Structure and hard magnetic properties of TbCu7-type SmFe8.95−xGa0.26Nbx nitrides

Nanocrystalline SmFe_(8.95-x)Ga_(0.26)Nb_xN_δ(x=0, 0.1, 0.2, 0.3) were prepared using rapid-quenching,annealing and nitriding. The magnetic properties and crystal structures were systematically studied under various wheel velocities to investigate the influence of Nb doping for the compounds. It is found that TbCu7-type structure is able to be obtained even though the wheel velocity is reduced to 20 m/s(x = 0.3). An significant increase(△T_c=70 ℃) of the Curie temperature is obtained with Nb doping at x = 0.1 due to the lattice expansion revealed by Rietveld analysis. The optimum coercivity with the value H_(cj) of 810 kA/m is achieved at x = 0.2 in the nitrides, in which a reasonable distribution of grain sizes of both TbCu_7-type SmFe_9 N_δ and α-Fe can be found. However, an excess of Nb doping may lead to the increase of the weight fraction of α-Fe, which in turn deteriorates the magnetic properties.     

Structure and permanent magnetic properties of SmFex （x=3-8） melt spun ribbons during heat treatment

Compounds with the composition SmFex（x=3–8） were prepared by melt spun method at a velocity of 40 m/s and subsequent annealing at temperature between 600–1000 ℃. The crystal structures of the as-quenched and as annealed powders were investigated by XRD methods with following Rietveld analysis. The glass forming ability could be enhanced by the increase of Sm content to x≤5.Metastable Sm5Fe17-type structure existed when 3≤x≤5 and temperature was lower than 800 ℃. SmFe2-type structure could be stable up to 1000 ℃ when x〉3 and temperature was under 800 ℃. The content of SmFe2-type decreased with the increase of x value and increased with temperature lower than 800 ℃, from which SmFe2-type started to bring the transition to SmFe3-type. As for Sm5Fe17-type compounds with x=3.4, the highest coercivity of 33.6 kOe could be obtained under a velocity of 30 m/s and heat treated under 700 ℃×1h.     

Phase and microstructure of TbCuT-type SmFe melt-spun powders

The SmxZr0.3Fe9.1-xCo0.6(x=0.8,0.9,1.0) powders were prepared by melt-spun method with different quenching velocities.The phase and microstructure were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The Th2Zn17-type structure of the as-cast state changed to TbCu7-type after quenching to a rotating molybdenum roll under certain velocity,and the formation of TbCu7-type phase was strictly depending on the Sm content and roll speed.The SEM morphology showed that the Fe-rich zone was typically fish-bone structure and TEM diffraction pattern indicated the nano-scale crystal size with TbCu7-structure when x=0.9,and FCC type γ-Fe on the basis of α-Fe formed in the non-equilibrium solidification could be detected by selected area electron diffraction(SAED) indexing in the x=0.8 samples.     

Magnetic properties and magnetic exchange interactions in Gd_(1–x)RE_x(RE=Pr,Nd) alloys

The effect of Pr,Nd addition on the magnetic properties and magnetic exchange interaction of gadolinium alloys was systematically studied.Curie temperature T_C and magnetic moment of Gd_(1–x)RE_x(RE=Pr,Nd)systems with x0.05 were investigated.When x0.05,Pr and Nd formed respectively with Gd continuous solid solution which has the crystalline structure HCP.Study on the magnetic behavior indicated that at near room temperature,the simple ferromagnetism prevailed in these two systems of alloy.The Curie temperature and magnetic moment of Gd_(1–x)RE_x alloy decreased with RE(RE= Pr,Nd)content x increasing.The de Gennes factor of Gd_(1–x)RE_x alloy which was associated with the exchange interaction between magnetic spin components also decreased with RE content increasing.The above results showed that the magnetic exchange interaction between magnetic atoms in gadolinium could be effectively changed by the Pr,Nd addition.     

Influence of partial substitution of cerium for lanthanum on magnetocaloric properties of La_(1–x)Ce_xFe_(11.44)Si_(1.56) and their hydrides

