Temperature Dependence Of Coercivity Research Articles

Magnetism of metastable γ-Fe85Pd15 nanowire arrays across an unusually broad temperature range (5 K to 800 K).

Arrays of 50 nm diameter Fe85Pd15 cylindrical nanowires were electrochemically grown, crystallizing in a metastable γ-Fe(Pd) fcc A1 disordered solid solution. After performing a heating-cooling thermal cycle between 300 K and 1000 K, the γ-Fe(Pd) fcc metastable phase still predominates (97%), coexisting with a not-fully-identified minority phase. The thermal cycling induces a moderate increase in the crystallite size and a reduction of the lattice parameter although leading to a significant heating-cooling magnetic hysteresis. No further changes in temperature-dependent magnetization, M(T), are observed during subsequent cycling. The full-range (5 K to 800 K) saturation magnetization Ms(T) curve is quite accurately described by a phenomenological expression, which provides a Bloch-type contribution as T → 0 and undergoes the critical behavior near the Curie temperature TC. An upturn in Ms(T) is observed below 100 K which is described by a spin-glass-like second contribution, with freezing temperature Tf = (80 ± 2) K, and kBTf comparable to the exchange interactions in Fe-Pd systems. A Curie temperature of TC = 830 K, and a critical exponent value β = 0.42 ± 0.05 are estimated. These regimes (below and above 100 K) are also observed in the magnetization process. The temperature dependence of coercivity between 100 K and 800 K is consistent with a nucleation/propagation remagnetization mechanism, with activation energy of (320 ± 20) kJ mol-1 and critical field for magnetization reversal of (65 ± 1) mT, at 0 K. The analysis of the effective magnetic anisotropy as a function of temperature allows us to conclude that it essentially arises from the balance between different magnetostatic contributions.

Reliable Accessibility of Intermediate Polarization States in Textured Ferroelectric Al0.66Sc0.34N Thin Film

AbstractFerroelectric materials are promising candidates for neuromorphic computing synaptic devices due to the nonvolatile multiplicity of spontaneous polarization. To ensure a sufficient memory window, ferroelectric materials with a large coercivity are urgently required for practical applications in highly scaled multi‐bit memory devices. Herein, a remarkable reliability of intermediate ferroelectric polarization states is demonstrated in a textured Al0.66Sc0.34N thin film with a coercive field of 2.4 MV cm−1. Al0.66Sc0.34N thin films are prepared at 300 °C on Pt (111)/Ti/SiO2/Si substrates using a radio frequency reactive sputtering method. Al0.66Sc0.34N thin films exhibit viable ferroelectricity with a large remanent polarization value of >100 µC cm−2. Through the conventional current–voltage characteristics, polarization switching kinetics, and temperature dependence of coercivity, the reproducibility of multiple polarization states with apparent accuracy is attributed to a small critical volume (3.7 × 10−28 m3) and a large activation energy (3.3 × 1027 eV m−3) for nucleation of the ferroelectric domain. This study demonstrates the potential of ferroelectric Al1‐xScxN for synaptic weight elements in neural network hardware.

Magnetic and magnetocaloric properties of polycrystalline Pr[formula omitted]Ca[formula omitted]MnO[formula omitted] (x [formula omitted] 0.85, 0.90, 0.95) compounds: Emergence of large inverse and conventional magnetocaloric effects

Magnetic and magnetocaloric effect have been investigated for the electron doped polycrystalline Pr1−xCaxMnO3 (x ∼ 0.85, 0.90, 0.95) compounds. The experimental outcomes indicate that the nature of the ground state is very much sensitive with the doping concentration. Pr0.15Ca0.85MnO3 compound exhibits antiferromagnetic ground state. In contrast to that ferromagnetic nature was found for the Pr0.10Ca0.90MnO3 and Pr0.05Ca0.95MnO3 compounds. Although, for x∼0.90 ferromagnetic phase was stable. However for x∼ 0.95, the ferromagnetic state is quite unstable. Additionally for the doping concentration x∼ 0.95, the anomalous nature was found in temperature dependent coercivity. Magnetocaloric effect study indicate that antiferromagnetic Pr0.15Ca0.85MnO3 compound exhibits large inverse magnetocaloric effect (10.5 J/kg-K at H = 70 kOe magnetic field). On the other hand, ferromagnetic Pr0.10Ca0.90MnO3 and Pr0.05Ca0.95MnO3 compounds show conventional magnetocaloric effect. The magnetocaloric entropy change for x∼0.90 compound is significantly large (7.5 J/kg-K at H = 70 kOe magnetic field) and comparable with the other reported magnetic refrigerant materials. Universal master curve for magnetic entropy change have been constructed for all the materials having inverse and conventional magnetocaloric effect by proper scaling of temperature axis.

