High-Throughput Computational Search for Half-Metallic Oxides.

Half metals are a peculiar class of ferromagnets that have a metallic density of states at the Fermi level in one spin channel and simultaneous semiconducting or insulating properties in the opposite one. Even though they are very desirable for spintronics applications, identification of robust half-metallic materials is by no means an easy task. Because their unusual electronic structures emerge from subtleties in the hybridization of the orbitals, there is no simple rule which permits to select a priori suitable candidate materials. Here, we have conducted a high-throughput computational search for half-metallic compounds. The analysis of calculated electronic properties of thousands of materials from the inorganic crystal structure database allowed us to identify potential half metals. Remarkably, we have found over two-hundred strong half-metallic oxides; several of them have never been reported before. Considering the fact that oxides represent an important class of prospective spintronics materials, we have discussed them in further detail. In particular, they have been classified in different families based on the number of elements, structural formula, and distribution of density of states in the spin channels. We are convinced that such a framework can help to design rules for the exploration of a vaster chemical space and enable the discovery of novel half-metallic oxides with properties on demand.

1. Introduction

Spintronics attempts to employ electron’s charge and spin degrees of freedom in novel computing and data storage applications, assumed to be faster and more energy efficient than their conventional counterparts [1,2]. Successful development of such devices strongly depends on the availability and integration of diverse materials which would enable harnessing of electrons spins. Among several obstacles on the route to the practical realization of both spin logic and memory elements is the lack of sufficiently spin-polarized magnetic electrodes that could act as spin current sources. The mixed spin current may not only impede the efficient spin injection, but also limit the performance of devices utilizing either giant (GMR) or tunneling magnetoresistance (TMR) [3]. In this regard, half-metals (HM), which are fully spin-polarized at the Fermi level [4,5,6,7] and only pass a spin-up or spin-down current, emerge as natural candidates to use as electrodes in such devices. The identification of half-metallic compounds integrable with mature architectures based on complementary metal oxide semiconductors (CMOS) would therefore represent an important step towards broader implementation of spintronics. Regrettably, half metals are extremely rare in nature, and discovery of materials with suitable properties is challenging, either from experimental or theoretical side [8,9,10,11].

Most representative half-metallic compounds belong to either heusler/half-heusler alloys (e.g., NiMnSb and PtMnSb [12]) or transition metal oxides (TMO) in multiple structural forms, such as simple rutiles (CrO [13]), spinels (FeO [14]), perovskites (LaSrMnO [15]), pyrochlores (TlMnO [16]), or double perovskites (SrFeMoO [17]). Interestingly, extensive studies of these and similar materials have clearly shown that half-metallicity, despite being quite peculiar, does not manifest in an obvious way in magnetic, electrical, or optical properties that could be measured. The simplest indicator is perhaps the integer magnetic moment per unit cell at absolute zero, which is a necessary condition for half-metallicity in a stoichiometric compound. However, it does not permit unambiguously distinguishing a half metal from a standard ferromagnet. Observation of conducting electrons of one spin direction could certainly provide a more direct proof, but experimental techniques such as spin-polarized photoemission or transport measurements in point contacts and tunnel junctions are flawed by uncertainty and full spin polarization is rarely confirmed. Electronic structure calculations are thus the most useful tool to identify half metals, even though the existence of a band gap for just one spin channel is by some means fortuitous and hard to predict. In fact, the design rules so far have strongly relied on investigations in specific crystal families based on a prototype.

Here, we have identified a large number of new candidates for half metals by performing a high-throughput (HT) screening of the electronic structure information from the aflowlib database [18,19,20]. Currently, the repository contains over 3,000,000 different materials entries, among which 60,324 belong to the inorganic crystal structure database (ICSD) [21], representing fully determined and synthesized compounds. The data from this subset have been explored based on the analysis of their calculated density of states (DOS); the presence of a band gap in just one spin channel has been adapted as a major criterion for half-metallicity. The search revealed in total over one-thousand potential half metals, including most of the known examples. We have selected and described in detail a subgroup of oxides, among which 223 materials have been predicted to be strongly half-metallic. We have further classified them according to the number of elements and atomic structure, recognizing new crystal prototypes. Remarkably, the reported electronic structure parameters have shed more light on the mechanisms of hybridization and origin of half-metallicity. Figure 1 shows the schematic diagram of the full search procedure as well as the classification of the identified compounds.

Figure 1

Left: Scheme illustrating steps of the high-throughput (HT) search for half-metallic oxides starting with the aflowlib database. Right: Diagram representing the distribution of different structural groups among the identified half-metal candidates.

2. Results

Before we start with an overview of the HT search, let us remark that materials repositories usually do not provide complete information on the magnetic ordering. Thus, some ferromagnetic compounds may be classified as paramagnetic and they cannot be included in this search. Conversely, several materials labeled as ferromagnetic may actually display different magnetic orders. As the ground state needs to be verified experimentally or theoretically via more realistic ab initio calculations, we will hereafter refer to potential half metals. In the screening procedure, we have first selected materials whose spin polarization at the Fermi level (E) is different from zero. This can be expressed as P(E) = [N N)]/[N + N] ≠ 0, where N denotes the density of states for spin majority/minority at E. Second, we have analyzed electronic states around the Fermi level, and classified the compounds as metals, half-metals, and semiconductors/insulators. We have further restricted the analysis to “half-metallic semiconductors” determined by conditions on the band gap (E < 3.5 eV), valence band maximum (VBM > −1.5 eV), and conduction band minimum (CBM < 2.5 eV) of the insulating channel. Finally, in order to unveil materials that are strongly half-metallic, we have put a constraint on the spin polarization in the energy region limited by VBM and CBM, namely, P(E) > 0.8. The spin-flip excitation energies of the transition from majority to minority spin, and vice versa, have been also constrained to ensure robustness of half-metallicity with respect to thermal fluctuations [4].

