Influence of Interface Roughness, Film Texture, and Magnetic Anisotropy on Exchange Bias in [Pt/Co]3/IrMn and IrMn/[Co/Pt]3 Multilayers

Co/Pt multilayers can exhibit considerable exchange bias along the film normal when grown onto or covered by an antiferromagnetic IrMn layer. The magnitude of the bias effect depends strongly on the selection of the buffer layers and is largest for Co/Pt multilayers with small perpendicular anisotropy. This counterintuitive result is explained by opposite dependencies of exchange bias and magnetic anisotropy on the degree of film texture and grain size. Interface roughness also influences the perpendicular exchange bias field. The largest biases are measured on smooth films. Interlayer mixing during post-deposition annealing procedures decreases the exchange bias field of IrMn-capped Co/Pt multilayers.     

Magnetic softness and interparticle exchange interactions of (Fe65Co35)1 − x(Al2O3)x (x = 0-0.50) nanogranular films

The effect of Al₂O₃ content on the structure, electrical properties, magnetic properties, and interparticle exchange interactions of (Fe₆₅Co₃₅)_1 − x(Al₂O₃)_x films with Al₂O₃ volume fractions x ranging from 0 to 0.50 was systematically investigated. Among the films with x between 0 and 0.25, the lowest coercivity of 0.56 kA/m was achieved in the (Fe₆₅Co₃₅)_0.82(Al₂O₃)_0.18 film. This is ascribed to the strongest exchange interactions between the Fe₆₅Co₃₅ nanoparticles in this film. Combined with the microstructure analysis of the (Fe₆₅Co₃₅)_1 − x(Al₂O₃)_x films, the modified Herzer's model was extended to interpret the variation of the coercivity with x and analyze the effect of the exchange interactions between the Fe₆₅Co₃₅ nanoparticles on the magnetic softness. The remanence curves confirm the existence of the exchange interactions and reveal the evolution of the exchange interaction strength with Al₂O₃ content.     

Hysteresis loops of polarization and magnetization in (BiNd0.05)(Fe0.97Mn0.03)O3/Pt/CoFe2O4 layered epitaxial thin film grown by pulsed laser deposition

(BiNd_0.05)(Fe_0.97Mn_0.03)O₃ (BNFM)/Pt/CoFe₂O₄ (CFO) layered thin film was fabricated on (100) SrTiO₃ substrate by pulsed laser deposition. BNFM, Pt, and CFO layers were epitaxially grown on the substrate. Almost no increase of leakage current due to the formation of heteroepitaxial structure was found, and well-saturated hysteresis loops in the polarization vs electric field and magnetization vs magnetic field curves coexist at room temperature. The remnant polarization and remnant magnetization values were 55 μC/cm², and 70-145 mA/m, respectively.     

Onset of hard magnetic L10 FePt phase with (001) texture

The perpendicular anisotropic magnetic properties of in-situ deposited FePt/Pt/Cr trilayer films were elucidated as functions of the deposition temperature and the sputtering rate of the FePt magnetic layer. Ordered L1₀ FePt thin films with perpendicular anisotropy and a (001) texture can be developed at a temperature as low as 300 °C with the sputtering of a FePt layer at a low rate. The larger Pt(001)[100] lattice induced an expansion of the FePt a- and b-axis, leading to the contraction of the FePt c-axis, enabling the epitaxial growth of the L1₀ FePt(001) texture to occur. A low rate of sputtering of the FePt thin film promotes the formation of the magnetically hard FePt(001) texture on the surface of the Pt(001) buffer layer at low temperature, while the high sputtering rate of FePt layer suppresses the phase transformation.     

