Showing papers on "Antiferromagnetism published in 1991"
IBM1
TL;DR: It is shown that Co slabs are indirectly exchanged coupled via thin Cu layers with a coupling that alternates back and forth between antiferromagnetic and ferromagnetic superlattices, confirming theoretical predictions more than 25 years old.
Abstract: Confirming theoretical predictions more than 25 years old, we show that Co slabs are indirectly exchange coupled via thin Cu layers with a coupling that alternates back and forth between antiferromagnetic and ferromagnetic Four oscillations are observed with a period of \ensuremath{\simeq}10 \AA{} Moreover, the antiferromagnetically coupled Co/Cu superlattices exhibit extraordinarily large saturation magnetoresistances at 300 K of more than 65%
1,141 citations
TL;DR: Threedimensional antiferromagnetic exchange of the donor and acceptor spins resulting in ferrimagnetic behavior appears to be the mode of magnetic coupling.
Abstract: The reaction of bis(benzene)vanadium with tetracyanoethylene, TCNE, affords an insoluble amorphous black solid that exhibits field-dependent magnetization and hysteresis at room temperature. The critical temperature could not be estimated as it exceeds 350 kelvin, the thermal decomposition temperature of the sample. The empirical composition of the reported material is V(TCNE)x.Y(CH(2)Cl(2)) with x approximately 2 and Y approximately 1/2. On the basis of the available magnetic and infrared data, threedimensional antiferromagnetic exchange of the donor and acceptor spins resulting in ferrimagnetic behavior appears to be the mode of magnetic coupling.
802 citations
TL;DR: In this paper, the authors reported giant values of saturation magnetoresistance in sputtered antiferromagnetic Co/Cu multilayers containing thin Co and Cu layers 8-10 A.
Abstract: We report giant values of saturation magnetoresistance in sputtered antiferromagnetic Co/Cu multilayers containing thin Co and Cu layers 8–10 A thick. We discuss the key importance of the buffer layer in controlling the growth of flat Co and Cu layers. As shown by cross‐section transmission electron microscopy high‐quality structures are found for growth on Fe buffer layers. Such structures display saturation magnetoresistance at 300 K of more than 65% with saturation fields of ≂10 kOe. These values are several times larger than previously found for any magnetic material at room temperature.
537 citations
TL;DR: Oscillations of ferromagnetic to antiferromagnetic exchange coupling between two Fe layers separated by a Cr spacer of linearly increasing thickness were investigated by imaging the magnetic domains with scanning electron microscopy with polarization analysis (SEMPA).
Abstract: Oscillations of ferromagnetic to antiferromagnetic exchange coupling between two Fe layers separated by a Cr spacer of linearly increasing thickness were investigated by imaging the magnetic domains with scanning electron microscopy with polarization analysis (SEMPA). Up to six oscillations in the coupling with a period of 10\char21{}12 Cr layers were observed, and, in the case of an extremely well ordered Cr interlayer, additional oscillations with a period of 2 Cr layers were observed.
501 citations
TL;DR: In this article, the magnetic and magneto-transport properties of Co/Cu multilayers prepared by sputtering were studied and a giant magnetoresistance was observed in the half-periods with antiferromagnetic interlayer coupling.
496 citations
TL;DR: UPd2Al3 as discussed by the authors is a new heavy-fermion superconductor with a recordTc of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atTN=14 K.
Abstract: UPd2Al3 is a new heavy-fermion superconductor with a recordTc of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atTN=14 K. Its Sommerfeld coefficient is reduced from γp=210mJ/K2 mole in the paramagnetic to γ0=150mJ/K2 mole in the antiferromagnetic phase.
348 citations
TL;DR: A general form of the mean-field theory is developed for dealing with {ital n}-component classical vector spins on any lattice and Calculations for the pyrochlore problem show that the Fourier modes of the system are completely degenerate for all wave vectors in the first Brillouin zone.
