Search

We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRu

A wide variety of intriguing and exciting effects observed in solids results from the coupling of their magnetic sublattice with an applied magnetic field. In particular, it concerns the magneto-thermodynamic phenomenon well-known as the magnetocaloric effect (MCE), which manifests itself in the absorption and emission of heat by a solid under an external magnetic field change. To date, significant efforts have been put into searching for materials with advanced magnetocaloric properties, which show promise as refrigerants that might ensure the reliable and efficient cooling technology being alternative to the conventional gas-compression-based technology. To fulfill the expectations, new compounds with improved and tailored MCE characteristics are designed and efficient procedures for their manufacturing are developed. A great deal of attention has been paid to searching for magnetic refrigerants in view of industrial applications. Systems showing large MCEs at low temperatures are of importance due to their potential utilization in refrigeration for gas liquefaction. We find that the Laves-phase rare-earth-nickel solid solutions give the possibility of reaching high magnetocaloric potentials in a wide temperature range, which is demonstrated by an example of the Ho1xErxNi2 compositions used as components of magnetocaloric composites.

The nature of magnetism in the doubly-diluted spinel ZnTiCoO

Using the analysis of the temperature and magnetic field dependence of the magnetization (

Herein, we report unusual magnetic behavior in arc melted bulk stoichiometric Fe3Al alloy with a D03 structure. The temperature variation in the magnetization measurements revealed two transitions, i.e. one at ∼763 K and another at ∼830 K. Below 763 K, it exhibits ferromagnetic ordering and the nature of the transition is second order. However, the second transition is more complex and detailed analysis of the magnetic data suggested the coexistence of ferromagnetic and paramagnetic phases (two-phase: α + D03/α + B2) and structural transitions triggered by temperature. We observed dual peaks in the magnetic entropy change curve, in accordance with the magnetization results, which corroborate the occurrence of a two-phase system. We believe that the concurrent magnetic ordering and the complex two-phase are associated with the evolution of short-range ordered magnetic clusters having a magnetic moment of ∼103μB in the host matrix. A cluster hypothesis is proposed to explain the observed intricate magnetic behavior of this alloy at high temperature. The estimated critical exponents using a modified Arrott plot, Kouvel–Fisher plot and critical isotherm analysis lie in between the 3D-Heisenberg and 3D-Ising model, indicating a short-range interaction and magnetic inhomogeneity, which again are consistent with the magnetization results. The obtained critical exponents follow the universal scaling behavior, which indicates the renormalization of interactions around the magnetic ordering transition (TC). Despite the obvious larger thermal entropy at very high temperature, our synthesized Fe3Al alloy showed enhanced magnetic entropy changes. The obtained magnetic entropy change for binary Fe3Al alloy shows twice the value of that of other binary/ternary Fe-based alloys.

This report introduces both synthesis and in vitro biological behaviour of dual magnetic-fluorescent silica nanoparticles. The amino group-decoration of 78 nm sized silica nanoparticles enables their efficient internalization into motoneurons, which is visualized by the red fluorescence arising from [Ru(dipy)3]2+ complexes encapsulated into a silica matrix. The internalized nanoparticles are predominantly located in the cell cytoplasm as revealed by confocal microscopy imaging. The magnetic function of the nanoparticles resulted from the incorporation of 17 nm sized superparamagnetic iron oxide cores into the silica matrix, enabling their responsivity to magnetic fields. Fluorescence analysis revealed the "on-off" switching of Ca2+ influx under the application and further removal of the permanent magnetic field. This result for the first time highlights the movement of the nanoparticles within the cell cytoplasm in the permanent magnetic field as a promising tool to enhance the neuronal activity of motoneurons.

