66 research outputs found
Spin reorientation behavior in YMn1-xMxO3 (M = Ti, Fe, Ga; x = 0, 0.1)
The structural and magnetic properties of polycrystalline YMn1-xMxO3 (M = Ti,
Fe, Ga; x = 0, 0.1) have been studied by neutron powder diffraction and
magnetic measurements to probe the effect of Mn site doping on the frustration
behavior and magnetic structure of these compounds. The compounds are
isostructural and crystallize with hexagonal structure in P63cm space group. We
find that doping with these three ions, Ti4+ (d0), Fe3+ (d5) and Ga3+ (d10),
influences both the TN and magnetic structure, unlike other Mn-site dopants
reported previously. The magnetic structure of YMnO3 is described by
considering a linear combination of irreducible representations {\Gamma}3 and
{\Gamma}4 below TN ~ 75K and with decrease in temperature the ratio of
{\Gamma}3 and {\Gamma}4 changes. The mixing ratio of these two irreducible
representations remains constant on lowering of temperature in the Ga doped
compounds. The magnetic structure is modified on doping with nonmagnetic ion
Ti4+ (d0). It is described by the basis vectors of the irreducible
representation {\Gamma}2 with moment 2.3{\mu}B at 6 K. On doping with Fe3+ (d5)
the magnetic structure immediately below TN is explained by considering the
{\Gamma}3 irreducible representation. On further lowering of temperature, a
spin reorientation at ~ 35 K is observed. Below this temperature, the magnetic
structure of YMn0.9Fe0.1O3 is explained by considering {\Gamma}3 representation
with 51% mixing of {\Gamma}4. The ordered moments are found to be reduced from
the expected value for a Mn3+ ion in all these compounds indicating the
frustrated nature of these compounds. However, the frustration parameter, f is
significantly reduced in the case of Ti doped compound with {\Gamma}2
representation
Antiferromagnetic Coupling between Surface and Bulk Magnetization and Anomalous Magnetic Transport in Electro-deposited Co Film
We report an interesting magnetic behavior of a Co film (thickness ~ 350
{\AA}) grown on Si/Ti/Cu buffer layer by electro-deposition (ED) technique.
Using depth sensitive X-ray reflectivity and polarized neutron reflectivity
(PNR) we observed two layer structures for the Co film grown by ED with a
surface layer (thickness ~ 100 {\AA}) of reduced density (~ 68% of bulk)
compared to rest of the Co film (thickness ~ 250 {\AA}). The two layer
structure is consistent with the histogram profile obtained from atomic force
microscope (AFM) of the film. Interestingly, using PNR, we found that the
magnetization in the surface Co layer is inversely (antiferomagnetically)
coupled (negative magnetization for surface Co layer) with the rest of the Co
layer for the ED grown film. While we compare PNR result for a Co film of
similar layered structure grown by sputtering, the film showed a uniform
magnetization as expected. We also show that the depth dependent unusual
magnetic behavior of ED grown Co film may be responsible for anomalous
anisotropic magnetoresistance observed in low field in this film as compared to
the Co film grown by sputtering. Combining X-ray scattering, AFM,
superconducting quantum interface device magnetometry (SQUID), PNR and
magneto-transport measurements we attempted to correlate and compare the
structural, magnetic and morphological properties with magneto-transport of Co
films grown by ED and sputtering. The study indicates that the interesting
surface magnetic property and magneto-transport property of the ED film is
caused by its unique surface morphology
Magnetic and dielectric behavior in YMn1-xFexO3 (x less than or equal to 0.5)
The role of doping Fe on the structural, magnetic and dielectric properties
of frustrated antiferromagnet YMn1-xFexO3 (x less than or equal to 0.5) has
been investigated. The neutron diffraction analysis shows that the structure of
these polycrystalline samples changes from hexagonal phase (space group P63cm)
to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter
decreases with Fe substitution. All the compounds are antiferromagnetic and the
magnetic structure is described as a mixture of {\Gamma}3 and {\Gamma}4
irreducible representation (IR) in the hexagonal phase and the ratio of these
two IRs is found to vary with Fe doping (x less than or equal to 0.2). A
continuous spin reorientation as a function of temperature is observed in these
samples. The magnetic ground state in the orthorhombic phase of the higher
doped samples (x greater than or equal to 0.3) is explained by taking {\Gamma}1
(GxCyAz) representation of Pnma setting. In YMnO3 suppression of dielectric
constant {\epsilon} is observed below Tn indicative of magnetoelectric
coupling. This anomalous behavior reduces in Fe doped samples. The dielectric