The structure and magnetocaloric properties of La1–xCexFe11.44Si1.56 and their hydrides La1–xCexFe11.44Si1.56Hy（x=0, 0.1, 0.2, 0.3, 0.4） were investigated.The samples crystallized mainly in the cubic Na Zn13-type structure with a small amount of α-Fe phase as impurity.The lattice constants and Curie temperature presented the same change tendency with increasing of Ce content.For the hydrides, the influence of Ce content on lattice constants was weakened and the values of H concentration y were approximate to be 1.56.The La1–xCexFe11.44Si1.56 compounds exhibited large values of isothermal entropy change –ΔSm around the Curie temperature TC under a low magnetic field change of 1.5 T.The value of –ΔSm increased and then decreased with increasing Ce content, reached the maximum, 26.07 J/kg·K for x=0.3.TC increased up to the vicinity of room temperature by hydrogen absorption for the Ce substituted compounds, but TC only slightly decreased with increasing Ce content.The first-order metamagnetic transition was still kept in the hydrides and the maximum values of –ΔSm were lower than those of the La1–xCexFe11.44Si1.56 compounds, but still remained large values, about 10.5 J/kg K under a magnetic field change of 1.5 T.The values of –ΔSm were nearly independent of the Ce content and did not increase with increasing x for the hydrides.The La1–xCexFe11.44Si1.56Hy（x=0–0.4） hydrides exhibited large magnetic entropy changes, small hysteresis loss and effective refrigerant capacity covered the room temperature range from 305 to 317 K.These hydrides are very useful for the magnetic refrigeration applications near room temperature under low magnetic field change.     

Magnetic properties and magnetization behavior of SmCo-based magnets with TbCu7-type structure

The nanocrystalline magnets with nominal compositions of Sm1-xLuxCo6.8Zr0.2(x=0,0.2,0.4,0.6)were prepared directly by the intensive milling.The effects of Lu content on the phase structure,the magnetic properties,and magnetization behaviors were also investigated.The XRD patterns of the as-milled samples showed a single SmCo7 phase with TbCu7 structure.Lu addition was proved to result in relevant improvements in the microstructure and magnetic properties,especially in the maximum energy product(BH)max.It was shown that a higher maximum energy product and coercivity of about 17.47 kJ/m3 and 473.45 kA/m were obtained in the sample with x=0.2.From the analysis of the magnetization reversal behavior,it was found that a stronger intergrain exchange coupling interaction was observed in the samples with Lu-doping.From the studies of the coercivity mechanism,it was shown that nucleation model was the dominant magnetization reversal process at the elevated temperature.     

Effect of titanium addition on the magnetic property of Nd2Fe14B/α-Fe nanocomposite alloys

Rapidly solidified nanocrystalline α-Fe/Nd2Fe14B alloys with enhanced coercivity were obtained by melt spinning.The effects of Ti addition on the microstructore and magnetic properties of the nanocomposite α-Fe/Nd2Fe14B alloys were investigated by X-ray diffraction(XRD)and superconducting quantum interference device(SQUID)magnetometer.The analysis of XRD showed that Vα-Fe estimated to be about 35.3% in the Ti-free α-Fe/Nd2Fe14B nanocomposites decreased down to 26.5% as the addition of was 5 at.% Ti.Accordingly,adding Ti resulted in relevant improvements of magnetic properties,especially of the coercivity Hc from 595 kA/m up to 1006 kA/m.The dependence of Mirrev(H)/2Mr on the reverse field H indicated that nucleation was the dominating mechanism for the magnetization reversal in these nanocomposites.The analysis of the temperature dependence of the demagnetization curve in the α-Fe/Nd2Fe14B nanocomposite magnets indicated that a reduction of αex could play a leading role in an increase in the coercivity of Ti-doped sample.     

Magnetocaloric Effect of Alloys Gd（Al1-x Cox ）2

The magnetocaloric effect in alloys Gd（Al1-xCox）2 with x = 0, 0.05, and 0.10 were investigated using X-ray diffraction （XRD） and magnetization measurements. It was found that three alloys crystallized in a single phase with MgCu2-type structure. The lattice parameter and Curie temperature decreased with increasing Co content, whereas the magnetic-entropy change increased. With a magnetic-field change of 2 T, the maximum of the magnetic-entropy change reached 4.6 J·kg^-1·K^-1 near Curie temperature at approximately 95 K in the alloy GdAl1.8Co0.2, which appeared to be an alternative candidate for active magnetic refrigerants working in the temperature range centered at 100 K.     

Effects of magnetic annealing on crystallization and magnetic properties of Nd2Fe14B/α-Fe nanocomposite magnets

Crystallization and magnetic properties of Nd2Fe14B/α-Fe nanocomposite magnets have been investigated by annealing the as-spun ribbons with magnetic field.The crystallization process was accelerated by field annealing.The hysteresis loop became to be fat by magnetic annealing at 645 oC for 4 min,which was 690 oC for ribbons annealing without magnetic field.The relative content of α-Fe phase was increased from the results of XRD.The strength of the magnetic field had no obvious influence on the remanence and coercivity,but modified the squareness of hysteresis loop.     

