Ac Initial Magnetic Susceptibility Research Articles

Thermoremanence and zero-field-cooled/field-cooled magnetization study ofCox(SiO2)1−xgranular films

A systematic study of $\mathrm{Co}({\mathrm{SiO}}_{2})$ granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented. The experimental results are compared with simulations of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length \ensuremath{\Lambda}, which results in a larger average ``magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ``particle clusters'' of volume ${\ensuremath{\Lambda}}^{3},$ involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature $〈{T}_{B}〉$ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Giant volume magnetostriction in CMR manganites R1−xSrxMnO3(R=Sm, Nd)

Magnetization, ac initial magnetic susceptibility, resistance, magnetoresistance, thermal expansion and magnetostriction measurements are made on R1−xSrxMnO3 (R=Sm, x=0.33, 0.40, 0.45; R=Nd, x=0.33, 0.45) compounds. For all compounds in the TC region we observe a large volume contraction ΔV/V≈0.1% and unusual behavior of the volume magnetostriction ω, namely, a peak of anomalous magnitude of negative volume magnetostriction on the ω(T) curve. We obtain a giant negative volume magnetostriction of ∼5×10−4 at a relatively low magnetic field B=0.9 T and ∼10−3 at a high magnetic field B=13 T for Sm samples. The magnetostriction of Nd compounds is one order of magnitude less. For all compounds the giant magnetostriction is accompanied by colossal negative magnetoresistance equal to 78, 72, and 44% at B=0.9 T for Sm compounds with x=0.33, 0.40, and 0.45, respectively. All of the observed properties are explained in the framework of an electronic phase separation model.

Influence of oxygen content on the structural, magnetotransport, and magnetic properties ofLaMnO3+δ

A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples, with $\ensuremath{\delta}=0,$ 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1--320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in ${\mathrm{LaMnO}}_{3}$ evolves towards a ferromagnetic order as \ensuremath{\delta} increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The ferromagnetic coupling weakens for $\ensuremath{\delta}g~0.1.$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in \ensuremath{\delta}: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\ensuremath{\delta}g~0.1.$ The sample $\ensuremath{\delta}=0.1$ shows a structural transition from rhombohedral to orthorhombic below ${T}_{S}\ensuremath{\approx}300\mathrm{K}$ with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for $\ensuremath{\delta}=0.15,$ in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below ${T}_{C}$ for $\ensuremath{\delta}g~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.10$ samples we found at temperatures below ${T}_{C}$ magnetic and structural anomalies that are characteristic of charge ordering.

Magnetic structures and magnetic phase diagram ofNdxTb1−xMn2Ge2

The various magnetic structures and magnetic phase transitions in the series ${\mathrm{Nd}}_{\mathrm{x}}$${\mathrm{Tb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{2}$${\mathrm{Ge}}_{2}$ have been thoroughly studied by means of macroscopic magnetic and thermal measurements (such as magnetization, ac initial magnetic susceptibility and linear thermal expansion) and microscopic neutron-diffraction experiments. As a result, the magnetic phase diagram has been determined over the whole temperature range. Large changes in the local Mn magnetic moments (e.g., \ensuremath{\Delta}${\mathrm{\ensuremath{\mu}}}_{\mathrm{Mn}}$/${\mathrm{\ensuremath{\mu}}}_{\mathrm{Mn}}$\ensuremath{\approx}16% in ${\mathrm{TbMn}}_{2}$${\mathrm{Ge}}_{2}$) have been detected at the magnetic phase transitions observed at low temperatures, \ensuremath{\approx}100--140 K, in the x=0--0.4 alloys. This variation, together with the appearance of magnetic ordering in the rare-earth sublattice, has been related to the volume anomalies found (e.g., \ensuremath{\Delta}V/V\ensuremath{\approx}0.3% in ${\mathrm{TbMn}}_{2}$${\mathrm{Ge}}_{2}$). A new magnetic structure of the Mn sublattice in the ${\mathrm{RMn}}_{2}$${\mathrm{X}}_{2}$ (R=rare earth, X=Si, Ge) family has been found in ${\mathrm{Nd}}_{0.4}$${\mathrm{Tb}}_{0.6}$${\mathrm{Mn}}_{2}$${\mathrm{Ge}}_{2}$ (140 K350 K) where two antiferromagnetic commensurate components within the (001) plane coexist with a ferromagnetic component along the c axis. The peculiar layered structure of the ${\mathrm{RMn}}_{2}$${\mathrm{Ge}}_{2}$ compounds favors a cancellation of the molecular field at the rare-earth sites in the case of antiferromagnetic arrangements of the Mn sublattice, effectively isolating the R atoms and making a paramagnetic behavior of these possible despite the presence of long-range order. The existence of a ferromagnetic component in the Mn sublattice has been concluded to be indispensable to allow the ordering of the rare-earth magnetic moments.

Ac susceptibility of a DyFe11Ti single crystal.