Hard Magnetic Properties and the Features of Nanostructure of High-Temperature Sm-Co-Fe-Cu-Zr Magnet with Abnormal Temperature Dependence of Coercivity

This paper presents methods and approaches that can be used for production of Sm-Co-Fe-Cu-Zr permanent magnets with working temperatures of up to 550 °C. It is shown that the content of Sm, Cu, and Fe significantly affects the coercivity (Hc) value at high operating temperatures. A decrease in the content of Fe, which replaces Co, and an increase in the content of Sm in Sm-Co-Fe-Cu-Zr alloys lead to a decrease in Hc value at room temperature, but significantly increase Hc at temperatures of about 500 °C. Increasing the Cu concentration enhances the Hc values at all operating temperatures. From analysis of the dependence of temperature coefficients of the coercivity on the concentrations of various constituent elements in this alloy, the optimum chemical composition that qualifies for high-temperature permanent magnet (HTPM) application were determined. 3D atom probe tomography analysis shows that the nanostructure of the HTPM is characterized by the formation of Sm2(Co,Fe)17 (2:17) cells relatively smaller in size along with the slightly thickened Sm(Co,Cu)5 (1:5) boundary phase compared to those of the high-energy permanent magnet compositions. An inhomogeneous distribution of Cu was also noticed in the 1:5 phase. At the boundary between 1:5 and 2:17 phases, an interface with lowered anisotropy constants has developed, which could be the reason for the observed high coercivity values.

Effect of spin-reorientation transition of cell boundary phases on the temperature dependence of magnetization and coercivity in Sm2Co17 magnets

Four Sm2Co17 magnets with spin-reorientation transition (SRT) of cell boundary phases (CBPs) are prepared by liquid-phase sintering. The temperature of the SRT of CBPs () is regulated from 125 K to 195 K by adding 0 wt.%, 3 wt.%, 6 wt.% and 9 wt.% Dy88Cu12 alloy powder. The effect of SRT of Sm2Co17 magnet CBPs on the temperature dependence of the magnetization (M–T) and coercivity (H–T) is systematically investigated. The temperature dependence of the magnetization is influenced by the SRT of CBPs. The M–T curves measured during the heating process are larger than those measured during the cooling process when . When there is a bifurcation point. When the M–T curves overlap and the M–T derivation curve shows that the magnetization of the magnet has low temperature dependence of magnetization above . With increasing , the initial temperature of the low temperature dependence of magnetization shifts towards a higher temperature. The coercivity temperature coefficient becomes positive as the SRT effect increases, and the temperature range of the positive coercivity temperature coefficient moves towards higher temperatures as increases. This reveals that SRT of CBPs has little effect on the temperature dependence of magnetization above , while the temperature dependence of coercivity is optimized. The temperature range of magnetization and coercivity with low temperature dependence tends towards higher temperatures, which is conducive to the preparation of magnets with a low temperature coefficient at higher temperatures.

Room and low-temperature magnetic characterization of Cr doped CoFe2O4 nanostructures

The macroscopic magnetism in nanoparticulated systems is the consequence of numerous contributions, ranging from the spatial arrangement of nanoparticles to their intrinsic nanostructure. Understanding and unraveling these temptations is essential to scientifically and technologically introduce nanomaterials. In the present work, we investigate how the magnetic properties of Co ferrite can be significantly ameliorated by Cr substitution. At first, applied the co-precipitation strategy to synthesize stoichiometric Co ferrite nanoparticles followed by the Cr substitution to achieve CoCrxFe2-xO4 (0 ≤ x ≤ 1) nanoparticles. The phase and structural analysis by X-ray diffraction identified the formation of spinel structure with the Fd-3m space group. An increasing behaviour in X-ray density with Cr substitution in Co ferrite has been observed due to lattice contraction. Field-emission scanning electron microscopic images confirmed that the particle size ranged from 14 to 17 nm. Room temperature magnetic investigations showed that the magnetic coercivity of the synthesized nanoparticles increased up to 40% Cr substitution, followed by a monotonic decrease in the magnetic properties. Low-temperature magnetic properties were carried out for 40% Cr substituted Co ferrite. The increase in saturation magnetization with a reduction in temperature followed Bloch's law, and the trend of temperature-dependent coercivity was confirmed with Kneller's law.