We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe 1 − x Ru x )2O4 by means of detailed d c -magnetization and a c -susceptibility studies combined with the heat capacity analysis. Two specific compositions (x = 0.5, 0.75) have been investigated in detail along with the substitution of Jahn–Teller (JT) active spin-1/2 Cu2+ ions at B-sites. Measurements based on the frequency and temperature dependence of a c -susceptibility ( χ a c ( f , T ) ) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel–Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperature T S G (17.77 K (x = 0.5) and 14 K (x = 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida–Thouless (AT)-line approach ( T S G ( H ) = T S G ( 0 ) ( 1 − A H 2 / ϕ ) ) , with an ideal value of φ = 3. Nevertheless, the analysis of temperature dependent high field d c -susceptibility, χ h f ( 2 kOe ⩽ H DC ⩽ 20 kOe , T) provides evidence for Gabay–Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of Cu0.2Zn0.8FeRuO4 system, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature ( T F i M ) and their volume fraction vanishes completely across ∼6 T F i M . This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling ( J S ) between the spins which yields J S = − 3.02 × 10 − 5 eV for Cu0.2Zn0.8FeRuO4 representing the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cu2+ ions. However, the antiferromagnetic coupling is predominant in ZnFeRuO4 and Cu0.2Zn0.8Fe0.5Ru1.5O4 systems. Finally, we deduced the magnetic phase diagram in the H D C − T plane using the characteristic parameters obtained from the field variations of both a c - and d c -magnetization measurements.

This work presents the magnetic field-temperature (

The present paper reports the synthesis of iron-oxide nanoparticles (diameter 12.8±2.2 nm) coated with silica shell doped with paramagnetic Gd(III)-based complexes. The resulting nanoparticles with a silica shell thickness of about 45 nm have an average diameter of 113.1±14.3 nm and feature high transverse and longitudinal relaxivities (356 and 25 mM−1 s−1, respectively) at 1.5 T and 25 °C on a medical whole body NMR scanner. It has been also revealed using magnetic heating measurements that the prepared core-shell nanoparticles possess a high specific adsorption rate of around 236 W/g in aqueous media. The surface of the composite nanoparticles was decorated by amino-groups for a greater cellular uptake behaviour. The cell viability measurements reveal the concentration-dependent cytotoxicity of the nanoparticles, which agrees well with the high content of Gd(III) complexes in the nanomaterial. The obtained results show that the core-shell design of nanoparticles with superparamagnetic and paramagnetic parts can be promising for high transverse (and longitudinal) relaxivity as well as magnetic hyperthermia.

A detailed crystal structure analysis, temperature and field dependence of magnetic characteristics and phonon instabilities for different compositions (0.1 ⩽ x ⩽ 0.5) of Dy1−x Ce x CrO3 solid-solutions have been reported. All the investigated compounds exhibit distorted orthorhombic crystal structure with a distortion factor of d Oct/d Cell ∼ 6 × 10−3/3.5 ppm (for x ∼ 0.2) for Pbnm space group that follows Vegard’s law. The bonds between apical oxygen atoms (OA1) and Cr atoms stand more rigidly in comparison with the basal oxygen atoms (OB1/OB2) resulting the octahedral distortion and thereby causing the changes in phonon modes. The CrO6 octahedral tilt angle θ rotates with respect to the Miller pseudocubic axis [101] which varies from 10.36° (x = 0.1) to 12.25° (x = 0.5) and significantly influences the Ag(5) phonon stability by 3% for a change in A-site mean radius from 1.095 Å to 1.141 Å for x = 0.1 and 0.5, respectively. From the magnetization measurements we find that these series of compositions exhibit canted antiferromagnetic (AFM) ordering with Néel temperature, T N 1 that increases from 151.8 K (x = 0.1) to 162 K (x = 0.5) which also manifests as a significant reduction in the magneto-crystalline anisotropy (H K ∼ 2.58 kOe → 2.07 kOe, K 1 ∼ 36.47 J m−3 → 18.97 J m−3) while maintaining the stable Γ4(G x , A y , F z ) AFM configuration. Both Dzyaloshinskii–Moriya interaction method and modified Curie–Weiss law are employed to analyse the inverse paramagnetic susceptibility, χ −1(T > T N 1 ). Further, we have evaluated the symmetric (J S) and antisymmetric exchange (D AS) constants, which show progressively increasing trend (J S → 10.08 K to 11.18 K and D AS → 1.24 K to 1.73 K) with the incorporation of Ce inside the perovskite lattice. Furthermore, the role of Ce substitution on the low-temperature spin reorientation transition (T SR ∼ 3.5 K → 16.8 K pertaining to the Γ25 phase configuration) and emergence of Γ 2 ( F x , C y , G z ; F x R , C y R ) weak-FM phase between 31 K and 45.5 K are discussed in consonance with the phonon spectra.

