Antiferromagnetic resonance and high magnetic field properties of NaNiO2

Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on NaNiO₂ powders at low temperature. Also typical spectra above the Néel temperature are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. /Ni confirms the low spin state of the Ni³⁺ ions. The susceptibility exhibits a maximum at K with and K. NaNiO₂ is an A-type antiferromagnet: we derive K and K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with can be attributed to the Jahn-Teller effect for Ni³⁺ ions in the low spin state, which stabilises the occupation. We finally comment on the isomorphic controversial Li_1-xNi_1+xO₂ compound. Received 9 March 2000 and Received in final form 13 July 2000.     

X-ray detected ferromagnetic resonance in thin films

We discuss the physical content of X-ray Detected Magnetic Resonance (XDMR), i.e. a novel spectroscopy which uses XMCD to probe the resonant precession of the local magnetization in a strong microwave pump field. We focus on the simplest case of a steady-state precession of elemental moments in the non-linear regime of angular foldover. Like XMCD, XDMR is element and edge selective and could become a unique tool to investigate how precessional dynamics can locally affect the spin and orbital magnetization of p- or d-projected DOS. This should be possible only in the limit where there is no overdamping due to ultrafast orbit-lattice relaxation.     

On the 240 K anomaly in the magnetic properties of LiNiO2

In this work, results of X-band ESR spectroscopy, ac-magnetic susceptibility and X-ray powder diffraction measurements on Li_1-xNi _1+x O₂ (x = 0.02 and x = 0.07) are presented and discussed. While the susceptibility of the compound with x = 0.02 is shown to follow a Curie-Weiss law, with a Weiss temperature of the order of 30 K, the compound with x = 0.07 is found to order ferromagnetically below K. However, an additional anomaly is observed in the magnetic properties of this latter compound at around 240 K. We attribute this anomaly to the presence of macroscopic Ni-rich regions which order ferrimagnetically below this temperature. This phenomenon is different from the bulk ferromagnetism that occurs at much lower temperatures, and allows us to discard earlier suggestions proposed in the literature in which the 240 K anomaly has been considered as denoting an intrinsic phenomenon. Received 14 May 1999 and Received in final form 5 August 1999     

On the cross-over from half-metal to normal ferromagnet in NiMnSb

Magnetism and transport properties of the semi-Heusler compound NiMnSb are re-examined in great details. A wide set of experiments (elastic and inelastic neutron scattering, static magnetic measurements, magnetoresistance, Hall effect, thermopower, FMR) have been performed on polycrystals, single crystals or single-crystalline thin films, and the results are analysed. Special emphasis is given to the magnetic excitations and to the relaxation mechanisms in this metallic ferromagnet. At low temperatures, all experimental results hint at the existence of a fully spin-polarized conduction band (half metallic state). At higher temperature (T > 80 K), but well below the Curie temperature (730 K), a cross-over to a usual metallic ferromagnetic state is evidenced and discussed. Received 10 January 2000     

Colossal magnetostriction effect in HoMn2O5

We have investigated the detailed field and temperature dependence of the dielectric constant, electric polarization, magnetization and magnetostriction in orthorhombic HoMn₂O₅ single crystals. HoMn₂O₅ displays incommensurate antiferromagnetic ordering below 39 K, becoming commensurate on further cooling. The commensurate-incommensurate transition takes place at low temperatures. The inherent magnetic frustration in this material is lifted by a small lattice distortion, primarily involving shifts of the Mn³⁺ cations and giving rise to a canted antiferroelectric phase. Colossal magnetostriction effect was observed and a novel phase transition diagram was build.     

Quantitative model of large magnetostrain effect in ferromagnetic shapell memory alloys

A quantitative model describing the large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar types of materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa. Received 29 September 1999     

Classical magnetostriction of nickel in high magnetic field

A simple model is proposed as a possible explanation for the checkerboard pattern of modulations in the hole density observed in recent tunneling experiments on underdoped cuprates. Two assumptions are made; first, an enhanced hole density near the acceptor dopants and secondly short range correlations in the positions of these dopants caused by their electrostatic and anisotropic elastic interactions. Together these can lead to a structure factor in qualitative agreement with experiment.     

