Calibration of Coercive and Stray Fields of Commercial Magnetic Force Microscope Probes

Variable-field magnetic force microscope (MFM) is introduced to characterize the magnetic behavior of commercially available MFM probes that is relevant to interpret MFM imaging. A Nanotec Electronica S.L. microscope has been conveniently modified to apply magnetic fields in axial direction (up to 1.5 kOe) and in-plane direction (up to 2.0 kOe). Axial and transeverse hysteresis loops of the probes have been generated by measuring the changes in the MFM contrast observed when the magnetic field is applied. The variation of the MFM signal is ascribed to the modification of the magnetic state of the tips. This is enabled by the large coercitivity (~1.7 kOe) of the checked longitudinal recording media. The properties of the probes depend on the coating material, the macroscopic tip shape, and tip radius. In only a few cases, the magnetization of the probe can be oriented along in-plane orientation. In addition, the stray field of the tips has been deduced by measuring the influence of the probe in the magnetic state of the checked samples.     

Superconductivity of FeSr2YCu2O6+δ

Resistivity measurements of polycrystalline FeSr₂YCu₂O_6+δ under magnetic fields up to 160 kOe were made to study the superconductivity of FeSr₂YCu₂O_6+δ in detail. The resistivity began to decrease at 64 K and dropped to zero at 38 K under zero magnetic field. The superconductivity in inter grain was affected by the magnetic field and zero resistivity was observed below 12 K under H=10 kOe. Above 20 kOe, the superconductivity in the grain began to be affected. Even with increasing magnetic field up to 160 kOe, zero resistivity was persisted below 10 K.     

Experimental study of the Fermi surface and magnetic breakdown of vanadium

The results of magnetoresistance and thermopower investigations of a high- purity vanadium single crystal in fields up to 175 kOe are presented. The oscillations of thermopower appearing in fields above 50 kOe for [110] and [100] directions are explained by magnetic breakdown. It is shown that the topology of the Fermi surface deduced from both magnetoresistance and thermopower behavior is similar to that of niobium and tantalum.     

3He spin volume variations in magnetic field

The results of the investigation of high magnetic field (up to 78 kOe) effect on³He quantum liquid in the temperature range (0,55–1,2)K are given. The effect of liquid volume change is obtained which is related to spin-reorientation processes taking place in antiparallel-correlated system of nuclear magnetic moments.     

Switching of bistable magnetic states in (NdSmDy)(FeCo)B alloy in the vicinity of a spin-reorientation transition

Bistable magnetic states with two equiprobable orientations of the magnetization vector (corresponding to opposite polarities of a permanent magnet) exist in (NdSmDy)(FeCo)B magnetic alloy in the vicinity of a spin-reorientation transition. A critical value of the magnetic field strength ~1 kOe is determined, at which switching of these bistable magnetic states takes place. It is established that the polarity of polycrystalline sintered magnets of the Nd₂Fe₁₄B family in the vicinity of a spin-reorientation transition can be stabilized by a small external bias magnetic field, which opens up new possibilities for using these magnets in cryomagnetic systems.     

Current-carrying capacities of small V3 Ga pancakes

As an intermediate step leading to the production of V₃Ga tapes long enough for magnets of practical size, we have produced by surface diffusion V₃Ga tapes ranging in length from 5 to 100 meters. These were wound into single pancakes and their current-carrying capacities were measured in external magnetic fields up to 180 kOe. Results indicate that it should be possible in the near future, to construct V₃Ga magnets capable of generating fields as high as 180 kOe, which exceeds by at least 30 kOe the maximum generated by an Nb₃Sn magnet. A further study in the area of stability, however, would be helpful.     

A Capacitive Dilatometer for Measuring the Magnetostriction,Piezoelectric Effect,and Linear Thermal-Expansion Coefficient

We describe a capacitive dilatometer for measuring the magnetostriction, piezoelectric effect, and linear thermal-expansion coefficient in the temperature range from 1.85 to 350 K in external magnetic fields of up to 90 kOe under a voltage of 1 kV, which operates on the basis of a Quantum Design PPMS commercial facility for studying the properties of solids.     

Hexaferrite materials displaying ultra-high coercivity and sub-terahertz ferromagnetic resonance frequencies

Single-domain Sr_1−x/12Ca_x/12Fe_12−xAl_xO₁₉ (x = 4–6) particles are synthesized by a simple citrate auto-combustion method. The room temperature coercivity of the materials rises with aluminum content from 21.3 kOe (x = 4) to a maximum of 36 kOe (x = 5.5). This value is the highest among ferrite materials to date. Moreover, the magnetic alignment of the particles leads to further coercivity improvement up to 40 kOe. Due to large magnetic anisotropy the samples demonstrate sub-terahertz electromagnetic wave absorption by natural (zero-field) ferromagnetic resonance (NFMR). The absorption lines shift with aluminum substitution from 160 GHz (x = 4) to 250 GHz (x = 5.5), which is the record NFMR frequency known for a magnetic material. This research paves the way for development low-cost materials with extremely high coercivity and sub-terahertz NFMR.     

