Tapati Sarkar

Crystal structure and physical properties of half-doped manganite nanocrystals of less than 100-nm size

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped La 0.5 Ca 0.5 MnO 3 (LCMO) synthesized by chemical route, having particle size down to an average diameter of 15 nm. 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 to 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. 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.

Effect of size reduction on the ferromagnetism of the manganite La 1 x Ca x MnO 3 (x = 0.33)

In this paper, we report an investigation of the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of La0.67Ca0.33MnO3 using magnetic measurements and neutron diffraction. The investigation was performed on nanoparticles with crystal size down to 15nm. The neutron data show that even down to a size of 15nm the nanoparticles show finite spontaneous magnetization (MS) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic- to-paramagnetic transition temperature TC with size d and found that TC initially enhances upon size reduction, but for d < 50nm it decreases again. The initial enhancement in TC was related to an increase in the bandwidth that occurred due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined from 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.

Polar and Magnetic Mn2FeMO6 (M = Nb, Ta) with LiNbO3-type Structure - High Pressure Synthesis

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.

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, t...

Formation of magnetic glass in calcium-doped YBaCo2O5.5cobaltites

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.

Spectacular switching from ferrimagnetism to antiferromagnetism by zinc doping in “114” orthorhombic CaBaCo4O7

We report a spectacular switching from ferrimagnetism to antiferromagnetism in orthorhombic “114” CaBaCo4O7 by doping with a diamagnetic cation, Zn2+. Magnetic hysteresis loops, together with M(T) and χ′(T) curves at various frequencies, show that ferrimagnetism is abruptly converted to antiferromagnetism below T ∼ 80 K for less than 3% Zn doping, with the simultaneous appearance of local magnetic frustration. A model is proposed based on the ordered doping of Zn2+ at Co2+ sites in the ferromagnetic zig-zag chains of CaBaCo4O7, leading, by domino effect, to ferrimagnetic domains distributed at random and ordered at 180°. The boundaries of the junction of these domains contain Zn2+ and are magnetically frustrated. The crucial role of such ferromagnetic zig-zag chains for designing and tuning magnetic properties in oxides with a triangular metallic lattice is emphasized.

Magnetism of the “114” orthorhombic charge ordered CaBaCo4O7 doped with Zn or Ga: a spectacular valency effect

The substitution of zinc and gallium for cobalt in CaBaCo4O7 has allowed the oxide series CaBaCo4−xMxO7 (M = Zn, Ga) with 0.02 ≤ x ≤ 1.20, belonging to the “114” structural family, to be synthesized. From the structural view point two domains can be distinguished: for x ≤ 0.70 (for Zn) and x ≤ 0.90 (for Ga), the orthorhombic symmetry of CaBaCo4O7 is retained (albeit with reduced orthorhombic distortion for higher values of x), whereas for x ≥ 1.0 the symmetry is hexagonal. More importantly, we observe a contrasting effect of the valency of Zn2+ and Ga3+ upon the magnetic properties of the doped CaBaCo4−xMxO7 cobaltites in the low doping regime (x ≤ 0.40). While the doping of Ga3+ for Co3+ leads to a progressive disappearance of ferrimagnetism with a simultaneous appearance of magnetic frustration, the doping of Zn2+ for Co2+ induces an abrupt switching from ferrimagnetism to antiferromagnetism. We explain these effects as arising from a perturbation of the ferromagnetic Co2+ chains which form the magnetic structure of CaBaCo4O7 in the case of substitution by zinc, and a destruction of the magnetic order by substitution of Ga3+ on the Co3+ sites in the case of gallium doping. For higher substitution levels (x ≥ 0.50), both Zn and Ga substitutions lead to a spin glass behavior, irrespective of the symmetry, orthorhombic or hexagonal.

Phase diagram, structures and magnetism of the FeMnP1-xSix-system

The magnetic properties of the (Fe,Mn)2(P,Si)-system have been shown to be readily manipulated by small changes in composition. This study surveys the FeMnP1−xSix-system (0.00 ≤ x ≤ 1.00) reporting sample syntheses and investigations of crystallographic and magnetic properties using X-ray powder diffraction and magnetic measurements. Two single phase regions exist: the orthorhombic Co2P-type structure (x < 0.15) and the Fe2P-type structure (0.24 ≤ x < 0.50). Certain compositions have potential for use in magnetocaloric applications.

Complex magnetic phase separation induced by Li-doping in multiferroic CaBaCo4O7

The doping of lithium at the cobalt sites in the multiferroic orthorhombic cobaltite CaBaCo4O7 has been investigated. The oxides CaBaCo4-2xLixGaxO7 and CaBaCo4-xLixO7 keep the same polar space group as the parent phase. In contrast, they show a spectacular decrease of their ferrimagnetic properties for very low doping levels (0 < x ≤ 0.10), with the appearance of antiferromagnetism below TN ∼ 80 K and magnetic frustration at around Tf ∼ 20 K, which should have a great impact upon multiferroism in this system. This behavior is different from that of the Ga-doped phases CaBaCo4-xGaxO7, but remarkably similar to that of the Zn-doped phases CaBaCo4-xZnxO7. The actual role of the valence of the diamagnetic cation, Li+ is interpreted from its ability to sit in the Co2+ zig-zag chain, breaking the ferromagnetism along those chains in both series of oxides. Importantly, the Li-doping in CaBaCo4-xLixO7 oxides, introduces an excess Co3+ with respect to the other two series, CaBaCo4-xZnxO7 and CaBaCo4-2xLixGaxO7. Th...

Pb2MnTeO6 Double Perovskite: An Antipolar Anti-ferromagnet

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to ∼120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s(2) lone-pair electrons, together with a surprising off-centering of Mn(2+) (d(5)) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN ≈ 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near ∼150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.