Temperature Dependence Of Ac Susceptibility Research Articles

Incidence of the Brownian Relaxation Process on the Magnetic Properties of Ferrofluids.

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter), the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic-field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6 and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of Brownian relaxation for the sample of large-diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small-diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of Brownian relaxation. This peak vanishes below 273 K.

Spin glass state in Co doped NdMnO3

The temperature-dependent ac susceptibility studies of in-phase component (χ′ac) and out of phase component (χac″) was done on a single phase polycrystalline NdMn0.6Co0.4O3 sample. The measurements were conducted in 4 Oe ac field and 0 Oe dc field at frequencies of 111, 311, 1011, and 1311 Hz. As the frequency increases, the magnitude of χ′ac decreases, and its peak position, corresponding to Tf, moves towards higher temperatures. This strongly suggests the presence of a spin glass (SG) state and the absence of long-range ferromagnetic ordering. The value of relative shift in freezing temperature (δTf) is found to be δTf = 0.0089, which suggests the existence of canonical SG state in NdMn0.6Co0.4O3 sample. To confirm it the ac susceptibility data was further analyzed using Arrhenius, Vogel Fulcher, and Power–law.

Designing the stripe-ordered cuprate phase diagram through uniaxial-stress.

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate La[Formula: see text]Ba[Formula: see text]CuO[Formula: see text], with [Formula: see text] = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO[Formula: see text] plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature [Formula: see text] and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of [Formula: see text]0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal [Formula: see text]. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.

Magneto-electric effect in Y1-xRxFeO3 (R = Sm and Er) ceramics

In the present work, we synthesize a series of Y1-xRxFeO3R=Er,Sm&0.≤x≤1 polycrystalline compound by solid-state reaction method to study the A-site substitution effects on physical properties in YFeO3. The Rietveld refinement of X-ray diffraction analysis indicates that with Er3+ substitution, the lattice parameters have decreased, and more structural distortion has been shown compared to Sm3+ substitution. The obtained magnetic hysteresis loops reveal that Er3+ ions have induced an antiferromagnetic behavior in YFeO3 because of a reduction in the Dzyloshinskii-Moriya interaction. The Y1-xSmxFeO3 family, however, shows a weak ferromagnetic behavior. Furthermore, due to the spin reorientation (SR) transition, a distinct anomaly has been detected in the temperature dependence of Ac susceptibility and dielectric constant curves. With substituting Sm at the Y-site in the YFeO3 compound, SR transition happens at a temperature, TSR, in which the spin structure changes high-temperature Γ4 to Γ2 configuration, and therefore the starting and ending of the SR region depend on Sm concentration. Interestingly, we have observed a gigantic dielectric constant in Y0.7Sm0.3FeO3, Y0.5Er0.5FeO3, and Y0.25Er0.75FeO3. It, however, has demonstrated that Y0.7Sm0.3FeO3 is the only compound that shows a relaxor ferroelectric behavior. The observed peak in the temperature dependence of the dielectric constant has also described a magnetic transition temperature indicating the magneto-dielectric effect in our studied samples.

Non-equilibrium magnetic response of canonical spin glass and magnetic glass

Time and history dependent magnetization has been observed in a wide variety of materials, which are collectively termed as the glassy magnetic systems. However, such systems showing similar non-equilibrium magnetic response can be microscopically very different and they can be distinguished by carefully looking into the details of the observed metastable magnetic behavior. Canonical spin glass (SG) is the most well studied member of this class and has been extensively investigated both experimentally and theoretically over the last five decades. In canonical SGs, the low temperature magnetic state obtained by cooling across the SG transition temperature in presence of an applied magnetic field is known as the field cooled (FC) state. This FC state in canonical SG is widely believed as an equilibrium state arising out of a thermodynamic second order phase transition. Here, we show that the FC state in canonical SG is not really an equilibrium state of the system. We report careful dc magnetization and ac susceptibility measurements on two canonical SG systems, AuMn (1.8%) and AgMn (1.1%). The dc magnetization in the FC state shows clear temperature dependence. In addition, the magnetization shows a distinct thermal hysteresis in the temperature regime below the SG transition temperature. On the other hand, the temperature dependence of ac susceptibility has clear frequency dispersion below SG transition in the FC state prepared by cooling the sample in the presence of a dc-bias field. We further distinguish the metastable response of the FC state of canonical SG from the metastable response of the FC state in an entirely different class of glassy magnetic system namely magnetic glass, where the non-equilibrium behavior is associated with the kinetic-arrest of a first order magnetic phase transition.

