arXiv subjects

K. Mukherjee

Publications and source records attributed to K. Mukherjee.

At least 73 records · Page 4Linked to original sources

arXiv2011

Magnetic behavior of bulk and fine particles of RCr2Si2C (R = La, Ce)compounds: possible magnetic ordering from Cr

The magnetic behavior of the quaternary compounds, RCr2Si2C (R = La, Ce), has been investigated by magnetization (M) and heat-capacity (C) measurements (1.8-300 K) in the bulk polycrystals and nano forms (<1 μm) obtained by high-energy balling. Our finding is that Cr appears to exhibit magnetic ordering of an itinerant type at low temperatures (<20 K) in the bulk form, as inferred from a combined look at all the data. The magnetic ordering gets gradually suppressed with increasing milling time. Evidence for a mixed-valence state of Ce for the bulk form is obtained from the tendency of magnetic susceptibility to exhibit a maximum above 300 K. However, this feature vanishes in the nano form, which exhibits a Curie-Weiss behavior above 200 K as though Ce tends towards trivalency in these fine particles; in addition, there is a weak upturn in C/T below 10 K in the bulk, which becomes very prominent in the milled Ce-based specimens at lower temperatures, as though heavy-fermion behavior gets stronger in smaller particles.

cond-mat.str-el↗

arXiv2011

Isothermal magnetic entropy behavior in Tb5Si3: Sign reversal and non-monotonic variation with temperature, and implications

The magnetic entropy change (\DeltaS), a measure of the magnetocaloric effect, in Tb5Si3, a compound exhibiting unusual positive magnetoresistance following a magnetic-field-induced transition below magnetic transition temperature (~ 69 K), has been investigated. We found that \DeltaS is negative in the paramagnetic state. At the magnetic transition temperature, \DeltaS shows sign reversal from negative (in the paramagnetic state) to positive value in the magnetically ordered state. The high-field state which is interestingly the high resistive state is found to be associated with higher entropy i.e. large positive \DeltaS, behaving like a paramagnet. On the basis of this observation, we conclude that the magnetic field induces magnetic fluctuations in the system resulting in positive magnetoresistance, thereby rendering support to the idea of 'inverse metamagnetism' in this compound. In addition, we note that Arrott plots present an interesting scenario.

cond-mat.str-el↗

arXiv2011

Contrasting magnetic behavior of fine particles of some Kondo Lattices

We have investigated the magnetic behavior of ball-milled fine particles of well-known Kondo lattices, CeAu2Si2, CePd2Si2 and CeAl2, by magnetization and heat-capacity studies in order to understand the magnetic behavior when the particle size is reduced. These compounds have been known to order antiferromagnetically in the bulk form near (TN=) 10, 10 and 3.8 K respectively. We find that the features due to magnetic ordering get suppressed to temperatures below 1.8 K in the case of fine particles of ternary alloys, though trivalence of Ce as inferred from the effective moment remains unchanged. In contrast to this, in CeAl2, there appears to be a marginal enhancement of TN, when the particle size is reduced to less than a micron. These results can be consistently understood by proposing that there is relatively more 4f-localization as the particle size is reduced, resulting in weakening of exchange interaction strength.

cond-mat.str-el↗

arXiv2011

Evolution of a metastable phase with a magnetic phase coexistence phenomenon and its unusual sensitivity to magnetic field cycling in the alloys Tb5-xLuxSi3 (x <= 0.7)

Recently, we reported an anomalous enhancement of the positive magnetoresistance beyond a critical magnetic field in Tb5Si3 in the magnetically ordered state, attributable to 'inverse metamagnetism'. This results in unusual magnetic hysteresis loops for the pressurized specimens, which are relevant to the topic of 'electronic phase separation'. In this paper, we report the influence of small substitutions of Lu for Tb, to show the evolution of these magnetic anomalies. We find that, at low temperatures, the high-field high-resistivity phase could be partially stabilized on returning the magnetic field to zero in many of these Lu substituted alloys, as measured through the electrical resistivity (ρ). Also, the relative fractions of this phase and the virgin phase appear to be controlled by a small tuning of the composition and temperature. Interestingly, at 1.8 K a sudden 'switch-over' of the value of ρ for this mixed phase to that for the virgin phase for some compositions is observed at low fields after a few field cycles, indicating metastability of this mixed phase.

cond-mat.str-el↗

arXiv2011

4f-hybridization effect on the magnetism of Nd2PdSi3, an anomalous magnetic compound

