arXiv subjects

Tapati Sarkar

Publications and source records attributed to Tapati Sarkar.

13 recordsLinked to original sources

arXiv2024

The paramagnetic Lyddane-Sachs-Teller relation

In this letter, we derive an expression for magnetic dipole transitions that is analogous to the Lyddane-Sachs-Teller relation for dielectric polar lattice vibrations. We thereby define transverse and longitudinal optical frequencies at which paramagnetic resonance and antiresonance occurs, respectively. The relation found here thus permits non-invasive optical analysis of static magnetization properties in paramagnetic materials. Validated through terahertz electron paramagnetic resonance ellipsometry and superconducting quantum interference device measurements on Iron-doped Gallium Nitride, our findings show very good agreement between theory and experiment. We term the excitations associated with the paramagnetic transitions as paramagnetic polaritons which may find use in future photonic applications

cond-mat.mtrl-sci↗

arXiv2022

Intra-Atomic and Local Exchange Fields in the Van der Waals Magnet CrI3

We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic X-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra which is evidence of spin-flip in the dd excitation. Interestingly, the dd excitation energies are similar on both sides of the ferromagnetic transition temperature, TC of 61 K, although MD in RIXS is predominant at 0.4 tesla magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the intra-atomic exchange field, is vanishingly small compared to local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The investigation presented here demonstrate that the electronic structure of bulk CrI3 is complex in the sense that dynamical electron correlations are significant. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intra-orbital dd excitations that represent transitions between electronic levels that are heavily influenced by multi-configuration effects. Our calculations employing the crystal field TTmultiplet theory taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry, clearly reproduced the dichroic trend in experimental RIXS spectra.

cond-mat.str-el↗

arXiv2016

Magnetic structure of the magnetocaloric compound AlFe2B2

The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic $a$-axis. The magnetic entropy change from 0 to 800 kA/m was found to be -1.3 J/K kg at the magnetic transition temperature.

cond-mat.mtrl-sci↗

arXiv2012

Geometric and disorder -- type magnetic frustration in ferrimagnetic "114" Ferrites: Role of diamagnetic Li+ and Zn2+ cation substitution

The comparative study of the substitution of zinc and lithium for iron in the "114" ferrites, YBaFe4O7 and CaBaFe4O7, shows that these diamagnetic cations play a major role in tuning the competition between ferrimagnetism and magnetic frustration in these oxides. The substitution of Li or Zn for Fe in the cubic phase YBaFe4O7 leads to a structural transition to a hexagonal phase YBaFe4-xMxO7, for M = Li (0.30 < x < 0.75) and for M = Zn (0.40 < x < 1.50). It is seen that for low doping values i.e. x = 0.30 (for Li) and x = 0.40 (for Zn), these diamagnetic cations induce a strong ferrimagnetic component in the samples, in contrast to the spin glass behaviour of the cubic phase. In all the hexagonal phases, YBaFe4-xMxO7 and CaBaFe4-xMxO7 with M = Li and Zn, it is seen that in the low doping regime (x ~ 0.3 to 0.5), the competition between ferrimagnetism and 2 D magnetic frustration is dominated by the average valency of iron. In contrast, in the high doping regime (x ~ 1.5), the emergence of a spin glass is controlled by the high degree of cationic disorder, irrespective of the iron valency.

cond-mat.mtrl-sci↗

arXiv2012

Electrical transport properties of nanostructured ferromagnetic perovskite oxides La_0.67Ca_0.33MnO_3 and La_0.5Sr_0.5CoO_3 at low temperatures (5 K > T >0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La_0.67Ca_0.33MnO_3 (LCMO) (with size down to 15 nm) and La_0.5Sr_0.5CoO_3 (LSCO) (with size down to 35 nm) in the temperature range 0.3 K to 5 K and magnetic fields upto 14 T. The transport, magnetotransport and non-linear conduction (I-V curves) were analysed using the concept of Spin Polarized Tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (Φ_B). The magnetotransport data were used to find out the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the MR contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and the MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to the barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the non-linear I-V data in this temperature range and the I-V data was found to be strongly dependent on magnetic field. We made the important observation that a gap like feature (with magnitude ~ E_C, the Coulomb charging energy) shows up in the conductance g(V) at low bias for the systems with smallest nanocrystal size at lowest temperatures (T < 0.7 K). The gap closes as the magnetic field and the temperature are increased.

