J. J. Rhyne

Small‐angle neutron scattering evidence for the absence of long‐range magnetic order in amorphous Fe91Zr9

Small‐angle neutron scattering (SANS) studies of an amorphous Fe91Zr9 alloy have shown that there is no transition to long‐range magnetic order in contrast to previous bulk magnetization results which indicated a transition to a ferromagnetic state at 220 K followed by a reentrant spin‐glass state near 60 K. The scattering line shape is Lorentzian in Q down to below 150 K and yields a spin correlation length which reaches a maximum of only 27 A at the transition T C=210 K and exhibits an essentially constant plateau of 23 A down to helium temperature, with no evidence of a second transition in the 60–80 K range. The line shape departs from Lorentzian below about 120 K and can be represented by a Lorentzian plus Lorentzian‐squared cross section as appropriate for a system in which long‐range order has been replaced by a frozen cluster configuration. The absence of ferromagnetic order in Fe91Zr9 at Fe concentrations far above the percolation threshold suggests the presence of a very broad distribution of ex...

Occurrence of long‐range helical spin ordering in Dy‐Y multilayers (invited)

The magnetic ordering of highly perfect single‐crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 A) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long‐range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 A−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long‐range coupling is a conduction band spin‐density wave in bot...

Magnetic and crystallographic properties of UNiSn

Abstract We have performed neutron diffraction measurements on the compound UNiSn above and below its magnetic transition temperature. Below the transition, the magnetic structure is FCC type 1 antiferromagnetism with a uranium moment of (1.42±0.03)μ B parallel to the [100] axis. A small amount of second phase U 3 Ni 3 Sn 4 was present in the sample and its structure was also analysed as the Y 3 Au 3 Sb 4 structure, with space group 1 43d.

Neutron scattering studies of rare earth magnetic multilayers

Abstract Single crystal artificial superlattices prepared by molecular beam epitaxy and consisting of Dy, Er, and Gd layers alternated with Y all exhibit long-range magnetic order that is propagated through intervening non-magnetic Y layers as shown by neutron diffraction studies. In the case of Dy systems, the phase and chirality of the helical order are preserved from one Dy layer to another. The phase shift across the Y layer is proportional to the Y layer thickness, and so is in general a non-integral multiple of π, suggesting that the coupling is propagated by a spin-density-wave mechanism. The magnetic coherence range, obtained from a series of samples of constant Dy thickness and varying number of Y layers, shows a linear variation with the inverse of the Y layer thickness.

Inelastic neutron scattering studies of II‐VI diluted magnetic semiconductors (invited)

Inelastic neutron scattering studies of AII1−xMxBVI diluted magnetic semiconductor (DMS) alloys with low and high concentations of the magnetic component (M) are reviewed. Measurements of inelastic scattering from isolated pairs of magnetic ions makes possible a direct determination of the ion–ion exchange parameters. This method has been used for JNN determination in Zn1−xMnxB (B=S, Se, Te) and Zn1−xCoxB DMS alloy families. Inelastic scattering experiments on DMS alloys with high magnetic ion concentrations (Zn1−xMnxTe and Cd1−xMnxTe with x=0.4–0.7) reveal that at low T the magnetic excitation spectra of these systems are dominated by magnonlike modes despite the absence of long‐range magnetic order. A theoretical description of these excitations can be obtained using the numerical method developed by Ching and Huber. The model results reproduce remarkably well the dispersion and the line‐shape characteristics of the excitation modes seen in the experiments.

