Thermoremanent Magnetization Measurements Research Articles

Synthesis and magnetic properties of solid solutions of a diluted magnetic semiconductor (Zn1-xFex)3As2

The magnetic properties of the new semimagnetic semiconductor (Zn1-xFex)3As2 with x < 0.04 have been investigated over a wide temperature range. Single crystals of ZFA were grown using the modified Bridgeman technique. The results of X-ray analyses and the magnetometry data show that the border of solid solutions in this system lay near x = 0.015. Further increasing of x leads to the growing of the second phase, which was identified as Fe2As. A notable feature of our magnetometry results is the spin-glass-type freezing of magnetic moments in the temperature range of 10 - 50 K. The value of the freezing temperature decreases when the magnetic field increases and in 2 kOe magnetic field the effect is no longer observable. It is likely that the observed freezing phenomena are due to the cluster nature of magnetic properties of ZFA. The thermoremanent magnetization measurements give evidence for two types of clusters in the magnetic system of ZFA.

Strong anisotropy in the mixed antiferromagnetic system Mn1−xFexPSe3

We report the magnetic phase diagram of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$, which represents a random magnet system of two antiferromagnetic systems with mixed spin, mixed spin anisotropies, mixed nearest-neighbor magnetic interactions, and mixed periodicities in their respective antiferromagnetic structure. Bulk samples of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PSe}}_{3}$ have been prepared and characterized phase pure by powder x-ray and neutron diffraction and x-ray fluorescence. Nature and extent of magnetically ordered state has been established using powder neutron diffraction, dc magnetic susceptibility, and heat capacity. Long-range magnetic ordering exists between $x=0.0$ and 0.25 (${\mathrm{MnPSe}}_{3}$ type) and between $x=0.875$ and 1 (${\mathrm{FePSe}}_{3}$ type). A short-range magnetic order with the existence of both ${\mathrm{MnPSe}}_{3}$- and ${\mathrm{FePSe}}_{3}$-type nanoclusters has been established between $x=0.25$ and 0.875. Irreversibility in dc magnetization measurements, also characterized by isothermal and thermoremanent magnetization measurements, suggest similarities to magnetic nanoparticles where uncompensated surface spins result in diverging thermoremanent and isothermal remanent magnetization responses, further reinforcing existence of magnetic nanoclusters or domains. A spin-glass state, observed in analogous ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{PS}}_{3}$, has been ruled out, and formation of nanoclusters exhibiting both ordering types results from unusually high anisotropy values. The effect of ligand contributions to the spin-orbit interactions has been suggested as a possible explanation for high $D$ values in these compounds.

Topologically stable helices in exchange coupled rare-earth/rare-earth multilayer with superspin-glass like ordering

Existence of 2π-planar domain walls (DWs) are often reported for transition metal-rare-earth (TM/RE) layered systems. The magnetization process of such two-dimensional randomly anisotropical system in the form of 2π-DWs is directly correlated with topologically stable helices. Here, instead of TM/RE, we have investigated [Dy/Tb]10 multilayers involving two different anisotropic layers of rare-earth/rare-earth (RE/RE). Using magnetization and susceptibility as function of temperature along with thermo-remanent magnetization measurements we have confirmed superspin-glass type of behavior within this RE/RE system. Additionally, an exchange bias field up to –0.88 kOe (–88 mT) was also revealed for such rare-earth based multilayers. Interestingly, using detailed analysis of the polarized neutron reflectometry profiles, we find evidences of superimposed helical magnetic configurations within both materials of Dy and Tb associated with spin-frustrated interfaces. Furthermore, magnetization fluctuations around the mean magnetization from vertically uncorrelated domains were observed with polarized off-specular neutron scattering. We believe that coexistence of helical ground states with superspin-glass-like ordering are fundamentally instrumental for topologically stability in RE/RE systems, which in principle, can be exploited in all-spin-based technology.

