Antiferromagnetic-ferrimagnetic Transition Research Articles

Narrow transitional hysteresis in (Mn–Cr–Co)2Sb pnictides for room-temperature magnetic refrigeration

Mn1.96−xCrxCo0.04Sb with x = 0.05 and 0.09 has been investigated for its magnetocaloric properties using structural, magnetization and calorimetric methods. These compounds exhibit an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.09. Magnetocaloric properties are presented as entropy-change and direct temperature-change. We show that these compounds have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We find a nearly hysteresis-free transition with a 2.6 K temperature-change around 287 K making it attractive for magnetic-cooling.

The reversibility of the inverse magnetocaloric effect in Mn2−xCrxSb0.95Ga0.05

Mn2−xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.13, but an additional ferromagnetic MnSb impurity phase is usually inevitable at this concentration. To suppress the occurrence of this phase unfavorable for the magnetocaloric effect, we partially replace Sb by Ga. We show that Mn2−xCrxSb0.95Ga0.05 alloys have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We present here results on structural, magnetic, and magnetocaloric properties using isothermal magnetization and direct adiabatic temperature-change measurements. We find in particular for the sample with x = 0.13 a nearly hysteresis-free transition with a 2 K temperature-change around room temperature making it attractive for magnetic cooling technology.

The structural, magnetic, and magnetocaloric properties of In-doped Mn2−xCrxSb

Mn2−xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be changed by controlling the Cr concentration. Increasing the Cr content from x = 0.05 to 0.13 raises the transition temperature from about 180 K up to around room temperature. To suppress the inevitable ferromagnetic MnSb impurity phase, we partially replace Sb by In. Mn2−xCrxSb1−yIny alloys have an antiferromagnetic-ferrimagnetic transition and a narrow transition hysteresis and exhibit the inverse magnetocaloric effect. We examine the magnetocaloric effect from the magnetization and direct adiabatic temperature-change measurements.

Magnetic and electronic structure calculations of antiferromagnetic Mn 2As

Magnetic and electronic structure calculations are performed for Mn 2As with antiferromagnetic (AFM), ferromagnetic (FM), and ferrimagnetic (FIM) spin ordering, using the full-potential linearized augmented plane-wave (FLAPW) method based on the generalized gradient approximation (GGA). It is shown that AFM is the magnetic ground state of Mn 2As, which is in agreement with the experimental observations. At a low temperature (0 K), AFM–FIM transition is also predicted which is consistent with the previous predictions. The ground state stability of the magnetic structure of Mn 2As is attributed to the nearest Mn (I) and Mn (II) antiferromagnetic interaction. The calculated magnetic moment of Mn (II) 3.44 μ B is found to be in good agreement with the neutron diffraction experiment 3.50 μ B while there is a disagreement for the magnetic moment of Mn (I). The different magnetic moments are reflected in the electronic structures of Mn 2As and the exchange splitting between Mn atoms is shown to be an intra-atomic effect.

Anomalous electrical transport and magneto-transport properties at room temperature in Fe0.8Mn1.5As

For the Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic (AF) phase to a ferrimagnetic (FI) phase above Ts = 285 K, stabilizing the latter phase with the larger unit-cell volume. On cooling, the resistivity curve ρ(T) of Fe0.8Mn1.5As shows an upturn near the temperature-induced AF–FI transition temperature Ts. With an increase in the number of warming and cooling cycles, the resistivity ρ(T) shifts to higher resistivity values, and this anomalous shift is ascribed to a pronounced reduction of the mean-free path of electron scattering. At/near room temperature, the resistivity first decreases with increasing external magnetic field until a critical field, and then after a field-independent stage it increases abruptly with a further increase in the field. In the subsequent field-decreasing process, the resistivity increases almost linearly, resulting in a large hysteresis in resistivity of 56.4 µΩ cm at 290 K (or 20.2 µΩ cm at 295 K) at zero field.

Magneto−Structural Correlation in a Series of Bimetallic Alternating Chain Complexes of [CrIIIL(CN)4]n[MnIII(salpn)]n·nSolvents (L = 2,2‘-bipy or 9,10-phen, salpn = Substituted Salicyldehyde, Solvents = Water and Methanol)

Five chain compounds based on the building block of [Cr(L)(CN)4]- (L=2,2'-bipy, 1-4; L=9,10-phen, 5) and [Mn(salpn)]+ (salpn=substituted salicyldehyde-type Schiff base in Scheme 1) have been prepared and characterized structurally and magnetically. The four compounds (1-4) consisting of [Cr(bipy)(CN)4]- units possess straight bimetallic chains as the [Cr(bpy)(CN)4]- unit links the two neighbor [Mn(salpn)]+ units with the two trans-cyanide ligands, while in 5 the chain is zigzag because the [Cr(phen)(CN)4]- unit connects the [Mn(salpn)]+ units with its two cis-cyanide ligands. The bond angles of Mn-N-C-Cr are adjusted by different coligands of salpn and bipy/phen. The chains are stacking via mainly the aromatic pi-pi-type interactions. All compounds show 3D antiferromagnetic ordering with Néel temperatures ranging from 3.7 to 8.1 K, and they are metamagnets displaying antiferromagnetic to ferrimagnetic transition at critical fields of 4.0-13.1 kOe at 1.9 K. This is due to weak interchain antiferromagnetic interactions between the ferrimagnetic bimetallic chains in the materials. The intrachain couplings (J, in cm(-1)) in the materials, between cyanide-bridged CrIII and MnIII ions, from -1.84 to -5.35 cm(-1), follow a linear relationship (J=-33+0.18alpha) to the Mn-N-C angles (alpha, in deg). In addition, the weak interchain antiferromagnetic interactions and critical fields for antiferromagnetic-ferrimagnetic transition are closely related to some of their structural factors, which were studied very superficially only referring to the separations of nearest chains in each material.

Magnetic Domains in Cr-Modified Mn2Sb during the First-Order Phase Transition

A first-order antiferromagnetic-ferrimagnetic transition in Cr-modified Mn2Sb has been studied using the Bitter pattern technique. The composition Mn1.90Cr0.10Sb, which exhibits the transition at room temperature, has been examined. In the transition temperature range, a change in the magnetic domain pattern has been observed on a c-plane, by varying the temperature. Stripe patterns associated with the initiation of the ferrimagnetic phase appears at various points on a c-plane surface. With increasing temperature these patterns expand into the surrounding matrix with an increase of the domain width. In the case of decreasing temperature, a reversed process of the pattern variation has been observed. On reheating, a similar stripe pattern nucleates at the same point. This result is related to the inhomogeneous distribution of Cr content in the sample matrix.