arXiv subjects

K. G. Suresh

Publications and source records attributed to K. G. Suresh.

At least 55 records · Page 3Linked to original sources

arXiv2013

Observation of large positive magnetoresistance and its sign reversal in GdRhGe

Magnetic properties, heat capacity and magnetoresistance (MR) of polycrystalline GdRhGe are investigated. It shows two antiferromagnetic transitions, one at T1=31.8 K and the other at T2=24 K, and field induced metamagnetic transition over a wide temperature range. The ac susceptibility data reveal that the transition at 24 K is not simple antiferromagnetic. Dominant contributions to the heat capacity and the resistivity have been identified. MR is found to show sign reversal just below T1 and attains a large positive value of 48% at 2 K for 50 kOe. Like MR, the isothermal magnetic entropy change also undergoes a sign reversal as the temperature is varied, indicating a change of the magnetic structure and the moment amplitude in determining these properties.

cond-mat.str-el↗

arXiv2013

Observation of giant magnetocaloric effect in HoCoSi

We report the magnetic and magnetocaloric properties of HoCoSi, which shows second order ferro-para magnetic transition at 14 K. It is found to exhibit a large magnetic entropy change of 13 J/kg K for a field change of 20 kOe, which increases to 20.5 J/kg K at 50 kOe. The refrigerant capacity is found to be 135 and 410 J/kg for 20 and 50 kOe, respectively. The large, low-field adiabatic temperature change (~ 3 K) and a large refrigerant capacity make this compound a suitable candidate for magnetic refrigeration applications at around 15 K.

cond-mat.str-el↗

arXiv2013

Effect of annealing on the magnetic, magnetocaloric and magnetoresistance properties of Ni-Co-Mn-Sb melt spun ribbons

The structural, magnetic, magnetocaloric and magnetotransport properties of Ni46Co4Mn38Sb12 melt spun ribbons have been systematically investigated. The partially ordered B2 phase of the as-spun ribbon transforms to fully ordered L21 phase upon annealing, which signifies a considerable change of the atomic ordering in the system. The presence of atomic disorder in the as-spun ribbon gives rise to a higher martensitic transition temperature and a lower magnetization as compared to the bulk sample. However, annealing the ribbons helps in regaining the bulk properties to a large extent. Significant changes in magnetocaloric effect, exchange bias and magnetoresistance have been observed between the as-spun and the annealed ribbons, indicating the role of atomic ordering on the functional as well as fundamental properties in the Heusler system. Importantly, the study shows that one can reduce the hysteresis loss by preparing melt spun alloys and subjecting them to appropriate annealing conditions, which enable them to become practical magnetic refrigerants.

cond-mat.mtrl-sci↗

arXiv2013

In-plane and out of plane magnetic properties in Ni46Co4Mn38Sb12 Heusler alloys ribbons

Magnetic, magnetocaloric and exchange bias properties have been systematically investigated in Ni46Co4Mn38Sb12 ribbon by applying magnetic field along (IP) and perpendicular (OP) to the ribbon plane. From the thermo-magnetization curves, the sharpness of the martensitic transition is observed to be nearly the same for both IP and OP ribbons. The thermomagnetic irreversibility region is found to be larger in the OP ribbon at 500 Oe, indicating that the magnetic anisotropy is larger in this case. The OP ribbon shows the Hopkinson maximum at 500 Oe, both for the FCC and ZFC modes. The magnetization curve for IP ribbon shows a faster approach to saturation, compared to the OP ribbon. Isothermal magnetic entropy change at 50 kOe has been found to be nearly same for both the ribbons. At 5 K the coercivity and exchange bias values are larger for the OP ribbon. Crystallographic texturing of the ribbons and its effect in the easy magnetization direction are found to be the reason behind the differences between the two ribbons.

cond-mat.mtrl-sci↗

arXiv2013

Change in the order of magnetic transition in HoRhGe as probed by magnetoresistance and magnetocaloric studies

Magnetoresistance and magnetocaloric properties of polycrystalline HoRhGe have been studied. This compound orders antiferromagnetically with a Neel temperature (TN) of 5.5 K and undergoes a first order metamagnetic transition at 2 K. It shows a large negative magnetoresistance of 25% at TN, for a field of 50 kOe. However, at 2 K and in lower fields, the magnetoresistance is found to be positive with a magnitude of about 12%, which is attributed to the first order metamagnetic transition. The compound also shows large magnetocaloric effect near its Neel temperature. Field dependence of magnetic entropy change also reflects the change in the nature of magnetic transition as the field is increased. The value of magnetic entropy change (-ΔSM) is found to be 11.1 J/kg K for a field change of 50 kOe. Field dependence of magnetoresistance and magnetocaloric effect clearly shows the change in the order of the metamagnetic transition with increase in field.

cond-mat.str-el↗

arXiv2013

Magnetic, magnetocaloric and transport properties of HoRuSi compound

Magnetic, thermal, magnetocaloric and transport properties of polycrystalline HoRuSi have been studied. It was found that the compound shows magnetic transitions at T1=18 K and T2=8 K. Magnetization data reveal that the ferromagnetic ordering is dominant in this compound. Magnetocaloric effect has been estimated from M-H-T data and the -ΔSM has been found to be 12.8 J/kg K for field of 50 kOe, which is comparable to some potential refrigerant materials in same temperature regime.

