Showing papers on "Antiferromagnetism published in 1984"

Spectrum and scattering of excitations in the one-dimensional isotropic Heisenberg model

Abstract: The work gives a consistent and uniform exposition of all known results related to Heisenberg model. The classification of excitations is presented and their scattering is described both in ferromagnetic and the antiferromagnetic cases. It is shown that in the antiferromagnetic case there exists only one excitation with spin 1/2 which is a kink in the following sense: in physical states there is only an even number of kinks-spin waves, therefore they always have an integer spin. Thus, it is shown that the conventional picture of excitations Is wrong in the antiferromagnetic case and the spin wave has spin 1/2, matrix is calculated.

Nature of the Phase Transition of the Two-Dimensional Antiferromagnetic Plane Rotator Model on the Triangular Lattice

Abstract: A study is made on the antiferromagnetic plane rotator model on the triangular lattice by Monte Carlo simulations In sharp contrast to the ferromagnetic case the specific heat shows a logarithmic divergence at a critical temperature and the magnetic susceptibility is almost independent of temperature over a wide temperature range The origin of the divergent specific heat is ascribed to an order-disorder transition of chirality, which is introduced to characterize the two-fold degeneracy of the ground state A discussion is made on the possible existence of an intermediate phase, in which the translational spin order vanishes, but the chirality order remains

Magnetic ordering in CeM/sub 2/Si/sub 2/ (M = Ag,Au,Pd,Rh) compounds as studied by neutron diffraction

Abstract: We report the results of neutron-diffraction experiments on CeM/sub 2/Si/sub 2/ (M = Ag,Au,Pd,Rh) which were performed to explore the role of valence fluctuations and 4f hybridization in the magnetic ordering of cerium compounds. All four order antiferromagnetically, the first three exhibiting structures consisting of ferromagnetic layers with moments perpendicular to the layers, which are believed to be characteristic of 4f-4f interactions mediated through hybridization with conduction electrons. CePd/sub 2/Si/sub 2/ has an anomalously small moment (0.62..mu../sub B/) in the ordered state. CeAg/sub 2/Si/sub 2/ exhibits an incommensurate longitudinal, static magnetization wave with moment and propagation direction along the a axis. The fourth compound, CeRh/sub 2/Si/sub 2/, has the highest known transition temperature (39 K) reported for cerium ordering; it exhibits another second-order transition at 27 K to a complex commensurate structure with modulated moments. The results are discussed in terms of the effects that 4f hybridization can have on ordering.

Spin-glass behavior in an antiferromagnetic frustrated spinel: ZnCr 1.6 Ga 0.4 O 4

Abstract: A study of magnetic properties in the insulating spinel ${\mathrm{ZnCr}}_{1.6}$${\mathrm{Ga}}_{0.4}$${\mathrm{O}}_{4}$ (chromium ions in octahedral sites) has been made by carrying out low-field dc- and ac-susceptibility measurements as well as neutron diffraction, M\"ossbauer, and EPR experiments, which all give evidence of spin-glass-like behavior. The topological frustration, connected with the high ground-state degeneracy of the antiferromagnetic spinel octahedral sublattice, is the origin of the spin-glass ordering. Strong nearest-neighbor antiferromagnetic interactions are dominant in the system, leading to short-range magnetic order which appears at temperatures much higher than the freezing temperature defined by the acsusceptibility cusp [$T(\ensuremath{ u}=17 \mathrm{Hz})=2.50$ K is frequency dependent]. The thermoremanent magnetization does not vanish at ${T}_{f}$, suggesting the existence of uncompensated clusters blocked above ${T}_{f}$. A paramagnetic behavior is observed below $T=1$ K, providing evidence for the presence of unfrozen spins below ${T}_{f}$ (entropic clusters).

Phase transitions in diluted magnets: Critical behavior, percolation, and random fields

Abstract: Transition metal halides provide realizations of Ising,XY, and Heisenberg antiferromagnets in one, two, and three dimensions. The interactions, which are of short range, are generally well understood. By dilution with nonmagnetic species such as Zn++ or Mg++ one is able to prepare site-random alloys which correspond to random systems of particular interest in statistical mechanics. By mixing two magnetic ions such as Fe++ and Co++ one can produce magnetic crystals with competing interactions-either in the form of competing anisotropies or competing ferromagnetic and antiferromagnetic interactions. In this paper the results of a series of neutron scattering experiments on these systems carried out at Brookhaven over the past several years are briefly reviewed. First the critical behavior in Rb2Mn0.5Ni0.5F4 and FecZn1−cF2 which correspond to two-dimensional and three-dimensional random Ising systems, respectively, are discussed. Percolation phenomena have been studied in Rb2MncMgl−cF4, Rb2CocMgl−cF4, KMncZl-cF3, and MncZnl−cF2 which correspond to two-and three-dimensional Heisenberg and Ising models, respectively. In these casesc is chosen to be in the neighborhood of the nearest-neighbor percolation concentration. Application of a uniform field to the above systems generates a random staggered magnetic field; this has facilitated a systematic study of the random field problem. As we shall discuss in detail, a variety of novel, unexpected phenomena have been observed.

