We have compared and contrasted magnetic, magnetocaloric and magnetoresistive properties of Gd and Dy members of the rare-earth (R) series RFe5Al7, crystallizing in ThMn12 structure, known to order antiferromagnetically. Among other observations, we
would like to emphasize on the following novel findings: (i) There are multiple sign-crossovers in the temperature (T) dependence of isothermal entropy change (DeltaS) in the case of Dy compound; in addition to nil DeltaS at the magnetic compensation point known for two-magnetic-sublattice systems, there is an additional sign-crossover at low temperatures, as though there is a re-entrant inverse magnetocaloric phenomenon. Corresponding sign reversals could also be observed in the magnetoresistance data. (ii) The plots of magnetoresistance versus magnetic field are found to be highly asymmetric with the reversal of the direction of magnetic-field (H) well below TN for both compounds, similar to that known for an antiferromagnetic tunnel junctions. We attribute these to subtle changes in spin orientations of R and Fe moments induced by T and H.
We have investigated the magnetic, dielectric and magnetodielectric (MDE) behavior of a geometrically frustrated spin-chain system, Ca3Co1.4Rh0.6O6, in the single crystalline form for different orientations. The results bring out that the magnetic be
havior of this compound is by itself interesting in the sense that this compound exhibits an anisotropic glassy-like magnetic behavior with a huge frequency dependence of ac susceptibility peak for an orientation along the spin-chain in the range 30-60 K; this behavior is robust to applications of large external magnetic fields (H) unlike in canonical spin-glasses. The temperature dependence of dielectric constant also shows strong frequency dependence with similar robustness to H. The isothermal H-dependent dielectric results at low temperatures establishes anisotropic MDE coupling. It is intriguing to note that there is a step roughly at one-third of saturation values as in the case of isothermal magnetization curves for same temperatures (for orientation along spin-chain), a correlation hitherto unrealized for geometrically frustrated systems.
We report that the spin-chain compound Dy2BaNiO5 recently proven to exhibit magnetoelectric coupling below its Neel temperature (T_N) of 58 K, exhibits strong frequency-dependent behavior in ac magnetic susceptibility and complex dielectric propertie
s at low temperatures (<10K), mimicking reentrant multiglass phenomenon. Such a behavior is not known among undoped compounds. A new finding in the field of multiferroics is that the characteristic magnetic feature at such low temperatures moves towards higher temperatures in the presence of a magnetic-field (H), whereas the corresponding dielectric feature shifts towards lower temperatures with H, unlike the situation near T_N. This observation indicates that the alignment of spins by external magnetic fields tends to inhibit glassy-like slow electric-dipole dynamics, at least in this system, possibly arising from peculiarities in the magnetic structure.
The polycrystalline form of the compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure, which was earlier believe to order antiferromagnetically below 69 K, has been recently reported by us to exhibit interesting magnetoresistance (MR) an
omalies. In order to understand the magnetic anomalies of this compound better, we synthesized single crystals of this compound and subjected them to intense magnetization and MR studies. The results reveal that the magnetic behavior is strongly anisotropic as the easy axis is along a basal plane. There appear to be multiple magnetic features in the close vicinity of 70 K. In addition, there are multiple steps in isothermal magnetization (which could not be resolved in the data for polycrystalline data) for magnetic-field (H) along a basal plane. The sign of MR is positive in the magnetically ordered state, and, interestingly, the magnitude dramatically increases at the initial step for H parallel to basal plane, but decreases at subsequent steps as though the origin of these steps are different. However, for the perpendicular orientation (H || [0 0 0 1]), there is no evidence for any step either in M(H) or in MR(H). These results establish this compound is an interesting magnetic material.
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 red
uced. 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.
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 fo
r 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 ({rho}). 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 {rho} 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.
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 {mu}m) obtained by high-energy balling. Ou
r 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.
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 investi
gations 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.
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 o
f 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 Doniachs 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
We have investigated the magnetic behavior of the nanocrystalline form of a well-known Laves phase compound, ErCo2 - the bulk form of which has been known to undergo an interesting first-order ferrimagnetic ordering near 32 K - synthesized by high-en
ergy ball-milling. It is found that, in these nanocrystallites, Co exhibits ferromagnetic order at room temperature as inferred from the magnetization data. However, the magnetic transition temperature for Er sublattice remains essentially unaffected as though the (Er)4f-Co(3d) coupling is weak on Er magnetism. The net magnetic moment as measured at high fields, sat at 120 kOe, is significantly reduced with respect to that for the bulk in the ferrimagnetically ordered state and possible reasons are outlined. We have also compared the magnetocaloric behavior for the bulk and the nano particles.