Abstract: The metal atoms in the pyrochlore system of compounds ({ital A}{sub 2}{ital B2}O{sub 7}, where {ital A} and {ital B} are metals) form an infinite three-dimensional network of corner-sharing tetrahedra with cubic symmetry. For antiferromagnetic nearest-neighbor interactions and only {ital B} atoms magnetic, there is a very high degree of frustration, and no long-range order is predicted in the absence of further neighbor interactions. A general form of the mean-field theory is developed for dealing with {ital n}-component classical vector spins on any lattice. Calculations for the pyrochlore problem show that the Fourier modes of the system are completely degenerate for all wave vectors in the first Brillouin zone. In some cases further neighbor interactions will select the {bold q}={bold 0} or incommensurate modes. A comparison is made with long-range order known to exist in the pyrochlore form of FeF{sub 3}. The highly degenerate ordered phases of more complicated systems, where both {ital A} and {ital B} atoms are magnetic, will also be discussed. A comparison is made of the corner-sharing tetrahedral lattice and the more familiar stacked triangular antiferromagnets, with regard to the degree of frustration in both systems. Results for the Kagome lattice and the square lattice withmore » crossings, which are the two-dimensional analogs of the corner-sharing tetrahedral lattice, are also briefly discussed.« less
168 citations
TL;DR: In this article, the RBiPt (R=Ce−Lu with the exception of Pm and Eu) series has been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives.
Abstract: Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small‐gap semiconductor or semimetal seen in NdBiPt to that of a heavy‐fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2 mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.
145 citations
TL;DR: In this paper, the two-dimensional Heisenberg model with a Green's function decoupling scheme was studied in the ferromagnetic and the antiferromagnetic cases and the results were compared with those of the Monte Carlo simulations and the experiments of La 2 CuO 4.
Abstract: The two dimensional Heisenberg model is studied with a Green's function decoupling scheme in the ferromagnetic and the antiferromagnetic cases. We employ Kondo and Yamaji's formulation and improve it quantitatively. The present theory does not violate the rotational symmetry and the sum rule of the correlation function. We calculate excitation spectrum, long range order, correlation functions, correlation length, energy, and spin susceptibility. The results are compared with those of the Monte Carlo simulations and the experiments of La 2 CuO 4 . Qualitative and semi-quantitative agreements are obtained. In particular, the temperature dependence of the susceptibility indicated by Monte Carlo simulations is reproduced over all temperature region.
128 citations
TL;DR: In this article, the growth and magnetic properties of films of fcc cobalt on Cu(100) substrates have been characterized by a multitechnique approach, and the films are ferromagnetically ordered in-plane at temperatures below Tc.
115 citations
TL;DR: The ground state of the spin 1/2 nearest neighbor Heisenberg quantum antiferromagnet on the Kagome lattice probably lacks spin order; therefore, conventional spinwave analysis breaks down as discussed by the authors.
Abstract: The ground state of the spin‐1/2 nearest‐neighbor Heisenberg quantum antiferromagnet on the Kagome lattice probably lacks spin order; therefore, conventional spin‐wave analysis breaks down. To ascertain the ground state, we instead use a systematic 1/n expansion with SU(n) fermions. Two distinct states occur in the large‐n limit, depending on the size of the biquadratic interaction J. When J=0, there are an infinite number of degenerate ground states consisting of disconnected dimers. At finite n, however, this degeneracy is broken by local resonance. In contrast, a globally resonating chiral spin‐liquid phase with no spin‐Peierls modulation is the likely large‐n ground state at sufficiently large J. For intermediate values of J, a phase transition from the dimer state to the chiral phase occurs as the temperature increases. At a higher temperature, there is a second transition to a paramagnetic state. We comment on the possibility that these phases are experimentally realized by the nuclear magnetic moments of a second layer of 3He atoms lying on a graphite surface.
TL;DR: In this article, the magnetic excitations of a nearly ideal realization of a one-dimensional S = 1/2 Heisenberg antiferromagnet have been studied by inelastic neutron scattering.
