Showing papers on "Antiferromagnetism published in 1976"

Crystal and magnetic structure of NiMnGe

Abstract: NiMnGe is investigated by means of X-ray diffraction, neutron diffraction, and magnetometric methods. The orthorhombic distorsion of hexagonal lattice is observed below 470 K. The compound NiMnGe is antiferromagnetic with a Neel temperature TN = 346 K. The jump observed at temperature Tt = 185 K on the temperature dependence of magnetic susceptibility points at the change of a magnetic structure. From the analysis of the neutron diffraction patterns the following models of spiral magnetic structure are obtained; at T < Tt the spiral axis is in the bc-plane. The angle between the spiral axis and the b-axis is 45°, magnetic moment at 80 K is 2.75μB; at Tt < T < TN the spiral axis is along the a-axis. The magnetic moment at 295 K is 2.2μB.

Physical realizations of n ≥ 4 -component vector models. I. Derivation of the Landau-Ginzburg-Wilson Hamiltonians

Abstract: Certain phase transitions which involve an increase of the unit cell in one or more directions, are described by $n\ensuremath{\ge}4$-component vector models. According to universality, the critical behavior of a system should depend only upon a small number of parameters such as the dimensionality of space, the number of components of the order parameter, and the symmetry of the Hamiltonian. We suggest that it would be of great interest to study these $n\ensuremath{\ge}4$ systems experimentally, and examine the effect of the dimensionality of the order parameter and the anisotropy of the system on the critical behavior. We use the group-theoretical method of landau and Lifshitz to derive the Landau-Ginzburg-Wilson Hamiltonians corresponding to the following $n\ensuremath{\ge}4$ systems: type-II antiferromagnets TbAs, TbP, TbSb ($n=4$), and MnO, MnSe, NiO, and ErSb ($n=8$); type-I antiferromagnet U${\mathrm{O}}_{2}$ ($n=6$); type-III antiferromagnet ${\mathrm{K}}_{2}$Ir${\mathrm{Cl}}_{6}$ ($n=6$); sinusoidal magnetic systems Dy${\mathrm{C}}_{2}$ and Tb${\mathrm{Au}}_{2}$ ($n=4$), and Tb${\mathrm{D}}_{2}$ and Nd ($n=6$); and an $n=4$ system, Nb${\mathrm{O}}_{2}$, which exhibits a structural transition. In the following paper (II) we use the exact renormalization-group technique in $d=4\ensuremath{-}\ensuremath{\epsilon}$ dimensions to study the critical behavior of these Hamiltonians.

Spin-reorientation transitions in rare-earth magnets

Abstract: This review treats the little-studied magnetic phase transitions of the order-order, spin-reorientation type, in which a rotation of the magnetization vector (or of die antiferromagnetism vector) with respect to the crystallographic axes occurs upon change of temperature, magnetic field, or external pressure. It presents theoretical ideas about magnetic phase transitions of the spin-reorientation type; it considers the effect of domain structure on spin-reorientation phase transitions, aad critical fluctuations during such transitions. Taking as an example rare-earth magnets (orthoferrites, iron-garnets, and rare-earth metals and intermetallic compounds), it considers the experimental data on various types of spin-reorientation transitions and on the anomalies of physical properties at such transitions.

Lattice Distortion and Elastic Properties of Antiferromagnetic γ Mn-Ni Alloys

Abstract: Lattice parameters and elastic moduli of polycrystalline γ Mn alloys containing 12–40 at% Ni have been measured in a temperature range between –180° and 250°C. The face-centred cubic lattice undergoes a tetragonal distortion, either c / a 1, or an orthorhombic distortion at low temperatures. Both Young's modulus and shear modulus exhibit a step-type change at the Neel point and a broad minimum near the transition temperature of lattice distortion. The elastic behaviour is discussed on the basis of the so-called Δ E effect due to the displacement of antiferromagnetic domain walls. This effect may possibly be enhanced by the lattice softening in the γ Mn–Ni alloys.

