Showing papers on "Antiferromagnetism published in 1970"

Exact Results in the Kondo Problem. II. Scaling Theory, Qualitatively Correct Solution, and Some New Results on One-Dimensional Classical Statistical Models

Abstract: The simplest Kondo problem is treated exactly in the ferromagnetic case, and given exact bounds for the relevant physical properties in the antiferromagnetic case, by use of a scaling technique on an asymptotically exact expression for the ground-state properties given earlier. The theory also solves the $n=2$ case of the one-dimensional Ising problem. The ferromagnetic case has a finite spin, while the antiferromagnetic case has no truly singular $T\ensuremath{\rightarrow}0$ properties (e.g., it has finite $\ensuremath{\chi}$).

Study of (La 1− x Ca x )MnO 3 . I. Magnetic Structure of LaMnO 3

Abstract: Magnetic properties of the perovskite-type oxide LaMnO 3 were studied in detail, experimentally and theoretically. The properly stoichiometric LaMnO 3 had a weak ferromagnetic moment in addition to the antiferromagnetic character with a Neel temperature of 141°K. It was found to be a parasitic weak ferromagnet. These results can be successfully explained using a phenomenological approach or the Spin-Hamiltonian formalism. The theoretical results show that the antiferromagnetic spin axis should be along the b -axis of the orthorhombic crystal axes and the ferromagnetic moment along the c -axis. The relation between a layer-type ( A -type) antiferromagnetic superexchange interaction and the Jahn-Teller distortion of Mn 3+ ions is also discussed.

The metal-nonmetal transition

Abstract: An account is given of some of the mechanisms which can lead to a transition from a metallic to a nonmetallic state, when a parameter such as the interatomic distance or temperature is varied. The simplest of these is the band overlap or Wilson transition, which occurs when a conduction band overlaps a valence band; this is discussed in § 2 and for noncrystalline systems in § 15. These transitions can be described in the Hartree-Fock approximation. If the insulating property is due essentially to the repulsion between electrons (e2/r12), then the nonmetallic state is normally antiferromagnetic. The possibility of describing it by normal band theory with a spin-dependent potential is discussed in § 5. It is emphasized that antiferromagnetism can exist in the metallic state, and that the conditions for the appearance of conductivity and the disappearance of antiferromagnetism are not always the same. The nonmetallic behaviour, that is the existence of a Hubbard gap, normally persists above the Neel temperature (as in NiO), as does the gap in some metals, but not in chromium. Disordered systems, such as doped semiconductors, are discussed; here in the metallic state we suggest that the two Hubbard bands overlap, and that the metal-nonmetal transition can be described as an Anderson transition (§ 16). This model gives a simple explanation of the negative magnetoresistance. In some materials a transition occurs which does not involve magnetic moments or structural change, and for d bands, following Halperin and Rice, and Weger, we introduce the concept of an `orbital orientation wave' in degenerate d bands (§§ 5, 19.3, 19.4). A number of specific materials are discussed.

Band-Structure Effects in Itinerant Antiferromagnetism

Abstract: The effect of imperfect "nesting" of the band structure on itinerant antiferromagnetism is considered. A model band structure with spherical electron and hole pockets of unequal radii and a "nonmagnetic" reservoir is examined. A first-order commensurate-incommensurate transition is found, but the paramagnetic transition remains second order. The influence of generalized Kohn anomalies on the transition is investigated, and it is found that though they lead to anomalous behavior in the limit of vanishing antiferromagnetism, there is no change in the order of the transition.

Ising‐Model Spin Correlations on the Triangular Lattice. IV. Anisotropic Ferromagnetic and Antiferromagnetic Lattices

Abstract: A detailed discussion of pair correlations ω2(r) = 〈σ0σr〉 between spins at lattice sites 0 and r on the axes of anisotropic triangular lattices is given. The asymptotic behavior of ω2(r) for large spin separation is obtained for ferromagnetic and antiferromagnetic lattices. The axial pair correlation for the ferromagnetic triangular lattice has the same qualitative behavior as that for the ferromagnetic rectangular lattice: There is long‐range order below the Curie point TC and short‐range order above. It is shown that correlations on the anisotropic antiferromagnetic triangular lattice must be given separate treatment in three different temperature ranges. Below the Neel point TN (antiferromagnetic critical point), the completely anisotropic lattice exhibits antiferromagnetic long‐range order along the two lattice axes with the strongest interactions. Spins along the third axis with the weakest interaction are ordered ferromagnetically. Between TN and a uniquely located temperature TD, there is antiferro...