Relieving geometrical frustration through doping in the Dy1−xCaxBaCo4O7 swedenborgites

The geometrically frustrated antiferromagnet DyBaCo4O7 is investigated through a combination of x-ray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy1−xCaxBaCo4O7 causes a lifting of the geometrical frustration resulting in a structural transition from a trigonal P31c to an orthorhombic Pbn21 symmetry at x = 0.4. This structural transition can also be accessed as a function of temperature, and all our orthorhombic specimens exhibit this transition at elevated temperatures. The temperature at which this structural transition occurs is observed to scale linearly with the mean ionic radius of the R site ion. However, CaBaCo4O7 which has an equal number of Co2+ and Co3+ ions clearly violates this quasilinear relationship, indicating that charge ordering could also play a critical role in stabilizing the orthorhombic distortion in this system. Using thermoremanent magnetization measurements to circumvent the problem of the large paramagnetic background arising from Dy3+ ions, we chart out the phase diagram of the Dy1−xCaxBaCo4O7 series.

Positive exchange bias and upward magnetic relaxation in a Fe-film/CoO-nanoparticle hybrid system

Unusual positive exchange bias found in Fe/CoOx nanoparticle bilayer films is correlated to a characteristic magnetic spin-glass (SG) in CoOx, with the SG magnetization coupled antiparallel with the Fe magnetization upon field cooling. This SG magnetization has strong field- and time-dependence which displays unusual upward magnetic relaxation behavior in thermoremanent magnetization measurements. The antiparallel coupling is shown to result predominantly from the antiferromagnetic superexchange of the Fe2+-O-Co3+ couple, of the oxygen-terminated CoOx at the interface. These experimental results reveal the possibility of manipulating the exchange bias effect via an indirect exchange coupling mechanism.

Synthesis of magnetic nanoparticles by pulsed laser ablation

Magnetic nanoparticles were prepared by laser ablation of Fe foil in ethanol. The nanoparticles consisted of Fe3O4 and Fe3C and were superparamagnetic with a saturation magnetization Ms=124emu/g. Zero field cooled (ZFC) measurements collected at an applied field of 50Oe displayed a maximum magnetic susceptibility at 120K with a broad distribution. Field cooled (FC) measurements collected during cooling and heating showed a thermal hysteresis indicative of temperature dependent magnetic viscosity. The magnetic viscosity was calculated from thermoremanent magnetization (TRM) plots and it increased with decreasing temperature. The activation volume of these non uniform magnetic states was calculated from TRM measurements and it was found to decrease with decreasing temperature. The decrease in activation diameters was interpreted as a decrease in exchange length, and hence a decrease in particle-particle interactions.

Ac field-switchable magnetic properties of two-dimensional networked nanosize magnets

The magnetic properties of interacting magnetic nanoparticles were investigated by examining two kinds of two-dimensional networked single-molecule magnets (SMMs). Through nonlinear magnetic susceptibility measurements, we observed a switchable transformation of the magnetic response upon applying an ac magnetic field to the SMMs. The magnetic properties depending on the measurement time-scale can be understood as a competition of the magnetic interaction between the constituents (i.e., SMMs or SMM aggregates) and the strong Ising nature of the individual constituents. In high-frequency ac fields, the Ising nature of the SMM induces magnetic behavior due to either the aggregate consisting of several SMMs or the individual SMMs, rather than a static response due to magnetic ordering. The magnetic state at the low-frequency limit was investigated through thermoremanent magnetization measurements, and a relaxation of the aggregates in a manner similar to the Griffiths phase was observed in the first material, [Mn4(hmp)4Br2(OMe)2(dcn)2]⋅0.5H2O⋅2THF. The second material, [Mn4(hmp)6(dcn)2](ClO4)2, with strong dipolar interaction between aggregates, exhibited a glass-type system-wide response rather than a relaxation of the aggregates. Our present study on these regularly networked SMM systems demonstrated the existence of a switchable magnetic phenomenon induced by ac magnetic fields. It is difficult to investigate this kind of phenomenon in diffused magnetic nanoparticles with random location and distribution of both particle size and interparticle distance.

