J.I. Arnaudas

Cobalt nanoparticles deposited and embedded in AlN: Magnetic, magneto-optical, and morphological properties

We present a structural, morphological, magnetic, and magneto-optical study of cobalt nanoparticles deposited on 50A AlN∕c-sapphire substrates and embedded in an AlN matrix. The dependence of the properties of Co nanoclusters deposited on AlN with growth temperature and amount of deposited Co are studied and discussed. Also we directly compare the properties of as grown and AlN embedded Co nanoclusters and show that the AlN matrix has a strong impact on their magnetic and magneto-optical properties.

Spin reorientation in RECo5 compounds: A.C. susceptibility and thermal expansion

Abstract Thermal expansion (th.e.) and low frequency (15 Hz) initial susceptibility χ∥ (real, χ∥, and imaginary, χ∥ components) measurements were performed between 3.2 and 300 K for the uniaxial RECo5 permanent magnet compounds (RE = Pr, Nd, Ho and Y) in the form of oriented powder samples along the c-axis, and also for polycrystalline ones of NdCo5 and DyCo5.2. Anomalies in the th.e. and χ∥ were detected in the spin reorientation (SR) regime, in particular for temperatures at the beginning (TSR1) and ending (TSR2) of the SR. We also present simple models which explain reasonably well such anomalous behaviour.

Intrinsic low field susceptibility in pseudobinary DyxY1−>x>Al2andTbxGd1−xAl2 cubic intermetallic compounds

Abstract Low field ( H ac ≈ 3.5 x 10 -6 T ) and low frequency (15 Hz) ac susceptibilities, χ i , have been measured between 3.3 K and well above the Curie temperatures, for the intermetallic Laves phase compounds Dy x Y 1- x Al 2 and Tb x Gd 1- x Al 2 . It is concluded that χ i is, at low enough temperatures, dominated by the domain wall excursions around the minima of the anisotropy Peierls potential wells. The derived model relation χ i ( T ) H c ( i ) ( T ) = const., where H c (i) is the intrin sic pinning coercive field, is confirmed at low enough temperatures. The shape of the observed susceptibility cusps around the Curie temperatures in the series Dy x Y 1- x Al 2 ( x 0.35) suggests that these systems are magnetically inhomogenous. The magnetic domain sizes estimated from these measurements seem quite small (between around 0.01 and 0.1 μm).

Strong interfacial magnetoelastic stress in holmium - lutetium (0001) superlattices

The saturation magnetoelastic stress (MS), , breaking the basal plane cylindrical symmetry of the hexagonal structure, has been measured for the series of superlattices (n = 8 to 85 atomic planes, with separation c/2). The MS was directly measured using a low-temperature cantilever technique. The Ho block MSs in the superlattices are larger than in an Ho film of A thickness and in bulk Ho. An analysis accounting for the contributions to coming from: the volume, , the interface, with magnetoelastic parameter , and the epitaxial strain shows that the interfacial magnetoelastic stress is strong, up to about six times larger than for n = 8, and of the opposite sign.

Magnetostriction and thermal expansion of high-Tc magnetic superconductors REBa2Cu3O7−x (RE = Sm, Eu, Gd, Dy, Ho, Er, Tm and Y)

Abstract The magnetostriction, parallel (λ‖) and perpendicular (λ⊥) to the applied magnetic field, has been measured in the HTC magnetic superconductors REBa2Cu3O7−x (RE = Sm, Sm, Eu, Gd, Dy, Ho, Er, Tm and Y), between 3.8 K up to above Tc with applied magnetic fields up to 2.45 T. The anisotropic magnetostriction, λt = λ‖ − λ⊥, is very weak (≤ 10-6) (except for Dy and Ho compounds, where it is large, around -120×10−6 at 4.2 K and 2.45 T), indicative, for anisotropic ions, of weak effective field penetration at the RE sites. Besides λt is of single-ion origin for Dy and Ho compounds, where performing such a test is feasible. The volume striction, ω = λ‖ + 2λ⊥, is of the same order of magnitude (around 20×10-6 ffor most of the systems, indicative of a diamagnetic effect. The thermal expansion (th.e) has been measured for this series, showing a Gruneisen T3 regime at low temperatures, and the Debye temperatures (in the range of ≅ 345–391 K) have been obtained. Noticeable, is that th.e. strains collapse to a universal temperature dependence over most of the range of temperatures explored (between around 25–200 K), indicative of a common anharmonic phonon lattice origin, that of YBa2Cu3O7−x compound.

