Magnetostatic interaction studied by force microscopy in ultrahigh vacuum

3 (FeGe)₅ O₁₂ magnetic garnet, Co/Pt multilayer sample and the high coercivity BaFe₁₂O₁₉ single crystal. We have prepared in UHV by electron beam evaporation magnetic sensors/tips which allowed us to reach high sensitivity and high resolution of our microscope. Ideal UHV conditions (pressure 5×10^-11 \text{Torr}) helped us to observe domain wall contrast on all of studied samples by using a dynamic mode of operation of the force microscope. Received: 28 October 1996/Accepted: 5 November 1996     

Experimental determination of ultra-sharp stray field distribution from a magnetic vortex core structure

The fine magnetic stray field from a vortex structure of micron-sized permalloy (Ni₈₀Fe₂₀) elements has been studied by high-resolution magnetic force microscopy. By systematically studying the width of the stray field gradient distribution at different tip-to-sample distances, we show that the half-width at half-maximum (HWHM) of the signal from vortex core can be as narrow as ∼21 nm at a closest tip-to-sample distance of 23 nm, even including the convolution effect of the finite size of the magnetic tip. a weak circular reverse component is found around the center of the magnetic vortex in the measured magnetic force microscope (MFM) signals, which can be attributed to the reverse magnetization around the vortex core. Successive micromagnetic and MFM imaging simulations show good agreements with our experimental results on the width of the stray field distribution.     

Domain observation and transition from giant magnetoresistance to anisotropy magnetoresistance in Ag1−x(Ni0.8Co0.2)x granular films

A series of Ag_1−x(Ni_0.8Co_0.2)_x granular film samples were prepared using an ion-beam cosputtering technique. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed to investigate the microstructure of these samples. The results measured using a vibrating sample magnetometer (VSM) show a gradual change from superparamagnetism to ferromagnetism as x increases in these samples. Magnetoresistance was measured using a conventional four terminal method at room temperature. As x increases, a transition from giant magnetoresistance (GMR) to anisotropic magnetoresistance (AMR) has been observed. The stripe-type domains have been observed using magnetic force microscopy (MFM) in the high x samples, and the domains gradually disappear as x decreases. It suggests that the transition from GMR to AMR may result from intergranular interaction (not only dipolar) in the samples as x increases.     

Magnetic contrast from domain walls in BaFe12O19 by tunneling stabilized magnetic force microscopy

The magnetic structure of BaFe₁₂O₁₉ is imaged with a scanning tunneling microscope having a flexible, magnetic tip. We find that Fe thin films evaporated on a silicon tip, integrated with a cantilever, behave as magnetically soft tips. Therefore, we are able to image domain walls with high lateral resolution. A different contrast along the domain wall due to surface magnetic charges is observed. We explain the data using previously established models for wavy domain walls. The obtained images are the first experimental evidence of magnetic charges induced on the wavy domain walls in BaFe₁₂O₁₉.     

Spin-spin and spin-other-orbit effects of optical spectra and the spin-Hamiltonian parameters for Cr ions in MgO crystals

An extended complete diagonalization method/microscopic spin-Hamiltonian (CDM/MSH) program has been developed, which is applicable for d³ ions at sites of tetragonal symmetry type I (C_4v, D_2d, D₄, D_4h) and trigonal symmetry type I (C_3v, D₃, D_3d). The Hamiltonian includes the spin-spin (SS) and spin-other-orbit (SOO) magnetic interactions besides the spin-orbit (SO) magnetic interaction usually taken into account. Utilizing the extended CDM/MSH program, the optical spectra, the spin-Hamiltonian (SH) parameters of the ground state ⁴B₁, and the splitting δ(²E) of the first excited ²E state for Cr³⁺ (3d³) ions at C_4v symmetry sites in MgO crystals have been successfully investigated. It is found that although the SO magnetic interaction is the most important one, the contributions to the SH parameters and the optical spectra from the SS and SOO magnetic interactions for Cr³⁺:MgO crystals are appreciable and should not be omitted, especially reaching 27.8% for the zero field splitting parameter D.     