A study of the ac initial magnetic susceptibility of a roughly spherical single crystal of ${\mathrm{DyFe}}_{11}$Ti is reported. Both the real and imaginary parts of the complex susceptibility have been measured in absolute units along the principal crystallographic directions, the results being practically identical for [100] and [110]. The underlying mechanisms---coherent magnetization rotation and domain-wall motion---are considered. It is demonstrated that the thermally activated domain-wall displacements provide the clue to understanding the main features of the temperature dependence of both the real and imaginary parts of the observed susceptibility. \textcopyright{} 1996 The American Physical Society.

Large Magnetovolume Effect in Yttrium Doped La-Ca-Mn-O Perovskite

ac initial magnetic susceptibility, thermal expansion, and magnetostriction measurements were performed on a polycrystal of ${\mathrm{La}}_{0.60}{\mathrm{Y}}_{0.07}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ manganese oxide. We observed a spontaneous volume magnetic contribution ( $\ensuremath{\cong}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}160\ifmmode\pm\else\textpm\fi{}5\mathrm{K}$). This extra contribution sharply decreases at ${T}_{C}$ as a consequence of the metal ( $M$)--insulator ( $I$) transition, and is suppressed under an applied magnetic field. This shows that the transition to the $M$ state is induced by the field in the $I$ regime, producing strong magnetoelastic effects. The present results demonstrate the close relationship between magnetoelastic and magnetotransport properties in this compound.

Splitting of the Hopkinson maximum by heat treatment in Gd foils

Measurements of the ac initial magnetic susceptibility [χac ≡ χ(ω = constant, T)] were carried out on polycrystalline Gd foils in UHV. χ was measured before and after annealing the Gd foils for 16 h at 700°C in a 10−8 Torr vacou. The χ vs. T spectrum of Gd foils shows, after annealing, two Hopkinson maxima at the temperatures TH1 = 287.5 K and TH2 = 291.5 K. The Arrhenius plots indicate that the thermal activation contributions to χ may be partially responsible for the occurence of the Hopkinson effect in agreement with recent studies of Popov and Mikov[J. Magn. Magn. Mater. 82 (1989) 29].

A study of the spin reorientation transitions in (Er xHo 1 −x) Fe 10V 2 intermetallics

AC initial magnetic susceptibility (χ AC) measurements on the (Er x Ho 1- x )Fe 10V 2 series show peak anomalies for x≥ 0.25, which are an indication of the existence of spin reorientation phase transitions (SRT). No anomaly is observed for HoFe 10V 2, the SRT temperature decreasing with increasing Ho concentration. The critical exponents have been determined, with a value of γ = 1.20 ± 0.05 for all the compounds studied.

First-order magnetic phase transition in quasi-bulk Gd(0001)

The UHV-compatible AC initial magnetic susceptibility ( chi AC identical to chi ( omega =constant, T)) technique is used to study (with nanometer resolution) the magnetic behaviour of Gd(0001)/W(110) thin films. The abruptness of the drop in chi AC within 4 K above the Hopkinson maximum is interpreted as indicating the presence of a first-order magnetic phase transition.

Thermal expansion and ac initial magnetic susceptibility in RECu intermetallic compounds (abstract)

We have carried out a systematic study of the series RECu cubic intermetallic compounds where RE=Y, Gd, Tb, Dy, and Ho by means of the thermal expansion and ac initial magnetic susceptibility measurements. The measurements clearly show in some of these compounds (RE=Y, Gd, and Tb) very large anomalies associated with a martensitic structural phase transition, from the cubic CICs at room temperature to the tetragonal FeB structure. These anomalies exhibit very large hysteresis effects which make it necessary to reobtain the original CICs crystal structure to anneal the samples up to about 600 K. In the case of TbCu a magnetic-phase transition occurs simultaneously to the structural one; the trigger mechanism which drives this transition has been studied on the basis of the strong influence of the magnetoelastic energy using a single-ion crystal electric-field model within the mean-field approximation. In the case of RE=Dy and Ho only slight anomalies have been observed in the thermal expansion measurements at the region of the magnetic-phase transition and no hysteresis effects were observed. The existence or nonexistence of the spontaneous magnetostriction at the order temperature constitutes a suitable test of the nature of the magnetic structure (collinear or multiaxis) in these systems.

Spin reorientation processes in hard magnetic pseudoternaries (ErxNd1−x)2Fe14B

The effect of the different kinds of anisotropies (axial, conical, and planar) competing in the same compound with different grades of influence with the concentration, x, produces a wide range of peculiar behaviors. We have studied the magnetic anisotropy in this series by means of the thermal evolution of the spin reorientation transition (SRT) with concentration. We have measured the ac initial magnetic susceptibility for the concentrations x=0, 0.2, 0.4, 0.6, 0.8, 0.9, and 1. The results clearly show peaklike anomalies which allow us to obtain the critical temperature for the phase transition. We have also measured the magnetization parallel and perpendicular to the applied magnetic field on magnetically aligned powder samples in order to determine the thermal evolution of the orientation of the easy magnetization direction. The compounds with intermediate concentrations (0.2≤x≤0.8) present two peaklike anomalies in the range of temperature measured (3.5–350 K), which have been associated with critical phenomena related to the SRT (axial→cone 1 and cone 1→cone 2). The remaining concentrations display SRT axial→planar in the case of Er-rich compounds and axial→cone in the Nd-rich ones.