Magnetic properties of β-FeSi2 single crystals

We investigated temperature and magnetic field dependences of the magnetization of β-FeSi₂ single crystals in the temperature range of 5-300 K in magnetic fields up to 15 kOe. The temperature dependence of the magnetic susceptibility of the Cr- and Ni-doped sample can be explained by temperature-dependent contributions due to paramagnetic centres and due to the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature as well as activation energy of the donor and acceptor levels are estimated.     

Mössbauer spectrometry study of early stage spinodal decomposition in Fe-Cr-Co alloy under high magnetic field

Local structure in a low-cobalt-type Fe-25Cr-12Co-1Si ferromagnetic alloy spinodal decomposed under an external magnetic field up to 120 kOe was investigated by Mössbauer spectrometry. The high magnetic field was found to significantly affect the local structure in the alloy formed at the early stage of phase decomposition. It was found that high magnetic field favors the acceleration of phase decomposition of ferromagnetic alloy at the early stage, resulting in the enhancement of average hyperfine field. The effect of high magnetic field on spinodal decomposition in ferromagnetic alloy was initially interpreted based on the free energy analyses.     

Magnetic, transport and magneto-optical properties ofUxSb1-x amorphous films

It is shown experimentally that amorphous films of U-Sb and films with 20% Co and Mn added are all ferromagnetic with Curie temperatures as high as 135 K with magnetic moments per uranium of up to 1.4 μ_B. Magnetooptical polar Faraday and Kerr rotations at 10 K and 40 kOe have values up to +2×10⁶°/cm and -3°, respectively. The magnetooptical rotation correlates with Hall angles of up to 17°. It is suggested that U-Sb magnetooptical effects and Hall effects are due to an intraband transition     

Built-in getter-ion pumps

It is shown that theory of the Penning discharge essentially involves taking into account the collective interaction of the electrons in the near-anode region. Following investigations of the Penning discharge in a strong magnetic field (density up to 12 kOe) empirical formulae are proposed for calculating the pumping speed and the minimum operating pressures of getter-ion pumps. Methods of calculation for built-in getter-ion pump parameters and design peculiarities are considered.     

Magnetic Domain Evolution in Sintered Nd–Fe–B Magnet during Magnetization Process

In the present study, the magnetic domains and their evolution during magnetization process have been investigated for sintered Nd–Fe–B permanent magnets with Kerr microscopy. Observation of the magnetic domain evolution process during magnetization process shows that some domain walls were pinned at the grain boundary area under magnetic field up to 5 kOe. It is suggested that magnetic interaction between individual Nd₂Fe₁₄B grains contacting to each other leads to appearance of small closed domains near the grain boundary area, which are responsible for the pinning effect.     

Hard magnetic phase in rapidly quenched Zr-Co-B alloys

In order to identify a hard magnetic phase in rapidly quenched Zr-Co-B alloys and clarify its magnetic properties, Zr-Co-B ribbons, Zr-Co ribbons, and Zr-Co ingots were studied. The hard magnetic phase is interpreted as a Zr₂Co₁₁ intermetallic compound. This compound has a Curie temperature of 500°C and uniaxial magnetic anisotropy with an anisotropy field of 34 kOe. The magnetization of this compound was estimated to be 67 emu/g at 15 kOe. In addition to the hard magnetic phase, the low and high T_c phases appear in both binary and ternary alloys. The low T_c phase is FCC Zr₆Co₂₃ with T_c=180°C. The magnetization of Zr₆Co₂₃ was estimated to be 44 emu/g at 15 kOe. The high T_c phase is cobalt including a small amount of zirconium. In Zr-Co-B alloys, suitable boron addition is shown to enhance the coercive force. On the other hand, the addition increases the magnetization. While the boron addition produces cobalt, it reduces Zr₆Co₂₃ or quenches its ferromagnetism     

