H. A. Alperin

Magnetic Structure of EuTiO3

Europium titanate has the cubic perovskite structure containing divalent Eu (7 μB) and tetravalent Ti. From magnetic measurements we find that EuTiO3 is one of the few antiferromagnetic materials with a positive θ (TN=5.3°K, θ=3.8°K). At 1.3°K the magnetic moment (σ) increases linearly with field to 10 kOe; above 14 kOe the moment saturates and σ=156 emu/gm (6.93 μB) at 20 kOe. Powder neutron‐diffraction work indicates that EuTiO3 has the Type G magnetic structure in which a given Eu++ has six nearest‐neighbor europium ions antiparallel and 12 next‐nearest‐neighbor europium ions parallel. In a perovskite structure where only the 12‐coordinated ion is magnetic, i.e., Eu++, the molecular field relations for a two sublattice model yield J1/k=−0.021°K, where J1 is the effective intersublattice exchange interaction, and J2/k=0.040°K, where J2 is the effective intrasublattice exchange interaction. The signs of J1 and J2 are opposite to those found in the europium chalcogenide series. The chalcogenides, however,...

Magnetic Structure and Exchange Interactions in Cubic Gadolinium Compounds

The two groups of metallic compounds Gd3+X3− (X=N, P, As, Sb, Bi) and Gd3+Z2− (Z=S, Se, Te) have the rocksalt structure and except for ferromagnetic GdN they are antiferromagnetic. Powder neutron diffraction measurements on GdS, GdSe, GdSb, and GdBi show them to have order of the second kind. The exchange interactions are discussed on the basis of the magnetic structure and susceptibility measurements.

Magnetic Properties and Structure of GdN and GdN1−xOx

GdN is shown to be ferromagnetic but small amounts of oxygen (GdN1−xOx with x≈0.05) cause paramagnetism and short‐range antiferromagnetic order to set in. We attribute this to nearest‐neighbor interactions having opposite sign.

Neutron Diffraction Investigation of a Gadolinium Single Crystal

A single‐crystal disk of Gd cut parallel to (00.1) has been studied in zero magnetic field. A careful search along and perpendicular to the c* axis in the neighborhood of the (00.2) reflection has failed to reveal any satellites in the temperature range 77° to 290°K. Thus unlike the rare‐earth elements from Tb to Tm, gadolinium is a normal ferromagnet. Above 248°K, the moment is aligned along the c axis. Below this temperature, the moments make an angle with the c axis which reaches a maximum of 75° at 195°K and approaches 30° at 4°K. The coherent nuclear scattering amplitude has been estimated by scaling the nuclear scattering to the magnetic scattering in the (00.2) reflection. The result is |b|=1.5±0.2×10−12 cm.

Aspherical Spin Density in the Ferrimagnetic Compound Mn2Sb

The spatial distribution of the unpaired electrons about each manganese atom in the ferrimagnetic compound Mn2Sb (space group P4/nmm) has been measured by diffraction of polarized neutrons from single crystals at room temperature. It is found that this distribution is aspherical and highly compact. The moments for the manganese atoms at the 2a and 2c positions are measured to be −1.48±0.15 μB and 2.66±0.15 μB, respectively. Single‐crystal x‐ray determination of the manganese and antimony 2‐c position parameters for this structure yield the values 0.2897±0.0006 and 0.7207±0.0002, respectively. An unusual feature of the spin density of the manganese atom at the 2‐c position is the displacement of the center of the distribution from its nuclear position.

Magnetization and neutron scattering measurements on amorphous NdFe2

A bulk sputtered sample of NdFe2 was investigated by magnetization and neutron scattering measurements and compared to similar previous measurements for heavy rare‐earth alloys. The amorphous structure was found here to be more open but in all other respects the results were found to agree qualitatively with the results for TbFe2 and HoFe2. Magnetic ordering from the neutron measurements was observed below Tc=305 K with a net moment of 1.3 μB/atom at T=0. The coercive force increased to 11.4 kOe at 5 K.

Observation of the Dispersion Relation for Spin Waves in Hexagonal Cobalt

The acoustic branch of the spin‐wave spectrum of hexagonal cobalt has been measured for values of the magnon wave vector q up to qc/2π=0.2 by observing the inelastic scattering of polarized neutrons by the diffraction method. The results indicate a decided departure from the dispersion law expected for a Heisenberg ferromagnet. In particular, if the energy is expressed as a sum of q2 and q4 terms, then the q4 term should only contribute 3% to the energy, whereas a contribution of 35% is observed. The coefficient of the q2 term yields an exchange stiffness parameter D=490±20 meV·A2.

Rare‐Earth Sublattice Canting in DyIG, ErIG, and YbIG

Neutron diffraction data taken on DyIG, ErIG, and YbIG at show superlattice reflections at low temperatures that indicate a decomposition of the rare‐earth sublattice. The data are interpreted with a model in which rare‐earth spins tilt away from [111] in {110} planes, with a double conical arrangement, although this model is not unique.

Spin‐Wave Scattering of Polarized Neutrons from Nickel and Cobalt

The polarized neutron technique provides a very powerful method of distinguishing ferromagnetic spin wave scattering from other elastic and inelastic intensity components. The diffraction method in which the size of the scattering surface is measured as a function of offset angle from the Bragg position has been employed to investigate the magnon scattering from nickel and cobalt single crystals. In the case of cobalt, the observed inelastic scattering proved to be entirely of magnon character for misset angles greater than 3.5°. For nickel, the constant D in the spin wave dispersion relation E=Dq2 was measured to be 360±75 meV A2.

Magnetic Structures of Ferrimagnetic RbNiF3 and CsFeF3

Powder neutron diffraction measurements have been taken on the hexagonal fluorides RbNiF3 and CsFeF3. The results are consistent with a ferrimagnetic model having antiparallel 2a and 4f site spins, the spin axis being perpendicular to and 75° from the c axis, respectively. Possible origins of the discrepancy of this latter value from magnetization data are discussed.