Spin dynamics and relaxation in 7.6 nm thin film of La0.7Sr0.3MnO3/SrTiO3: ac magnetic susceptibility and magnetic viscosity investigations

Analysis of the results from our investigations of the temperature dependence of ac susceptibilities (χ′ and χ′′) and magnetic viscosity in a 7.6 nm thin film of La0.7Sr0.3MnO3 grown on SrTiO3 (001) substrate is presented. The Curie temperature (TC) of this film is magnetic field (H) dependent, varying as [TC(H)−TC(0)]∼Hn with TC(0) = 270 K and n = 0.15. Temperature dependence of χ′ and χ′′ measured at ac frequencies fm in the range of 0.1–10 kHz shows a broad peak near 230 K associated with the blocking temperature (TB) of spin clusters present in the 1.4 nm surface layer and a frequency dependent peak near 270 K associated with TC. The mean relaxation time τ determined from the Cole–Cole analysis of the temperature dependent χ′ and χ′′ is shown to fit the Vogel–Fulcher law: τ=τ0exp[ΔE/kB(T−T0)] with T0 = 245 K, ΔE/kB = 270 K = TC, and τ0 = 1.2 × 10−9 s. Magnetic viscosity S (measured in H = 0 for a sample cooled in H = 50 Oe) determined from the fit to the magnetization: M (t) = M (0) – S ln t, with time t up to 2 h, shows a peak at 230 K above which M (2 h) switches to negative values for temperatures up to TC(0). It is argued that this negative magnetization results from an interaction between the superparamagnetic spin clusters and the ferromagnetic phase, both being present in the temperature range of TB < T < TC.

Surface driven exchange bias in nanocrystalline CoCr2O4

This article reports exchange bias (EB) in nanocrystalline cobalt chromite CoCr2O4, - a phenomenon manifested by loop shift followed by increase in coercivity after cooling the sample in static magnetic field. The average particle size of the nanocrystalline CoCr2O4 is 30 nm, confirmed from transmission electron microscopy images. Thermal variation of magnetization data M(T), in zero field cooling (ZFC) and field cooling (FC) modes depict long range order below the lock-in transition at TL≈16 K, spin-spiral transition at TS≈28 K and a collinear ferrimagnetic state at TC≈100 K. Magnetic hysteresis loops are recorded in ZFC and FC modes at different cooling fields (). In addition, hysteresis loops are recorded in FC mode for a wide temperature range. In both cases, a clear signature of conventional exchange bias effect is observed. Analysis of the training effect in magnetic hysteresis loop and temperature dependent ac susceptibility (χac) at different frequencies suggest that the surface spins of CoCr2O4 nanoparticles play an important role in controlling the magnetic properties, and enhance the EB effect in collaboration with competition between non-collinear spiral spin state and collinear long range ferrimagnetic order.

Correlating spin freezing and magnetic properties in [Co/Ni]n/PtMn multilayers

Exchange-coupled ferromagnetic/antiferromagnetic (FM/AF) bilayers are widely used in spin valves, magnetic tunnel junctions and spin–orbit torque devices. While most studies focused on AF layers exchange coupled with single FM layer, the exchange interaction with FM multilayer were not well understood. In this work, [Co/Ni]n/PtMn multilayers with different PtMn thickness were prepared. Polycrystalline face-centered cubic Co, Ni, and PtMn phases were evidenced by the grazing incidence x-ray diffraction measurements. At room temperature, the [Co/Ni]n/PtMn multilayers show similar soft ferromagnetism as the reference [Co/Ni]2 layers. Reducing the temperature results in broadening of the hysteresis loops, and exchange bias was not observed until 100 K. Temperature-dependent DC magnetization and AC susceptibility revealed blocking temperatures of 210–240 K and a spin freezing temperature of 140 K. This work has revealed the temperature evolution of exchange interaction in [Co/Ni]n multilayer interfacing with A1-phase PtMn. The results are important for understanding the physical process of the establishment of exchange coupling in various FM/AF interfaces.

