Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

The Nonanuclear [Mo(IV){(CN)Fe(III)(3-ethoxy-saldptn)}8]Cl4 Complex Compound Exhibits Multiple Spin Transitions Observed by Mössbauer Spectroscopy

The hyperfine interactions at ¹⁸¹Ta ions on Fe³⁺ sites in α-Fe₂O₃ (hematite) were studied in the temperature range 11–1100 K by means of the perturbed angular correlation (PAC) technique. The ¹⁸¹Hf(β⁻)¹⁸¹Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B_hf and the electric field gradient (EFG) at the impurity sites. The angles between B_hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using ¹¹¹Cd probe. ^aAlso at Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

Decoupling experiments for the study of electric and magnetic hyperfine interactions in ferromagnetic compounds

The decoupling experiments involving the time-differential perturbed angular correlation (TDPAC) of the 133–482 keV γ-γ cascade in the presence of an external magnetic field applied along the quantization axis have been performed to measure the electric quadrupole and the magnetic hyperfine interactions experienced by the¹⁸¹Ta nuclei at Hf sites in the pseudobinary compounds HfFe_2-xSi_x, withx=0.1 andx=0.3. The hyperfine magnetic fields measured at 298 K areH _hf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe_1.9Si_0.1 andH _hf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe_1.7Si_0.3. The decoupling technique has also been used to obtain a —ve sign for the hyperfine field experienced by¹⁸¹Ta nuclei at the Ti or Hf sites in the Heusler compound Co₂Ti_0.8Hf_0.2Sn and a+ve sign for the hyperfine field at Zr sites in the cubic (C15) Laves phase compound ZrFe₂.     

Temperature dependence of electric field gradient in TbCoO3

The temperature dependence of the electric field gradient (efg) in TbCoO₃ perovskite was measured by perturbed angular correlation (PAC) technique using ¹¹¹Cd and¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced into the compound through a chemical process during sample preparation. The electric quadrupole interactions at ¹¹¹Cd show two different sites, assigned to probe substituting Tb and Co atoms. The temperature dependence of quadrupole frequencies show sharp discontinuities which have been interpreted as thermally activated spin state transitions from low-spin ground state configuration to the intermediate-spin state and from intermediate-spin to high-spin state of Co^3?+? ion. For ¹⁸¹Ta only one interaction was observed, which was assigned to probe at Co site. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out. No magnetic order was observed till 77 K.     

Perturbed angular correlation study of a nanostructured HfO2 film

The hyperfine field at ¹⁸¹Ta lattice sites in a nanostructured HfO₂ thin film was studied by the perturbed angular correlation (PAC) technique. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The thickness was about 25 nm. The radioactive ¹⁸¹Hf ions were produced by neutron activation of the very thin film in the Portuguese research reactor by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out at room temperature after annealing at different temperatures up to 1,473 K in air. The PAC technique allows determining the electric field gradient at the ¹⁸¹Ta probe sites. The ¹⁸¹Ta isotopes appear in the sample as disintegration product of ¹⁸¹Hf.     

Hyperfine interaction experienced by181Ta nuclei at the Hf site in HfFe2−xSix compounds through a decoupling experiment

The time-differential perturbed angular correlation (TDPAC) experiments involving the 133–482 keV γ-γ cascade in¹⁸¹Ta have been performed in the presence of an external magnetic field along the quantization axis to measure the hyperfine magnetic field experienced by¹⁸¹Ta nuclei at the Hf site in the pseudobinary compounds HfFe_2?xSi_x with x=0.1 and x=0.3. The hyperfine magnetic fields measured at 298 K are H_hf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe_1.9Si_0.1 and H_hf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe_1.7Si_0.3. The measured hyperfine fields are discussed within the framework of the Campbell-Blandin model.     