Magnetic properties and microstructure of Fe_(69.5-x)Nd_7B_(21)Nb_(2.5)Ga_x(x=0-1)alloys

The Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)permanent magnets in the form of rods were prepared by annealing the bulk amorphous alloys.The magnetic properties,phase evolution and microstructure of the alloys were investigated systematically.It is found that the glass forming ability(GFA), microstructure and magnetic properties are sensitive to Ga content for Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)bulk alloys.The annealed alloys are mainly composed of soft α-Fe,hard Nd_2 Fe_(14)B and nonmagnetic Nd_(1.1)Fe_4 B_4 phases.When x = 0.3,the optimally annealed magnets exhibit magnetic properties of the remanence Br = 0.63 T,intrinsic coercivity H_(cj) = 368.68 kA/m and maximum energy product(BH)_(max) = 33.73 kJ/m~3.Furthermore,magnetic field heat treatment at the temperature close to Curie temperature of Nd_2 Fe_(14)B phase was applied to the annealed Fe_(69.2)Nd_7 B_(21)Nb_(2.5)Ga_(0.3) magnet.The results of X-ray diffraction(XRD)and transmission electron microscopy(TEM)indicate that the magnetic field heat treatment can be beneficial for the precipitation of α-Fe.Thus,the B_r,H_(cj) and(BH)_(max) are enhanced by 8.7%,6.3% and 16.3%,respectively.     

Effect of niobium substitution on microstructures and thermal stability of TbCu7-type Sm–Fe–N magnets

This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm_(8.5)Fe_((85.8-x)Co_(4.5)Zr_(1.2)Nb_x(x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu_7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase a-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous micro structure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H_(cj) is achieved in the 1.2 at% substituted samples.The irreversible flux loss for 2 h exposure at 120 ℃ declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution.     

Effect of cobalt substitution on structure and magnetic properties of Nd_(0.4)Zr_(0.6)Fe_(10-x)Co_xSi_2(x =0-3) alloys and their ribbons

This study reports the stabilization of the RFe_(12)-type based compounds where part of R and Fe are substituted with Zr and Co and Si, respectively, in order to examine whether these rare-earth-lean materials are suitable for applications as permanent magnets. Structural and magnetic characterization of the family of alloys with the general formula Nd_(0.4) Zr_(0.6) Fe_(10-x)Co_xS_i2(x = 0 -3) and their melt-spun ribbons were carried out using X-ray diffraction and M€ossbauer spectroscopy. The ThMn_(12)-type structure is obtained for all samples as the majority phase with a minority a-Fe(CoSi) phase(less than 5 wt%) as it was estimated by XRD for x = 1 and 2. The Curie temperature increases linearly with Co substitution from 561 K for x = 0 to 712 K for x = 3. The saturation magnetization decreases slightly from 130.5(x =1)to 129.1 A·m~2/kg(x=3), while the anisotropy field is following the same trend.     

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH_2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH_2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd₂Fe₁₄B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd₂Fe₁₄B grains. The nano-sized α-Fe inside Nd₂Fe₁₄B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.     

Coercivity kinetics upon step annealing of sintered Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets

Behavior of the coercivity of the high-temperature Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets depending on the temperature and time of annealing with the temperature decreasing stepwise from 700 to 400℃ was investigated.It is shown that the growth rate of coercivity abruptly increases at the initial stage of annealing in the vicinity of the Curie temperature of the SmCo_5 phase.The origin of the effect is the counter diffusion of Cu and Co atoms through dislocation tubes,which form because of enhanced stresses and a partial breakage of coherent coupling at the interface of the Sm_2 Co_(17) and SmCo_5 phases.Diffusive enrichment of the SmCo_5 phase in Cu close to the interface with Sm_2 Co_(17) leads to relaxation of stresses and increases in the gradient of the magnetic domain-wall energy and coercivity.     

Magnetic properties of melt-spun ribbons(Sm_(1-x)Zr_x)(Fe_(0.92)Ti_(0.08))_(10) with ThMn_(12) structure and their hydrides

The structure and magnetic hysteresis properties of the cast Sm_(1-x)Zr_x(Fe_(0.92)Ti_(0.08))_(10)(x = 0-0.3)alloys and melt-spun ribbons prepared from them were studied.In the cast alloy with x0.2, a considerable amount of the eutectic phase is found in the SEM micrographs.Analysis of the temperature dependences of the magnetic susceptibility and XRD patterns allows amorphous state in the as-spun ribbons with x0.2 to be determined.The specific magnetization measured in a field of 17 kOe and remanence decrease with increasing annealing temperature from 800 to 900 ℃ and weakly depend on Zr concentration.The maximal value of coercivity Hc = 4.7 kOe is obtained on the ribbons with x = 0.2 after annealing at 850℃ for 10 min.After additional hydrogenation of the ribbons,both the coercivity and remanence increase by 54% and 7%,respectively.