cond-mat.mes-hall↗

arXiv2012

Gallium Substituted "114" YBaFe4O7: From a ferrimagnetic cluster glass to a cationic disordered spin glass

The study of the ferrites YBaFe4-xGaxO7 shows that the substitution of Ga for Fe in YBaFe4O7 stabilizes the hexagonal symmetry for 0.40 < x < 0.70, at the expense of the cubic one. Using combined measurements of a. c. and d. c. magnetization, we establish that Ga substitution for Fe in YBaFe4O7 leads to an evolution from a geometrically frustrated spin glass (for x = 0) to a cationic disorder induced spin glass (x = 0.70). We also find an intermediate narrow range of doping where the samples are clearly phase separated having small ferrimagnetic clusters embedded in a spin glass matrix. The origin of the ferrimagnetic clusters lies in the change in symmetry of the samples from cubic to hexagonal (and a consequent lifting of the geometrical frustration) as a result of Ga doping. We also show the presence of exchange bias and domain wall pinning in these samples. The cause of both these effects can be traced back to the inherent phase separation present in the samples.

cond-mat.mtrl-sci↗

arXiv2011

Formation of magnetic glass in calcium doped YBaCo2O5.5 cobaltites

The d.c. magnetization and magnetic relaxation studies of the calcium doped samples, Y0.95Ca0.05BaCo2O5.5 and YBa0.95Ca0.05Co2O5.5, show the existence of a magnetic glass like behaviour in the family of cobaltites for the first time. Our investigations reveal glass-like arrest of kinetics at low temperature which prevents the system from reaching its magnetic ground state. We show that the low temperature state of these calcium doped phases, which consists of coexisting antiferromagnetic and ferro (or ferri) magnetic phase fractions, can be tuned in a number of ways. Our observations establish that the low temperature state of this oxide is not in thermal equilibrium. The glassy state is formed with the assistance of an external magnetic field, which makes it distinctly different from the more well known metastable state, the spin glass state. The cooling field can tune the fractions of the coexisting phases, and the glass-like state formed at low temperature can also be devitrified by warming the sample. The role of Ca doping in the appearance of these phenomena is discussed in terms of phase separation, involving Co3+ disproportionation into Co4+ ferromagnetic clusters and Co2+ antiferromagnetic clusters.

cond-mat.mtrl-sci↗

arXiv2011

Oxygen hyperstoichiometric hexagonal ferrite CaBaFe4O7+δ(δ\approx 0.14) : coexistence of ferrimagnetism and spin glass behavior

An oxygen hyperstoichiometric ferrite CaBaFe4O7+δ(δ\approx 0.14) has been synthesized using "soft" reduction of CaBaFe4O8. Like the oxygen stoichiometric ferrimagnet CaBaFe4O7, this oxide also keeps the hexagonal symmetry (space group: P63mc), and exhibits the same high Curie temperature of 270 K. However, the introduction of extra oxygen into the system weakens the ferrimagnetic interaction significantly at the cost of increased magnetic frustration at low temperature. Moreover, this canonical spin glass (Tg ~ 166 K) exhibits an intriguing cross-over from de Almeida-Thouless type to Gabay-Toulouse type critical line in the field temperature plane above a certain field strength, which can be identified as the anisotropy field. Domain wall pinning is also observed below 110 K. These results are interpreted on the basis of cationic disordering on the iron sites.

cond-mat.str-el↗

arXiv2011

Effect of size reduction on the ferromagnetism of the manganite La1-xCaxMnO3 (x = 0.33)

In this paper we report an investigation on the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of $\mathrm{La_{0.67}Ca_{0.33}MnO_3}$ using magnetic measurements and neutron diffraction. The investigation was made on nanoparticles with crystal size down to $15$ nm. The neutron data show that even down to a size of $15$ nm the nanoparticles show finite spontaneous magnetization ($M_S$) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic to paramagnetic transition temperature $T_C$ with size $d$ and found that $T_C$ initially enhances on size reduction, but for $d < 50$ nm it decreases again. The initial enhancement in $T_C$ was related to an increase in the bandwidth that occured due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined form the neutron data. The size reduction also changes the nature of the ferromagnetic to paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.