Magnetic phase diagram of Fe 8 0 − x Ni x Cr 20 ( 10 ≤ x ≤ 30 ) alloys

Abstract: Detailed magnetic measurements and a few neutron-diffraction experiments were performed on polycrystalline ${\mathrm{Fe}}_{80\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{Cr}}_{20}$ alloys, with $x=30,26,23,21,19,17,14$, and $10$ All the alloys are fcc except the one with $x=10$, which is a mixture of fcc and bcc phases The system is ferromagnetic for $x=30$ The alloys with $x=26$ and $23$ make transitions from a ferromagnetic to a Gabay-Toulouse ferromagnetic---spin-glass mixed phase at a lower temperature Clear evidence of the presence of a spontaneous ferromagnetic bulk magnetization in this mixed phase supports recent theories predicting $Z$ ferromagnetism and $X\ensuremath{-}Y$ spin-glass freezing For $x=21$ and $19$ there is a spin-glass phase in low fields with some evidence of a field-induced ferromagnetism at high fields Long-range antiferromagnetism takes over for the alloys with $x=17,14$, and $10$ This sequence of compositional phase transition is due to competing ferromagnetic and antiferromagnetic interactions present in this system

Spin-Wave Analysis of the Antiferromagnetic Plane Rotator Model on the Triangular Lattice–Symmetry Breaking in a Magnetic Field

Abstract: Lowest-order spin-wave analysis is applied to the antiferromagnetic plane rotator model on the triangular lattice in order to study the low-temperature properties in the presence of magnetic fields. Equilibrium spin configuration in the zero-temperature limit is calculated as a function of the magnetic field. It is found that the thermal-fluctuation effect reduces a continuous degeneracy of the ground state to a discrete one at non-zero temperatures.

Magnetic transitions in Mn2−xMxSb (M=3d metals)

Abstract: Magnetic and crystallographic measurements have been carried out for Mn2−xMxSb (M=Ti, V, Cr, Fe, and Co) in order to study the nature of the transition from ferrimagnetic (Fr) to antiferromagnetic (AF) states. The magnetic phase diagrams have been determined. In the cases of M=Ti and Fe, only the Fr phase is observed. In the case of M=Co, the transition between Fr and AF states is observed in the composition range of x<0.35, but for the specimens with x larger than 0.35 only the AF ordering is found. The phase diagrams obtained are discussed on the basis of the molecular field theory.

Antiferro-Quadrupolar Ordering in CeB6

Abstract: The existence of an antiferro-quadrupolar ordering in the intermediate temperature region in CeB 6 is suggested by a model calculation. CeB 6 has two transition temperatures, T m =3.3 K and T N =2.3 K, enabling us to define three phases I, II and III in the sequence of decreasing temperature. The phase II shows unusual properties: A weak antiferromagnetism is induced and enhanced by the external magnetic field in the phase II, making the I-II phase boundary shift to the high-temperature side with increasing field. These properties can be explained by the present model based on the combined effect of the crystal field and the antiferro-quadrupolar interaction among 4f electrons on Ce 3+ ions. The molecular-field approximation with the two-sublattice model is used. Detailed comparison is made with experiment for various thermal and magnetic quantities.

Antiferromagnetic classical XY model: A mean-field analysis

Abstract: The antiferromagnetic classical $\mathrm{XY}$ (planar-rotator) model is analyzed under the mean-field approximation. Phase diagrams are obtained and found to be strongly dependent on the underlying lattice geometry. For bipartite lattices, there exists a second-order transition across a unique phase boundary. For tripartite lattices, there exist two phase boundaries, separating an intermediate "nonhelical" phase from a low-temperature "helical" phase and the high-temperature paramagnetic phase. The two phase boundaries merge into a single critical point at finite temperature and zero magnetic field.

