S. Brennan

Spin flop in goethite

A spin-flop transition is observed when a field of 20 T is applied parallel to the b axis of a specimen of natural goethite at 4.2 K. The goethite orders in a four-sublattice antiferromagnetic structure with the sublattice magnetizations inclined at +or-13 degrees to b, and an iron moment of 3.90 mu B. There is a small net ferromagnetic moment of 0.004 mu B along b which has the effect of more than doubling the threshold field for spin flop. The anisotropy field is deduced to be 0.11 T, corresponding to a uniaxial anisotropy energy K=6*104 J m-3. The origin of the weak moment is discussed, and it is suggested that mode superparamagnetism may arise in non-collinear antiferromagnets when two transverse spin configurations are effectively degenerate.

Anisotropy of easy-plane Y2Fe17, Y2Fe17N3 and Sm2Fe17

Abstract The magnetocrystalline anisotropy of the easy-plane compounds Y 2 Fe 17 , Y 2 Fe 17 N 3 and Sm 2 Fe 17 are measured. To simplify the analysis of the magnetisation data, the metallic powders are aligned in a magic cylinder device. Based on the Sucksmith-Thompson method, K 1 and K 2 are determined between 4.2 K and room temperature. The values are used to separate the transition metal anisotropy contribution from the rare-earth contribution in Sm 2 Fe 17 .

Non-equilibrium gas-phase nitrogenation

Non-equilibrium nitrogenation of transition-metal rich rare-earth intermetallics can be used to increase the nitrogen content beyond its equilibrium concentration. For example, nitrogenation of Sm/sub 2/Fe/sub 17/ using ammonia yields volume expansions up to 8.3 vol.%, which correspond to a composition Sm/sub 2/Fe/sub 17/N/sub x/ with x/spl ap/4. This effect is related to catalytic action at the metal surface where ammonia is decomposed into nitrogen and hydrogen, with nitrogen absorption on a site where it is weakly-bound, as an intermediate step. For overloaded Sm/sub 2/Fe/sub 17/N/sub x/ with x>3 the net reaction energy U/sub 0/=5/spl plusmn/10 kJ/mole indicates a nearly temperature-independent occupation of the non-9e sites. >

Magnetic properties of Sm2Fe17Nx, x = 3.9

Abstract Hydrogen decrepitation of Sm 2 Fe 17 followed by treatment in ammonia produces interstitial nitrides with nitrogen concentrations greater than three per formula unit. Measurements of the lattice parameters and magnetic properties are consistent with interstitial nitrogen occupation of the 9e and either the 3b or 18g sites. The effect of the extra nitrogen on the iron sublattice is to reduce the overall moment, particularly of the 18h sites. The magnetocrystalline anisotropy constants K 1 and K 2 indicate a significant decrease in the easy-axis character compared to Sm 2 Fe 17 N 3 material.

Determination of crystal-field parameters of Sm/sub 2/Fe/sub 17/N/sub 3/ and Sm/sub 2/Fe/sub 17/C/sub 3/

The crystal-field parameters A/sub 20/ and A/sub 40/ were determined from the magnetocrystalline anisotropy constants K/sub 1/ and K/sub 2/ in a linear approximation for Sm/sub 2/Fe/sub 17/Z/sub x/ (Z=N,C) compounds. For the materials investigated, the values of K/sub 1/ and K/sub 2/ in a temperature range from 4.2-533 K have been obtained by a fit of demagnetization curves. The parameters A/sub n0/ are calculated in two ways: (i) from the K/sub i/ at zero temperature and (ii) from the temperature dependence of the K/sub i/. Both methods give equivalent results. The value A/sub 20//spl ap/-400 Ka/sub 0//sup -2/ has been obtained which corresponds to an inverse THOMAS-FERMI screening length q/spl ap/2.3 /spl Aring//sup -1/ calculated within the framework of the screened charge model.

Magnetic properties of R2Fe17 treated with SiH4

Abstract Powders of R2Fe17 (R  Y, Sm) treated in silane form 2:17 compounds which exhibit a small lattice expansion ΔV/V ≈ 0.6%. Curie temperatures increase by 60 K but the anisotropy of the Sm2Fe17(Si) compounds remains easy-plane. It is inferred from Mossbauer spectra and SEM analysis that there is a little substitution of Si on Fe sites with the silicon concentrated very close to the surface.

Sm2Fe17Nx with discontinuous nitrogen profiles

Abstract Microprobe analysis and theoretical calculations based on Hookes law are used to investigate Sm 2 Fe 17 N x nitrides produced by N 2 and NH 3 interstitial modification. In the case of molecular nitrogen, stress and strain as well as Curie temperature, magnetization and anisotropy exhibit a smooth spatial variation, while nitrogen overloading due to the presence of ammonia leads to large excess strain and lattice incoherency. Discontinuous nitrogen profiles are associated with the occurrence of microcracks.

Is Sm2Fe17Nx a two-phase system?

Abstract Interstitial nitrides Sm 2 Fe 17 N x with x ≤ 3 form gas-solid solutions independent of whether NH 3 or N 2 is used to produce them. The volume of well homogenised samples follows Vegards law, and the Curie temperature is also linear in x . Ammonia facilitates the initial stage of the reaction by improving the effective surface activity of the metallic powder. A clear distinction is made between nitrides allowed to reach quasi-equlibrium at 425°C and those quenched to stop the reaction.