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H–T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6 are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6 sample verified its phase purity. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Néel temperature T N = 5.97 K determined from the peak in the computed ∂(χT)/∂T vs T plot, in agreement with the T N = 6.00 K determined from the peak in the C P vs T data. The experimental data of C P vs T near T N is fitted to C P = A|T − T N|−α yielding the critical exponent α = 0.10(0.13) for T > T N (T < T N). The χ vs T data for T > 25 K fits well with the modified Curie–Weiss law: χ = χ 0 + C/(T − θ) with χ 0 = −2.12 × 10−4 emu mol−1 Oe−1 yielding θ = −24 K, and C = 4.44 emu K mol−1 Oe−1, the later giving magnetic moment μ = 5.96 μ B per Mn2+ ion. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes of θ and T N and molecular field theory, the antiferromagnetic exchange constants J 0/k B = −1.5 ± 0.2 K and J ⊥/k B = −0.85 ± 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The χ vs T data when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The H–T phase diagram is mapped using the M–H isotherms and M–T data at different H yielding the tricritical point T TP (H, T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange field H E = 206.4 kOe and spin-flop field H SF = 23.5 kOe yield the anisotropy field H A = 1.34 kOe. These results for MnTa2O6 are compared with those reported recently in the isostructural MnNb2O6.

As the magnetocaloric characterization of polycrystalline Tb1−xHoxNi2 intermetallic compounds (with x = 0.25, 0.5, and 0.75), their relative cooling power (RCP) and refrigerant capacity (RC) are estimated based on comprehensive investigations of the magnetic and thermodynamic properties of the compositions, the structure of which was studied in detail by x-ray diffraction analysis and scanning electron microscopy. The taken x-ray diffraction patterns demonstrate the similarity between the Ho-depleted compound (x = 0.25) and the parent TbNi2 and HoNi2 compounds, i.e., the regular C15 cubic structure forms in them, whereas, in the case of x = 0.5 and 0.75, the C15-based superstructure is found. Both magnetic and heat capacity measurements showed that all of the studied compounds exhibit the transition from the ferromagnetic to paramagnetic state, which is confirmed to be the second order magnetic phase transition with the Curie temperature that decreases from 31.1 to 18.6 K in passing from the Tb-enriched Tb0.75Ho0.25Ni2 to the Ho-enriched Tb0.25Ho0.75Ni2 composition. The high values of RCP and RC are shown can be reached for the compositions and experimentally are demonstrated based on the performed measurements of magnetic and thermodynamic parameters. Under an external magnetic field change of 0–5 T, the maximum magnetic entropy change -ΔSmag for the Tb1−xHoxNi2 compounds increases from 13.9 to 19.5 J/kgK as x increases from 0.25 to 0.75. These experimental results are compared with and confirmed by available theoretical data. The maximum adiabatic temperature change ΔTad for Tb0.75Ho0.25Ni2 and Tb0.5Ho0.5Ni2 at the 2 T magnetic field change reaches 3.0 K, and substantially increases, namely, to 4.5 K in the case of the Tb0.25Ho0.75Ni2 composition near its Curie temperature TC. The effect of increasing Ho content in the Laves-phase compounds on their magnetic and magnetocaloric properties is discussed.

