Antiferromagnetic TN Research Articles

Synchrotron x-ray scattering of magnetic and electronic structure of UN and U2N3 epitaxial films

We examine the magnetic ordering of UN and of a closely related nitride, U2N3, by preparing thin epitaxial films and using synchrotron x-ray techniques. The magnetic configuration and subsequent coupling to the lattice are key features of the electronic structure. The well-known antiferromagnetic (AF) ordering of UN is confirmed, but the expected accompanying distortion at Tn is not observed. Instead, we propose that the strong magneto-elastic interaction at low temperature involves changes in the strain of the material. These strains vary as a function of the sample form. As a consequence, the accepted AF configuration of UN may be incorrect. In the case of cubic a-U2N3, no single crystals have been previously prepared, and we have determined the AF ordering wave-vector. The AF Tn is close to that previously reported. In addition, resonant diffraction methods have identified an aspherical quadrupolar charge contribution in U2N3 involving the 5f electrons; the first time this has been observed in an actinide compound.

Effect of Pr–O hybridization on the anomalous magnetic properties of Pr1+xBa2−xCu3O7−y system

The Pr L3-edge x-ray absorption near edge structure spectra and antiferromagnetic TN(Pr) data for the parent system PrBa2Cu3O7−y (0⩽y<1) and the oxygenated system Pr1+xBa2−xCu3O7−y (0⩽x⩽1; −0.27⩽y⩽0.05) are reported. The formal Pr valence in the PrO8 cage decreases from 3.18 for PrBa2Cu3O6.95 [TN(Pr)=18.5 K], to 3.12 for Pr1.2Ba1.8Cu3O6.98 (15.5 K), 3.03 for Pr1.6Ba1.4Cu3O7.16 (8 K), and 3.00 for Pr2BaCu3O7.27 [TN(Pr) not observed]. The Pr valence depends both on the number of holes available in the PrO8 cage and the Pr 4f-O 2pπ hybridization strength. On the other hand, anomalous TN(Pr) is a direct result of exchange interaction though Pr–O–Pr hybridization. However, the relative contribution from PrIV–PrIV superexchange via strong Pr–O orbital hybridization and PrIII–PrIII/PrIII–PrIV superexchange paths is unclear.

Proton Resonance and Critical Relaxation in NH4MnF3 and NH4ZnF3

Proton NMR line widths and relaxation have been studied in powders of almost simple cubic NH4ZnF3 and NH4MnF3. The activation energy for NH4+ reorientation is Ea/kB = 650K above Tc = 115 K in NH4ZnF3 and Ea/kB = 795 K below Tc, and probably about the same in NH4MnF3. The averaging of local fields because of rapid reorientation narrows the line and permits observation of critical relaxation near the antiferromagnetic TN = 75.8 K in NH4MnF3. If T1 ∝ (T - TN)ρ we find ρ in the range 0.1 to 0.3, compared to the expected ρ ≈ 0.33 for cubic Heisenberg antiferromagnets.

Abstract: Magnetic susceptibility and electrical resistivity of Gd(Cu1−xGax) pseudo binary alloys

We have measured at low temperatures the ac differential susceptibility and the electrical resistivity for a series of Gd(Cu1−xGax) pseudo binary alloys for x between 0.10 and 0.22. The Gd(CuGa) system is particularly interesting in this concentration region since high temperature susceptibility measurements have shown a zero paramagnetic ϑ for x?0.15. The intermetallic compound GdCu is antiferromagnetic TN?140 K while GdGa is ferromagnetic Tc?210 K. It was hoped by randomly mixing the Cu and Ga in the x region where Tc and TN→0, a spin glass-like state could be formed. The susceptibility exhibits a broad maximum around 38 K for all alloys which is relatively concentration independent. At very low temperatures (1–10 K) a shoulder develops in the susceptibility which does vary with x. The resistivity shows an x dependent minimum at high temperatures 80–120 K, followed by a lower temperature maximum at 45–80 K. These behaviours indicate that a simple spin glass freezing is absent and that a complicated form of long range magnetic order is present.