constant is found to be correlated with the magnetic moment (M) obtained from
neutron diffraction experiments and follows a M^2 behavior close to Tn in
agreement with Landau theory.Comment: To appear in Journal of Apllied Physic
Superconductivity-driven magnetization modulation in YBa2Cu3O7-{\delta} /SrTiO3/La0.67Sr0.33MnO3 heterostructures
Using spin polarized neutron reflectivity experiments, we demonstrate an
unusual proximity behaviour when the superconductor (SC) and the ferromagnet
(FM) are coupled through an insulator (I) in YBa2Cu3O7-{\delta} (SC)/SrTiO3
(I)/La0.67Sr0.33MnO3 (FM) heterostructures. We have observed an unexpected
magnetic modulation at the interface region of the FM below the superconducting
transition temperature. The magnetization of the FM layer at the I/FM interface
was drastically reduced as compared to the magnetization in the rest of the FM
layer. This result indicates that the Cooper pairs tunnel across the insulator
and interact with the local magnetization at the interface region (extending ~
30 {\AA}) of the FM causing modification of the magnetization at the interface.
This unexpected magnetic behavior cannot be explained on the basis of the
existing theoretical models. However, the length scale associated here clearly
suggests the long range proximity effect as a result of tunneling of Cooper
pairs.Comment: 6 figure
Griffiths like Robust Ferromagnetism in Co3-xMnxTeO6; (x = 0.5, 1, 2)
We report near room temperature ferromagnetic as well as low temperature
antiferromagnetic correlations in Mn doped Cobalt Tellurate (CMTO) solid
solutions using thorough magnetization studies. For all the composition the
solid solutions show not only short range robust FM order at 185 K but also
show long range enhanced AFM order less than or equal to 45 K. Scaling of
inverse magnetic susceptibility data provide clear indication of Griffiths like
FM phase extended over large thermal region and its robustness against magnetic
field. Variations in both the phases as a function of Mn concentration also
support our observation of anomalous behavior in the average bond distances and
charge states (JAP 116: 074904 (2014)). Further an attempt towards the
structural insight into the observed complex magnetic behavior by using network
like structural analysis has been drawn. These observations make this an
interesting magnetic system from fundamental and application perspective
Annealing driven positive and negative exchange bias in Fe-Cu-Pt heterostructures at room temperature
We report annealing induced exchange bias in Fe-Cu-Pt based heterostructures
with Cu as an intermediate layer (Fe/Cu/Pt heterostructure) and capping layer
(Fe/Pt/Cu heterostructure). Exchange bias observed at room temperature (300 K)
is found to be dependent on the annealing temperature. We obtained positive
exchange bias of 120 Oe on annealing both the heterostructures at 400 oC, while
on annealing these heterostructures at 500 and 600 oC a negative exchange bias
of ~ -100 Oe was found. X-ray reflectivity and polarized neutron reflectivity
measurements provided evolution of depth dependent structure and magnetic
properties of the heterostructures on annealing at different temperatures and
revealed coexistence of soft and hard (alloy) magnetic phases across the
thickness of the films. Rapid and long range interdiffusion at interfaces on
annealing the systems at a temperature above 400 oC resulted into formation of
a ternary alloy phase. These results can be understood within the context of a
very unusual interface exchange interaction at the interface of hard/soft
magnetic phases, which are dependent on the annealing temperature.Comment: 10 figure
Phase separated behavior in Yttrium doped CaMnO3
The effect of electron doping on the structural, transport, and magnetic
properties of Mn (IV) - rich Ca1-xYxMnO3 (x < 0.2) samples have been
investigated using neutron diffraction, neutron depolarization, magnetization
and resistivity techniques. The temperature dependence of resistivity follows
the small polaron model and the activation energy exhibits a minimum for x=0.1
sample. A phase separated magnetic ground state consisting of ferromagnetic
domains (~7microns) embedded in G-type antiferromagnetic matrix is observed in
the sample, x = 0.1. The transition to the long range magnetically ordered
state in this sample is preceded by a Griffiths phase. On lowering temperature
below 300K a structural transition from orthorhombic structure (Pnma) to a
monoclinic structure (P21/m) is observed in the case of x=0.2 sample. The
ferromagnetic behavior in this case is suppressed and the antiferromagnetic
ordering is described by coexisting C-type and G-type magnetic structures
corresponding to the monoclinic and orthorhombic phases, respectively.
