show CMR. The above observation indicates that oxidation state of Mn in TI 2 Mn 2 O 7 should be very close to 4+ and has neither mixed valency for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ . Both were thought to play an essential role in CMR materials. We propose that CMR in Tl 2 Mn 2 O 7 is based on magnetic ordering driven by superexchange and strong spin fluctuation scattering above T c .

Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl23+Mn24+O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.

Quantitative evaluation of the contribution of the double-exchange magnetic interaction in the total exchange integral in spinels Cd 1− xCu xCr 2Se 4 ( x = 0.05, 0.15, 0.20, 0.90, 0.95)

Using the high-temperature expansion of the magnetic susceptibility the hopping integrals for the first and second coordination sphere were evaluated. The two integrals are positive and several times greater than the superexchange integrals.