arXiv subjects

Geetha Balakrishnan

Publications and source records attributed to Geetha Balakrishnan.

54 records · Page 3Linked to original sources

arXiv2019

Structural and magnetic properties of GdCo$_{5-x}$Ni$_x$

GdCo$_5$ may be considered as two sublattices - one of Gd and one of Co - whose magnetizations are in antiparallel alignment, forming a ferrimagnet. Substitution of nickel in the cobalt sublattice of GdCo$_5$ has been investigated to gain insight into how the magnetic properties of this prototype rare-earth/transition-metal magnet are affected by changes in the transition metal sublattice. Polycrystalline samples of GdCo$_{5-x}$Ni$_x$ for 0 $ \leq x \leq $ 5 were synthesized by arc melting. Structural characterization was carried out by powder x-ray diffraction and optical and scanning electron microscope imaging of metallographic slides, the latter revealing a low concentration of Gd$_2$(Co, Ni)$_7$ lamellae for $x \leq 2.5$. Compensation - i.e. the cancellation of the opposing Gd and transition metal moments is observed for $1 \leq x \leq 3$ at a temperature which increases with Ni content; for larger $x$, no compensation is observed below 360 K. A peak in the coercivity is seen at $x \approx 1$ at 10K coinciding with a minimum in the saturation magnetization. Density-functional theory calculations within the disordered local moment picture reproduce the dependence of the magnetization on Ni content and temperature. The calculations also show a peak in the magnetocrystalline anisotropy at similar Ni concentrations to the experimentally observed coercivity maximum.

cond-mat.mtrl-sci↗

arXiv2019

Experimental signatures of emergent quantum electrodynamics in Pr$_2$Hf$_2$O$_7$

In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales [1-2]. Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal fundamentally novel physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory [3-7]. This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr$_2$Hf$_2$O$_7$ that provide evidence for a quantum spin ice ground state. We find a quasi-elastic structure factor with pinch points - a signature of a classical spin ice - that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite temperature. Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr$_2$Hf$_2$O$_7$ can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a three-dimensional quantum spin liquid - a topical state of matter which has so far mostly been explored in lower dimensionalities.

cond-mat.str-el↗

arXiv2018

Magnetic phases of skyrmion-hosting GaV$_4$S$_{8-y}$Se$_{y}$ ($y = 0, 2, 4, 8$) probed with muon spectroscopy

We present the results of a muon-spin spectroscopy investigation of GaV$_4$S$_{8-y}$Se$_{y}$ with $y=0, 2, 4$ and 8. Zero-field measurements suggest that GaV$_{4}$Se$_{8}$ and GaV$_{4}$S$_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature-dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and $4$ materials showing glassy magnetic behaviour at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of GaV$_4$Se$_{8}$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

cond-mat.mtrl-sci↗

arXiv2018

Investigating the magnetic ground state of the skyrmion host material Cu$_{2}$OSeO$_{3}$ using long-wavelength neutron diffraction

We present long-wavelength neutron diffraction data measured on both single crystal and polycrystalline samples of the skyrmion host material Cu$_{2}$OSeO$_{3}$. We observe magnetic satellites around the $(0\bar{1}1)$ diffraction peak not accessible to other techniques, and distinguish helical from conical spin textures in reciprocal space. We confirm successive transitions from helical to conical to field polarised ordered spin textures as the external magnetic field is increased. The formation of a skyrmion lattice with propagation vectors perpendicular to the field direction is observed in a region of the field-temperature phase diagram that is consistent with previous reports. Our measurements show that not only the field-polarised phase but also the helical ground state are made up of ferrimagnetic clusters instead of individual spins. These clusters are distorted Cu tetrahedra, where the spin on one Cu ion is anti-aligned with the spin on the three other Cu ions.

cond-mat.mtrl-sci↗

arXiv2018

Field-induced canting of magnetic moments in GdCo5 at finite temperature: first-principles calculations and high-field measurements

We present calculations and experimental measurements of the temperature-dependent magnetization of a single crystal of GdCo$_5$ in magnetic fields of order 60 T. At zero temperature the calculations, based on density-functional theory in the disordered-local-moment picture, predict a field-induced transition from an antiferromagnetic to a canted alignment of Gd and Co moments at 46.1 T. At higher temperatures the calculations find this critical field to increase along with the zero-field magnetization. The experimental measurements observe this transition to occur between 44-48 T at 1.4 K. Up to temperatures of at least 100 K, the experiments continue to observe the transition; however, at variance with the calculations, no strong temperature dependence of the critical field is apparent. We assign this difference to the inaccurate description of the zero-field magnetization of the calculations at low temperatures, due to the use of classical statistical mechanics. Correcting for this effect, we recover a consistent description of the high-field magnetization of GdCo$_5$ from theory and experiment.

