arXiv subjects

A. K. Bera

Publications and source records attributed to A. K. Bera.

47 records · Page 3Linked to original sources

arXiv2015

Controlling structural distortion in the geometrically frustrated layered cobaltate YBaCo4O7+δ by Fe substitution and its role on magnetic correlations

Effects of Fe-substitution on the crystal structure and magnetic correlations of the geometrically frustrated antiferromagnets YBaCo4-xFexO7+δ (x = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) have been studied by neutron diffraction, Mössbauer spectroscopy, and ac susceptibility. The compounds YBaCo4-xFexO7+δ have a special layered-type crystal structure with an alternating Kagomé (6c site) and triangular (2a site) layers along the c axis. Fe3+ ions are found to be substituted at both the crystallographic 2a and 6c sites of Co ions. Mössbauer results show a high spin state of Fe3+ ions in a tetrahedral coordination. A reduction in the distortion of the Kagomé lattice has been observed with the Fe-substitution. The correlation length of the short-range antiferromagnetic ordering decreases with the Fe-substitution. The sharpness of the magnetic transition also decreases with the Fe-substitution.

cond-mat.str-el↗

arXiv2014

Understanding the multiple magnetic structures of the intermetallic compound NdMn1.4Co0.6Si2

Magnetic phases for the intermetallic compound NdMn1.4Co0.6Si2 have been investigated at various temperatures by dc magnetization, neutron diffraction and neutron depolarization. Our study shows multiple magnetic phase transitions with temperature (T) over 1.5-300 K. In agreement with dc-magnetization and neutron depolarization results, the temperature dependence of the neutron diffraction patterns shows five distinct regions with different magnetic phases. These temperature regions are (i) T >= 215 K, (ii) 215 K > T >= 50 K, (iii) 50 K > T >= 40 K, (iv) 40 K > T > 15 K, and (v) T =< 15 K. The corresponding magnetic structures are paramagnetic, commensurate collinear antiferromagnetic (AFM-I), incommensurate AFM (AFM-II), mixed ferromagnetic and AFM (FM+AFM-II), and incommensurate AFM (AFM-II), respectively.

cond-mat.str-el↗

arXiv2014

Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets Sr$M_2$V$_2$O$_8$ ($M$ = Co, Mn)

Magnetic correlations of two iso-structural quasi-one-dimensional (1D) antiferromagnetic spin-chain compounds Sr$M_2$V$_2$O$_8$ ($M$ = Co, Mn) have been investigated by magnetization and powder neutron diffraction. Two different collinear antiferromagnetic (AFM) structures, characterized by the propagation vectors, $k$ = (0 0 1) and $k$ = (0 0 0), have been found below $\sim$ 5.2 K and $\sim$ 42.2 K for the Co- and Mn-compounds, respectively. For the Mn-compound, AFM chains (along the $c$ axis) order ferromagnetically within the $ab$ plane, whereas, for the Co-compound, AFM chains order ferro-/antiferromagnetically along the $a/b$ direction. The critical exponent study confirms that the Co- and Mn-compounds belong to the Ising and Heisenberg universality classes, respectively. For both compounds, short-range spin-spin correlations are present over a wide temperature range above $T_N$. The reduced ordered moments at base temperature (1.5 K) indicate the presence of quantum fluctuations in both compounds due to the quasi-1D magnetic interactions.

cond-mat.str-el↗

arXiv2013

Field Induced Magnetic Ordering and Single-ion Anisotropy in the Quasi-1D Haldane Chain Compound SrNi2V2O8: A Single Crystal investigation

Field-induced magnetic ordering in the Haldane chain compound SrNi$_{2}$V$_{2}$O$_{8}$ and effect of anisotropy have been investigated using single crystals. Static susceptibility, inelastic neutron scattering, high-field magnetization, and low temperature heat-capacity studies confirm a non-magnetic spin-singlet ground state and a gap between the singlet ground state and triplet excited states. The intra-chain exchange interaction is estimated to be $J \sim 8.9{\pm}$0.1 meV. Splitting of the dispersions into two modes with minimum energies 1.57 and 2.58 meV confirms the existence of single-ion anisotropy $D(S^z){^2}$. The value of {\it D} is estimated to be $-0.51{\pm}0.01$ meV and the easy axis is found to be along the crystallographic {\it c}-axis. Field-induced magnetic ordering has been found with two critical fields [$μ_0H_c^{\perp c} = 12.0{\pm}$0.2 T and $μ_0H_c^{\parallel c} = 20.8{\pm}$0.5 T at 4.2 K]. Field-induced three-dimensional magnetic ordering above the critical fields is evident from the heat-capacity, susceptibility, and high-field magnetization study. The Phase diagram in the {\it H-T} plane has been obtained from the high-field magnetization. The observed results are discussed in the light of theoretical predictions as well as earlier experimental reports on Haldane chain compounds.

cond-mat.str-el↗

arXiv2013

Low-energy magnetic excitations in the quasi-one-dimensional spin-1 chain compound SrNi2V2O8

Multi-frequency electron spin resonance (ESR) transmission spectra have been measured as function of temperature and magnetic field on single crystals of the quasi-one-dimensional spin-1 chain compound SrNi2V2O8 in the GHz frequency range. Magnetic resonance modes above 50 K have been observed with an effective g-factor of 2.24 at 100 K. Below 30 K, intra-triplet excitations have been observed in the ESR spectra, which reveal the presence of single-ion anisotropy with D = -0.29 meV.

