arXiv subjects

S. Giri

Publications and source records attributed to S. Giri.

At least 37 records · Page 2Linked to original sources

arXiv2013

Nature of the glassy magnetic state in Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$ shape memory alloy

The magnetic ground state of the ferromagnetic shape memory alloy of nominal composition Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$ was investigated. The sample shows reentry of a glassy magnetic phase below the martensitic transition temperature, which is found to have complex character with two distinct anomalies in the temperature dependent ac susceptibility data. The sample retains its glassy phase even below the second transition as evident from the magnetic memory measurements in different protocols. Existence of two transitions along with their observed nature suggest that the system can be described by the mean field Heisenberg model of reentrant spin glass as proposed by Gabay and Toulous. \cite{rsg-GT1} The sample provides a fascinating example where a Gabay-Toulous type spin glass state is triggered by a first order magneto-structural transition.

cond-mat.str-el↗

arXiv2013

True reentry of the glassy state in geometrically frustrated LiCr(1-x)Mn(x)O(2)

The development of spin glass like state in a geometrically frustrated (GF) magnet is a matter of great debate. We investigated the effect of magnetic (Mn) and nonmagnetic (Ga) doping at the Cr site of the layered GF antiferromagnetic compound LiCrO2. 10% Ga doping at the Cr site does not invoke any metastability typical of a glassy magnetic state. However, similar amount of Mn doping certainly drives the system to a spin glass state which is particularly evident from the relaxation, magnetic memory and heat capacity studies. The onset of glassy state in 10% Mn doped sample is of reentrant type developing out of higher temperature antiferromagnetic state. The spin glass state in the Mn-doped sample shows a true reentry with the complete disappearance of the antiferromagnetic phase below the spin glass transition. Mn doping at the Cr site can invoke random ferromagnetic Cr-Mn bonds in the otherwise 120 degree antiferromagnetic triangular lattice leading to the non-ergodic spin frozen state. The lack of spin glass state on Ga doping indicates the importance of random ferromagnetic/antiferromagnetic bonds for the glassy ground state in LiCrO2. Spin glass state in GF system has been earlier observed even for small non-magnetic disorder, and our result indicates that the issue is quite nontrivial and depends strongly on the material system concerned.

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arXiv2013

Spin correlated dielectric memory and rejuvenation in relaxor ferroelectric CuCrS2

CuCrS2, a Heisenberg antiferromagnet with layered edge sharing triangular lattice, exhibits a spiral magnetic order. Dielectric (E) and polarization studies show magnetoelectric (ME) coupling at Neel temperature (T_N=38 K) where simultaneous dielectric and magnetic long range order occur. The compound shows a diffused ferroelectric (FE) transition and slow relaxation dynamics above T_N, indicative of relaxor FE behavior. Interestingly, memory effect and magnetic field induced rejuvenation are also observed in E, establishing cooperative glassy dynamics and ME coupling even above T_N. We discuss the role of geometrical frustration and metal ligand hybridization for these unusual properties.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic and electric properties of CaMn7O12 based multiferroic compounds: effect of electron doping

The mixed-valent multiferroic compound CaMn7O12 is studied for its magnetic and electric properties. The compound undergoes magnetic ordering below 90 K with a helimagnetic structure followed by a low temperature magnetic anomaly observed around 43 K. The present study shows that the magnetic anomaly at 43 K is associated with thermal hysteresis indicating first order nature of the transition. The compound also shows field-cooled magnetic memory and relaxation below 43 K, although no zero-field-cooled memory is present. Clear magnetic hysteresis loop is present in the magnetization versus field measurements signifying the presence of some ferromagnetic clusters in the system. We doped trivalent La at the cite of divalent Ca expecting to enhance the fraction of Mn$^{3+}$ ions. The La doped samples show reduced magnetization, although the temperatures associated with the magnetic anomalies remain almost unaltered. Interestingly, the spontaneous electrical polarization below 90 K increases drastically on La substitution. We propose that the ground state of the pure as well as the La doped compositions contain isolated superparamagnetic like clusters, which can give rise to metastability in the form of field-cooled memory and relaxation. The ground state is not certainly spin glass type as it is evident from the absence of zero-field-cooled memory and frequency shift in the ac suceptibility measurements.