cond-mat.mtrl-sci↗

arXiv2018

Fermi surfaces in Kondo insulators

We report magnetic quantum oscillations measured using torque magnetisation in the Kondo insulator YbB$_{12}$ and discuss the potential origin of the underlying Fermi surface. Observed quantum oscillations as well as complementary quantities such as a finite linear specific heat capacity in YbB$_{12}$ exhibit similarities with the Kondo insulator SmB$_6$, yet also crucial differences. Small heavy Fermi sections are observed in YbB$_{12}$ with similarities to the neighbouring heavy fermion semimetallic Fermi surface, in contrast to large light Fermi surface sections in SmB$_6$ which are more similar to the conduction electron Fermi surface. A rich spectrum of theoretical models is suggested to explain the origin across different Kondo insulating families of a bulk Fermi surface potentially from novel itinerant quasiparticles that couple to magnetic fields, yet do not couple to weak DC electric fields.

cond-mat.str-el↗

arXiv2018

Calculating the Magnetic Anisotropy of Rare-Earth-Transition-Metal Ferrimagnets

Magnetocrystalline anisotropy, the microscopic origin of permanent magnetism, is often explained in terms of ferromagnets. However, the best performing permanent magnets based on rare earths and transition metals (RE-TM) are in fact ferrimagnets, consisting of a number of magnetic sublattices. Here we show how a naive calculation of the magnetocrystalline anisotropy of the classic RE-TM ferrimagnet GdCo$_5$ gives numbers which are too large at 0 K and exhibit the wrong temperature dependence. We solve this problem by introducing a first-principles approach to calculate temperature-dependent magnetization vs. field (FPMVB) curves, mirroring the experiments actually used to determine the anisotropy. We pair our calculations with measurements on a recently-grown single crystal of GdCo$_5$, and find excellent agreement. The FPMVB approach demonstrates a new level of sophistication in the use of first-principles calculations to understand RE-TM magnets.

cond-mat.mtrl-sci↗

arXiv2018

Coupling between Spin and Charge Order Driven by Magnetic Field in Triangular Ising System LuFe2O4+δ

We present a study of the magnetic-field effect on spin correlations in the charge ordered triangular Ising system LuFe2O4+δ through single crystal neutron diffraction. In the absence of a magnetic field, the strong diffuse neutron scattering observed below the Neel temperature (TN = 240 K) indicates that LuFe2O4+δ shows short-range, two-dimensional (2D) correlations in the FeO5 triangular layers, characterized by the development of a magnetic scattering rod along the 1/3 1/3 L direction, persisting down to 5 K. We also found that on top of the 2D correlations, a long range ferromagnetic component associated with the propagation vector k1 = 0 sets in at around 240 K. On the other hand, an external magnetic field applied along the c-axis effectively favours a three-dimensional (3D) spin correlation between the FeO5 bilayers evidenced by the increase of the intensity of satellite reflections with propagation vector k2 = (1/3, 1/3, 3/2). This magnetic modulation is identical to the charge ordered superstructure, highlighting the field-promoted coupling between the spin and charge degrees of freedom. Formation of the 3D spin correlations suppresses both the rod-type diffuse scattering and the k1 component. Simple symmetry-based arguments provide a natural explanation of the observed phenomenon and put forward a possible charge redistribution in the applied magnetic field.

cond-mat.str-el↗

arXiv2017

Rare-earth/transition-metal magnetic interactions in pristine and (Ni,Fe)-doped YCo5 and GdCo5