Ferromagnetic resonance in arrays of highly anisotropic nanoparticles

We present in this study computational simulations of the ferromagnetic resonance response of magnetic nanoparticles with a uniaxial anisotropy considerably larger than the microwave excitation frequency (in field units). The particles are assumed to be randomly oriented in a two dimensional lattice, and are coupled by dipolar interactions through an effective demagnetization field, which is proportional to the packing fraction. We have included in the model fluctuations in the anisotropy field (H_K) and allowed variations in the demagnetizing field. We then analyzed the line shape and line intensity as a function of both fields. We have found that when H_K is increased the line shape changes drastically, with a structure of two lines appearing at high fields. The line intensity has a maximum when H_K equals the frequency gap and decreases considerably for larger values of the anisotropy. The effects of fluctuations in H_K and variations in the packing fraction have been also studied. Comparison with experimental data shows that the overall observed behavior is dominated by the particles with lower anisotropy.     

Ferromagnetic resonance study of sputtered Co|Ni multilayers

We report on room temperature ferromagnetic resonance (FMR) studies of [ t Co|2t Ni]  × N sputtered films, where 0.1 ≤ t ≤ 0.6 nm. Two series of films were investigated: films with the same number of Co|Ni bilayer repeats (N = 12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N = 1.2/t). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field normal to the plane, and as a function of angle to the plane for several frequencies. For both sets of films, we find evidence for the presence of first and second order anisotropy constants, K₁ and K₂. The anisotropy constants are strongly dependent on the thickness t, and to a lesser extent on the total thickness of the magnetic multilayer. The Landé g-factor increases with decreasing t and is practically independent of the multilayer thickness. The magnetic damping parameter α, estimated from the linear dependence of the linewidth ΔH, on frequency, in the field in-plane geometry, increases with decreasing t. This behaviour is attributed to an enhancement of spin-orbit interactions with decreasing Co layer thickness and in thinner films, to a spin-pumping contribution to the damping.     

Ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies

We develop a model for ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies. This model applies to (i) magnetic materials with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with effective core and surface anisotropies; We numerically compute the resonance frequency as a function of the field and the resonance field as a function of the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial anisotropy. We also provide some approximate analytical expressions in the case of weak cubic anisotropy. We propose a method that uses these expressions for estimating the uniaxial and cubic anisotropy constants, and for determining the relative orientation of the cubic anisotropy axes with respect to the crystal principle axes. This method is applicable to the analysis of experimental data of resonance type measurements for which we give a worked example of an iron thin film with mixed anisotropy.     

Uniform spin wave modes in antiferromagnetic nanoparticles with uncompensated moments

In magnetic nanoparticles the uniform precession (q = 0 spin wave) mode gives the predominant contribution to the magnetic excitations. We have calculated the energy of the uniform mode in antiferromagnetic nanoparticles with uncompensated magnetic moments, using the coherent potential approximation. In the presence of uncompensated moments, an antiferromagnetic nanoparticle must be considered as a kind of a ferrimagnet. Two magnetic anisotropy terms are considered, a planar term confining the spins to the basal plane, and an axial term determining an easy axis in this plane. Excitation energies are calculated for various combinations of these two anisotropy terms, ranging from the simple uniaxial case to the planar case with a strong out-of-plane anisotropy. In the simple uniaxial case, the uncompensated moment has a large influence on the excitation energy, but in the planar case it is much less important. The calculations explain recent neutron scattering measurements on nanoparticles of antiferromagnetic α-Fe₂O₃ and NiO.     

Thermal expansion and magnetostriction in an anomalous Gd intermetallic compound

Thermal expansion and forced magnetostriction measurements are reported on two Gd intermetallic compounds which order magnetically below 10 K. The relative influence of the electronic, lattice and magnetic degrees of freedom was determined using results obtained on a non-magnetic isostructural compound. A Grüneisen analysis revealed that whilst the magnetic contribution to the specific heat is similar for both Pd₂GdIn and Cu₂GdIn the spontaneous magnetostriction was significantly smaller in the Pd compound. Forced magnetostriction measurements suggest that the thermal expansion in Pd₂GdIn is primarily associated with spin fluctuations in the Pd 4d band. It is suggested that these additional degrees of freedom give rise to the enhanced specific heat observed in Pd₂GdIn. Received 28 July 1999     

Nonlinear transport in {\sf \beta}-Na 0.33V 2O 5

Transport properties of the charge ordering compound β-Na_0.33V₂O₅ are studied in the temperature range from 30 K to 300 K using current driven DC conductivity experiments. It is found that below the metal-insulator transition temperature ( ) this material shows a nonlinear charge density modulation behavior. The observed conductivity is discussed in terms of a classical domain model for charge density modulation transport.     