Influence of magnetic (Fe) and non-magnetic (Ga) ion doping at Mn-site on the transport and magnetic properties of La0.7Ca0.3MnO3

The modifications in electrical and magnetic properties of polycrystalline bulk La_0.7Ca_0.3Mn_1−xT_xO₃ (T=Fe, Ga) samples at relatively higher doping concentration (x=0.08-0.12) are investigated. All the synthesized, single phase samples were subjected to resistivity measurements in the temperature range 50-300 K. No insulator-metal transition (T_P) was observed for Fe doped samples with x=0.12. For all the other samples the transition temperature decreased with increase in doping concentration. The small polaron hoping energy was found to increase, rather slowly, with increase in doping concentration. The effect on magnetic properties is also prominently observed with respect to doping element and doping concentration. Interestingly, with the increase in doping concentration, the Curie temperature (T_C) and T_P separate out significantly indicating decoupling of electric and magnetic properties. Changes in these properties have been analyzed on the basis of magnetic disorder introduced in the system due to the magnetic and nonmagnetic nature of these ions rather than strong lattice effects which is insignificant due to similar ionic radii of Fe⁺³ and Ga⁺³ when compared to that of Mn⁺³.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

Effect of swift heavy ion irradiation on dilute Fe-doped Sb0.95Se0.05 magnetic semiconductor

A thin film of dilute Fe (0.008)-doped Sb_0.95Se_0.05 alloy was grown on silicon substrate using the thermal evaporation technique. This film was irradiated with swift heavy ions (SHIs) Ag⁺¹⁵ having 200?MeV energy at ion fluences of 1?×?10¹² and 5?×?10¹² ions per cm², respectively. The thickness of the thin film was ～500?nm. We study the effect of irradiation on structural, electrical, surface morphology and magnetic properties of this film using grazing angle XRD (GAXRD), DC resistivity, atomic force microscopy (AFM) and magnetic force microscopy (MFM), respectively. GAXRD suggests that no significant change is observed in this system due to SHI irradiation. The average crystallite size increases with fluence, whereas the AFM image shows the rms roughness decreases due to irradiation with respect to the un-irradiated thin film. The MFM image shows that the magnetic interaction in irradiated film decreases due to the irradiation effect. Although the un-irradiated sample shows metal to semiconducting transition, but after irradiation with fluence of 5?×?10¹² ions per cm², the sharpness of the metal to semiconducting phase transition is observed to increase dramatically at ～300?K. This characteristic of the thin film makes it a promising candidate for an electrical switching device after irradiation.     

Spectral signatures of hyperfine and isotopic effects and of Tm-Tm pairs in LiYF4:Tm

We report on the high-resolution optical Fourier-transform spectroscopy of the LiYF₄:Tm³⁺ crystals. Splitting of several lines in the optical low-temperature polarized spectra was observed. We show that these splittings are caused by (i) the hyperfine interaction, (ii) the isotopic disorder in the lithium sublattice, and (iii) the interionic interaction between neighboring Tm ions. It is the first observation of the hyperfine splitting in the spectra of the Tm³⁺ ions in crystals. From the experimentally measured hyperfine splitting we evaluate the magnetic field at the thulium nucleus and calculate the magnetic g-factors of the excited crystal-field levels.     

Spin domain mapping of a CrO2 thin film using spin-polarized current microscopy

We investigate spin domain mapping of a CrO₂ thin film using spin-polarized current microscopy at room temperature, where conductive atomic force microscopy (CAFM) with a CrO₂-coated tip is used. The nanoscale spin domains of the CrO₂ thin film were crosschecked by magnetic force microscopy (MFM). Notably, the CAFM exhibits the spin domains of the CrO₂ thin film with higher resolution than the MFM, which may result from a local point contact between the nanoscale CrO₂-coated tip and surface of the CrO₂ thin film.     

Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu ions

We investigated the crystal growth, electron paramagnetic resonance (EPR) and optical absorption spectra of l-threonine doped with Cu²⁺. The quality, size and habit of the single crystals grown from aqueous solution by the slow solvent evaporation and by the cooling methods vary when the impurities are introduced during the growth process. The variations with the magnetic field orientation of the EPR spectra of single-crystal samples at room temperature and 9.77 GHz in three crystal planes (ab, bc and ac) show the presence of copper impurities in four symmetry-related sites of the unit cell. These spectra display well resolved hyperfine couplings of the of Cu²⁺ with the I_Cu= of the copper nuclei. Additional hyperfine splittings, well-resolved only for specific orientations of the magnetic field, indicate that the copper impurity ions in the interstitial sites have two N ligands with similar hyperfine couplings. The principal values of the g and A_Cu tensors calculated from the EPR data are g₁=2.051(1), g₂=2.062(2), g₃=2.260(2), A_Cu,1=16.9(5)×10⁻⁴ cm⁻¹, A_Cu,2=21.8(6)×10⁻⁴ cm⁻¹, A_Cu,3=180.0(5)×10⁻⁴ cm⁻¹. The principal directions corresponding to g₃ and to A_Cu,3 are coincident within the experimental errors, reflecting the orientation of the bonding planes of the copper ions in the crystal. The values of the crystal field energies are evaluated from the optical absorption spectrum, and the crystal field and bonding parameters of the Cu impurities in the crystal are calculated and analyzed. The EPR and optical absorption results are discussed in terms of the crystal structure of l-threonine and the electronic structure of the Cu²⁺ ions, and compared with data reported for other systems. The effects of the impurities in the growth and habit of the crystals are also discussed.     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular Co_xAg_1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×10⁵ ergs/cc for as-deposited Co₆₂Ag₃₈ and about 6×10⁵ ergs/cc for as-deposited Co₄₅Ag₅₅ was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