The study of the electric and magnetic properties of PbZr0.2Ti0.8O3-BiFeO3 multilayers

The results of the electric and magnetic measurements performed on PbZr_0.2Ti_0.8O₃-BiFeO₃ symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO₃ films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Investigations on the interlayer coupling in Co/Pt multilayers with perpendicular anisotropy via the extraordinary Hall effect

The interlayer coupling in Co/Pt multilayers has been investigated via the measurements of extraordinary Hall effect. In the conventional [Co/Pt]_n multilayer, the coercivity H_C has been observed to increase exponentially for n < 6 and almost linearly for n > 6 with the decrease of temperature. Surprisingly, H_C for n = 1 shows the fastest increase at low temperatures, and becomes the largest one at T < 40 K. As a function of the repetition number n, the almost temperature-independent oscillation of H_C has been observed, being indicative of the Ruderman-Kittel-Kasuya-Yosida type ferromagnetic interlayer coupling in the [Co/Pt]_n multilayer. However, the antiferromagnetic interlayer coupling can be realized via the spin-valve configuration of the Co/Pt multilayer. In the Co/Pt/[Co/Pt]_n multilayer, the antiferromagnetic-to-ferromagnetic transition of the interlayer coupling has been observed at low temperatures. These observed phenomena are strongly related to the temperature-dependent polarization in the Co/Pt multilayers.     

Role of La0.5Sr0.5CoO3 template layers on dielectric and electrical properties of pulsed-laser ablated Pb(Nb2/3Mg1/3)O3-PbTiO3 thin films

Relaxor ferroelectric thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) deposited on platinized silicon substrates with and without template layers were studied. Perovskite phase (80% by volume) was obtained through proper selection of the processing conditions on bare Pt/Ti/SiO₂/Si substrates. The films were initially grown at 300 °C using pulsed-laser ablation and subsequently annealed in a rapid thermal annealing furnace in the temperature range of 750-850 °C to induce crystallization. Comparison of microstructure of the films annealed at different temperatures showed change in perovskite phase formation and grain size etc. Results from compositional analysis of the films revealed that the films initially possessed high content of lead percentage, which subsequently decreased after annealing at temperature 750-850 °C. Films with highest perovskite content were found to form at 820-840 °C on Pt substrates where the Pb content was near stoichiometric. Further improvement in the formation of perovskite PMN-PT phase was obtained by using buffer layers of La_0.5Sr_0.5CoO₃ (LSCO) on the Pt substrate. This resulted 100% perovskite phase formation in the films deposited at 650 °C. Dielectric studies on the PMN-PT films with LSCO template layers showed high values of relative dielectric constant (3800) with a loss factor (tan δ) of 0.035 at a frequency of 1 kHz at room temperature.     

(Fe,Ti) 16N2 films with high saturation magnetization prepared by facing target sputtering

(Fe,Ti)-N films with a Ti concentration of 10 at.% were prepared on Si(100) and NaCl substrates by facing targets sputtering. The effects of the nitrogen pressure (P_N) and the substrate temperature (T_s) on the formation of various (Fe,Ti)-N phases and their microstructures were investigated in detail. X-ray diffractometer and transmission electron microscope provided complete identification of the phases present in the films and the characterization of their microstructures. Films deposited at a lower P_N = 1 3 × 10⁻² Pa or a lower T_s = RT consist of mainly -phase. Films deposited at a higher P_N = 1.3 2 × 10⁻¹ Pa or a higher T_s = 200 °C contain a great many γ' and Fe₂N phases with a higher nitrogen content. When P_N = 4 7 × 10⁻² Pa and T_s = 100 150 °C, it is advantageous to the formation of ′' phase. These films exhibit a high saturation magnetization (M_s) up to the range of 2.3 2.5 T, which is larger than that of pure iron.     

Elastic coupling and anelastic relaxation associated with multiple phase transitions in para-chloroanilinium tetrachlorocuprate, [p-ClC6H4NH3]2CuCl4