Study of hyperfine interactions in the intermetallic compound CePd2Si2 using PAC technique with 111Cd as probe nuclei

Perturbed γ???γ angular correlation spectroscopy (PAC) has been used to investigate the hyperfine interactions in the intermetallic compound CePd₂Si₂ using ¹¹¹In→¹¹¹Cd probe nuclei. Samples of CePd₂Si₂ were prepared by melting constituent elements in an arc furnace under pure argon atmosphere. Carrier-free ¹¹¹In nuclei were introduced into the samples by thermal diffusion at 800°C in vacuum during 12 h. The measurements were performed in the temperature range of 4.2–300 K. Above the magnetic transition temperature (T _N ?=?10 K), the results show two distinct and well defined quadrupole interactions that were assigned to probe nuclei occupying Ce and Si sites in the compound. The quadrupole frequencies were found to decrease linearly with increasing temperature. The PAC spectra taken below 10 K were analyzed with a model including combined electric quadrupole plus magnetic dipole interactions, from which the hyperfine magnetic field was determined.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Perturbed Angular Correlation Study of Magnetic and Electric Hyperfine Interactions at 111Cd in GdNi2 and SmNi2

The magnetic hyperfine field B _hf of the closed-shell probe nucleus ¹¹¹Cd in the C15 Laves phase RNi₂ (R= Gd, Sm) has been investigated as a function of temperature by perturbed angular correlation (PAC) spectroscopy. The saturation magnetic hyperfine fields at 10 K are B _hf=7.7(2) and 3.9(1) T for GdNi₂ and SmNi₂, respectively. Although the probe nucleus resides on the cubic rare-earth site, a strong axially symmetric electric quadrupole interaction (QI) is observed in the paramagnetic phase at T300 K. The possible relation of this unexpected QI to the structural instability of RNi₂ is discussed.     

Hydrogen and deuterium trapping at nitrogen in tantalum studied by PAC

PAC experiments using¹⁸¹Hf as probe atom were performed in Ta samples containing 1.1 at% of N and loaded with 0.2 at% of H or of 1.6 at% of D. At room temperature only the known frequency due to N is present. Below 100K or 140K, for H and D respectively, the amplitude of this interaction decreases and two new interaction frequencies characterized by V₀=240MHz, η=0.80 and V₀=250 MHz, η=0.40 appear. These frequencies are attributed to H(D) trapped by N in the vicinity of the probe atom¹⁸¹Hf. The disappearance of these interactions at 100K and 140K is interpreted as a release of H (D) from Hf but not from N. From the data the jump rates of H(D) around N in Ta are derived.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

Hf3Al2 and Zr3Al2 Isostructural Aluminides Studied by PAC with 181Ta and 111Cd Probes

The electric quadrupole hyperfine interactions for ¹⁸¹Hf/¹⁸¹Ta and for ¹¹¹In/¹¹¹Cd probes in polycrystalline Zr₃Al₂ and Hf₃Al₂ compounds were measured in the temperature range 24–1100 K. The hyperfine quadrupole interaction parameters were determined after different doping techniques and heat treatments of the samples.¹⁸¹Hf/¹⁸¹Ta was found to substitute the Hf/Zr sites and the ¹¹¹In/¹¹¹Cd impurities appear to substitute both the 8(j) Al sites and the three nonequivalent Hf/Zr sites.     

Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe ¹⁸¹Hf(¹⁸¹Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 10¹² at/cm². PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000oC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 340 MHz for 30% of the probe nuclei. Annealing up to 600oC reduces the damping of this frequency without an increase of the probe atom fraction f_s in these sites. Above 600oC f_s grows rapidly until after the 900oC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of ν_Q= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800oC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000oC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

PAC detection of oxide formation in zirconium and hafnium hydrides

The formation of HfO₂ and ZrO₂ oxides in Hf and Zr hydrides has been detected by¹⁸¹Ta PAC spectroscopy. The oxides were identified by their¹⁸¹Ta quadrupole frequency and asymmetry parameter. The growth of the oxide contamination in a H₂ atmosphere has been investigated as a function of temperature and time.     

Magnetic fields at 181Ta nuclei in Laves phases of RFe2 (R=Nd, Pr, Sm, Gd, Dy, Yb, Lu)

By studying the perturbed angular correlations of γ-rays emitted during the decay of ¹⁸¹Hf impurities in the Laves phases of PrFe₂, DyFe₂, and YbFe₂, we have investigated the magnetic hyperfine interaction between these compounds and its daughter nucleus ¹⁸¹Ta, and have determined the temperature dependence of the magnetic hyperfine fields. At room temperature we obtained the following values of these magnetic hyperfine fields B _hf:B _hf(PrFe₂)=7.6(1) T, B _hf(DyFe₂)=15.5(5) T, and B _hf(YbFe₂)=18.8(3) T. When taken together with data obtained previously, the results of our experiments show that for Ta nuclei in the RFe₂ Laves phases the values of B _hf depend strongly on whether R is a light or a heavy rare-earth element, which allows us to conclude that in these phases the value of the magnetic moment induced at the impurity Ta nuclei depends on the interatomic distance. Zh. éksp. Teor. Fiz. 111, 1085–1091 (March 1997)     