Investigation of the structural, morphological, and magnetic properties of small crystalline Co–Cu ferrite nanoparticles in the single-domain regime

Single-domain Co0.5Cu0.5Fe2O4 ferrite nanoparticles with a crystallite size of 23 nm were hydrothermally prepared and characterized using x-ray diffraction, transmission electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and vibrating sample magnetometry. According to the Rietveld refinement results, the prepared nanoparticles were single-phase with spinel type structures. The transmission electron microscope measurements demonstrated that the nanoparticles were spherical in shape. The FT-IR spectrum showed two principle absorption bands, confirming the characteristic features of cubic spinel ferrites. Magnetization data revealed that the prepared nanoparticles were ferrimagnetic from room temperature to 10 K, with well-defined saturation magnetization, coercivity, and remanence magnetization. The remanence magnetization and coercivity were found to increase with decreasing temperature. The value of room temperature squareness ratio (Mr/Ms) of 0.42 was found to be somewhat similar to that expected (0.5) for a system of noninteracting single-domain nanoparticles, suggesting that the prepared nanoparticles are in a single-domain regime. The temperature dependence of coercivity was found to have slight deviations from Kneller’s law, possibly due to interactions between nanoparticles. The zero field cooled–field cooled curves indicated that below 150 K, the nanoparticles were ferrimagnetic dressed with spin-glass behavior, resulting from interactions between the ferrimagnetic nanoparticles and/or random freezing of surface spins.

Charge-transfer-induced 2D ferromagnetism and realization of thermo-remnant memory effect in ultrathin ß-NiOOH-encapsulated graphene

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagnetic β-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thin β-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure of β-NiOOH/graphene/β-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

Quantitative analysis of pinning-hardened intrinsic coercivity of Sm(CoFeCuZr)z (z = 7.0–7.8) high-temperature permanent magnets

SmCo-based permanent magnets possessing high Curie temperature and outstanding magnetocrystalline anisotropy are of great scientific and technological value on high temperature applications. It is widely accepted the coercivity mechanism as well as its temperature dependence is closely related to the microstructure and microchemistry. However, direct domain wall pinning observation with Lorentz microscopy is extremely difficult to explore the strength of domain wall pinning under high temperature and strong magnetic field. The quantitatively understanding of coercivity mechanism such as micromagnetic simulations cannot be carried out without the intrinsic magnetism of 1:5H and 2:17R inside the cellular microstructure of SmCo sintered magnets. Up to nowadays, there are no methods to directly measure the intrinsic magnetism of nanometer scale 1:5H and 2:17R, which retards the exploring of coercivity mechanism especially at high temperature. Herein, we prepared Sm(CoCuFeZr)z high-temperature permanent magnets with the ratio z values ranging from 7.0 to 7.8 and measured the microchemistry of 1:5H and 2:17R phases with transmission electron microscopy. Then single-phase solid solution samples with the same composition as 1:5H and 2:17R phases were prepared and the high temperature magnetic properties are measured directly. The coercivity mechanism can be well explained based on the difference of domain wall energy density qualitatively, including the abnormal temperature dependence of coercivity. More importantly, the quantitatively calculated coercivity according to the pinning-hardened coercivity theory with the obtained intrinsic parameters were found to agree well with the experiment results. Our work on the intrinsic magnetism of 1:5H and 2:17R phases at varying temperatures may offer important guidance for compositions design of the magnets with higher performance.

Micromagnetics of Nd–Fe–B magnets at finite temperature

A three-dimensional micromagnetic model for Nd–Fe–B magnets, where the polycrystalline grain size and grain boundary width are adjustable, is built to calculate the finite temperature magnetic properties by the Hybrid Monte Carlo method. At 300 K, it is found that the increase of the energy product (BH)max of Nd–Fe–B is mainly contributed by the higher saturation magnetization M s. M–H loops are simulated from 300 to 450 K, and the measured temperature-dependent coercivity H c(T) and remanence M r(T) are explained quantitatively and fit well with experiments.

Temperature Dependence and Microstructure Effects on Magnetic Properties of FePt(B, Ag, C) Film.