Large magnetocaloric effect and magnetoresistance in ErNi single crystal

The magnetic properties,magnetocaloric effect and magnetoresistance in ErNi single crystal have been investigated in detail.With decreasing temperature,ErNi single crystal undergoes two successive mag-netic transitions:a paramagnetic to ferromagnetic transition at Tc=11 K and a spin-reorientation transition at TSR=5 K.Meanwhile,a sharp field-induced metamagnetic transition is observed below the Tc along the a axis.ErNi single crystal possesses a giant magnetocaloric effect around Tc.The maximum magnetic entropy change is-36.1 J(kg K)-1 along the a axis under the field change of 0-50 kOe.In par-ticular,the rotating magnetocaloric effect in ErNi single crystal reaches its maximum under a relatively low field,and the maximum rotating entropy change with a value of 9.3 J(kg K)-1 is obtained by rotat-ing the applied field from the[011]to[100]directions under 13 kOe.These results suggest that ErNi could be a promising candidate for magnetic refrigeration working at liquid-helium temperature region.Moreover,a complicated transport behavior is uncovered in ErNi single crystal,which is attributed to the complex magnetic states and magnetic polaronic effect.Both positive and negative magnetoresistance are observed.A considerable large magnetoresistance with the value of-34.5％is acquired at 8 K under 50 kOe when the field is along the[100]direction.     

Mechanisms of High Coercivity in Ni/NiO Composite Films by Post Annealing

A coercivity as large as 2.4 kOe has been achieved in the Ni/NiO composite film after an annealing under a magnetic field of 10 kOe and an O2 partial pressure of 0.001 torr. The coercivity was attributed to the strong exchange coupling of Ni and NiO. Small grain size of Ni and NiO was observed after the post-annealing. The enhanced coercivity is probably associated with the domain wall pinning by local energy minima, the distribution of Ni and NiO, and the domain structure in the interface of Ni/NiO generated under the presence of the magnetic field during the post-annealing.     

Magnetic properties of textured CoPd nanocrystalline thin films

CoPd is an important nanomaterial for magnetic and magneto-optic storage of information. In this work, CoPd alloyed thin films are grown via radio frequency magnetron sputtering on silicon, glass and polyimide substrates in a vacuum chamber with base pressure of 5 x 10(-8) mbar. The films are nanocrystalline with grain size between 4 and 80 nm. The magnetic properties of thoroughly textured CoPd alloyed thin films are compared to random polycrystalline ones. Magnetization hysteresis loops recorded under fields up to 12 kOe via a home-made magneto-optic Kerr-effect magnetometer reveal strong tendency for perpendicular magnetic anisotropy for the textured film. This anisotropy leads to the formation of well-defined stripe or labyrinthine ferromagnetic domains with the local spins oriented perpendicular to the film plane. The domain patterns and the hysteresis loops are simulated with micromagnetic calculations. Finally, an induced magnetic moment of 0.44 microB/atom is measured for Pd via X-ray magnetic circular dichroism and it is separated into spin and orbital magnetic moment contributions.     

Heat treatment of strontium hexaferrite powder in nitrogen,hydrogen and carbon atmospheres: a novel method of changing the magnetic properties

Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.     

High coercivities in Sm-Fe-Tm magnets

High coercivities have been obtained in three Sm-Fe-TM phases (TM=V, Ti, Zr) prepared by mechanical alloying and a subsequent heat treatment at relatively low temperatures. In Sm-Fe-V with the ThMn₁₂ crystal structure, coercivities up to 11.8 kOe were achieved; in Sm-Fe-Zr with the PuNi₃ crystal structure, coercivities up to 14.8 kOe were achieved; and in ω-phase Sm-Fe-Ti, giant coercivities up to 64 kOe (at room temperature) were achieved. The temperature dependence of the coercivity also differs favorably from that of Nd-Fe-B     

Interface effect of magnetic properties in Ni nanoparticles with a hcp core and fcc shell structure

We have fabricated hexagonal close-packed (hcp) Ni nanoparticles covered by a face-centered cubic (fcc) Ni surface layer by polyol method. The magnetic properties have been investigated as a function of temperature and applied magnetic field. The magnetic behavior reveals that the system should be divided magnetically into three distinct phases with different origins. The fcc Ni phase on the shell contributes to the superparamagnetism through a wide temperature range up to 360 K. The hcp Ni phase at the core is associated with antiferromagnetic nature below 12 K. These observations are in good agreement with the X-ray absorption spectroscopy and magnetic circular dichroism measurements. In our particular case, the unique hcp core and fcc shell structure gives rise to an additional anomaly at 20 K in the zero-field-cooled magnetization curve. Its position is barely affected by the magnetic field but its structure disappears above 30 kOe, showing a metamagnetic transition in the magnetization versus magnetic field curve. This new phase originates from the magnetic exchange at the interface between the hcp and fcc Ni sublattices.