The high-throughput procedure revealed a total number of 223 candidates for strongly half-metallic oxides. They have been categorized according to the number of elements; we have classified 13 binary, 105 ternary, 101 quaternary, and four quinary compounds. Each of these categories has been further divided into families based on a crystalographic structure. The full list of the identified materials is provided in Table A1, Table A2, Table A3 and Table A4 in the Appendix A along with the descriptors and parameters essential for the analysis of the electronic structure phenomena behind half-metallicity. We have reported crystal lattice, saturation magnetization (M), spin polarization (P), energy gap (E), as well as VBM and CBM of the insulating spin channel (either in spin majority or minority, as denoted by the arrows preceding the values). Finally, we have extracted quantities useful to determine the type of hybridization and strength of half-metallicity. The atom- and orbital-resolved spin magnetic moments are defined as fractions of the total magnetic moment per unit cell. We have also listed the overlaps of partial density of states functions projected on different orbitals, calculated as a percentage of a common area between each pair of functions in the metallic spin channel. These additional data are provided in the Supplementary Materials.

Table A1

Binary half-metallic oxides. P(E) and P(E) denote spin polarization at the Fermi level and within the band gap, respectively. E, VBM, and CBM are calculated for the insulating channel either in spin majority or minority (identified by ↑ and ↓). M refers to the saturation magnetization.

Name	ICSD	Lattice	P_0(E_F)	P_0(E_gap)	E_gap	VBM	CBM	M_S
Fe_3O_4	92356	RHL	1.0	1.000	↑ 2.90	↑−1.08	↑ 1.82	28.00
Ce_7O_12	88754	RHL	1.0	0.978	↓ 3.09	↓−0.99	↓ 2.10	4.00
Fe_3O_4	31156	ORCI	1.0	1.000	↑ 2.67	↑−1.08	↑ 1.59	28.00
CoO	53059	BCT	1.0	1.000	↑ 2.22	↑−0.78	↑ 1.44	3.00
Fe_3O_4	77589	FCC	1.0	1.000	↑ 2.73	↑−1.20	↑ 1.53	27.99
CoO	245320	FCC	1.0	0.999	↑ 2.31	↑−0.27	↑ 2.04	3.00
Fe_3O_4	98086	MCL	1.0	0.999	↑ 2.76	↑−0.88	↑ 1.88	52.00
CrO_2	155832	ORC	1.0	1.000	↓ 3.03	↓−0.63	↓ 2.40	4.00
Fe_3O_4	164813	ORC	1.0	1.000	↑ 2.61	↑−0.72	↑ 1.89	95.97
V_6O_13	16779	MCLC	1.0	0.997	↓ 2.34	↓−1.48	↓ 0.86	4.00
VO_2	34417	MCLC	1.0	1.000	↓ 2.13	↓−1.05	↓ 1.08	4.00
CrO_2	166023	TET	1.0	1.000	↓ 3.03	↓−0.65	↓ 2.39	4.00
Fe_2O_3	108905	BCC	1.0	0.999	↓ 1.60	↓−1.35	↓ 0.25	55.99

Table A2

Ternary half-metallic oxides. Parameters are the same as in Table A1.