A FePt(B, Ag, C) granular film was formed from post-annealed B4C(1.0 nm)/FePt(Ag, C) layers at a substrate temperature of 470 °C for 2 min. The 6 nm thick FePt(B, Ag, C) film demonstrates high perpendicular magnetic anisotropy (Ku = 2.83 × 107 erg/cm3 at 100 K) and out-of-plane coercivity (Hc = 38.0 kOe at 100 K). The Ku and out-of-plane Hc are respectively increased from 38% and 46% between 350 K and 50 K. The sample with a thickness of 8 nm also shows a similar trend for magnetic properties; however, the tiny magnetization kink which may come from rare Fe-B or disordered FePt grains was observed in the easy axis loop. The intrinsic (ΔHint = 12.6 kOe) and extrinsic switching field distribution (ΔHext = 1.62 kOe) were characterized by major and minor loops to correlate the microstructural grains. The coupled FePt grains grown on a single MgTiON grain were observed in a high-resolution transmission electron microstructure (HRTEM) image. This small intergranular exchange coupling was defined by estimating the magnetic cluster size (46.6 nm) from ΔHext and the average grains size (28.2 nm) from TEM images. The temperature dependence of coercivity was fitted to further understand the magnetization reversal process. The lower microstructural parameter was evidenced in the imperfect grain morphology.

Magnetic ordering of Ho and its role in the magnetization reversal and coercivity double peaks in the Ho3Fe5O12 garnet

Rare-earth based garnet system, Ho3Fe5O12 shows magnetization compensation phenomenon at Tcomp = 138 K and a magnetic transition at 50 K. In the present work, we reveal the physics of these low temperature magnetic transitions by carrying out detailed dc magnetization, ac susceptibility, neutron depolarization, and neutron diffraction studies. Interestingly, our dc magnetization study under low (≲ 50 Oe) magnetic fields has shown a previously unknown sign reversal of magnetization (below the Tcomp) that persists down to 5 K, the lowest measured temperature. Magnetic measurements under moderate fields between 50 Oe and 1 kOe reveal two compensation temperatures (in the range of 105 – 150 K depending upon the applied field value) leading to double magnetization reversals. Interestingly, for magnetic fields ≥ 1 kOe, these two compensation temperatures merge to a single Tcomp (=138 K). A modified Stoner-Wohlfarth model has been used to explain the asymmetrical nature of double peak around the Tcomp in the temperature dependent coercivity data. The ac susceptibility study supports the existence of both the transitions with an evidence of deep minimum at the Tcomp (138 K) and a broad peak at 50 K. Mesoscopic neutron depolarization study has revealed a zero domain-magnetization state at the Tcomp. Temperature dependent neutron diffraction study has revealed that Ho carries an induced moment above the Tcomp due to polarizing effect under the internal magnetic field of the two magnetically ordered Fe sublattices at 567 K, whereas a single umbrella type ordering of Ho sublattice moments appears below the Tcomp, resulting in the distortion of magnetic unit cell from cubic to rhombohedral symmetry. However, below 50 K, Ho3+ sites are divided into two inequivalent magnetic sublattices, having different moments and canting angles, and it leads to double umbrella type magnetic structure. Neutron diffraction study has revealed an asymmetric variation of the tetrahedral, octahedral, and dodecahedral sublattice moments resulting in a magnetization reversal at ~ 138 K which is very well supported by the mean field theory calculations. In the present study, we have resolved the ambiguity of magnetic ordering of the rare-earth (Ho3+) sublattice and its role in multiple magnetic transitions. The utility of such compensated ferrimagnetic materials in spin polarizers/analyzers and fast switching memory devices has been emphasized.

Coercivity enhancement of Ce-containing hot-deformed magnets by grain boundary diffusion of DyF3

The samples with full density were prepared by hot pressing the the melt-spun powders mixed with DyF3 powders of different mass fractions followed by hot-deformation process. The magnetic properties and temperature dependence of coercivity were obtained by BH tracer and VSM, respectively. The microstructure were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coercivity of Ce-containing hot-deformed magnets is increased from 1.41 to 1.95 T by grain boundary diffusion of 3 wt% DyF3, and is further enhanced to 2.05 T after annealing treatment. The thermal stability of coercivity and remanence is improved. The annealing condition in this work crucially plays a role in thickening the grain boundary phase. Microstructure analysis reveals that the continuous and thick grain boundary phase formed after DyF3 diffusion can weaken the magnetic coupling between grains, and suppress the platelet shaped grain size and the aspect ratio. The Dy-containing shell structure formed by the partial diffusion of Dy into the main phase can increase the magnetic anisotropy field, which is the main reason for the coercivity improvement. After optimizing the structure by DyF3 diffusion, the “dendritic-like” reverse domain is transformed into the “dot scattered-like” reverse domain.