We report a systematic study of the magnetic phase diagram in the H–T plane, negative magnetization (NM), exchange interactions and field-induced spin–flop transitions in the distorted perovskite Y1−x Ce x CrO3. Locked AFM and weak-FM configurations in Γ4(G z , F y , A x ) phase of YCrO3 (S = 3/2 ground state) unlocks into the Γ2 (F z , G y , C x ; F z R , C x R ) phase of the canted AFM and FM structures with the dilute substitution of Ce (x ⩾ 0.05). The asymmetric and symmetric exchange interaction (J AS ∼ 0.11 meV and J S ∼ 0.85 meV) between the trivalent Ce and Cr enable the positive quartic-anisotropy field ( H K 4 ∼ 2.85 × 102 Oe) along with the second order anisotropy field ( H K 2 ∼ 5.93 × 102 Oe). Unlike the pristine YCrO3 compound, the Ce incorporated system exhibits a giant fourth-order anisotropy constant (K 4 = 1.35 × 105 erg/c.c.) due to the asymmetric exchange interaction between the trivalent Ce–Cr which further lifts the free energy of the system and causes lag in the onset of AFM ordering showing the significant thermal hysteresis (ΔT ∼ 10 K) in the field-cooled (FC)-warming measurement protocol as compared to the FC-cooling mode. The H–T phase diagram, mapped from the isothermal magnetization data and differential magnetic susceptibility data with different measurement protocols clearly distinguishes three prominent regions below the T N (∼150 K), viz (i) long-range canted AFM + weak FM phase (Γ4 (G z , F y , A x )), (ii) Γ24 mixed phase and (iii) robust Γ2 (F z , G y , C x ; F z R , C x R ) AFM + FM phases. Tunable spin–flopped transition (∼ 30 kOe), significant negative exchange-bias field (H EB ∼ 2.5 kOe), huge coercive field (H C ∼ 22 kOe) and large NM (ΔM ∼ 280 emu/mole) are the unique characteristic features of the current investigated system.

This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6 polycrystals. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Neel temperature TN = 4.33 K determined from the peak in the computed d(χT)/dT vs. T plot in agreement with the transition in the CP vs. T data at TN = 4.36 K. The experimental data of CP vs. T near TN is fitted to CP = A|T-TN|-α yielding the critical exponent α = 0.12(0.15) for T > TN (T 50 K to χ = χ0+C/(T-θ) with χ0 = -1.85 Χ 10-4 emu.mol-1Oe-1 yields θ = -17 K, and C = 4.385 emu.K mol-1Oe-1, the latter giving magnetic moment μ = 5.920μB per Mn2+ ion. This confirms the effective spin S = 5/2 and g = 2.001 for Mn2+ and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes of θ and TN and molecular field theory (MFT), the exchange constants J0/kB = -1.08 K for Mn2+ ions along the chain c-axis and J⊥/kB = -0.61 K as the interchain coupling perpendicular to c-axis are determined. These exchange constants are consistent with the expected χ vs. T variation for the Heisenberg linear chain. The H-T phase diagram, mapped using the M-H isotherms and M-T data at different H combined with the reported data of Nielsen et al:, yields a triple-point TTP (H, T) = (18 kOe, 4.06 K). The spin flopped state above TTP and the forced ferromagnetism for H > 192 kOe are used to estimate the anisotropy energy HA ≈ 0.8 kOe.

The Dy-Sc nitride clusterfullerene Dy2ScN@C80-Ih exhibits slow relaxation of magnetization up to 76 K. Above 60 K, thermally-activated relaxation proceeds via the fifth-excited Kramers doublet with the energy of 1735 ± 21 K, which is the highest barrier ever reported for dinuclear lanthanide single molecule magnets.