Molecular Field Coefficients of Substituted Yttrium Iron Garnets

Abstract: By fitting the Neel theory of ferrimagnetism to previously reported magnetic moment‐temperature data for several {Y3}[RxFe2−x](QyFe3−y)O12 compositions, where R and Q represent diamagnetic octahedral and tetrahedral substitutions, i.e., Sc3+, In3+, Ga3+, and Al3+, the molecular field coefficients were determined to have the following linear relations with the levels of substitution: Ndd=−30.4(1−043x),Naa=−65.0(1−042y),Nad=97.0(1−0.125x−0.127y) mole/cm3, for x≤0.70 and y≤1.95. Since both intrasublattice coefficients are affected only by substitutions in the opposite sublattice, a strong similarity to Geller's random canting concepts is apparent. Below the antiferromagnetic transitions it is demonstrated that a clear correlation exists between the decrease in sublattice moment from canting and the reduction in magnitude of the molecular field constant. For both sublattices, the antiferromagnetic transition occurs when Ndd, Naa∼−20 mole/cm3. This observation lends further credence to the notion that canting ...

Ising-Model Spin Correlations on the Triangular Lattice. III. Isotropic Antiferromagnetic Lattice

Abstract: The asymptotic behavior of the pair correlation ω2(r) = 〈σ0σr〉 between two spins at sites 0 and r on an axis of an isotropic antiferromagnetic triangular lattice is investigated with the aid of the theory of Toeplitz determinants as developed by Wu. The leading terms in the asymptotic expansion are obtained for large spin separation at fixed nonzero temperature. Evidence is presented that the zero‐point behavior of the correlation is of the form ω2(r) ∼ e0r−½ cos ⅔πr, where r = |r| is the spin separation and e0=212(E0T)2=0.632226080…,E0T being the decay amplitude of the pair correlation at the Curie point (critical point) of an isotropic ferromagnetic triangular lattice. A special class of fourth‐order correlations ω4(r) = 〈σ0σδσr σr+δ〉 − 〈σ0σδ〉 〈σrσr+δ〉 between the four spins at sites 0, δ, r, and r + δ on the same lattice axis, where δ is a lattice vector, is reconsidered. The asymptotic form of the correlation for large separation of pairs of spins r = |r| is obtained for all fixed temperatures.

Room‐Temperature Ferromagnetic Materials Transparent in the Visible

Abstract: The only known crystals which have a spontaneous ferromagnetic moment at room temperature and are transparent well into the visible spectrum are ferric borate, FeBO3, and ferric fluoride, FeF3. Each of these rhombohedral materials is a canted antiferromagnet with a small moment lying in the plane perpendicular to its unique axis. The Curie temperatures are 348° and 363°K and the room‐temperature moments are 115 and 40 G, respectively. Both materials contain ferric ions in a slightly distorted octahedral environment. Both are green with a series of absorption bands in the visible, a fundamental absorption edge in the violet (FeBO3) or ultraviolet (FeF3), and a large Faraday rotation per unit absorption at room temperature, as high as 14°/dB at 5250 A for FeBO3 and 16°/dB at 4050 A for FeF3. Recent measurements of the magnetic, optical, magneto‐optical, and microwave resonance properties of these materials are discussed. Potential applications of room temperature transparent ferromagnets include microwave magneto‐optical modulation of visible lasers with very low modulation power per unit bandwidth, optical deflection and isolation, magneto‐optic displays, and holograms. Each of these applications is discussed along with the limitations imposed by the optical absorption and birefringence of the available materials. Birefringence interferes with the Faraday effect in most noncubic crystals. Methods for minimizing this interference are described. Finally, requirements for useful transparent ferromagnets are defined and guidelines for finding new materials are suggested.

