arXiv subjects

R. Mallik

Publications and source records attributed to R. Mallik.

11 recordsLinked to original sources

arXiv2002

Do we understand electron correlation effects in Gadolinium based intermetallic compounds?

Recognising the difficulties in systematic understanding of the physical characteristics of strongly correlated f-electron systems, we considered it worthwhile to subject the so-called "normal" f-electron systems like those of Gd to careful investigations. We find that the spin-disorder contribution to electrical resistivity ($ρ$) in the paramagnetic state, instead of remaining contant, surprisingly increases with decreasing temperature (T) in the paramagnetic state in some of the Gd alloys. In some cases, this "excess resistance" is so large that a distinct minimum in the plot of $ρ$ versus T can be seen, mimicking the behaviour of Kondo lattices. This excess resistance can be suppressed by the application of a magnetic field, naturally resulting in large magnetoresistance. In addition, these alloys are found to exhibit heavy-fermion-like heat-capacity behavior. These unusual findings imply hither-to-unexplored electron correlation effects even in Gd-based alloys.

cond-mat.str-el↗

arXiv2000

Silence of magnetic layers to magnetoresistive process and electronic separation at low temperatures in (La, Sm)Mn$_2$Ge$_2$

A closer look at the temperature (T) dependence of magnetoresistance (MR) of two polycrystalline magnetic compounds, LaMn$_2$Ge$_2$ and SmMn$_2$Ge$_2$, previously reported by us, is made. A common feature for both these compounds is that the low temperature MR is positive (say, below, 30 K) in spite of the fact that both are ferromagnetic at such low temperatures; in addition, MR as a function of magnetic field (H) does not track magnetization (M) in the sense that M saturates at low fields, while MR varies linearly with H. These observations suggest that the magnetic layers interestingly do not dominate low temperature magnetotransport process. Interestingly enough, as the T is increased, say around 100 K, these magnetic layers dominate MR process as evidenced by the tracking of M and MR in SmMn$_2$Ge$_2$. These results tempts us to propose that there is an unusual "electronic separation" for MR process as the T is lowered in this class of compounds.

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arXiv1999

Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of Gd$_{2}$PdSi$_3$

Electrical resistivity ($ρ$), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are reported. The well-defined minimum in $ρ$ at a temperature above Néel temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., two-step first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.

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arXiv1999

Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above Curie temperature

We report here the results of magnetic susceptibility, electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an AlB_2-derived hexagonal structure. The results establish that Eu ions are divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An interesting observation is that the sign of MR is negative even at temperatures close to 3T_C, with increasing magnitude with decreasing temperature exhibiting a peak at T_C. This observation, being made for a Cu containing magnetic rare-earth compound for the first time, is of relevance to the field of collosal magnetoresistance.

cond-mat.str-el↗

arXiv1999

La substitution induced linear temperature dependence of electrical resistivity and Kondo behavior in the alloys, Ce_{2-x}La_{x}CoSi_{3}

The results of electrical resistivity, heat capacity and magnetic susceptibility behavior of new class of alloys, Ce_{2-x}La_{x}CoSi_{3}, are reported. The x= 0.0 alloy is mixed valent and La substitution for Ce (x= 0.25) induces linear temperature dependence of resistivity at low temperatures, an observation of relevance to the topic of non-Fermi liquid behavior. The modifications of Kondo effect for all the alloys are also presented.

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arXiv1998

Magnetic precursor effects in Gd alloys

The behaviour of electrical resistivity ($ρ$) and magnetoresistance in the vicinity of respective magnetic ordering temperatures in a number of Gd alloys is reported. In some compounds, e.g., GdNi_2Sn_2 and GdPt_2Ge_2, there is an enhancement of $ρ$ prior to long range magnetic order over a wide temperature range which can be highlighted by the suppression of $ρ$ caused by the application of a magnetic field. However, such features are absent in many other Gd compounds, e.g., GdCu_2Ge_2, GdAg_2Si_2, GdAu_2Si_2, GdPd_2Ge_2 and GdCo_2Si_2. Attempts to relate such features to magnetic precursor effects in heat capacity are made. On the basis of our studies, we suggest that better understanding of magnetic precursor effects in Gd alloys will be helpful to throw light on some of the current trends in magnetism. Various other interesting findings in the magnetically ordered state in some of these alloys are also brought out.