Temperature Dependence of Coercivity and Magnetic Relaxation in a Mn–Al–C Permanent Magnet

The kinetics of magnetization reversal in a rare-earth-free Mn-Al-C permanent magnet is reported. The L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -phase material was prepared using a combination of the melt spinning and spark plasma sintering techniques and characterized with a vibrating sample magnetometer. Magnetic relaxation and susceptibility experiments were performed. Magnetic viscosity, fluctuation field, and activation volume were measured in a wide temperature range. These data were used to analyze the magnetization reversal and temperature dependence of the coercive field. The thermally activated mechanism of the reversed nucleus formation was confirmed for the intermediate temperatures. The role of wall thickness and crystallite size as additional characteristic lengths in magnetic-domain nucleation is discussed.

Comparison of anomalous magnetic properties of non-collinear CoCr2O4 and NiCr2O4 nanoparticles

Temperature-dependent magnetic properties of cobalt chromite (CoCr2O4) and nickel chromite (NiCr2O4) nanoparticles (NPs) have been compared by using magnetic measurements. The average particle size of CoCr2O4 and NiCr2O4 NPs deduced from TEM images was found to be 48 and 46 nm, respectively. The NiCr2O4 NPs showed paramagnetic to ferrimagnetic transition (Tc) at 84 K which is relatively lower than the Tc = 98 K for CoCr2O4 NPs. The temperature dependent Ms value for CoCr2O4 NPs showed deviation from Bloch’s law at low temperatures which may be due to the existence of lock-in and strong conical spin spiral state. NiCr2O4 NPs showed a good Bloch’s law fit with high value of Bloch’s constant B = 0.00183 K−1 and low value of Bloch’s exponent b = 1.23 than their respective bulk value which indicates the presence of disordered surface spins and finite size effect in these NPs. Interestingly, a high value of Hc (5 K) was observed for NiCr2O4 NPs relative to CoCr2O4 NPs which is attributed to the large canted angle between the oxygen and metallic cations in NiCr2O4 NPs. Temperature dependent coercivity (Hc) for both the NPs were fitted with Kneller’s law. The Kneller’s law fit to Hc vs. T data revealed an exponent α = 0.46 for CoCr2O4 NPs which indicates the non-interacting particles with uni-axial symmetry, while it was 0.74 for NiCr2O4 NPs indicating the assembly of randomly oriented particles. In summary, CoCr2O4 and NiCr2O4 NPs showed a different low temperature magnetic response which is also relatively different than counter ferrite NPs.

Possible Effects of Antiferromagnetic Crystalline Phases on the Temperature Dependence of Coercivity for Ni and Co Nanowires Obtained by Electrodeposition

Herein, the structural and magnetic properties of Ni and Co nanowires obtained by electrodeposition on alumina membranes are studied. Structural analysis shows the coexistence of Ni and Co metallic phases as well as NiO and CoO, which are associated with the preparation method. The magnetic study suggests the possibility of unidirectional anisotropy as a possible mechanism on the magnetization reversal in nickel and cobalt nanowires. For the experimental curves of coercivity versus temperature, there exists a critical value, below which only the dipole interactions are predominant. Above this temperature, the dipole magnetocrystalline and magnetostrictive energy compete in a complicated way. Alongside these three contributions, the coexistence of the “local unidirectional coupling” of antiferromagnetic nanoparticles that engage in random directions on nanowires is proposed.