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arXiv2012

Glassy magnetic phase driven by short range charge and magnetic ordering in nanocrystalline La$_{1/3}$Sr$_{2/3}$FeO$_{3-δ}$: Magnetization, Mossbauer, and polarised neutron studies

The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-δ}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, Mössbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the Mössbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe$^{3+}$ and Fe$^{5+}$ are about 3.15$μ_B$ and 1.57$μ_B$ for bulk, and 2.7$μ_B$ and 0.53$μ_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\sim$ 70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass like transition around $\sim$ 65 K, below which EB appears. Overall results propose that finite size effect directs the complex glassy magnetic behavior driven by unconventional short range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.

cond-mat.mtrl-sci↗

arXiv2012

Structural domain and spin ordering induced glassy magnetic phase in single layered manganite Pr$_{0.22}$Sr$_{1.78}$MnO$_4$

The single layered manganite Pr$_{0.22}$Sr$_{1.78}$MnO$_4$ undergoes structural transition from high temperature tetragonal phase to low temperature orthorhombic phase below room temperature. The orthorhombic phase was reported to have two structural variants with slightly different lattice parameters and Mn-3$d$ levels show orbital ordering within both the variants, albeit having mutually perpendicular ordering axis. In addition to orbital ordering, the orthorhombic variants also order antiferromagnetically with different Néel temperatures. Our magnetic investigation on the polycrystalline sample of Pr$_{0.22}$Sr$_{1.78}$MnO$_4$ shows large thermal hysteresis indicating the first order nature of the tetragonal to orthorhombic transition. We observe magnetic memory, large relaxation, frequency dependent ac susceptbility and aging effects at low temperature, which indicate spin glass like magnetic ground state in the sample. The glassy magnetic state presumably arises from the interfacial frustration of orthorhombic domains with orbital and spin orderings playing crucial role toward the competing magnetic interactions.

cond-mat.str-el↗

arXiv2011

Investigation of weak itinerant ferromagnetism and critical behavior of Y$_2$Ni$_7$

The weak itinerant ferromagnetic behavior of Y$_2$Ni$_7$ is investigated through magnetic, transport and calorimetric measurements. The low value of saturation moment, large Rhodes-Wohlfarth ratio, large value of the linear term in heat capacity and Fermi liquid like resistivity behavior with enhanced electron-electron scattering contribution establish firmly the weak itinerant ferromagetic nature of the sample. The critical exponents associated with the paramagnetic to ferromagnetic transition are also investigated from magnetization isotherms using modified Arrott plot, Kouvel-Fisher plot and critical isotherm technique. The more accurate Kouvel-Fisher plot provides the critical exponents to be $β$ = 0.306, $γ$ = 1.401 and $δ$ = 5.578. These values are markedly different from the mean field values and correspond closely to the two dimensional Ising spin system with long range spin spin interaction.

cond-mat.str-el↗

arXiv2011

Spin-glass-like state in GdCu: role of phase separation and magnetic frustration

We report investigations on the ground state magnetic properties of intermetallic compound GdCu through dc magnetization measurements. GdCu undergoes first order martensitic type structural transition over a wide temperature window of coexisting phases. The high temperature cubic and the low temperature orthorhombic phases have different magnetic character and they show antiferromagnetic and helimagnetic orderings below 145 K and 45 K respectively. We observe clear signature of a glassy magnetic phase below the helimagnetic ordering temperature, which is marked by thermomagnetic irreversibility, aging and memory effects. The glassy magnetic phase in GdCu is found to be rather intriguing with its origin lies in the interfacial frustration due to distinct magnetic character of the coexisting phases.

cond-mat.str-el↗

arXiv2011

Magnetic and magnetoresistive properties of cubic Laves phase HoAl$_2$ single crystal

We report the magnetization ($M$) and magnetoresistance (MR) results of HoAl$_2$ single crystals oriented along $<100>$ and $<110>$ directions. Although HoAl$_2$ has cubic Laves phase structure, a large anisotropy is observed in $M$ and MR results below Curie temperature ($T_C$) at 31.5 K. A satisfactory correlation between magnetic entropy change ($ΔS_M$) and MR could be established along $<110>$ direction and in $<100>$ direction except for the temperature ($T$) region, around which spin reorientation takes place. Large inverse magnetocaloric effect is observed at low-$T$ which is associated with the spin reorientation process in $<100>$ direction. A theoretical model based on Landau theory of phase transition can describe $T$-variation of -$ΔS_M$ for $T > T_C$.

