| The importance of magnetic materials in the fields of manufactury,space devices,national defenses,military gears,electronic components,advanced medical equipments etc is highly focused.However,due to the relatively low hardness,poor thermal stability and insufficient chemical inertia of traditional magnetic materials,heir applications in extreme environments is highly hinded.At present,there is no effective methods to maintain magnetic property and to improve its mechanical properties at same time.Therefore,it is urgent to find a scientific way to improve the magnetic and mechanical properties of a material scientifically,and make the underlying physics clear.Meanwhile,it is necessary to obtain new magnetic multifunctional materials with excellent mechanical properties,and measure their related application parameters.High pressure and high temperature(HPHT)method enables to synthesize new compounds that cannot be prepared traditional other conditions.In the synthesizing process,the structure,electronic structure and various properties can be modulated by thermodynamic conditions such as precursor chemical ratio,temperature and pressure,so as to obtain new materials with a variety of excellent functions.In this work,potential high hardness magnetic iron borides were synthesized by high pressure and high temperature method.Then,on the based of Fe-B compounds,other transition metal elements were added into Fe-B system.The magnetic properties,mechanical properties and bonding feature of the obtained samples were investigated with the combination of experiments and first principle simulations.We obtained following innovative results.1.We synthesized the high hardness magnetic material Fe2B(space group:I4/mcm)with HPHT method,and found the competitive relationship between hardness and magnetism within this system.Cu Al2-type polycrystalline Fe2B bulk samples were synthesized at 5 GPa and 1500 K.The saturation magnetization of Fe2B sample was156.9 emu/g at room temperature and its Vickers hardness was 12.4 GPa.X-ray photoelectron spectroscopy(XPS)analysis combined with first-principle simulations show that the high hardness mainly stems from the strong hybridization between Fe 3d orbitals and B 2p orbitals.The strong magnetism mainly comes from the spin polarization caused by the bonding/non-bonding difference between the up and down spin electrons in Fe-Fe bonds.The competition for Fe 3d electrons to fall into Fe-Fe bonds or Fe-B bonds is the key factor that affect its hardness and magnetic.Therefore,we propose that there are two basic principles to be followed in the designation of high hardness magnetic materials:(i)high-strength bonding skeleton to ensure the hardness,(ii)appropriate light element concentration to reduce the amount of electron transfer of magnetic metals.2.We successfully prepared hard material?-Fe B with excellent magnetic properties with HPHT method,and found that the key factor that modulates the hardness,magnetism and conductivity is the spin-selected electrons which transfer between Fe 3d orbitals and the zigzag B chains.The single-phase?-Fe B polycrystalline material was successfully synthesized at 5 GPa and 1800 K.The saturation magnetization of?-Fe B is 79.54 emu/g,and the oxidation resistance temperature is above 800 K.Moreover,the hardness of?-Fe B was 15.6 GPa,and the resistivity was3.4×10-6?·m;?-Fe B is an antioxidant magnetic material with high hardness and good conductivity.Subsequently,we performed X-ray photoelectron spectroscopy analysis combined with first-principle calculations,and found that its high hardness comes from the covalent zigzag boron chains,and strong magnetism originates from the unpaired electrons in Fe-3d orbitals.The valence electrons transferred from Fe atoms stabilize the zigzag boron chains,and the spin-selection effect occurs during electron transfer and bonding process,with majority spin electrons are the main participants.The spin-selective effect between Fe and B atoms is the key factor to modulate the hardness,magnetism and conductivity of the system.3.We successfully synthesized high hardness and excellent thermal stability spin glass material Cr Fe B by high pressure and high temperature method for the first time and clarified the underlying physics of its magnetic characteristics.XRD and EDS analysis found that Cr Fe B has orthorhombic structure with a Fddd space group,where Cr and Fe are randomly arranged in the positions of 16e and 16f sites.The asymptotic hardness of Cr Fe B is 14.2 GPa,which is higher than any of the binary metal borides Fe2B or Cr2B.Its high hardness comes from four different bonding framework between metal and boron.The AC and DC susceptibilities show that it is a spin glass behaviour caused by magneto-resistance frustration occurs in the test temperature range of 2-400K,and the evolution of freezing temperature with frequency follows the Vogel-Fulcher law.The room temperature saturation magnetization of Cr Fe B is 4.79 emu/g,and the coercivity is 118.89 Oe.XPS analysis combined with first principles simulation show that magnetism may derive from the short-range magnetic exchange between the nearest neighboring Fe and Cr atoms.The random occupation for the two transition metal atoms leads to the disordered distribution of ferromagnetic,ferrimagnetic and antiferromagnetic region,which conforms to typical spin glass characteristics.Moreover,Cr Fe B has an excellent thermal stability(>1200 K),which make it a potential high temperature resistant magnetic material.These results will be helpful to understand the complex magnetic behavior of Cr Fe B and lay a foundation for the design of multifunctional ternary transition metal borides.4.We synthesized a ternary layered transition metal boride Fe(Mo B)2 by high temperature and high pressure method,and determined the origin of its ferromagnetism.Fe(Mo B)2 exhibited a class of atomically laminated structure,and the Fe B2 and Mo layers alternating stacking along the c axis direction.The measured Vickers hardness value of Fe(Mo B)2 was 10.72 GPa,and its excellent mechanical properties mainly originated from strong covalent B2 short chains.The anisotropic index indicated that Fe(Mo B)2 was more anisotropic along the(100)and(010)planes,and less anisotropic along the(001)plane.These analyses are consistent with the characteristics of its layered structure.Fe(Mo B)2 exhibited ferromagnetic metastable properties with a saturation magnetization of 8.35 emu/g and a coercivity of 96.12 Oe at room temperature.The magnetism was attributed the d–d hybridizations between Fe-3d egorbitals and Mo-4d orbitals.Fe(Mo B)2 has a magnetizable axis along the c-axis,and Mo atom acts as a medium to realize the exchange between two Fe atoms.In summary,by comparing the properties of the four Fe-based borides in this paper,we found that their hardness is mainly dominated by the internal light element skeleton,and the bond strength between metal and boron will also affect its hardness.The strong magnetism of the system mainly comes from the spin polarization of Fe 3d electrons.The transfermation and hybridization of Fe 3d electrons are the key factors to tune the hardness and magnetism of Fe-based borides.With the increasing of Fe 3d electron transfer,a stronger bonded skeleton tends to be formed in the system,which plays an important role in the hardness of the material.However,due to the existence of spin-selection effect in the process of the electron transfer,the magnetism of the system gradually decreases with electron transformation increasing.Therefore,to choose an appropriate amount of Fe 3d electron transformation to balance the contradiction between hardness and magnetism is the key point on Fe-based light element compounds.Moreover,the d-d hybridization between different metals shows a profound impact on the magnetism of the system,which results in some novel magnetic behaviors.Such as the spin glass behavior of Cr Fe B and the ferromagnetic metastable behavior of Fe(Mo B)2.By introducing different magnetic metal into Fe-B system,the magnetic diversities of Fe-based boride systems will be greatly expanded.This work is helpful to improve the understanding the of high hardness magnetic materials,and provide a theoretical basis for the further design of new hard multifunctional magnetic materials. |
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