Magnetic and transport properties of Pr2Pd3Si5

We have investigated the magnetic and transport properties of a new ternary intermetallic compound Pr₂Pd₃Si₅ which forms in U₂Co₃Si₅-type orthorhombic structure (space group Ibam). At low field (0.01 T) magnetic susceptibility exhibits an abrupt increase below 7 K and peaks at 5 K, revealing a magnetic phase transition. The onset of magnetic order is also confirmed by well defined anomalies in the specific heat and electrical resistivity data. Apart from the sharp λ-type anomaly, magnetic part of specific heat also shows a broad Schottky-type hump due to crystal field effect. Magnetoresistance data as a function of temperature exhibits a pronounced peak in paramagnetic state which could be interpreted in terms of crystal field effect and short-range ferromagnetic correlations.     

Compression of CdCu3Ti4O12 perovskite to 55 GPa

Synchrotron x-ray diffraction measurements of CdCu₃Ti₄O₁₂ (CDCTO) were performed up to 55.5 GPa. There is no structural phase transformation in this pressure range. The irregular curvature shifts of the P-V curve are attributed to the grain surface effect. Analysis indicates that the grain surface of CDCTO is stiffer than the grain interior at higher pressures. We point out that the atoms on grain surfaces must be either densely packed or have a strong correlation with the gain interior in order to have a high dielectric constant, as in CaCu₃Ti₄O₁₂. The derived bulk modulus of CDCTO is approximately 235±7 GPa with K^′=5.1±0.4.     

Magnetic properties of field-annealed FeCo thin films

Fe₅₀Co₅₀ thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 °C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films.     

Magnetic force microscopic study of the magnetic field induced antiferro to ferrimagnetic transition in Mn1.85Co0.15Sb

Magnetic field induced first order antiferromagnetic (AFM) to ferrimagnetic (FRI) transition in polycrystalline Mn_1.85Co_0.15Sb has been studied using magnetic force microscopy (MFM) at 60 K and up to 8 T magnetic fields. Our MFM studies provide real space visualization of AFM to FRI transition. It shows growth (decay) of FRI phase with increasing (decreasing) magnetic field. The hysteretic behavior and co-existing FRI and AFM phases across the critical field required for FRI-AFM transition in Mn_1.85Co_0.15Sb are highlighted. This study demonstrates the potential of MFM for studying phase co-existence at high field and low temperatures.     

Enhanced atomic-scale contrast on Fe3O4 (100) observed with an Fe STM tip

Clean (100) surfaces of a synthetic single crystal of magnetite (Fe₃O₄) have been prepared in situ using current pulses in a scanning tunneling microscope without subsequent annealing. We have observed atomically resolved terraces with rows of Fe²⁺ and Fe³⁺ ions of the B-sublattice (octahedrally coordinated lattice sites). Along these rows a long-distance corrugation (∼12 ?) has been observed at 300 K using in situ prepared Fe tunneling tips. This corrugation is interpreted as a Wigner localization associated with a Verwey transition above 300 K in the top surface layer. Received: 26 September 2000 / Accepted: 27 October 2000 / Published online: 3 May 2001     

Magnetic domain structure in small diameter magnetic nanowire arrays

Fe_0.3Co_0.7 alloy nanowire arrays were prepared by ac electrodepositing Fe²⁺ and Co²⁺ into a porous anodic aluminum oxide (PAO) template with diameter about 50 nm. The surface of the samples were polished by 100 nm diamond particle then chemical polishing to give a very smooth surface (below ±10 nm/μm²). The morphology properties were characterized by SEM and AFM. The bulk magnetic properties and domain structure of nanowire arrays were investigated by VSM and MFM respectively. We found that such alloy arrays showed strong perpendicular magnetic anisotropy with easy axis parallel to nanowire arrays. Each nanowire was in single domain structure with several opposite single domains surrounding it. Additionally, we investigated the domain structure with a variable external magnetic field applied parallel to the nanowire arrays. The MFM results showed a good agreement with our magnetic hysteresis loop.