cond-mat.str-el↗

arXiv2010

Hysteretic "Magnetic-Transport-Structural" Transition in "114" Cobaltites: Size Mismatch Effect

The triple "magnetic-transport-structural" transition versus temperature in three series of "114" cobaltites - Y1-xYbxBaCo4O7, Y1-xCaxBaCo4O7 and Yb1-xCaxBaCo4O7 - has been studied using magnetic, transport and differential scanning calorimetric measurements. The effect of the size mismatch σ2, due to cationic disordering at the Ln sites upon such a transition is shown for the first time in a triangular lattice. We show that increasing has an effect of increasing TS dramatically, while the size mismatch σ2 at the Ln sites decreases TS substantially. Moreover, the cationic mismatch at the Ln sites modifies the nature of the hysteretic transition by turning the sharp first order transition seen in the undoped samples into an intermix of first and second order transitions. These results are discussed on the basis of the particular nature of the high temperature form which exhibits a hexagonal close packed structure (space group: P63mc) with respect to the low temperature orthorhombic form (space group: Pbn21), the latter corresponding to a distortion of the former due to a puckering of the kagomé layers.

cond-mat.mtrl-sci↗

arXiv2010

Competition between Ferrimagnetism and Magnetic Frustration in Zinc Substituted YBaFe4O7

The substitution of zinc for iron in YBaFe4O7 has allowed the oxide series YBaFe4-xZnxO7, with 0.40 < x < 1.50, belonging to the "114" structural family to be synthesized. These oxides crystallize in the hexagonal symmetry (P63mc), as opposed to the cubic symmetry (F-43m) of YBaFe4O7. Importantly, the d.c. magnetization shows that the zinc substitution induces ferrimagnetism, in contrast to the spin glass behaviour of YBaFe4O7. Moreover, a.c. susceptibility measurements demonstrate that concomitantly these oxides exhibit a spin glass or a cluster glass behaviour, which increases at the expense of ferrimagnetism, as the zinc content is increased. This competition between ferrimagnetism and magnetic frustration is interpreted in terms of lifting of the geometric frustration, inducing the magnetic ordering, and of cationic disordering, which favours the glassy state.

cond-mat.mtrl-sci↗

arXiv2008

Size induced change-over from first to second order magnetic phase transition in La0.67Ca0.33MnO3 nanoparticles

In this report we show that in the perovskite manganite La_{1-x}Ca_{x}MnO_3 for a fixed x \approx 0.33, the magnetic transition changes over from first order to second order on reducing the particle size to nearly few tens of a nanometer. The change-over is brought about only by reducing the size and with no change in the stoichiometry. The size reduction to an average size of about 15 nm retains the ferromagnetic state albeit with somewhat smaller saturation magnetization and the ferromagnetic transition temperature T_{C} is suppressed by a small amount (4%). The magnetization of the nanoparticles near T_{C} follow the scaling equation $M/|ε|^β= f_\pm(H/|ε|^{γ+β})$, where, $ε= |T-T_C|/T_C$. The critical exponents, associated with the transition have been obtained from modified Arrott plots and they are found to be $β=0.47\pm 0.01$ and $γ=1.06\pm 0.03$. From a plot of M vs H at T_{C} we find the exponent $δ=3.10 \pm 0.13$. All the exponents are close to the mean field values. The change-over of the order of the transition has been attributed to a lowering of the value of the derivative dT_{C}/dP due to an increased pressure in the nanoparticles arising due to size reduction. This effect acts in tandem with the rounding off effect due to random strain in the nanoparticles.

cond-mat.str-el↗

arXiv2008

Crystal structure and physical properties of half-doped manganite nanocrystals with size < 100nm

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped $\mathrm{La_{0.5}Ca_{0.5}MnO_3}$(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. It was observed that the size reduction leads to change in crystal structure and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison with observed crystal structure data under hydrostatic pressure it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze-in. The size reduction also does not allow the long supercell modulation needed for the Charge Ordering, characteristic of this half-doped manganite, to set-in. The magnetic and transport measurements also show that the Charge Ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.

cond-mat.str-el↗