A new quasi-Ising magnet

We have performed measurements of the magnetization and differential magnetic susceptibility of Dy_0.62Y_2.38Fe₅O₁₂ single crystals in pulsed magnetic fields up to 45 T at liquid-helium temperature for three orientations of the external field: H‖[100], H‖[110], and H‖[111]. It was found that the magnetization reversal in the rare-earth magnetic subsystem occurs via several phase transitions, whose number depends on the direction of the external field, as is characteristic for Ising magnets. The anomalies in the field dependences of the magnetization are interpreted on the assumption of quasi-Ising ordering of the rare-earth ions. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 552–556 (25 April 1998)     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

Magnetic structure of a RbDy(WO4)2 single crystal

The results of measuring the temperature and field dependences of the magnetization of a RbDy(WO₄)₂ single crystal in the temperature range from 4.2 to 50 K and in magnetic fields up to 1.6 T are presented. The energies of the exchange and dipole-dipole interactions are estimated. The magnetic structure of the ground state is determined. Fiz. Tverd. Tela (St. Petersburg) 41, 672–676 (April 1999)     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Magnetism and structural phase transitions in LiTmF4 powders

The field (0–5.5 T) and temperature (2–300 K) dependences of the magnetization of LiTmF₄ powders with particle sizes of 1 μm and 56–400 μm are investigated experimentally and theoretically. It is concluded that a transition layer exists between the thulium ions in the bulk and the ions at the surface. Two magnetic-field-induced structural phase transitions are observed at low temperatures, and the temperature dependence of the critical magnetic fields is established. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 247–250 (25 August 1997)     

Influence of a nonmagnetic impurity on the properties of the quasi-one-dimensional antiferromagnet CsNiCl3

Various magnetic properties of the diluted quasi-one-dimensional antiferromagnet CsNi_1−x MgxCl₃ are investigated experimentally for several impurity concentrations. The antiferromagnetic resonance spectrum and the phase diagrams are found to depend significantly on the amount of added Mg. The field and temperature dependences of the static magnetization is measured for crystals with two different contents x. A substantial increase in the magnetization is observed at low temperature, where the additional susceptibility is approximately proportional to the concentration. The physical mechanisms underlying the observed strong influence of magnetic defects formed at breaks in the spin chains in a quasi-one-dimensional antiferromagnet on its magnetic properties in the ordered state and for T<T _N are discussed. Zh. éksp. Teor. Fiz. 112, 209–220 (July 1997)     

Magnetic and crystalline properties of GdxLa1−x FeSi compounds

The magnetic and crystalline properties of the polycrystalline compounds Gd_xLa_1−x FeSi (x=0, 0.1, 0.3, 0.45, 0.55, 0.65, 0.8, 1) and single crystal GdFeSi are studied. All the compounds have the CeFeSi tetragonal structure. The temperature dependences of the magnetization and initial susceptibility show that ferromagnetic behavior is characteristic of these compounds; the exception is the Pauli paramagnetic LaFeSi. Substitution of gadolinium by the nonmagnetic lanthanum leads to a sharp drop in the Curie temperature T _c, and to a reduction in the saturation magnetic moment μ ₀, effective magnetic moment μ _eff, and paramagnetic Curie temperature Θ_p. Measurements on single crystal GdFeSi imply that the easy magnetization axis of this compound is the c axis and the anisotropy field is H _a∼4 kOe. No magnetic moment was observed on the iron ion in any of these compounds. Fiz. Tverd. Tela (St. Petersburg) 39, 325–329 (February 1997)     

Magnetization-odd nonreciprocal reflection of light from the magnetoelectric—ferromagnet LiFe5O8

Circular dichroism, which changes sign under magnetization reversal and exhibits a 120° periodicity, is observed in the reflection of light from the (111) plane of a noncentrosymmetric LiFe₅O₈ crystal in the transverse geometry k⊥M, where the linear Kerr effect is forbidden. It is shown that this phenomenon is due to the manifestation of optical magnetoelectric susceptibility and a Kerr effect of third order in the magnetization. The spectral dependences of the circular dichroism in the range 1.4–3.1 eV show that this phenomenon is of a resonance character. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 65–70 (10 January 1997)     

Conductivity jumps and low-temperature magnetoresistance in the new semiconducting perovskites La1− x BixMnO3+δ