Spin Ordering in a Spin-Pair System

Abstract: A new type of spin ordering in a spin-pair system is theoretically investigated. In the system, the intra-pair exchange interaction is antiferromagnetic and is much stronger than the inter-pair exchange interaction. In an external magnetic field, the excited triplet of the pair splits, and the lowest component of the triplet crosses the ground singlet at a certain magnitude of the field. In the vicinity of the point of level crossing, even the weak inter-pair exchange interaction causes a considerable amount of mixing between the singlet and the lowest component of the triplet. Due to the mixing, an ordering of the spin component perpendicular to the external field occurs. This ordering explains an anomaly which has been found by Haseda et al. in their experiment of cooling by adiabatic magnetization of Cu(NO 3 ) 2 2.5H 2 O.

Spin Ordering in a Spin-Pair System

Abstract: A new type of spin ordering in a spin-pair system is theoretically investigated. In the system, the intra-pair exchange interaction is antiferromagnetic and is much stronger than the inter-pair exchange interaction. In an external magnetic field, the excited triplet of the pair splits, and the lowest component of the triplet crosses the ground singlet at a certain magnitude of the field. In the vicinity of the point of level crossing, even the weak inter-pair exchange interaction causes a considerable amount of mixing between the singlet and the lowest component of the triplet. Due to the mixing, an ordering of the spin component perpendicular to the external field occurs. This ordering explains an anomaly which has been found by Haseda et al. in their experiment of cooling by adiabatic magnetization of Cu(NO 3 ) 2 2.5H 2 O.

New Approach to the Theory of Itinerant Electron Ferromagnets with Local‐Moment Characteristics

Abstract: The understanding of ferromagnets, like iron, which exhibit localized moment behavior above the Curie point yet show itinerancy has long stood as a major theoretical problem. An account will be given of recent progress on this problem which was achieved through functional integral methods. This technique transforms the interacting electron system into an average over a system of noninteracting electrons moving in a Gaussian‐weighted external ``magnetic'' field which acts only on the electronic spins. For a single magnetic impurity in a free electron metal, a single approximation allows one to go from Pauli paramagnetism to localized moment behavior in a smooth manner as the atomic exchange interaction is increased. The two impurity problem leads to an effective exchange coupling as in the Heisenberg model, which is antiferromagnetic for the nondegenerate orbital case studied here. Application of the technique to homogeneous systems leads to damped spin waves in the ferromagnet in lowest approximation.

The antiferromagnetic-ferromagnetic transition in iron-rhodium alloys

Abstract: The critical field required to induce this transition in Fe-Rh alloys close to the equiatomic composition has been measured using pulsed magnetic fields up to 280 kOe. Measurements of the thermal expansion and high field magnetization have also been carried out. It is found that application of Kittel's theory of exchange inversion to the results leads to serious numerical discrepancies. A four sublattice model is developed which indicates the importance of the excess entropy of the ferromagnetic over the antiferromagnetic phase in exciting the antiferromagnetic-ferromagnetic transition. The exchange iteration β between iron sublattices is small and negative and, although its lattice parameter dependence is large and positive, there is no suggestion of exchange inversion corresponding to a change in sign of β at the antiferromagnetic-ferromagnetic transition. Thus the ferromagnetic state is stabilized by the excess entropy even in the presence of antiferromagnetic interactions and this stability is maintained right up to the Curie temperature

Spin Fluctuations in Nearly Antiferromagnetic Metals

Abstract: It is shown theoretically that the linear contribution to the specific heat enhanced by spin fluctuations in nearly antiferromagnetic metals exhibits no logarithmic increase as the critical boundary is approached, nor does a term like ${T}^{3}\mathrm{ln}T$ in the specific heat exist, in contrast to the case of nearly ferromagnetic metals.

Effect of Pressure on Curie Temperature of Calcogenide Spinels CuCr2X4(X=S, Se and Te)

Abstract: The pressure coefficient of the Curie temperature of CuCr 2 X 4 was obtained from the result of measurement of the variation with pressure in the self inductance of search coil wounded on the specimen vessel. The coefficients, d T c /d p , were founded to be -11.3×10 -4 °Kkg -1 cm 2 for CuCr 2 S 4 , -4.1×10 -4 °Kkg -1 cm 2 for CuCr 2 Se 4 and nearly zero for CuCr 2 Te 4 , respectively. If the main exchange interaction in a pair of Cr ions is thought to be the ferromagnetic superexchange of the Cr-X-Cr type and the antiferromagnetic direct interaction of the Cr-Cr type, it can be seen from the results of the present experiment that the direct Cr-Cr interaction is more sensitive to lattice compression than the superexchange Cr-X-Cr. The magnetic properties of those compounds are also reported.

