arXiv subjects

V. Caignaert

Publications and source records attributed to V. Caignaert.

24 records · Page 2Linked to original sources

arXiv2007

Spin-locking effect in the nanoscale ordered perovskite cobaltite LaBaCo2O6

A new nanoscale ordered perovskite cobaltite, which consists of 90 degree ordered domains of the layered-112 LaBaCo2O6 has been evidenced by high resolution- transmission electron microscopy. This new form, like the disordered La0.5Ba0.5CoO3 and ordered LaBaCo2O6, exhibits a ferromagnetic transition at TC around 179 K. However, it differs from the two previous forms by its strong magnetic anisotropy, and correlatively by its high value of coercivity (0.42 Tesla) at low temperature. We suggest that this behaviour originates from the locking of magnetic spins in the 90 degree oriented nano-domain. Moreover, one observes a semi-metal/semi-metal transition at TC with a maximum magnetoresistance of 6.5 % at this temperature.

cond-mat.mtrl-sci↗

arXiv2006

Enhancement of ferromagnetism by nickel doping in the 112 cobaltite EuBaCo2O5.50

The study of the ordered oxygen deficient perovskite EuBaCo2-xNixO5.50 shows that the doping of cobalt sites by nickel induces a strong ferromagnetic component at low temperature in the antiferromagnetic matrix of EuBaCo2O5.50. This system exhibits indeed phase separation, i.e. consists of ferromagnetic domains embedded in the antiferromagnetic matrix of EuBaCo2O5.50. Besides, a magnetic transition is observed for the first time at 40K in the undoped and nickel doped phases, which can be attributed to the ferromagnetic ordering of the Eu3+ moments below this temperature. Moreover sharp ultra magnetization multisteps are observed below 5K, characteristic of motion of domain walls in a strong pinning system and very different from any metamagnetic transition.

cond-mat.mtrl-sci↗

arXiv2006

Enhancement of giant magnetoresistance effect in the Ruddlesden-Popper phase Sr3Fe2-xCoxO7-d: Predominant role of oxygen nonstoichiometry and magnetic phase separation

The magnetic and magnetotransport properties of the Sr3Fe2-xCoxO7-d system (0.2 <= x <= 1.0) were systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (d). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disordering leads to enhancement in the negative MR effect. It is suggested that the compound segregates into F clusters embedded in a non-F matrix, being a naturally occurring analog of the artificial granular-GMR materials, as in the doped perovskite cobaltites, La1-xSrxCoO3 (x < 0.18).

cond-mat.str-el↗

arXiv2005

The impact of niobium doping upon the magnetotransport properties of the oxygen-deficient perovskite SrCo1-xNbxO3-d

The oxygen-deficient perovskite cobaltite SrCo1-xNbxO3-d was synthesized by direct solid-state reaction and its magnetotransport properties were investigated. This cobaltite exhibits an unusual ferromagnetic behavior with a transition temperature Tm = 130-150 K and a spin glass like behavior below Tm. Importantly, this phase reaches a large magnetoresistance (MR) value, MR = -(rH - r0) / r0 = 30% at 5 K in 7 T. The large MR effect is believed to be related to the disordered magnetic state induced by the Nb-for-Co substitution.

cond-mat.str-el↗

arXiv2005

Competition between ferromagnetism and spin glass: the key for large magnetoresistance in oxygen deficient perovskites SrCo1-xMxO3-d (M = Nb, Ru)

The magnetic and magnetotransport properties of the oxygen deficient perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and Ru-substituted cobaltites are weak ferromagnets, with transition temperatures Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass behavior at temperatures below Tf = 80-90 K. It is demonstrated that there exists a strong competition between ferromagnetism and spin glass state, where Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites induces magnetic disorder, and this particular magnetic behavior is the origin of large negative magnetoresistance of these oxides, reaching up to 30% at 5 K in 7 T. The differences between Nb- and Ru-substituted cobaltites are discussed on the basis of the different electronic configuration of niobium and ruthenium cations.

cond-mat.str-el↗

arXiv2003

Structural evidence against current-induced destruction of charge-ordering in manganese oxides

High-resolution x-ray scattering, in the presence of an applied current, has been used for studying the stability of the charge ordered phase of manganese oxides upon current biasing. We find that the charge ordered structure is unchanged when a current is flowing in the sample. Such a result indicates that the non-linear conduction observed in charge ordered manganites can not be ascribed to a current-induced destabilisation of charge ordering.

cond-mat↗