arXiv subjects

A. K. Nigam

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

At least 19 recordsLinked to original sources

arXiv2022

Non-equilibrium magnetic response in concentrated spin-glass AuFe(11%) alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline AuFe(11%) alloy. The temperature variation of ZFC and FC dc magnetization at low fields show a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. The ZFC ac susceptibility shows a sharp cusp at Tf, which shifts towards higher temperatures with an increase in the frequency of the ac magnetic field. When ac susceptibility is recorded after cooling the sample from high temperature in the presence of dc bias magnetic field, the susceptibility cusp gets broadened. In this case, no perceptible frequency dependency of the Tf has been observed, but a significant dispersion in the ac susceptibility is present below Tf. This clearly indicates the non-equilibrium nature of the FC state.In addition, the FC state of AuFe(11%) alloy exhibits a pronounced memory effect which further underlines that the FC state is not an equilibrium state. In contrast to the general perception obtained through the meanfield theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of AuFe(11%) alloy is a nontrivial one

cond-mat.mtrl-sci↗

arXiv2022

Antisite disorder and phase segregation in Mn$_{2}$NiSn

A systematic study of crystal structure, local structure, magnetic and transport properties in quenched and temper annealed Ni$_{2-x}$Mn$_{1+x}$Sn alloys indicate the formation of Mn$_3$Sn type structural defects caused by an antisite disorder between Mn and Sn occupying the Y and Z sublattices of X$_2$YZ Heusler structure. The antisite disorder is caused by the substitution of Ni by Mn at the X sites. On temper annealing, these defects segregate and phase separate into $L2_1$ Heusler and $D0_{19}$ Mn$_3$Sn type phases.

cond-mat.mtrl-sci↗

arXiv2020

Lattice strain accommodation and absence of pre-transition phases in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$

The stoichiometric Ni$_{50}$Mn$_{25}$In$_{25}$ Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ alloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value of $x$ = 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.

cond-mat.mtrl-sci↗

arXiv2020

Non-equilibrium magnetic response of canonical spin glass and magnetic glass

Time and history dependent magnetization has been observed in a wide variety of materials, which are collectively termed as the glassy magnetic systems. However, such systems showing similar non-equilibrium magnetic response can be microscopically very different and they can be distinguished by carefully looking into the details of the observed metastable magnetic behavior. Canonical spin glass is the most well studied member of this class and has been extensively investigated both experimentally and theoretically over the last five decades. In canonical spin glasses, the low temperature magnetic state obtained by cooling across the spin glass transition temperature in presence of an applied magnetic field is known as the field cooled (FC) state. This FC state in canonical spin glass is widely believed as an equilibrium state arising out of a thermodynamic second order phase transition. Here, we show that the FC state in canonical spin glass is not really an equilibrium state of the system. We report careful dc magnetization and ac susceptibility measurements on two canonical spin glass systems, AuMn (1.8%) and AgMn (1.1%). The dc magnetization in the FC state shows clear temperature dependence. In addition, the magnetization shows a distinct thermal hysteresis in the temperature regime below the spin glass transition temperature. On the other hand, the temperature dependence of ac susceptibility has clear frequency dispersion below spin glass transition in the FC state prepared by cooling the sample in the presence of a dc-bias field. We further distinguish the metastable response of the FC state of canonical spin glass from the metastable response the FC state in an entirely different class of glassy magnetic system namely magnetic glass, where the non-equilibrium behavior is associated with the kinetic-arrest of a first order magnetic phase transition.

cond-mat.str-el↗

arXiv2020

The field cooled state of canonical spin-glass revisited

Canonical spin-glass (SG) is an enigmatic system in condensed matter physics. In spite of the intense activities of last five decades several questions regarding the nature of the SG phase transition and the SG ground state are yet to be resolved completely. In this backdrop we have revisited the field cooled state of canonical spin-glass. We have experimentally studied magnetic response in two canonical spin-glass systems AuMn(1.8%) and AgMn(1.1%), both in the field cooled (FC) as well as zero field cooled (ZFC) state. We show that the well known magnetic memory effect, which clearly established earlier the metastable nature of the ZFC state in SG, is also present in the FC state. The results of our experimental study indicate that the FC state also is a non-equilibrium state, and hence the energy landscape involved is a non-trivial one. This in turn seriously questions the picture of spin-glass transformation as a second order thermodynamic phase transition.

cond-mat.str-el↗

arXiv2020

$d$ band filling and magnetic phase separation in transition metal-doped Mn$_3$SnC

