Giant intrinsic magnetic hardness in RFe5−x Ni x (R=rare earth,x=4 to 5)

Giant low temperature intrinsic magnetic hardness is observed in structurally homogeneous CaCu₅ type compounds RFe_5−x Ni _x . In SmFe_5−x Ni _x , this magnetic hardness peaks approximately at a composition SmFe_0.2Ni_4.8, with an extrapolated coercive force of 230 kOe at absolute zero. The transition metal sublattice is not anisotropic. Thus, the rare earth alone creates giant coercivity. Only compounds withc-axis preference exhibit substantial magnetic hardness (Sm, Er, Tm). Partial substitution of a tetravalent rare earth to produce crystal field anisotropy fluctuations apparently increases coercivity somewhat in the axis-preference compound SmFeNi₄, but has no effect in the plane-preferred compound TbFeNi₄.     

Improvement of magnetic properties and read/write characteristics in SmCo5 perpendicular thin films

An SmCo₅ alloy is a promising candidate for ultra-high density magnetic recording media because of its strong uniaxial magnetocrystalline anisotropy, whose constant, K_u, is more than 1.1×10⁸ erg/cm³. Recently, we successfully obtained high perpendicular magnetic anisotropy for a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, it is necessary to improve magnetic properties and read/write (R/W) characteristics for applying SmCo₅ thin films to perpendicular magnetic recording media. In this study, we focused on reduction of magnetic domain size and change of a magnetization reversal process of SmCo₅ perpendicular magnetic thin films by introducing carbon (C) atoms into the constituent Cu underlayer. The magnetic domain size became small and the ratio of coercivity (H_c) against magnetic anisotropy (H_k) which is an index of the magnetization reversal process was increased by adding C atoms. We also evaluated the R/W characteristics of SmCo₅ double-layered media including C atoms. The medium noise was decreased and signal-to-noise ratio increased by introducing the C. The addition of C into the Cu underlayer is effective for changing the magnetization reversal process, reducing medium noise and increasing SNR.     

Melt-spun magnetically anisotropic SmCo5 ribbons with high permanent performance

We have succeeded in preparing magnetically anisotropic SmCo₅ ribbons with high permanent performance by single-roller melt spinning at low wheel velocity. The anisotropy is associated with a crystallographic texture formed during melt-spinning process, with the c-axis parallel to the longitudinal direction of the ribbons. The formation of the crystallographic texture is attributed to a directional solidification process resulting from a thermal gradient. A remanence of 9.1 kG, remanence ratio of 0.9, intrinsic coercivity of 16.2 kOe and energy product of 18.2 MGOe at room temperature are obtained in the melt-spun and subsequently annealed SmCo₅ ribbons prepared at 5 m/s.     

Dipole magnetic anisotropy fields in rhombohedral antiferromagnetic materials

Results of calculations of the contribution of magnetic dipole interactions to the effective uniaxial anisotropy fields of antiferro- and ferromagnetism vectors in rhombohedral antiferromagnetic materials with theS ions are given as functions of the ratio of the hexagonal crystal cell parameters c_H/a_H. There is a strong dependence of the calculated curves on the lattice parameters of real compounds. From the dependences obtained the effective anisotropy fields are calculated for FeF₃, FeBO₃, and MnCO₃. L. V. Kirenskii Institute of Physics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 82–86, May, 1999.     