This report introduces both synthesis and in vitro biological behaviour of dual magnetic-fluorescent silica nanoparticles. The amino group-decoration of 78 nm sized silica nanoparticles enables their efficient internalization into motoneurons, which is visualized by the red fluorescence arising from [Ru(dipy)3]2+ complexes encapsulated into a silica matrix. The internalized nanoparticles are predominantly located in the cell cytoplasm as revealed by confocal microscopy imaging. The magnetic function of the nanoparticles resulted from the incorporation of 17 nm sized superparamagnetic iron oxide cores into the silica matrix, enabling their responsivity to magnetic fields. Fluorescence analysis revealed the "on-off" switching of Ca2+ influx under the application and further removal of the permanent magnetic field. This result for the first time highlights the movement of the nanoparticles within the cell cytoplasm in the permanent magnetic field as a promising tool to enhance the neuronal activity of motoneurons.

Herein, we report unusual magnetic behavior in arc melted bulk stoichiometric Fe

The magnetostructural transition (MST) can be tuned close to room temperature for an isostructurally alloyed (MnNiGe)1−x(Fe2Ge)x (x = 0.1) compound by partially substituting a small amount of Si for Ge (7 at. %). In this study, the effect of hydrostatic pressure (p) on MST is investigated. In comparison to purely magnetically induced phase transition, pressure initiates structural transition more abruptly, which results in an increase in the isothermal entropy change by a factor of 2 from −Δs = 25.6 (p = 0) to 45.6 J/kg K (p = 190 MPa) for a magnetic field change of 2 T. Since the direct assessment of the adiabatic temperature change, ΔTad, is difficult due to the large volume change and subsequent structural breakdown at MST, an indirect method has been employed to estimate ΔTad.

Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ0ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔTad, and to determine the effective temperature change at the surrounding steel jacket, ΔTeff, during the field pulse. An inverse thermal hysteresis is observed for ΔTad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to shed light on their impact on ΔTeff at the outside of the jacket in comparison to ΔTad provided by the core.

The coupling between magnetic ordering and electric polarization in magnetoelectrics is technically relevant and not yet completely understood. The elucidation of this requires a comprehension of underlying magnetic interactions and the electronic structure. Here, the authors investigate the magnetoelectric honeycomb compound Mn${}_{4}$Ta${}_{2}$O${}_{9}$, by means of neutron scattering and electronic structure calculations, to reveal the ingredients that determine the ground state. For this compound, the ground state is determined by an electron correlation assisted magnetic dipole-dipole interaction. The investigation here offers detailed insight into the factors that determine the ground-state manifold and the role of electron correlation on the strength of anisotropy.

Magnetocaloric composites made from La-Fe-Co-Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite-element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La-Fe-Co-Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented.

We report the processing, analysis and testing of magnetocaloric composite materials consisting of La–Fe–Co–Si particles of various size fractions and a polymer matrix. All of the composites have working temperatures close to room temperature. The composites were pressed into thin plates, a geometry favorable for testing the composites in an active magnetic regenerator (AMR). In order to investigate the influence of particle size and binder type (epoxy), eight different epoxy-bonded La–Fe–Co–Si plates were made and analyzed. We found that the higher filling factor that can be achieved by using a mixture of several particle size fractions has beneficial influence on the thermal conductivity. Tests in the AMR revealed that a maximum temperature span of approximately ΔT=10 K under magnetic field change of μ0H=1.15 T can be obtained at no cooling load conditions. The stability of the measured ΔT values and the mechanical integrity of sample after cyclic application of a magnetic field have been monitored for 90,000 cycles and showed no significant changes. We therefore conclude that epoxy-bonded La–Fe–Co–Si magnetocaloric composites have good magnetocaloric properties at low material-processing costs and hence represent a competitive way to produce magnetocaloric materials to be used in AMR.