Random Ordered Phase Characteristic of Quenched Mixtures of Ising Spins

Abstract: A new ordered phase, tentatively called “random ordered phase” (ROP) is proposed in the quenched Ising spin system in which ferro- and antiferromagnetic exchange integrals exist. In the ROP each spin points to the same direction as the molecular field which is either up or down and is altered from site to site according as the various situations of surroundings. The phase transition is characterized by “random magnetization” and “random susceptibility”, whose expressions are given.

Specific Heat Study of Magnetic Ordering and Band Structure of 3d Transition Metal Disulfides Having the Pyrite Structure

Abstract: Specific heats of FeS 2 -CoS 2 -NiS 2 Solid solutions and of Ni 0.95 Cu 0.05 S 2 have been measured from 10 K to 350 K. Debye temperatures, coefficients of electronic contribution γ, magnetic ordering temperatures, and magnetic entropies have been determined. The magnetic entropy of ferromagnetic Fe 1- x Co x S 2 is proportional to cobalt concentration and is 40% of that expected for a localized spin 1/2 per Co atom, suggesting an itinerant character of e g electrons. The magnetic entropy of NiS 2 is smaller than that of a localized spin 1 and is in good agreement with that expected for the magnetic moment observed by neutron diffraction. The magnetic entropy of antiferromagnetic Co 1- x Ni x S 2 Suggests that only Ni atoms have magnetic moment in Ni-rich phase. The density-of-states curve of e g band for NiS 2 has been constructed from γ values assuming a rigid band. The band width of lower half of e g band is 0.6 eV. The Debye temperature decreases monotonously from 605 K of FeS 2 to 455 K of NiS 2 .

Itinerant f-electron antiferromagnetism in NpSn/sub 3/

Abstract: We report measurements of the specific heat, Moessbauer effect, and electrical resistivity of NpSn/sub 3/, which identify its weak antiferromagnetism (T/sub N/=9.5 K) as arising from itinerant 5f electrons. This is the first known verification of itinerant f-electron antiferromagnetism in any system. (AIP)

New magnetic compounds with Heusler and Heusler-related structures

Abstract: Magnetic and structural properties are presented for new ferromagnetic compounds of the form ${\mathrm{Rh}}_{2}\mathrm{Mn}X$ where $X$ is Al, Ga, In, Tl, Sn, Pb, and Sb. The exchange is described in terms of competing ferromagnetic Mn-Rh-Mn interactions and antiferromagnetic Mn-Mn interactions.

Spin-Peierls transition in quasi-one-dimensional crystals

Abstract: A review is given of theoretical concepts and experimental data concerning the spin-Peierls transition in a one-dimensional spin system with antiferromagnetic exchange interaction (an analog of Peierls instability of a one-dimensional metal). Analysis of experimental data confirms the existence of the spin-Peierls transition in TTF-CuBDT, TTF-AuBDT, and MEM(TCNQ)2 crystals. The magnitude of the spin-phonon interaction in crystals undergoing the spin-Peierls transition at low temperatures is discussed together with the role of fluctuations in transitions of this type. The influence of magnetic fields on spin-Peierls transitions is examined.

Interlayer Exchange Field in (CnH2n+1NH3)2CuCl4 with n=1-6 and (C6H5CmH2mNH3)2CuCl4 with m=1, 2 Determined by Parallel Pumping Experiment