Name	ICSD	Lattice	P_0(E_F)	P_0(E_gap)	E_gap	VBM	CBM	M_S
FeNaO_2	167376	RHL	1.0	0.996	↑ 2.52	↑−1.41	↑ 1.11	1.00
CoLaO_3	180176	RHL	1.0	0.997	↓ 2.42	↓−0.51	↓ 1.91	4.00
Ca_3Co_2O_6	82175	RHL	1.0	0.995	↓ 1.32	↓−0.83	↓ 0.49	12.00
LaNiO_3	173477	RHL	1.0	0.999	↓ 2.28	↓−0.64	↓ 1.64	2.00
BaRuO_3	91075	RHL	1.0	0.999	↑ 1.84	↑−0.55	↑ 1.29	6.00
CuNdO_2	18104	RHL	1.0	1.000	↓ 3.02	↓−0.92	↓ 2.10	3.00
PuSrO_4	31974	RHL	1.0	1.000	↓ 2.25	↓−0.98	↓ 1.27	2.00
Ba_2Co_9O_14	161771	RHL	1.0	0.999	↓ 1.58	↓−0.53	↓ 1.05	25.00
SrV_13O_18	97949	RHL	1.0	0.993	↓ 1.41	↓−0.72	↓ 0.69	25.00
RhSr_4O_6	109297	RHL	1.0	0.999	↑ 2.62	↑−0.20	↑ 2.43	2.00
FeLiO_2	51207	RHL	1.0	0.994	↑ 2.62	↑−1.39	↑ 1.23	1.00
AgNiO_2	73974	RHL	1.0	0.999	↓ 2.39	↓−0.89	↓ 1.50	1.00
Cu_2PO_4	80181	TRI	1.0	0.992	↑ 1.98	↑−0.33	↑ 1.65	4.00
CaIrO_3	159027	ORCC	1.0	0.999	↑ 2.55	↑−0.31	↑ 2.24	2.00
Fe_4Sr_4O_11	249009	ORCC	1.0	0.999	↓ 1.14	↓−0.15	↓ 0.99	18.00
UYO_4	16492	ORCC	1.0	1.000	↓ 3.06	↓−1.02	↓ 2.04	1.00
Bi_3Mn_2O_7	184382	ORCC	1.0	1.000	↓ 2.46	↓−0.84	↓ 1.62	18.00
K_3Pd_2O_4	245610	ORCC	1.0	0.998	↑ 2.54	↑−0.22	↑ 2.31	1.99
CaRhO_3	164774	ORCC	1.0	0.999	↑ 2.08	↑−0.20	↑ 1.89	2.00
Eu_3OsO_7	170874	ORCC	1.0	1.000	↓ 1.61	↓−0.81	↓ 0.79	42.77
MgVO_3	15927	ORCC	1.0	0.997	↓ 0.95	↓−0.15	↓ 0.80	2.00
Cl_5U_2O_2	23084	ORCC	1.0	1.000	↓ 3.05	↓−1.04	↓ 2.01	3.00
RuSr_2O_4	73394	BCT	1.0	0.995	↑ 0.69	↑−0.65	↑ 0.04	2.00
Nd_4Ni_3O_8	51097	BCT	1.0	0.962	↓ 1.41	↓ v0.52	↓ 0.88	14.00
NdVO_4	15610	BCT	1.0	1.000	↓ 3.12	↓−1.47	↓ 1.65	6.00
FeSr_2O_4	74419	BCT	1.0	0.998	↓ 1.34	↓−0.42	↓ 0.92	4.00
KRu_4O_8	1562	BCT	1.0	0.988	↑ 2.01	↑ v0.66	↑ 1.35	7.00
La_3Ni_2O_6	249209	BCT	1.0	0.942	↓ 1.85	↓−0.39	↓ 1.46	1.00
IrSr_2O_4	45974	BCT	1.0	0.994	↑ 0.34	↑−0.19	↑ 0.15	1.00
K_2Ru_8O_16	61378	BCT	1.0	0.989	↑ 2.01	↑−0.64	↑ 1.37	7.00
CoSr_2O_4	246483	BCT	1.0	0.997	↓ 0.98	↓−0.21	↓ 0.77	3.00
Pr_2TeO_2	89559	BCT	1.0	1.000	↓ 2.04	↓−0.54	↓ 1.50	4.02
CrSr_2O_4	245595	BCT	1.0	0.999	↓ 1.56	↓−0.11	↓ 1.46	2.00
BaFeO_3	50869	HEX	1.0	1.000	↓ 1.22	↓−0.14	↓ 1.08	23.99
BaRhO_3	15520	HEX	1.0	1.000	↑ 1.48	↑−0.19	↑ 1.29	4.00
BaRuO_3	84652	HEX	1.0	0.999	↑ 1.68	↑−0.42	↑ 1.26	8.00
AgNiO_2	415451	HEX	1.0	0.999	↓ 2.40	↓−0.89	↓ 1.52	2.00
InMnO_3	186856	HEX	1.0	1.000	↓ 2.52	↓−1.49	↓ 1.03	24.01
BaCrO_3	35029	HEX	1.0	1.000	↓ 2.32	↓−0.42	↓ 1.90	8.00
HNiO_2	169980	HEX	1.0	0.994	↓ 3.38	↓−0.93	↓ 2.44	1.00
Fe_2NaO_3	200009	HEX	1.0	1.000	↑ 2.77	↑−1.02	↑ 1.75	9.00
Fe_12SrO_19	16158	HEX	1.0	0.994	↓ 0.86	↓−0.31	↓ 0.54	77.92
CuMn_2O_4	174000	FCC	1.0	1.000	↓ 2.97	↓−1.01	↓ 1.96	14.01
Ir_2Pr_2O_7	156436	FCC	1.0	0.998	↑ 2.07	↑−0.24	↑ 1.83	12.00
Co_2GeO_4	21115	FCC	1.0	1.000	↑ 2.04	↑−1.40	↑ 0.64	12.00
Mn_2Sb_2O_7	247301	FCC	1.0	0.941	↓ 1.26	↓−1.25	↓ 0.02	20.00
Ni_2Sb_2O_7	247303	FCC	1.0	0.997	↑ 0.97	↑−0.53	↑ 0.45	8.00
Eu_2Mo_2O_7	173946	FCC	1.0	0.999	↓ 1.42	↓−0.88	↓ 0.54	32.00
Pr_2Te_2O_7	92444	FCC	1.0	0.998	↓ 1.61	↓−0.41	↓ 1.20	7.99
Cd_2Tc_2O_7	180008	FCC	1.0	0.999	↓ 1.86	↓−1.20	↓ 0.66	8.00
Ru_2Tl_2O_7	51158	FCC	1.0	0.998	↑ 0.88	↑−0.72	↑ 0.17	8.00
CuRh_2O_4	88962	FCC	1.0	0.980	↑ 2.13	↑−0.25	↑ 1.87	1.99