Abstract: The magnetic excitations of a nearly ideal realization of a one-dimensional S=1/2 Heisenberg antiferromagnet, ${\mathrm{KCuF}}_{3}$, have been studied by inelastic neutron scattering. The experimments were performed using the pulsed neutron source ISIS and show that the magnetic scattering occurs at energies well above the top of the linearized spin-wave band. This result shows that it is essential to take account of quantum effects in discussing the excitations. The results are in excellent accord with theoretical predictions for the dynamical correlation function.
TL;DR: In this paper, the exchange interaction of two Fe films across a Cr film as a function of the Cr thickness dCr was investigated by means of light scattering from spin waves and magneto-optical Kerr measurements.
Abstract: By means of light scattering from spin waves and magneto-optical Kerr measurements we investigated the exchange interaction of two Fe films across a Cr film as a function of the Cr thickness dCr. Using good-quality epitaxial samples with a special wedge geometry of the interspacer we observed up to four full periods of the long-range oscillations of the exchange, including its ferromagnetic parts. The decay of the oscillations does not follow the dCr-2-dependence predicted by classical two-dimensional RKKY-type theory. A fine structure in the first antiferromagnetic region is also observed.
TL;DR: It is shown that large quantum fluctuations induce an anisotropy in Heisenberg spiral structures, driving a biaxial-uniaxial transition to a spin nematic; this state is characterized by tensor spin order and a gapless Goldstone mode of spin-pair excitations.
Abstract: Treating antiferromagnetism as a spin superfluid, we discuss the possibility of moment-free magnetism. Specifically we show that large quantum fluctuations induce an anisotropy in Heisenberg spiral structures, driving a biaxial-uniaxial transition to a spin nematic; this state is characterized by tensor spin order and a gapless Goldstone mode of spin-pair excitations. Experimental signatures and realizations of this phenomena are suggested.
TL;DR: An interplay of the covalent bonding, scalar relativistic effects, electronegativity, and the charge transfer is shown to be responsible for a continuous transition from itinerant to local-moment magnetism.
Abstract: The electronic structure of several ordered phases of the Pt-Fe system is studied using the self-consistent linear muffin-tin-orbital method. The phases studied include nonmagnetic and ferromagnetic ${\mathrm{PtFe}}_{3}$, ferromagnetic ${\mathrm{Pt}}_{2}$${\mathrm{Fe}}_{2}$, antiferromagnetic and ferromagnetic ${\mathrm{Pt}}_{3}$Fe, and ferromagnetic ${\mathrm{Pt}}_{5}$${\mathrm{Fe}}_{3}$. The electronic structure, the densities of states, and the ground-state properties, such as equilibrium lattice constants, bulk moduli, local and average magnetic moments, and the state equations are calculated. The calculated parameters compare favorably with existing experimental data. The cohesive properties, chemical bonding, and the moment-stabilizing mechanism are discussed. A variation of all these aspects with alloy composition illustrates a continuous transition from itinerant to local-moment magnetism. An interplay of the covalent bonding, scalar relativistic effects, electronegativity, and the charge transfer is shown to be responsible for this transition. Homogeneity of the magnetic state of ${\mathrm{PtFe}}_{3}$ Invar is explained. On the basis of the calculated elastic properties and recently reported critical pressures at which the magnetic moment of ${\mathrm{PtFe}}_{3}$ collapses, the total energy difference between the ferromagnetic and nonmagnetic phase is estimated.
TL;DR: The nuclear spin-lattice relaxation time T 1 of 63 Cu in La 2- x Sr x CuO 4 was measured in the concentration range of x = 0.075∼0.15 near the magnetic phase boundary around x =0.05 when θ is extrapolated to zero as discussed by the authors.
