Sunil Labroo

Magnetic and electrical properties of R2Mo2O7 pyrochlore compounds

Abstract A combination of ac susceptibility, dc magnetization in a variety of applied fields, and electrical resistivity data is reported for five members of the R 2 Mo 2 O 7 series of cubic pyrochlore structure oxides, R = Nd, Sm, Gd, Tb, and Y. These measurements extend published studies on the compounds and aid in the clarification and interpretation of the magnetic and electrical transport properties. The metallic ferromagnetics, R = Nd, Sm, and Gd, order at 96, 80, and 57 K, respectively. For R = Nd and Sm this is due most likely to ordering of Mo(IV) moments of about 1 μ B . Near 30 K both compounds show hysteresis in the dc magnetization due to complex R Mo coupling which give rise to resistivity minimum. For Gd 2 Mo 2 O 7 there is no low temperature hysteresis, indicating that both sublattices order ferromagnetically at the same temperature. A magnetization anomaly in semiconducting Tb 2 Mo 2 O 7 at 25 K which is attributed to long range or short range magnetic order is reported for the first time and ac susceptibility data confirm a spin-glass like transition in Y 2 Mo 2 O 7 reported previously. A correlation between the cubic lattice constant and the electrical and magnetic properties is noted.

Magnetism of rare‐earth disilicides

Magnetic and electrical properties of polycrystalline RSi2 compounds, where R=La‐Yb and Y, have been investigated at temperatures ranging from 1.8 K to room temperature using superconducting quantum‐interference device (SQUID) magnetometry, ac susceptibility, resistivity, and magnetoresistance measurements. Our data indicate that while PrSi2, ErSi2, and TmSi2 order ferromagnetically at 11, 2.8, and below 1.8 K, respectively, other RSi2 compounds with R=Nd, Eu, Gd, Tb, Dy, and Ho order antiferromagnetically with TN≤39 K. All compounds show Curie–Weiss behavior in the paramagnetic region and the effective moments are found to be close to the free‐ion values, gJ [J(J+1)]1/2. Another interesting feature is the observation of second transitions in the cases of Pr, Nd, Eu, Gd, Tb, and Dy compounds. SmSi2, CeSi2, and YbSi2 do not show any magnetic order down to 1.8 K. In the ordered state, all antiferromagnetic compounds have a T3 dependence of resistivity and show a positive magnetoresistance ranging from a few...

Magnetic properties of RMn2 compounds (R=heavy rare earths)

Magnetic and electrical properties of RMn2 compounds, where R=Gd–Tm, have been investigated via SQUID magnetometry, ac‐susceptibility, thermal‐expansion, electrical‐resistivity, and magnetoresistance experiments from 4.2 K to room temperature. All samples with R=Gd, Tb, Dy, Ho, Er, and Tm are found to order ferromagnetically at Tc =109, 54, 35, 24, 15, and 8 K, respectively. The paramagnetic‐to‐ferromagnetic phase transitions in case of Gd and Tb compounds are of first order, shown as a sharp volume change in the thermal‐expansion curve, but the transitions in the cases of other compounds are second order in nature. The first‐order transitions are similar in nature to that in the case of YMn2, where the transition is believed to be associated with the collapse of the Mn moment. The transition temperatures in RMn2, where R=Gd–Tm, are in fairly good agreement with the prediction by the de Gennes rule. In GdMn2 and TbMn2, the Mn moment is found to be finite and it appears that Mn and Gd or Tb sublattices ord...

Observation of valence fluctuation phenomena in YbSi2

Abstract Electrical and magnetic properties of YbSi2 have been investigated experimentally by electrical resistivity and a.c. susceptibility measurements. The resistivity data may be summarized as following: 1. (i) at high temperatures the resistivity shows a tendency to saturate; 2. (ii) there is a negative curvature above T ≅ 65 K ( d 2 ρ dT 2 and ρ exhibits an inflection point at 65 K; 3. (iii) below 37 K the resistivity shows a T2 dependence. The a.c. susceptibility shows a broad peak at about 90 K. The low temperature T2 resistivity and the broad peak in a.c. susceptibility suggest that YbSi2 is a valence fluctuating system. The results will be discussed in terms of valence fluctuating phenomena. Measurements of the magnetic moment in YbSi2 will be carried out in a SQUID magnetometer.