Abrupt jumps are observed on the temperature dependences of the electrical conductivity of La_{1− x} Bi_xMnO_3+δ (0.1<x<0.7) and La_0.3Bi_0.7Na_0.1MnO_3+δ ceramics. The temperatures at which the jumps take place can be reduced by several degrees by applying an external magnetic field of 15 kOe. It can be seen from the magnetization data that the poorly conducting low-temperature phase is not antiferromagnetic (it is likely ferromagnetic) and hence the real-space charge ordering mechanism is not sufficient for explaining the experimental results. An interpretation in terms of magnetic phase nonuniformity is proposed. All the samples studied exhibit high low-temperature magnetoresistance. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 250–253 (25 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Anomalous magnetic properties of the complex (ET)2C60

Investigations of the temperature dependences of the magnetic permeability and dielectric permittivity in the temperature range 4.2 K⩽T ⩽300 K and the field dependence of the magnetization M(B) in fields B⩽50 T show that the magnetic properties of the complex (ET)₂C₆₀ cannot be described on the basis of the standard model, which assumes that the paramagnetic oxygen impurity makes the dominant contribution. It is found that the magnetism in (ET)₂C₆₀ is due to the diamagnetic properties of the C₆₀ and ET molecules and to specific paramagnetic centers of the type C ₆₀ ⁻ , which possess an anomalously low g factor |g|≈0.14. An experimentum crucis is proposed for checking the oxygen paramagnetic center model for pure C₆₀ films. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 733–738 (25 May 1999)     

Magneto-optical phenomena in MnAs/CaF2/Si(111) epitaxial films in a transverse magnetic field

Field and angular dependences of the rotation of the plane of polarization in a transverse magnetic field H⊥k under normal reflection of light (λ=633 nm) have been studied in MnAs ferromagnetic epitaxial films grown by MBE on CaF₂/Si(111) substrates. The angle of rotation of the plane of polarization a is shown to be determined by contributions even and odd in the magnetization M. The odd contribution is associated with the deviation of the easy plane of magnetic anisotropy from the film plane, which originates from misorientation of the Si surface from the (111) plane, or from a presence of small regions of ( )-oriented MnAs. The even contribution is due to the optical anisotropy of films connected with quadratic-in-M terms in the dielectric permittivity tensor ɛ _ij of manganese arsenide. A method based on measuring the angular dependences of a in a rotating magnetic field is proposed to separate these contributions. Fiz. Tverd. Tela (St. Petersburg) 41, 110–115 (January 1999)     

Magnetization curves of DyN clusters

Magnetic properties of isolated Dy_N clusters are studied in a molecular beam performing Stern-Gerlach experiments. The magnetizations μ _z of Dy_N are measured in dependence of the magnetic field strength B = 0-1.6 T and at nozzle temperatures T _n = 18 K and T _n = 300 K. At room temperature the magnetization augments linear with the field following a simple paramagnetic model. At T _n = 18 K the magnetization curves saturate at field strengths B ≥0.8 T. To explain the magnetization process at low temperatures two models are discussed: A model for adiabatic magnetization based on cluster rotation effects and a modified van-Vleck model. Received 30 November 2000     

Magnetic properties of an easy-plane trigonal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> antiferromagnet

The field and temperature dependences of magnetization and the temperature dependences of the initial magnetic susceptibility have been theoretically studied for three crystallographic directions in a trigonal NdFe₃(BO₃)₄ antiferromagnetic crystal. The calculations were performed using a molecular field approximation and a crystal field model for the rare-earth subsystem. The obtained theoretical expressions are applied to the interpretation of recent experimental data [1–4] on the magnetic properties of NdFe₃(BO₃)₄. The results of calculations show a good agreement with experiment. The proposed theory adequately describes (i) anomalies of the Schottky type in the temperature dependence of the magnetic susceptibility, (ii) nonlinear curves of magnetization in the basal plane in a magnetic field up to 1 T (showing evidence of the first-order phase transitions) and their evolution with the temperature, and (iii) the field and temperature dependences of magnetization in a magnetic field up to 9 T.     

