34 research outputs found
Enhanced ferroelectric polarization by induced Dy spin-order in multiferroic DyMnO3
Neutron powder diffraction and single crystal x-ray resonant magnetic
scattering measurements suggest that Dy plays an active role in enhancing the
ferroelectric polarization in multiferroic DyMnO3 above TNDy = 6.5 K. We
observe the evolution of an incommensurate ordering of Dy moments with the same
periodicity as the Mn spiral ordering. It closely tracks the evolution of the
ferroelectric polarization which reaches a maximum value of 0.2 muC/m^2. Below
TNDy, where Dy spins order commensurately, the polarization decreases to values
similar for those of TbMnO3
Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53
Synchrotron X-ray diffraction investigations of two single crystals of
Na_xCoO_2 from different batches with composition x = 0.525-0.530 reveal
homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53
0) at room-temperature. The incommensurate (qq0) superstructure exists between
220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a
broad plateau at the latter value between 260 K and 360 K. On cooling, unusual
reversible phase segregation into two volume fractions is observed. Below 220
K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50
superstructure, while a second volume fraction with x = 0.55 exhibits another
commensurate superstructure, presumably with a 6a x 6a x c hexagonal supercell.
We argue that the commensurate-to-incommensurate transition is an intrinsic
feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.Comment: Corrected/improved versio
Magnetic field induced effects on the electric polarization in RMnO3 R Dy,Gd
X-ray resonant magnetic scattering studies of rare earth magnetic ordering
were performed on perovskite manganites RMnO3 (R = Dy, Gd) in an applied
magnetic field. The data reveal that the field-induced three-fold polarization
enhancement for H || a (H approx. 20 kOe) observed in DyMnO3 below 6.5 K is due
to a re-emergence of the Mn-induced Dy spin order with propagation vector k(Dy)
= k(Mn) = 0.385 b*, which accompanies the suppression of the independent Dy
magnetic ordering, k(Dy) = 1/2 b*. For GdMnO3, the Mn-induced ordering of Gd
spins is used to track the Mn-ordering propagation vector. The data confirm the
incommensurate ordering reported previously, with k(Mn) varying from 0.245 to
0.16 b* on cooling from T_N(Mn) down to a transition temperature T'. New
superstructure reflections which appear below T' suggest a propagation vector
k(Mn) = 1/4 b* in zero magnetic field, which may coexist with the previously
reported A-type ordering of Mn. The Gd spins order with the same propagation
vector below 7 K. Within the ordered state of Gd at T = 1.8 K we find a phase
boundary for an applied magnetic field H || b, H = 10 kOe, which coincides with
the previously reported transition between the ground state paraelectric and
the ferroelectric phase of GdMnO3. Our results suggest that the magnetic
ordering of Gd in magnetic field may stabilize a cycloidal ordering of Mn that,
in turn, produces ferroelectricity.Comment: 8 Figures, v2: improved figure layou
Magnetic field induced transitions in multiferroic TbMnO3 probed by resonant and non-resonant X-ray diffraction
Multiferroic TbMnO3 is investigated using x-ray diffraction in high magnetic
fields. Measurements on first and second harmonic structural reflections due to
modulations induced by the Mn and Tb magnetic order are presented as function
of temperature and field oriented along the a and b-directions of the crystal.