Abstract: To determine the interlayer exchange field H ' E in quasi-two-dimensional magnets (C n H 2 n +1 NH 3 ) 2 CuCl 4 with n =1-6 and (C 6 H 5 C m H 2 m NH 3 ) 2 CuCl 4 with m =1, 2, the lowest Brillouin-zone-boundary magnons are parametrically excited by parallel pumping technique in the temperature range from 1.2 to 4.2 K and at a pumping frequency of 8.9 GHz. The values of H ' E extrapolated to T =0 K are as follows: 45±5 Oe( n =1), -850±20 Oe( n =2), -17±3 Oe( n =3), -17±3 Oe( n =4), -16±3 Oe( n =5), -11±3 Oe( n =6), 9±3 Oe( m =1) and 6±3 Oe( m =2), where positive and negative values correspond to the ferromagnetic and antiferromagnetic fields, respectively. The ratio of the interlayer exchange field to the intralayer exchange field for the m =2 compond is 1.1×10 -5 , which is the smallest value among these componds. The orthorhombic anisotropy fields for these compounds are determined by means of ferromagnetic resonance. The relation between the transition temperature and H ' E is discussed.

Electronic and magnetic properties of the pyrite-structure compound NiS2: influence of vacancies and copper impurities

Abstract: The compounds NiSx (1.91 or approximately=2.0. The transition to the paramagnetic state occurs at a temperature which increases roughly proportional to x. The presence of at least two sites characterized by different hyperfine fields indicates the existence of some inhomogeneity in the antiferromagnetic phase for T>31K.

The antiferromagnetic and ferromagnetic properties of Fe2MnSi

Abstract: The results of extensive magnetization, susceptibility, X-ray and neutron powder diffraction measurements on the stoichiometric intermetallic compound Fe2MnSi are reported. At room temperature the compound is paramagnetic enabling the crystallographic structure to be established as L21 with some degree of preferential DO3 disorder. Between 69 and 214 K the compound is ferromagnetic but below 69 K the manganese atoms become antiferromagnetically ordered whilst the iron atoms remained ferromagnetically aligned. This coincides with a change in the configurational magnetic symmetry to rhombohedral, in which the antiferromagnetically aligned moments are directed along the [111] direction.

Short range antiferromagnetic ordering in vitreous Fe2O3P2O5

Abstract: An outline is given of the theory of neutron magnetic scattering from amorphous materials, and data are presented for vitreous 0.79 Fe 2 O 3 · P 2 O 5 . The magnetic correlation function shows that at low temperatures the glass exhibits short-range antiferromagnetic ordering with Fe 3+ −Fe 3+ interionic distances similar to those found in crystalline FePO 4 . The neutron data are not consistent with a previous suggestion that the material is microcrystalline.

Magnetic investigation of the system Mn5Ge3Mn5Si3

Abstract: Investigations of the system Mn5(Ge1-xSix)3, (with 0 ≦ X ≦ 1) show that a continuous series of solid solutions can be obtained over the complete concentration range. Curie temperature and magnetization measurements at 4.2 K show that the system is ferromagnetic in the concentration range 0 ≦ X ≦ 0.75. For X = 0.8 it becomes antiferromagnetic, however Weiss temperature determinations indicate that the ferromagnetic interaction is still prevalent. Thus the magnetic structure goes over into a ferromagnetic arrangement at about 40 kOe at 4.2 K. In the concentration range 0 ≦ X ≦ 0.8 the measured mean ferromagnetic moment per manganese atom decreases slowly from 2.60 to 2.07 μB/Mn atom. Then with increasing Si concentration the antiferromagnetic interaction becomes more and more important. Notre etude montre que dans tout l'intervalle de concentration 0 ≦ X ≦ 1, le systeme Mn5(Ge1−xSix)3 donne lieu a une serie continue de solutions solides. Des mesures de points de Curie et d'aimantation a 4,2 K montrent que le systeme est ferromagnetique dans l'intervalle de concentration 0 ≦ X ≦ 0,75. Pour X = 0,8 la structure devient antiferromagneetique avec cependant une interaction ferromagnetique preponderante comme le laisse prevoir la determination des temperatures de Weiss 0, il en resulte qu'un basculement vers une disposition ferromagnetique est observe dans un champ de l'ordre de 40 kOe. Jusqu'a cette concentration le moment ferromagnetique moyen mesure decroit lentement de 2,60 a 2,07 μB/Mn at. Pour des concentrations en Si superieures a 0,8 les interactions antiferro-magnetiques deviennent de plus en plus fortes.