Dynamic magnetization of <Emphasis Type="Bold">γ</Emphasis>- nanoparticles isolated in an amorphous matrix

We have studied the magnetization of a system of γ-Fe₂O₃ (0.68 vol.%) nanoparticles isolated in an SiO₂ amorphous matrix placed in an alternating magnetic field with a frequency of 640 Hz and in the temperature range of (77-300) K. Compared to temperatures closer to 300 K (where the system has a superparamagnetic behaviour), at lower temperatures, the magnetization has a dynamic hysteresis loop due to the magnetization's phase shift between the field and the magnetization. The delay of the magnetization (attributed to the Néel relaxation processes) increases with the decrease of temperature. It has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant ( K ) that takes into account the magnetocrystalline anisotropy, as well as the shape, surface and strain anisotropies. In the following we will show that the surface and strain anisotropy components have the most significant influence. When the temperature decreases from 300 to 77 K, the relative increase of the saturation magnetization of the nanoparticles is much higher than that of the (spontaneous) saturation magnetization of bulk γ-Fe₂O₃. This increase is due to the increase of the mean magnetic diameter of the particles attached to the core of aligned spins, from 10.16 nm to 11.70 nm, as a result of the modification of the superexchange interaction in the surface layer. Received 25 April 2002 / Received in final form 11 August 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: ccaizer@physics.uvt.ro     

Magnetoelastic and elastocaloric effects in rare-earth metals,their alloys and compounds in the region of magnetic phase transitions

This article discusses experimental data and their theoretical interpretation concerning the volume magnetostriction, spontaneous magnetostriction, variation of magnetization under the action of pressure, and elastocaloric effects in rare-earth metals, as well as their alloys and compounds. Particular attention is paid to the region of phase transitions. The volume magnetostriction ω of true magnetization was investigated near the Curie temperature Θ as a function of magnetization and determined from the change of magnetostriction under the action of pressure. From these data we obtained the dependence of the exchange integrals on the unit cell volume. Giant volume magnetostriction and magnetoelastic elastocaloric effects were discovered in the rare-earth metals and alloys in the region of their magnetic phase transitions. It was established that giant volume magnetostriction in RCo₂ compounds is caused by a critical increase of the magnetic moment of the 3d sublattice of cobalt in magnetic fields that exceeds the critical field at T > Θ. Giant volume magnetostriction in R₂Fe₁₇ compounds near the temperature Θ is shown to occur due to strong deformational dependences of exchange interaction and the value of the 3d electron bandwidth.     

Critical behavior of electron spin resonance in La0.8Sr0.2MnO3 — a material with colossal magnetoresistance

High-frequency absorption in the colossal magnetoresistance material La_0.8Sr_0.2MnO₃ is investigated near the critical temperature in a magnetic field. The temperature dependences of the width, position, and intensity of the ESR line are studied in the near-critical region. Pis’fma Zh. éksp. Teor. Fiz. 67, No. 12, 1000–1004 (25 June 1998)     

Critical behavior of a degenerate ferromagnet in a uniaxial random field: Exact results in a space of arbitrary dimension

The critical behavior of the transverse (with respect to the field) magnetization component in classical degenerate magnets with only nearest-neighbors interaction in a uniaxial random magnetic field at zero temperature is found exactly. For a Gaussian distribution of the random field the asymptotic transverse magnetization in strong fields does not depend on the dimension of the space and is of the form m _⊥ ∝ 1nh ₀/h ₀ ² , where h ₀ is the width of the distribution. For a bimodal distribution, where only the field direction is random and the amplitude is fixed, the transverse magnetization behaves as m _⊥∝exp(−const/(H _c −H) ^D/2), where H is the amplitude of the random field, D is the dimension of the space, and H _c is the critical field. Zh. éksp. Teor. Fiz. 115, 2143–2159 (June 1999)     

Investigation of metamagnetic transitions in the itinerant d subsystem of the intermetallics RCo2 in superstrong magnetic fields up to 300 T

The time derivatives of the magnetization of intermetallic compounds RCo₂ (R = Y, Tm, Er, Ho, or Dy) with a metamagnetic subsystem of itinerant d electrons have been measured in pulsed magnetic fields up to 300 T generated by an explosive method. Peaks associated with abrupt demagnetization or magnetization of the d subsystem are found in the field dependences of dM/dH. The results obtained are compared with theoretical estimates and the values derived for the critical fields from measurements performed on substituted compounds. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 188–192 (10 August 1996)     

Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.