Among the members of the series R2PdSi3 (R= Rare-earths), the magnetic behavior of the Nd compound is interesting in some respects. This compound is considered to order ferromagnetically (below 16 K), unlike other members of this series which order antiferromagnetically. In addition, magnetic ordering temperature (To) is significantly enhanced with respect to de Gennes scaled value. In order to understand the magnetism of this compound better, we have investigated the magnetic behavior in detail (under external pressure as well) and also of its solid solutions based on substitutions at Nd and at Si sites, viz., on the series, Nd(2-x)(Y,La)(x)PdSi(3-y)Ge(y) (x, y= 0.2, 0.4, 0.8, and 1.2) by bulk measurements. The results overall establish that Nd2PdSi3 orders ferromagnetically below 16 K, but antiferromagnetic component seems to set in at very low temperatures. Notably, there is a significant suppression of To for Y and Ge substitutions, compared to La substitution, for a given magnitude of unit-cell volume change, however qualitatively correlating with the separation between the layers of Nd and Pd-Si(Ge). On the basis of this observation, we conclude that 4f(Nd) hybridization plays a major role on the magnetism of the former solid solutions. To our knowledge, this work serves as a rare demonstration of 4f-hybridization effects on the magnetism of a Nd-based intermetallic compound.

cond-mat.str-el↗

arXiv2011

A first-order magnetic phase transition near 15 K with novel magnetic-field-induced effects in Er5Si3

We present magnetic characterization of a binary rare-earth intermetallic compound Er5Si3, crystallizing in Mn5Si3-type hexagonal structure, through magnetization, heat-capacity, electrical resistivity, and magnetoresistance measurements. Our investigations confirm that the compound exhibits two magnetic transitions with decreasing temperature, first one at 35 K and the second one at 15 K. The present results reveal that the second magnetic transition is a disorder-broadened first-order transition, as shown by thermal hysteresis in the measured data. Another important finding is that, below 15 K, there is a magnetic-field-induced transition with a hysteretic effect with the electrical resistance getting unusually enhanced at this transition and the magnetorsistance (MR) is found to exhibit intriguing magnetic-field dependence indicating novel magnetic phase-co-existence phenomenon. It thus appears that this compound is characterized by interesting magnetic anomalies in the temperature-magnetic-field phase diagram.

cond-mat.mtrl-sci↗

arXiv2011

Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder

Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H ruthenates. Local structural study reveals that the possible Fe/Ru-site disorder further extends to create local chemical inhomogeneity, affecting the high temperature magnetism of this material. There is a gradual decrease of 57Fe Mössbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glass-like magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2011

Enhanced electrical resistance at the field-induced magnetic transitions in some stoichiometric and nonstoichiometric Tb-based ternary germanides

We present the magnetic and transport behavior of some Tb compounds, viz., TbIrGe2, TbFe(0.4)Ge2, and TbCo(0.4)Ge2. The stoichoometric germanide TbIrGe2 exhibits at least two distinct magnetic transitions in a close temperature interval around 10 K. The non-stoichiometric compounds, TbFe(0.4)Ge2 and TbCo(0.4)Ge2, undergo magnetic ordering around 17 and 19 K respectively. The magnetic state of these compouds appears to be antiferromagnetic-like. Qualitatively, there is a correlation between the field response of the magnetization (M), the magnetoresistance (MR) and the entropy change curve in all these compouds. That is, these Tb compounds exhibit a "positive" MR and entropy change beyond a magnetic field where M also shows a field-induced transition. On the basis of this correlaion, we conclude that magnetic disorder/fluctuations beyond a critical field - "a phenomenon called inverse metamagnetism" - rather than metamagnetism,is induced in these compounds.

cond-mat.str-el↗

arXiv2011

Magnetism of fine particles of Kondo lattices, obtained by high-energy ball-milling

Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles of the Kondo lattices. In this article, we review the results of our investigation on the fine particles (less than 1 micron) of some of the alloys obtained by high-energy ball-milling to bring out that this synthetic method paves a way to study strong electron correlations in nanocrystals of such alloys. We primarily focus on the alloys of the series, CeRu(2-x)Rh(x)Si2, lying at different positions in Doniach's magnetic phase diagram. While CeRu2Si2, a bulk paramagnet, appears to become magnetic (of a glassy type) below about 8 K in fine particle form, in CeRh2Si2, an antiferromagnet (T_N= 36 K) in bulk form, magnetism is destroyed (at least down to 0.5 K) in fine particles. In the alloy, CeRu(0.8)Rh(1.2)Si2, at the quantum critical point, no long range magnetic ordering is found

cond-mat.str-el↗

arXiv2011

Transport and magnetic behavior under pressure and high-resolution photoemission studies of Ce2Rh(o.7)Co(0.3)Si3, an alloy on the verge of quantum critical point