Abstract: The nuclear spin-lattice relaxation time T 1 of 63 Cu in La 2- x Sr x CuO 4 was measured in the concentration range of x =0.075∼0.15 near the magnetic phase boundary around x =0.05. At low Sr concentration and low temperature, ( T 1 T ) -1 was found to increase markedly with a development of strong antiferromagnetic ( AF ) spin correlation among Cu spins. In normal state, ( T 1 T ) -1 exhibits a temperature dependence of C /( T +θ) associated with the Curie-Weiss law of the staggered susceptibility χ Q ( T ) at the zone boundary, Q =(π/ a , π/ a ), above 60 K and approaches a nearly constant value in a narrow T -region just above T c . T c decreases with decreasing θ, becoming zero at x =0.05 when θ is extrapolated to zero. The rapid decrease of 1/ T 1 below T c has commonly been observed irrespectively of further development of the AF correlation with decreasing Sr content.
Book•
01 Jan 1991
TL;DR: Localized magnetism associated with the ion core magnetism is associated with band electrons techniques of making magnetic measurements magnetic scattering of neutrons and magnetic excitations antiferromagnetic, ferromagnetism and non-collinear magnetic order exchange interactions in magnetism and hyperfine fields domain magnetism applications of magnetism.
Abstract: Localized magnetism associated with the ion cores magnetism associated with band electrons techniques of making magnetic measurements magnetic scattering of neutrons and magnetic excitations antiferromagnetism, ferromagnetism and non-collinear magnetic order exchange interactions in magnetism and hyperfine fields domain magnetism applications of magnetism.
TL;DR: In this article, the effect of magnetoresistance on the number and thickness of the magnetic layers in a layered Fe/Cr structure was investigated and the experimental results and their possible interrelations were discussed.
TL;DR: In this article, the anomalous spin-splitting phenomenon which exhibits the field-dependent spin splitting separation has been observed, assuming an antiferromagnetic exchange interaction between the electrons in the spin-up and spin-down states.
TL;DR: In this paper, it was shown that a powdered CuFeO 2, a quasi-two-dimensional antiferromagnet on a triangular lattice (AFT), has two successive magnetic phase transitions at low temperatures.
Abstract: Neutron diffraction study revealed that a powdered CuFeO 2 , a quasi-two-dimensional antiferromagnet on a triangular lattice (AFT), has two successive magnetic phase transitions at low temperatures. In the high-temperature phase below T N1 =16 K, it has a monoclinic magnetic unit cell with five spins in a layer (\(\sqrt{7}a\times\sqrt{7}a\times2{c}\), γ=141.78°). At T N2 =10 K, it shows a discontinuous transition and enters the low-temperature phase with an orthorhombic magnetic unit cell with four spins in a layer (\(\sqrt{3}a\times2{a}\times2{c}\)). In both magnetic structures, spins are collinear and parallel to the c axis.
TL;DR: In this paper, the authors used a zero-field neutron polarimeter to confirm that the Cu2+ moments are parallel to the monoclinic b axis in the commensurate antiferromagnetic phase stable below 213 K.
Abstract: Single-crystal neutron diffraction measurements on cupric oxide, CuO, using a zero-field neutron polarimeter have confirmed that the Cu2+ moments are parallel to the monoclinic b axis in the commensurate antiferromagnetic phase stable below 213 K. Any component of moment in the a-c plane must have a magnitude less than 0.03 of the total. The incommensurate antiferromagnetic phase, stable between 213 K and the Neel temperature of 230 K, has a propagation vector (0.506, 0, -0.483) as previously reported, but the elliptical envelope of the helical modulation at 215.5 K is shown to have its axes parallel to (010) and in the a-c plane making an angle of 28.2(8) degrees to (001) beta obtuse. The envelope is almost circular with the ratio of the b to a-c components of 1.03(1) and the plane of the moments makes an angle of 73.0(5) degrees to the propagation vector. The new magnetic model produces an equally good fit to the original unpolarized neutron diffraction intensities as the previous model, in which the Cu2+ moments were confined to the a-c plane with a more elliptical envelope, and gives a root mean square Cu2+ effective moment of 0.27(1) mu B.
TL;DR: It is shown that in a system composed of alternating superconducting and magnetic layers the ground state may be the so-called ``\ensuremath{\pi} phase,'' wherein thesuperconducting order parameter changes its sign as the authors go from one superconducted layer to another.
