Ferromagnetic resonance of magnetic field oriented Fe3O4 nanoparticles in frozen ferrofluids

Measurements of the orientational dependence of the ferromagnetic resonance (FMR) spectra are made on Fe₃O₄ nanoparticles in ferrofluids solidified in dc magnetic fields. The in field solidification locks the direction of magnetization parallel to the direction of the cooling field enabling measurements as a function of orientation with respect to the direction of magnetization in the frozen state. The g value of the FMR spectra at 77 K is 2.16 and the anisotropy constant is −1.23 J/m³. A marked reduction of the difference between the field position in the parallel and perpendicular orientation onsets on warming to 140 K well below the melting temperature of the fluid carrier and is attributed to the onset of fluctuations in the direction of the magnetization in the solid phase. The phase transition of the magnetic symmetry observed in bulk Fe₃O₄ occurs at much lower temperature in the nanoparticles.     

Magnetic and magneto-transport investigations of Co37Ni34Al29 alloy

Ferromagnetic shape memory alloy with nominal composition Co₃₇Ni₃₄Al₂₉ is investigated by transport and magnetic measurements. The anomaly due to the martensitic transition is observed around 130-210 K. The thermal hysteresis, observed due to martensitic transition in the dc magnetization versus temperature data, gets suppressed at higher applied field. Below 50 K, magnetization varies with temperature perfectly as T^3/2, which signifies that spin wave excitations are largely responsible for thermal demagnetization. The sample shows small negative magneto-resistance, which varies non-monotonically with temperature showing largest value at around 200 K.     

Magnetic dynamics of charge ordered Nd0.80Na0.20MnO3 compound

The magnetic dynamics of charge ordered Nd_0.8Na_0.2MnO₃ compound was studied by measuring the temperature variation of magnetization for different magnetic fields up to 7 T and, the field variation of magnetization at different temperatures down to 5 K. This sample exhibits a charge-ordering transition at 180 K, followed by a weak ferromagnetic (FM) transition at around 100 K and a spin glass like transition below 40 K. Suppression of charge-ordering and spin glass like transition and increase in FM T_C were observed with an increase in magnetic field. A reversible metamagnetic transition above a threshold field (H_f) of 4.5 T was observed at 130 K, followed by a saturation magnetization of 3.2 μ_B/f.u. However at 5 K, an irreversible field induced first order phase transition from charge ordered state to FM state was observed at H_f=5 T. For comparison, the temperature and field variations of magnetization were studied on a FM compound from the same series with the composition Nd_0.90Na_0.10MnO₃. A clear FM transition with a T_C of 113 K and a saturation magnetization of 4.3 μ_B/f.u was observed.     

Spin glass like behavior in Cd0.42Mn0.58In2S4

The magnetic behavior of the diluted magnetic semiconductor Cd_0.42Mn_0.58In₂S₄ has been study by dc magnetization and ac susceptibility experiments. Zero field cooled and field cooled measurements reveal irreversibility below T_irr=2.60±0.15 K. Ac susceptibility data, performed as a function of the temperature and the frequency, confirm the spin-glass like behavior of the material with T_f=2.75±0.15 K. High temperature susceptibility data follow a typical Curie-Weiss law with θ=−74±1 K which suggests predominant antiferromagnetic interactions. The randomness of the magnetic ions, necessary to explain the magnetic behavior of the material, has been determined by X-ray powder diffraction experiments.     

Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Ferromagnetism and spin reorientation in Sm12Fe14Al5

The single crystal of the new ternary compound Sm₁₂Fe₁₄Al₅ was grown and its crystallographic and magnetic properties were investigated. Sm₁₂Fe₁₄Al₅ has a hexagonal structure of the space group p-3m1 and shows ferromagnetism with a Curie temperature of 245 K. The easy direction of magnetization is parallel to the c-axis at temperatures between 245 and 85 K; however, it changes to the c-plane below 85 K through a first-order-like phase transition. No saturation is observed in the magnetization curve even under the applied field of 55 kOe at 5 K. Sm₁₂Fe₁₄Al₅ seems to have a large coercive field at very low temperatures. The anisotropy field was estimated at 5 and 120 K and the saturation magnetization of low temperature phase is explained assuming a ferromagnetic coupling between Fe and Sm sublattices.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Probing the magnetic state by linear and non-linear ac magnetic susceptibility measurements in under-doped manganite Nd0.8Sr0.2MnO3