In-situ synchrotron XRD measurements of the magnetocaloric material LaFe11.8Si1.2 are used to understand virgin effects and asymmetry of the underlying first order magnetovolume transition. A remarkable change of the transition kinetics occurs after the first cycle, which we attribute to the formation of cracks originating from the volume change. Tomographic imaging revealed that the bulk material disintegrates via an interlocked state where fragments are loosely connected. Though cracks have opened between the fragments, the transition is sharp, which we attribute to magnetostatic interactions. In the cycled sample we find a strong asymmetry between the transition interval upon heating and cooling, which we explain by isostatic pressure acting on parts of the sample during the cooling transition. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

We report a detailed single-crystal and powder neutron diffraction study of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)${}_{\text{M}}$ reflection due to ferrimagnetic ordering, which sets in below ${T}_{\text{N}}=48.6$ and 41 K for ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$, respectively. An additional low intensity magnetic reflection (200)${}_{\text{M}}$ was noticed in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we noticed a significant broadening of the magnetic (111)${}_{\text{M}}$ reflection, which possibly results from the disordered character of the Ti and Co atoms on the $B$ site. Practically, the same peak broadening was found for the neutron powder data of ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$. On the other hand, from our single-crystal neutron diffraction data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Co}}^{3+}$, the $2{t}_{g}$ levels split into a lower ${d}_{xy}$ level yielding a higher twofold degenerate ${d}_{xz}/{d}_{yz}$ level. As a consequence, one can expect a tetragonal distortion in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ with $c/al1$, which we could not significantly detect in the present work.

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping ([Formula: see text]) on the electronic structure and magnetic properties of the spinel [Formula: see text]O

The exchange bias effect is commonly used to shift the coercive field of a ferromagnet. This technique is crucial for the use of magnetic tunnel junctions as logic or memory devices. Therefore, an independent switching of the two ferromagnetic electrodes is necessary to guarantee a reliable readout. Here, we demonstrate that the intrinsic exchange bias effect of Ni-Mn-Sn can be used to apply a unidirectional anisotropy to magnetic tunnel junctions. For this, we use epitaxial Ni-Mn-Sn films as pinning layers for microfabricated CoFeB/MgO/CoFeB magnetic tunnel junctions. We compare the exchange bias field ($H_{\text{EB}}$) measured after field cooling in $-10$\,kOe external field by magnetization measurements with $H_{\text{EB}}$ obtained from tunnel magnetoresistance measurements. Consistent for both methods we find an exchange bias of about $H_{\text{EB}}=130$\,Oe at 10\,K, which decreases with increasing temperature and vanishes above 70\,K., 5 pages, 4 figures

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

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni$_{41}$Co$_{10.4}$Mn$_{34.8}$Al$_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5\,J\,kg$^{-1}$K$^{-1}$ for $\mu_0 \Delta H=5\,\text{T}$., Comment: 8 pages, 8 figures

Thermal management is one of the crucial issues in the development of magnetocaloric refrigeration technology for application. In order to ensure optimal exploitation of the materials “primary” properties, such as entropy change and temperature lift, thermal properties (and other “secondary” properties) play an important role. In magnetocaloric composites, which show an increased cycling stability in comparison to their bulk counterparts, thermal properties are strongly determined by the geometric arrangement of the corresponding components. In the first part of this paper, the inner structure of a polymer-bonded La(Fe, Co, Si)13-composite was studied by X-ray computed tomography. Based on this 3D data, a numerical study along all three spatial directions revealed anisotropic thermal conductivity of the composite: Due to the preparation process, the long-axis of the magnetocaloric particles is aligned along the xy plane which is why the in-plane thermal conductivity is larger than the thermal conductivity along the z-axis. Further, the study is expanded to a second aspect devoted to the influence of particle distribution and alignment within the polymer matrix. Based on an equivalent ellipsoids model to describe the inner structure of the composite, numerical simulation of the thermal conductivity in different particle arrangements and orientation distributions were performed. This paper evaluates the possibilities of microstructural design for inducing and adjusting anisotropic thermal conductivity in magnetocaloric composites.