The perovskite-like salt [p-ClC₆H₄NH₃]₂CuCl₄1 exhibits a wealth of magnetic and structural phase transitions which have been probed by variable temperature single crystal X-ray diffraction, SQUID magnetometry, resonant ultrasound spectroscopy (RUS), EPR spectroscopy, DSC measurements and DFT calculations. Single crystal X-ray diffraction studies between room temperature and 3 K reveal a rich tapestry of structural changes; at 298 K the structure conforms to a monoclinic setting but undergoes a first order phase transition upon cooling below ∼275 K to a higher symmetry orthorhombic cell. This is facilitated by a transition to an intermediate phase at ∼277 K. Whilst the intermediate phase has a limited stability window (∼2 K) and has not been structurally determined, the two discrete phase transitions at 275.5 K and 277 K have been clearly detected by differential scanning calorimetry, EPR spectroscopy and RUS studies. On further cooling a dynamic relaxation process is observed in RUS measurements, evidenced by a Debye-like peak in the dissipation at ∼140 K. Residual electron density maps from single crystal X-ray diffraction studies reveal that this may be associated with a freezing out of the NH₃?Cl hydrogen-bonding between cation and anion frameworks upon cooling. The activation barrier for this order/disorder process was estimated to be at least 27 kJ mol⁻¹ from the RUS data. Variable temperature dc SQUID data reveal that 1 is a 2D ferromagnet with antiferromagnetic interactions between layers below 9 K. Analysis of the temperature dependence of the magnetic susceptibility for T > 40 K reveals that 1 exhibits Curie–Weiss behaviour with θ = +22.8 K indicative of dominant ferromagnetic interactions. Good agreement is observed between the strength of the ferromagnetic interaction extracted from the Weiss constant (J/k = +22.8 K) and that calculated by DFT (J/k = +25 to +28 K) and from EPR studies (J/k = +17 K). The presence of short range ferromagnetic interactions is reflected in a marked temperature dependence of the g-factors determined from EPR spectroscopy below 20 K and possibly a small elastic anomaly in the RUS data. RUS studies indicate a very small elastic anomaly associated with the transition to long range order, implying weak or no magnetoelastic effect. At fields above ∼15 G a spin flip transition is induced and 1 displays metamagnetic behaviour, with a saturation magnetization of 0.96 μ_B.     

Thickness dependence of structural and ferroelectric properties of sol-gel Pb(Zr0.56Ti0.44)0.90(Mg1/3Nb2/3)0.10O3 films

Sol-gel Pb(Zr_0.56Ti_0.44)_0.90(Mg_1/3Nb_2/3)_0.10O₃ (PZT-PMN) films were prepared onto the Ti/Pt/Ti bottom electrode by multilayer spin coating. The film thickness ranged from 0.22 to 0.88 μm. The Pt top electrodes were deposited on the PZT-PMN films by DC sputtering. The structural and ferroelectric properties of PZT-PMN films were investigated as a function of film thickness by X-ray diffraction (XRD), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and by measuring the relative permittivity. The film retains the tetragonal perovskite structure with the [111] and [100] preferred orientations perpendicular to the film surface independent of film thickness. The [100] texture increases with increasing film thickness although the [111] texture is always predominant. The film consists of columnar grains. The average grain size is nearly independent of film thickness. The surface layer containing fine grains about 30 nm in diameter is induced on the top of the film. As the film thickness exceeds 0.44 μm, the number of the fine grains decreases remarkably. The crystalline interface layer about 10 nm thick is formed between the film and the bottom electrode. This interface layer is composed of Pt, Pb, Zr, Ti and O, while it is rich in Ti and deficient in Pb and O as compared with the inside of the film. The measured relative permittivity of the film increases with increasing film thickness, following the low permittivity interface model. On the basis of this model, the relative permittivity is estimated to be 3200 for the intrinsic PZT-PMN film, 750 for the surface layer and 50 for the interface layer.     

Gold embedding influence on the magnetic behaviour of iron in Fe/Si3N4 multilayers prepared by sputtering

Au/Fe/Au/Si₃N₄ multilayers have been prepared in order to improve ferromagnetism in Fe/Si₃N₄ system. A huge enhancement of magnetization is observed when the iron layers are embedded into gold ones, showing saturation magnetization values near the bulk ones. In this paper, a careful microstructural study has been carried out to explain the observed magnetic behaviour of this ferromagnetic system prepared in the transparent silicon nitride matrix.     