The structural and magnetic properties of transition metal-doped Mn$_3$SnC are studied with an aim to understand the effect of transition metal atom on magnetostructural properties of the antiperovskite compound. The doped Mn$_{2.8}$T$_{0.2}$SnC (T = Cr, Fe, Co, Ni and Cu) compounds show a distinctly different magnetic behavior which can be related to electronic filling of the $d$ band of the transition metal atom. While Cr and Fe doped Mn$_3$SnC show properties similar to that of Mn$_3$SnC, the Co, Ni and Cu doped compounds exhibit nucleation of secondary phases which are devoid of carbon and having Heusler and DO19 type hexagonal structure. A strong magnetic interaction is observed between the impurity phases and the major antiperovskite phase leading to a sharp decrease in magnetostructural transition temperature of the antiperovskite phase and a cluster glassy ground state.

cond-mat.mtrl-sci↗

arXiv2020

Modulations in magnetostructural coupling in C and Sn deficient Mn$_3$SnC

The structural and magnetic interactions in stoichiometric (Mn$_3$SnC), carbon deficient (Mn$_3$SnC$_{0.8}$) and tin deficient (Mn$_3$Sn$_{0.85}$C) antiperovskite compounds are studied using x-ray absorption fine structure spectroscopy and neutron diffraction. The study confirms the presence of local structural distortions only around Mn atoms in the antiperovskite compounds irrespective of their stoichiometry. The distortions in the Mn$_6$C octahedra are such that only Mn atoms are displaced from their crystallographic positions resulting in long and short Mn-Mn bonds. These long and short Mn-Mn bonds are responsible for presence of ferromagnetic and antiferromagnetic moments on Mn atoms. The C deficiency at the center of the octahedra increases the strain on the Mn$_6$C octahedra and results in a wide variation of Mn-Mn bond distances as a function of temperature and large hysteresis in magnetic properties. On the other hand, Sn deficiency tends to relax strain by giving more space for the octahedra to distort leading to temperature independent Mn-Mn bond distances.

cond-mat.mtrl-sci↗

arXiv2019

Possible glass-like random singlet magnetic state in 1T-TaS2

Two-dimensional layered transition-metal-dichalcogenide compound 1T-TaS2 shows the rare coexistence of charge density wave (CDW) and electron correlation driven Mott transition. In addition, atomic-cluster spins on the triangular lattice of the CDW state of 1T-TaS2 give rise to the possibility of the exotic spin-singlet state in which quantum fluctuations of spins are strong enough to prevent any long range magnetic ordering down to absolute zero ( 0 K). We present here the evidences of a glass-like random singlet magnetic state in 1T-TaS2 at low temperatures through a study of temperature and time dependence of magnetization. Comparing the experimental results with a representative canonical spin-glass system Au(1.8%Mn), we show that this glass-like state is distinctly different from the well established canonical spin-glass state.

cond-mat.str-el↗

arXiv2019

Absence of first order magnetic transition, a curious case of Mn3InC

The volume expanding magnetostructural transition in Mn$_3$GaC and Mn$_3$SnC has been identified to be due to distortion of Mn$_6$C octahedra. Despite a similar lattice volume as Mn$_3$SnC and similar valence electron contribution to the density of states as in Mn$_3$GaC, Mn$_3$InC does not undergo a first order magnetostructural transformation like the Ga and Sn antiperovskite counterparts. A systematic investigation of its structure and magnetic properties using probes like x-ray diffraction, magnetization measurements, neutron diffraction and extended x-ray absorption fine structure (EXAFS) reveal that though the octahedra are distorted resulting in long and short Mn -- Mn bonds and different magnetic moments on Mn atoms, the interaction between them remains ferromagnetic. This has been attributed to the strain on the Mn$_6$C octahedra produced due to the relatively larger size of In atom compared to Sn and Ga. The size of In atom constricts the deformation of Mn$_6$C octahedra giving rise to Mn -- Mn distances that favor only ferromagnetic interactions in the compound.

cond-mat.mtrl-sci↗

arXiv2018

Measurement independent magnetocaloric effect in Mn-rich Mn-Fe-Ni-Sn(Sb/In) Heusler alloys

We report a systematic study on the magneto-structural transition in Mn-rich Fe-doped Mn-Fe-Ni-Sn(Sb/In) Heusler alloys by keeping the total valence electron concentration (e/a ratio) fixed. The martensitic transition (MT) temperature is found to shift by following a proportional relationship with the e/a ratio of the magnetic elements alone. The magnetic entropy change across MT for a selected sample (Mn49FeNi40Sn9In) has been estimated from three different measurement methods (isofield magnetization (M) vs temperature (T), isothermal M vs field (H) and heat capacity (HC) vs T). We observed that though the peak value of magnetic entropy change changes with the measurement methods, the broadened shape of the magnetic entropy change vs T curves and the corresponding cooling power (~140 Jkg-1) remains invariant. The equivalent adiabatic temperature change ~ -2.6 K has been obtained from indirect measurements of temperature change. Moreover, an exchange bias field ~ 783 Oe at 5 K and a magnetoresistance of -30% are also obtained in one of these materials.