Angular dependence of the coercive force of textured rare earth permanent magnets

On the basis of model calculations experimental results on the angular dependence of the coercivity H_c and the remanence coercivity H_R of hard magnetic materials of the type SmCo₅, Sm₂(Co, Fe, Cu, Zr)₁₅ and Nd₂Fe₁₄B are discussed. In the model coherent rotation as well as incoherent magnetization jumps (e.g. 180°-Bloch walls) are included. The texture is described by an axial symmetric distribution of the easy axes with only oneparameter. For Sm₂(Co, Fe, Cu, Zr)₁₅ the model explains irreversible (H_R (θ)-curves) as well as reversible (H_R(θ)−H_c(θ)) magnetization processesin good agreement with the experiments, whereas stronger deviations exist for SmCo₅ and Nd₂Fe₁₄B, especially in the H_c(θ)-curves. These deviations should be caused by other reversible magnetization processes     

A new TiW seed layer for SmCo5 films with perpendicular magnetic anisotropy

A new seed layer TiW is proposed for SmCo₅ films with perpendicular magnetic anisotropy. The influence of a TiW seed layer on the microstructure and the surface morphology of Cu underlayer are studied. The grain size and surface roughness dependence of Cu underlayer on the thickness and the annealing of the TiW seed layer are also investigated. The improvement in the perpendicular magnetic properties of SmCo₅ film from the TiW seed layer is approved. The results show that a 5 nm Ti₃W₇ seed layer improves the microstructure and surface morphology of Cu underlayer, and significantly improves the perpendicular magnetic properties of SmCo₅ film. The diffusion barrier and a high melting point of the TiW seed layer are regarded as the physical mechanism of the improvement for SmCo₅ film with perpendicular magnetic anisotropy.     

Low temperature magnetic hardness due to Co in CaCu5 type bulk materials

The coercive force at 4.2 K of bulk specimens YCo_5?xNi_x and LaCo_5?xNi_x increases from negligible values (of order H_c < 1 kOe) for x = 0 to values of H_c of 11 and 27 kOe for the Y and La analog compounds with x = 3. This indicates that a transition metal alone can be responsible for the phenomenon of strong low temperature magnetic hardness in bulk specimens formerly found in various systems with anisotropic and magnetic rare earths. The coercive force of SmCo₂Ni₃ is larger than 50 kOe at 4.2 K while the analogous compound on the basis of Tb does not show coercive forces begond 0.5 kOe. The origins of the phenomenon are discussed briefly.     

Influence of partial rare-earth substitution for Sm in the giant intrinsic magnetic hardness compound SmCo2Ni3

The phenomenon of giant intrinsic magnetic hardness is investigated in compounds R_1−x Sm _x Co₂Ni₃ with R=Y, Pr, Gd, Tb, Er. Partial Er substitution for Sm actually increases magnetic hardness while all other substitutions decrease magnetic hardness. The strength of coercivity is thus dependent on both the sign and magnitude of the crystal field interaction. The temperature dependence of coercivity is complex in the case of Pr substitution as a result of competing effects from thermal activation and a decrease in anisotropy at low temperatures. This study has been supported by a grant from the National Science Foundation.     

Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T_N, coercive field H_c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H_c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy.     

Specific features of the states of cobalt fluoride in the vicinity of the critical field

The state of cobalt fluoride in the vicinity of the critical value H _c of a longitudinal magnetic field H, in which the magnetic subsystem of a CoF₂ crystal with a strong Dzyaloshinskii interaction is transformed from the antiferromagnetic phase into the canted phase, has been investigated taking into account the increasing number of experimental studies related to the use of cobalt fluoride. It has been found that, despite the unusually high magnetic anisotropy of the crystal, the state of the magnetic subsystem at H = H _c is extremely sensitive to a small deviation of the vector H from the C ₄ axis. Another feature is that the high sensitivity disappears with an increase or decrease in the magnetic field by only a few thousandths of H _c . The results of the investigations performed in this work are applicable to magnetically ordered crystals FeF₃ and Cu₂OSeO₃, which, as well as the CoF₂ crystals, are characterized by a strong Dzyaloshinskii interaction and a significant magnetic anisotropy. The revealed anomaly in the reduction of the effective magnetic anisotropy is of interest in connection with numerous attempts to decrease the magnetic anisotropy in crystals with giant magnetostriction, which are necessary for the use as sensors and vibrators.     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Magnetic studies on RCo3B2 (R=Sm,Gd, Er)