Abstract: We show that in a system composed of alternating superconducting and magnetic layers the ground state may be the so-called ``\ensuremath{\pi} phase,'' wherein the superconducting order parameter changes its sign as we go from one superconducting layer to another. This phase is similar to the Larkin-Ovchinnikov-Fulde-Ferrell phase. We find that the \ensuremath{\pi} phase exists, under certain conditions, in the presence of both ferro- and antiferromagnetism.
TL;DR: In this paper, the ground state phase diagram of the Kondo lattice Hamiltonian for arbitrary J/W, and conduction electron concentrationnc (J is the kondo coupling and W the bandwidth) was investigated.
Abstract: We use the variational method to investigate the ground state phase diagram of the Kondo lattice Hamiltonian for arbitraryJ/W, and conduction electron concentrationnc (J is the Kondo coupling andW the bandwidth). We are particularly interested in the question under which circumstances the globally singlet (collective Kondo) Fermi liquid type ground state becomes unstable against magnetic ordering. For the collective Kondo singlet we use the lattice generalization of Yosida's wavefunction which implies the existence of a large Fermi volume, in accordance with Luttinger's theorem. Using the Gutzwiller approximation, we derive closed-form results for the ground state energy at arbitraryJ/W andnc, and for the Kondo gap atnc=1. We introduce simple trial states to describe ferromagnetic, antiferromagnetic, and spiral ordering in the small-J (RKKY) regime, and Nagaoka type ferromagnetism at largeJ/W. We study three particular cases: a band with a constant density of states, and the (tight binding) linear chain, and square lattice periodic Kondo models. We find that the lattice enhancement of the Kondo effect, which is described in our theory of the Fermi liquid state, pushes the RKKY-to-nonmagnetic phase boundary to much smaller values ofJ/W than it was previously thought. In our study of the square lattice case, we also find a region of itinerant, Nagaoka-type ferromagnetism at largeJ/W fornc ≦1/3.
TL;DR: Properties of incommensurate spiral spin phases are calculated at the mean-field level for a single-band Hubbard Hamiltonian with variable hole density, by adapting both the Hartree-Fock decoupling and the Kotliar-Ruckenstein slave-boson approach to a regular twist of the spin quantization axes from site to site in a two-dimensional square lattice.
Abstract: Properties of incommensurate spiral spin phases are calculated at the mean-field level for a single-band Hubbard Hamiltonian with variable hole density, by adapting both the Hartree-Fock decoupling and the Kotliar-Ruckenstein slave-boson approach to a regular twist of the spin quantization axes from site to site in a two-dimensional square lattice. The relative stability of the (1,1) and (1,0) spiral phases, the coexistence of the antiferromagnetic and the spiral phases over a finite range of hole density, and the stiffness of the spirals against fluctuations of their direction and pitch are discussed within the model Hamiltonian over a wide range of hole density and interaction strength.
TL;DR: A rigorous proof is given that the ground state of the Kondo-lattice model with one conduction electron forms an incomplete ferromagnetic order with S tot =(N-1)/2 for antiferromagnetic exchange coupling.
Abstract: The ground state of the Kondo-lattice model with one conduction electron is analyzed. A rigorous proof is given that this system forms an incomplete ferromagnetic order with ${\mathit{S}}_{\mathrm{tot}}$=(N-1)/2 for antiferromagnetic exchange coupling. The wave function of the ground state is derived and some of its properties are discussed.
TL;DR: In this paper, the antiferromagnetic arrangement of the magnetic moments of Fe layers in sputtered Fe/Cr multilayered structures is directly demonstrated from polarized neutron reflectometry studies.
Abstract: The antiferromagnetic arrangement of the magnetic moments of Fe layers in sputtered Fe/Cr multilayered structures is directly demonstrated from polarized neutron reflectometry studies. Such an antiferromagnetic interlayer exchange coupling is also consistent with magnetization studies on a series of [Fe/Cr]N structures. A remanent magnetization is observed for structures containing an odd number of bilayers but no remanent moment is found for an even number of bilayers. By examining the dependence of saturation field on the number of bilayers it is shown that the antiferromagnetic coupling strength is independent of the number of bilayers and is the same for superlattice and sandwich structures.
