Metamagnetism in La2CuO4.

PHYSICAL REVIEW B VOLUME 39, NUMBER 7 Metamagnetism MARCH 1989 in LazCuO4 S-W. Cheong, J. D. Thompson, and Z. Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 19 September 1988) A careful study of the metamagnetic transition in single-crystalline La2Cu04 is presented. From magnetic susceptibility measurements the critical exponent (P) of the weak ferromagnetic state, which terminates at a triple point, has been estimated to be P=0. 5+0. 02, a value consistent with mean-field theory. Furthermore, the pressure dependence of the critical magnetic field for the tran- sition has been examined from magnetoresistance measurements under hydrostatic pressure. The magnetic properties of LazCu04 have been exam- ined in detail, ' particularly in an attempt to under- between magnetism stand any possible interrelationship in La2Cu04- found in La2Cu04 and superconductivity based compounds. An anomaly in the susceptibility' of La2Cu04 suggested the presence of an antiferromagnetic phase trarisition, which has been confirmed by neutron- scattering experiments that find three-dimensional Bragg peaks with unit-cell coupling. The Neel temperature sensitive to the oxygen content. ' ( T~ ) is extremely have also established Neutron-scattering experiments the existence of strong two-dimensional (2D) magnetic correlations even far above T&, in addition, two-magnon Raman scattering experiments show the presence of strong, magnetic intralayer coupling. Furthermore, a field-induced transition, which occurs when a magnetic field is applied perpendicular to the CuO planes at a tern- perature below T&, has been reported. The origin of this metamagnetic, field-induced transition is from the cant- ing of Cu spins out of the CuO planes due to the rotation- al distortion of elongated octahedra of oxygen atoms around the divalent Cu ions. The susceptibility peak or- around T& has been attributed to antiferromagnetic in- dering in the presence of the Dzyaloshinski-Moriya teraction which is allowed by the distorted coordination of oxygen atoms. Since the critical behavior around the Neel tempera- ture has not been explored in detail, and the driving force producing the Neel state has not been established well, we have performed careful measurements of the magnetic around Tz on a crystal susceptibility (g)/magnetization of La2Cu04 which exhibits a very sharp susceptibility peak, indicative of a well-ordered homogeneous sample. Large crystals of LazCu04 (as large as 3 X 3 XO. 3 cm ) were grown from a CuO flux. After quenching from high temperatures, crystals were removed from the CuO flux and annealed in an appropriate gas atmosphere according to the oxygen content desired. We found that proper an- nealing is necessary to ensure a sharp magnetic transition and, furthermore, reduces the temperature-independent in y, as well as supp ress es the low- background temperature Curie-tail frequently observed in less well- ordered samples. We observed the peak temperature ( Tz ) in y as high as 326 K in crystals annealed in a nitro- gen atmosphere. The distinction between T& and T will be discussed herein. The data shown here are representa- tive of crystals annealed in air with T -257 K. Suscepti- bility and magnetization were measured with a Quantum superconducting-quantum-interference-device Design susceptometer capable of magnetic fields to 5 T. The temperature-dependent magnetic susceptibility of a La2Cu04 crystal, measured in 0. 2-T magnetic field ap- plied in the CuO-plane direction (yl ) and in the perpen- dicular direction (yi), is displayed in the left panel of Fig. 1. Even though divalent Cu ions are good Heisenberg ions, because of large spin-orbit coupling and the fact that the ground state is a Kramer's doublet, there is ap- preciable susceptibility anisotropy. gz shows a sharper peak, with T =257+0. 5 K, than y~~ and the full width at half maximum of the peak is — 13 K, indicating good homogeneity of oxygen distribution in the sample. The right panel in Fig. 1 exhibits the magnetic field depen- dence of yi (0. 2 T data are the same as shown in the left panel). A clear difference in the two susceptibility curves is evident. We note that in the case of y~~ there is no field dependence except for a slight depression of T . In Fig. 2, we plot isothermal magnetization curves at various temperatures with the applied field perpendicular the field- to the CuO planes. As reported earlier, induced transition, indicated by the deviation from linear shifts to lower-field M(H) behavior found for T Above T„, M(H) values with increasing temperature. curves are linear with a slight tendency toward saturation at high-field values. However, it is not clear, from data like these, precisely at what temperature the jump in M(H) curves disappears. The inset will be discussed later. The critical fields (H, ), defined from the maximum of ~dM/dH~, as a function of temperature are plotted in the left panel of Fig. 3. The right panel displays the tempera- ture dependence of the jump (M, ) in M (H) curves at the critical field values of M, were determined by the difference at H, between smooth extrapolations of low- field and high-field portions of the M(H) curves. We found from careful analysis'that H, (T) does not extrapo- late smoothly to zero at T =257+0. 5 K and that M, ex- trapolates to zero at T =251.5+0. 5 K. This behavior has been observed in a number of samples that show a ( T, The American Physical Society