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M. S. Ramachandra Rao

Publications and source records attributed to M. S. Ramachandra Rao.

26 records · Page 2Linked to original sources
arXiv2017

Microstrain induced deviation from Néel's 1/d behaviour: Size-dependent magnetization in Bi1-xCaxFe1-yTiyO3-delta nanoparticles

Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to Néel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1-xCaxFe1-yTiyO3-delta (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. A complex interdependence of doping concentration (x or y), annealing temperature (T), microstrain (epsilon) and particle size (d) is established. X-ray diffraction studies reveal a linear variation of microstrain with inverse particle size, 1/d nm-1 (i.e. epsilon.d = 16.5 nm.%). A rapid increase in the saturation magnetization below a critical size dc ~ 35 nm, exhibiting a (1/d)^alpha (alpha ~ 2.6) dependence, is attributed to the influence of microstrain. We propose an empirical formula M is proportional to (1/d)epsilon^beta (beta ~ 1.6) to highlight the contributions from both the size and microstrain towards the total magnetization in the doped systems. The magnetization observed in nanoparticles is thus, a result of competing magnetic contribution from the terminated spin cycloid on the surface and counteracting microstrain present at a given size. Large magnetodielectric response of ~ 9.5 % is observed in spark plasma sintered pellets with optimal size and doping concentration, revealing a strong correlation between magnetic and ferroelectric order parameters.

cond-mat.mtrl-sci↗
arXiv2016

Pure spin current transport in gallium doped zinc oxide

We study the flow of a pure spin current through zinc oxide by measuring the spin Hall magnetoresistance (SMR) in thin film trilayer samples consisting of bismuth-substituted yttrium iron garnet (Bi:YIG), gallium-doped zinc oxide (Ga:ZnO), and platinum. We investigate the dependence of the SMR magnitude on the thickness of the Ga:ZnO interlayer and compare to a Bi:YIG/Pt bilayer. We find that the SMR magnitude is reduced by almost one order of magnitude upon inserting a Ga:ZnO interlayer, and continuously decreases with increasing interlayer thickness. Nevertheless, the SMR stays finite even for a $12\;\mathrm{nm}$ thick Ga:ZnO interlayer. These results show that a pure spin current indeed can propagate through a several nm-thick degenerately doped zinc oxide layer. We also observe differences in both the temperature and the field dependence of the SMR when comparing tri- and bilayers. Finally, we compare our data to predictions of a model based on spin diffusion. This shows that interface resistances play a crucial role for the SMR magnitude in these trilayer structures.

cond-mat.mes-hall↗
arXiv2014

Magnetic phases of erbium orthochromite

Erbium orthochromite, ErCrO3, is a distorted-perovskite which has antiferromagnetic ground state below 10 K while Cr3+ magnetic moments order at 133 K. The temperature dependence of magnetization is studied across different magnetic phases for ErCrO3 and different magnetization isotherms are analyzed. In the presence of external magnetic field, polycrystalline ErCrO3 develops weak ferromagnetism from antiferromagnetic ground state. These magnetic phase transitions are observed to be of first order which is justified by thermal hysteresis and Arrott-plots.

cond-mat.str-el↗
arXiv2014

Magnetostructural and magnetocaloric properties of bulk LaCrO3 system

We studied the magnetic properties of bulk LaCrO3; a GdFeO3-type distorted perovskite, with a predominant antiferromagnetic phase transition at ~ 290 K. The bulk LaCrO3 exhibits intrinsic weak ferromagnetism at room temperature, which may arise due to the tilting of CrO6 octahedra, resulting in a non-zero net magnetic moment, as confirmed from the magnetization measurements. A broad magnetically-induced entropy change (-ΔS) is observed with the maximum at 290 K, close to room temperature in LaCrO3 system.

cond-mat.str-el↗
arXiv2009

Epitaxial Zn(x)Fe(3-x)O(4) Thin Films: A Spintronic Material with Tunable Electrical and Magnetic Properties

The ferrimagnetic spinel oxide Zn(x)Fe(3-x)O(4) combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial thin films with 0<=x<=0.9 on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an Ar/O2 mixture by laser molecular beam epitaxy. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range during growth. Our extensive characterization of the films provides a clear picture of the underlying physics of this spinel ferrimagnet with antiparallel Fe moments on the A and B sublattice: (i) Zn substitution removes both Fe3+ moments from the A sublattice and itinerant charge carriers from the B sublattice, (ii) growth in finite oxygen partial pressure generates Fe vacancies on the B sublattice also removing itinerant charge carriers, and (iii) application of both Zn substitution and excess oxygen results in a compensation effect as Zn substitution partially removes the Fe vacancies. A decrease (increase) of charge carrier density results in a weakening (strengthening) of double exchange and thereby a decrease (increase) of conductivity and the saturation magnetization. This scenario is confirmed by the observation that the saturation magnetization scales with the longitudinal conductivity. The combination of tailored films with semiconductor materials such as ZnO in multi-functional heterostructures seems to be particularly appealing.

cond-mat.mtrl-sci↗
arXiv2008

Anomalous Hall Effect in Magnetite: Universal Scaling Relation Between Hall and Longitudinal Conductivity in Low-Conductivity Ferromagnets

The anomalous Hall effect (AHE) has been studied systematically in the low-conductivity ferromagnetic oxide Fe$_{3-x}$Zn$_x$O$_4$ with $x = 0$, 0.1, and 0.5. We used (001), (110), and (111) oriented epitaxial Fe$_{3-x}$Zn$_x$O$_4$ films grown on MgO and sapphire substrates in different oxygen partial pressure to analyze the dependence of the AHE on crystallographic orientation, Zn content, strain state, and oxygen deficiency. Despite substantial differences in the magnetic properties and magnitudes of the anomalous Hall conductivity $σ_{xy}^{\rm AHE}$ and the longitudinal conductivity $σ_{xx}$ over several orders of magnitude, a universal scaling relation $σ_{xy}^{\rm AHE} \propto σ_{xx}^α$ with $α= 1.69 \pm 0.08$ was found for all investigated samples. Our results are in agreement with recent theoretical and experimental findings for ferromagnetic metals in the dirty limit, where transport is by metallic conduction. We find the same scaling relation for magnetite, where hopping transport prevails. The fact that this relation is independent of crystallographic orientation, Zn content, strain state, and oxygen deficiency suggests that it is universal and particularly does not depend on the nature of the transport mechanism.

cond-mat.mtrl-sci↗
arXiv2004

Study of Magnetic Properties of A_2B^'NbO_6 (A=Ba,Sr, (BaSr): and B^'=Fe and Mn) double perovskites

We have studied the magnetic properties of Ba_2FeNbO_6 and Ba_2MnNbO_6. it is seen that Ba_2FeNbO_6 is an antiferromagnet with a weak ferromagnetic behaviour at 5K while Ba_2MnNbO_6 shows two magnetic transitions one at 45 K and the other at 12K. Electron spin resonance (ESR) measurements at room temperature show that the Mn compound does not show any Jahn-Teller distortion. It is also seen that the Neel temperature of the A_2FeNbO_6 (A=Ba,Sr, BaSr) compounds do not vary significantly. However variations in the average A-site ionic radius influence the formation of short range correlations that persist above T_N.

cond-mat.str-el↗