Ferrimagnetism as a Consequence of Unusual Cation Ordering in the Perovskite SrLa2FeCoSbO9.

A polycrystalline sample of SrLa2FeCoSbO9 has been prepared in a solid-state reaction and studied by a combination of electron microscopy, magnetometry, Mössbauer spectroscopy, X-ray diffraction, and neutron diffraction. The compound adopts a monoclinic (space group P21/ n; a = 5.6218(6), b = 5.6221(6), c = 7.9440(8) Å, ß = 90.050(7)° at 300 K) perovskite-like crystal structure with two crystallographically distinct six-coordinate sites. One of these sites is occupied by 2/3 Co2+, 1/3 Fe3+ and the other by 2/3 Sb5+, 1/3 Fe3+. This pattern of cation ordering results in a transition to a ferrimagnetic phase at 215 K. The magnetic moments on nearest-neighbor, six-coordinate cations align in an antiparallel manner, and the presence of diamagnetic Sb5+ on only one of the two sites results in a nonzero remanent magnetization of ∼1 µB per formula unit at 5 K.

The magnetic structures of GdCuSn, GdAgSn and GdAuSn.

We have determined the magnetic structures of GdCuSn, GdAgSn and GdAuSn using a combination of [Formula: see text]Gd Mössbauer spectroscopy and neutron powder diffraction. Each compound shows the same antiferromagnetic ordering of the Gd sublattice. The magnetic cell is doubled along the crystallographic a-axis (propagation vector [Formula: see text]) with the moments aligned along the hexagonal c-axis, forming alternating ferromagnetic sheets of up/down Gd moments along the a-axis.

Magnetic structure and spin reorientation of quaternary Dy2Fe2Si2C.

We have investigated the low temperature magnetic properties of Dy2Fe2Si2C by using magnetisation, specific heat, x-ray diffraction, neutron powder diffraction and 57Fe Mössbauer spectroscopy measurements over the temperature range 1.5 K-300 K. Dy2Fe2Si2C exhibits two magnetic transitions at low temperatures: an antiferromagnetic transition at [Formula: see text] K and a spin-reorientation transition at [Formula: see text] K. The magnetic structure above T t can be described with a propagation vector [Formula: see text] with the ordering of the Dy magnetic moments along the monoclinic b-axis whereas on cooling below T t the Dy moment tips away from the b-axis towards the ac-plane. We find that the spin-reorientation in Dy2Fe2Si2C is mainly driven by the competition between the second-order crystal field term B 20 and the higher-order terms, in particular B 40 and B 64.

Complex incommensurate helicoidal magnetic ordering of EuNiGe3.

(151)Eu Mössbauer spectroscopy and neutron powder diffraction are combined to show that the tetragonal (I4mm #107) compound EuNiGe3 orders magnetically below [Formula: see text] K and adopts a complex incommensurate helicoidal magnetic structure at 3.6 K, with a propagation vector [Formula: see text] and a Eu moment of 7.1(2) [Formula: see text]. On warming through 6 K an incommensurate sinusoidal modulation develops and dominates the magnetic order by 12 K.

Europium and manganese magnetic ordering in EuMn2Ge2.

The antiferromagnetic structures of both the manganese and europium sublattices in EuMn2Ge2 have been determined using thermal neutron diffraction. T(N)(Mn) = 714(5) K with the 3.35(5) µ(B) (at 285 K) Mn moments ordering according to the I4'/m'm'm space group. The Eu order is incommensurate with the 6.1(2) µ(B) (at 3.6 K) Eu moments oriented parallel to the c-axis with a propagation vector of k = [0.153(2) 0 0]. Both neutron diffraction and (151)Eu Mössbauer spectroscopy reveal evidence of magnetic short-range ordering of the Eu sublattice around and above T(N)(Eu) ∼ 10 K. The ordering temperature of the Eu sublattice is strongly affected by the sample's thermal history and rapid quenching from the melting point may lead to a complete suppression of that ordering.

The magnetic structure of EuCu2Sb2.