Reentrant spin glass behaviour in Nd 0.84K 0.12MnO 3

Polycrystalline Nd 0.84K 0.12MnO 3 was prepared in single phase form with Pbnm space group. The magnetic properties are studied from magnetization, linear and non-linear susceptibility, and thermoremanent magnetization measurements. The sample exhibits paramagnetic to ferromagnetic transition followed by low temperature spin glass like transition. From frequency variation of ac susceptibility measurements, the spin glass transition temperature is found to be 97.6±0.1 K with critical exponents zυ=1.13±0.06. The critical exponent γ corresponding to spin glass transition has been determined from the third harmonic susceptibility analysis and it is found to be 3.09±0.05. The effective number of spins blocked under frustration and their correlation length are determined from the analysis of thermoremanent magnetization.

Intrinsic exchange bias inZnxMn3−xO4(x≤1)solid solutions

Bulk specimens of the het\ae{}rolite solid solution ${\text{Zn}}_{x}{\text{Mn}}_{3\ensuremath{-}x}{\text{O}}_{4}$ with $x=0$, 0.25, 0.5, 0.75, and 1 have been prepared as homogeneous, phase-pure polycrystalline samples as ascertained by neutron-diffraction measurements. Samples with $x=0.25$, 0.5, and 0.75 exhibit shifted magnetic hysteresis loops at low temperature, characteristic of exchange bias typically seen in magnetic composites. We propose that the unusual magnetic behavior arises as a result of a nanoscale mixture of ferrimagnetic and antiferromagnetic regions that are distinct but lack long-range order. While some glassy behavior is seen in ac magnetic measurements, its magnitude is not sufficient to account for the observed dramatic exchange bias. Furthermore, isothermal and thermoremanent magnetization measurements distinguish this material from a pure spin glass. The title system offers insights into the alloying of a ferrimagnet ${\text{Mn}}_{3}{\text{O}}_{4}$ with an antiferromagnet ${\text{ZnMn}}_{2}{\text{O}}_{4}$ wherein distinct magnetic clusters grow and percolate to produce a smooth transition between competing orders.

Inhomogeneous structure and magnetic properties of granularCo10Cu90alloys

Granular ${\mathrm{Co}}_{10}{\mathrm{Cu}}_{90}$ alloys displaying giant magnetoresistance have been obtained by melt spinning followed by an appropriate heat treatment in the range 0--700 \ifmmode^\circ\else\textdegree\fi{}C. Their structural and magnetic properties have been studied on a microscopic scale using ${}^{59}\mathrm{Co}$ NMR technique and thermoremanent magnetization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained Co particles (60% of the entire Co population) and a composition modulated CoCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properties. One part consists of ferromagnetic alloy (average Cu concentration of about 20%) with a blocking temperature of about 35 K (involving 6% of the entire Co population in a sample). The other part, containing the remaining 34% of the entire Co population, is a paramagnetic alloy with a blocking temperature below 4.2 K. The ferromagnetic alloy is magnetically soft---its transverse susceptibility is lower by a factor of 7 than the transverse susceptibility of the quenched-in Co particles. The latter population has a blocking temperature of about 150--200 K. During the heat treatment, each of the two main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in dissolving the quenched-in Co particles after annealing at around 400 \ifmmode^\circ\else\textdegree\fi{}C, followed at higher temperatures by a nucleation and growth of the more regular in shape Co particles. The other process resembles a spinodal decomposition of the quenched-in CoCu alloy, resulting in sharpening of the concentration profile and eventually leading to Co cluster formation in samples annealed above 450 \ifmmode^\circ\else\textdegree\fi{}C. Both processes end at about ${T}_{\mathrm{an}}=700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.