cond-mat.mtrl-sci↗

arXiv2018

Role of Tin and Carbon in the magnetic interactions in Mn$_3$SnC

In this paper we attempt to understand the role of tin and carbon in magnetic interactions in Mn$_3$SnC. Mn$_3$SnC exhibits a time dependent magnetic configuration and a complex magnetic ground state with both ferromagnetic and antiferromagnetic orders. Such a magnetic state is attributed to presence of distorted Mn$_6$C octahedra with long and short Mn--Mn bonds. Our studies show that C deficiency increases the tensile strain on the Mn$_6$C octahedra which elongates Mn--Mn bonds and strengthens ferromagnetic interactions while Sn deficiency tends to ease out the strain resulting in shorter as well as longer Mn--Mn bond distances in comparison with stoichiometric Mn$_3$SnC. Such a variation strengthens both, ferromagnetic and antiferromagnetic interactions. Thus the structural strain caused by both Sn and C is responsible for complex magnetic ground state of Mn$_3$SnC.

cond-mat.mtrl-sci↗

arXiv2018

Phase separation and effect of strain on magnetic properties of Mn$_3$Ga$_{1-x}$Sn$_x$C

While the unit cell volume of compounds belonging to the Mn$_3$Ga$_{1-x}$Sn$_x$C, (0 $ \le x \le $ 1) series shows a conformity with Vegard's law, their magnetic and magnetocaloric properties behave differently from those of parent compounds Mn$_3$GaC and Mn$_3$SnC. A correlation between the observed magnetic properties and underlying magnetic and local structure suggests that replacing Ga atoms by larger atoms of Sn results in the formation of Ga-rich and Sn-rich clusters. As a result, even though the long range structure appears to be cubic, Mn atoms find themselves in two different local environments. The packing of these two different local structures into a single global structure induces tensile/compressive strains on the Mn$_{6}$C functional unit and is responsible for the observed magnetic properties across the entire solid solution range.

cond-mat.mtrl-sci↗

arXiv2018

Reentrant cluster glass and stability of ferromagnetism in Ga2MnCo Heusler alloy

We present here a detailed investigation into the magnetic ordering of full Heusler alloy Ga$_2$MnCo using dc, ac magnetization measurements, neutron diffraction and neutron depolarization experiments. Crystal structure at room temperature was first confirmed to be L2$_1$ using the highly intense synchrotron X-ray diffraction (XRD) technique. Temperature dependent magnetization reveals that Ga$_2$MnCo enters a ferromagnetic (FM) state at $T_C = $154 K, characterized by a sharp increase in magnetization and a plateau-like region hereafter. As the temperature is decreased further, a sharp drop in magnetization is observed at $T_f$ = 50 K, hinting towards an antiferromagnetic (AFM) phase change. Neutron diffraction (ND) recorded over the range of temperature from 6 to 300 K, provides combined information regarding crystal as well as magnetic structure. Accordingly, an increase in the intensity of the ND pattern is seen at 150 K, signaling onset of long range FM order. However, there is no sign of appearance of superlattice reflections corresponding to the AFM phase, in the patterns recorded below 50 K. An unusual discontinuity in the unit cell volume is seen around $T_f$ indicating a coupling of this second transition with the contraction of the lattice. Attempts to unravel this interesting magnetic behaviour using ac susceptibility measurements lead to the existence of glassy magnetism below $T_f$. Systematic analysis of the susceptibility results along with neutron depolarization measurement, identifies the low temperature phase as a reentrant cluster glass.

cond-mat.str-el↗

arXiv2018

Coupled magnetic and ferroelectric states in the distorted honeycomb system Fe$_{4}$Ta$_{2}$O$_{9}$

We report on the magnetic, thermodynamic, dielectric, and pyroelectric measurements on the hitherto unreported Fe${_4}$Ta${_2}$O${_9}$. This system is seen to exhibit a series of magnetic transitions, many of which are coupled to the emergence of ferroelectric order, making Fe${_4}$Ta${_2}$O${_9}$ the only genuine multiferroic in its material class. We suggest that the observed properties arise as a consequence of an effective reduction in the dimensionality of the magnetic lattice, with the magnetically active Fe${^{2+}}$ ions preferentially occupying a quasi 2D buckled honeycomb structure. The low temperature $H$-$T$ phase diagram of Fe${_4}$Ta${_2}$O${_9}$ reveals a rich variety of coupled magnetic and ferroelectric phases, in similarity with that observed in the distorted Kagome systems.

