Field-cooled Protocols Research Articles

Trapped flux in pure and Mn-substituted CaKFe4As4 and MgB2 superconducting single crystals

Measurements of temperature dependent magnetization associated with trapped magnetic flux in single crystals of CaKFe4As4, CaK(Fe0.983Mn0.017)4As4 and MgB2 using zero-field-cooled and field-cooled protocols are presented. The results allow for the determination of the values of superconducting transition temperature, lower critical field and self-field critical current density. These are compared with the literature data. Possible experimental concerns are briefly outlined. Our results, on these known superconductors at ambient pressure, are qualitatively similar to those recently measured on superhydrides at megabar pressures (Minkov et al 2023 Nat. Phys. https://doi.org/10.1038/s41567-023-02089-1) and, as such, hopefully serve as a baseline for the interpretation of high pressure, trapped flux measurements.

Large exchange bias in Mn-Ni-Sn Heusler alloys: Role of cluster spin glass state

We report a combined experimental and theoretical study of the exchange bias (EB), structural and magnetic properties of manganese-rich ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{41.5+x}{\mathrm{Sn}}_{8.5\ensuremath{-}x}$ $(x\phantom{\rule{4pt}{0ex}}=0, 0.75, \text{and} 1)$ Heusler alloys. All these alloys crystallize in a tetragonal structure at room temperature. Ab initio simulation using density functional theory (DFT) on off-stoichiometric ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{42.1875}{\mathrm{Sn}}_{7.8125}$ alloy (very close to the actual composition) predicts the tetragonal structure with the ferrimagnetic arrangement as the ground state, in good agreement with the experimental results. These alloys are found to exhibit a large exchange bias at low temperatures, with a maximum EB field of 7.1 kOe at 2 K for $x\phantom{\rule{4pt}{0ex}}=1$, which is significantly larger than that reported for any Mn-Ni--based Heusler systems. Frequency dependence of spin freezing temperature obeying the scaling law confirms the presence of a cluster spin glass (CSG)--like state in these alloys. The latter is also confirmed from the memory and aging effect studies in zero-field cooling and field-cooled protocols for these alloys. The large EB is attributed to the strong exchange coupling between the CSG clusters embedded in an otherwise ferrimagnetic matrix. The exchange mechanism behind the large EB effect is discussed in the light of varying bond lengths between different magnetic pairs, and hence the magnetic coupling, due to the large tetragonal distortion. The present experimental findings are found to be in good agreement with the DFT results.

Magnetization of Magnetically Inhomogeneous Sr2FeMoO6-δ Nanoparticles

In this work, we describe the magnetization of nanosized SFMO particles with a narrow size distribution around ca. 70 nm fabricated by the citrate-gel technique. The single-phase composition and superstructure ordering degree were proved by X-ray diffraction, the superparamagnetic behavior by magnetization measurements using zero-field cooled and field-cooled protocols, as well as by electron magnetic resonance. Different contributions to the magnetic anisotropy constant and the temperature dependence of the magnetocrystalline anisotropy are discussed.

Giant exchange bias effect in all-3d-metal Ni38.8Co2.9Mn37.9Ti20.4 thin film

In this work, a giant exchange bias field of over 1 kOe, which is uncommon in film systems, is obtained in all-3d-metal Heusler-type Ni38.8Co2.9Mn37.9Ti20.4 thin films. The thermomagnetic curves show an unambiguous noncoincidence between zero-field-cooled and field-cooled protocols, indicating the presence of magnetically inhomogeneous phases in the film in the low temperature range. Magnetic measurements, including magnetic relaxation and magnetic memory effects, demonstrate that the exchange bias should be attributed to the exchange interaction between ferromagnetic clusters and the antiferromagnetic host in the Ni38.8Co2.9Mn37.9Ti20.4 thin film at low temperature. The local Mn-Co-Mn configuration provides the ferromagnetic contribution, which is formed in the antiferromagnetic Heusler main phase due to the small amount of Co-substitution for Ni. The achievement of the large exchange bias effect in the Ni38.8Co2.9Mn37.9Ti20.4 thin film is a significant addition to the existing multiple magneto-responsive effects in all-3d-metal Ni-Mn-based Heusler systems.

Cluster spin-glass behavior in Ni2In-type Mn–Cu-Ga alloys

We report the structural and magnetic properties of Ni2In-type (Mn0.75Cu0.25)66Ga34 alloy by using powder X-ray diffraction, DC- and AC-susceptibility, aging effect and magnetic memory effect measurements. From the results of the DC-susceptibility measurements, an irreversible temperature Tirr(0) = 109.1 ± 1.4 K and a crossover exponent Φ = 4.9 ± 0.3 are obtained, which indicate that our system favors a canonical Heisenberg spin-glass state. From the AC-susceptibility results, the shift of freezing temperature with frequency is characterized by a medium Mydosh parameter of 0.03. Furthermore, a relaxation time of the spin flip τ0=1.1×10−9 s and a critical exponent zv=7.5 are obtained by the fitting of power law. A significant aging effect and a magnetic memory effect in both zero-field-cooled and field-cooled protocols are observed. In addition, the memory effect existing in a negative T cycle but missing in a positive T cycle favors the hierarchical model in our sample. All the above observations render a cluster spin-glass behavior in (Mn0.75Cu0.25)66Ga34 alloy.