We have thoroughly investigated the entire magnetic states of under-doped ferromagnetic-insulating manganite Nd_0.8Sr_0.2MnO₃ through temperature-dependent linear and non-linear complex ac magnetic susceptibility measurements. This ferromagnetic-insulating manganite is found to have frequency-independent ferromagnetic to paramagnetic transition temperature at around 140 K. At around 90 K (≈T_?) the sample shows a second frequency-dependent re-entrant magnetic transition as explored through complex ac susceptibility measurements. Non-linear ac susceptibility measurements (higher harmonics of ac susceptibility) have also been performed (with and without the superposition of a dc magnetic field) to further investigate the origin of this frequency dependence (dynamic behavior at this re-entrant magnetic transition). Divergence of 3rd harmonic of ac susceptibility in the limit of zero exciting field indicates a spin-glass-like freezing phenomena. However, large value of spin-relaxation time (τ₀=10⁻⁸ s) and small value of coercivity (∼22 Oe) obtained at low temperature (below T_?) from critical slowing down model and dc magnetic measurements, respectively, are in contrast with what generally observed in a canonical spin glass (τ₀=10⁻¹²-10⁻¹⁴ s and very large value of coercivity below freezing temperature). We have attributed our observation to the formation of finite size ferromagnetic clusters which are formed as consequence of intrinsic phase separation and undergo cluster glass-like freezing below certain temperature in this under-doped manganite. The results are supported by the electronic- and magneto-transport data.     

Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

Metastable states in phase separated electron doped Sm0.15Ca0.85MnO3

In order to study the mechanism behind the phase separation scenario in the Sm_0.15Ca_0.85MnO₃ compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K<T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM (P2₁/m) phase resulting in field-induced insulator-metal transition, which is irreversible below T₁=75 K. In zero field the content of a G-type phase in the mixed C-G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T₁ depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.     

Electrical and magnetic properties of ε-Fe3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe₃N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe₃O₄ phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe₃N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ(T) for compacted ε-Fe₃N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.     

Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

On the ferromagnetism of AlFe2B2

Based on extensive Mossbauer effect (ME) and magnetization measurements, the orthorhombic AlFe₂B₂ was characterized as a ferromagnet (FM) because this character is evident as an onset of a FM transition at T_c=320 K and characteristic magnetizations isotherms below T_c. At liquid helium temperatures, the magnetization saturates to μ_sat≈1μ_B per Fe atom; a value which is half the one reported for the iron metal indicating a relatively more filled 3d band. The ME analysis revealed a hyperfine field H(0) of 88(2) kOe, an isomer shift (relative to Fe) of 0.50(2) mm/s, and a quadrupole parameter of 0.02 mm/s: all parameters extrapolated to zero Kelvin. The itinerant character of the magnetic moment will be discussed.     

Magnetic properties of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix

Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO₂ matrix with manganese concentration 0.7 and 3 at% have been synthesized by a sol-gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10-20 nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10 nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO₂. At temperatures below T_C=43 K strong ferrimagnetism originating from the minor Mn₃O₄ phase masks the relatively weak magnetism of λ-MnO₂ with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40 K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3 at% Mn suggest that the magnetic moments of the smaller Mn₃O₄ nanoparticles with dimensions below 10 nm are exposed to thermally activated blocking process just below the Curie temperature T_C. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10 K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO₂ crystal structure.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Field-induced structural transition and the related magnetic entropy change in Ni43Mn43Co3Sn11 alloy

Magnetic properties and magnetic entropy change ΔS were investigated in Heusler alloy Ni₄₃Mn₄₃Co₃Sn₁₁. With decreasing temperature this alloy undergoes a martensitic structural transition at T_M=188 K. The incorporation of Co atoms enhances ferromagnetic exchange for parent phases. Austenitic phase with cubic structure shows strong ferromagnetic behaviors with Curie temperature T_C^A at 346 K, while martensitic phase shows weak ferromagnetic properties. An external magnetic field can shift T_M to a lower temperature at a rate of 4.4 K/T, and a field-induced structural transition from martensitic to austenitic state takes place at temperatures near but below T_M. As a result, a great magnetic entropy change with positive sign appears. The size of ΔS reaches 33 J/kg K under 5 T magnetic field. More important is that the ΔS displays a table-like peak under 5 T, which is favorable for Ericsson-type refrigerators.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Magnetoelectric behavior in the complex CaMn7O12 perovskite

We report a magnetoelectric effect in the double perovskite CaMn₇O₁₂, that shows a complex magnetic behavior below 90 K with two magnetic phases coexisting (one ferrimagnetic and the other modulated). A second magnetic transition, associated with changes in the magnetic modulation and magnetic ordering coherence lengths of the two magnetic phases occurs at 50 K (T_N2). A detailed structural characterization of this compound, that we have carried out by means of high-resolution X-ray powder diffraction, reveals an anisotropic thermal expansion of its lattice parameters at 50 K (T_N2). In addition, our study of the complex permittivity of this sample as a function of temperature, frequency and magnetic field shows very interesting results below 90 K and specially below 50 K: the dielectric constant ε′_r that was decreasing continuously on cooling experiences an upturn, and even more, on application of a magnetic field it shows a moderate magnetoelectric response. We attribute such dielectric behavior to the formation of electric dipoles by magnetostriction in this charge and spin ordered system, that are sensible to the presence of an external magnetic field.