R. Nathans

Spin Waves in 3d Metals

A review is given of recent measurements of spin‐wave dispersion relations of the 3d metals by the Brookhaven neutron diffraction group using the diffraction technique and triple‐axis spectrometry. The parameters D and β in the relation ħω = Dq2(1 − βq2) have been determined at 295°K for Fe, Co, Ni, and some of their alloys. These values are compared with those obtained by thin‐film resonance and small‐angle scattering.The most extensive measurements were carried out on Fe using a triple‐axis spectrometer. The dispersion relation was measured along the three principal symmetry directions for wavevectors up to q/qmax = 0.4. The stiffness constant D as well as the linewidth of selected spin waves were studied for the temperature range between 77°K and the Curie temperature, 1042°K. Well‐defined magnons were observed up to a reduced temperature T/TC = 0.995, but not above TC.

Neutron-diffraction study of antiferromagnetic FeTi03 and its solid solutions with α-Fe2O3☆

Abstract While FeTiO 3 and α-Fe 2 O 3 are both antiferromagnetic, some solid solutions of the two possess strong magnetic moments which are believed to be related to an ordering of the Fe and Ti atoms. Neutron powder patterns confirm the X-ray picture of FeTiO 3 as alternating layers of Fe and Ti along the [111] axis of the rhombohedral cell with intervening oxygen layers and provide more accurate oxygen parameters. Data taken below the Neel point of 68°K suggest a model for the spin structure in which Fe 2+ moments are ferromagnetically coupled within a (111) sheet and directed perpendicular to it. Studies on the solid solutions connect the presence of the spontaneous moment with ordering of the Ti and Fe atoms and show that the spin arrangement in most of these compositions is of the α-Fe 2 O 3 type with spins ferromagnetically coupled and lying within (111) sheets. The fact that no long-range ordered spin structure is observed down to 4.2°K for the sample containing 12 per cent hematite can be explained on the basis of an inhomogeneous magnetic structure.

Magnetic Structure of Cr2O3

Pratt and Bailey have recently explained the optical and anomalous magnetic properties of Cr2O3 by means of a model consisting of the basic Cr2O3 antiferromagnetic structure with moments canted away from the c axis and forming some kind of spiral. Our neutron powder‐diffraction data confirm the original Brockhouse moment configuration but indicate that the c‐axis component of the Cr moment is 8% lower than the spin‐only value. This result can be interpreted as supporting the canted model of Pratt and Bailey. A careful search has been made, using single crystals, to find evidence for an ordered or spiraling perpendicular component. Polarization studies establish that there is no magnetic contribution to the fundamental reflections other than that arising from the c‐axis components. A survey of reciprocal space revealed extra spots at forbidden positions, but these could be accounted for semiquantitatively in terms of double‐Bragg scattering. We conclude, therefore, that either the Cr moment is intrinsicall...

Antiferromagnetic structure of EuTe

Abstract EuTe has been shown by neutron diffraction to undergo at 7.8°K a transition to anti-ferromagnetism. The antiferromagnetic ordering is of the second kind with the spins lying within the (111) planes. The temperature dependence of the magnetic scattering was analyzed by adding a biquadratic exchange interaction to the usual bilinear term.

Neutron Diffraction Investigation of Pt‐Based Alloys of the First Transition Series

Polarized neutron diffraction measurements show that the radial spin density of the Mn atom in ferromagnetic MnPt3 is contracted relative to the Mn2+ distribution, while that of Cr in ferrimagnetic CrPt3 is more expanded. An antiferromagnetic structure was detected in the ordered CrPt phase at room temperature in which the spins are antiferromagnetic within each layer.

Inelastic Magnetic Scattering from RbMnF3 in the Neighborhood of its Néel Point

A study of the inelastic neutron scattering from antiferromagnetic RbMnF3 in the vicinity of the Neel point reveals the presence of a diffusive‐type peak, accompanied by spin‐wave modes that persist above the transition temperature. The width of the diffusive peak for small‐momentum transfer is found to be strongly temperature‐dependent above TN, indicating a slowing down of the spin fluctuations near the transition.

Hyperfine Fields and Magnetic Moments in the Fe–Rh System

Hyperfine fields and isomer shifts of Fe57 in the Fe–Rh system have been measured by the Mossbauer technique in the composition range between 0 and 52 at. % Rh. When the Rh concentration exceeds 25%, chemical ordering of CsCl type develops and two distinct hyperfine fields are observed, corresponding to Fe on Fe sites (Fe I) and Fe on Rh sites (Fe II). For example, Hi(I) = 277 kOe and Hi(II) = 384 kOe in an alloy containing 48% Rh. Neutron diffraction study of this alloy gives μFeI = 3.2 μB and μRh = 0.9 μB. It is suggested that Fe II, which is surrounded by 8 Fe nearest neighbors, may have a smaller magnetic moment than Fe I despite its higher hyperfine field. These results are consistent with a model in which the polarization of conduction electrons is negative and its magnitude is reduced when rhodium replaces iron as nearest neighbors.

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.

NEUTRON MAGNETIC SCATTERING FROM F.C.C. IRON ALLOYS

Paramagnetic diffuse scattering of thermal neutrons by two gamma phase iron alloys (Fe/sub 0.88/Mn/sub 0.12/ and Fe/sub 0.71/Cr/sub 0.18/ Ni/sub 0.11/) measured at room and elevated temperatures, indicates that the iron carries a moment of approximately one-half Bohr magneton. Measurement of the coherent scattering at 4.2 deg K showed no evidence for antiferromagnetism in the stainless steel. (auth)