TL;DR: In this article, Parkin, More, and Roche confirmed the oscillatory behavior of the antiferromagnetic-type interlayer coupling and a new magnetoresistance (MR) effect due to antiparallel magnetization alignment.
Abstract: Layered Fe/Cr structures are known to display antiferromagnetic‐type interlayer coupling and a new magnetoresistance (MR) effect due to antiparallel magnetization alignment. The strength of the coupling is found to be similar in multilayered structures and in double layers. The oscillatory behavior of the coupling, previously found by Parkin, More, and Roche [Phys. Rev. Lett. 64, 2304 (1990)] on sputtered polycrystalline samples, is here confirmed for epitaxial samples, obtained by thermal evaporation. The new MR effect is interpreted as due to a spin‐dependent scattering of the electrons at the Fe‐Cr interfaces. The investigations have been extended to Fe/V, Fe/Mn, Fe/Cu, Co/Au, Co/Cr, and Co/Cu structures where the antiparallel alignment of the ferromagnetic layers is obtained via hysteresis effects. A MR effect due to antiparallel alignment, which is strong for Co/Au and Co/Cu but weak in the other cases, has been found.
TL;DR: Ferromagnetic Fe/Mo superlattices grown on sapphire exhibit an oscillatory magnetic coupling as a function of Mo thickness as mentioned in this paper, but have negative magnetoresistive anomalies that are only ≲2% at 4.2 K.
Abstract: Sputtered Fe/Mo superlattices grown on sapphire exhibit an oscillatory magnetic coupling as a function of Mo thickness. Ferromagnetic Fe layers 25 A thick couple across the nonmagnetic Mo layers ferro‐ or antiferromagnetically with a period of ∼11 A Mo. Ferromagnetically aligned films show the anticipated trend in longitudinal Kerr‐rotation values based on calculation, while the antiferromagnetically coupled films in zero field yield only weak Kerr signals due to their characteristically ‘‘pinched’’ hysteresis loops. The films are well ordered and exhibit up to seven low‐angle, x‐ray diffraction peaks, but have negative magnetoresistive anomalies that are only ≲2% at 4.2 K.
TL;DR: Chakraborty et al. as discussed by the authors reported that the magnetic properties in La2Cu04 are explainable in terms of a classical spin model, although with slightly renormalized However, an ( 20%) parameters.
Abstract: PHYSICAL REVIEW B VOLUME 44, NUMBER 17 Magnetic dilution study in La2Cu04. Comparison with other two-dimensional NOVEMBER 1991-I magnets S-W. Cheong, A. S. Cooper, L. W. Rupp, Jr. , and B. Batlogg ATc%T Bell Laboratories, Murray Hill, New Jersey 07974 J. D. Thompson and Z. Fisk Los Alamos National Laboratory, Los Alarnos, New Mexico 87545 (Received 22 May 1991) The reduction of the Neel point T& of LazCu04 by substitution of nonmagnetic Mg or Zn for Cu has been studied, with particular emphasis on keeping the oxygen content unaffected. In the low- concentration range (~10%), the rate of Tg suppression is found to be the same as in other two- dimensional (2D) Heisenberg magnets such as KqCuF4 (S= —, ) and K2MnF4 (S= —, ). While this suppression rate is larger than for 2D Ising magnets, it is much smaller than the rapid T~ reduction of holes in LazCu04 through oxygen excess or heterovalent substitution for La. upon introduction Thus, no unusual quantum magnetic dilution effects are observed in the S = 2 LaqCu04 magnet. The problem of site-diluted magnets is a simple form of the more general percolation or connectivity problem. In spite of the simplicity of the magnetic dilution problem, only a limited number of experiments have been reported. This can be attributed in part to the diSculty in finding magnetic materials that can be diluted without drastic changes of the magnetic interactions (e.g. , due to local symmetry and bond-length variations). Examples of two- dimensional (2D) magnets where these problems could be minimized are K2CoF4 and Rb2CoF4 in which the reduc- tion of the Neel temperature Tz follows closely