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arXiv1998

The magnetoresistance in GdNi: Magnetic-polaronic-like effect near Curie temperature and low temperature sign reversal

The results of magnetoresistance ($Δρ/ρ$)measurements in GdNi, in the temperature range 4.2 to 300 K are reported. The sign of $Δρ/ρ$ above the Curie temperature (Tc= 70 K) is negative and its magnitude in a magnetic field of 80kOe grows with decreasing temperature below 150 K with a peak value of about -20% at Tc. These features, qualitatively resembling those in giant magnetoresistance systems (manganates), are attributed to the formation of some kind of magnetic polarons induced by Gd. The magnetoresistance changes sign below 12 K which is attributed to subtle band structure effects.

cond-mat.str-el↗

arXiv1998

Residual resistivity ratio and its relation to the positive magnetoresistance behavior in natural multilayer LaMn2Ge2; relevance to artificial multilayer physics

Results of low temperature magnetoresistance ($Δρ/ρ$) and isothermal magnetization (M) measurements on polycrystalline ferromagnetic (T_C close to 300 K) natural multilayers, LaMn_{2+x}Ge_{2-y}Si_y, are reported. It is found that the samples with large residual resistivity ratio, $ρ(300K)/ρ(4.2K)$, exhibit large positive magnetoresistance at high magnetic fields. The Kohler's rule is not obeyed in these alloys. In addition, at 4.5 K, there is a tendency towards linear variation of $Δρ/ρ$ with magnetic field with increasing $ρ(300K)/ρ(4.2K$); however, the field dependence of $Δρ/ρ$ does not track that of M, thereby suggesting that the magnetoresistance originates from non-magnetic layers. It is interesting that these experimental findings on bulk polycrystals are qualitatively similar to what is seen in artificially grown multilayer systems recently.

cond-mat.str-el↗

arXiv1998

Large Low Temperature Magnetoresistance and Magnetic Anomalies in Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$

The results of heat-capacity, magnetic susceptibility, electrical resistivity and magnetoresistance $(Δρ/ρ)$ measurements on the compounds Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$, are reported. The results establish that these compounds undergo long-range magnetic ordering (presumably with a complex magnetic structure) below (Tc=) 23 and 8 K respectively. The $Δρ/ρ$ is negative in the vicinity of Tc and the magnitude grows as Tc is approached from higher temperature as in the case of well-known giant magnetoresistance systems (La manganite based perovskites); this is attributed to the formation of some kind of magnetic polarons. The magnitude of magnetoresistance at low temperatures is quite large, for instance, about 30% in the presence of 60 kOe field at 5 K in the Dy sample.

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arXiv1998

Observation of a temperature dependent electrical resistance minimum above the magnetic ordering temperature in Gd$_2$PdSi$_3$

Results on electrical resistivity, magnetoresistance, magnetic Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat capacity and Gd Mossbauer measurements on a Gd-based intermetallic compound, Gd$_{2}$PdSi$_{3}$ are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined minimum at about 45 K, well above the long range magnetic ordering temperature (21 K), a feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents an interesting scenario. On the basis of our results, we propose electron localization induced by s-f (or d-f) exchange interaction prior to long range magnetic order as a mechanism for the electrical resistance minimum.

cond-mat.str-el↗

arXiv1998

Transport and magnetic anomalies due to A-site ionic size mismatch in La$_{0.5}$Ca$_{0.5-x}$Ba_{x}MnO$_3$

We present results of electrical resistivity, magnetoresistance and ac and dc magnetic susceptibility on polycrystalline samples of the type La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment conditions. The substitution of larger Ba ions for Ca results in a non- monotonic variation of the curie temperature as the system evolves from a charge ordered insulating state for x=0 to a ferromagnetic metallic state for x=0.5. An intermediate compositino, x=0.1, interestingly exhibits ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit semiconducting like behavior as the temperature is lowered below 300K, with a broad peak in rho around 80-100K: These compositions exhibit a weak increase in rho as the temperature lowered below 30K, indicative of electron localization effects. These compositions also undergo ferromagnetic transitions below about 200 and 235K respectively, though these are non-hysteretic; above all, for these compositions, MR is large and conveniently measurable over the entire tempera- ture range of measurement below Tc. This experimental finding may be of interest from the application point of view. We infer that the A-site ionic-size mismatch plays a crucial role in the deciding these properties.

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