I. Nowik

New multiple magnetic phase transitions and structures in RMn2X2, X = Si or Ge, R = rare earth

Abstract Magnetometry and dilute 57 Fe Mossbauer spectroscopy studies of RMn 2 X 2 (X = Si or Ge, R = La, Ce, Pr, Nd, Sm and Gd) at temperatures 4.2–650 K yield the following results; Fe in RMn 2 X 2 is nonmagnetic. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields. The compounds LaMn 2 Si 2 , LaMn 2 Ge 2 , CeMn 2 Ge 2 , PrMn 2 Ge 2 , NdMn 2 Ge 2 and SmMn 2 Ge 2 , known to be ferromagnets with T C = 300–350 K, are antiferromagnetically ordered above their corresponding T C . Their T N values extend from 385 K (SmMn 2 Ge 2 ) to 470 K (LaMn 2 Si 2 ), similar to the T N values of the antiferromagnetic heavy rare earth compounds. At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In SmMn 2 Ge 2 we find five magnetic phase transitions, T C (Sm) = 30 K and T C (Mn) at 105 and 345 K and T N (Mn) at 155 and 385 K. In this compound, a superposition of two six-line 57 Fe Mossbauer patterns is seen between 90 and 155 K with changing relative intensities, indicating a competition of two easy magnetization axes, with an anisotropic transferred hyperfine field at the Fe nucleus. In NdMn 2 Ge 2 we find four phase transitions, T C (Nd) = 21 K, T C (Mn) = 335 K, T N (Mn) = 415 K, and one more very sharp transition at 210 K, associated with a discontinuity in 57 Fe hyperfine interaction parameters and a sharp drop in bulk magnetization; this seems to be a transition from pure ferromagnetism to canted antiferromagnetism. The results for antiferromagnetic CeMn 2 Si 2 , PrMn 2 Si 2 and GdMn 2 Ge 2 revealed no new phenomena and are in full agreement with previous magnetization studies. In GdMn 2 Ge 2 the transferred hyperfine field at the 57 Fe nucleus is smaller at 4.2 K (below the ordering temperature of Gd) than at 90 K, proving that the transferred hyperfine field from Gd is opposite to that produced by Mn.

Magnetism and hyperfine interactions of 57Fe, 151Eu, 155Gd, 161Dy, 166Er and 170Yb in RM4Al8 compounds (R = rare earth or Y, M = Cr, Mn, Fe, Cu)☆

Abstract Mossbauer and magnetic susceptibility studies of sixty tetragonal RM 4 Al 8 compounds ( R = 4 f , M = 3 d element), show a wide variety of magnetic phenomena in the behaviour of 3 d transition elements. The rare earths order antiferromagnetically at temperatures below 10–30 K in all compounds. The 3 d elements, however, all behave differently. Fe in R Fe 4 Al 8 has a localized moment (effective moment of 4.4 μ B ) and orders independently of the rare earth sublattice. Mn in R Mn 4 Al 8 has also a localized moment (∼1 μ B ) but orders only when the rare earths order. Cr in R Cr 4 Al 8 has no moment of its own, but it has an induced moment (.1 μ B ) by its magnetic rare earth neighbours. Cu in R Cu 4 Al 8 is nonmagnetic. The Mossbauer studies of 151 Eu, 155 Gd, 161 Dy, 166 Er, 170 Yb and a 57 Fe probe yield all hyperfine interaction parameters including the orientation of the hyperfine field relative to the crystallographic c -axis. In addition, the studies yield the Ce, Eu and Yb valencies in the various compounds. Eu in EuFe 4 Al 8 and in EuMn 4 Al 8 and Yb in YbCr 4 Al 8 are in a mixed valent state.

Mössbauer spectroscopy of 57Fe in high Tc superconductors YbA2Fe3xCu3(1−x)O7−δ

Abstract Mossbauer spectroscopy of 57 Fe in both tetragonal and othorhombic phases of YBa 2 ( Fe x Cu 1− x ) 3 O 7− δ , with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x =0.01 in the “quenched” tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x =0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.

Crystal structure magnetic properties and hyperfine interactions in RFe4Al8 (R = rare earth) systems

Abstract X-ray, magnetic susceptibility and 57Fe, 151Eu, 155Gd and 170Yb Mossbauer studies were performed. Detailed analysis of X-ray intensities yields all ion locations and interatomic distances in the body centered tetragonal structure (space group I4/MMM). The unit cell contains two formula units. The rare earth, iron and aluminum occupy the 2(a), 8(f) and 8(i) and 8(j) crystallographic sites, respectively. The susceptibility and Mossbauer studies indicate the existence of two independent magnetic sublattices. The iron sublattice orders into an antiferromagnetic structure at about 120 K, whereas the rare earth sublattice orders (excluding those with La, Ce, Eu, Y and Lu) antiferromagnetically at about 20 K. The 57Fe, 151Eu, 155Gd and 170Yb Mossbauer studies yield, in addition to the hyperfine interaction parameters, also the direction along which the moments are aligned. In EuFe4Al8 the Eu ion is in a mixed valent state.

Sonochemical synthesis of nanocrystalline LaFeO3

Nanocrystalline perovskite-type LaFeO3 with particle size of about 30 nm was prepared by a sonochemical method using iron pentacarbonyl and lanthanum carbonate as starting materials. The overall process involves three steps: formation of lanthanum carbonate using lanthanum nitrate and urea; reaction of the so-formed lanthanum carbonate with iron pentacarbonyl resulting in the formation of a precursor; calcination of the precursor to obtain nanocrystalline particles of LaFeO3. Transmission electron microscopy revealed the particles to have a mean size of about 30 nm. Study of the magnetic properties of nanocrystalline LaFeO3 particles shows a coercivity of ∼250 Oe, while the saturation magnetization is ∼40 memu g−1.