Effect of Fe substitution in YBaCuO superconductors

Abstract We have studied the electrical resistivity and ac susceptibility of iron substituted YBaCuO superconductors. The superconducting transition temperature T c of YBa 2 (Cu 1- x Fe x ) 3 O 7-δ decreases linearly with increasing x , while the width of the superconducting transition (Δ T c ) increases linearly with x . Normal state resistivity remains linear (metallic) in T for x = 0.01 but for x ⩾ 0.05 the normal state resistivity exhibits a semiconducting behavior. For x = 0.20 the material remains non-superconducting down to 4.2 K. The semiconducting behavior for samples with x ⩾ 0.05 suggests it to be the property of the tetragonal structure, since it has been established in the literature that YBa 2 (Cu 1- x Fe x ) 3 O 7-δ has the tetragonal phase for x ⩾ 0.03.

Localized magnetism in YMn2 below the Néel temperature

We have measured magnetic susceptibility, thermal expansion, electrical resistivity, and magnetoresistance of the intermetallic compound YMn2. YMn2 orders antiferromagnetically belowTN≃110 K. There is a large thermal hysteresis such that the ordering temperature isTN=86 K on cooling andTN=116 K on warming. This is a first-order phase transition from the paramagnetic to antiferromagnetic phase. Our electrical resistivity and magnetoresistance measurements in the ordered state show a localized magnetic moment for Mn atoms in YMn2 belowTN. The Mn moments collapse atTN which is reflected in the sharp decrease in the thermal expansivity atTN. AboveTN the Mn moment gradually recovers with increasing temperature which is reflected as an increase in χ with increasing temperature in the paramagnetic state. This behavior atT>TN could be described by weak itinerant electron magnetism.

Influence of Mn moments on the properties of RMn2 compounds (R=Y and light rare earths)

The influence of Mn moments on the magnetism of RMn2 compounds, where R=Y, Pr, Nd, and Sm, has been investigated using SQUID magnetometry, thermal expansion, electrical resistivity, and magnetoresistance measurements. RMn2 compounds, with R=Y, Pr, Nd, and Sm, order antiferromagnetically at 116, 115, 105, and 94 K, respectively. According to thermal expansion data all these materials undergo a first‐order phase transition from a paramagnetic to a magnetically ordered state, and this effect is believed to be caused by the collapse of the Mn moment at the transition. For the YMn2 compound, the resistivity has a T3 temperature dependence below 38 K and a positive magnetoresistance in the ordered phase, indicating that the Mn moments are localized below TN in this compound. Above TN, however, the Mn moments become itinerant which is reflected as a gradual increase in susceptibility χ with increasing temperature. In PrMn2, NdMn2, and SmMn2 the resistivity does not follow a simple temperature power law and the m...

Effect of fe and Ni substitution in high-Tc YBaCuO superconductors☆

Abstract Effects of iron and nickel substitutions in YBa 2 (Cu 1 − x M x ) 3 O 7 − δ (when M  Fe and Ni ) have been investigated through electrical resistivity and a.c. susceptibility measurements. The superconducting transition temperature ( T c ) decreases linearly with increasing iron and nickel concentrations, while the width of the transition ( ΔT c ) increases with iron and nickel concentrations. The normal state resistivity remains metallic for x = 0.01 but for x ⩾ 0.05 the normal state resistivity exhibits a semiconducting behavior. The semiconducting behavior for samples with x ⩾ 0.05 suggests it to be the property of the tetragonal structure, since it has been recently established, that YBa 2 (Cu 1 − x M x ) 3 O 7 − δ has a tetragonal phase for x ⩾ 0.03 for M  Fe and Ni . The data do not indicate a clear role for the effect of the magnetism of iron and nickel ions in depressing T c .

Electrical and magnetic properties of R2Mo2O7 (R Nd, Sm, Gd, Tb, Y)

Abstract Electrical and magnetic properties of rare earth molybdenum pyrochlores (R 2 Mo 2 O 7 , R  Nd , Sm , Gd , Tb and Y ) have been investigated using electrical resistivity and a.c. susceptibility measurements. For R  Nd , Sm , Gd and Tb , the magnetic ordering is observed at 96 K, 80 K, 57 K and 28 K respectively. The a.c. susceptibility data for R  Nd , Sm and Gd exhibit some very unusual behavior in comparison with earlier d.c. susceptibility results. A Kondo-type minimum is observed in the resistivity of the pyrochlores for R  Nd , Sm and Gd at 25 K , 20 K and 34 K respectively. The Kondo behavior is quite consistent with the a.c. susceptibility data. We have clearly observed a magnetic ordering at 28 K for Tb 2 Mo 2 O 7 . The resistivity data for Y 2 Mo 2 O 7 and Tb 2 Mo 2 O 7 show a semiconducting behavior as seen by earlier authors.