Magnetic structure of holmium

Abstract: The magnetic structure of high purity single crystals of holmium has been studied by neutron diffraction techniques. Although the general characteristics of the magnetic structure have been found to agree with earlier measurements, some discrepancies have been resolved and new features have been observed. The magnetic form factor has been measured and compared with relativistic atomic calculations. The low temperature structure (T<20 K) is that of a conical ferromagnet with wave vector (1/6)(2π/c) along the c axis. The basal plane moment is 9.7 μB and the c‐axis ferromagnetic component is 1.6 μB at T=6 K. Bunching of the basal plane moments around the easy hexagonal direction has been observed below T=50 K. Evidence for asphericity in the magnetization density is presented and discussed. The wave vector of the basal plane modulation decreases monotonically with temperature in general accordance with the Elliott–Wedgewood theory. Several inflection points were observed, however, which correspond to commens...

Magnetic properties of cubic La(FexAl1−x)13 intermetallic compounds

Abstract: We report susceptibility and magnetization measurements for La(FexAl1−x)13 which can be stabilized over the x range 0.46–0.92. At low x values a mictomagnetic regime occurs with distinct cusps in the ac susceptibility. Upon increasing the Fe concentration a soft ferromagnetic phase is found which at lower temperatures shows anisotropy effects related to mictomagnetic behavior. Finally for x>0.86 antiferromagnetic order appears along with a very sharp metamagnetic transition in external fields of a few Teslas. The saturation moment increases linearly with x from 1.3μB to 2.3μB throughout the ferro‐ and metamagnetic regimes. These unusual properties are discussed in terms of local moment magnetism and the particular crystal structure which permits a large Fe‐Fe coordination number at small distances.

Magnetic and electrical properties of ilvaite

Abstract: Electrical conductivity, magnetization, magnetic susceptibility, and Mossbauer spectra are reported for three samples of ilvaite, a mixed‐valence iron silicate with ideal composition CaFe2+2Fe3+Si2O8(OH). The Neel temperature is 118±3 K, but there is evidence from the susceptibility for competing antiferromagnetic and ferromagnetic exchange interactions. Analysis of the Mossbauer spectra at 4.2 K for Fe2+ at A and B sites, shows that e2qQ is positive and η∼0, indicating pseudotetragonal local symmetry and orbital singlet ground states for both ions. A peak found at 40 K in the susceptibility measured with the field perpendicular to the c axis may be related to the temperature dependence of the Fe2+ single‐ion anisotropy.

NMR Study of Coexistence of Ferro- and Antiferromagnetism in the Itinerant Electron System (Zr 1− x Nb x )Fe 2 . II

Abstract: Pulsed NMR investigations of 93 Nb have been performed in the hexagonal Laves phase intermetallic compound (Zr 1- x Nb x )Fe 2 with 0.8 ≤ x ≤1.0. From the measurements of the Knight shift and the nuclear spin-lattice relaxation time T 1 , it is demonstrated that NbFe 2 is a nearly ferromagnetic compound and (Zr 1- x Nb x )Fe 2 with x =0.95 and 0.85 are weakly ferromagnetic compounds having a Curie temperature of 30 K and 40 K, respectively, in contrast with the itinerant electron antiferromagnetism for 0.5 ≤ x ≤0.7. In the composition of x =0.8 where the antiferromagnetic order occurs below 85 K and ferromagnetic moment appears below 30 K, it was found out from T 1 measurement that the system is in a coexistent state of ferro- and antiferromagnetism.

Spin-density waves and itinerant antiferromagnetism in metals

Abstract: The present state of the theory and the main experimental results are reviewed for systems in which the antiferromagnetic ordering is directly due to a peculiar topology of the Fermi surface, i.e., to the presence of nested electron and hole sheets. A familiar example of such a system is chromium, which supports a spin-density wave (SDW) that is incommensurate with the lattice. For this reason much of the experimental material discussed in this review is for chromium and its alloys with transition and nontransition metals. The theoretical description of SDWs of this type is based on the excitonic insulator model, which is discussed in detail not only in connection with the phase diagrams and SDW structure but also in regard to the electronic properties of systems of this type (measurements of the de Haas-van Alphen effect, optical measurements, band-structure calculations from first principles, etc.). The review concludes with a discussion of less-studied transition-metal compounds which apparently also support a SDW state due to a peculiar topology of the Fermi surface.

Metamagnetic transitions in cubic La(FexAl1-x)13 intermetallic compounds

Abstract: Cubic NaZn13-type compounds of the form La(FexAl1-x)13 was stabilised with compositions between LaFe6Al7. For compositions above LaFe11.2Al1.8 (x=0.861) a low-temperature antiferromagnetic state is present in small external fields. However, upon increasing the field to a few tesla, an exceedingly sharp spin-flip transition with remarkably large hysteresis occurs to the fully saturated ferromagnetic state. The origins of this unusual metamagnetic transition are discussed in terms of the special crystal structure.

