Showing papers by "S. M. Yusuf published in 2010"

Coexistence of sign reversal of both magnetization and exchange bias field in the core-shell type La0.2Ce0.8CrO3 nanoparticles

Abstract: We report an extraordinary coexistence of sign reversal of both magnetization and exchange bias field in the La0.2Ce0.8CrO3 nanoparticles. From the high resolution transmission electron microscopy image, and field dependence of thermoremanent and isothermoremanent magnetization measurements, the nanoparticles are found to be of core-shell nature. The core-shell configuration with an antiferromagnetic core of the Cr3+ and Ce3+ spins and a disordered shell with the uncompensated spins, explains the sign reversal of both magnetization and exchange bias field. The present study shows an excellent way of tuning the sign of both magnetization and exchange bias field in a single magnetic system.

Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn0.5Co0.5O3.

Abstract: Two distinct ferromagnetic phases of LaMn0.5Co0.5O3 having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature Tc 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 Tc. Raman scattering suggests the presence of spin–phonon coupling for both the phases around the Tc. Electrical transport properties of both the phases have been investigated based on the lattice distortion.

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

Abstract: 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.

Enhancement of Curie temperature in electrochemically prepared crystalline thin films of Prussian blue analogs KjFekII[CrIII(CN)6]l⋅mH2O

Abstract: Structural and magnetic properties of electrochemically prepared crystalline films of Prussian blue analogs (PBAs) KjFekII[CrIII(CN)6]l⋅mH2O, with varying deposition time and electrode voltage, which result into change in film thickness and stoichiometry, respectively, have been investigated by using x-ray diffraction (XRD), infrared (IR) spectroscopy, and dc magnetization measurement techniques. An atomic force microscopy (AFM) and XRD study reveal uniform and crystalline nature of all films. As the film thickness increases from 1 μm to 5 μm, the Curie temperature (TC), coercive field, and maximum magnetization increase from 11 K to 21 K, 20 Oe to 160 Oe, and 5.7 μB to 6.5 μB, respectively. For the films prepared with variation in electrode voltage, it has been found that the alkali metal ions are introduced into the films just by using suitable electrode voltage, contrary to usual method where alkali metal ions are intentionally introduced into the lattice by using additional compounds of alkali metals ...

Two- and three-dimensional magnetic correlations in the spin- (1)/(2) square-lattice system Zn 2 VO(PO 4 ) 2

Abstract: The magnetic correlations in the quasi-two-dimensional (2D) spin-$\frac{1}{2}$ square-lattice system ${\text{Zn}}_{2}\text{VO}{({\text{PO}}_{4})}_{2}$ have been investigated by neutron-diffraction technique. A long-range antiferromagnetic (AFM) ordering below 3.75 K $({T}_{N})$ has been observed with a reduced moment of $0.66(2)\text{ }{\ensuremath{\mu}}_{B}$ per V ion at 1.5 K. In a given $ab$ plane, the AFM spin arrangement is N\'eel 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 ${T}_{N}$. Interestingly, the coexistence of diffuse magnetic scattering and three-dimensional antiferromagnetic Bragg peaks has been found below ${T}_{N}$, 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-$\frac{1}{2}$ square-lattice system.

Cyanide-bridged RuxNi3-3x/2[Cr(CN)6]2⋅zH2O molecular magnets: Controlling structural disorder and magnetic properties by a 4d ion (ruthenium) substitution

Abstract: We report the effect of a 4d ion (Ru3+) substitution on structural disorder and magnetic ordering in the cyanide-bridged molecular magnets, RuxNi3-3x/2[Cr(CN)6]2⋅zH2O (0≤x≤0.5). Structural analysis, by employing Rietveld refinement of x-ray diffraction patterns, reveals a face centered cubic structure for all these compounds. In the present study, we have controlled the [Cr(CN)6] vacancies by substituting Ru at the Ni site and then the role of varying [Cr(CN)6] vacancies on the structural correlations and consequently on the magnetic properties has been investigated. We have succeeded in increasing the magnetic ordering temperature (TC) with increasing Ru concentration up to x=0.2. The IR spectra of the Ru doped compounds indicate the presence of Cr2+–C≡N–Ru4+ type sequences amounting to an additional structural disorder. The magnetic Bragg scattering in the neutron diffraction patterns is masked by a large diffuse scattering arising due to the structural disorder. The reverse Monte Carlo calculations qua...

Pole-reversal of magnetization in core-shell type La1−xCexCrO3(x = 0.8-1.0) nanoparticles

Abstract: We report, for the first time, temperature dependent pole-reversal of magnetization for La1−xCexCrO3 (x = 0.8, 1.0) nanoparticles. The Rietveld refinement of the x-ray diffraction patterns confirmed the single phase nature of the samples (Orthorhombic: Pbnm). From the transmission electron microscopy (TEM) image, the average particle size is found to be ~ 44 nm. High resolution TEM reveals surface defects/roughness which corroborates a core-shell nature. The magnetic properties of these nanoparticles have been tuned by Ce3+ substitution. The novel pole-reversal phenomenon has been explained using the core-shell model, and can be exploited to fabricate advanced magnetic memory devices.

Cu1.5[Cr(CN)6]⋅6.5H2O nanoparticles: synthesis, characterization, and magnetic properties

Abstract: We have synthesized nanoparticles of Cu1.5[Cr(CN)6]⋅6.5H2O of varying size by using poly(vinylpyrrolidone) (PVP) as a protecting polymer. The particle size variation has been achieved by varying the amount of the PVP surfactant with the reactants. The prepared nanoparticles have been investigated by using X-ray diffraction, transmission electron microscopy, and direct-current magnetization techniques. The nanoparticles crystallize in a face centred cubic structure (space group: Fm3m). The approximate particle sizes for the three samples are 18, 9, and 5 nm, respectively. Non-PVP nanoparticles (18 nm) show a magnetic ordering temperature of 65 K. A decrease in the magnetic ordering temperature was observed with decreasing particle size. These nanoparticles are magnetically very soft, showing negligibly small values of the coercivity and remanent magnetization. The maximum magnetization and spontaneous magnetization values at 5 K are found to decrease with decreasing particle size. The observed magnetization behaviour of the nanoparticles has been attributed to the increasing surface spin disorder with decreasing particle size.

Magnetism of the spin-chain compound Ca3Co1.9Fe0.1O6: Magnetization and neutron diffraction study

Abstract: We report the results of the dc magnetization and neutron diffraction studies of the quasi-one-dimensional spin-chain compound Ca3Co19Fe01O6 Rietveld refinement of neutron powder diffraction pattern at 100 K confirmed that Fe ion was located at trigonal prism site 6a (0, 0, 1/4) The dc susceptibility obeys the Curie-Weiss law The appearance of additional magnetic Bragg peaks in the low temperature neutron powder diffraction patterns mainly indicates an antiferromagnetic ordering The magnetic reflections can be indexed with a propagation vector K = (0, 0, 1), referred to the space group Rc (hexagonal setting), and indicate that the centering translations have been lost in the magnetic structure