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Register a new userProbing low temperature non-equilibrium magnetic state in Co$_{2.75}$Fe$_{0.25}$O$_{4+delta}$ spinel oxide using dc magnetization, ac susceptibility and neutron diffraction experiments
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The low temperature lattice structure and magnetic properties of Co$_{2.75}$Fe$_{0.25}$O$_4$ ferrite have been investigated using experimental results from synchrotron x-ray diffraction (SXRD), dc magnetization, ac susceptibility, neutron diffraction and neutron depolarization techniques. The samples have been prepared by chemical co-precipitation of the Fe and Co nitrates solution in high alkaline medium and subsequent thermal annealing of the precipitates in the temperature range of 200- 900 $^circ$C. Rietveld refinement of the SXRD patterns at room temperature indicated two-phased cubic spinel structure for the samples annealed at temperatures 200-600 $^circ$C. The samples annealed at temperatures 700 $^circ$C and 900 $^circ$C (CF90) have been best fitted with single phased lattice structure. Refinement of the neutron diffraction patterns in the temperature range of 5-300 K confirmed antiferromagnetic (AFM) Co$_3$O$_4$ and ferrimagnetic (FIM) Co$_{2.75}$Fe$_{0.25}$O$_4$ phases for the sample annealed at 600 $^circ$C and single FIM phase of Co$_{2.75}$Fe$_{0.25}$O$_4$ for the CF90 sample. Magnetic measurements have shown a non-equilibrium magnetic structure, consisting of the high temperature FIM phase and low temperature AFM phase. The magnetic phases are sensitive to magnetic fields, where high temperature phase is suppressed at higher magnetic fields by enhancing the low temperature AFM phase, irrespective of annealing temperature of the samples.
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We report high temperature synchrotron X-ray diffraction (SXRD), dc magnetization and current-voltage (I-V) characteristics for the samples of Co$_{2.75}$Fe$_{0.25}$O$_4$ ferrite. The material was prepared by chemical reaction of the Fe and Co nitrate solutions at pH = 11 and subsequent annealing at temperatures 200 0C, 500 0C and 900 0C. The measurements were performed by cycling the temperature from 300 K to high temperature (warming mode) and return back to 300 K (cooling mode). The SXRD patterns indicated a fine bi-phased cubic spinel structure in the highly Co rich spinel oxide. Magnetization curves showed intrinsic ferrimagnetic features and defect induced additional ferromagnetic phase at higher temperatures. Electrical conductivity showed thermal hysteresis loop between warming and cooling modes of temperature variation. The samples exhibited new information on the irreversibility phenomena of lattice structure, magnetization and electrical conductivity on cycling the measurement temperatures.
We have thoroughly investigated the entire magnetic states of under doped ferromagnetic insulating manganite Nd0.8Sr0.2MnO3 through temperature dependent linear and non linear complex ac magnetic susceptibility measurements. This ferromagnetic insulating manganite is found to have frequency independent ferromagnetic to paramagnetic transition temperature at around 140 K. At around 90 K (approx T_f) the sample shows a second frequency dependent re - entrant magnetic transition as explored through complex ac susceptibility measurements. Non linear ac susceptibility measurements (higher harmonics of ac susceptibility) have also been performed (with and without the superposition of a dc magnetic field) to further investigate the origin of this frequency dependence (dynamic behavior at this re-entrant magnetic transition). Divergence of 3rd order susceptibility in the limit of zero exciting field indicates a spin glass like freezing phenomena. However, large value of spin relaxation time (?0= 10-8 s) and small value of coercivity (~22 Oe) obtained at low temperature (below T_f) from critical slowing down model and dc magnetic measurements, respectively, are in contrast with what generally observed in a canonical spin glass (?0 = 10-12 - 10-14 s and very large value of coercivity below freezing temperature). We have attributed our observation to the formation of finite size ferromagnetic clusters which are formed as consequence of intrinsic separation and undergo cluster glass like freezing below certain temperature in this under doped manganite. The results are supported by the electronic - and magneto - transport data.
Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of Ca(Co$_{1-x}$Fe$_{x}$)$_{y}$As$_{2}$, $0leq xleq1$, $1.86leq y leq 2$, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the $c$ axis, persists for $xlesssim0.12(1)$. The antiferromagnetic order is smoothly suppressed with increasing $x$, with both the ordered moment and N{e}el temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for $xleq0.25$, nor does ferromagnetic order for $x$ up to at least $x=0.104$, and a smooth crossover from the collapsed-tetragonal (cT) phase of CaCo$_{1.86}$As$_{2}$ to the tetragonal (T) phase of CaFe$_{2}$As$_{2}$ occurs. These results suggest that hole doping CaCo$_{1.86}$As$_{2}$ has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca.
A systematic study using neutron diffraction and magnetic susceptibility are reported on Mn substituted ferrimagnetic inverse spinel Ti$_{1-x}$Mn$_{x}$Co$_2$O$_4$ in the temperature interval 1.6 K $leq$ $T$ $leq$ 300 K. Our neutron diffraction study reveals cooperative distortions of the $T$O$_6$ octahedral for all the Jahn-Teller active ions $T$ = Mn$^{3+}$, Ti$^{3+}$ and Co$^{3+}$, which are confirmed by the X-ray photoelectron spectroscopy. Two specific compositions ($x$ = 0.2 and 0.4) have been chosen because of their unique features: noncollinear Yafet-Kittel type ordering, and weak tetragonal distortion with ${c/a}$ $<$ 1, in which the apical bond length $d_c$($T_B$-O) is longer than the equatorial $d_{ab}$($T_B$-O) due to the splitting of the $e_g$ level of Mn$^{3+}$ ions into $d_{x^2-y^2}$ and $d_{z^2}$. For $x$ = 0.4, the distortion in the $T_B$O$_6$ octahedra is stronger as compared to $x$ = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in $x$ = 0.4 and $x$ = 0.2 sets in at 110.3 and 78.2 K, respectively due to the unequal magnetic moments of cations, where Ti$^{3+}$, Mn$^{3+}$, and Co$^{3+}$ occupying the octahedral, whereas, Co$^{2+}$ sits in the tetrahedral site. In addition, weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon at $T_{COMP}$ = 25.4 K in $x$ = 0.2 and a reentrant spin-glass behaviour in $x$ = 0.4 with a freezing temperature $sim$110.1 K. The compensation phenomenon is characterized by sign reversal of magnetization and bipolar exchange bias effect below $T_{COMP}$ with its magnitude depending on the direction of external magnetic field and the cooling protocol.
The magnetic structure of the mixed antiferromagnet NdMn$_{0.8}$Fe$_{0.2}$O$_3$ was resolved. Neutron powder diffraction data definitively resolve the Mn-sublattice with a magnetic propagation vector ${bf k} = (000)$ and with the magnetic structure (A$_x$, F$_y$, G$_z$) for 1.6~K~$< T < T_N (approx 59$~K). The Nd-sublattice has a (0, f$_y$, 0) contribution in the same temperature interval. The Mn sublattice undergoes spin-reorientation transition at $T_1 approx 13$~K while the Nd magnetic moment keep ordered abruptly increases at this temperature. Powder X-ray diffraction shows a strong magnetoelastic effect at $T_N$ but no additional structural phase transitions from 2~K to 300~K. Density functional theory calculations confirm the magnetic structure of the undoped NdMnO$_3$ as part of our analysis. Taken together, these results show the magnetic structure of Mn-sublattice in NdMn$_{0.8}$Fe$_{0.2}$O$_3$ is a combination of the Mn and Fe parent compounds, but the magnetic ordering of Nd sublattice spans over broader temperature interval than in case of NdMnO$_3$ and NdFeO$_3$. This result is a consequence of the fact that the Nd ions do not order independently, but via polarization from Mn/Fe sublattice.
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