cond-mat.str-el↗

arXiv2012

Anomalous magnetic, transport and thermal properties of Gd5Ge3

We have studied the magnetic, thermal and magnetotransport properties of polycrystalline Gd5Ge3. It is found that the compound is a strong antiferromagnet and does not undergo any strong metamagnetic transition even in a field of 90 kOe. However, a small but visible ferromagnetic component is found to coexist with the antiferromagnetic order at low temperatures, as revealed by the anomalies in magnetization, magnetoresistance and heat capacity data. Our data suggest that the ferromagnetic component is of magnetostructural in origin, as evidenced by the strong field induced irreversibility of the magnetoresistance and heat capacity isotherms. The first order like magnetostructural distortion established in this material is found to result in a martensitic like scenario wherein the kinetic arrest controls the field/temperature dependence of physical properties such as heat capacity and magnetoresistance.

cond-mat.str-el↗

arXiv2012

Structural, magnetic, magnetocaloric and magneto-transport properties in Ge doped Ni-Mn-Sb Heusler Alloys

The effect of Ge substitution on the magnetic, magnetocaloric and transport properties of Ni45Co5Mn38Sb12-xGex (x=0-3) has been investigated. The decrease in the exchange interaction brought by Ge substitution can be seen from the reduction in the magnetization of austenite phase and the increase in the martensitic transition temperature. Large magnetocaloric effect and magnetoresistance have been observed at room temperature, making it a potential material system for various applications.

cond-mat.str-el↗

arXiv2012

Anomalous magnetoresistance and magnetocaloric properties of NdRu2Ge2

It is found that the polycrystalline NdRu2Ge2 undergoes two successive magnetic transitions at Tt=10 K and TN=19 K. Evidence of metamagnetic transition is detected in the magnetization isotherm data in the antiferromagnetic regime. Temperature dependence of magnetoresistance (MR) show that the relative magnitudes of MR at TN and Tt change considerably as the field is increased from 10 kOe to 30 kOe. Moreover, the MR is found to be positive below 9 K for 30 kOe field although the material is ferromagnetic at these temperatures. The highest value of negative MR near TN is about 42% in a field of 30 kOe, while the positive MR is about 35 % at 3 K in a field of 50 kOe. Like MR, the magnetocaloric effect at TN and Tt also shows anomalous behavior. The relative magnitudes of MCE at these temperatures are found to change with increase in field. It appears that the high field (>10 kOe) magnetic state below TN is complex, giving rise to some antiferromagnetic-like fluctuations, affecting the MR and MCE behavior.

cond-mat.str-el↗

arXiv2012

Magnetic, magnetocaloric and magnetotransport properties of RSn_{1+x}Ge_{1-x} compounds (R=Gd, Tb, Er; x=0.1)

We have studied the magnetic, magnetocaloric and magnetotransport properties of RSn1+xGe1-x(R=Gd, Tb, Er; x=0.1) series by means of magnetization, heat capacity and resistivity measurements. It has been found that all the compounds crystallize in the orthorhombic crystal structure described by the centrosymmetric space group Cmcm (No. 63). The magnetic susceptibility and heat capacity data suggest that all the compounds are antiferromagnetic. Large negative values of θp in case of GdSn1.1Ge0.9 and TbSn1.1Ge0.9 indicate that strong antiferromagnetic interactions are involved, which is also reflected in the magnetization isotherms. On the other hand ErSn1.1Ge0.9 shows weak antiferromagnetic interaction. The heat capacity data have been analyzed by fitting the temperature dependence and the values of θD and γ have been estimated. Among these three compounds, ErSn1.1Ge0.9 shows considerable magnetic entropy change of 9.5 J/kg K and an adiabatic temperature change of 3.2 K for a field of 50 kOe. The resistivity data in different temperature regimes have been analyzed and the dominant contributions have been identified. All the compounds show small but positive magnetoresistance.

cond-mat.str-el↗

arXiv2011

Effect of Fe substitution on the magnetic, transport, thermal and magnetocaloric properties in Ni50Mn38-xFexSb12 Heusler alloys

The structural, magnetic, transport, thermal and magnetothermal properties of quaternary Heusler alloys Ni50Mn38-xFexSb12 have been studied. Powder x-ray diffraction and temperature dependence of magnetization studies reveal that with addition of Fe in Mn site, the martensitic transition shifts to lower temperatures. It is also found that the martensitic transition becomes broader for the higher Fe concentrations. The metamagnetic transition in M(H) isotherms becomes very prominent in x=2 and vanishes for x=3 and 4. A maximum positive magnetic entropy change of 14.2 J/kg K is observed for x=2 at 288 K for 50 kOe. Resistivity shows an abrupt decrease across the martensitic transition in all the alloys, except x=6, which does not have the martensitic transition. Maximum negative magnetoresistance of 21% has been obtained for x=2 at 50 kOe. The same alloy also shows an exchange bias field of 288 Oe.