Spin Dynamics in Itinerant Antiferromagnetic γFeMn Alloys

Abstract: The spin wave excitations in the itinerant antiferromagnetics of γFeMn alloys have been studied by neutron scattering. The isotropic dispersion relation has been observed for γFe 0.7 Mn 0.3 and γFe 0.5 Mn 0.5 . The velocities of spin waves at room temperature are 230±30 meVA, 220±30 meVA and 160±30 meVA for Fe 0.7 Mn 0.3 , Fe 0.5 Mn 0.5 and Fe 0.3 Mn 0.7 , respectively. All samples have the spin wave energy gaps of about 10 meV at 0 K which shows a similar temperature dependence. The damping of the spin wave is significant and it depends on the momentum transfer of the spin waves. The magnon-like excitation persists even 200 degrees above the Neel temperature for Fe 0.5 Mn 0.5 . The results are favorable to the itinerant electron model where the multiple spin density wave state is assumed to exist.

Magnetic Transition of the Mn2Sb-Mn2As System

Abstract: The system Mn 2 Sb (1- x ) As ( x ) (0≦ x ≦1) is found to be solid solutions of the Cu 2 Sb type by X-ray analysis A magnetic phase diagram has been produced by magnetic measurments in the temperature range 80 to 1100 K Transition from antiferromagnetic to ferrimagnetic states with increasing temperature was observed for the compounds with x ≦04 and the compounds with x ≧055 were antiferromagnetic at all temperatures below the Neel temperatures It is shown that these magnetic transition are explained qualitatively by the molecular field theory

Supertransferred hyperfine interaction: Perturbed angular correlation of 111 Cd m in antiferromagnetic NiO, CoO, and MnO

Abstract: The time-differential perturbed angular correlation of $^{111}\mathrm{Cd}^{m}$ substituted as a dilute impurity into antiferromagnetic NiO, CoO, and MnO has been observed. The following magnetic fields are found at the Cd nucleus (4\ifmmode^\circ\else\textdegree\fi{}K): NiO (191.1 \ifmmode\pm\else\textpm\fi{} 2.5 kOe), CoO (170.8 \ifmmode\pm\else\textpm\fi{} 3.0 kOe), MnO (194.7 \ifmmode\pm\else\textpm\fi{} 2.5 kOe). They are compared with the Cd hyperfine fields in the antiferromagnetic perovskites KNi${\mathrm{F}}_{3}$:Cd, KCo${\mathrm{F}}_{3}$:Cd, and RbMn${\mathrm{F}}_{3}$:Cd reported earlier. The oxides are found to be more covalent than the fluorides. From the ratio of the hyperfine fields in MnO:Cd and NiO:Cd a new value of the spin density parameter ${f}_{\ensuremath{\sigma}}(\mathrm{Mn}\ensuremath{-}\mathrm{O})=8.1%$ has been estimated.

Magnetic Structure of TbCu1-XZnX

Abstract: Magnetic structure of TbCu 1- X Zn X system with the cubic CsCl type structure has been studied by neutron diffraction and magnetic measurements. In Cu-rich region ( X <0.5). the antiferromagnetic structure is conserved, which is TbCu type with (π π 0) mode. When X increases further, the magnetic state suddenly transforms to a ferromagnetic state with canted spin arrangement of which antiferromagnetic mode is (π π 0). In the region with the canted spin arrangement (0.5< X <0.7), the magnetic state approaches to a simple ferromagnetic state with increasing temperature, and transforms not only to a complete ferromagnetic state with increasing temperature, and transforms not only to a complete ferromagnetic state, but also directly to a paramagneticstate. This magnetic phase diagram and properties are fairly well explained by the theory based on the s-f exchange interaction with the low density 4s-electrons, assuming the electronic band structure of YZn.