the theoretical expectations for Ising magnets on a square lattice. Here we report magnetic dilution studies on La2Cu04, which is isostructural with the cobalt mag- nets, but with the cuprates being a Heisenberg system. Particular attention has been given to keep the oxygen concentrations constant because T~ of La2Cu04 is known to be very sensitive to this parameter. Our results, to- gether with previous studies on other 2D Heisenberg mag- nets such as KqCuF4 and K2MnF4, indicate a universal initial reduction of the magnetic ordering temperature that is significantly faster than in 2D Ising magnets. Extensive investigations of the magnetic behavior of La&Cu04 have been performed, especially to explore pos- sible quantum magnetic phenomena, expected to be most pronounced in this S = It appears, however, system. that the magnetic properties in La2Cu04 are explainable in terms of a classical spin model, although with slightly renormalized However, an ( 20%) parameters. anomalous rapid depression of the Neel point upon dilu- tion of the magnetic Cu sites has been reported. The main focus of the present study is to clarify this apparent anomaly. Even though an ideal 2D Heisenberg system does not develop long-range order at finite temperatures, LaqCu04 undergoes 3D antiferromagnetic (AF) ordering due to the presence of a slight deviation from the ideal system, possibly due to interlayer coupling. Chakraborty et a/. reported that nonmagnetic, divalent Zn substitu- tion on Cu sites in La2Cu04 induces a rapid decline of T~. The slope of the initial T~ reduction curve in their report S= S= was times larger than the theoretical predication for an Ising square lattice and times larger than that of the 2D Heisenberg such as K2CuF4 and magnets K2MnF4. ' The T~ reduction due to Cu dilution is to be contrasted to the effects of charge carriers introduced through either oxygen excess or heterovalent substitution The initial T~ reduction by the (e.g. , Sr, Ba) for La. introduction of hole carriers was found to be comparable with that by Zn substitution reported by Chakraborty et al. Furthermore, for La2Cu04+~ and La2, . Sr, Cu04, the threshold in 6 and x occurs at of the conventional per- colation limit (for the 2D square lattice, the site percola- tion threshold is predicted to correspond to 41% removal of magnetic sites). This fast T~ reduction has been at- tributed to magnetic frustration arising from the competi- tion between AF order in the CuOq planes and ferromag- netic coupling among Cu ions induced via hole carriers. Among nonmagnetic and divalent atoms, the ionic radii of Mg and Zn ions are very close to that of Cu. According to Shannon's study, ' ' the ionic radii of Mg, Cu, and Zn in octahedral coordinations are 0. 72, 0. 73, and 0. 74 A, re- spectively. In our investigation of the magnetic dilution by Mg or Zn substitution on Cu sites in La2Cu04, we find that Mg atoms are very good substitutional ions for simple magnetic dilution study in La2Cu04, i. e. , Mg ions have a large solubility limit in the LazCu04 structure and Mg substitution in La2Cu04 induces little change of lattice constants and lattice symmetry, especially orthorhombici- ty. The results of our study also indicate that the rapid Tz reduction by Zn substitution reported by Chakraborty et al. is probably due to extra oxygen in their specimens and that Tz variation, by Zn substitution only, is similar to that produced by Mg doping. At the end of our report, we compare the studied magnetic dilution effect on Tz by Mg and Zn with that in more conventional 2D magnets. Polycrystalline pellets of La2Cu~ — „Mg(or Zn), 04 were prepared by conventional solid-state reaction in air. The sintered pellets were annealed at 650 C for 10 h under a Np atmosphere to ensure the least amount of extra oxygen in the compounds, which can affect Tz rather drastically. It is noteworthy that extensive investigations of the oxy- The American Physical Society