Study of A-site disorder dependent structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3

This paper reports the studies on the effect of A-site substitution by strontium on the structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3 (x = 0.1, 0.2 and 0.3). The investigated samples are prepared by conventional solid-state reaction technique. XRD analysis at room temperature has confirmed orthorhombic structure of the sample with space group Pnma. The dependence of structural parameter, Curie temperature and coercivity on Sr doping content has been thoroughly investigated. It is observed that substitution of Sr2+ for Ca2+ increases lattice parameter, tolerance factor and the Curie temperature. However, the coercivity (Hc) decreases with increasing Sr content while the charge ordering process is weakened with increasing Sr content. Field cooled (FC) and zero-field cooled (ZFC) dc magnetizations measurements at low field and low temperature indicate that there is a spin-glass (SG) like state occurred. Temperature dependent ac susceptibility at different frequency indicates a spin-glass-like transition of the sample.

Effect of praseodymium doping on the phase transitions of barium orthoferrites

In this article, an effort was made to explore the properties of praseodymium-substituted barium orthoferrites with a better performance in absorption of microwaves. Orthoferrites with BaPrxFe2−xO4 (0 ≤ x ≤ 0.1) were prepared using an ultrasound-assisted sonochemical reactor. In this study, ultrasonic technique was used for the preparation and characterization of BaPrxFe2−xO4 (0 ≤ x ≤ 0.1) materials. X-ray diffraction (XRD) pattern was used to determine the structural properties such as phase purity and crystallite parameters. Analysis of the XRD pattern indicated that the prepared BaPrxFe2−xO4 ferrites have orthorhombic structure with the Bb21 m space group. The temperature-dependent AC susceptibility (χac) and in situ ultrasonic measurements were carried out to explore the Curie temperature of the BaPrxFe2−xO4 samples. Magnetic properties of the prepared BaPrxFe2−xO4 orthoferrites showed narrow M − H hysteresis loops. The coercivity of barium-based samples was found to increase with praseodymium substitution whereas the remenance and saturation magnetization decreased.

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to M\"ossbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.

Low temperature ferromagnetic behavior and temperature dependent anomalous dielectric relaxation of Zn0.90Ni0.05Mn0.05O diluted magnetic semiconductor

We report structural, magnetic and temperature dependent dielectric properties of diluted magnetic semiconductor Zn0.90Ni0.05Mn0.05O prepared by solid state reaction technique. X-ray diffraction analysis revealed formation of single phase hexagonal wurtzite structure. Scanning electron microscopy and atomic force microscopy images indicated increase in grain size and roughness respectively with increasing sintering temperature. Field dependent DC magnetization at low temperature exhibited ferromagnetic ordering with coercivity ~ 6 × 104 A/m and remanence ~ 17 A/m. Complex initial permeability values were found to be positive for the measurement frequency range (1 kHz–120 MHz) with a relaxation at lower frequency. Temperature dependent DC magnetization and AC susceptibility followed curie law with curie temperature below 65 K. Temperature dependent dielectric constants ($$\varepsilon ^{\prime}\,\& \,\varepsilon ^{\prime\prime}$$) and loss tangents ($$tan\delta$$) measured for selected frequencies were found to be an increasing function of temperature and decreasing function of frequency. AC conductivity ($${\sigma }_{ac}$$) values were found to be an increasing function of frequency and temperature. Clear signatures of relaxations were observed in $$\varepsilon ^{\prime},\,\varepsilon ^{\prime\prime},$$ $$tan\delta$$ and $${\sigma }_{ac}$$ for temperatures above 200 °C.

X-ray Photoelectron Spectroscopy, Magnetotransport and Magnetisation Study of Nb2PdS5 Superconductor

In the present report, we investigate various properties of the Nb2PdS5 superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb2PdS5 while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity ( $T_{c}^{\text {onset}} = 7$ K and T cρ= 0 = 6.2 K) of the studied Nb2PdS5 material is quite robust against magnetic field with the upper critical field (H c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb2PdS5 is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H c1) is extracted from DC magnetisation measurements at various T below T c. It is found that H c1(T) of Nb2PdS5 material seemingly follows the multiband nature of superconductivity.

Magnetocaloric effect in (La0.7Sr0.3MnO3)1−x –(BaTiO3) x solid solution spin-glass system

(La0.7Sr0.3MnO3)1−x –(BaTiO3) x solid solution samples (x = 0, 0.03, and 0.08) were synthesized by sol–gel route. Pure La0.7Sr0.3MnO3 has orthorhombic structure with Pbnm space group whereas solid solution samples possess rhombohedral structure with R-3c space group. Temperature-dependent ac susceptibility at different frequencies and memory effect confirmed the spin-glass behavior in these samples. The samples exhibited magnetocaloric coefficient around room temperature. The maximum values of magnetic entropy change observed in our samples are 0.45, 0.86, and 1.07 J Kg−1 K−1 for x = 0.03 and 0.62, 1.23 and 1.56 J Kg−1 K−1 for x = 0.08 compounds at 2, 4, and 5 T applied magnetic fields, respectively. The relative cooling power values for x = 0.03 and 0.08 are 55.9 and 116.5 J Kg−1, respectively, at 5 T. The observed multifunctional properties of the studied system give a new direction to explore the magnetocaloric effect in similar such systems.