Magnetic interaction between magnetite particles dispersed in calciumsilicate glasses

The effect of the measuring process on the magnitude of the inter-particle interaction has been investigated by measuring the superparamagnetic relaxation for magnetite particles dispersed in calciumsilicate glasses using thermoremanent magnetization (TRM), AC susceptibility and Mössbauer spectroscopy measurements: The temperature dependence of the remanent magnetization in TRM measurements can be explained by Néel's theory which describes the superparamagnetic relaxation of a noninteracting particle. On the other hand, the relation between log 10 τ m ( τ m: the measurement time) and 1 T B ( T B: the average blocking temperature) in AC susceptibility measurements can be interpreted using Dormann's model in which the inter-particle interaction is taken into account. Therefore, it can be concluded that the measuring process for the superparamagnetic relaxation can be more or less sensitive to the inter-particle interaction.

Fine magnetic particles in layered silicates and zeolites

Ultrafine Ni particles have been generated at ambient temperatures in clay (montmorillonite) and zeolite (mordenite) matrices. The synthesis involves a reduction in aqueous solutions of the ion-exchanged clay or zeolite with sodium borohydride. SQUID magnetometer measurements reveal single domain Ni particles with superparamagnetic properties. While the Ni particles in clay have a narrow distribution of blocking temperatures T/sub b/ and particle size, they seem to have in zeolites a broader distribution of T/sub b/ and particle size. Time decay measurements of thermoremanent magnetization also support single domain characteristics.

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mössbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

Remanence magnetization in powders and ceramics of YBaCuO

Thermoremanent magnetization (TRM), isothermoremanent magnetization (IRM) and time dependence of IRM have been investigated in powders and ceramics of YBa 2Cu 3O 7−δ in low magnetic fields. Measurements of TRM of powders with different particle sizes show trapping of single flux quanta in microcrystals. Time dependence of IRM for powders and ceramics shows the difference in dynamics of vortices when there were present in the sample only Abrikosov vortices or both Abrikosov and Josephson vortices, respectively. Also a reversible temperature dependence of the remanent magnetization, observed only in powders but not in ceramics, is reported. This reversibility is explained by the increase of the penetration depth λ when the temperature of the sample is lowered.

Noncritical behavior and remanent magnetization in magnetically frustrated FeSbO4.

Low-field dc-magnetization and thermoremanent-magnetization measurements have been recorded from the rutile-related iron antimonate of composition ${\mathrm{FeSbO}}_{4}$. The zero-field-cooled susceptibility curve at low magnetic fields (H=20 Oe) shows a small peak at \ensuremath{\sim}72 K and a much broader peak at lower temperatures of \ensuremath{\sim}25 K. The temperature dependence of the remanent magnetization below \ensuremath{\sim}72 K indicates the existence of two different relaxation regimes which depend on the strength of the applied field in which the sample was cooled. The thermoremanent-magnetization measurements as a function of time show aging effects and the characteristic parameters are quite similar to those found in canonical spin glasses. The temperature dependence of the first nonlinear coefficient ${\mathit{b}}_{3}$ in the expansion of the magnetization in odd powers of (${\mathit{a}}_{1}$H) shows no divergence around the peak at 72 K, which is indicative of the absence of a true spin-glass transition. The moderate degree of frustation associated with an inhibited antiferromagnetic transition at \ensuremath{\sim}72 K, is nevertheless insufficient to produce a true spin-glass transition where the magnetic correlation length is even much shorter.