Influence of synthesis procedure on the YIG formation

The influence of synthesis procedure on the yttrium iron garnet (YIG; Y3Fe5O12) formation has been investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Mossbauer and magnetization measurements. The samples were prepared by coprecipitation or ceramic processing using the starting molar ratio Y2O3/Fe2O3=3:5. The fractions of Y2O3, α-Fe2O3, YFeO3 and YIG present in the samples depended on the method of materials processing and the calcination temperature. XRD of the thermally treated hydroxide coprecipitate at 1173 K showed the formation of YIG as a dominant phase, and YFeO3 and Y2O3 as associated phases, whereas upon heating at 1473 K, YIG and a small amount of YFeO3 were found. The samples produced by combining ball-milling of the starting powder and ceramic processing at 1573 K contained YIG and a smaller amount of YFeO3, as found by XRD. It was shown that high-energy ball-milling with stainless steel can be substituted by milling with agate bowl and balls, thus decreasing the contamination of the oxide system due to wear. FT-IR and 57Fe Mossbauer spectroscopic measurements were in agreement with XRD; however, the smaller amount of YFeO3 produced at 1573 K could not be detected with certainty by means of FT-IR and 57Fe Mossbauer spectroscopies. The magnetization values of end-products measured at 5 K were in agreement with their phase composition.

Local and itinerant magnetism and superconductivity in R Rh2si2 (R = rare earth)

Abstract Magnetization and Mossbauer studies reveal that R Rh 2 Si 2 ( R = rare earth) have two magnetic phase transitions, one corresponding to the ordering of the rare earth ( T N = 27−130 K ) and the other to the itinerant electron ordering of the Rh sublattice ( T M = 5−17 K ). LaRh 2 si 2 has also been studied by resistivity, specific heat and a.c. susceptibility measurements. All studies indicate that LaRh 2 Si 2 orders magnetically at T M = 7 K and becomes superconducting, type II, at T c = 3.8±0.2 K .

Itinerant and local magnetism, superconductivity and mixed valency phenomena in RM2Si2, (R = rare earth, M = Rh, Ru)°☆

Abstract X-Ray, magnetization and Mossbauer ( 151 Eu, 155 Gd, 161 Dy and dilute 57 Fe) studies of RM 2 Si 2 reveal that when R is a magnetic ion the compounds order antiferromagnetically. For M = Rh a second antiferromagnetic phase transition is observed, corresponding to Rh itinerant electron magnetic ordering. In EuRh 2 Si 2 the Eu ion is predominantly divalent with a mixed valent component. In EuRu 2 Si 2 the Eu is predominantly trivalent. LaRu 2 Si 2 and LuRu 2 Si 2 display enhanced electron paramagnetism and become superconducting at 3.5 K and 2.4 K respectively. LaRh 2 Si 2 , YRh 2 Si 2 and LuRh 2 Si 2 display an itinerant electron magnetic phase transition, T M (LaRh 2 Si 2 ) = 7 K, and at lower temperatures a superconducting phase transition, T c (LaRh 2 Si 2 ) = 3.8 K. There is evidence that in the superconducting phase the itinerant magnetic order survives.

Magnetic order and hyperfine interactions in RFe6Al6 (R = rare earth)

Abstract The systems R Fe 6 Al 6 ( R = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) crystallize in the tetragonal body centered I 4/ mmm structure. In striking contrast to the magnetic behaviour of R Fe 4 Al 8 (weakly coupled R and Fe sublattices, complicated magnetic structure, low T c ~ 130 K), in the R Fe 6 Al 6 systems all magnetic sublattices order simultaneously at a relatively high temperature. The magnetization curves start with low values at low temperatures and rise to very high values at T max ~ 230 K and then drop to 0 at T c ~ 330 K. All samples show strong hysteresis effects at temperatures just below T max . Mossbauer studies of 57 Fe in the ( f ) and ( j ) sites and 151 Eu, 155 Gd, 161 Dy, 166 Er and 170 Yb in the ( a ) site yield all hyperfine interaction parameters and temperature dependence of the local magnetic moments. All Mossbauer and magnetization experimental results can be explained in a self consistent way with a simple molecular field model. The Fe in the ( j ) site plays the dominant role in its strong intrasublattice ferromagnetic exchange and its strong antiferromagnetic exchange with the rare earth site. The Fe in the ( f ) site have an antiferromagnetic intrasublattice exchange, they have a canted strcuture with the ferromagnetic component parallel to the ( j ) sublattice magnetization.

Annealing study of Fe2O3 nanoparticles: Magnetic size effects and phase transformations

Sonochemically synthesized Fe2O3 nanoparticles were annealed in air or in vacuum while their magnetization was continuously recorded. Annealing in vacuum at temperatures Ta between 240 and 450 °C produced nanophases of γ-Fe2O3 with average particle size ranging from 4 to 14 nm, depending on Ta. Phase transformation into α-Fe2O3 occurred directly by annealing in air, or via an intermediate Fe3O4 phase by annealing in vacuum at temperatures higher than 450 °C. Mapping the correlation between the magnetic properties and the annealing conditions, enables control of the annealing process to obtain nanocrystals of γ-Fe2O3, α-Fe2O3, or Fe3O4 with different particle size and magnetic properties.