Antiferromagnetic ordering of EuCu2Sb2 which forms in the tetragonal CaBe2Ge2-type structure (space group P4/nmm #129) has been studied using neutron powder diffraction and (151)Eu Mössbauer spectroscopy. The room temperature (151)Eu isomer shift of -12.8(1) mm s(-1) shows the Eu to be divalent, while the (151)Eu hyperfine magnetic field (B(hf)) reaches 28.7(2) T at 2.1 K, indicating a full Eu(2+) magnetic moment. B(hf)(T) follows a smooth S = 7/2 Brillouin function and yields an ordering temperature of 5.1(1) K. Refinement of the neutron diffraction data reveals a collinear A-type antiferromagnetic arrangement with the Eu moments perpendicular to the tetragonal c-axis. The refined Eu magnetic moment at 0.4 K is 7.08(15) µ(B) which is the full free-ion moment expected for the Eu(2+) ion with S = 7/2 and a spectroscopic splitting factor of g = 2.

Determination of the magnetic structure of Gd2Fe2Si2C by Mössbauer spectroscopy and neutron diffraction.

We have determined the magnetic structure of the intermetallic compound Gd2Fe2Si2C using neutron powder diffraction, (155)Gd and (57)Fe Mössbauer spectroscopy. This compound crystallizes in a monoclinic (C2/m) structure and its magnetic structure is characterized by antiferromagnetic order of the Gd sublattice along the b-axis, with cell-doubling along the c-axis. The propagation vector is k = [0 0 ½]. At 3.6 K the Gd moment reaches 6.2(2) µ(B). Finally, (57)Fe Mössbauer spectroscopy shows no evidence of magnetic ordering of the Fe sublattice.

Magnetic ordering in TmGa.

We have determined the magnetic structure of the intermetallic compound TmGa by high-resolution neutron powder diffraction and (169)Tm Mössbauer spectroscopy. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure and its magnetic structure is characterized by magnetic order of the Tm sublattice along the a-axis. The initial magnetic ordering occurs at 15(1) K and yields an incommensurate antiferromagnetic structure described by the propagation vector k1 = [0 0.275(2) 0]. At 12 K the dominant ferromagnetic ordering of the Tm sublattice along the a-axis develops in what appears to be a first-order transition. At 3 K the magnetic structure of TmGa is predominantly ferromagnetic but a weakened incommensurate component remains. The ferromagnetic Tm moment reaches 6.7(2) µB at 3 K and the amplitude of the remaining incommensurate component is 2.7(4) µB. The (169)Tm hyperfine magnetic field at 5 K is 631(1) T.

Ferrimagnetism in GdCo(12-x)Fe(x)B6.

The effects of iron substitution on the structural and magnetic properties of the GdCo(12-x)Fe(x)B6 (0 ≤ x ≤ 3) series of compounds have been studied. All of the compounds form in the rhombohedral SrNi12B6-type structure and exhibit ferrimagnetic behaviour below room temperature: T(C) decreases from 158 K for x = 0 to 93 K for x = 3. (155)Gd Mössbauer spectroscopy indicates that the easy magnetization axis changes from axial to basal-plane upon substitution of Fe for Co. This observation has been confirmed using neutron powder diffraction. The axial to basal-plane transition is remarkably sensitive to the Fe content and comparison with earlier (57)Fe-doping studies suggests that the boundary lies below x = 0.1.

The magnetic structure of EuPdSn.

The TiNiSi-type structure, antiferromagnetic ordering and divalent state of europium in EuPdSn have been confirmed by neutron powder diffraction. The Néel temperature is 16.2(3) K. The magnetic diffraction peaks can be indexed with a propagation vector k = [0, 0.217, q(z)] (q(z) ≤ 0.02) at 13.2 K, and k = [0, 0.276, 0] at 3.6 K, indicating an incommensurate antiferromagnetic structure at both temperatures. At 13.2 K, the best refinement is obtained with a sinusoidally modulated magnetic structure and europium magnetic moments oriented in the (a,b) plane with an azimuthal angle ϕ of 66(4)°relative to the a-axis. By 3.6 K, the magnetic structure of EuPdSn has transformed to an (a,b) planar helimagnetic structure (a 'flat spiral').

Magnetic ordering in GdAgSb2.

