Unveiling the correlation between structural and magnetic ordering in nano Co1-xNixTeO4.

Nanomaterials with unique structures and exotic magnetic phenomena are always intriguing; however, the direct correlation of structural and magnetic ordering up to a few nanometers remains critical. We report structural and magnetic properties of sol-gel grown Co1-xNixTeO4 (x = 0, 0.5 and 1) nanoparticles. An increase in the calcination temperature leads to the enhancement of the particle size and structural ordering. This is accompanied by changes in the magnetic interactions as well. Calcination at lower temperatures retains the short-range non-crystalline structure and superparamagnetic behavior, while calcination at higher temperatures results in long-range ordering in both the crystal and magnetic structures. Superparamagnetic to antiferromagnetic ordering observed from temperature- and field-dependent magnetization is attributed to the changes in structural ordering. This study presents a new family of nanomaterials displaying stable magnetic order up to ∼6 nm, where the magnetic properties can be uniquely controlled by changing the structural ordering.

Nearly compensated ferrimagnetic behaviour and giant exchange bias of hexagonal Mn2PtAl: experimental and theoretical studies.

We have investigated the Mn2PtAl Heulser alloy to unravel its structural, magnetic, calorimetric and electronic structure properties. At room temperature, the alloy crystallizes in a hexagonal structure. Magnetization reveals a weak martensitic transition at 307 K, followed by a long range ferrimagnetic transition at 90 K. Griffiths phase-like signature and positive Weiss temperature in dc-magnetization, isothermal magnetic hysteresis loops and a frequency-independent peak confirm a nearly compensated ferrimagnetic order of Mn2PtAl. The theoretical electronic structure calculations also reveal the ferrimagnetic ground state of Mn2PtAl and Mn ions (occupying different sites) with a very small total magnetic moment. A giant exchange bias field of 2.73 kOe, at a temperature of 3 K and a cooling field of 70 kOe, has been estimated and is attributed to the unidirectional anisotropy associated with possible ferromagnetic clusters formed by the field cooling process in the ferrimagnetic matrix.

Magnetic behavior of Ru substituted skyrmion metal MnSi.

We report on the structural and magnetic properties of Ru substituted skyrmion metal MnSi i.e. Mn1-xRuxSi for the nominal compositions of0⩽x⩽0.5. The composition-temperature (x-T) phase diagram illustrates the substitution-driven changes in the magnetic behavior. It is confirmed that the magnetic ordering temperature (para-to helimagnetic)Ttrand the effective magneticµeffmoment decrease with increasingx. This indicates the suppression of magnetic order by the substitution of Ru in MnSi. However, the magnetic nature is sustained up to a concentration of aboutx= 0.1 above which the system exhibits spin-glass like nature as inferred from the negative Curie-Weiss temperatureθCW, reduced magnetic moment (of the order 10-2 µBf.u.-1) and linearM-H(at 2 K) inx= 0.5. Mn1-xRuxSi is found to avoid the quantum phase transition and exhibits a composition-driven magnetic to spin-glass like transition.

Experimental and Theoretical Investigations of Fe-Doped Hexagonal MnNiGe.

We report a comprehensive investigation of MnNi0.7Fe0.3Ge Heusler alloy to explore its magnetic, caloric, and electrical transport properties. The alloy undergoes a ferromagnetic transition across T C ∼ 212 K and a weak-antiferromagnetic transition across T t ∼ 180 K followed by a spin-glass transition below T f ∼ 51.85 K. A second-order phase transition across T C with mixed short and long-range magnetic interactions is confirmed through the critical exponent study and universal scaling of magnetic entropy and magnetoresistance. A weak first-order phase transition is evident across T t from magnetization and specific heat data. The frequency dependent cusp in χAC(T) along with the absence of a clear magnetic transition in specific heat C(T) and resistivity ρ(T) establish the spin glass behavior below T f. Mixed ferromagnetic and antiferromagnetic interactions with dominant ferromagnetic coupling, as revealed by density functional calculations, are experimentally evident from the large positive Weiss temperature, magnetic saturation, and negative magnetic-entropy and magnetoresistance.

Non-collinear antiferromagnetism to compensated ferrimagnetism in Ti(Fe1-xCox)2 (x = 0, 0.5 and 1) alloys: experiment and theory.

The manifestation of the structural and magnetic properties of Co substituted TiFe2 is investigated using powder X-ray diffraction, magnetization and density functional theory calculations. The alloys TiFe2 and TiFeCo crystallize in the hexagonal structure (P63/mmc) with a reduction in the lattice parameters of TiFeCo (by about 0.51% in a and 0.64% in c) when compared to TiFe2. On the other hand, TiCo2 crystallizes in the cubic structure (Fd3[combining macron]m). A structural transition from hexagonal to cubic is anticipated for a composition with x ∈ [0.5, 1]. The non-collinear antiferromagnetic (AFM) spin structure (formed by 6h Fe atoms) of TiFe2 with Néel temperature TN ∼ 275 K is reported at zero magnetic field H. Meanwhile, a magnetic field-induced collinear antiferromagnetic spin structure is suggested by magnetization measurements and supported by density functional theory calculations. The magnetization of TiFeCo shows a weak-ferromagnetic (FM)-like transition around 204 K, followed by a broad hump at 85.5 K and H = 200 Oe. Ferromagnetic interactions are weakened, causing the hump to disappear due to the possible transfer of electrons between Fe and Co. TiCo2 shows compensated ferrimagnetism with magnetization of the order of 10-5µB f.u.-1 and a linear increase of M with H at 5 K. The presence of a non-collinear AFM spin structure in TiFe2, a reduced magnetic moment in TiFeCo due to the charge transfer between Co and Fe, and compensated ferrimagnetism in TiCo2 promise a rich phase diagram of Ti(Fe1-xCox)2 alloys and the possible potential of these alloys for use in spintronics applications.

