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Using ferromagnetic-resonance spectroscopy (FMR), we investigate the anisotropic properties of epitaxial 3 nmPt/2 nmAg/10 nmFe/10 nmAg/GaAs(001) films in fully saturated meta-stable states at temperatures ranging from 70 K to 280 K. By comparison to spin-wave theory calculations, we identify the role of thermal fluctuation of magnons in overcoming the energy barrier associated with these meta-stable states. We show that the energy associated with the size of the barrier that bounds the meta-stable regime is proportional to the heat stored in the magnonic bath. Our findings offer the possibility to measure the magnonic contribution to the heat capacity by FMR, independent of other contributions at temperatures ranging from 0 K to ambient temperature and above. The only requirement being that the selected sample exhibits magnetic anisotropy, here, magnetocrystalline anisotropy.
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Specific heat measurements constitute one of the most powerful experimental methods to probe fundamental excitations in solids. After the proposition of Einsteins model, more than one century ago (Annalen der Physik textbf{22}, 180 (1907)), several theoretical models have been proposed to describe experimental results. Here we report on a detailed analysis of the two-peak specific heat anomalies observed in several materials. Employing a simple multilevel model, varying the spacing between the energy levels $Delta_i$ = $(E_i$ $-$ $E_{0})$ and the degeneracy of each energy level $g_i$, we derive the required conditions for the appearance of such anomalies. Our findings indicate that a ratio of $Delta_2$/$Delta_1$ $thickapprox$ 10 between the energy levels and a high degeneracy of one of the energy levels define the two-peaks regime in the specific heat. Our approach accurately matches recent experimental results. Furthermore, using a mean-field approach we calculate the specific heat of a degenerate Schottky-like system undergoing a ferromagnetic (FM) phase transition. Our results reveal that as the degeneracy is increased the Schottky maximum in the specific heat becomes narrow while the peak associated with the FM transition remains unaffected.
Specific heat measurements were used to study the magnetic phase transition in Ga1-xMnxAs. Two different types of Ga1-xMnxAs samples have been investigated. The sample with a Mn concentration of 1.6% shows insulating behavior, and the sample with a Mn concentration of 2.6% is metallic. The temperature dependence of the specific heat for both samples reveals a pronounced lambda-shaped peak near the Curie temperature, which indicates a second-order phase transition in these samples. The critical behavior of the specific heat for Ga1-xMnxAs samples is consistent with the mean-field behavior with Gaussian fluctuations of the magnetization in the close vicinity of TC.
We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260meV. An analysis with the local-moment J_1-J2 model implies very different in-plane nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in the orthorhombic and tetragonal phases. However, the spectrum calculated from the J1-J2 model deviates significantly from our data. We show that the qualitative features that cannot be described by the J1-J2 model are readily explained by calculations from a 5-band itinerant mean-field model.
In recent years, there has been increasing interest in the specific heat $C$ of insulators and semiconductors because of the availability of samples with different isotopic masses and the possibility of performing textit{ab initio} calculations of its temperature dependence $C(T)$ using as a starting point the electronic band structure. Most of the crystals investigated are elemental (e.g., germanium) or binary (e.g., gallium nitride) semiconductors. The initial electronic calculations were performed in the local density approximation and did not include spin-orbit interaction. Agreement between experimental and calculated results was usually found to be good, except for crystals containing heavy atoms (e.g., PbS) for which discrepancies of the order of 20% existed at the low temperature maximum found for $C/T^3$. It has been conjectured that this discrepancies result from the neglect of spin-orbit interaction which is large for heavy atoms ($Delta_0sim$1.3eV for the $p$ valence electrons of atomic lead). Here we discuss measurements and textit{ab initio} calculations of $C(T)$ for crystalline bismuth ($Delta_0sim$1.7 eV), strictly speaking a semimetal but in the temperature region accessible to us ($T >$ 2K) acting as a semiconductor. We extend experimental data available in the literature and notice that the textit{ab initio} calculations without spin-orbit interaction exhibit a maximum at $sim$8K, about 20% lower than the measured one. Inclusion of spin-orbit interaction decreases the discrepancy markedly: The maximum of $C(T)$ is now only 7% larger than the measured one. Exact agreement is obtained if the spin-orbit hamiltonian is reduced by a factor of $sim$0.8.
We present measurements of pure spin current absorption on lateral spin valves. By varying the width of the absorber we demonstrate that spin current absorption measurements enable to characterize efficiently the spin transport properties of ferromagnetic elements. The analytical model used to describe the measurement takes into account the polarization of the absorber. The analysis of the measurements allows thus determining the polarization and the spin diffusion length of a studied material independently, contrarily to most experiments based on lateral spin valves where those values are entangled. We report the spin transport parameters of some of the most important materials used in spinorbitronics (Co60Fe40, Ni81Fe19, Co, Pt, and Ta), at room and low (10 K) temperatures.
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