Hysteresis shift in Fe-filled carbon nanotubes due to γ-Fe

Abstract

The phase distribution of high aspect ratio, Fe-filled carbon nanotubes prepared by pyrolyzing a mixture of powered ferrocene and C 60 has been determined by means of Mössbauer spectroscopy. Our results for that characterization are closely related to the observation, after field cooling processes, of a hysteresis loop shift and clearly suggest a spatial phase distribution which includes the presence of a ␥-Fe/␣-Fe interface. The temperature dependence of the hysteresis loop shift is discussed in terms of localized regions at that interface exhibiting uncompensated antiferromagnetism within reduced dimensions. DOI: 10.1103/PhysRevB.65.113405 PACS number͑s͒: 75.70.Cn, 76.80.ϩy, 81.07.De Since their first synthesis carbon nanotubes have been appreciated by their unique combination of electronic and mechanical properties 1,2 that make them a very promising candidate system to be used on the development of a broad range of new devices, which already includes field emission lamps, nanotransistors, spin polarized electron sources, flat display panels, and hydrogen storing systems. 3,4 Nevertheless, one of the most natural and attractive possibilities of these materials, closely linked to their morphology and extremely high aspect ratio, is their use as nanocontainers for secondary phases and, particularly, for magnetically ordered ones. 5 In fact, the encapsulation of magnetic phases in carbon nanotubes could constitute, due to the very large magnetic shape anisotropies acting on the encapsulated material, a feasible approach to, on the one side, the achievement of magnetic order stabilization against thermal fluctuations in systems having extremely reduced dimensions and, on the other, the exploration of the physics of the magnetic order in close-to-one-dimensional structures. From the point of view of the practical uses, carbon nanotubes filled with magnetic phases could also develop giant coercivities ͑coercivity values above those predictable on the basis of the bulk magnetocrystalline anisotropy͒ linked to their reduced transverse dimension and the consequential large influence on the magnetization reversal phenomenology of the reduced symmetry, high anisotropy interfaces. The control of the coercivity of these magnetic phase-filled nanotubes would make possible their use as very high density recording media, as magnetoresistive elements suitable to be packed to high densities, as probes for magnetic force microscopy, and, even, as recording heads.In this work, we present and discuss results about the structural and magnetic properties of Fe-filled nanotubes. We will show how the presence in the as-prepared samples of antiferromagnetic fcc ␥-Fe results, through the coupling to bcc ␣-Fe, on the occurrence of the so-called exchange biasing a phenomenology underlying the operation of the spin valves, one of the most relevant developments of the magnetoelectronics.Fe-filled carbon nanotubes were prepared by pyrolyzing a 1:1 mixture ͑by weight͒ of powered ferrocene and C 60 at atmospheric pressure under an Ar atmo...

Iron-containing microphases in multiwalled carbon nanotubes

“…This conclusion is favored by Fe (5) singlet line broadening observed in the low temperature spectrum ( 3 (300 K) = 0.26 mm/s → 3 (77 K) = 0.32 mm/s), which can be caused by the beginning of magnetic ordering of γ-Fe particles [3]. The temperature of the appearance of the line broadening at T ≈ 85 ± 5 K, obtained from recent temperature dependence of 3 (77 ÷ 300 K), is very close to the Neel temperature (T N = 90 K) of polycrystalline γ-Fe particles [3], which shows that γ-iron particles are not amorphous or superparamagnetic. Unexpectedly, the γ-Fe phase, which is thermodynamically unstable at T < 1011 K [6], is characterized by a substantial relative contribution of S 3 ≈ 24% (Table 1).…”

“…The situation with two paramagnetic subspectra Fe(3) and Fe(5) is unambiguous. The isomer shift (δ(5)) and quadrupole spitting ( (5)) of the Fe(5) doublet can correspond either to Fe 1−x C x superparamagnetic particles [3] or to the iron-graphite complex already mentioned above [4]. The high quadruple splitting value for Fe(5) component is evidence of a substantial distortion of the local environment of iron atoms, and broadened linewidth ( (5) ≈ 0.40 mm/s) shows that its nearest environment is nonuniform.…”

“…The high quadruple splitting value for Fe(5) component is evidence of a substantial distortion of the local environment of iron atoms, and broadened linewidth ( (5) ≈ 0.40 mm/s) shows that its nearest environment is nonuniform. The isomer shift of the Fe(3) subspectrum can be assigned either to the face-centered γ-Fe polymorph, which becomes magnetically ordered only at low temperatures [3], or to small α-Fe phase particles in the superparamagnetic state (d ∼ 60 Å) [5]. To solve the problem of the origin of the Fe (3) …”

Preparation temperature effect on the synthesis of various carbon nanostructures

“…8(d)) compared with the value of the coercivity Hc (0.9 Oe) of bulk polycrystalline Fe [35] and Hc of nanocrystalline (23 Oe) [36]. During the last few years, a number of research works have been published dealing with magnetic behaviour of ferromagnetic materials (Fe, Ni, and Co) filled in CNTs [37][38][39][40][41][42]. However, the exact nature of filling of these magnetic materials in the core region of CNTs during preparation is not clear.…”

Synthesis, Properties, and Applications of Ferromagnetic‐Filled Carbon Nanotubes

“…Ferromagnetic-filled carbon nanotubes may find applications ranging from magnetic data-storage devices, [1] to implementation of individual filled tubes in a sensor system for magnetic force microscopy. [2] Another possible use of filled carbon nanotubes is seen in biomedicine as ferromagnetic nanocontainers initiating a new antitumor therapeutic concept in the treatment of cancer. [3] These filled carbon nanotubes represent a suitable material for magneticallyguided hyperthermia and, functionalized inside and outside the tube, a unique drug delivery/carrier system.…”