cond-mat.str-el↗

arXiv2017

Quasi Static Remanence in Dzyaloshinskii-Moriya Interaction driven Weak Ferromagnets and Piezomagnets

We explore remanent magnetization ($μ$) as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFM) which are also weak ferromagnets (WFM) and piezomagnets (PzM). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasi static in nature and exhibits a counter-intuitive magnetic field dependence. These observations unravel an ultra-slow magnetization relaxation phenomenon related to this quasi static remanence. This feature is also observed in a defect free single crystal of $α$-Fe$_2$O$_3$, which is a canonical WFM and PzM. Notably, $α$-Fe$_2$O$_3$ is not a typical geometrically frustrated AFM and in single crystal form, it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFM which are either symmetry allowed WFM, driven by Dzyaloshinskii-Moriya Interaction (DMI) or can generate this trait by tuning of size and interface. The qualitative features of the quasi static remanence indicate that such WFM are potential piezomagnets, in which magnetization can be tuned by \textit{stress} alone.

cond-mat.str-el↗

arXiv2017

3d Transition Metals and Oxides within Carbon Nanotubes by Co-Pyrolysis of Metallocene & Camphor: High Filling Efficiency and Self-Organized Structures

We demonstrate that a single zone furnace with a modified synthesis chamber design is sufficient to obtain metal (Fe, Co or Ni) filled carbon nanotubes (CNT) with high filling efficiency and controlled morphology. Samples are formed by pyrolysis of metallocenes, a synthesis technique that otherwise requires a dual zone furnace. Respective metallocene in all three cases are sublimed in powder form, a crucial factor for obtaining high filling efficiency. While Fe@CNT is routinely produced using this technique, well-formed Ni@CNT or Co@CNT samples are reported for the first time. This is achieved by sublimation of nickelocene (or cobaltocene) in combination with 'camphor'. These samples exhibit some of the highest saturation magnetization (Ms) values, at least an order of magnitude higher than that reported for Ni or Co filled CNT, by aerosol assisted pyrolysis. The results also elucidate on why Ni or Co@CNT are relatively difficult to obtain by pyrolyzing powder metallocene alone. Overall, a systematic variation of synthesis parameters provides insights for obtaining narrow length and diameter distribution and reduced residue particles outside filled CNT - factors which are important for device related applications. Finally, the utility of this technique is demonstrated by obtaining highly aligned forest of Fe2O3@CNT, wherein Fe2O3 is a functional magnetic oxide relevant to spintronics and battery applications.

cond-mat.mtrl-sci↗

arXiv2017

Mechanism of magnetostructural transformation in multifunctional Mn$_3$GaC

Mn$_3$GaC undergoes a ferromagnetic to antiferromagnetic, volume discontinuous cubic-cubic phase transition as a function of temperature, pressure and magnetic field. Through a series of temperature dependent x-ray absorption fine structure spectroscopy experiments at the Mn K and Ga K edge, it is shown that the first order magnetic transformation in Mn$_3$GaC is entirely due to distortions in Mn sub-lattice and with a very little role for Mn-C interactions. The distortion in Mn sub-lattice results in long and short Mn-Mn bonds with the longer Mn-Mn bonds favoring ferromagnetic interactions and the shorter Mn-Mn bonds favoring antiferromagnetic interactions. At the first order transition, the shorter Mn-Mn bonds exhibit an abrupt decrease in their length resulting in an antiferromagnetic ground state and a strained lattice.

cond-mat.mtrl-sci↗

arXiv2017

Phase separated magnetic ground state in Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C

Existence of non-ergodic ground states is considered as a precursor to a first order long range magnetostructural transformation. Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C lies compositionally between two compounds, Mn$_3$GaC and Mn$_3$SnC, undergoing first order magnetic transformation. Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C though crystallizes in single phase cubic structure, exhibits more than one long range magnetic transitions. Using a combination of magnetization, ac susceptibility, neutron diffraction and XAFS techniques it is shown that, though Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C exhibits long range magnetic order, it presents a cluster glassy ground state due to formation of magnetically ordered Ga rich and Sn rich clusters. The clusters are big enough to present signatures of long range magnetic order but are distributed in such way that it limits interactions between two clusters of the same type leading to a frozen magnetic state at low temperatures. The main reason for such a cluster glass state is the difference in local structure of Mn atoms that find themselves in Ga rich and Sn rich clusters.

cond-mat.mtrl-sci↗