Effects of random fields on the phase transitions and phase diagram of Mn 0.75 Zn 0.25 F 2

Abstract: The magnetic phase transitions and phase diagram of ${\mathrm{Mn}}_{075}$${\mathrm{Zn}}_{025}$${\mathrm{F}}_{2}$ are investigated by dilatometric, ultrasonic-attenuation, and magnetization measurements The random fields, generated by the magnetic field $H$, affect all three phase boundaries: the spin-flop line ${H}_{\mathrm{sf}}(T)$ separating the antiferromagnetic (AF) phase from the spin-flop (SF) phase; the boundary ${T}_{c}^{\ensuremath{\parallel}}(H)$ between the paramagnetic (P) and AF phases; and the P-SF boundary ${T}_{c}^{\ensuremath{\perp}}(H)$ At $T=0$, ${H}_{\mathrm{sf}}=582\ifmmode\pm\else\textpm\fi{}03$ kOe At high temperatures the boundary ${H}_{\mathrm{sf}}(T)$ is reentrant, in disagreement with the mean-field prediction Near the P-AF transitions the differential thermal expansion and differential magnetostriction exhibit hysteresis effects when the transition field is above about 12 kOe In low fields the P-AF transitions are quite sharp, in agreement with earlier data In high fields the P-AF transitions are smeared if the sample is cooled at constant $H$ or if $H$ is reduced at constant $T$ The P-AF transitions at high $H$ are sharper if the sample is warmed in a field after it has been cooled in zero field The hysteretic behavior at high $H$ is consistent with recent theory The transition temperature ${T}_{c}^{\ensuremath{\parallel}}$ is depressed substantially by the random fields The crossover exponent obtained from the low-field results for ${T}_{c}^{\ensuremath{\parallel}}$ is $\ensuremath{\varphi}=125\ifmmode\pm\else\textpm\fi{}007$, which agrees with the Fishman-Aharony prediction The N\'eel temperature is 460 K The P-SF transition temperature ${T}_{c}^{\ensuremath{\perp}}$ increases with increasing $H$ up to 131 kOe This increase of ${T}_{c}^{\ensuremath{\perp}}$ is much larger than in pure Mn${\mathrm{F}}_{2}$, but is consistent with other data in siterandom uniaxial antiferromagnets The phase diagram in the bicritical region is qualitatively different from that in pure Mn${\mathrm{F}}_{2}$ Also, we do not observe the two separate critical lines which surround the intermediate phase suggested by Aharony

Mean-field simulation of field-induced domains and hysteretic behavior in dilute Ising antiferromagnets

Abstract: A mean-field free-energy surface of a dilute Ising antiferromagnet on two-, three-, and four-dimensional lattices is studied numerically as a function of temperature and an applied magnetic field. In the presence of the field parallel to spins, the dilute Ising antiferromagnet is believed to map onto the random-field Ising model providing a convenient experimental realization. From the hysteretic behavior we identify three temperature regions: (1) a high-temperature equilibrium region above the hysteresis boundary ${T}_{H}$ where an accessed state is independent of the sample history; (2) a low-temperature region below ${T}_{c}$ where the antiferromagnetic state has the lowest free energy; (3) an intermediate-temperature region ${T}_{c}lTl{T}_{H}$ where the accessed state depends on the sample history, but the domain state has a lower free energy than the antiferromagnetic state. When the sample is cooled in a finite field, the domains are formed and stabilized below ${T}_{H}$. Thus the antiferromagnetic state is inaccessible in our simulation. In contrast, once the antiferromagnetic order is prepared at low temperatures and the temperature is raised in the finite field, the sample remains in the antiferromagnetic state until ${T}_{c}$ and then the domains nucleate above ${T}_{c}$, and finally the sample recovers the high-temperature equilibrium state at ${T}_{H}$. In recent neutron scattering experiments on three-dimensional (3D) Ising antiferromagnets, the analogous behavior is observed. Using the simulation results, we argue that strong domain-wall pinning prevents long-range order from occurring in 3D samples when cooled with $H\ensuremath{ e}0$, despite the theoretical consensus that the lower critical dimension in equilibrium is 2 for the random-field Ising model.

Current trends in amorphous magnetism

Abstract: Recent research on some topics in amorphous magnetism is reviewed and questions are posed which should be answered by future work. The topics are i) appearance of magnetism in binary alloys of the 3d transition elements Fe, Co and Ni with sp elements, ii) conflicting ground states of amorphous iron (ferromagnetic, asperomagnetic, paramagnetic), inferred by extrapolating different Fe x M 100-x system to x = 100, iii) influence of hydrogen on the magnetic properties of metallic glasses, iv) random magnetic anisotropy and v) amorphous ionic compounds with purely antiferromagnetic interactions.