Anomalous magnetic properties in LaFe11.5Al1.5.

We report the magnetic relaxation, DC magnetization, heat capacity, and X-ray powder diffraction studies of a melt-spun LaFe11.5Al1.5 compound executed across a temperature range of 5 to 300 K. We have found three magnetic transitions (at temperatures T1, T2, and Tord) in this compound in the zero-field cooled (ZFC) mode, and two magnetic transitions (at T2 and Tord) in the field-cooled cooling/warming (FCC/FCW) mode. The ferromagnetic transition (FM) at temperature T2 indicating hysteresis alludes to a magnetic transition of the first-order (FOT) at lower temperatures. The magnetization study reveals that meta-stable states exist in the low-temperature antiferromagnetically ordered state. Partially reversible behaviour is also observed in the 120-140 K temperature range. The heat capacity data indicates that the magnetic state of this compound is clearly different from that of spin glasses. The magnetocaloric properties of the compound are determined in the form of the isothermal magnetic entropy change (SM) and the adiabatic temperature change (ΔTad) and the utmost values of SM and Tad for a field variation of 25 kOe observed were 3.1 J kg-1 K-1 and 1.12 K respectively. The relative cooling power (RCP) is ascertained to be 216 J kg-1 for an enforced field of 25 kOe.

Magnetism of 3d and 4d doped Mn0.7T0.3NiGe (T = Fe, Co, Ru and Rh): bulk magnetization and ab initio calculations.

We compare the magnetic properties of 3d (Fe and Co) and 4d (Ru and Rh) transition metals doped MnNiGe using the combined results of magnetization and ab initio calculations. The alloys crystallize in austenite Ni2In-type hexagonal phase (space group: P63/mmc) with insignificant difference in the lattice parameters. Mn0.7Fe0.3NiGe and Mn0.7Co0.3NiGe exhibit spin-glass behavior, resulting from the competing ferro- and antiferromagnetic interactions. These alloys exhibit spontaneous exchange bias field of about [Formula: see text] Oe and 323 Oe, respectively. From the 4d-metal doped alloys, Mn0.7Ru0.3NiGe shows glassy behavior while long-range ferromagnetic order is confirmed in Mn0.7Rh0.3NiGe. In Mn0.7Rh0.3NiGe, in agreement with experiment and the theoretical calculations, the ground state is confirmed to be ferromagnetic because of the FM exchange interactions of the Mn magnetic moments. But in Mn1-x (Fe,Co,Ru) x NiGe alloys the calculations revealed the competing and comparable FM and AFM exchange interaction parameters, resulting in the formation of spin-glassy characteristics.

Revelation of spin glass behavior in Ru doped MnNiGe: experiment and theory.

We report on the nature of the magnetism in Ru substituted MnNiGe using the combined results of x-ray diffraction, dc-magnetization, ac-susceptibility and ab initio calculations. Mn0.7Ru0.3NiGe crystallizes in Ni2In-type hexagonal structure (P63/mmc) at room temperature with lattice parameters a = b = 4.099 [Formula: see text] and c = 5.367 [Formula: see text]. From the dc-magnetization; a broad peak around 46.55 K, separation between zero-field cooled and field-cooled warming state and non-saturating isothermal magnetization with typical S-type hysteresis indicate glassy behavior. A cusp in [Formula: see text] is observed to shift toward high temperatures with increasing frequency. Mydosh parameter ([Formula: see text]), single-relaxation time ([Formula: see text] s) obtained through critical slowing-down analysis, [Formula: see text] from the Vogel-Fulcher law and Tholence criterion [Formula: see text], confirm that Mn0.7Ru0.3NiGe belongs to the short-range interaction spin-glass systems with strong coupling between the magnetic clusters. LSDA+U calculations confirmed the competing exchange interactions between large magnetic moments of the Mn ions in Mn0.7Ru0.3NiGe compound resulting in the formation of spin-glassy characteristics.

Magnetization, resistivity, specific heat and ab initio calculations of Gd5Sb3.

We report on the combined results of the structural, magnetic, transport and calorimetric properties of Mn5Si3-type hexagonal Gd5Sb3, together with ab initio calculations. It exhibits a ferromagnetic (FM)-like transition at 265 K, antiferromagnetic (AFM) Néel transition at 95.5 K followed by a spin-orientation transition at 62 K. The system is found to be in AFM state down to 2 K in a field of 70 kOe. The FM-AFM phase coexistence is not noticeable despite large positive Curie-Weiss temperature ([Formula: see text] K). Instead, low-temperature AFM and high-temperature FM-like phases are separated in large temperatures. Temperature-magnetic field (H-T) phase diagram reveals field-driven complex magnetic phases. Within the AFM phase, the system is observed to undergo field-driven spin-orientation transitions. Field-induced tricritical and quantum critical points appear to be absent due to the strong AFM nature and by the intervention of FM-like state between paramagnetic and AFM states, respectively. The metallic behavior of the compound is inferred from resistivity along with large Sommerfeld parameter. However, no sign of strong electron-correlations is reasoned from the Kadowaki-Wood's ratio [Formula: see text] [Formula: see text] cm · (mol · K)2(mJ)-2, despite heavy γ. Essentially, ab initio calculations accounting for electronic correlations confirm AFM nature of low-temperature magnetic state in Gd5Sb3 and attainable FM ordering in agreement with experimental data.