Ordering in Two‐ and Three‐Dimensional Diluted Heisenberg Antiferromagnets

Abstract: The ordering temperatures of the magnetically diluted antiferromagnetic systems K2MnpMg1−pF4 and KMnpMg1−pF3 are presented. From a magnetic point of view these systems can be looked upon as being simple quadratic and simple cubic, respectively. Near p=1, (d/dp)[TN(p)/TN(1)] is clearly larger than 1, as is theoretically expected for diluted Heisenberg systems. The curves for TN as a function of the concentration show that the critical concentrations for ordering at T=0 are clearly different for the two magnetic systems; they may be equal to the values predicted for the percolation problem. The low‐field perpendicular susceptibility strongly depends on p, at the lower temperatures it can be approximately described by a kind of Curie‐Weiss law. Extrapolating the Curie‐Weiss behavior down to T=0, one obtains values for the inverse perpendicular susceptibility that, if plotted as a function of the concentration of magnetic atoms, show a clear similarity to the concentration dependence of the ordering temperature itself. Some results of the two‐dimensional nearly Ising system K2CopMg1−pF4 are also presented.

Magnetic Properties of Several Phases of Barium Orthoferrate, BaFeOx

Abstract: Various barium orthoferrates over wide Fe 4+ concentrations were prepared and their magnetic properties were investigated. The hexagonal phase with large Fe 4+ concentration is ferromagnetic above the anomaly point. With increasing Fe 3+ , it tends to be antiferromagnetic. The cubictetragonal phases are paramagnetic with positive paramagnetic Curie temperature. In these phases, the Fe 4+ -Fe 4 interaction is ferromagnetic, but the Fe 3+ -Fe 4+ and Fe 3+ -Fe 3+ interactions are antiferromagnetic. Some of the Fe 3+ and Fe 4+ ions in these phases take low spin states due to strong uniaxial crystalline fields caused by neighbouring oxygen vacancies. The triclinic I phase seems to have a magnetic structure of one-dimensional antiferromagnet. The magnetic structures of the triclinic II and rhombohedral phases resemble that of the triclinic I.

Magnetic Studies of C1 b -Compounds CuMnSb, PdMnSb and Cu 1− x (Ni or Pd) x MnSb

Abstract: Magnetic properties of manganese compounds with C1 b -type crystal structure are studied. CuMnSb (lattice parameter a =6.09 5 A) is antiferromagnetic with a Neel temperature of 55°K and an asymptotic Curie temperature of -160°K. The paramagnetic Bohr magneton number per Mn is 5.4 7 . PdMnSb ( a =6.24 6 A) is ferromagnetic with a Curie temperature of 500°K and the magnetic moment per Mn is 3.9 5 µ B . Magnetic studies are also carried out for the compounds in which Ni or Pd are substituted for Cu in CuMnSb, resulting in a conclusion that the concentration of conduction electrons is very important in determining the magnetic interaction. Qualitative interpretation of experimental results is given based on the theory of resonance scattering by Friedel, Caroli and Blandin.

Cooperative Magnetic Transitions in the Titanium‐Oxygen System: A New Approach

Abstract: Semiconductor‐to‐metal‐type transitions were studied by magnetic susceptibility measurements on pure single phases of Ti3O5, Ti4O7, Ti5O9, and Ti6O11. This work represents a more detailed investigation than those reported previously. From this and related studies the following model is proposed to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups the d electrons are delocalized; however, ``constrained‐type'' antiferromagnetism sets in between specific neighboring d electrons through homopolar bonding of cations. Neighboring groups interact via thermal excitation of electrons. Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3d overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. The behavior is described by the free‐electron gas model. In addition, in all but Ti3O4, a Curie‐Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (<11%) interactions between the unpaired spins. In proposing this model, calculations of the effective electronic mass and Goodenough's ideas have been used.

