Polycrystalline La2NiMnO6 prepared by sol–gel auto combustion technique is found to possess dual crystallographic phases R-3c (71 vol. %) and Pbnm (29 vol. %), with respective ferromagnetic transitions at 277 K and 160 K, respectively. The impedance data are analysed via Havriliak-Negami equivalent for impedance (HNEI). Using HNEI equation, the resistance “R,” relaxation time “τ,” and symmetric “α” and asymmetric “β” broadening parameters, for the two crystallographic phases are obtained. The dielectric relaxations for the two crystallographic phases are found to be independent of each other. The activation energy of relaxation for the R-3c and Pbnm phases is found to be 147 meV and 88 meV, respectively. Strong correlations of dielectric relaxations with the magnetic phases are seen. Before the onset of ferromagnetic transition, i.e., above 160 K for Pbnm and 277 K for R-3c phase, relaxations are dominantly non-Debye type. Once the ferromagnetic state is achieved, relaxations attain Debye nature. Hopping type conduction mechanism is confirmed from electrical transport measurements: nearest neighbour hopping near room temperature and gradual transition to variable range hopping (VRH) at low temperature. Signature of short range magnetic ordering, reminiscent to Griffiths like phase, is seen in the temperature range 295 K > T > 277 K.

Dielectric relaxations in La2NiMnO6 with signatures of Griffiths phase

Structural, magnetic and magnetocaloric properties in R2CoMnO6 (R = Dy, Ho, and Er)

Several double perovskite materials of the form A_2BB'O_6 exhibit high ferromagnetic T_c, and significant low field magnetoresistance. They are also a candidate source of spin polarized electrons. The potential usefulness of these materials is, however, frustrated by mislocation of the B and B' ions, which do not organise themselves in the ideal alternating structure. The result is a strong dependence of physical properties on preparative conditions, reducing the magnetization and destroying the half-metallicity. We provide the first results on the impact of spatially correlated antisite disorder, as observed experimentally, on the ferromagnetic double perovskites. The antisite domains suppress magnetism and half-metallicity, as expected, but lead to a dramatic enhancement of the low field magnetoresistance.

Antisite Domains in Double Perovskite Ferromagnets: Impact on Magnetotransport and Half-metallicity

Preparation, characterization and dielectric tunability of La2NiMnO6 ceramics

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_{6}$ stands out as presenting the largest ZEB reported so far, while for ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoMnO}}_{6}$ the exchange bias field (${H}_{\text{EB}}$) is one order of magnitude smaller. Here we show that ${\mathrm{La}}_{1.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoMnO}}_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate ${H}_{\text{EB}}$ value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ and ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ are already present at the ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ parent compound, and that ${\mathrm{Ba}}^{2+}/{\mathrm{Ca}}^{2+}/{\mathrm{Sr}}^{2+}$ partial substitution at the ${\mathrm{La}}^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples ($0.05\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.1$). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

Zero-field-cooled exchange bias effect in phase-segregated La 2 − x A x CoMnO 6 − δ ( A = Ba,Ca,Sr ; x = 0 , 0.5 )

Magnetic properties, resistivity and magnetoresistance effects of double perovskite La2Co1−xFexMnO6

Single-phase polycrystalline La2NiMnO6 was studied by means of magnetization and specific heat measurements. This monoclinic perovskite material shows ferromagnetic behavior with Tc about 270 K. ac Susceptibility of the sample shows a strong frequency dependent behavior. Fitting results of the relationship between freezing temperature and frequency indicate that the present sample can be better described as a system with superparamagnetic (SPM) behavior. This dynamic magnetic behavior was further confirmed by the weak magnetic relaxation and memory effect. The SPM behavior was explained in terms of formation of clusters in the magnetic background. Moreover, specific heat at 0 T and 9 T both shows broad transition around Tc.

Unusual dynamic magnetic behavior of polycrystalline La2NiMnO6

Exchange bias and magnetoresistance effects in La2-xSrxCoMnO6 (0.05 ≤ x ≤ 0.5)

Polycrystalline layered perovskite Sr2CoO4 sample was synthesized by high temperature and high pressure method. The staircaselike behavior has been observed in the magnetization and resistivity versus field curves of Sr2CoO4 at low temperature. The main features of the steps can be obtained from the measured results: (i) the positions of the external magnetic field at which steps occur are varying in different measurement runs, (ii) the steps only appear at low temperature and disappear with a slight increase of the temperature, (iii) the steps are dependent on the temperature and field sweep rate. Based on the features of the magnetization and magneto-transport staircaselike behavior in Sr2CoO4, the unusual phenomenon can be ascribed to an avalanche of flipping domains in terms of the random field theory.

/pdf/magnetization-and-magneto-transport-staircaselike-behavior-315zlw32ru.pdf

Magnetization and magneto-transport staircaselike behavior in layered perovskite Sr2CoO4 at low temperature.

https://iopscience.iop.org/article/10.1088/2053-1591/aa6920/pdf

Qiuhang Li

Papers

Magnetic properties, resistivity and magnetoresistance effects of double perovskite La2Co1−xFexMnO6

Unusual dynamic magnetic behavior of polycrystalline La2NiMnO6

Exchange bias and magnetoresistance effects in La2-xSrxCoMnO6 (0.05 ≤ x ≤ 0.5)

Magnetization and magneto-transport staircaselike behavior in layered perovskite Sr2CoO4 at low temperature.

Magnetic properties and magnetoresistance effect of La 2 Co 1-x Mg x MnO 6 (0.1 ⩽ x ⩽ 1.0)