Magnetic behavior of single crystalline Ho2_2PdSi3_3

The magnetic behavior of single-crystal Ho$_2$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure, is investigated by magnetic susceptibility ($\chi$) and electrical resistivity ($\rho$) measurements along two directions. There is no dramatic anisotropy in the high temperature Curie-Weiss parameter or in the $\rho$ and isothermal magnetization data, though there is a noticeable anisotropy in the magnitude of $\rho$ between two perpendicular orientations. The degree of anisotropy is overall less prominent than in the Gd (which is an S-state ion!) and Tb analogues. A point of emphasis is that this compound undergoes long range magnetic ordering below 8 K as in the case of analogous Gd and Dy compounds. Considering this fact for these compounds with well-localised f-orbital, the spin glass freezing noted for isomorphous U compounds in the recent literature could be attributed to the role of the f-ligand hybridization, rather than just Pd-Si disorder.Comment: Physical Review B, in pres

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.Comment: Accepted for publication in Solid State Communication

Probing core and shell contributions to exchange bias in Co/Co3O4 nanoparticles of controlled size

Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with a total average diameter around ∼27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co3O4 crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias HE and coercive HC fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both phases. The EB effects persist up to temperatures above the ordering temperature of the oxide shell. Results of our atomistic Monte Carlo simulations of particles with the same size and composition as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal, giving rise to the observed maximum in HE and HC

Self organized criticality of magnetic avalanches in disordered ferrimagnetic material

We observe multiple step-like jumps in a Dy-Fe-Ga-based ferrimagnetic alloy in its magnetic hysteresis curve at 2 K. The observed jumps have a stochastic character with respect to their magnitude and the critical field of occurrence, and the jumps do not show any temporal effect. The jump size distribution follows a power law variation indicating the scale invariance nature of the jumps. We have invoked a simple two-dimensional random bond Ising-type spin system to model the dynamics. Our computational work can qualitatively reproduce the jumps and their scale invariant character. It also elucidates that the flipping of antiferromagnetically coupled Dy and Fe clusters is responsible for the observed discrete avalanche-like features in the hysteresis loop. These characteristics indicate that the present phenomenon can be well described within the realm of self-organized criticalityComment: 12 pages, 13 figure

Magnetocaloric effect in the low hysteresis Ni-Mn-In metamagnetic shape-memory Heusler alloy

"We have studied magnetocaloric properties of a Ni-Mn-In metamagnetic shape-memory alloy especially designed in order to display low thermal hysteresis. Magnetization and calorimetric measurements under a magnetic field have been used in order to determine isothermal magnetic field-induced entropy changes. Results obtained indirectly from magnetization data, quasi-directly from isofield calorimetric measurements, and directly from isothermal calorimetric runs are systematic and agree well with each other. We have analyzed the reproducibility of magnetocaloric properties with cycling from direct isothermal calorimetric measurements. Due to low thermal hysteresis, we have found that about 80% of the transition entropy change, ΔSt ≃ 25 J/kg K, can be reversibly induced under successive application and removal of a field of 6 T.

Magnetoresistance in ordered and disordered double perovskite oxide, Sr2_2FeMoO6_6

We have prepared crystallographically ordered and disorder specimens of the double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder x-ray diffraction patterns and reconfirmed by M\"{o}ssbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the magnetoresistance at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.Comment: accepted in Solid State Communicatio

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.Comment: To appear in PRB, RevTex, 4 pages text + 6 psFigs. Related to our earlier work on Gd systems (see cond-mat/9811382, cond-mat/9811387, cond-mat/9812069, cond-mat/9812365

Giant multicaloric response of bulk Fe49Rh51

We report on the multicaloric response of the Fe49Rh51 alloy under the combined application of hydrostatic pressure and magnetic field. Experimental data are complemented by a mean field model that takes into account the interplay between structural and magnetic degrees of freedom. A large multicaloric strength has been found for this alloy, and it is shown that a suitable combination of pressure and magnetic field enables the sign of the entropy change to be reversed and thus the multicaloric effect can be tuned from conventional to inverse. It is also shown that an extended temperature window for the multicaloric effect can be achieved by taking advantage of the coupling between structure and magnetism which enables a cross response of the alloy under the application of different external fields. Mean field calculations remarkably reproduce experimental results