New lead vanadium phosphate with langbeinite-type structure: Pb1.5V2(PO4)3

The new lead vanadium phosphate Pb_1.5V₂(PO₄)₃ was synthesized by solid state reaction and characterized by X-ray powder diffraction, electron microscopy, and magnetic susceptibility measurements. The crystal structure of Pb_1.5V₂(PO₄)₃ (a = 9.78182(8) Å, S.G. P2₁3, Z = 4) was determined from X-ray powder diffraction data and belongs to the langbeinite-type structures. It is formed by corner-linked V³⁺O₆ octahedra and tetrahedral phosphate groups resulting in a three-dimensional framework. The lead atoms are situated in the structure interstices and only partially occupy their positions. An electron microscopy study confirmed the structure solution. Magnetic susceptibility measurements revealed Curie-Weiss (CW) behavior for Pb_1.5V₂(PO₄)₃ at high temperature whereas at around 14 K an abrupt increase on the susceptibility was observed.     

Hydrothermal synthesis, crystal structure and spectroscopic and magnetic properties of (C2H10N2)[Mn2.09Co0.91(HPO3)4]

(C₂H₁₀N₂)[Mn_2.09Co_0.91(HPO₃)₄] has been synthesized using mild hydrothermal conditions under autogeneous pressure. The compound crystallizes in the triclinic P-1 space group. The unit-cell parameters are a = 5.4061(8), b = 5.4150(7), c = 14.136(2) Å, α = 80.84(1), β = 85.41(1), γ = 60.00(1) and Z = 1. The compound shows a layered structure constructed from M₃O₁₂ trimer units linked thorough the (HPO₃)²⁻ phosphite oxoanions with the ethylenediammonium cations located between the sheets compensating the anionic charge of the inorganic framework. The IR and Raman spectra confirm the presence of the ethylenediammonium cation and phosphite anion. The diffuse reflectance spectrum is in accordance with the presence of Co(II) and Mn(II) high spin cations in slightly distorted octahedral symmetry. The calculated Dq and Racah parameters for the Co(II) cations are Dq = 710, B = 870 and C = 4100 cm⁻¹. The magnetic measurements indicate the existence of antiferromagnetic interactions as the major interactions. Hysteresis observed at low temperature indicates a weak ferromagnetic component, due to a non-cancellation of spins, with coercitive field of 900 G and magnetization of 700 emu/mol.     

强磁场退火处理对La0.5Ca0.5MnO3薄膜晶粒尺寸演化和磁性能的影响

采用脉冲激光沉积法制备了La_0.5Ca_0.5MnO₃(LCMO)薄膜,然后在不同强磁场作用下进行原位后退火处理。利用XRD、FESEM和SQUID对薄膜进行微结构表征和磁特性测量。结果显示,强磁场退火使得薄膜的面外晶格参数拉长,晶粒尺寸明显增大,同时使低温磁化强度降低。通过建立强磁场下的晶粒生长速率方程对晶粒尺寸演化机理做了分析。分析认为,强磁场引入的额外驱动力降低了LCMO薄膜的临界晶粒尺寸,有利于晶粒生长。此外,本文通过建立一个基于相分离的反铁磁-铁磁核壳模型对薄膜的磁结构演化进行了探讨。认为晶粒尺寸增大导致铁磁相体积分数下降,从而导致低温磁化强度下降。     

Ferroelectric properties of BiFeO3 films grown by sol–gel process

Several methods have been used to prepare ferroelectromagnetic BiFeO₃ films. In this paper, we adopted a sol–gel process to fabricate BiFeO₃ films on indium tin oxide (ITO)/glass substrates. X-ray diffraction pattern indicated that the samples are randomly oriented. Cross section scanning microscopy showed that the thicknesses of both films were about 1.2 μm and no apparent diffusion between the BiFeO₃ films and ITO/glass substrates. Remnant polarization of 2.0 and 1.75 μC/cm² were identified by the measuring of electric hysteresis loops for the films annealed at 500 and 600 °C respectively at an applied field of 108 kV/cm. Dielectric property and loss factor were investigated as a function of frequency. In addition, magnetism was detected at 77 K.     