Magnetic couplings vs. stress and strain in epitaxial (La, Sr)MnO3

Based on ab initio calculations, we quantify the magnetic couplings and the stress tensor in ferromagnetic Sr-doped LaMnO₃ upon combined application of built-in epitaxial and external uniaxial strains. We suggest YAlO₃ as a substrate suited to change magnetic order in manganite film with practicable external strains. The effect could lead to strain-activated switches based on piezoelectric-piezomagnetic heterojunctions.     

High magnetic field properties and ESR of the compounds

compounds have a layered structure made of alternating Ni-O and Li-O slabs. An amount z of extra divalent Ni ions is always present in the Li-O layers. We show, using high field magnetisation, static and dynamic susceptibility and high frequency ESR, that the magnetic properties are driven by the z parameter. The compounds can be described as ferromagnetic Ni-O layers, bridged by clusters possessing a net ferromagnetic moment. Received: 24 July 1998 / Revised and Accepted: 23 September 1998     

Spontaneous SU(2) symmetry breaking in one-dimensional quantum antiferromagnet

We present a Bethe Ansatz based investigation of a one-dimensional (1D) Heisenberg spin chain in a real 3D crystal lattice. We have shown that due to an influence of the lattice distortion on a crystalline field of ligands of magnetic ions, a Heisenberg antiferromagnetic spin chain is unstable under the appearance of a magnetic anisotropy of the “easy-plane” type. The effects of an external magnetic field and nonzero temperature onto such a phase transition are studied. Received: 19 January 1998 / Revised: 16 March 1998 / Accepted: 17 March 1998     

On dissipation mechanisms in micromagnetics

Within the framework of a dynamic version of micromagnetics [1,2], the space-time evolution of magnetization in a rigid, saturated ferromagnet is governed by the following equation: γ^-1 = ×( + + div ), where the interaction couple × and the couple stress are to be constitutively specified. Under constitutive assumptions for , , and the free energy ψ, that allow for equilibrium response and viscosity out of equilibrium and agree with the dissipation principle - ^. + ^. ∇ - ≥ 0, the above evolution equation yields a broad generalization of the standard Gilbert equation. In particular, while the standard Gilbert equation only incorporates relativistic dissipation, it is shown that the dissipation mechanisms compatible with the generalized Gilbert equation include exchange dissipation [2], dry-friction dissipation [3], and others. It is also shown that the additional term proposed in [4] to account for exchange dissipation, rather than having a genuine dissipative nature, modifies instead the nature of possible equilibria; and that such a modification is an automatic side effect when dry-friction dissipation is incorporated in the manner of [3]. Received 31 October 2000     

Interplay of metamagnetic and structural transitions in Ca2-xSrxRuO4

Metamagnetism in layered ruthenates has been interpreted as a novel kind of quantum critical behavior. In an external magnetic field, Ca_2-xSr_xRuO₄ undergoes a metamagnetic transition accompanied by a pronounced magnetostriction effect. In this paper we present a mean-field study for a microscopic model that naturally reproduces the key features of this system. The phase diagram calculated is equivalent to the experimental T-x phase diagram. The presented model also gives a good basis to discuss the critical metamagnetic behavior measured in the system.     

Influence of hydrogen distribution on magnetic properties of amorphous samples

We have studied the effect of the homogeneity of hydrogen distribution on magnetic properties of hydrogen charged amorphous samples (Metglass 2826MB: Fe₄₀Ni₄₀Mo₄B₁₈). The results suggest that, in high magnetostrictive samples, the induced internal stresses and hence the magnetic anisotropy are related to the hydrogen distribution and not to the total quantity of hydrogen inside the sample. To perform this study, the homogeneity of hydrogen distribution in the sample was improved by using a very low rate of hydrogen charging by means of a pulsed electrolytic current. Furthermore, this pulsed electrolytic current allows us to introduce a high quantity of hydrogen without breaking the sample. Received 7 May 1999 and Received in final form 11 October 1999