We present an investigation into the intrinsic magnetic properties of the compounds YCo5 and GdCo5, members of the RETM5 class of permanent magnets (RE = rare earth, TM = transition metal). Focusing on Y and Gd provides direct insight into both the TM magnetization and RE-TM interactions without the complication of strong crystal field effects. We synthesize single crystals of YCo5 and GdCo5 using the optical floating zone technique and measure the magnetization from liquid helium temperatures up to 800 K. These measurements are interpreted through calculations based on a Green's function formulation of density-functional theory, treating the thermal disorder of the local magnetic moments within the coherent potential approximation. The rise in the magnetization of GdCo5 with temperature is shown to arise from a faster disordering of the Gd magnetic moments compared to the antiferromagnetically aligned Co sublattice. We use the calculations to analyze the different Curie temperatures of the compounds and also compare the molecular (Weiss) fields at the RE site with previously published neutron scattering experiments. To gain further insight into the RE-TM interactions, we perform substitutional doping on the TM site, studying the compounds RECo4.5Ni0.5, RECo4Ni, and RECo4.5Fe0.5. Both our calculations and experiments on powdered samples find an increased/decreased magnetization with Fe/Ni doping, respectively. The calculations further reveal a pronounced dependence on the location of the dopant atoms of both the Curie temperatures and the Weiss field at the RE site.

cond-mat.mtrl-sci↗

arXiv2017

Coulomb spin liquid in anion-disordered pyrochlore Tb$_2$Hf$_2$O$_7$

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. In the pyrochlore Tb$_2$Hf$_2$O$_7$, we demonstrate that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly-fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2016

Field Induced Magnetic States in Holmium Tetraboride

A study of the zero field and field induced magnetic states of the frustrated rare earth tetraboride, HoB$_4$, has been carried out using single crystal neutron diffraction complemented by magnetisation measurements. In zero field, HoB$_4$ shows magnetic phase transitions at $T_{\mathrm{N1}}$ = 7.1 K to an incommensurate state with a propagation vector ($δ$, $δ$, $δ'$), where $δ$ = 0.02 and $δ'$ = 0.43 and at $T_{\mathrm{N2}}$ = 5.7 K to a non-collinear commensurate antiferromagnetic structure. Polarised neutron diffraction measurements in zero field have revealed that the incommensurate reflections, albeit much reduced in intensity, persist down to 1.5 K despite antiferromagnetic ordering at 5.7 K. At lower temperatures, application of a magnetic field along the $c$-axis initially re-establishes the incommensurate phase as the dominant magnetic state in a narrow field range, just prior to HoB$_4$ ordering with an up-up-down ferrimagnetic structure characterised by the $(h,k,\frac{1}{3})$-type reflections between 18 and 24 kOe. This field range is marked by the previously reported $M/M_{\mathrm{sat}}$= $\frac{1}{3}$ magnetisation plateau, which we also see in our magnetisation measurements. The region between 21 and 33 kOe is characterised by the increase in the intensity of the antiferromagnetic reflections, such as (100), the maximum of which coincides with the appearance of the narrow magnetisation plateau with $M/M_{\mathrm{sat}}\approx$ $\frac{3}{5}$. Further increase of the magnetic field results in the stabilisation of a polarised state above 33 kOe, while the incommensurate reflections are clearly present in all fields up to 59 kOe. We propose the $H-T$ phase diagram of HoB$_4$ for the $H \parallel c$ containing both stationary and transitionary magnetic phases which overlap and show significant history dependence.

cond-mat.str-el↗

arXiv2016

Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore

Large single crystals of the pyrochlore Pr2Hf2O7 have been successfully grown by the floating zone technique using an optical furnace equipped with high power Xenon arc lamps. Structural investigations have been carried out by both synchrotron X-ray and neutron powder diffraction to establish the crystallographic structure of the materials produced. The magnetic properties of the single crystals have been determined for magnetic fields applied along different crystallographic axes. The results reveal that Pr2Hf2O7 is an interesting material for further investigations as a frustrated magnet. The high quality of the crystals produced make them ideal for detailed investigations, especially those using neutron scattering techniques.

cond-mat.mtrl-sci↗

arXiv2016

Candidate quantum spin ice in the pyrochlore Pr$_2$Hf$_2$O$_7$

We report the low temperature magnetic properties of the pyrochlore Pr$_2$Hf$_2$O$_7$. Polycrystalline and single-crystal samples are investigated using time-of-flight neutron spectroscopy and macroscopic measurements, respectively. The crystal-field splitting produces a non-Kramers doublet ground state for Pr$^{3+}$, with Ising-like anisotropy. Below 0.5 K ferromagnetic correlations develop, which suggests that the system enters a spin ice-like state associated with the metamagnetic behavior observed at $μ_0H_c\sim2.4$~T. In this regime, the development of a discrete inelastic excitation in the neutron spectra indicates the appearance of spin dynamics which are likely related to cooperative quantum fluctuations.