A new borophosphate chain anion in an organo-templated iron(III) borophosphate: Synthesis, crystal structure and magnetic properties of (C4H12N2)3Fe6(H2O)4[B6P12O50(OH)2]·2H2O

The organo-templated iron(III) borophosphate (C₄H₁₂N₂)₃Fe^III₆(H₂O)₄[B₆P₁₂O₅₀(OH)₂]·2H₂O was prepared under mild hydrothermal conditions (443 K). The crystal structure was determined from single-crystal X-ray data at 295 K (orthorhombic, Pbca (No. 61), Z=4, a=17.8023(7) Å, b=16.1037(5) Å, c=19.1232(6) Å, V=5482.3(3) Å³, R1=0.055, wR2=0.104, 6576 observed reflections with I>2σ(I)) and contains a new type of borophosphate anion: a mixed open- and loop-branched zehner single chain, , built from heptamers [B₂P₅O₂₁] interconnected by BO₃(OH) tetrahedra sharing their third oxygen corners with additional (terminal) PO₄ tetrahedra to form open branchings. The mixed open- and loop-branched single chains running along [0 0 1] are interconnected by three crystallographically independent iron coordination octahedra to form a 3D framework structure containing eight-membered ring channels running along [0 1 0] and cages, which are occupied by two crystallographically independent piperazine cations and H₂O molecules. The displacement parameters of C and N atoms in the piperazine cations are dramatically influenced by the strength of the hydrogen bond reflecting the shape of the cavities. The magnetic investigations indicate the existence of antiferromagnetic interactions as the major components. A narrow hysteresis at low temperatures indicates a weak ferromagnetic component, due to a non-cancellation of spins.     

Electrical and optical properties of (Pb0.9La0.1)TiO3 films prepared by the sol–gel method with ultrasound application

(Pb_0.9La_0.1)TiO₃ thin films were deposited onto indium tin oxide-coated glass and Pt/Ti/SiO₂/Si substrates by spin coating. One set of the solutions was sonication-treated during the sol preparation, while the other remained untreated for comparison. The results were analyzed in terms of perovskite developments, dielectric constants and losses, polarization behavior, and optical properties. Perovskite formation temperatures became lower by application of the sonication process. Furthermore, dielectric/ferroelectric and optical transmittance properties were improved substantially.     

Exchange-coupled Fe3O4/L10-FePt bilayer films by controlled oxidation of Fe/Pt multilayer

A Fe₃O₄/L1₀-FePt bilayer thin-film magnet was fabricated via a simple one-step process by annealing Fe/Pt multilayer thin films in a N₂ gas flow. X-ray diffraction and plane-view selected-area electron diffraction results are identified with magnetite phase (Fe₃O₄) and L1₀-FePt. Cross-sectional transmission electron microscopy images show the two phases form a bilayer structure. Magnetic hysteresis loops of the bilayer show single phase behavior which is interpreted as a result of the soft Fe₃O₄ phase exchange-coupled with the hard L1₀-FePt phase, consistent with micromagnetic simulation prediction. Such bilayer structure may have potential for coercivity control in high density magnetic recording application.     

Synthesis, structure and magnetic properties of a new iron phosphonate-oxalate with 3D framework: [Fe(O3PCH3)(C2O4)0.5(H2O)]

A new iron phosphonate-oxalate [Fe(O₃PCH₃)(C₂O₄)_0.5(H₂O)] (1), has been synthesized under hydrothermal condition. The single-crystal X-ray diffraction studies reveal that 1 consists of layers of vertex-linked FeO₆ octahedra and O₃PC tetrahedra, which are further connected by bis-chelate oxalate bridges, giving to a 3D structure with 10-membered channels. Crystal data: monoclinic, P2₁/n (no. 14), a = 4.851(2) Å, b = 16.803(7) Å, c = 7.941(4) Å, β = 107.516(6)°, V = 617.2(5) Å³, Z = 4, R₁ = 0.0337 and wR₂=0.0874 for 1251 reflections [I > 2σ(I)]. Mössbauer spectroscopy measurement confirms the existence of high-spin Fe(III) in 1. Magnetic studies show that 1 exhibits weak ferromagnetism with T_N = 30 K due to a weak spin canting.