cond-mat.str-el↗

arXiv2010

Effect of Si and Ga substitutions on the magnetocaloric properties of NiCoMnSb quaternary Heusler alloys

The effect of Si and Ga substitutions on the magnetic and the magnetocaloric properties in Heusler based system Ni46Co4Mn38Sb12-xZx (Z=Si and Ga) has been studied. From the M(T) plots it is found that Si substitution stabilizes the austenite phase, whereas, Ga substitution stabilizes the martensite phase. Strong metamagnetic behaviour is observed in the M(H) isotherms for Si=0.75 and 1, whereas, such a behaviour is absent in the Ga substituted alloys. Associated with magneto-structural transition, large MCE of 58 J/kg K and 70 J/kg K is observed for x=0.75 and 1, respectively in the case of Si. Though the MCE observed in x=0.5 and 1 in the case of Ga is much lower, the MCE peak is found to be quite broad.

cond-mat.str-el↗

arXiv2010

Magnetic, magnetocaloric and neutron diffraction studies on TbNi5-xMx (M = Co and Fe) compounds

The effect of substitution of Co and Fe for Ni in TbNi5 on the structural, magnetic and magneto-thermal properties has been investigated. Considerable enhancement of Curie temperature is observed with Fe substitution, whereas the increase is nominal in the case of Co. Neutron diffraction measurements reveal the redistribution of moments and site preference of substitutional ions in Ni 2c and 3g sites. In TbNi4Fe, both Ni and Fe as well as Tb are found to carry moment while in the case of TbNi4Co, mainly Tb carries the moment. Magnetocaloric behavior has been investigated from the magnetization and the heat capacity measurements. The magnetic and magnetocaloric properties are found to be strongly correlated in these compounds.

cond-mat.str-el↗

arXiv2010

Observation of spontaneous magnetization jump and field-induced irreversibility in Nd5Ge3

We report the observation of spontaneous and ultra-sharp jumps in the low temperature magnetization isotherms of polycrystalline Nd5Ge3. Field-induced and ultra-sharp jumps are also seen in resistivity and heat capacity data. These jumps are accompanied by field-induced irreversibilities. The consistency seen in these three data clearly shows that the spin, electronic and lattice states are strongly coupled. Time-induced growth of the ferromagnetic phase is observed at a constant field and temperature, implying the metastability of the magnetic phase in the low field region. Various experimental findings point towards a strong field-induced magneto-structural irreversibility in this compound.

cond-mat.str-el↗

arXiv2010

Observation of re-entrant spin glass behavior in (Ce1-xErx)Fe2 compounds

Clear experimental evidence of re-entrant spin glass state has been revealed in Er doped CeFe2 compounds. The zero field cooled - field cooled bifurcation in dc magnetization, frequency dependence of freezing temperature, relaxation in zero field cooled magnetization and presence of large remanence confirm the spin glass state in these compounds. Frequency dependence is found to follow the critical slowing down mechanism. The random substitution of Er and the change in the valence state of Ce along with an enhancement of the ferromagnetic component in the Fe sublattice seem to be responsible for the spin glass state. Using detailed experimental protocols, we also prove that the low temperature state in these compounds is not a magnetic glass. The absence of exchange bias gives an indication that there is no coexistence of ferromagnetism and spin glass state in these compounds. The RSG state is found to be associated with the randomly magnetized clusters instead of atomic level randomness.

cond-mat.str-el↗

arXiv2010

Pressure induced magnetic and magnetocaloric properties in NiCoMnSb Heusler alloy

The effect of pressure on the magnetic and the magnetocaloric properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant magnetocaloric effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications.

cond-mat.str-el↗

arXiv2010

Irreversibility of field-induced magnetostructural transition in NiCoMnSb shape memory alloy revealed by magnetization, transport and heat capacity studies

The effects of magnetic field on the martensitic transition have been studied in Ni45Co5Mn38Sb12. We find a large field-induced irreversibility in this system, as revealed by the field dependence of resistivity, magnetization, and heat capacity data. At the critical temperature, the field-induced conversion of the martensitic to austenite phase is not reversible under any field variation. At this temperature any energy fluctuation induces nucleation and growth of the equilibrium austenite phase at the expense of the metastable martensitic phase and gets arrested. All these three measurements completely rule out the coexistence of austenite and martensitic phases in the irreversibility regime.

cond-mat.str-el↗

arXiv2010

Observation of enhanced exchange bias behavior in NiCoMnSb Heusler alloys

We report the observation of large exchange bias in Ni50-xCoxMn38Sb12 Heusler alloys with x=0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic and antiferromagnetic phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature ferromagnetic phase and the predominant antiferromagnetic component in the martensitic phase. The presence of exchange bias is well supported by the observation of training effect. The exchange bias field increases with Co concentration. The maximum value of 480 Oe at T=3K is observed in x=5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the antiferromagnetic coupling after Co substitution is found to be responsible for the increase in the exchange bias.

cond-mat.str-el↗