Effects of Cu doping on the electronic structure and magnetic properties of MnCo2O4 nanostructures

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping () on the electronic structure and magnetic properties of the spinel O4. A detailed comparison is given for the and cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ()A]B i.e. substitutes for on the octahedral B-sites. For the bulk samples, the ferrimagnetic K for is lowered to K for the sample, this decrease being due to the effect of Cu doping. For the nanosize () sample, the lower K ( K) is observed using analysis, this lowering being due to finite size effects. For , fits of dc paramagnetic susceptibility data of versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: K (4.1 K), K (16.3 K) and K (13.8 K) for . The temperature dependence of ac susceptibilities and at different frequencies shows that in bulk samples of and , the transition at TC is the normal second order transition. But for the nanosize and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at TC is followed by a re-entrant spin-glass transition at lower temperatures K (138 K) for (). Analysis of the ac susceptibilities, and , versus T data is done in terms of two scaling laws: (i) Vogel–Fulcher law ; and (ii) power law of critical slowing-down . These fits confirm the existence of glassy behavior below TSG with the parameters (8.91), (9.6 × 10) and K (∼138 K) for the samples (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in versus (AT-line) further supports the existence of glassy states in nanosize samples. For , the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with x ⩾ 0.1 have the coercivity HC and remanence . Since the results reported here in these nanostructures are significantly different from those in bulk , further investigations of their magnetic structures using neutron diffraction are warranted.

Electrical, magnetic, and magnetotransport behavior of inhomogeneous Nd1-xCaxMnO3 (0.0 ≤ x ≤ 0.8) manganites

We have systematically investigated the electrical, magnetic and magnetotransport properties of Nd1-xCaxMnO3 (where x=0.0 to 0.8) manganites. The substitution of Ca at Nd-site changes the Mn3+/Mn4+ ratio, which in turn influences the electrical and magnetic transport behavior. The temperature dependence of AC susceptibility (ACS) measurements confirmed the inhomogeneity in the magnetic phases for all the compounds. Mostly Nd1-xCaxMnO3 behaves as an insulator under zero and higher magnetic fields. Exemption to that, the intermediate compositions, where x=0.33–0.5, exhibit a magnetic field induced metal-insulator transition. A significant value of magnetoresistance, 100%, along with a hysteretic nature is obtained for the compositions x=0.3 to x=0.5, which attribute to the metamagnetic transitions in the materials. The results have also been explained from the ACS measurements, which are corroborated by the earlier reports.

The effect of film thickness on Curie temperature distribution and magnetization reversal mechanism for granular L10 FePt films

Structural and magnetic properties of granular L10 FePt films are investigated as a function of film thickness in the range 3–11 nm. Scanning transmission electron microscopy observations indicate an increasing contribution of in-plane variants to the L10 FePt crystalline structure with increasing film thickness, which may explain the noticed degradation of the perpendicular anisotropy. The temperature dependence of coercivity suggests a progressive deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm, corresponding to a domain-wall width. The anisotropy field distribution (〈HK〉, ) is estimated using Monte Carlo (MC) simulations of AC transverse susceptibility (ACTS). The average anisotropy field 〈HK〉 displays a similar temperature dependence to (MS)1.1, where MS is the film saturation magnetization, consistent with the theoretical predictions for L10 FePt. The Curie temperature distribution (〈TC〉, ) is evaluated using two methods: fast (ns) thermal erasure (ThEr) of remanent magnetization and MC modeling of the temperature dependence of AC susceptibility (ACS). The values evaluated by the two methods are in agreement, indicating an increase of in the range 2–6% with decreasing film thickness. The observed increase of 〈TC〉 in the range 620–680 K with increasing film thickness is explained using the finite-size scaling law with the critical exponent set to the theoretical value for L10 FePt. A general procedure to extract 〈TC〉 and using MC modeling of ACS, essentially independent of the magnetization reversal mechanism, is proposed. MC simulations of ACS confirm the deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm.