GdAgSb(2) has been studied using (155)Gd Mössbauer spectroscopy and neutron powder diffraction at a wavelength of 2.3672(1) Å. Commensurate antiferromagnetic order develops below T(N) = 13.8(4) K with the 6.2(3) µ(B) Gd moments lying in the ab-plane of the tetragonal cell. The magnetic ordering is characterized by a [½ 0 0] propagation vector (i.e. the magnetic cell is doubled along one of the crystallographic basal plane axes) with the Gd moments oriented perpendicular to the doubled direction. These results are fully consistent with an earlier determination by x-ray resonant magnetic exchange scattering.

A study on the magnetic behaviour of polymorphic YbFe6Ge6.

The intermetallic compound YbFe(6)Ge(6) adopts two very closely related hexagonal crystal structures, HfFe(6)Ge(6)-type and YCo(6)Ge(6)-type. In both structures the Fe sublattice orders antiferromagnetically at 485(2) K. The Yb sublattice does not order magnetically, down to 1.5 K. In the HfFe(6)Ge(6)-type structure, the Fe magnetic moments undergo a spin reorientation away from the C-axis upon cooling, commencing at around 60 K, whereas in the YCo(6)Ge(6)-type structure the Fe moments remain ordered along the C-axis, or quite close to it.

Valence change and magnetic order in YbMn6Ge(6-x)Sn(x).

The YbMn(6)Ge(6-x)Sn(x) compounds (0 < x < 6) have been investigated using x-ray diffraction, magnetic measurements, neutron diffraction and (170)Yb Mössbauer spectroscopy. The YbMn(6)Ge(6-x)Sn(x) system comprises three solid solutions: (i) 0 < x ≤ 1.1, (ii) 3.2 ≤ x ≤ 4.6 and (iii) 5.3 ≤ x < 6, all of which crystallize in the hexagonal (P6/mmm) HfFe(6)Ge(6)-type structure. The substitution of Sn for Ge yields changes in the type of magnetic order (antiferromagnetic, helimagnetic, ferromagnetic, conical and ferrimagnetic), in the easy magnetization direction (from easy axis to easy plane) as well as in the valence state of Yb (from trivalent to divalent). The Mn moments order at or above room temperature, while magnetic ordering of the Yb sublattice is observed at temperatures up to 110 K. While Yb is trivalent for x ≤ 1.1 and divalent for x ≥ 5.3, both magnetic and (170)Yb Mössbauer spectroscopy data suggest that there is a gradual reduction in the average ytterbium valence through the intermediate solid solution (3.2 ≤ x ≤ 4.6), and that intermediate valence Yb orders magnetically, a very unusual phenomenon. Analysis of the (170)Yb Mössbauer spectroscopy data suggests that the departure from trivalency starts as early as x = 3.2 and the loss of ytterbium moment is estimated to occur at an average valence of ∼2.5+.

Neutron powder diffraction determination of the magnetic structure of Gd(3)Ag(4)Sn(4).

Natural gadolinium is the strongest neutron-absorbing element and neutron diffraction studies of Gd-containing materials rely on the use of either enriched Gd isotopes or short neutron wavelengths where the absorption is weaker but, unfortunately, the neutron flux is also weak. We have employed a new sample-mounting technique to obtain neutron powder diffraction patterns from the intermetallic compound Gd(3)Ag(4)Sn(4) containing natural Gd, at a neutron wavelength of ∼ 2.37 Å where there is much greater flux. Here, we report the magnetic structure of Gd(3)Ag(4)Sn(4). The magnetic ordering temperature is 28.8(2) K. At 2.8 K the Gd(4e) sublattice is antiferromagnetically ordered along the crystal c-axis, commensurate with the crystal lattice. The Gd(2d) sublattice is also ordered along the c-axis but its magnetic structure is incommensurate with the crystal lattice.

Structural transitions in RNi(10)Si(2) intermetallics.

Intermetallic compounds of the type RFe(10)Si(2) and RCo(10)Si(2) crystallize in the ThMn(12) structure (space group I4/mmm) whilst the heavy rare earth series RNi(10)Si(2) crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi(10)Si(2) and ErNi(10)Si(2) which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 °C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients α(11), α(33) and α(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order-disorder and displacive transitions, ß = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.