Ir_2Y_2O_7	187534	FCC	1.0	1.000	↑ 2.29	↑−0.17	↑ 2.13	4.00
AlFe_2O_4	76977	FCC	1.0	1.000	↑ 3.36	↑−1.38	↑ 1.98	18.00
FeNi_2O_4	109150	FCC	1.0	0.998	↓ 1.52	↓−0.27	↓ 1.25	4.00
Ca_2Ru_2O_7	156409	FCC	1.0	0.997	↑ 1.56	↑−0.12	↑ 1.44	12.00
Mn_3P_2O_8	415107	MCL	1.0	0.996	↓ 3.03	↓−0.87	↓ 2.16	22.00
Ru_3Sr_4O_10	96729	MCL	1.0	0.999	↑ 1.11	↑−0.70	↑ 0.41	12.03
Co_3La_3O_8	86176	MCL	1.0	0.994	↓ 1.11	↓−0.66	↓ 0.45	18.04
Co_3La_4O_10	51177	MCL	1.0	0.982	↓ 1.26	↓−0.60	↓ 0.66	18.00
Co_3P_2O_8	9850	MCL	1.0	0.995	↓ 1.44	↓−0.66	↓ 0.78	14.00
CoSrO_3	108896	MCL	1.0	0.999	↓ 2.06	↓−0.38	↓ 1.68	0.00
BFe_3O_5	25101	MCL	1.0	1.000	↑ 2.97	↑−1.14	↑ 1.83	26.00
Cr_3HO_8	156386	MCL	1.0	0.998	↓ 2.67	↓−1.28	↓ 1.40	6.00
CaRhO_3	183583	MCL	1.0	0.998	↑ 2.02	↑−0.23	↑ 1.80	6.00
As_2Co_3O_8	59000	MCL	1.0	0.996	↑ 2.24	↑−1.26	↑ 0.97	36.01
Bi_3Ru_3O_11	74382	CUB	1.0	0.998	↑ 1.65	↑−0.36	↑ 1.29	28.00
CaSnO_3	27777	CUB	1.0	1.000	↑ 2.49	↑−0.57	↑ 1.92	2.01
PbVO_3	187637	CUB	1.0	1.000	↓ 2.13	↓−1.31	↓ 0.83	1.00
CrSrO_3	245834	CUB	1.0	0.998	↓ 2.61	↓−0.36	↓ 2.25	2.00
Fe5LiO_8	35769	CUB	1.0	0.946	↓ 2.04	↓−0.62	↓ 1.42	76.00
Na_11U_5O_16	15,137	CUB	1.0	0.981	↓ 2.80	↓−0.41	↓ 2.40	18.00
LaRu_3O_11	100517	CUB	1.0	1.000	↑ 2.36	↑−0.44	↑ 1.92	28.00
CaRuO_3	99451	ORC	1.0	1.000	↑ 1.44	↑−0.31	↑ 1.12	8.00
NaRh_2O_4	170598	ORC	1.0	0.996	↑ 1.46	↑−0.24	↑ 1.22	3.99
RuSrO_3	56697	ORC	1.0	1.000	↑ 1.03	↑−0.50	↑ 0.54	8.00
NiYbO_3	151949	ORC	1.0	0.999	↓ 2.13	↓−0.50	↓ 1.63	0.00
LiRuO_2	48007	ORC	1.0	0.999	↑ 1.53	↑−0.36	↑ 1.17	2.00
NaRu_2O_4	172608	ORC	1.0	1.000	↑ 1.60	↑−0.55	↑ 1.05	12.00
CaFeO_3	92349	ORC	1.0	0.993	↓ 1.59	↓−0.45	↓ 1.14	16.00
Mn2Pb_2O_5	174474	ORC	1.0	1.000	↓ 2.36	↓−0.72	↓ 1.64	32.02
Ba_3FeO_5	281029	ORC	1.0	0.998	↓ 2.60	↓−0.33	↓ 2.27	16.00
FeYO_3	23822	ORC	1.0	0.999	↓ 1.68	↓−0.59	↓ 1.09	16.00
Cu_2Li_3O_4	66509	MCLC	1.0	1.000	↓ 1.55	↓−0.79	↓ 0.75	1.00
Co_2Na_7O_6	414128	MCLC	1.0	0.987	↑ 1.95	↑−0.82	↑ 1.13	14.00
MoYb_2O_6	99574	MCLC	1.0	0.998	↑ 2.45	↑−0.19	↑ 2.25	8.00
Ba_2Mn_8O_16	62096	MCLC	1.0	1.000	↓ 2.73	↓−0.80	↓ 1.94	14.00
BaMn_3O_6	93226	MCLC	1.0	0.998	↓ 2.44	↓−0.27	↓ 2.17	22.00
IrNa_2O_3	187130	MCLC	1.0	0.999	↑ 2.34	↑−0.33	↑ 2.01	2.00
HgV_4O_10	418029	MCLC	1.0	0.995	↓ 2.22	↓−1.38	↓ 0.84	2.00
MnPbO_3	246351	MCLC	1.0	1.000	↓ 2.49	↓−0.10	↓ 2.38	18.04
Fe_2K_9O_8	174307	MCLC	1.0	0.897	↓ 2.94	↓−0.70	↓ 2.24	36.01
Eu_2ReO_5	88686	TET	1.0	0.998	↓ 0.89	↓−0.10	↓ 0.78	48.27
FeSrO_2	418603	TET	1.0	0.974	↓ 1.25	↓−1.23	↓ 0.02	4.00
CaFeO_2	173438	TET	1.0	0.970	↓ 1.09	↓−1.00	↓ 0.09	4.00
Fe5Y_3O_12	23855	BCC	1.0	1.000	↑ 2.35	↑−1.38	↑ 0.97	68.00
La_4Ru_6O_19	100098	BCC	1.0	0.975	↑ 2.47	↑−1.00	↑ 1.47	14.00
La_4Os_6O_19	100099	BCC	1.0	0.986	↑ 1.23	↑−0.61	↑ 0.62	8.00
Fe5Pr_3O_12	248013	BCC	1.0	0.993	↑ 2.42	↑−1.37	↑ 1.05	92.00
Mn_7NaO_12	151587	BCC	1.0	1.000	↓ 3.32	↓−0.98	↓ 2.34	26.00
Bi_12MnO_20	75079	BCC	1.0	0.999	↓ 2.47	↓−1.23	↓ 1.25	3.00
Ba_2Ir_3O_9	54725	BCC	1.0	0.988	↑ 2.50	↑−0.21	↑ 2.29	10.00
Bi_12MnO_20	75390	BCC	1.0	1.000	↓ 2.37	↓−1.14	↓ 1.23	3.00
Cs_18Tl_8O_6	421376	BCC	1.0	0.984	↓ 0.27	↓−0.18	↓ 0.09	1.99
Nd_4Os_6O_19	200870	BCC	1.0	0.943	↑ 1.20	↑−0.65	↑ 0.55	20.00