cond-mat.mtrl-sci↗

arXiv2010

Giant magneto-caloric effect near room temperature in Ni-Mn-Sn-Ga alloys

We report the observation of giant magneto-caloric effect (MCE) in ferromagnetic shape memory alloys (FSMAs) of nominal compositions Ni$_2$Mn$_{1.36}$Sn$_{0.64-x}$Ga$_{x}$ ($x$ =0.24, 0.28 and 0.32). Irrespective of the Ga doping, all the samples undergo long range ferromagnetic ordering below around 330 K. However, the martensitic transition temperature ($T_{MS}$) of the samples varies strongly with Ga concentration. Clear signature of field induced transition around $T_{MS}$ is visible for all the samples. The observed MCE ($ΔS$ = 13.6 J/kg K for field changing from 0 to 50 kOe) is found to be highest for $x$ = 0.28 with peak near 249 K, while $x$ = 0.32 shows $ΔS$ = 12.8 J/kg K at 274 K. This series of Ga-doped alloys are found to be interesting materials with high value of $ΔS$ over a varied range of temperature

cond-mat.str-el↗

arXiv2009

Exchange bias effect and intragranular magnetoresistance in Nd$_{0.84}Sr_{0.16}CoO_3

Electrical transport properties as a function of magnetic field and time have been investigated in polycrystalline, Nd_{0.84}Sr_{0.16}CoO_3. A strong exchange bias (EB) effect is observed associated with the fairly large intragranular magnetoresistance (MR). The EB effect observed in the MR curve is compared with the EB effect manifested in magnetic hysteresis loop. Training effect, described as the decrease of EB effect when the sample is successively field-cycled at a particular temperature, has been observed in the shift of the MR curve. Training effect could be analysed by the successful models. The EB effect, MR and a considerable time dependence in MR are attributed to the intrinsic nanostructure giving rise to the varieties of magnetic interfaces in the grain interior.

cond-mat.str-el↗

arXiv2009

Exchange bias effect involved with tunneling magnetoresistance in polycrystalline La_{0.88}Sr_{0.12}CoO_3

We report the exchange bias (EB) effect along with tunneling magnetoresistance (MR) in polycrystalline La_{0.88}Sr_{0.12}CoO_3. Analogous to the shift in the magnetic hysteresis loop along the field (H)-axis a shift is clearly observed in the MR-H curve when the sample is cooled in a static magnetic field. Training effect (TE) is a significant manifestation of EB effect which describes the decrease of EB effect when sample is successively field-cycled at a particular temperature. We observe TE in the shift of the MR-H curve which could be interpreted by the spin configurational relaxation model. A strong field-cooled (FC) effect in the temperature as well as time dependence of resistivity is observed below spin freezing temperature. The unusual MR results measured in FC mode are interpreted in terms of intragranular interface effect between short range ferromagnetic clusters and spin-glass matrix giving rise to the EB effect. EB effect in MR has been observed in bilayer or multilayer films which has not yet seen in a polycrystalline compound. EB effect involved with tunneling MR and semiconducting transport property attributed to the intragranular intrinsic nanostructure is promising for the spintronic applications.

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arXiv2009

Possible magnetoelectric coupling in the half doped charge ordered manganite, Pr_0.5Ca_0.5MnO_3-d

Magnetization, magnetoresistance, and magnetodielectric measurements have been carried out on the half doped charge ordered manganite, Pr_0.5Ca_0.5MnO_3. The low temperature state is found to be strongly dependent on the oxygen stoichiometry whereas the high temperature state remains almost unaltered. A disorder driven phase separation in the low temperature state is noticed in the magnetic, magnetoresistance, and dielectric measurements which is attributed to the oxygen deficiency in the compound. A considerable magnetodielectric (MD) effect is noticed close to room temperature at 280 K which is fascinating for technological applications. The strongest MD effect observed in between 180 K and 200 K is found to be uncorrelated with magnetoresistance but it is suggested to be due to a number of intricate processes occuring in this temperature range which includes paramagnetic to antiferromagnetic transition, incommensurate to commensurate charge ordering and orbital ordering. The strongest MD effect seems to emerge from the high sensitivity of the incommensurate state to the external perturbation such as external magnetic field. The results propose the possible magnetoelectric coupling in the charge ordered compound.

cond-mat.str-el↗

arXiv2009

Intragrain electrical inhomogenities and compositional variation of static dielectric constant in LaMn_{1-x}Fe_xO_3