Field-Assisted Slow Magnetic Relaxation in a Six-Coordinate Co(II)-Co(III) Complex with Large Negative Anisotropy.

The reaction of Co(CH3COO)2·4H2O with the Schiff base ligand LH4 derived from o-vanillin and tris(hydroxymethyl)aminomethane produces the dinuclear mixed-valence complex [CoIICoIII(LH2)2(CH3COO)(H2O)](H2O)3 (1), which has been investigated using IR spectroscopy, X-ray crystallography, temperature-dependent magnetic susceptibility, magnetization, HFEPR spectroscopy, and ac susceptibility measurements at various frequencies, temperatures, and external magnetic fields. The structure of 1 consists of neutral molecules in which two cobalt ions with distorted octahedral geometries, CoIIO6 and CoIIIN2O4, are bridged by two deprotonated -CH2O- groups of the two LH22- ligands. 1 completes a series with Cl, Br, NO3, and NCS anions published before by different authors. Low-temperature HFEPR measurements reveal that the ground electronic state of the Co(II) center in 1 is a highly anisotropic Kramers doublet; the effective g values of 7.18, 2.97, and 1.96 are frequency-independent over the frequency ranges 200-630, 200-406, and 200-300 GHz for the highest, intermediate, and lowest geff values, respectively. The two lower values were not seen at higher frequencies because the magnetic field was not high enough. Temperature-dependent magnetic susceptibility and field-dependent magnetization data confirm high magnetic anisotropy of the easy axis type. Complex 1 behaves as a single-ion magnet under a small applied external field and demonstrates two relaxation modes that strongly depend on the applied static dc field. The observation of multiple relaxation pathways clearly distinguishes 1 from the Cl and Br analogues.

Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles

Effect of silica (SiO2) coating concentration on structural and magnetic properties of multiferroic cobalt chromite (CoCr2O4) nanoparticles have been studied. The nanoparticles with average crystallite size in the range 19 to 28 nm were synthesised by sol-gel method. X-ray diffraction (XRD) analysis has verified the composition of single-phase cubic normal spinel structure of CoCr2O4 nanoparticles. The average crystallite size and cell parameter decreased with increasing SiO2 concentration. TEM image revealed that the shape of nanoparticles was non-spherical. Zero field cooled/field cooled (ZFC/FC) curves revealed that nanoparticles underwent a transition from paramagnetic (PM) state to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition temperature decreased from 101 to 95 K with increasing SiO2 concentration or decreasing crystallite size. A conical spin state at Ts = 27 K was also observed for all the samples which decreased with decreasing average crystallite size. Low temperature lock-in transition was also observed in these nanoparticles at 12 K for uncoated nanoparticles which slightly shifted towards low temperature with decreasing average crystallite size. Saturation magnetization (Ms) showed decreasing trend with increasing SiO2 concentration, which was due to decrease in average crystallite size of nanoparticles and enhanced surface disorder in smaller nanoparticles. The temperature dependent AC-susceptibility also showed the decrease in the transition temperature (Tc), broadening of the Tc peak and decrease in magnetization with increasing SiO2 concentration or decreasing average crystallite size. In summary, the concentration of SiO2 has significantly affected the structural and magnetic properties of CoCr2O4 nanoparticles.

High Density Heat-Assisted Magnetic Recording Media and Advanced Characterization—Progress and Challenges

Heat-assisted magnetic recording (HAMR) is being developed as the next generation magnetic recording technology. Critical components of this technology, such as plasmonic near-field transducer (NFT) and high anisotropy granular FePt media, as well as the performance and reliability of fully integrated drives have been reported. This paper will focus on the progress and challenges of HAMR media, including microstructure and thermal design as well as the testing and characterization at high field and high temperature. Due to the importance of the Curie temperature distribution, $\sigma T_{C}$ , for HAMR, we present a newly developed temperature-dependent complex ac susceptibility method to extract $\sigma T_{C}$ for HAMR media. Such novel magnetic characterization methods have been used in combination with other high field magnetic metrology and spin-stand recording to provide feedback for continuous improvements of HAMR media. Together with NFT and write head design, the thermal design, $\sigma T_{C}$ , and microstructure of the media are key factors to reduce the transition jitter below 2 nm as demonstrated in a previously reported 1 Tb/in2 HAMR demonstration. Here, we report the further improvements by significantly enabling higher linear density (>2500 kfci) HAMR and steady progress in areal density to 1.402 Tb/in2.