We report the influence of external pressure on the temperature dependence of magnetization and electrical resistivity as well as high-resolution photoemission studies for an alloy, Ce2Rh(0.7)Co(0.3)Si3, ordering magnetically below 3 K. It is found that the external pressure has the same effect as that induced by (further) Co substitution for Rh in the series, Ce2Rh(1-x)Co(x)Si3, resulting in qualitative changes in the features in the magnetic and transport data, with a suppression of magnetic ordering followed by quantum critical point effect. The high-resolution spectra reveal signature of Kondo feature at ambient feature. These findings support the validity of spin-density-wave picture in this series.

cond-mat.str-el↗

arXiv2010

Synthesis of fine particles of a geometrically frustrated spin-chain system, Ca3Co2O6, through a pyrophoric route and its magnetic behavior

We report the synthesis of fine particles of a well-known geometrically frustrated spin-chain compound Ca3Co2O6 through a new route, namely, a pyrophoric method employing triathanolamine (TEA) and studied the magnetic behavior of such specimens. We find that this synthesis yields particles made up of rods whose diameter appears to be controllable by the fraction of TEA during synthesis. It is seen that the two well-known magnetic transitions (near 24 and 8 K) remain unaffected for all TEA concentrations used. The most notable finding is that the multi-step feature in the M versus magnetic-field isotherm reported for single crystalline form tends to smoothen out gradually with decreasing rod thickness (from about 1 micron to about a few hundred nm). Further, unlike for the single crystalline form known in the literature, there is no tendency for the relaxation time (tau) to remain constant at low temperatures (<10 K) and tau remains temperature dependent up to the lowest temperature measured (1.8K) in all the specimens. These findings suggest that the multistep magnetization anomaly in this system may be characterized by a magnetic correlation length.

cond-mat.str-el↗

arXiv2010

An anomalous butterfly-shaped magnetoresistance loop in an alloy, Tb4LuSi3

Magnetic-field (H) induced first-order magnetic transition and the assiciated electronic phase-separation phenomena are active topics of research in magnetism. Magnetoresistance (MR) is a key property to probe these phenomena and, in literature, a butterfly-shaped MR loop has been noted while cycling the field, with the envelope curve lying below the virgin curve in MR versus H plots of such materials. Here, we report an opposite behavior of MR loop for an alloy, Tb4LuSi3, at low temperatures (<<20 K) in the magnetically ordered state. Such an anomalous curve reveals unexpected domination of higher resistive high-field phase in electronic conduction, unlike in other materials where conducion is naturally by low-resistive high-field phase that follows first-order transition. The observed features reveal an unusual electronic phase separation, namely involving high-resistive high-field phase and low-resistive virgin phase.

cond-mat.str-el↗

arXiv2010

Ferromagnetic feature from Mn near room temperature in the fine particles of GdMn2Ge2 and TbMn2Ge2

The magnetization behaviors of GdMn2Ge2 and TbMn2Ge2 in the bulk and in the fine particles obtained by high-energy ball-milling are compared. Pronounced modificayions in the spontaneous, remnent and high-field magnetization in the fine particle form, attributable to Mn are observed. The results indicate that the antiferromagnetism of Mn sub-lattice known for the bulk form in the range 100-300 K gets weakened in favor of ferromagnetism in the fine particles. On the basis of this observation, we infer that there are other factors like size (and possibly defects) also play a role to decide the exact nature of magnetic ordering of Mn in this ternary family of compounds, contrasting the traditionally held view that the basal plane Mn-Mn distance is the crucial controlling parameter.

cond-mat.str-el↗

arXiv2009

Magnetic and electrical transport anomalies in RMAs2 (R= Pr and Sm, M= Ag and Au)

The results of magnetization, heat-capacity, and electrical resistivity (rho) studies of the compounds, RMAs2 (R= Pr and Sm; M= Ag, Au), crystallizing in HfCuSi2-derived structure are reported. PrAgAs2 orders antiferromagnetically at T_N= 5 K. The Au analogue, however, does not exhibit long range magnetic order down to 1.8 K. We infer that this is due to subtle differences in their crystallographic features, particularly noting that both the Sm compounds with identical crystal structure as that of former order magnetically nearly at the same temperature (about 17 K). It appears that, in PrAgAs2, SmAgAs2, and SmAuAs2, there is an additional magnetic transition at a lower temperature, as though the similarity in the crystal structure results in similarities in magnetism as well. The rho for PrAgAs2 and PrAuAs2 exhibits negative temperature coefficient in some temperature range in the paramagnetic state. SmAuAs2 exhibits magnetic Brillouin-zone gap effect in rho at T_N, while SmAgAs2 shows a well-defined broad minimum well above T_N around 45 K. Thus, these compounds reveal interesting magnetic and transport properties.

cond-mat.str-el↗

arXiv2009

Influence of pressure on the magnetic behavior and the anomalous magnetoresistance in Tb5Si3

The compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure, was recently reported by us to exhibit a sudden and huge enhancement in electrical resistivity (rho) at a critical magnetic field (H_cr) in the magnetically ordered state (<70 K) tracking isothermal magnetization (M) behavior. We have investigated the influence of external pressure (<15 kbar) and negative chemical pressure induced by Ge substitution for Si on M and rho as a function of temperature (5-300 K) and magnetic field (<120 kOe), with the primary aim of understanding the field-induced anomalies. Focussing on isothermal M and magnetoresistance (MR) at two temperatures, 5 and 20K, we find that this rho anomaly persists under external as well as negative chemical pressures, however with a large change in the H_cr. The pressure-derivative of H_cr is negative and this trend and the MR behavior at the H_cr are comparable to that observed in some Laves phase itinerant magnetic systems. On the basis of this observation, we speculate that the magnetic fluctuations induced at this critical field could be responsible for the MR anomal.ies

cond-mat.str-el↗

arXiv2009

Excess specific heat and evidence of zero point entropy in magnetic glassy state of half-doped manganites

We show that specific heat C$_p$ has non-Debye behavior for glassy states in half-doped manganites. Irrespective of the magnetic order or electronic states, these magnetic glasses have higher C$_p$ compared to their equilibrium counterparts. The excess C$_p$ contributed by the glassy state varies linearly with temperature similar to conventional glasses indicating tunneling in the two-level systems. These glassy states show signature of zero point entropy. Magnetic glasses can be produced simply by different field cooling protocols and may be considered ideal magnetic counterpart of the conventional glass.

cond-mat.str-el↗

arXiv2008

Selective substitution in orbital domains of a low doped manganite : an investigation from Griffiths phenomenon and modification of glassy features

An effort is made to study the contrast in magnetic behavior resulting from minimal disorder introduced by substitution of 2.5% Ga or Al in Mn-site of La${_{0.9}}$Sr$_{0.1}$MnO${_3}$. It is considered that Ga or Al selectively creates disorder within the orbital domains or on its walls, causing enhancement of Griffiths phase (GP) singularity for the former and disappearance of it in the later case. It is shown that Ga replaces Mn$^{3+}$ which is considered to be concentrated within the domains, whereas Al replaces Mn$^{4+}$ which is segregated on the hole-rich walls, without causing any significant effect on structure or ferromagnetic transition temperatures. Thus, it is presumed that the effect of disorder created by Ga extend across the bulk of the domain having correlation over similar length-scale resulting in enhancement of GP phenomenon. On the contrary, effect of disorder created by Al remains restricted to the walls resulting in the modification of the dynamics arising from the domain walls and suppresses the GP. Moreover contrasting features are observed in the low temperature region of the compounds; a re-entrant spin glass like behavior is observed in the Ga doped sample, while the observed characteristics for the Al doped sample is ascribed only to modified domain wall dynamics with the absence of any glassy phase. Distinctive features in third order susceptibility measurements reveals that the magnetic ground state of the entire series comprises of orbital domain states. These observations bring out the role of the nature of disorder on GP phenomenon and also reconfirms the character of self-organization in low-doped manganites.

cond-mat.str-el↗

arXiv2007

Changeover from Glassy ferromagnetism of the orbital domain state to long range ferromagnetic ordering in La{_{0.9}}Sr_{0.1}MnO{_3}

An attempt is made to resolve the controversy related to the low temperature phase (ground state) of the low-doped ferromagnetic (FM)- insulator(I) manganite through bulk magnetic measurements on La${_{0.9}}$Sr$_{0.1}$MnO${_3}$ sample. It is shown that the FM phase, formed out of well defined transition in the low-doped system, becomes inhomogeneous with decrease in temperature. This inhomogeniety is considered to be an outcome of the formation of orbital domain state of e$_g$-electrons having hole rich (metallic) walls separating the hole deficient (insulating) regions. The resulting complexity brings in metastability and glassy behaviour within the FM phase at low temperature, however, with no resemblance to spin glass, cluster glass or reentrant phases. It shows ageing effect without memory but magnetic relaxation shows signatures of inter-cluster interaction. The energy landscape picture of this glassy phase is described in terms of hierarchical model. Further, it is shown that this inhomogeneity disappear in La${_{0.9}}$Sr$_{0.1}$MnO$_{3.08}$ where, the orbital domain state is destroyed by self doping resulting in reduction of Mn$^{3+}$ and hence e$_g$-electrons. The ferromagnetic phase of the non-stoichiometric sample, does not show glassy behaviour. It neither follows 'hierarchical model' nor 'droplet model' generally used to explain glassy or inhomogeneous systems. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferromagnet.

cond-mat.str-el↗