Bulk dc resistivity and dielectric constant measurements with temperature and frequency have been performed in LaMn_{1-x}Fe_xO_3 (0 <= x <= 1.0) as a result of Fe substitution both for the as-synthesized and oxygen annealed samples. Temperature dependence of real part of dielectric constant at different frequencies show a frequency and temperature independent value (epsilon_s) at low temperature for x >= 0.15 where epsilon_0 rises with x reaching a maximum at x = 0.5 and then it decreases sharply at x = 0.70 showing a further increasing trend with the further increase in x. The sharp drop of epsilon_s at x = 0.70 is correlated with the structural change from rhombohedral to orthorhombic structure. A considerable increase of epsilon_s ~ 43% is observed for an increase of x from 0.15 to 0.50. Furthermore, epsilon_s is increased considerably (up to ~ 17% at x = 0.5) due to the oxygen annealing for x <= 0.50. The low temperature resistivities satisfying Variable Range Hopping model are found to be related with the increase of epsilon_s with x for x <= 0.50. The analysis of the complex impedance and modulus planes at low temperature indicates the electrical inhomogeneities in the grain interior of the compounds.

cond-mat.str-el↗

arXiv2008

Reentrant spin glass state in Mn doped Ni2MnSn shape memory alloy

The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.

cond-mat.str-el↗

arXiv2008

Exchange bias effect in the phase separated Nd_{1-x}Sr_{x}CoO_3 at the spontaneous ferromagnetic/ferrimagnetic interface

We report the new results of exchange bias effect in Nd_{1-x}Sr_{x}CoO_3 for x = 0.20 and 0.40, where the exchange bias phenomenon is involved with the ferrimagnetic (FI) state in a spontaneously phase separated system. The zero-field cooled magnetization exhibits the FI (T_{FI}) and ferromagnetic (T_C) transitions at ~ 23 and \sim 70 K, respectively for x = 0.20. The negative horizontal and positive vertical shifts of the magnetic hysteresis loops are observed when the system is cooled through T_{FI} in presence of a positive static magnetic field. Training effect is observed for x = 0.20, which could be interpreted by a spin configurational relaxation model. The unidirectional shifts of the hysteresis loops as a function of temperature exhibit the absence of exchange bias above T_{FI} for x = 0.20. The analysis of the cooling field dependence of exchange bias field and magnetization indicates that the ferromagnetic (FM) clusters consist of single magnetic domain with average size around \sim 20 and ~ 40 Å~ for x = 0.20 and 0.40, respectively. The sizes of the FM clusters are close to the percolation threshold for x = 0.20, which grow and coalesce to form the bigger size for x = 0.40 resulting in a weak exchange bias effect.

cond-mat.str-el↗

arXiv2008

Polaron relaxation and hopping conductivity in LaMn$_{1-x}$Fe$_{x}$O$_3$

Dc and ac transport properties as well as electric modulus spectra have been investigated for the samples LaMn$_{1-x}$Fe$_{x}$O$_3$ with compositions 0 $\leq x \leq$ 1.0. The bulk dc resistivity shows a temperature variation consistent with the variable range hopping mechanism at low temperature and Arrhenius mechanism at high temperatures. The ac conductivity has been found to follow a power law behavior at a limited temperature and frequency region where Anderson-localization plays a significant role in the transport mechanism for all the compositions. At low temperatures large dc resistivities and dielectric relaxation behavior for all the compositions are consistent with the polaronic nature of the charge carriers. Scaling of the modulus spectra shows that the charge transport dynamics is independent of temperature for a particular composition but depends strongly on different compositions possibly due to different charge carrier concentrations and structural properties.

cond-mat.str-el↗

arXiv2008

Metastability and magnetic memory effect in Ni-Mn-Sn alloy

Magneto-structural instability in the ferromagnetic shape memory alloy of composition Ni$_2$Mn$_{1.4}$Sn$_{0.6}$ is investigated by transport and magnetic measurements. Large negative magnetoresistance is observed around the martensitic transition temperature (90-210 K). Both magnetization and magnetoresistance data indicate that upon the application of an external magnetic field at a constant temperature, the sample attains a field-induced arrested state which persists even when the field is withdrawn. We observe an intriguing behavior of the arrested state that it can remember the last highest field it has experienced. The field-induced structural transition plays the key role for the observed anomaly and the observed irreversibility can be accounted by the Landau-type free energy model for the first order phase transition.

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