Mössbauer spectroscopical investigation of the exchange biased Fe/MnF2 interface

Two different Fe/MnF₂ samples have been prepared by e-beam evaporation on MgO(001) substrates. The Fe layer in the samples includes a 10 Å thick ⁵⁷Fe probe layer either at the Fe/MnF₂ interface (interface sample) or 35 Å away from the interface (center sample). The samples are characterized by X-ray diffraction, conversion electron Mössbauer spectroscopy (CEMS) and SQUID magnetometry. ⁵⁷Fe CEMS has been employed to study the depth dependent hyperfine interactions in Fe/MnF₂ as a function of temperature between 18 K to 300 K. The hyperfine field B _hf has been obtained for the interfacial and off-interfacial ⁵⁷Fe layers. At the interface, besides B _hf of bcc-Fe, the presence of a component with a distribution P(B _hf ) is observed. The latter is assigned to interfacial ⁵⁷Fe atoms, indicating some (～15%, equivalent to ～1 Fe atomic layer) intermixing at the Fe/MnF₂ interface and a decrease of the average hf > by 21%. The influence of the interface disappears as the ⁵⁷Fe probe layer is placed away from the interface. The temperature dependence of the average hf > of the interface has been measured. The Fe spins, at remanence, are found to lie in the film plane.     

Characterization of nanostructured HfO2 films using Perturbed Angular Correlation (PAC) technique

The hyperfine field at ¹⁸¹Ta lattice sites in nanostructured HfO₂ thin films was studied by the Perturbed Angular Correlation (PAC) technique. Thin oxide films were deposited by Electron Beam Evaporation on a silicon substrate. The thickness of the films was ~100 nm and ~250 nm. Radioactive ¹⁸¹Hf nuclei were produced by neutron activation of the film samples in the Brazilian Research Reactor (IPEN IEA-R1) by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out after annealing at 1473 K. The PAC technique allows the determination of the electric field gradient (EFG) at the probe sites.     

Measurements of the hyperfine magnetic fields at Ta and Fe in the Laves compounds (Zrx Hf1−x)Fe2 for 0≤x≤1

The hyperfine magnetic fields at¹⁸¹Ta and⁵⁷Fe in the ferromagnetic Laves intermetallic compounds (Zr_xHf_1?x)Fe₂ (0≤x≤1) have been measured by the methods of TDPAC and Mössbauer effect, respectively, and shown to be practically independent of x at x≥0.4. An average value B_hf (Ta)=?6.52 T at 300 K was obtained for samples with x≥0.4, and ?14.2 T for pure HfFe₂ in the hexagonal C14 modification. For 0hf(Ta) in (Zr_0.9 Hf_0.1)Fe₂ from that for B_hf (Fe) and the bulk magnetization was confirmed, studied in detail, and shown to exist for all x≥0.4. The temperature dependence of B_hf(Ta) in HfFe₂ was close to that of B_hf (Fe).     

Electric field gradient in nanostructured SnO2 studied by means of PAC spectroscopy using 111Cd or 181Ta as probe nuclei

Electric quadrupole interactions were studied in pure and Mn-doped powder samples and thin films of SnO₂ using perturbed γγ angular correlation spectroscopy (PAC). The powder samples were prepared by Sol gel method and the thin film were prepared on the Si (100) substrate by sputtering technique using Sn in the oxygen atmosphere. The samples were characterized by x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The thickness of the film was 100 nm. The average particle size of the SnO₂ powder samples was determined to be smaller than 60 nm. The radioactive ¹¹¹In and ¹⁸¹Hf tracers were introduced in the powder samples during the sol gel chemical process. Radioactive ¹¹¹In was implanted on the SnO₂ thin films using the University of Bonn ion implanter (BONIS). PAC measurements were carried out in a four BaF₂ detector spectrometer in the temperature range of 77–973 K for samples annealed at different temperatures. The PAC results for both nuclear probes show the presence of two electric quadrupole interactions. The major fractions in both cases correspond to the substitutional sites in the rutile phase of SnO₂. The results are compared with previous PAC measurements.