Table A3

Quaternary half-metallic oxides. Parameters are the same as in Table A1.

Name	ICSD	Lattice	P_0(E_F)	P_0(E_gap)	E_gap	VBM	CBM	M_S
Ba_4Ru_3ZrO_12	47132	RHL	1.0	0.998	↑ 2.78	↑−0.75	↑ 2.03	6.00
Ba_7Cl_2Ru_4O_15	71755	RHL	1.0	0.993	↑ 2.15	↑−0.72	↑ 1.42	10.03
Ba_4Ru_3TbO_12	160870	RHL	1.0	0.982	↑ 2.98	↑−1.03	↑ 1.95	7.00
B_4Fe_3NdO_12	83507	RHL	1.0	0.998	↓ 2.30	↓−0.72	↓ 1.58	18.00
CoPb_2TeO_6	169195	RHL	1.0	0.998	↑ 1.60	↑−1.20	↑ 0.41	3.00
Ca_3CoMnO6	93775	RHL	1.0	0.994	↑ 2.08	↑−1.29	↑ 0.79	12.00
Ba_7Br2Ru_4O_15	73,069	RHL	1.0	0.996	↑ 2.15	↑−0.72	↑ 1.42	10.07
InNiSr_3O_6	81660	RHL	1.0	0.945	↓ 3.20	↓−0.81	↓ 2.39	2.00
BaEu_2NiO_5	68086	ORCI	1.0	1.000	↓ 2.46	↓−0.45	↓ 2.01	14.00
Ca_2GaMnO_5	51466	ORCI	1.0	1.000	↓ 3.15	↓−0.84	↓ 2.31	8.00
BaCoEu_2O_5	78173	ORCI	1.0	0.999	↓ 3.00	↓−0.56	↓ 2.44	15.00
Ba_2GdMoO_6	236363	TRI	1.0	0.982	↑ 0.95	↑−0.20	↑ 0.75	8.00
Ag_2Co_3P_4O_14	93591	TRI	1.0	0.997	↑ 2.60	↑−1.35	↑ 1.24	18.03
NiOsSr_2O_6	152442	BCT	1.0	0.999	↓ 1.36	↓−0.93	↓ 0.43	4.00
Cl_2Fe_2Sr_3O_4	174532	BCT	1.0	0.999	↑ 1.11	↑−0.72	↑ 0.39	8.00
Fe_2La_2Se_2O_3	183143	BCT	1.0	0.992	↑ 2.91	↑−1.26	↑ 1.65	8.00
Fe_2Pr_2Se_2O_3	183145	BCT	1.0	0.999	↑ 2.64	↑−1.25	↑ 1.39	12.00
BaFe_4YO_7	262842	BCT	1.0	0.996	↑ 2.27	↑−1.19	↑ 1.08	17.00
Fe_2Pr_2S_2O_3	181168	BCT	1.0	0.991	↑ 2.70	↑−1.33	↑ 1.37	11.99
Cl_10Cs_3Re_2O	231	BCT	1.0	0.999	↑ 1.23	↑−0.89	↑ 0.35	2.99
As_2Ba_2Mn_3O_2	32011	BCT	1.0	0.989	↓ 0.77	↓−0.54	↓ 0.23	10.99
CoMoSr_2O_6	153546	BCT	1.0	0.999	↑ 2.30	↑−1.43	↑ 0.87	3.00
AlNd_2NO_3	201358	BCT	1.0	1.000	↓ 1.97	↓−0.32	↓ 1.65	6.00
Ba_3Ir_2MgO_9	245252	HEX	1.0	1.000	↑ 2.59	↑−0.19	↑ 2.40	8.01
Ba_3NaOs_2O_9	281248	HEX	1.0	0.984	↓ 1.25	↓−0.10	↓ 1.14	6.00
Ba_3LiOs_2O_9	281247	HEX	1.0	1.000	↓ 2.12	↓−1.28	↓ 0.84	10.01
Ba_3Ru_2ZrO_9	172754	HEX	1.0	1.000	↑ 2.61	↑−0.66	↑ 1.95	8.05
Ba_2NiOsO_6	16406	HEX	1.0	0.998	↓ 1.60	↓−1.00	↓ 0.60	11.99
Ba_3CoRu_2O_9	50830	HEX	1.0	0.999	↑ 2.46	↑−0.19	↑ 2.27	18.00
Ba_3CeRu_2O_9	94024	HEX	1.0	0.998	↑ 2.17	↑−0.76	↑ 1.41	8.02
Ba_9Cl_2Cu_7O_15	9628	HEX	1.0	1.000	↓ 0.51	↓−0.29	↓ 0.23	5.00
FeMo_2RbO_8	245666	HEX	1.0	0.999	↑ 2.68	↑−1.42	↑ 1.26	1.00
Ba_3Ir_2NiO_9	33530	HEX	1.0	1.000	↑ 2.35	↑−0.24	↑ 2.11	12.00
K_3NaOs_2O_9	423654	HEX	1.0	0.999	↓ 1.51	↓−1.16	↓ 0.36	4.00
Ba_3LiRu_2O_9	281126	HEX	1.0	1.000	↓ 1.47	↓−0.48	↓ 0.99	10.01
Ba_3Ru_2YbO_9	400598	HEX	1.0	1.000	↑ 2.97	↑−0.81	↑ 2.16	12.00
Ba_3NaRu_2O_9	281127	HEX	1.0	1.000	↓ 1.58	↓−0.51	↓ 1.07	10.02
Ba_3CaIr_2O_9	246280	HEX	1.0	0.999	↑ 2.12	↑−0.46	↑ 1.65	4.00
Ba_5ClCo_5O_13	93657	HEX	1.0	0.993	↓ 1.19	↓−0.23	↓ 0.96	24.00
CrEuTeO_6	164940	HEX	1.0	0.995	↓ 3.30	↓−0.86	↓ 2.45	18.00
Ba_2CoWO_6	27425	FCC	1.0	0.992	↑ 3.00	↑−0.89	↑ 2.11	3.00
Ba_2NaOsO_6	412143	FCC	1.0	1.000	↓ 1.35	↓−1.11	↓ 0.24	1.00
ReSr_2YbO_6	25407	FCC	1.0	0.996	↓ 1.22	↓−0.79	↓ 0.42	1.00
NiRuSr_2O_6	181753	FCC	1.0	0.997	↓ 1.20	↓−0.13	↓ 1.07	4.00
Ba_2NbNdO_6	109152	FCC	1.0	0.995	↓ 3.32	↓−1.12	↓ 2.19	3.00
CoPb_2TeO_6	169196	FCC	1.0	0.992	↑ 1.97	↑−1.19	↑ 0.78	3.00
Ba_2BiIrO_6	174289	FCC	1.0	1.000	↑ 2.18	↑−0.56	↑ 1.62	2.00
Ba_2CoMoO_6	184910	FCC	1.0	0.999	↑ 2.19	↑−0.79	↑ 1.40	3.00
Ba_2MnReO_6	109256	FCC	1.0	0.986	↓ 0.54	↓−0.38	↓ 0.17	2.02
