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Register a new userWeak Ferromagnetism and Time-Stable Remanence in Hematite: Effect of Shape, Size and Morphology
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We have recently established that a number of Dzyaloshinskii-Moriya interaction driven canted antiferromagnets or weak ferrromagnets (WFM) including hematite exhibit an ultra-slow magnetization relaxation phenomenon, leading to the observation of a time-stable remanence (Phys. Rev. B 96, 104422 (2017)). In this work, our endeavor is to optimize the magnitude of this time-stable remanence for the hematite crystallites, as a function of shape size and morphology. A substantial enhancement in the magnitude of this unique remanence is observed in porous hematite, consisting of ultra-small nano particles, as compared to crystallites grown in regular morphology, such as cuboids or hexagonal plates. This time-stable remanence exhibits a peak-like pattern with magnetic field, which is significantly sharper in porous sample. The extent and the magnitude of the spin canting associated with the WFM phase can be best gauged by the presence of this remanence and its unusual magnetic field dependence. Temperature variation of lattice parameters bring out correlations between strain effects that alter the bond length and bond angle associated with primary super exchange paths, which in-turn systematically alter the magnitude of the time-stable remanence. This study provides insights regarding a long standing problems of anomalies in the magnitude of magnetization on repeated cooling in case of hematite. Our data caps on these anomalies, which we argue, arise due to spontaneous spin canting associated with WFM phase. Our results also elucidate on why thermal cycling protocols during bulk magnetization measurements are even more crucial for hematite which exhibits both canted as well as pure antiferromgnetic phase.
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The interface of two dissimilar materials is well known for surprises in condensed matter, and provides avenues for rich physics as well as seeds for future technological advancements. We present some exciting magnetization (M) and remnant magnetization ($mu$) results, which conclusively arise at the interface of two highly functional materials, namely the graphitic shells of a carbon nanotube (CNT) and $alpha$-Fe$_2$O$_3$, a Dzyaloshinskii-Moriya Interaction (DMI) driven weak ferromagnet (WFM) and piezomagnet (PzM). We show that the encapsulation inside CNT leads to a very significant enhancement in M and correspondingly in $mu$, a time- stable part of the remanence, exclusive to the WFM phase. Up to 70% of in-field magnetization is retained in the form of $mu$ at the room temperature. Lattice parameter of CNT around the Morin transition of the encapsulate exhibits a clear anomaly, confirming the novel interface effects. Control experiments on bare $alpha$-Fe$_2$O$_3$ nanowires bring into fore that the weak ferromagnets such as $alpha$-Fe$_2$O$_3$ as are not as weak, as far as their remanence and its stability with time is concerned, and encapsulation inside CNT leads to a substantial enhancement in these functionalities.
We explore remanent magnetization ($mu$) as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFM) which are also weak ferromagnets (WFM) and piezomagnets (PzM). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasi static in nature and exhibits a counter-intuitive magnetic field dependence. These observations unravel an ultra-slow magnetization relaxation phenomenon related to this quasi static remanence. This feature is also observed in a defect free single crystal of $alpha$-Fe$_2$O$_3$, which is a canonical WFM and PzM. Notably, $alpha$-Fe$_2$O$_3$ is not a typical geometrically frustrated AFM and in single crystal form, it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFM which are either symmetry allowed WFM, driven by Dzyaloshinskii-Moriya Interaction (DMI) or can generate this trait by tuning of size and interface. The qualitative features of the quasi static remanence indicate that such WFM are potential piezomagnets, in which magnetization can be tuned by textit{stress} alone.
We investigate a model of excitonic ordering (i.e electron-hole pair condensation) appropriate for the divalent hexaborides. We show that the inclusion of imperfectly nested electron hole Fermi surfaces can lead to the formation of an undoped excitonic metal phase. In addition, we find that weak ferromagnetism with compensated moments arises as a result of gapless excitations. We study the effect of the low lying excitations on the density of states, Fermi surface topology and optical conductivity and compare to available experimental data.
Magnetization and neutron diffraction measurements indicate long-range antiferromagnetic ordering below TN=4 K in the 2D, S=1/2 Heisenberg antiferromagnet K2V3O8. The ordered state exhibits ``weak ferromagnetism and novel, field-induced spin reorientations. These experimental observations are well described by a classical, two-spin Heisenberg model incorporating Dzyaloshinskii-Moriya interactions and an additional c-axis anisotropy. This additional anisotropy can be accounted for by inclusion of the symmetric anisotropy term recently described by Kaplan, Shekhtman, Entin-Wohlman, and Aharony. This suggests that K2V3O8 may be a very unique system where the qualitative behavior relies on the presence of this symmetric anisotropy.
Weak itinerant ferromagnetism in YCo9Si4 below about 25 K is studied by means of magnetisation, specific heat, and resistivity measurements. Single crystal X-ray Rietveld refinements at room temperature reveal a fully ordered distribution of Y, Co and Si atoms within the tetragonal space group I4/mcm isostructural with LaCo9Si4. The latter exhibits itinerant electron metamagnetism with an induced moment of about 1 mu_B/f.u. above 6 T, whereas YCo9Si4 exhibits a spontaneous magnetisation M0~12 Am^2/kg at 2 K which corresponds to an ordered moment of about 1.6 mu_B/f.u. indicating weak itinerant ferromagnetism.
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