Deuterium site occupation and magnetism in Ho6Fe23Dx, compounds

Abstract High resolution neutron diffraction techniques were used to establish the hydrogen (deuterium) site occupation and sublattice magnetization of a series of Ho 6 Fe 23 D x compounds ( x = 1.5, 8.2, 12.1 and 15.7). The f.c.c. structure of the parent compound was maintained except for the D 12.1 sample which exhibited a tetragonal distortion. The f.c.c. lattice was reestablished for the higher D 15.7 composition. Diffraction and inelastic hydrogen vibration spectra data demonstrate conclusively that the deuterium atoms in the D 1.5 material fully occupy the octahedral a site with the residual in the tetrahedral f site. At the D 8.2 and higher concentration level, the a site is empty and the deuterium atoms partially occupy only the tetrahedral sites. The 4 K holmium magnetic moment was near the free-ion value of 10 μ B for all compounds and was oriented antiparallel to all four iron site moments (b, d, f 1 and f 2 ). For the cubic materials, the magnetization on the iron sites varied with deuteration in the range 0.8–2.4 μ B , with the D 15.7 compound showing the highest iron moment for all sites. A spin reorientation is observed in the tetragonal material between room temperature and 4 K with the holmium and iron b and n 1 moments rotating out of the basal plane.

Amorphous YFe2—A concentrated spin glass

Magnetic susceptibility, Mossbauer, and neutron scattering studies of the same alloy sample all indicate that amorphous YFe2 (a‐YFe2) exhibits spin‐glass behavior with a truly thermodynamic spin‐glass transition at TS.G.=58K. These studies show that spin‐glass behavior can persist to quite high magnetic concentrations in an amorphous alloy system which is dominated by competing ferromagnetic and antiferromagnetic exchange interactions. At a lower temperature of T?20K, the susceptibility versus T curve shows a break and the coercive field is anomalous. We interpret this phenomenon near T=20K as arising from magnetic ’’blocking’’ of individual spins or regions of correlated spins which persist below TS.G.. This produces a cluster‐freezing‐type ’’pseudo‐transition’’ which is merely a manifestation of the slowing down of spin fluctuations to a time interval comparable with the experimental susceptibility measurement time. The temperature dependence of the spin‐glass order parameter is obtained from the suscep...

Evolution from ferromagnetism to spin-glass behavior

Abstract The magnetic properties of amorphous (Fe x Ni 100−x ) 75 P 16 B 6 Al 3 have been investigated in the critical concentration region ( x c ≈ 17) via neutron scattering. For x > x c a transition to ferromagnetism is first observed followed at lower temperatures by an evolution to spin-glass behavior. Associated with this low temperature state are two distinct time scales corresponding to a resolution-limited elastic peak and to excitations. The temperature dependence of the elastic component is directly related to the spin-glass order parameter and indicates that there is a region of temperature where ferromagnetism and spin-glass order coexist.

Spin waves in amorphous Fe1−xBx alloys

The temperature dependence of spin excitations has been studied in amorphous Fe1−xBx (x = 0.18 and 0.14) by inelastic neutron scattering. The spin‐wave stiffness constant D was determined directly from the magnon dispersion curves (E = Dq2) over the temperature range from below room temperature (T/TC = 0.48 and 0.36, respectively, for x = 0.18 and 0.14 alloys) up to 548 K (T/TC = 0.75 and 0.99), which is just below the crystallization temperature. For both alloys the temperature dependence of D was found to be proportional to (T/TC)5/2 up to (T/TC) greater than 0.8. The extrapolated values of D at T = 0 were D = 167 and 138 meV A2, respectively, and were nearly twice as large as those determined from the T3/2 coefficient found from magnetization studies. These anomalies may be related to the Invar characteristics of the thermal expansion in these alloys. Limited data were also obtained for an alloy Fe76B24 which exhibits a higher D (≳174 meV A2).

Antiferromagnetism in epilayers and superlattices containing zinc‐blende MnSe and MnTe

New zinc‐blende Mn‐based magnetic semiconductors offer unique insights into the magnetic order and critical behavior of frustrated antiferromagnets. Molecular beam epitaxy grown samples exist both in the form of strained superlattices, with strain induced anisotropies (MnSe/ZnSe and MnTe/ZnTe), and in the form of near‐isotropic bulk MnTe. The tetragonally distorted strained superlattices exhibit a second‐order phase transition, consistent with symmetry arguments favoring such behavior. In contrast, the bulk‐like epitaxial layers of MnTe have a first‐order phase transition, associated with magnetostriction, unknown to an unaccounted for by the previous studies.