Ba_2RuTmO_6	55713	FCC	1.0	0.997	↑ 3.03	↑−0.60	↑ 2.43	20.00
Ba_2CaOsO_6	171988	FCC	1.0	0.997	↓ 1.77	↓−1.01	↓ 0.77	2.00
Ba_2LiOsO_6	412142	FCC	1.0	1.000	↓ 1.28	↓−1.08	↓ 0.19	1.00
ClCu_6YO_8	188351	FCC	1.0	0.989	↓ 1.32	↓−0.86	↓ 0.47	4.00
ClCu_6InO_8	69612	FCC	1.0	1.000	↓ 1.23	↓−0.87	↓ 0.36	4.00
CoMoSr_2O_6	181517	FCC	1.0	0.992	↑ 2.06	↑−0.85	↑ 1.20	3.00
Ba_2CoUO_6	245141	FCC	1.0	0.980	↑ 2.58	↑−0.77	↑ 1.81	3.00
Ba_2NdRuO_6	155551	FCC	1.0	0.997	↓ 1.41	↓−1.05	↓ 0.36	5.99
MnNbSr_2O_6	181751	FCC	1.0	0.997	↓ 2.92	↓−1.27	↓ 1.65	3.98
Ba_2ReYbO_6	25399	FCC	1.0	0.994	↓ 1.64	↓−1.21	↓ 0.42	0.73
Ba_2PrPtO_6	80636	FCC	1.0	0.992	↓ 2.64	↓−0.33	↓ 2.31	1.00
CoSr_2WO_6	28598	FCC	1.0	0.995	↑ 2.82	↑−0.96	↑ 1.86	3.00
BaMn_2TbO_6	154009	MCL	1.0	1.000	↓ 3.12	↓−0.66	↓ 2.46	14.00
CdCsN_3O_6	28649	CUB	1.0	1.000	↓ 1.67	↓−0.69	↓ 0.97	4.00
CdN_3TlO_6	28650	CUB	1.0	1.000	↓ 1.31	↓−0.77	↓ 0.54	4.00
CdKN_3O_6	28647	CUB	1.0	1.000	↓ 1.70	↓−0.84	↓ 0.86	4.00
CdN_3RbO_6	28648	CUB	1.0	1.000	↓ 1.79	↓−0.94	↓ 0.84	4.00
BaCo_2YO_5	171435	ORC	1.0	0.999	↓ 1.54	↓−0.15	↓ 1.40	10.00
CKLaO_4	90735	ORC	1.0	0.998	↑ 1.44	↑−0.75	↑ 0.69	8.00
BaEuFe_2O_5	416716	ORC	1.0	0.946	↑ 0.95	↑−0.10	↑ 0.84	33.15
Ca_2FeMnO_5	85125	ORC	1.0	0.998	↓ 2.46	↓−1.14	↓ 1.32	35.96
Ca_2GaMnO_5	51464	ORC	1.0	0.996	↓ 3.33	↓−0.93	↓ 2.40	16.04
MnPb_2ReO_6	182002	MCLC	1.0	0.975	↑ 0.45	↑−0.15	↑ 0.30	13.99
Ag_4CuTeO_6	416931	MCLC	1.0	0.999	↓ 0.26	↓−0.20	↓ 0.06	2.01
BaMn_2NdO_6	158890	TET	1.0	1.000	↓ 2.81	↓−0.31	↓ 2.49	10.00
BaErMn_2O_5	188495	TET	1.0	1.000	↓ 2.70	↓−1.42	↓ 1.28	18.01
BaMn_2PrO_5	158885	TET	1.0	0.998	↓ 2.62	↓−1.47	↓ 1.16	11.00
BaMn_2PrO_6	150704	TET	1.0	0.997	↓ 2.96	↓−0.51	↓ 2.45	9.00
CuPrSO	92,494	TET	1.0	0.998	↓ 2.16	↓−0.76	↓ 1.40	4.00
ClCoSr_2O_3	90122	TET	1.0	0.996	↓ 2.15	↓−0.10	↓ 2.04	4.00
BaLaMn_2O_6	150703	TET	1.0	0.998	↓ 2.94	↓−0.45	↓ 2.49	7.00
Cu_3NdRu_4O_12	202061	BCC	1.0	0.993	↑ 1.63	↑−0.39	↑ 1.25	13.00
Cu_3LaRu_4O_12	51897	BCC	1.0	0.998	↑ 1.61	↑−0.39	↑ 1.21	10.00
Cu_3HoMn_4O_12	153872	BCC	1.0	0.999	↓ 2.58	↓−0.45	↓ 2.13	10.00
Cu_3Ru_4SrO_12	51895	BCC	1.0	0.997	↑ 1.44	↑−0.32	↑ 1.12	11.00
Cu_3Fe_4SrO_12	262855	BCC	1.0	0.999	↓ 0.63	↓−0.45	↓ 0.18	19.00
CeCu_3Mn_4O_12	169043	BCC	1.0	0.999	↓ 1.68	↓−0.58	↓ 1.09	11.00
Cu_3Mn_4YO_12	38418	BCC	1.0	0.999	↓ 2.59	↓−0.43	↓ 2.16	10.00
CaCu_3Ru_4O_12	95715	BCC	1.0	0.998	↑ 1.47	↑−0.30	↑ 1.17	11.00
Cu_3Mn_4YbO_12	153874	BCC	1.0	0.999	↓ 2.37	↓−0.27	↓ 2.10	9.00
Cu_3NaRu_4O_12	95716	BCC	1.0	0.998	↑ 1.33	↑−0.27	↑ 1.07	12.00
CaCo_4Cu_3O_12	169095	BCC	1.0	0.988	↓ 1.28	↓−0.24	↓ 1.04	7.00
Cu_3Mn_4ThO_12	34316	BCC	1.0	0.999	↓ 2.82	↓−0.49	↓ 2.33	11.00
Cu_3DyMn_4O_12	153871	BCC	1.0	0.999	↓ 2.60	↓−0.44	↓ 2.16	10.00
Fe_2Mn_3Si_3O_12	27381	BCC	1.0	0.999	↑ 2.47	↑−0.92	↑ 1.56	68.00
Mn_3Si_3V_2O_12	27380	BCC	1.0	1.000	↑ 2.16	↑−0.83	↑ 1.33	59.99
CaCu_3V_4O_12	250094	BCC	1.0	0.999	↓ 1.53	↓−0.88	↓ 0.65	1.00
FeMoSr_2O_6	150701	BCT	1.0	0.999	↑ 2.67	↑ -1.80	↑ 0.87	4.00
FeMoSr_2O_6	157603	FCC	1.0	0.988	↑ 2.70	↑ -1.81	↑ 0.88	4.00
GdReSr_2O_6	25400	FCC	1.0	0.996	↑ 2.87	↑ -2.00	↑ 0.87	5.00
Ba_2ReYO_6	94215	FCC	1.0	0.998	↓ 2.87	↓ -1.92	↓ 0.95	2.00
DyReSr_2O_6	25402	FCC	1.0	0.997	↓ 2.91	↓ -1.98	↓ 0.93	2.00

