Quasi-static remanence as a generic-feature of spin-canting in Dzyaloshinskii-Moriya Interaction driven canted-antiferromagnets

We consistently observe a unique pattern in remanence in a number of canted-antiferromagnets (AFM) and piezomagnets. A part of the remanence is $\textit{quasi-static}$ in nature and vanishes above a critical magnetic field. Present work is devoted to exploring this $\textit{quasi-static}$ remanence ($\mu$) in a series of isostructural canted-AFMs and piezomagnets that possess progressively increasing N\'eel temperature ($T{_N}$). Comprehensive investigation of remanence as a function of $\textit{magnetic-field}$ and $\textit{time}$ in CoCO$_{3}$, NiCO$_{3}$ and MnCO$_{3}$ reveals that the magnitude of $\mu$ increases with decreasing $T{_N}$, but the stability with time is higher in the samples with higher $T{_N}$. Further to this, all three carbonates exhibit a universal scaling in $\mu$, which relates to the concurrent phenomenon of piezomagnetism. Overall, these data not only establish that the observation of $\textit{quasi-static}$ remanence with $\textit{counter-intuitive}$ magnetic-field dependence can serve as a foot-print for spin-canted systems, but also confirms that simple remanence measurements, using SQUID magnetometry, can provide insights about the extent of spin canting - a non trivial parameter to determine. In addition, these data suggest that the functional form of $\mu$ with $\textit{magnetic-field}$ and $\textit{time}$ may hold key to isolate Dzyaloshinskii Moriya Interaction driven spin-canted systems from Single Ion Anisotropy driven ones. We also demonstrate the existence of $\mu$ by tracking specific peaks in neutron diffraction data, acquired in remnant state in CoCO$_{3}$

Verwey type transition in nano-particle spinel ferrite Fe_2.2Co_0.8O₄

343-348Results are presented on magnetization and Mössbauer measurements on two multi-disperse nano-particle samples of Co substituted spinel ferrite Fe2.2Co0.8O4 with average particle sizes of ~ 50 Ǻ and ~ 80 Ǻ. In these nono-particle samples Co occupies the octahedral site B with ~ 60% preference. The 50 Ǻ average particle sized sample Co50 is largely in super-paramagnetic state at 300 K and its blocking temperature TB  is ~ 260 K. 80 Ǻ average particle sized sample Co80 has its TB above 300 K. The two samples exhibit magnetic relaxation. Much smaller moments of the two samples than saturation magnetization for the bulk particle sample and a loop shift observed in M-H curves of Co50 recorded in zero field and field cooling modes are suggestive of spin disordered surfaces. The nano-particle sample Co80, of the ferrite Fe2.2Co0.8O4 which in its bulk particle state is known to have more than an order of magnitude higher magneto-crystalline anisotropy than Fe3O4, shows signature of a ‘Verwey’ type transition with enhanced magnetic anisotropy below ~ 90 K. Co50 does not show a ‘Verwey’ type transition. A weak hump in M-T curve at a lower temperature of ~ 50 K in Co50 is attributed to disordered spin freezing

Role of annealing atmosphere on structure, dielectric and magnetic properties of La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>

Polycrystalline samples of La<SUB>2</SUB>MMnO<SUB>6</SUB> (M = Co and Mg) were prepared by a combined gel-combustion and high temperature reaction method. The samples were annealed in different oxygen partial pressure (p<SUB>O2</SUB>) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P2<SUB>1</SUB>/n) and rhombohedral (Requation image) lattices were observed for La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>, respectively. On annealing in inert atmosphere, La<SUB>2</SUB>MgMnO<SUB>6</SUB> partially converted to monoclinic La<SUB>2</SUB>CoMnO<SUB>6</SUB> type structure, whereas no structural change was observed in La<SUB>2</SUB>CoMnO<SUB>6</SUB>. Dielectric studies of La2CoMnO6 indicated relaxor like behavior with polaronic conduction, which systematically decreased with the increase in p<SUB>2</SUB> of the annealing atmosphere. Magnetic studies indicated multiple ferromagnetic phase transitions in La<SUB>2</SUB>CoMnO<SUB>6</SUB> and a spin-glass like phase transition in La<SUB>2</SUB>MgMnO<SUB>6</SUB>. The fraction of ferromagnetic phases of La<SUB>2</SUB>CoMnO<SUB>6</SUB> was significantly dependent on the annealing environments. The variations of magnetic and dielectric properties of samples were related to the fluctuation of oxidation state of transition metal ions and oxygen vacancies in the samples