MAGNETIC INTERACTION IN SELF-REVERSING ANDESITIC PUMICE IN RELATION TO IRON ALLOYS

Andesitic pumice erupted in 1985 from Nevado del Ruiz volcano (Colombia) has been magnetized in opposite direction t o the present geomagnetic field. This self-reversal is probably due t o an exchange mechanism between two phases of the haematite-ilmenite series, which is similar to the magnetic coupling observed in synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films produced under ultra high vacuum conditions. One of the fundamental principles of palaeomagnetism and rock magnetism states that a rock specimen when cooled in the earth's magnetic field will acquire a natural remanent magnetization (NMR) which is usually parallel to the applied field. However, the NMR of andesitic pumice emitted during the 1985 eruption of Nevado del Ruiz volcano (Colombia) shows a direction opposite of the present geomagnetic field [I]. For the first time these rocks present firm evidence that a self-reversal process actually has controlled the acquisition of NMR. The self-reversing properties are demonstrated in the laboratory during repeated cycles of heating (up to 165 OC) and subsequent cooling (to room temperature) in air and zero magnetic field, see figure 1. The reversal from negative to positive polarity near 130 OC upon heating and from positive back to negative polarity on cooling can be clearly seen. From the reversibility of the NMR behaviour during heat treatment and from additional strong field magnetization measurements it is concluded that the magnetic phases are stable up to at least 700 OC. Self-reversing remanent magnetization was first discovered in dacitic rocks from Japan by Nagata [2] and co-workers [3-51. Two major interaction models leading to self-reversing properties have been proposed [6, 71; firstly NBel's N-type with two sublattices in a one phase model, aad secondly different types of two phase models with magnetostatic (i.e. dipole-dipole) or (super)exchange interaction. The energies involved in these interactions are usually assumed to be different by orders of magnitude. Estimates of the interaction energy may be obtained from the applied field amplitude needed to suppress the self-reversing mechanism. This was achieved by thermoremanent magnetization measurements, which simulate the natural magnetization process between the maximum Curie temperatures measured in the samples (ca 400 OC) and room temperature in magnetic DC fields of increasing amplitude. Figure 2 shows that the critical field necessary to completely suppress self-reversal is very weak, about +,., Heating i Cooling i Heating

Superparamagnetism and spin-glass freezing in nickel-manganese alloys

An experimental study of the ac susceptibility, magnetization, and thermoremanent magnetization has been made for atomically disordered and weakly ordered ${\mathrm{Ni}}_{3}$Mn. Superparamagnetism with cluster interaction is demonstrated by Curie-Weiss behavior above the susceptibility peak temperature and a Langevin dependence of the magnetization. Blocking is suggested by a weak frequency dependence of the susceptibility peak and an increase in hysteresis below the peak temperature. Spin freezing, which fixes the direction of the local anisotropy, is examined by thermoremanent magnetization measurements.

Thermoremanent magnetization and blocking temperatures in Ni3Mn

Initial susceptibility and thermoremanent magnetization measurements have been made on atomically disordered and weakly ordered Ni3Mn. The blocking of superparamagnetic clusters yields a peak in initial susceptibility near 110 K. Ferromagnetic clusters with Curie temperatures near 265 K cause a break in the susceptibility vs. temperature curves at 265 K. Exchange anisotropy effects appear below 35 K.

The origin of thermoremanent magnetization: Contribution of pseudo-single-domain magnetic moments

The pseudo-single-domain (PSD) intensity and stability of thermoremanent magnetization (TRM) in multidomain magnetite grains 0.05–15 μm in size are attributed to residual magnetic moments not removed by demagnetization. Barkhausen discreteness in domain wall positions is a possible source of such moments, but the observed grain size and applied field dependences of TRM in the lower PSD range are more convincingly explained by a new theory (F.D. Stacey and S.K. Banerjee, 1974) in which the moments of domain walls and the surface terminations of domain walls play a central role. The magnitudes of PSD moments and the average number of moments per grain required by this theory are deduced from low-field (0–25 Oe) TRM measurements on magnetite grains of controlled sizes between 0.04 and 0.22 μm. The predicted maximum PSD moment is about equal to the saturation moment in grains ≤ 0.1 μm in size but is only 10% of the saturation moment in 0.22-μm grains. Since blocking temperature and hysteresis data independently suggest two-domain structure in 0.22-μm grains and wall-like domain structure in smaller grains, the predicted PSD moments are quantitatively reasonable.