Compounds RCo₃B₂ withR=Sm, Gd, and Er were prepared and identified structurally by X-ray diffraction. Magnetic properties of these materials were studied down to liquid He temperature. The materials order ferromagnetically (e.g.,T _c for GdCo₃B₂ is 58 K). The magnitude of saturation moments indicates that Co is nonmagnetic in this environment. Bulk specimens of the compound SmCo₃B₂ show extraordinarily high coercive forces at 4.2 K (H _c =50 kOe). The second-order crystal field term of Sm in this compound is calculated on the point charge crystal field model and is found to be considerably larger than the one of Sm in SmCo₅. This is mainly due to the unusual ratio of the lattice constants. This work was supported by a grant from the National Science Fundation.     

Superheating,supercooling, surface superconductivity and Ginzburg-Landau parameters of pure type-I superconductors and their alloys

We have measured the low-frequency differential susceptibility of several pure type 1 superconductors, viz. Sn, In, Tl, Pb and Hg, as a function of increasing or decreasing applied magnetic fields. Special emphasis was placed on the observation of metastable (superheated and supercooled) states. Experimental values of the Ginzburg-Landau parameter? at the transition temperatureT _c and of suitably defined temperature dependent parameters for superheating,? _sh(t), and for supercooling,? _sc(t), have been obtained and compared with theoretical estimates. The rapid drop in? _sh close toT _c observed in materials with??1 such as Sn, In, and Tl is interpreted as a manifestation of nonlocal electrodynamics. Supercooling of the normal state down to the bulk nucleation fieldH _c2, rather thanH _c3, has been achieved close toT _c in Cu-plated samples of Sn and In. The parameters? and?(t) have also been determined for a series of SnIn alloys spanning the range between type I and type II behavior. Supercooling of the superconducting surface sheath below the thermodynamic critical fieldH _c has been observed in these alloys as well as in pure Pb. The measured values of? are compared with estimates based on normal state parameters extracted from independent low temperature experiments. Corrections for energy gap anisotropy and electron-phonon retardation are typically below 10%. The strong temperature dependence of? _sc(t), observed in all the materials studied, is not explained by present theories, even in the concentrated alloys where anisotropy effects should be washed out.     

Magnetic anisotropy of an extended double exchange model for layered manganites: A Monte Carlo study

The magnetic ground state properties of layered manganites are investigated on the basis of an anisotropic double exchange model using a Monte Carlo technique. The temperature dependence of magnetization and spin-spin correlations are calculated in a highly anisotropic hopping integral t _c/t _ab regime. The ferromagnetic ordering temperature (T_c) is suppressed by introducing t _c/t _ab, and eventually a layered ferromagnetic structure appears along the c-axis, but there are block-walls. The significant change of magnetic anisotropy is also observed for the antiferromagnetic superexchange integral J _c/t _ab. We discuss the connection of these results to the magnetic anisotropy observed in the La_1.4Sr_1.6Mn₂O₇.     

Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H _c2(T)|,T ∼ 0 andT →T _c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|H_c2(0)|/[-Tδ|H _c2 T|δT]_T{s0} for the case of a FS open ink _z-direction with thek _z-bandwidth, 4t ₃, very small compared to the Fermi energy,E _F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT _c and the temperature dependence of H _c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa₂Cu₃O₇ used elsewhere, we find reasonable values for the upper critical fieldH _c2(0), the slope (dH _c2/dT)_{T c0}, anisotropic coherence lengths ζ_i(T=0),i=x, y, z, and (dT _c/d|H|)_{|H| → 0}.     