Table A4

Quinary half-metallic oxides. Parameters are the same as in Table A1.

Name	ICSD	Lattice	P_0(E_F)	P_0(E_gap)	E_gap	VBM	CBM	M_S
H_4K_2N_4PdO_10	164218	TRI	1.0	0.996	↓ 0.60	↓−0.39	↓ 0.21	0.00
La_3MnS_3WO_6	380406	HEX	1.0	1.000	↓ 1.55	↓−0.25	↓ 1.29	8.00
CuH_12Mn_2N_4O_8	61243	MCL	1.0	0.980	↓ 0.74	↓−0.68	↓ 0.06	17.99
CH_2P_2PuO_6	262902	MCLC	1.0	0.998	↓ 3.42	↓−1.40	↓ 2.02	8.00

2.1. Binary Oxides

Binary compounds are the simplest half-metallic structures; yet no element can be a half metal. However, few binary oxides beyond the representative CrO are known to exhibit half-metallicity. The high-throughput search has not greatly improved this status, as binaries account for only a small fraction of revealed materials. Moreover, among the 13 compounds listed in Table A1, several are polytypes or almost identical structures. One example is the well-studied rutile CrO (ORC) whose half-metallic properties are a consequence of the exchange splitting larger than the occupied bandwidth. The same mechanism leads to full spin polarization in CrO (TET), which can be considered a strained variant of the former; note nearly identical parameters in Table A1. Similarly, most of the FeO phases can be associated with the cubic magnetite structure (FCC) above the Verwey transition, which is ferrimagnetic due to the presence of two different ions Fe(3+) and Fe(2+); FeO is a well known half metal with the highest reported > 800 K. Last, stoichiometrically different FeO was previously found to be antiferromagnetic and insulating, thus the crystal ground state is not a half metal.

Table A1 contains several materials that have never been proposed as potential half metals. However, a detailed consideration of their properties indicates that the half-metallicity might be unfeasible in most of them. Both reported phases of CoO are antiferromagnetic [22]. Moreover, the vanadates do not manifest ferromagnetic ground states. The simpler VO is non-magnetic and shows a strong metal-to-insulator (MIT) transition at 340 K, accompanied by a structural change from tetragonal to monoclinic. The latter was previously suggested to be metallic, but we emphasize that its electronic structure is still under debate. VO with its mixed-valence state V(4+) and V(5+) is again a MIT system, shown to be antiferromagnetic below 50 K and ferrimagnetic at higher temperatures; half-metallicity of the ferrimagnetic phase has never been reported. Finally, we conclude that the only binary compound that indeed seems to be half-metallic is CeO, one of few rare-earth oxides revealed in this study. The crystal structure is rather complex, whereby inequivalent Ce sites are likely to cause a ferrimagnetic ordering.

2.2. Ternary Oxides

The diversity of the revealed ternaries is reflected in complex chemical formulas that can be found in Table A2. Although most of the structures belong to one of three main crystal families, including (i) spinels (ABO), (ii) perovskites (ABO), and (iii) pyrochlores (ABO), there are materials containing more than seven and up to 20 oxygen atoms in the unit cell; many of them have not been considered as candidates for half metals and can be thus regarded as new prototypes. Below, we have briefly characterized the known structural families in which we have found several new HM.

2.2.1. Spinels

Spinels are cubic lattice structures characterized by a general formula ABO [23,24]. Multiple degrees of freedom present in these complex crystals can be used to engineer their physical properties. In particular, they exhibit complex magnetic properties ranging from ferrimagnetism and ferromagnetism, to strong-magnetostructural coupling which is often related to the occupation of the magnetic ions in two different sublattices. According to their cation distributions, spinels can be categorized as “normal” and “inverted” spinel structures [25]. In the normal spinel structures, one-eighth of the tetrahedral interstices in oxygen sublattice are occupied by the A atoms and one-half of the octahedral interstices are occupied by the B atoms. In the inverted spinel structures, tetahedral interstices are occupied by B atoms and the octahedral interstices are occupied by both A and B atoms. The HT search revealed in total 18 spinels; most of them were never proposed as HM candidates.

2.2.2. Perovskites

Half-metallic ternary perovskite oxides are very rare. The crystals sharing the general formula ABO contain BO octahedra whereby B cations are surrounded by oxygen atoms. Such a configuration causes a superexchange mechanism mediated by dominating oxygen atoms, which makes the magnetic state more likely to be antiferromagnetic than ferromagnetic. Thus, the electronic structure is usually semiconducting and insulating. It has been though proven that the half-metallic state could emerge upon doping, strain, or intrinsic defects. The most known example is a non-stoichiometric LaSrMnO, based on antiferromagnetic and insulating LaMnO in which the sufficient Sr doping may yield half-metallicity [15]. The HT search revealed 23 potentially half-metallic perovskites that could be interesting for similar non-stoichiometric-doped configurations. Indeed, we have identified BaFeO recently reported to be ferromagnetic and half-metallic below ∼180 K [26]. We have also listed BaRuO, which was shown to be ferromagnetic [27], while the previous prediction of half-metallicity still awaits experimental verification [28]. Finally, we have revealed stoichiometric SrRuO, which was theoretically predicted to be half-metallic under doping with Sn or Ti [29,30].

2.2.3. Pyrochlores

The most representative example of a half-metallic pyrochlore is TlMnO [16]. However, it has not been listed in Table A2, as the electronic structure from the aflowlib database does not comply with the criteria for robust half-metallicity adapted in the HT search. In particular, the stringent condition imposed on the density of states eliminates the materials that would have more than 0.005 states/eV at energies within the band gap of the insulating channel. The closer inspection of the calculated electronic structure confirms that TlMnO indeed does not fit in this regime; this compound could only be included upon increasing the threshold. Nevertheless, we have found 11 different pyrochlores with the half-metallic electronic structure.

