M. Bacmann

Magnetoelastic transition and antiferro-ferromagnetic ordering in the system MnFeP1-yAsy

Abstract Neutron diffraction experiments have been performed on a range of hexagonal structures in the MnFeP 1− y As y system which exhibit a magnetoelastic transformation. Local transformations accompanying both the hexagonal crystal structure and the magnetic ordering have been measured. The anisotropic changes observed in the unit cell parameters arise primarily from the expansion of the shortest Fe-Fe distance when the compounds transform from the antiferro- to the ferromagnetic state. Substitution of As for P favours such as a transition via the preferential expansion of selected basal interatomic bond lengths. The incommensurate type of antiferromagnetic ordering indicates some competing interatomic exchange interactions. The net result of these phenomena is that the magnetic moment of iron increases markedly and adopts a more localized weak ferromagnetic state.

Magnetism of Fe2P investigated by neutron experiments and band structure calculations

Abstract Neutron diffraction and magnetisation experiments have been performed on powdered Fe 2 P. The electronic band structure of Fe 2 P and the previously measured FeMnP 1- x As x has been computed. The fully self-consistent KKR-CPA method was used in all calculations. The magnetic structure of Fe 2 P was refined with the new neutron data and ab initio calculated magnetic form factors.

Crystallographic and magnetic properties of fe2p

Summary The crystallographic and magnetic properties of Fe 2 P have been studied by neutron diffraction experiments, magnetization measurements and KKR method for band structure calculations. The measured atomic magnetic moments of tetrahedral Fe(3f) and pyramidal Fe(3g) sites are respectively (0.59±0.02)μ B and (2.22±0.01)μ B , resulting in a net moment of (2.82±0.03)μ B . This value is very close to that obtained from saturation magnetization measurements (H=150kOe, T=4K) which yield 2.87μ B per formula unit. The results obtained from the KKR method are 0.80, 2.33, −0.03(1), −0.02(8)μ B for Fe and P sites respectively. The values are comparable with the measured atomic magnetic moments. Thermal evolution of magnetic moments, magnetization at saturation and cell parameters presents a sharp drop at the Curie temperature (217±2)K indicating a first order ferro - paramagnetic transition.

Single crystal neutron diffraction investigations of the crystal and magnetic structures of R2Fe14B (R=Y, Nd, Ho, Er)

Abstract The compounds of general formula R 2 Fe 14 B form a series of magnetic materials, the prototype of which, Nd 2 Fe 14 B, is used to make high performance permanent magnets. These compounds crystallise in the tetragonal space group P4 2 /mnm and with R=Nd, Ho, Er, Tm and Yb a spin reorientation transition was found. Precise structural analyses using neutron diffraction on single crystals were undertaken. It is clearly demonstrated that lowering of symmetry takes place for non-axial compounds. Besides, large deviations to collinearity of the magnetic arrangements were determined.

Influence of hydrogen on the structural and magnetic properties of the RFe10.5Mo1.5 compounds (R=rare earth)

Abstract The structural and magnetic properties of RFe 10.5 Mo 1.5 alloys and their hydrides were studied using X-ray diffraction and magnetic measurements. Hydrogen occupies the octahedral interstitial 2b site of the tetragonal ThMn 12 -type structure (space group I4/mmm, Z = 2 ), which allows a maximum uptake of one hydrogen atom per formula unit. The unit cell volume expands about 1% and the Curie temperature is increased about 10% by the hydrogen uptake. The mean magnetic moment of the iron sublattices is increased. On hydrogenation, (i) the magnetocrystalline anisotropy of the 3d sublattice is enhanced, (ii) in the compounds containing the R elements with a positive second-order Stevens factor α J , the c -axis is the easy axis, and (iii) for the compounds with R having a negative second-order Stevens factor, the easy direction lies in the ab -plane. The Dy and Tb alloys show temperature induced spin reorientation transitions (SRT). On hydrogen uptake, their hydrides have a basal plane behaviour. This change in the series can be understood by the donor character of H together with a narrowing of the 3d metal bands induced by the volume expansion.

Structure and magnetism in the polymorphous MnFeAs

Abstract Two crystallographic phases of MnFeAs were stabilised: the tetragonal Fe 2 As-type structure or the hexagonal Fe 2 P-type one, if synthesis is performed under normal conditions, or high pressure and temperature. The structural, magnetic and electronic properties of both MnFeAs polytypes are investigated using magnetisation, neutron diffraction measurements as well as band structure calculations by the Korringa–Kohn–Rostoker method. The tetragonal phase is antiferromagnetic ( T N =470 K) with the magnetic cell doubled along c -axis, with respect to the chemical one. The local magnetic moment on Mn(2c) refined from neutron study is 3.36 μ B , while no significant magnetic moment is observed on the Fe(2a) site. The hexagonal phase is ferromagnetic with T C near 190 K. The magnetic moments determined by neutron diffraction are 3.14 and 1.54 μ B on Mn(3g) and Fe(3f), respectively. Upon applying a magnetic field as large as 10 T the saturation magnetisation of 4.5 μ B /f.u. is measured at 5 K. From non-polarised KKR calculations we deduce, that the Fermi level falls into a deep minimum of density of states (DOS) in the tetragonal as well as in the hexagonal variants of MnFeAs, which may tentatively explain the structural stability of both phases. The spin-polarised band structure study of the tetragonal MnFeAs (AFM) shows DOS splitting exclusively on the Mn(2c) sites, resulting in the local magnetic moment of 3.42 μ B , unlike the hexagonal MnFeAs (FM) where the magnetic moments of 3.13 and 1.10 μ B are found on Mn(3g) and Fe(3f), respectively. Moreover, the influence of lattice parameter variation on electronic and magnetic properties is discussed based on total energy KKR calculations.

Magnetic structures and magnetoresistivity of MnRhAs

Abstract Measurements of the magnetisation, resistivity, magnetoresistivity and neutron diffraction experiments have been made on a powder sample of MnRhAs. The compound exhibits a metamagnetic behaviour below 160 K and shows both ferro- and antiferromagnetic couplings between 160 and 200 K. The magnetoresistivity reflects both transitions. Above T c = 200 K strong magnetic correlations are pointed out.

Magnetic phase diagram of the (Fe1−xMnx)2P system

Abstract The ac susceptibilities of the (Fe 1− x Mn x ) 2 P series have been measured in the range 77–300 K. The magnetic ordering temperatures agree well with previous results. For the iron-rich compositions, anomalies in X ′ and X ″ ( T ) indicate the existence of a new magnetic phase. Comparison of ac and dc susceptibility results with those of neutron diffraction allows the main exchange interactions to be characterized as a function of composition.

Magnetic study of the hexagonal FeMnP1-xAsx system

Abstract For the hexagonal phase FeMnP 1- x As x , neutron diffraction and Mossbauer spectroscopy results are compared. The magnetoelastic transition is accompanied by a drastic change in the 57 Fe hyperfine field that is more than twice as large in the ferro (F) than in the antiferromagnetic (AF) state. Such trends exist in FeMnX (X = As, P) and can be related to the changes in the FeFe distances.

A magnetic Kosterlitz-Thouless transition in quasi 2-d MnRhAs?

Abstract For the magnetic 2-d compound MnRhAs we find a Cp anomaly at Tp=(240.7±0.3) K, where χ is continuous, and a χ divergence at Tc=(190.5 ± 0.5) K, with no accompanying Cp peak. The results are analyzed in terms of the Kosterlitz-Thouless transition model. Comparisons with Monte Carlo simulations are performed.