Intrinsic coercive field in cubic intermetallic compounds. II: TbxGd1−xAl2 and ErAl2

Abstract The thermal dependence of the coercive field in the pseudobinary cubic Laves phase compounds Tb x Gd 1− x Al 2 (0 ⩽ x ⩽1.0) and ErAl 2 has been measured from 3.7 K up to de Cirie temperatures. The coercive field has two contributions: one from the intrinsic pinning of narrow domain walls in the Peierls crystal potential wells; this contribution is preponderant at low temperatures, decaying exponentially with the domain walls width. The other, which for x ⩾ 0.4 is dominant, comes out from the pinning of domain walls by defects (Kersten mechanism). The domain walls seem very narrow in those materials, between 2 and 5 interlayer atomic spacings.

Magnetoelastic study of Ho/Y and Ho/Lu metallic superlattices

Abstract Magnetoelastic stress measurements in Ho/Y and Ho/Lu superlattices were performed by using a capacitive cantilever technique. The magnetic phase diagrams obtained from in-field transitions observed in the stress isotherms show helical, fan and ferromagnetic phases. At zero-field, the ferromagnetic phase is suppressed in Ho/Y and stabilised in Ho/Lu. Magnetoelastic coupling coefficients corresponding to the breaking of the cylindrical symmetry of the basal plane have been determined. Surface magnetostriction is observed for {Ho 31 Lu 19 } 50 .

Magnetic correlations in random anisotropy DyxY1−xAl2 magnets

Abstract Coexistence of short range antiferromagnetic order and random axes of magnetic anisotropy is found in the crystalline Laves phase compounds Dy x Y 1− x Al 2 , for x = 0.35. A neutron diffraction Bragg peak ( 1 2 , 1 2 , 1 2 ) , which vanishes at temperatures much higher than the RMA one, T SG , is observed, together with strong small angle neutron scattering below T SG .

Spin glass order parameter in RE40Y23Cu37 amorphous alloys (RE = rare earth)

Abstract The low field ( H a.c. ⋍ 0.05 Oe) and low frequency (15 Hz) a.c. susceptibility has been measured for the amorphous alloys RE 40 Y 23 Cu 37 , with RE = Gd , Tb , Dy , Ho , and Er . The susceptibility exhibits a cusp, characteristic of a spin-glass phase transition from paramagnetic state. Gd 40 Y 23 Cu 37 seems to exhibit a mixed ferromagnetic-spin glass phase below 62.5 K. Using the Sherrington-Kirkpatrick mean field theory, we have determined, from the observed cusps, the temperature dependence of the Edwards-Anderson spin glass order parameter, the dependence, consistent with theory, being in the form q ( T ) = A ( T SG − T ) β , with β ⋍ 1.

Ferro-, quasiferro- and antiferromagnetic spin-glass orders in random anisotropy crystalline DyxY1−xAl2 compounds

Abstract The nature of the different kinds of magnetic order in the magnetic phase diagram (MPD) of the cubic DyxY1−xAl2 weak random magnetic anisotropy series is clarified. At the spin-glass side of the MPD the system is formed by “antiferromagnetic spin-glass” clusters, with random-sign pair correlations (≅25 A size), with randomized net magnetic moments, but correlated over long distances (≈103 A) (cluster-glass). At the quasiferromagnetic side, ferromagnetic-like Bragg peaks persist down to x=0.40, but a first-order phase transition at x≅0.625 signals the borderline for existence of good ferromagnetic order.