cond-mat.str-el↗

arXiv2016

CDW slips and giant frictional dissipation peaks at the NbSe$_2$ surface

Accessing, controlling and understanding nanoscale friction and dissipation is a crucial issue in nanotechnology, where moving elements are central. Recently, ultra-sensitive noncontact pendulum Atomic Force Microscope (AFM) succeeded in detecting the electronic friction drop caused by the onset of superconductivity in Nb, raising hopes that a wider variety of mechanisms of mechanical dissipation arising from electron organization into different collective phenomena will become accessible through this unconventional surface probe. Among them, the driven phase dynamics of charge-density-waves (CDWs) represents an outstanding challenge as a source of dissipation. Here we report a striking multiplet of AFM dissipation peaks arising at nanometer distances above the surface of NbSe$_2$ - a layered compound exhibiting an incommensurate CDW. Each peak appears at a well defined tip-surface interaction force of the order of a nN, and persists until T=70K where CDW short-range order is known to disappear. A theoretical model is presented showing that the peaks are connected to tip-induced local 2$π$ CDW phase slips. Under the attractive potential of the approaching tip, the local CDW surface phase landscape deforms continuously until a series of 2$π$ jumps occur between different values of the local phase. As the tip oscillates to and fro, each slip gives rise to a hysteresis cycle, appearing at a selected distance, the dissipation corresponding to "pumping" in and out a local slip in the surface CDW phase of NbSe$_2$.

cond-mat.mes-hall↗

arXiv2015

Electric Field Controlled Magnetization and Charge-Ordering in Pr0.6Ca0.4MnO3

In this paper, we present the observation of the electric field control on the charge-ordering and metamagnetic transitions during the magnetization measurements in a single crystal of Pr0.6Ca0.4MnO3 (PCMO). We have demonstrated that the complete melting of charge ordering can be realized in a single crystal of PCMO by applying a voltage as small as 2.5 V, which otherwise needs magnetic fields in excess of 11 T. The maximum change in magnetization with applied voltage occurs across the charge-ordering transition temperature (TCO = 235 K). Even though the electric field does not seem to affect the magnetic ordering, we see a clear evidence at low temperatures for the occurrence of the metamagnetic transitions at higher fields with the application of electric field.

cond-mat.mes-hall↗

arXiv2014

Aging of the surface of Bi(2)Se(3)

We study the electronic structure of Bi(2)Se(3) employing high resolution photoemission spectroscopy and discover the dependence of the behavior of Dirac particles on surface terminations. The Dirac cone apex appears at different energies and exhibits contrasting shift on Bi and Se terminated surface with complex time dependence emerging from subtle adsorbed oxygen-surface atom interactions. These results uncover the surface states behavior of real systems possessing topologically ordered surface.

cond-mat.mtrl-sci↗

arXiv2014

Crystal growth and properties of the non-centrosymmetric superconductor, Ru7B3

We describe the crystal growth of high quality single crystals of the non-centrosymmetric superconductor, Ru7B3 by the floating zone technique, using an optical furnace equipped with xenon arc lamps. The crystals obtained are large and suitable for detailed measurements, and have been examined using x-ray Laue patterns. The superconducting properties of the crystals obtained have been investigated by magnetisation and resistivity measurements. Crystals have also been grown starting with enriched 11B isotope, making them suitable for neutron scattering experiments.

cond-mat.supr-con↗

arXiv2011

Magnetisation Studies of Geometrically Frustrated Antiferromagnets SrLn2O4, with Ln = Er, Dy and Ho

We present the results of susceptibility χ(T) and magnetisation M(H) measurements performed on single crystal samples of the rare-earth oxides SrLn2O4 (Ln = Er, Dy and Ho). The measurements reveal the presence of magnetic ordering transition in SrHo2O4 at 0.62 K and confirm that SrEr2O4 orders magnetically at 0.73 K, while in SrDy2O4 such a transition is absent down to at least 0.5 K. The observed ordering temperatures are significantly lower than the Curie-Weiss temperatures, θ_{CW}, obtained from the high-temperature linear fits to the 1/χ(T) curves, which implies that these materials are subject to geometric frustration. Strong anisotropy found in the χ(T) curves for a field applied along the different crystallographic directions is also evident in the M(H) curves measured both above and below the ordering temperatures. For all three compounds the magnetisation plateaux at approximately one third of the magnetisation saturation values can be seen for certain directions of applied field, which is indicative of field-induced stabilisation of a collinear {\it two-up one-down} structure.

cond-mat.str-el↗