Doping Dependent Magnetic Behavior in MBE Grown GaAs1-xSbx Nanowires

Intrinsic and Te-doped GaAsSb nanowires with diameters ~100–120 nm were grown on a p-type Si(111) substrate by molecular beam epitaxy (MBE). Detailed magnetic, current/voltage and low-energy electron energy loss spectroscopy measurements were performed to investigate the effect of Te-doping. While intrinsic nanowires are diamagnetic over the temperature range 5–300 K, the Te-doped nanowires exhibit ferromagnetic behavior with the easy axis of magnetism perpendicular to the longitudinal axis of the nanowire. The temperature dependence of coercivity was analyzed and shown to be in agreement with a thermal activation model from 50–350 K but reveal more complex behavior in the low temperature regime. The EELS data show that Te doping introduced a high density of states (DOS) in the nanowire above the Fermi level in close proximity to the conduction band. The plausible origin of ferromagnetism in these Te-doped GaAsSb nanowires is discussed on the basis of d0 ferromagnetism, spin ordering of the Te dopants and the surface-state-induced magnetic ordering.

Magnetic finite size effects, coercive field and irreversibility in sintered (1-x)BaTiO3–xCoFe2O4 nano-composites

We present detailed magnetic properties of (1-x)BaTiO3–xCoFe2O4 (x = 1, 0.5, 0.4, 0.3, 0.2, 0.1, 0.75, 0.05, 0.025) nanocomposites, synthesized through multiple-step chemical synthesis technique. Stepwise synthesis and structural optimization of composites include oxalate route synthesis of BaTiO3 with specifically chosen tetragonality followed by mixing in citric acid solution, and a final addition of the precursor for CoFe2O4. Heat treatment temperature were appropriately chosen to obtain composites with distinct spinel and tetragonal perovskite phases. Peak broadening has been observed in composites being a consequence of presence of strain. Main goal of the present work is to investigate magnetic response as a function of temperature for various compositions and to compare them with the magnetic properties of CoFe2O4 nanoparticles. BaTiO3 acts as a barrier for the growth of the CoFe2O4 due to the phase separation between these constituent phases in the composites. The nanoscopic features are consistently observed in the magnetic properties of the composites. These features include increase in coercivity, stronger temperature dependence of coercivity and magnetic irreversibility. CoFe2O4 and BaTiO3 composites exhibit strong temperature dependence with a more than an order of magnitude increase in coercivity at low temperatures.

Magnetic interaction and anisotropy axes arrangement in nanoparticle aggregates can enhance or reduce the effective magnetic anisotropy

The magnetic response of nanostructures plays an important role on biomedical applications being strongly influenced by the magnetic anisotropy. In this work, we investigate the role of the temperature, particle concentration and nanoparticles arrangement forming aggregates in the effective magnetic anisotropy of Mn-Zn ferrite-based nanoparticles. Electron magnetic resonance and coercivity temperature dependence analyses, were critically compared for the estimation of the anisotropy. We found that the temperature dependence of the anisotropy follows the Zener-Callen model, while the symmetry depends on the particle concentration. At low concentration one observes only an uniaxial term, while increasing a cubic contribution has to be added. The effective anisotropy was found to increase the higher the particle concentration on magnetic colloids, as long as the easy axis was at the same direction of the nanoparticle chain. Increasing even further the concentration up to a highly packed condition (powder sample) one observes a decrease of the anisotropy, that was attributed to the random anisotropy axes. Further, room temperature anisotropy decreased on the order of 50% in comparison to the low temperature values, thus impacting the theoretical estimations of the magnetic response of nanoparticles.

Coupling of pinned magnetic moments in an antiferromagnet to a ferromagnet and its role for exchange bias

The interaction between uncompensated pinned magnetic moments within an antiferromagnetic (AFM) layer and an adjacent ferromagnetic (FM) layer responsible for the existence of exchange bias is explored in epitaxially grown trilayers of the form FM2/AFM/FM1 on Cu3Au(0 0 1) where FM1 is ~12 atomic monolayers (ML) Ni, FM2 is 21–25 ML Ni, and AFM is 27 ML or 50 ML Ni~25Mn~75. Field cooling for parallel or antiparallel alignment of the out-of-plane magnetizations of the two FM layers does not make a difference for the temperature-dependent coercivity (HC), magnitude of exchange bias field (Heb), AFM ordering temperature (TAFM), and blocking temperature for exchange bias (Tb). We explain this by a model in which the uncompensated pinned magnetic moments distributed within the volume of the AFM layer interact with both of the FM layers, albeit with different strength. Parallel and antiparallel coupling between the magnetization of the pinned moments and the FM layers equally exists. This leads to the experimentally observed independence of HC, Heb, as well as of TAFM and Tb on the magnetization direction of the FM layers during field cooling. These results provide new and detailed insight into revealing the subtle and complex nature of the exchange bias effect.