Insights into the magnetic dead layer in La0.7Sr0.3MnO3 thin films from temperature, magnetic field and thickness dependence of their magnetization

Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La0.7Sr0.3MnO3 (LSMO) are reported. The dc magnetization (M) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature TB in different H. Isothermal hysteresis loops at different T are used to determine the temperature dependence of saturation magnetization (MS), remanence (MR) and coercivity HC. The MS vs. T data are fit to the Bloch law, MS (T) = M0 (1 – BT 3/2), showing a good fit for T < 100 K and yielding the nearest-neighbor exchange constant J/kB ≅ 18 K. The variations of TB vs. H and HC vs. T are well described by the model often used for randomly oriented magnetic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness (D) variation of Curie temperature (TC) and MS of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O2 are analyzed in terms of the finite-size scaling, with MS vs. D data fit to MS (D) = MS(b)(1-d/D) yielding the dead layer thickness d = 1.1 nm, 1.4 nm and 2.4 nm respectively. Brief discussion on the significance of these results is presented.

Kinetic arrest of first-order transition between charge-ordered and ferromagnetic phases in Gd0.5Sr0.5MnO3 single crystals: magnetization relaxation studies

We have studied the span and nature of first-order phase transition (FOPT) between charge-ordered insulating and ferromagnetic metallic phases in oriented single crystals of Gd0.5Sr0.5MnO3. Magnetic field—temperature phase diagram was formulated from magnetization data for different crystallographic axes and non-monotonic variation of supercooling limit was observed at low temperature. A peculiar nature of magnetization was observed as irreversible open hysteresis loops during thermal cycling. We perceive that the nature of metastable states responsible for open hysteresis loops is different from that of supercooled ones. Further, thermal cycling magnetization data reveal that magnetic phases formed at 8 K after zero-field or field-cooled protocols (89 kOe) are not in equilibrium. Relaxation time constant is found to increase below 30 K in magnetization relaxation measurements made across the FOPT. The non-monotonic variation of relaxation time constant is a manifestation of kinetic arrest of the FOPT. We propose that the non-equilibrium, glass-like magnetic phase (at 8 K and 89 kOe) is a consequence of kinetic arrest.

Magnetic Memory Effects in ${\rm Fe}/\gamma\hbox{-}{\rm Fe}_{{2}}{\rm O}_{{\bf 3}}$ Nanostructures

We report an unusual consequence of magnetic memory effect in Fe/γ-Fe2O3 nanostructures, which is prepared using an electrochemical route. Chemical phases of Fe/γ-Fe2O3 nanostructures are characterized by X-ray diffraction, optical, and Mossbauer spectroscopic studies. The static and dynamic aspects of dc magnetization studies are carried out. Thermal variation of zero-field-cooled (ZFC) magnetization displays a signature of blocking temperature (TB) at 215 K. Another broadened peak is also evident in the field-cooled (FC) magnetization at 40 K (TG). Signature of memory effect is observed below TG after cooling the sample both in ZFC and FC protocols. Appearance of memory effect in the ZFC protocol confirms spin-glass-like state below TG. Interestingly, memory effect is absent below TB even in FC protocol, although a large particle size distribution is noticed in the Fe/γ-Fe2O3 nanostructures. The results address the fundamental question whether substantial particle size distribution is sufficient enough for the memory effect in the FC protocol as found in the literatures.

Ageing effects in nanocrystalline Co50Ni50 and Fe50Ni50 alloy: Role of magnetic anisotropy

Nanocrystalline Co50Ni50 (CN8.6) and Fe50Ni50 (FN9.5) alloys embedded in an amorphous silica host are considered for the study with same volume fraction, ϕ=10% and nearly close average particle sizes 8.6 and 9.5nm, respectively. The contrast magnetic properties together with anomalous ageing or memory effect are noticed for these nanocrystalline alloys. Dc magnetization results indicate superparamagnetic-like behaviour for CN8.6 and superspin glass-like behaviour for FN9.5. The ageing effect in field-cooled protocol is apparent below blocking temperature for CN8.6 which is unexpectedly absent for FN9.5. Rather, ageing effect is evident at low temperature below superspin-glass transition temperature for FN9.5. The ageing effect recorded in zero-field cooled protocol for FN9.5 suggests cooperative spin correlation as evident in atomic spin-glass system, which is absent for CN8.6. The contrast magnetic properties are correlated with the magnetic anisotropies of these nanocrystalline alloys.

Non-equilibrium effects in the magnetic behavior of Co 3O 4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co 3O 4 nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co 3O 4 nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.

Magnetic properties of MnAs nanocrystals embedded in GaAs

A phenomenological model for explaining the magnetic properties of MnAs nanocrystals embedded in GaAs is proposed. It is shown that experimental data of DC magnetization as a function of temperature, obtained according to zero-field-cooled and field-cooled protocols, can be understood assuming a transition of the system from a low temperature state in which very slow dynamics is observed (frozen state) to a high-temperature state in which dynamics is fast (quasi superparamagnetic state).

MnAs Nanocrystals Embedded in GaAs

Magnetic properties of MnAs nanocrystals embedded in GaAs are analyzed in the frame of phenomenological model proposed by Sasaki for ferritin superparamagnets. Our calculations explain qualitatively experimental data of magnetization versus temperature; obtained according to zero-field-cooled and field-cooled protocols. They show dynamics of magnetization of MnAs nanocrystals in range of temperature from 10 K to 320 K. There is transition from state in which very slow dynamics is observed (frozen state) to state in which dynamics is fast (quasi-superparamagnetic state).