Enhanced magnetism andtime-stableremanence at the interface of hematite and carbon nanotubes

Abstract

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 (µ) results, which conclusively arise at the interface of two highly functional materials, namely the graphitic shells of a carbon nanotube (CNT) and α-Fe2O3, 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 µ, 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 µ 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 α-Fe2O3 nanowires bring into fore that the weak ferromagnets such as α-Fe2O3 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.Hematite (or α-Fe2O3) is an earth abundant and environment-friendly oxide, generally considered as a menace, for its appearance as common rust over elemental Fe, but technologically, it is well known for a very diverse range of applications. [1] However, the observation of WFM [2,3] nearly six decades ago in hematite and its connection to spin orbit coupling (SOC) has had profound implications in the field of spintronics. A variety of nontrivial topological spin structures in chiral magnets stabilizing through DMI/SOC have triggered new research areas such as antiferromagnetic spintronics and spin orbitronics [4,9] .Generation of WFM in α-Fe2O3 is due to a slight canting of its inherent AFM sublattice [2,3] which persists from 950 K (TN) down to 265 K, well-known Morin Transition temperature (TM). Below TM, the spins turn from "a" axis to "c" axis (rhombohedral unit cell in hex setting) and the canting vanishes for hematite. Many of such canted AFM, either DMI driven or systems in which canting takes place due to other mechanism, are also known to exhibit the phenomenon of piezomagnetism. [10,14] Here, PzM implies that magnetization can be tuned by stress alone.Another important point is the occurrence of WFM which is concurrent with PzM as theoretically predicted [10] and experimentally observed [11,14] . Dzyaloshinskii also showed that the spin canting effect is larger for compounds with smaller TN [2] . Thus, WFM / PzM is seen to be the weakest in α-Fe2O3 with TN ~950 K as compared to MnCO3 or NiCO3 (TN below 50 K).This work centers around remanence µ, which, in general, is an important parameter for any magnetic material for a variety of practical applications related to permanent magnets, soft or hard, relate to this quantity [15,17] . In addition, it is an important tool for probing fundamental magnetic interactions in conventional lon...

Co0.85Se magnetic nanoparticles supported on carbon nanotubes as catalyst for hydrogen evolution reaction

“…Hence, as shown in Fig. 6(b), the Ms (0.118 emu g -1 ), Mr (0.035 emu g -1 ), and Hc (158 Oe) of MWCNTs/Co0.85Se increased compared with those of Co0.85Se due to the decrease in Co0.85Se size in the MWCNTMWCNTs/Co0.85Se nanocomposites [41], which is consistent with the XRD and TEM results. The Hc increased from 55.09 Oe for the Co0.85Se nanoparticles to 158 Oe for the MWCNTs/Co0.85Se nanocomposites ( Fig.…”

Weak ferromagnetism andtime-stable remanencein hematite: effect of shape, size and morphology

“…Heating in presence of H from below the T M should enable larger number of WFM domains to point in the direction of H. For sake of consistency, we have only shown the remanent state following FC protocol in this work. Remanent states prepared in both FC and ZFC protocols have been discussed in [41]. Overall, we infer that the uncertainties on M versus T data in hematite crucially relate to the WFM phase and can be understood by considering the presence of timestable remanence.…”

“…The presence of this time-stable µ as well as its H dependence explains the discrepancies on magnetization data on repeated cooling [13,40]. Depending on the magnitude of the H used while cooling and on the morphology of the sample, this contribution can be 50%-90% of the in-field M value [27,41]. This contribution to magnetization comes from the spontaneously canted AFM domains and it is best gauged by the presence of time-stable µ.…”

Synthetically encapsulated & self-organized transition metal oxide nano-structures inside carbon nanotubes as robust: Li-ion battery anode materials

“…To confirm that superior cyclic stability in these Oxides@CNT nano-structures arises from encapsulation within the core cavity of the CNT, we conducted control measurements on the bare-oxide. Here, the representative bare-oxide (α-Fe 2 O 3 ) is formed in the same morphology as Fe 2 O 3 @CNT by suitable annealing [41]. The bare-oxide template was obtained from the same batch as Fe 2 O 3 @CNT, on which electrochemical measurements were performed.…”

“…The D band is assigned to the disordered structures in the hexagonal sp 2 carbon network, and the G-band originates from the in-plane bond stretching motion of sp 2 carbon atoms[40]. The increase in the number of defects on the CNT surfaces may also have a contribution from strain associated with the oxide filling[41]. The contribution from Fe 3 O 4 is also identified in Raman data[42].…”