The relation to changes in ordering of the rare earth moments in applied field
is discussed. Observations below T_N(Tb) without and with applied magnetic
field point to a strong interaction of the rare earth order, the Mn moments and
the lattice. Also, the incommensurate to commensurate transition of the wave
vector at the critical fields is discussed with respect to the Tb and Mn
magnetic order and a phase diagram on basis of these observations for magnetic
fields H||a and H||b is presented. The observations point to a complicated and
delicate magneto-elastic interaction as function of temperature and field.Comment: 12 pages, 15 figures, 2 references adde
Revised superconducting phase diagram of hole doped Na(HO)CoOHO
We have studied the superconducting phase diagram of \NaH\space as a function
of electronic doping, characterizing our samples both in terms of Na content
and the Co valence state. Our findings are consistent with a recent report
that intercalation of \oxp\space ions into NaCoO, together with
water, act as an additional dopant indicating that Na sub-stochiometry alone
does not control the electronic doping of these materials. We find a
superconducting phase diagram where optimal \Tc\space is achieved through a Co
valence range of 3.24 - 3.35, while \Tc\space decreases for materials with a
higher Co valence. The critical role of dimensionality in achieving
superconductivity is highlighted by similarly doped non-superconducting
anhydrous samples, differing from the superconducting hydrate only in
inter-layer spacing. The increase of the interlayer separation between
CoO sheets as Co valence is varied into the optimal \Tc\space region is
further evidence for this criticality.Comment: Paper updated on 29/10/2004, 4 pages, 4 figures. Physical Review
Letters (in press
Low-temperature ferroelectric phase and magnetoelectric coupling in the underdoped La_2CuO_(4+x)
We report the discovery of a ferroelectric ground state below 4.5 K in highly
underdoped La_2CuO_(4+x) accompanied by slow charge dynamics which develop
below T~40 K. An anisotropic magnetoelectric response has also been observed,
indicating considerable spin-charge coupling in this lightly doped "parent"
high temperature copper-oxide superconductor. The ferroelectric state is
proposed to develop from polar nanoregions, in which spatial inversion symmetry
is locally broken due to non-stoichiometric carrier doping.Comment: 7 Pages, 6 Figures, supplementary materia
Cs2NaAl1-xCrxF6: A family of compounds presenting magnetocaloric effect
FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPIn this paper we explore the magnetocaloric effect (MCE) of chromium-doped elpasolite Cs2NaAl1-x,CrxF6 (x = 0.01 and 0.62) single crystals. Magnetization and heat capacity data show the magnetocaloric potentials to be comparable to those of garnets, perovskites, and other fluorides, producing magnetic entropy changes of 0.5 J/kg K (x = 0.01) and 11 J/kg K (x = 0.62), and corresponding adiabatic temperature changes of 4 and 8 K, respectively. These values are for a magnetic field change of 50 kOe at a temperature around 3 K. A clear Schottky anomaly below 10 K, which becomes more apparent when an external magnetic field is applied, was observed and related to the splitting of the Cr3+ energy levels. These results hint at a new family of materials with potential wide use in cryorefrigeration.90616FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPSem informaçãoSem informaçãoSem informaçãoSem informaçãoSem informaçãoWe thank Pedro von Ranke (UERJ, Brazil) and Walter Kalceff (UTS, Australia) for fruitful discussions. J.C.G.T.’s participation in this work was financed by the Science without Borders Program. Access to CICECO/Chemistry Department (Aveiro, Portugal), GPMR-UNICAMP (Campinas, Brazil), LMBT- UFF (Niter´oi, Brazil), LBT-UFRJ (Rio de Janeiro, Brazil), BERII facilities, and LaMMB MagLab (Berlin, Germany) are gratefully acknowledged by all authors. Financial support was provided by Proppi/UFF, FAPERJ, FAPESP, CAPES, CNPq, and FINEP
Cs2NaAl1-xCrxF6: A family of compounds presenting magnetocaloric effect
In this paper we explore the magnetocaloric effect (MCE) of chromium-doped elpasolite Cs2NaAl1-x,CrxF6 (x = 0.01 and 0.62) single crystals. Magnetization and heat capacity data show the magnetocaloric potentials to be comparable to those of garnets, perovskites, and other fluorides, producing magnetic entropy changes of 0.5 J/kg K (x = 0.01) and 11 J/kg K (x = 0.62), and corresponding adiabatic temperature changes of 4 and 8 K, respectively. These values are for a magnetic field change of 50 kOe at a temperature around 3 K. A clear Schottky anomaly below 10 K, which becomes more apparent when an external magnetic field is applied, was observed and related to the splitting of the Cr3+ energy levels. These results hint at a new family of materials with potential wide use in cryorefrigeration
The magnetic and electronic properties of oxyselenides—influence of transition metal ions and lanthanides
Magnetic oxyselenides have been a topic of research for several decades, firstly in the context of photoconductivity and thermoelectricity owing to their intrinsic semiconducting properties and ability to tune the energy gap through metal ion substitution. More recently, interest in the oxyselenides has experienced a resurgence owing to the possible relation to strongly correlated phenomena given the fact that many oxyselenides share a similar structure to unconventional superconducting pnictides and chalcogenides. The two dimensional nature of many oxyselenide systems also draws an analogy to cuprate physics where a strong interplay between unconventional electronic phases and localised magnetism has been studied for several decades. It is therefore timely to review the physics of the oxyselenides in the context of the broader field of strongly correlated magnetism and electronic phenomena. Here we review the current status and progress in this area of research with the focus on the influence of lanthanides and transition metal ions on the intertwined magnetic and electronic properties of oxyselenides. The emphasis of the review is on the magnetic properties and comparisons are made with iron based pnictide and chalcogenide systems