钇—镧石榴石型铁氧体系统的磁性和铁磁共振的研究

本文对钇-镧石榴石型铁氧体系Y_3(1-x)La_3xFe₅O₁₂(其中x=0,0.05,0.10,O.20,0.30,0.50,0.75和1.00)的磁性和铁磁共振进行了研究。由X射线粉末照相分析和金相观察确定了石榴石转构的单相区域。测量了饱和磁矩σ_s、起始磁导率μ₀、矫顽力H_c及有效g因子g_eff和共振线宽△H(3970和9160兆赫)与La含量x的关系。由磁性和铁磁共振的测量结果表明单相区城要比由X射线分析和金相观察的结果更窄一些。讨论了不同成分的σ_s的变化,μ₀与磁化机构的关系。由g_eff与频率的关系计算出内场H_i和材料的内禀g因子。从磁的不均匀性的观点解释了实验上观测到的△H、H_i和H_c在一定成分范围内与x的指数式关系。 关键词：     

Highly anisotropic resin-bonded magnets processed with surfactant-coated SmCo5 nanocrystalline powders

Highly anisotropic SmCo₅ nanocrystalline powders with grain size in the range 5-20 nm were processed through surfactant and magnetic field-assisted milling. The SmCo₅ nanocrystalline powders so obtained by this method possess unusual characteristics such as reduction in particle size, platelet-structure and high remanence values. A possible mechanism for achieving remanence enhancement with the surfactant-coated SmCo₅ powders has been discussed. Besides, the resin-bonded magnets processed with the surfactant-coated SmCo₅ powders showed relatively higher density, induction remanence and energy product with strong anisotropic behavior than those of the magnets processed with the conventionally milled SmCo₅ powders. Maximum values of H_ci (16 kOe), B_r (4.66 kG) and (BH)_max (5.5 MG Oe) were achieved for the resin-bonded magnets processed with the surfactant-coated powders.     

Upper critical field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>2</Subscript> in anisotropic superconductors with varying charge carrier density: Application of the theory to doped MgB<Subscript>2</Subscript>

A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed. The upper critical field H _c2^(ab) parallel to the ab plane and field H _c2^(c) parallel to the c axis are determined in the entire temperature range 0 < T < T _c . A considerable increase in upper critical field H _c2^(ab) as compared to H _c2^(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γ _H = H _c2^(ab) / H _c2^(c) is obtained as a function of temperature for pure MgB₂ and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential and critical magnetic fields H _c2^(ab) and H _c2^(c) is observed. The effect of doping on magnetic anisotropy is also determined.     

Influence of partial rare-earth substitution for Sm in the giant intrinsic magnetic hardness compound SmCo2Ni3

The phenomenon of giant intrinsic magnetic hardness is investigated in compounds R_1?x Sm _x Co₂Ni₃ with R=Y, Pr, Gd, Tb, Er. Partial Er substitution for Sm actually increases magnetic hardness while all other substitutions decrease magnetic hardness. The strength of coercivity is thus dependent on both the sign and magnitude of the crystal field interaction. The temperature dependence of coercivity is complex in the case of Pr substitution as a result of competing effects from thermal activation and a decrease in anisotropy at low temperatures.     

Microstructure and magnetic properties studies of SmCo5 thin films grown on MgO and glass substrates

In this work, SmCo₅ thin films are deposited on single crystal MgO (1 0 0) and amorphous glass substrates with a Cr underlayer at 400 °C by sputtering. A comparison study shows that the microstructures and magnetic properties are different in the two SmCo₅ films on the MgO (1 0 0) and glass substrates, respectively. An epitaxial growth of Cr-(2 0 0)〈1 1 0〉/SmCo₅-(1 1 2¯ 0)〈0 0 0 1〉 is achieved on the MgO (1 0 0) single crystal substrate with an average grain size of 20 nm for SmCo₅. On the amorphous glass substrate, no significant crystallographic texture is found in the Cr underlayer. After the deposition of SmCo₅, a weak texture of (1 1 2¯ 0) is observed with an average grain size of 8 nm. High remanence ratio value in this film is probably due to strong exchange coupling. Both SmCo₅ films show high in-plane coercivity, high in-plane anisotropy and remanence enhancement.