Antisymmetric magnetoresistance and helical magnetic structure in compensated Gd/Co multilayer
Using spin dependent specular and off-specular polarized neutron reflectivity
(PNR), we report the observation of a twisted helical magnetic structure with
planar 2{\pi} domain wall (DW) and highly correlated magnetic domains in a
Gd/Co multilayer. Specular PNR with polarization analysis reveals the formation
of planar 2{\pi}DWs below a compensation temperature (TComp), resulting to
positive exchange bias in this system. Off-specular PNR with spin polarization
showed development of magnetic inhomogenities (increase in magnetic roughness)
for central part (thickness ~ 25-30 {\AA}) of each Gd layer, where
magnetization is aligned perpendicular (in-plane) to an applied field. These
magnetic roughness are vertically correlated and results into Bragg sheet in
spin flip channel of Off-specular PNR data, which is contributing towards an
antisymmetric magnetoresistance at TComp in the system. The growth and
tunability of highly correlated magnetic inhomogeneities (roughness) and domain
structure around TComp in combination of twisted helical magnetic structure
with planar 2{\pi}DWs will be key for application in all-spin-based technology.Comment: 4 figure
A spin ladder compound doubles its superconducting TC under a gentle uniaxial pressure
Discovery of new high TC superconductors, with TC > 23 K, continues to be
challenging. We have doubled the existing TC of single crystal Sr3Ca11Cu24O41,
a spin ladder cuprate, from 12K to 24K, using a gentle uniaxial pressure ~ 0.06
GPa. In contrast, earlier works used a nearly 100 times larger hydrostatic
pressure 5 GPa, only to reach a maximum TC ~ 12K. Our work exposes large and
nearly equal, but opposing contributions to changes in TC, arising from
compressions along and perpendicular to ladder planes, in hydrostatic pressure
experiments. In our resistivity measurements, uniaxial pressure applied along
ladder planes increase TC, while that perpendicular to ladder planes decrease
TC. Our findings i) offers a new hope for further increase in TC in spin ladder
compounds and ii) calls for a large shift in phase boundaries of the currently
accepted pressure-temperature phase diagram.Comment: 5 figures and 24 page
Superconductivity-induced Magnetic Modulation in a Ferromagnet Through an Insulator in LaCaMnO3\SrTiO3\YBa2Cu3O7-{\delta} Hybrid Heterostructures
Coexistence of ferromagnetic and superconducting orders and their interplay
in ferromagnet-superconductor heterostructures is a topic of intense research.
While it is well known that proximity of a ferromagnet suppresses
superconducting order in the superconductor, there exist few studies indicating
the proximity of a superconductor suppressing ferromagnetic order in a
ferromagnet. Here we demonstrate a rare observation of the suppression of
ferromagnetic order in a LaCaMnO3 layer separated from a YBa2Cu3O7-{\delta}
layer by a thin insulator (SrTiO3). Polarized neutron reflectivity measurements
on LaCaMnO3\SrTiO3\YBa2Cu3O7-{\delta} trilayer deposited on [001] SrTiO3 single
crystal substrates shows the emergence of a thin magnetic dead layer in
LaCaMnO3 adjacent to the insulating layer below its superconducting transition
temperature of YBa2Cu3O7-{\delta}. Further, the magnetic dead layer grows in
thickness when the insulating layer is made thinner. This indicates a possible
tunneling of the superconducting order-parameter through the insulating SrTiO3
inducing modulation of magnetization in LaCaMnO3.Comment: 29 page
- …