cond-mat.str-el↗

arXiv2012

Quantum Phase Transition from a Spin-liquid State to a Spin-glass State in the Quasi-1D Spin-1 System Sr1-xCaxNi2V2O8

We report a quantum phase transition from a spin-liquid state to a spin-glass state in the quasi-one dimensional (1D) spin-1 system Sr1-xCaxNi2V2O8, induced by a small amount of Ca-substitution at Sr site. The ground state of the parent compound (x = 0) is found to be a spin-liquid type with a finite energy gap of 26.6 K between singlet ground state and triplet excited state. Both dc-magnetization and ac-susceptibility studies on the highest Ca-substituted compound (x = 0.05) indicate a spin-glass type magnetic ground state. With increasing Ca-concentration, the spin-glass ordering temperature increases from 4.5 K (for the x = 0.015 compound) to 6.25 K (for the x = 0.05 compound). The observed results are discussed in the light of the earlier experimental reports and the theoretical predictions for a quasi-1D spin-1 system.

cond-mat.str-el↗

arXiv2012

Effect of electron doping on the magnetic correlations in the bilayered brownmillerite compound Ca_{2.5-x}La_{x}Sr_{0.5}GaMn_{2}O_{8}: a neutron diffraction study

The effect of electron doping on the magnetic properties of the Brownmillerite-type bilayered compounds has been investigated by neutron powder diffraction in La substituted Ca2.5-xLaxSr0.5GaMn2O8 compounds (x = 0.05 and 0.1), in comparison with the undoped compound (x = 0). In all compounds, a long-range three dimensional collinear antiferromagnetic (AFM) structure is found below the Néel temperature TN of respective compound, whereas, well above TN, three dimensional short-range magnetic ordering is observed. In the intermediate temperature range just above TN, a strong effect of electron doping (La substitution) on the magnetic correlations has been observed. Here, a short-range AFM correlation with possible dimensionality of three has been found for substituted compounds (x = 0.05 and 0.1) as compared to the reported two dimensional long-range AFM ordering in the parent compound. With increasing the electron doping, a decrease in TN is also observed. The short-range magnetic correlations set in over a large temperature range above TN. A magnetic phase diagram in the x-T plane is proposed from these results.

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

Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn$_{0.5}$Co$_{0.5}$O$_{3}$

Two distinct ferromagnetic phases of LaMn$_{0.5}$Co$_{0.5}$O$_{3}$ having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature $\textit{T}_{c}$ is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn - O - Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in $\textit{T}_{c}$. Raman scattering suggests the presence of spin-phonon coupling for both the phases around the $\textit{T}_{c}$. Electrical transport properties of both the phases have been investigated based on the lattice distortion.

cond-mat.str-el↗

arXiv2010

Two- and three-dimensional magnetic correlations in the spin-1/2 square-lattice system Zn2VO(PO4)2

The magnetic correlations in the quasi-two dimensional spin-1/2 square-lattice system Zn2VO(PO4)2 have been investigated by neutron diffraction technique. A long-range antiferromagnetic (AFM) ordering below 3.75 K (TN) has been observed with a reduced moment of 0.66(2) μB per V ion at 1.5 K. In a given ab plane, the AFM spin arrangement is Néel type and the AFM layers are coupled ferromagnetically along the c axis. Remarkably, we have observed a pure 2D short-range AFM ordering in the ab plane above TN. Interestingly, the coexistence of diffuse magnetic scattering and three dimensional antiferromagnetic Bragg peaks has been found below TN, indicating the presence of spin-waves as confirmed by our calculation using the linear spin-wave theory. The observed results are discussed in the light of existing theory for a two-dimensional spin-1/2 square-lattice system.

cond-mat.mtrl-sci↗

arXiv2010

Ferromagnetic Clusters in the Brownmillerite Bilayered Compounds Ca2.5-xLaxSr0.5GaMn2O8: An Approach to Achieve Layered Spintronics Materials

We report the effect of La-substitution on the magnetic and magnetotransport properties of Brownmillerite-like bilayered compounds Ca2.5-xLaxSr0.5GaMn2O8 (x = 0, 0.05, 0.075, and 0.1) by using dc-magnetization, resistivity and magnetoresistance techniques. The Rietveld analysis of the room temperature x-ray diffraction patterns confirms no observable change of average crystal structure with the La-substitution. Both magnetic and magnetotransport properties are found to be very sensitive to the La-substitution. Interestingly, the La-substituted compounds show ferromagnetic-like behavior (due to the occurrence of a double exchange mechanism) whereas, the parent compound is an antiferromagnet (TN 150 K). All compounds show an insulating behavior, in the measured temperature range of 100 - 300 K, with an overall decrease in the resistivity with the substitution. A higher value of magnetoresistance has been successfully achieved by the La-substitution. We have proposed an electronic phase separation model, considering the formation of ferromagnetic clusters in the antiferromagnetic matrix, to interpret the observed magnetization and magnetotransport results for the La-substituted samples. The present study demonstrates an approach to achieve new functional materials, based on naturally occurring layered system like Ca2.5-xLaxSr0.5GaMn2O8, for possible spintronics applications.

cond-mat.mtrl-sci↗