2.3. Quaternary Oxides

The majority of the quaternary oxides reported in Table A3 belongs to the crystal family group of the double perovskites with the general formula ABB’O, consisting of two different perovskites—ABO and AB’O—arranged in a three-dimensional checkerboard pattern. The possibility of choosing two different transition metal ions opens up a wide range of possibilities to tailor the magnetic properties in this class of materials. The most known compound from this family is SrMoFeO with a large magnetic moment and Curie temperature of more than 420 K [17,31]. The HT search revealed over 30 double perovskites, including a similar SrMoCoO structure. The mechanism that determines half-metallicity in these compounds is quite complex and related to a combination of superexchange interaction in the B-O-B’ chains and the hybridization of transition metal orbitals with O 2p states. The magnetic properties of double perovskites are, however, quite well known [32]. In addition, Table A1 contains previously unrecognized prototypes, many of which seem to be good candidates for half metals.

2.4. Quinary Oxides

Finally, we note that the search revealed only four quinary compounds. Such a result does not mean that quinary half-metallic oxides would not occur in nature. The reason is mostly related to a currently limited number of quinary materials in the aflowlib repository. In particular, we have noticed that a known half-metallic quadruple perovskite CaCuFeReO has not been found because it is yet absent in the database [33]. The materials listed in Table A4 are rather difficult to analyze without performing additonal calculations. Although the radioactive CHPPuO is useless for spintronics, CuHMnNO, HKNPdO, and LaMnSWO indeed seem to be robust half metals. The latter can be considered a quasi-1D spin chain and would probably reveal short range magnetic ordering below 4 K. The ground states still need to be verified but these structures are clearly new prototypes of half metals (see Figure 2).

Figure 2

Crystal structures of quinary prototypes of half metals revealed in the HT search.

3. Discussion

The presented search based on the HT screening suggests that several new half-metallic oxides could be discovered. In fact, the choice of oxides as target materials for the above analysis was deliberate in a larger perspective of rapidly evolving oxide spintronics [34]. TMO often host a large variety of physical phenomena that emerge due to the complex interplay of the electronic charge, spin, and orbital degrees of freedom. Beyond magnetic properties, more exotic effects have been extensively studied, including multiferroicity, superconductivity, or magnetocaloric behavior [35,36,37]. Thus, the role of half-metallic oxides does not need to be limited solely to the generation of spin-polarized currents; being highly multifunctional, they could be capable to perform multiple tasks within one spin-based device [38]. The interplay of diverse phenomena in realistic half-metallic systems is therefore an interesting direction to explore in more specific studies.

Importantly, realization of either novel or conventional spintronics devices operating at room temperature requires robust half-metallicity. One could therefore raise a question, how many of the identified compounds will be still half-metallic at 300 K? Our search and analysis based primarily on the electronic structure information from the aflowlib cannot give a precise answer at the present stage. As we have previously explained, most of the magnetic compounds in materials databases are in a ferromagnetic configuration, whereas, in reality, oxides often exhibit antiferromagnetic or more complex magnetic ground states. Although a large number of the identified materials may indeed be ferromagnetic and several are confirmed half metals, a complete verification of the magnetic phase along with the transition temperature would be desirable [39,40,41]. Multi-step calculations of numerous hypothetical magnetic configurations for each compound are though a great challenge. In a short-term perspective, exploring particular oxide-based interfaces could be more appropriate than the verification of the whole dataset ground states.

4. Methods

The high-throughput screening has been performed utilizing the aflux search engine, which helps to extract the electronic structure data from the aflowlib database [42]. In particular, we have analyzed the output files of density functional calculations performed within the aflow framework [43,44,45], which leverages the Vienna Ab initio Simulation Package (VASP) [46,47]. Projector-augmented-wave pseudopotentials were used to treat electron–ion interactions [48]; kinetic energy cut-offs were set to highest recommended value among corresponding pseudopotentials. The exchange–correlation interaction was treated in the generalized gradient approximation in the parametrization of Perdew, Burke, and Ernzerhof (PBE) [49]. LSDA+U approach in formulation of Dudarev [50] was used to account for electronic correlations of transition metal ions (U parameters are listed in [19]). Spin–orbit interaction was not included in the calculations.

The search procedure has been described in Results; we hereby give technical details which determine the exact output of each phase, necessary to reproduce the list of compounds extracted from aflowlib. In the first step, based on the condition P(E) = ≠ 0, we select thousands of magnetic materials with spin imbalance at the Fermi level. Certainly, such a descriptor cannot provide sufficient information about the half-metallic state. Several materials that satisfy this criterion may not have a robust band gap. For instance, binary compound CuO (ICSD-628616) switches the conductance between two spin channels just around , and reveals semi-metallic rather than half-metallic properties. The essential phase of screening relies on the analysis of DOS. Materials with more (less) than 0.005 states/eV at energies within the band gap in the insulating (conducting) channel around are screened out as not half-metallic (note that this criterion eliminates a known half metal TlMnO, see Section 2.2.3). As an outcome of this phase, we have revealed 1061 compounds, among which 494 are oxides. Finally, we have selected materials referred to as strong half metals potentially useful for spintronics, whose spin imbalance is sufficiently large within the energy window of the band gap and robust against thermal fluctuations. Screening based on the set of constraints (P(E) > 0.8; E < 3.5 eV; −0.01 > VBM > −1.5 eV; 0.1 < CBM < 2.5 eV) revealed 223 oxides reported in Table A1, Table A2, Table A3 and Table A4.

5. Conclusions

In summary, we have explored the aflowlib repository containing electronic structure data of thousands of materials, and identified over one hundred potentially half-metallic oxides. A large number of these compounds were not previously recognized as half metals. Remarkably, we have revealed new crystal prototypes, suggesting a way to design additional half-metallic oxides sharing the same structure. Finally, we have also indicated numerous crystals belonging to the same families as some of the known half-metallic compounds. These include newly identified spinels, perovskites, pyrochlores, and double perovskites. The quantitative analysis of the electronic structure has indicated a strong p-d hybridization as a common mechanism behind half-metallicity in a vast majority of the considered compounds. We believe that this study will stimulate further exploration of a larger chemical space as well as an ultimate confirmation of half-metallicity in selected structures.