Peggy Hill

Neutron diffraction study of the magnetic structures of CeMn2Ge2 and CeMn2Si2

We have performed high‐resolution neutron powder diffraction measurements on CeMn2Si2 and CeMn2Ge2 at temperatures between 12 and 550 K. Our measurements indicate that at high temperatures both compounds are paramagnetic. Below TN=380 K CeMn2Si2 becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is τ=(0 0 1). CeMn2Ge2, on the other hand, exhibits two different magnetic transitions. At TN≊415 K there is a transition to a collinear AF phase characterized by the commensurate propagation wave vector τ=(1 0 1). At TC≊318 K there is a transition to a conical structure with a ferromagnetic component along the c axis and a helical component in the ab plane. The helical component is characterized by the incommensurate propagation vector τ=(1 0 1−qz), where qz is temperature dependent.

Magnetic and magnetocaloric properties of the new rare-earth–transition-metal intermetallic compound Gd3Co29Ge4B10

The compounds Gd3-xYxCo29Ge4B10 (x = 0, 0.5, 1.0, 1.5, and 3.0), Gd3Co29Al4B10, and Gd3Co29Al4B10 were synthesized by arc melting, and their magnetic properties investigated as a function of temperature and applied magnetic field. X-ray measurements showed primarily single-phase samples with the tetragonal crystal structure P4/nmm. It was found that Gd3Co29Ge4B10 orders ferromagnetically at TC = 212 K and shows a compensation point at 128 K, indicating a ferrimagnetic ordering of the Co and Gd moments. An entropy change of −ΔS = 0.5 J/kgK was observed in a 5-T field at TC for this sample, while a change in sign for this quantity was observed both at the maximum value of magnetization (around 200 K) and then again at the compensation point. Substitution of Y for Gd in Gd3Co29Ge4B10 does not affect the Curie temperature, but shifts the compensation point to lower temperatures. This indicates that a decrease in Gd concentration does not affect the d-d exchange interaction, but has a pronounced effect on the ...

Local moments in the paramagnetic phase of dilute CrV alloys

The magnetic susceptibility of Cr+0.2 at.% V and Cr+0.4 at.% V alloys (nominal concentrations) is found to show a sharp increase at the Neel temperature with increasing temperature and to obey a Curie-Weiss law in the paramagnetic phase up to 400 K, corresponding to a moment per V atom of about one Bohr magneton. The relation of these results to the behaviour of other physical properties of dilute CrV alloys is discussed.

Magnetic properties of LaCr1−xMxSb3 (M=V, Mn, Fe, Cu, and Al)

The influence of Cr substitution by various metals (M=V, Mn, Fe, Cu and Al) on the magnetic state of the itinerant intermetallics La(Cr,M)Sb3 was studied by magnetization and magnetic susceptibility measurements up to 55 kG at 5 K and from 4.2 to 400 K, in a magnetic field of 1000 G, respectively. It was found that the Curie temperature (TC) and magnetization (M) of these compounds depend nonlinearly on the concentration, remaining in the vicinity of the values of TC and M measured for LaCrSb3. Curie temperatures and magnetization values at 55 kG are suppressed by Mn, Fe, V, and Cu, and have a slight maximum at low Al concentration (about 5%).

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.

Investigation of the transition from ferromagnetic to antiferromagnetic order in the CeMn2(GexSi1−x)2 system

Samples in the alloy system CeMn2(GexSi1−x)2 exhibit a crossover from antiferromagnetic ordering for CeMn2Si2 to ferromagnetic ordering for CeMn2Ge2 as a function of concentration. The low temperature behavior of samples in the range 0.0≤x≤1.0 using the techniques of dc magnetization, ac susceptibility, and thermal expansion were studied. From this we have determined the magnetic phase diagram which shows four concentration regions of qualitatively different low temperature behavior. Increasing substitution of Ge for Si in CeMn2Si2 shows a depression of TN and the gradual development of ferromagnetic order at T≊305 K. For 0.42≤x≤0.60 there is a clear ferromagnetic transition at T≊305 K followed by a sharp drop in magnetization at lower temperatures. For all samples ac susceptibility shows a sharp peak near 300 K indicating the onset of magnetic order at that temperature. Neither ac susceptibility nor thermal expansion measurements show clear anomalies at TN below Tc. There is evidence of mixed magnetic be...

Investigation of the magnetic-non-magnetic crossover region in the Kondo lattice system CeSix

Measurements of resistivity, susceptibility and magnetization for polycrystalline samples of the alloys CeSix (1.6 or=1.90 do not. This correlates well with previous studies of these samples. However, samples with x=1.70 and 1.75 show the presence of an anomalous second peak at a slightly higher temperature than the ferromagnetic ordering temperature. CeSix with x=1.90 shows interesting resistive behaviour with characteristics of both magnetic and non-magnetic samples. It exhibits a broad low temperature peak, a Kondo minimum at higher temperatures and the onset of Fermi liquid behaviour below the maximum. The magnitude of the room temperature resistivity is substantially larger than that of the other members of the series, indicating that this sample is very close to the critical concentration for the magnetic-non-magnetic transition in this series. High temperature anomalies are seen in the resistivities of samples with x<or=1.90; nonetheless, no magnetic correlation is observed. Negative magnetoresistance is observed for samples CeSi1.70 and CeSi1.90 over all fields and temperatures investigated.

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 .

Magnetization and thermoremanent magnetization of Tb2Mo2O7 and Y2Mo2O7 spin glasses

Abstract Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 are crystalline oxides having the pyrochlore structure. Magnetic susceptibility measurements have shown a magnetic transition which is similar to a spin glass below 27 and 22 K for Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 respectively. Experimental measurements of the low field susceptibility, the magnetic hysteresis loop, the thermoremanent magnetization (TRM) and the time decay of the remanent magnetization have been carried out. A shift from the origin in the magnetic hysteresis loop, the disappearance of TRM at the transition temperature and a log t decay of the remanent magnetization have been observed. All these experimental observations strongly suggest a spin glass behavior for Tb 2 Mo 2 O 7 and Y 2 Mo 2 O 7 below 27 and 22 K respectively.