Ordered moment of NiS 2

Abstract: The transition metal dichalcogenides exhibit a wide variety of both electrical and magnetic properties, from insulators to superconductors and from ferromagnetism through antiferromagnetism to diamagnetism NiS 2 is a semiconductor with an anomalous paramagnetic behavior which leaves in doubt the existence of a local moment Previous neutron diffraction data have failed to show any ordering down to 42°K Our neutron powder diffraction data on stoichiometric NiS 2 show a transition at 40°K to a structure which can be described as ordering of the first kind This is followed by an abrupt transition at 30°K in which additional diffraction peaks appear and these are consistent with ordering of the second kind Despite the fact that the powder data cannot indicate an unambiguous magnetic structure, the magnitude of the ordered moment can be fixed The rms moments associated with ordering of the first and second kinds are 10 and 060 µ B per nickel atom, respectively, and the total moment is 117 µ B per nickel atom

Antiferromagnetism in V2O3

Abstract: Neutron diffraction measurements on powder and single‐crystal samples of V2O3 have confirmed the existence of long‐range magnetic order in the low temperature (monoclinic) phase. Paoletti and Pickart1 discovered an extra diffraction peak at low temperature which they attributed to nuclear scattering. A polarization analysis experiment on a powder sample showed that this peak was magnetic in origin. In addition, several much weaker magnetic peaks were identified. These peaks index in the monoclinic cell given by McWahn,2 with indices satisfying the relation h+k+l=2n+1. The magnetic peaks show little temperature dependence from 77°K up to the crystallographic transition at 170°K, where they vanish abruptly. On cooling, they suddenly reappear at about 154°K. The data agree with a model in which the V moments are ferromagnetically aligned in (010)m layers (perpendicular to hexagonal a axis) with a reversal between adjacent layers. The ordered moment of 1.2 μB per V atom is oriented perpendicular to the hexago...

Linear Chain Antiferromagnetism in Copper Benzoate

Abstract: Various magnetic properties of one dimensional antiferromagnet Cu(C6HsC00)2 ·3H20 were investigated by means of magnetic susceptibility, magnetic torque, NMR, and ESR measurements. The results show that this compound is one of the best one dimensional antiferromagnets known at present. An intrachain exchange interaction J/K was estimated to be -8. 6±1 °K while no second order phase change was found down to 1. 4 °K. ESR line width at room temperature was not explained by an usual treatment and an anisotropic exchange effect was newly taken into account. However, other anomalies of the line width such as the frequency, temperature, and angular dependences appeared at low temperature regions were not yet explained.

Magnetic and Structural Properties of Some Rare‐Earth‐Sn3 Compounds

Abstract: The 23.8‐keV Mossbauer resonance of 119Sn and susceptibility measurements have been used to study the structural and magnetic properties of CeSn3, PrSn3, and NdSn3 intermetallic compounds between 293° and 1.6°K. The deterioration of the powder samples, which yields free tin and has led to inconsistent results in the past, is clearly demonstrated in these spectra. Both PrSn3 and NdSn3 exhibit magnetic hyperfine (hf) spectra below their Neel temperatures of 8.6° and 4.7°K, respectively. In PrSn3 one‐third of the Sn nuclei experience a transferred hf field of 67 kOe with the spin direction parallel to the principal electric‐field‐gradient axis while the remaining Sn nuclei display no magnetic splitting. This is interpreted as the result of antiferromagnetic ordering of the first kind. In NdSn3 the magnetic ordering appears to be more complex. In CeSn3 neither magnetic ordering nor the ``incipient ferromagnetism'' previously suggested is seen. The size of the quadrupole interaction and the isomer shift are ne...

Neutron Diffraction Study of the Magnetic Structure of BaNiF4

Abstract: The orthorhombic compounds BaMF4 (M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF4 reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer‐type structures. As the temperature is lowered, χ∥ along b decreases rapidly, while χ⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF4, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along the b and c directions (A21am orientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4‐A apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in the b direction depends upon interactions between second nearest sheets 14‐A apart. This is very similar to the situation in K2N...

Neutron Diffraction Study of γ‐FeOOH

Abstract: Results of the study of γ-FeOOH structure by means of neutron diffraction and Mossbauer effect are discussed. The positions of the ions are determined. The hydrogen ion occupies a central position between two oxygen ions separated from one another by 2.70 A. The Neel temperature determined from magnetic susceptibility is 50 °K. A model of antiferromagnetic ordering of the magnetic moments is suggested.